Network Working Group P. Almquist, Author
Request for Comments: 1716 Consultant
Category: Informational F. Kastenholz, Editor
FTP Software, Inc.
November 1994

Towards Requirements for IP Routers

Status of this Memo

This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.

Table of Contents

0. PREFACE ....................................................... 1
1. INTRODUCTION .................................................. 2
1.1 Reading this Document ........................................ 4
1.1.1 Organization ............................................... 4
1.1.2 Requirements ............................................... 5
1.1.3 Compliance ................................................. 6
1.2 Relationships to Other Standards ............................. 7
1.3 General Considerations ....................................... 8
1.3.1 Continuing Internet Evolution .............................. 8
1.3.2 Robustness Principle ....................................... 9
1.3.3 Error Logging .............................................. 9
1.3.4 Configuration .............................................. 10
1.4 Algorithms ................................................... 11
2. INTERNET ARCHITECTURE ......................................... 13
2.1 Introduction ................................................. 13
2.2 Elements of the Architecture ................................. 14
2.2.1 Protocol Layering .......................................... 14
2.2.2 Networks ................................................... 16
2.2.3 Routers .................................................... 17
2.2.4 Autonomous Systems ......................................... 18
2.2.5 Addresses and Subnets ...................................... 18
2.2.6 IP Multicasting ............................................ 20
2.2.7 Unnumbered Lines and Networks and Subnets .................. 20
2.2.8 Notable Oddities ........................................... 22
2.2.8.1 Embedded Routers ......................................... 22
2.2.8.2 Transparent Routers ...................................... 23
2.3 Router Characteristics ....................................... 24
2.4 Architectural Assumptions .................................... 27
3. LINK LAYER .................................................... 29
3.1 INTRODUCTION ................................................. 29
3.2 LINK/INTERNET LAYER INTERFACE ................................ 29
3.3 SPECIFIC ISSUES .............................................. 30
3.3.1 Trailer Encapsulation ...................................... 30
3.3.2 Address Resolution Protocol - ARP .......................... 31
3.3.3 Ethernet and 802.3 Coexistence ............................. 31
3.3.4 Maximum Transmission Unit - MTU ............................ 31
3.3.5 Point-to-Point Protocol - PPP .............................. 32
3.3.5.1 Introduction ............................................. 32
3.3.5.2 Link Control Protocol (LCP) Options ...................... 33
3.3.5.3 IP Control Protocol (ICP) Options ........................ 34
3.3.6 Interface Testing .......................................... 35
4. INTERNET LAYER - PROTOCOLS .................................... 36
4.1 INTRODUCTION ................................................. 36
4.2 INTERNET PROTOCOL - IP ....................................... 36

4.2.1 INTRODUCTION ............................................... 36
4.2.2 PROTOCOL WALK-THROUGH ...................................... 37
4.2.2.1 Options: RFC-791 Section 3.2 ............................. 37
4.2.2.2 Addresses in Options: RFC-791 Section 3.1 ................ 40
4.2.2.3 Unused IP Header Bits: RFC-791 Section 3.1 ............... 40
4.2.2.4 Type of Service: RFC-791 Section 3.1 ..................... 41
4.2.2.5 Header Checksum: RFC-791 Section 3.1 ..................... 41
4.2.2.6 Unrecognized Header Options: RFC-791 Section 3.1 ......... 41
4.2.2.7 Fragmentation: RFC-791 Section 3.2 ....................... 42
4.2.2.8 Reassembly: RFC-791 Section 3.2 .......................... 43
4.2.2.9 Time to Live: RFC-791 Section 3.2 ........................ 43
4.2.2.10 Multi-subnet Broadcasts: RFC-922 ........................ 43
4.2.2.11 Addressing: RFC-791 Section 3.2 ......................... 43
4.2.3 SPECIFIC ISSUES ............................................ 47
4.2.3.1 IP Broadcast Addresses ................................... 47
4.2.3.2 IP Multicasting .......................................... 48
4.2.3.3 Path MTU Discovery ....................................... 48
4.2.3.4 Subnetting ............................................... 49
4.3 INTERNET CONTROL MESSAGE PROTOCOL - ICMP ..................... 50
4.3.1 INTRODUCTION ............................................... 50
4.3.2 GENERAL ISSUES ............................................. 50
4.3.2.1 Unknown Message Types .................................... 50
4.3.2.2 ICMP Message TTL ......................................... 51
4.3.2.3 Original Message Header .................................. 51
4.3.2.4 ICMP Message Source Address .............................. 51
4.3.2.5 TOS and Precedence ....................................... 51
4.3.2.6 Source Route ............................................. 52
4.3.2.7 When Not to Send ICMP Errors ............................. 53
4.3.2.8 Rate Limiting ............................................ 54
4.3.3 SPECIFIC ISSUES ............................................ 55
4.3.3.1 Destination Unreachable .................................. 55
4.3.3.2 Redirect ................................................. 55
4.3.3.3 Source Quench ............................................ 56
4.3.3.4 Time Exceeded ............................................ 56
4.3.3.5 Parameter Problem ........................................ 57
4.3.3.6 Echo Request/Reply ....................................... 57
4.3.3.7 Information Request/Reply ................................ 58
4.3.3.8 Timestamp and Timestamp Reply ............................ 58
4.3.3.9 Address Mask Request/Reply ............................... 59
4.3.3.10 Router Advertisement and Solicitations .................. 61
4.4 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP .................... 61
5. INTERNET LAYER - FORWARDING ................................... 62
5.1 INTRODUCTION ................................................. 62
5.2 FORWARDING WALK-THROUGH ...................................... 62
5.2.1 Forwarding Algorithm ....................................... 62
5.2.1.1 General .................................................. 63
5.2.1.2 Unicast .................................................. 64

5.2.1.3 Multicast ................................................ 65
5.2.2 IP Header Validation ....................................... 66
5.2.3 Local Delivery Decision .................................... 68
5.2.4 Determining the Next Hop Address ........................... 70
5.2.4.1 Immediate Destination Address ............................ 71
5.2.4.2 Local/Remote Decision .................................... 71
5.2.4.3 Next Hop Address ......................................... 72
5.2.4.4 Administrative Preference ................................ 77
5.2.4.6 Load Splitting ........................................... 78
5.2.5 Unused IP Header Bits: RFC-791 Section 3.1 ................. 79
5.2.6 Fragmentation and Reassembly: RFC-791 Section 3.2 .......... 79
5.2.7 Internet Control Message Protocol - ICMP ................... 80
5.2.7.1 Destination Unreachable .................................. 80
5.2.7.2 Redirect ................................................. 82
5.2.7.3 Time Exceeded ............................................ 84
5.2.8 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP .................. 84
5.3 SPECIFIC ISSUES .............................................. 84
5.3.1 Time to Live (TTL) ......................................... 84
5.3.2 Type of Service (TOS) ...................................... 85
5.3.3 IP Precedence .............................................. 87
5.3.3.1 Precedence-Ordered Queue Service ......................... 88
5.3.3.2 Lower Layer Precedence Mappings .......................... 88
5.3.3.3 Precedence Handling For All Routers ...................... 89
5.3.4 Forwarding of Link Layer Broadcasts ........................ 92
5.3.5 Forwarding of Internet Layer Broadcasts .................... 92
5.3.5.1 Limited Broadcasts ....................................... 94
5.3.5.2 Net-directed Broadcasts .................................. 94
5.3.5.3 All-subnets-directed Broadcasts .......................... 95
5.3.5.4 Subnet-directed Broadcasts ............................... 97
5.3.6 Congestion Control ......................................... 97
5.3.7 Martian Address Filtering .................................. 99
5.3.8 Source Address Validation .................................. 99
5.3.9 Packet Filtering and Access Lists .......................... 100
5.3.10 Multicast Routing ......................................... 101
5.3.11 Controls on Forwarding .................................... 101
5.3.12 State Changes ............................................. 101
5.3.12.1 When a Router Ceases Forwarding ......................... 102
5.3.12.2 When a Router Starts Forwarding ......................... 102
5.3.12.3 When an Interface Fails or is Disabled .................. 103
5.3.12.4 When an Interface is Enabled ............................ 103
5.3.13 IP Options ................................................ 103
5.3.13.1 Unrecognized Options .................................... 103
5.3.13.2 Security Option ......................................... 104
5.3.13.3 Stream Identifier Option ................................ 104
5.3.13.4 Source Route Options .................................... 104
5.3.13.5 Record Route Option ..................................... 104
5.3.13.6 Timestamp Option ........................................ 105

6. TRANSPORT LAYER ............................................... 106
6.1 USER DATAGRAM PROTOCOL - UDP ................................. 106
6.2 TRANSMISSION CONTROL PROTOCOL - TCP .......................... 106
7. APPLICATION LAYER - ROUTING PROTOCOLS ......................... 109
7.1 INTRODUCTION ................................................. 109
7.1.1 Routing Security Considerations ............................ 109
7.1.2 Precedence ................................................. 110
7.2 INTERIOR GATEWAY PROTOCOLS ................................... 110
7.2.1 INTRODUCTION ............................................... 110
7.2.2 OPEN SHORTEST PATH FIRST - OSPF ............................ 111
7.2.2.1 Introduction ............................................. 111
7.2.2.2 Specific Issues .......................................... 111
7.2.2.3 New Version of OSPF ...................................... 112
7.2.3 INTERMEDIATE SYSTEM TO INTERMEDIATE SYSTEM - DUAL IS-IS
.............................................................. 112
7.2.4 ROUTING INFORMATION PROTOCOL - RIP ......................... 113
7.2.4.1 Introduction ............................................. 113
7.2.4.2 Protocol Walk-Through .................................... 113
7.2.4.3 Specific Issues .......................................... 118
7.2.5 GATEWAY TO GATEWAY PROTOCOL - GGP .......................... 119
7.3 EXTERIOR GATEWAY PROTOCOLS ................................... 119
7.3.1 INTRODUCTION ............................................... 119
7.3.2 BORDER GATEWAY PROTOCOL - BGP .............................. 120
7.3.2.1 Introduction ............................................. 120
7.3.2.2 Protocol Walk-through .................................... 120
7.3.3 EXTERIOR GATEWAY PROTOCOL - EGP ............................ 121
7.3.3.1 Introduction ............................................. 121
7.3.3.2 Protocol Walk-through .................................... 122
7.3.4 INTER-AS ROUTING WITHOUT AN EXTERIOR PROTOCOL .............. 124
7.4 STATIC ROUTING ............................................... 125
7.5 FILTERING OF ROUTING INFORMATION ............................. 127
7.5.1 Route Validation ........................................... 127
7.5.2 Basic Route Filtering ...................................... 127
7.5.3 Advanced Route Filtering ................................... 128
7.6 INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE .................. 129
8. APPLICATION LAYER - NETWORK MANAGEMENT PROTOCOLS .............. 131
8.1 The Simple Network Management Protocol - SNMP ................ 131
8.1.1 SNMP Protocol Elements ..................................... 131
8.2 Community Table .............................................. 132
8.3 Standard MIBS ................................................ 133
8.4 Vendor Specific MIBS ......................................... 134
8.5 Saving Changes ............................................... 135
9. APPLICATION LAYER - MISCELLANEOUS PROTOCOLS ................... 137
9.1 BOOTP ........................................................ 137
9.1.1 Introduction ............................................... 137
9.1.2 BOOTP Relay Agents ......................................... 137
10. OPERATIONS AND MAINTENANCE ................................... 139

10.1 Introduction ................................................ 139
10.2 Router Initialization ....................................... 140
10.2.1 Minimum Router Configuration .............................. 140
10.2.2 Address and Address Mask Initialization ................... 141
10.2.3 Network Booting using BOOTP and TFTP ...................... 142
10.3 Operation and Maintenance ................................... 143
10.3.1 Introduction .............................................. 143
10.3.2 Out Of Band Access ........................................ 144
10.3.2 Router O&M Functions ...................................... 144
10.3.2.1 Maintenance - Hardware Diagnosis ........................ 144
10.3.2.2 Control - Dumping and Rebooting ......................... 145
10.3.2.3 Control - Configuring the Router ........................ 145
10.3.2.4 Netbooting of System Software ........................... 146
10.3.2.5 Detecting and responding to misconfiguration ............ 146
10.3.2.6 Minimizing Disruption ................................... 147
10.3.2.7 Control - Troubleshooting Problems ...................... 148
10.4 Security Considerations ..................................... 149
10.4.1 Auditing and Audit Trails ................................. 149
10.4.2 Configuration Control ..................................... 150
11. REFERENCES ................................................... 152
APPENDIX A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS ................ 162
APPENDIX B. GLOSSARY ............................................. 164
APPENDIX C. FUTURE DIRECTIONS .................................... 169
APPENDIX D. Multicast Routing Protocols .......................... 172
D.1 Introduction ................................................. 172
D.2 Distance Vector Multicast Routing Protocol - DVMRP ........... 172
D.3 Multicast Extensions to OSPF - MOSPF ......................... 173
APPENDIX E Additional Next-Hop Selection Algorithms .............. 174
E.1. Some Historical Perspective .................................. 174
E.2. Additional Pruning Rules ..................................... 176
E.3 Some Route Lookup Algorithms ................................. 177
E.3.1 The Revised Classic Algorithm ............................... 178
E.3.2 The Variant Router Requirements Algorithm ................... 179
E.3.3 The OSPF Algorithm .......................................... 179
E.3.4 The Integrated IS-IS Algorithm .............................. 180
Security Considerations ........................................... 182
Acknowledgments ................................................... 183
Editor's Address .................................................. 186

0. PREFACE

This document is a snapshot of the work of the Router Requirements
working group as of November 1991. At that time, the working group had
essentially finished its task. There were some final technical matters
to be nailed down, and a great deal of editing needed to be done in
order to get the document ready for publication. Unfortunately, these
tasks were never completed.

At the request of the Internet Area Director, the current editor took
the last draft of the document and, after consulting the mailing list
archives, meeting minutes, notes, and other members of the working
group, edited the document to its current form. This effort included
the following tasks: 1) Deleting all the parenthetical material (such as
editor's comments). Useful information was turned into DISCUSSION
sections, the rest was deleted. 2) Completing the tasks listed in the
last draft's To be Done sections. As a part of this task, a new "to be
done" list was developed and included as an appendix to the current
document. 3) Rolling Philip Almquist's "Ruminations on the Next Hop"
and "Ruminations on Route Leaking" into this document. These represent
significant work and should be kept. 4) Fulfilling the last intents of
the working group as determined from the archival material. The intent
of this effort was to get the document into a form suitable for
publication as an Historical RFCso that the significant work which went
into the creation of this document would be preserved.

The content and form of this document are due, in large part, to the
working group's chair, and document's original editor and author: Philip
Almquist. Without his efforts, this document would not exist.

1. INTRODUCTION

The goal of this work is to replace RFC-1009, Requirements for Internet
Gateways ([INTRO:1]) with a new document.

This memo is an intermediate step toward that goal. It defines and
discusses requirements for devices which perform the network layer
forwarding function of the Internet protocol suite. The Internet
community usually refers to such devices as IP routers or simply
routers; The OSI community refers to such devices as intermediate
systems. Many older Internet documents refer to these devices as
gateways, a name which more recently has largely passed out of favor to
avoid confusion with application gateways.

An IP router can be distinguished from other sorts of packet switching
devices in that a router examines the IP protocol header as part of the
switching process. It generally has to modify the IP header and to
strip off and replace the Link Layer framing.

The authors of this memo recognize, as should its readers, that many
routers support multiple protocol suites, and that support for multiple
protocol suites will be required in increasingly large parts of the
Internet in the future. This memo, however, does not attempt to specify
Internet requirements for protocol suites other than TCP/IP.

This document enumerates standard protocols that a router connected to
the Internet must use, and it incorporates by reference the RFCs and
other documents describing the current specifications for these
protocols. It corrects errors in the referenced documents and adds
additional discussion and guidance for an implementor.

For each protocol, this final version of this memo also contains an
eXPlicit set of requirements, recommendations, and options. The reader
must understand that the list of requirements in this memo is incomplete
by itself; the complete set of requirements for an Internet protocol
router is primarily defined in the standard protocol specification
documents, with the corrections, amendments, and supplements contained
in this memo.

This memo should be read in conjunction with the Requirements for
Internet Hosts RFCs ([INTRO:2] and [INTRO:3]). Internet hosts and
routers must both be capable of originating IP datagrams and receiving
IP datagrams destined for them. The major distinction between Internet
hosts and routers is that routers are required to implement forwarding
algorithms and Internet hosts do not require forwarding capabilities.
Any Internet host acting as a router must adhere to the requirements
contained in the final version of this memo.

The goal of open system interconnection dictates that routers must
function correctly as Internet hosts when necessary. To achieve this,
this memo provides guidelines for such instances. For simplification
and ease of document updates, this memo tries to avoid overlapping
discussions of host requirements with [INTRO:2] and [INTRO:3] and
incorporates the relevant requirements of those documents by reference.
In some cases the requirements stated in [INTRO:2] and [INTRO:3] are
superseded by the final version of this document.

A good-faith implementation of the protocols produced after careful
reading of the RFCs, with some interaction with the Internet technical
community, and that follows good communications software engineering
practices, should differ from the requirements of this memo in only
minor ways. Thus, in many cases, the requirements in this document are
already stated or implied in the standard protocol documents, so that
their inclusion here is, in a sense, redundant. However, they were
included because some past implementation has made the wrong choice,
causing problems of interoperability, performance, and/or robustness.

This memo includes discussion and explanation of many of the
requirements and recommendations. A simple list of requirements would
be dangerous, because:

o Some required features are more important than others, and some
features are optional.

o Some features are critical in some applications of routers but
irrelevant in others.

o There may be valid reasons why particular vendor products that are
designed for restricted contexts might choose to use different
specifications.

However, the specifications of this memo must be followed to meet the
general goal of arbitrary router interoperation across the diversity and
complexity of the Internet. Although most current implementations fail
to meet these requirements in various ways, some minor and some major,
this specification is the ideal towards which we need to move.

These requirements are based on the current level of Internet
architecture. This memo will be updated as required to provide
additional clarifications or to include additional information in those
areas in which specifications are still evolving.

1.1 Reading this Document

1.1.1 Organization

This memo emulates the layered organization used by [INTRO:2] and
[INTRO:3]. Thus, Chapter 2 describes the layers found in the
Internet architecture. Chapter 3 covers the Link Layer. Chapters
4 and 5 are concerned with the Internet Layer protocols and
forwarding algorithms. Chapter 6 covers the Transport Layer.
Upper layer protocols are divided between Chapter 7, which
discusses the protocols which routers use to exchange routing
information with each other, Chapter 8, which discusses network
management, and Chapter 9, which discusses other upper layer
protocols. The final chapter covers operations and maintenance
features. This organization was chosen for simplicity, clarity,
and consistency with the Host Requirements RFCs. Appendices to
this memo include a bibliography, a glossary, and some conjectures
about future directions of router standards.

In describing the requirements, we assume that an implementation
strictly mirrors the layering of the protocols. However, strict
layering is an imperfect model, both for the protocol suite and
for recommended implementation approaches. Protocols in different
layers interact in complex and sometimes suBTle ways, and
particular functions often involve multiple layers. There are
many design choices in an implementation, many of which involve
creative breaking of strict layering. Every implementor is urged
to read [INTRO:4] and [INTRO:5].

In general, each major section of this memo is organized into the
following subsections:

(1) Introduction

(2) Protocol Walk-Through - considers the protocol specification
documents section-by-section, correcting errors, stating
requirements that may be ambiguous or ill-defined, and
providing further clarification or explanation.

(3) Specific Issues - discusses protocol design and
implementation issues that were not included in the walk-
through.

Under many of the individual topics in this memo, there is
parenthetical material labeled DISCUSSION or IMPLEMENTATION. This
material is intended to give a justification, clarification or

explanation to the preceding requirements text. The
implementation material contains suggested approaches that an
implementor may want to consider. The DISCUSSION and
IMPLEMENTATION sections are not part of the standard.

1.1.2 Requirements

In this memo, the Words that are used to define the significance
of each particular requirement are capitalized. These words are:

o MUST
This word means that the item is an absolute requirement of the
specification.

o MUST IMPLEMENT
This phrase means that this specification requires that the
item be implemented, but does not require that it be enabled by
default.

o MUST NOT
This phrase means that the item is an absolute prohibition of
the specification.

o SHOULD
This word means that there may exist valid reasons in
particular circumstances to ignore this item, but the full
implications should be understood and the case carefully
weighed before choosing a different course.

o SHOULD IMPLEMENT
This phrase is similar in meaning to SHOULD, but is used when
we recommend that a particular feature be provided but does not
necessarily recommend that it be enabled by default.

o SHOULD NOT
This phrase means that there may exist valid reasons in
particular circumstances when the described behavior is
acceptable or even useful, but the full implications should be
understood and the case carefully weighed before implementing
any behavior described with this label.

o MAY
This word means that this item is truly optional. One vendor
may choose to include the item because a particular marketplace
requires it or because it enhances the product, for example;
another vendor may omit the same item.

1.1.3 Compliance

Some requirements are applicable to all routers. Other
requirements are applicable only to those which implement
particular features or protocols. In the following paragraphs,
Relevant refers to the union of the requirements applicable to all
routers and the set of requirements applicable to a particular
router because of the set of features and protocols it has
implemented.

Note that not all Relevant requirements are stated directly in
this memo. Various parts of this memo incorporate by reference
sections of the Host Requirements specification, [INTRO:2] and
[INTRO:3]. For purposes of determining compliance with this memo,
it does not matter whether a Relevant requirement is stated
directly in this memo or merely incorporated by reference from one
of those documents.

An implementation is said to be conditionally compliant if it
satisfies all of the Relevant MUST, MUST IMPLEMENT, and MUST NOT
requirements. An implementation is said to be unconditionally
compliant if it is conditionally compliant and also satisfies all
of the Relevant SHOULD, SHOULD IMPLEMENT, and SHOULD NOT
requirements. An implementation is not compliant if it is not
conditionally compliant (i.e., it fails to satisfy one or more of
the Relevant MUST, MUST IMPLEMENT, or MUST NOT requirements).

For any of the SHOULD and SHOULD NOT requirements, a router may
provide a configuration option that will cause the router to act
other than as specified by the requirement. Having such a
configuration option does not void a router's claim to
unconditional compliance as long as the option has a default
setting, and that leaving the option at its default setting causes
the router to operate in a manner which conforms to the
requirement.

Likewise, routers may provide, except where explicitly prohibited
by this memo, options which cause them to violate MUST or MUST NOT
requirements. A router which provides such options is compliant
(either fully or conditionally) if and only if each such option
has a default setting which causes the router to conform to the
requirements of this memo. Please note that the authors of this
memo, although aware of market realities, strongly recommend
against provision of such options. Requirements are labeled MUST
or MUST NOT because experts in the field have judged them to be
particularly important to interoperability or proper functioning
in the Internet. Vendors should weigh carefully the customer

support costs of providing options which violate those rules.

Of course, this memo is not a complete specification of an IP
router, but rather is closer to what in the OSI world is called a
profile. For example, this memo requires that a number of
protocols be implemented. Although most of the contents of their
protocol specifications are not repeated in this memo,
implementors are nonetheless required to implement the protocols
according to those specifications.

1.2 Relationships to Other Standards

There are several reference documents of interest in checking the
current status of protocol specifications and standardization:

o INTERNET OFFICIAL PROTOCOL STANDARDS
This document describes the Internet standards process and lists
the standards status of the protocols. As of this writing, the
current version of this document is STD 1, RFC1610, [ARCH:7].
This document is periodically re-issued. You should always
consult an RFCrepository and use the latest version of this
document.

o Assigned Numbers
This document lists the assigned values of the parameters used
in the various protocols. For example, IP protocol codes, TCP
port numbers, Telnet Option Codes, ARP hardware types, and
Terminal Type names. As of this writing, the current version of
this document is STD 2, RFC1700, [INTRO:7]. This document is
periodically re-issued. You should always consult an RFC
repository and use the latest version of this document.

o Host Requirements
This pair of documents reviews the specifications that apply to
hosts and supplies guidance and clarification for any
ambiguities. Note that these requirements also apply to
routers, except where otherwise specified in this memo. As of
this writing (December, 1993) the current versions of these
documents are RFC1122 and RFC1123, (STD 3) [INTRO:2], and
[INTRO:3] respectively.

o Router Requirements (formerly Gateway Requirements)
This memo.

Note that these documents are revised and updated at different
times; in case of differences between these documents, the most
recent must prevail.

These and other Internet protocol documents may be obtained from
the:

The InterNIC
DS.INTERNIC.NET
InterNIC Directory and Database Service

+1 (800) 444-4345 or +1 (619) 445-4600

info@internic.net

1.3 General Considerations

There are several important lessons that vendors of Internet software
have learned and which a new vendor should consider seriously.

1.3.1 Continuing Internet Evolution

The enormous growth of the Internet has revealed problems of
management and scaling in a large datagram-based packet
communication system. These problems are being addressed, and as
a result there will be continuing evolution of the specifications
described in this memo. New routing protocols, algorithms, and
architectures are constantly being developed. New and additional
internet-layer protocols are also constantly being devised.
Because routers play such a crucial role in the Internet, and
because the number of routers deployed in the Internet is much
smaller than the number of hosts, vendors should expect that
router standards will continue to evolve much more quickly than
host standards. These changes will be carefully planned and
controlled since there is extensive participation in this planning
by the vendors and by the organizations responsible for operation
of the networks.

Development, evolution, and revision are characteristic of
computer network protocols today, and this situation will persist
for some years. A vendor who develops computer communications
software for the Internet protocol suite (or any other protocol
suite!) and then fails to maintain and update that software for
changing specifications is going to leave a trail of unhappy
customers. The Internet is a large communication network, and the
users are in constant contact through it. Experience has shown
that knowledge of deficiencies in vendor software propagates
quickly through the Internet technical community.

1.3.2 Robustness Principle

At every layer of the protocols, there is a general rule (from
[TRANS:2] by Jon Postel) whose application can lead to enormous
benefits in robustness and interoperability:

Be conservative in what you do,
be liberal in what you accept from others.

Software should be written to deal with every conceivable error,
no matter how unlikely; sooner or later a packet will come in with
that particular combination of errors and attributes, and unless
the software is prepared, chaos can ensue. In general, it is best
to assume that the network is filled with malevolent entities that
will send packets designed to have the worst possible effect.
This assumption will lead to suitably protective design. The most
serious problems in the Internet have been caused by unforeseen
mechanisms triggered by low probability events; mere human malice
would never have taken so devious a course!

Adaptability to change must be designed into all levels of router
software. As a simple example, consider a protocol specification
that contains an enumeration of values for a particular header
field - e.g., a type field, a port number, or an error code; this
enumeration must be assumed to be incomplete. If the protocol
specification defines four possible error codes, the software must
not break when a fifth code shows up. An undefined code might be
logged, but it must not cause a failure.

The second part of the principle is almost as important: software
on hosts or other routers may contain deficiencies that make it
unwise to exploit legal but obscure protocol features. It is
unwise to stray far from the obvious and simple, lest untoward
effects result elsewhere. A corollary of this is watch out for
misbehaving hosts; router software should be prepared to survive
in the presence of misbehaving hosts. An important function of
routers in the Internet is to limit the amount of disruption such
hosts can inflict on the shared communication facility.

1.3.3 Error Logging

The Internet includes a great variety of systems, each
implementing many protocols and protocol layers, and some of these
contain bugs and misfeatures in their Internet protocol software.
As a result of complexity, diversity, and distribution of
function, the diagnosis of problems is often very difficult.

Problem diagnosis will be aided if routers include a carefully
designed facility for logging erroneous or strange events. It is
important to include as much diagnostic information as possible
when an error is logged. In particular, it is often useful to
record the header(s) of a packet that caused an error. However,
care must be taken to ensure that error logging does not consume
prohibitive amounts of resources or otherwise interfere with the
operation of the router.

There is a tendency for abnormal but harmless protocol events to
overflow error logging files; this can be avoided by using a
circular log, or by enabling logging only while diagnosing a known
failure. It may be useful to filter and count duplicate
successive messages. One strategy that seems to work well is to
both:
o Always count abnormalities and make such counts accessible
through the management protocol (see Chapter 8); and
o Allow the logging of a great variety of events to be
selectively enabled. For example, it might useful to be able
to log everything or to log everything for host X.

This topic is further discussed in [MGT:5].

1.3.4 Configuration

In an ideal world, routers would be easy to configure, and perhaps
even entirely self-configuring. However, practical experience in
the real world suggests that this is an impossible goal, and that
in fact many attempts by vendors to make configuration easy
actually cause customers more grief than they prevent. As an
extreme example, a router designed to come up and start routing
packets without requiring any configuration information at all
would almost certainly choose some incorrect parameter, possibly
causing serious problems on any networks unfortunate enough to be
connected to it.

Often this memo requires that a parameter be a configurable
option. There are several reasons for this. In a few cases there
currently is some uncertainty or disagreement about the best value
and it may be necessary to update the recommended value in the
future. In other cases, the value really depends on external
factors - e.g., the distribution of its communication load, or the
speeds and topology of nearby networks - and self-tuning
algorithms are unavailable and may be insufficient. In some
cases, configurability is needed because of administrative
requirements.

Finally, some configuration options are required to communicate
with obsolete or incorrect implementations of the protocols,
distributed without sources, that persist in many parts of the
Internet. To make correct systems coexist with these faulty
systems, administrators must occasionally misconfigure the correct
systems. This problem will correct itself gradually as the faulty
systems are retired, but cannot be ignored by vendors.

When we say that a parameter must be configurable, we do not
intend to require that its value be explicitly read from a
configuration file at every boot time. For many parameters, there
is one value that is appropriate for all but the most unusual
situations. In such cases, it is quite reasonable that the
parameter default to that value if not explicitly set.

This memo requires a particular value for such defaults in some
cases. The choice of default is a sensitive issue when the
configuration item controls accommodation of existing, faulty,
systems. If the Internet is to converge successfully to complete
interoperability, the default values built into implementations
must implement the official protocol, not misconfigurations to
accommodate faulty implementations. Although marketing
considerations have led some vendors to choose misconfiguration
defaults, we urge vendors to choose defaults that will conform to
the standard.

Finally, we note that a vendor needs to provide adequate
documentation on all configuration parameters, their limits and
effects.

1.4 Algorithms

In several places in this memo, specific algorithms that a router
ought to follow are specified. These algorithms are not, per se,
required of the router. A router need not implement each algorithm
as it is written in this document. Rather, an implementation must
present a behavior to the external world that is the same as a
strict, literal, implementation of the specified algorithm.

Algorithms are described in a manner that differs from the way a good
implementor would implement them. For expository purposes, a style
that emphasizes conciseness, clarity, and independence from
implementation details has been chosen. A good implementor will
choose algorithms and implementation methods which produce the same
results as these algorithms, but may be more efficient or less
general.

We note that the art of efficient router implementation is outside of
the scope of this memo.

2. INTERNET ARCHITECTURE

This chapter does not contain any requirements. However, it does
contain useful background information on the general architecture of the
Internet and of routers.

General background and discussion on the Internet architecture and
supporting protocol suite can be found in the DDN Protocol Handbook
[ARCH:1]; for background see for example [ARCH:2], [ARCH:3], and
[ARCH:4]. The Internet architecture and protocols are also covered in
an ever-growing number of textbooks, such as [ARCH:5] and [ARCH:6].

2.1 Introduction

The Internet system consists of a number of interconnected packet
networks supporting communication among host computers using the
Internet protocols. These protocols include the Internet Protocol
(IP), the Internet Control Message Protocol (ICMP), the Internet
Group Management Protocol (IGMP), and a variety transport and
application protocols that depend upon them. As was described in
Section [1.2], the Internet Engineering Steering Group periodically
releases an Official Protocols memo listing all of the Internet
protocols.

All Internet protocols use IP as the basic data transport mechanism.
IP is a datagram, or connectionless, internetwork service and
includes provision for addressing, type-of-service specification,
fragmentation and reassembly, and security. ICMP and IGMP are
considered integral parts of IP, although they are architecturally
layered upon IP. ICMP provides error reporting, flow control,
first-hop router redirection, and other maintenance and control
functions. IGMP provides the mechanisms by which hosts and routers
can join and leave IP multicast groups.

Reliable data delivery is provided in the Internet protocol suite by
Transport Layer protocols such as the Transmission Control Protocol
(TCP), which provides end-end retransmission, resequencing and
connection control. Transport Layer connectionless service is
provided by the User Datagram Protocol (UDP).

2.2 Elements of the Architecture

2.2.1 Protocol Layering

To communicate using the Internet system, a host must implement
the layered set of protocols comprising the Internet protocol
suite. A host typically must implement at least one protocol from
each layer.

The protocol layers used in the Internet architecture are as
follows [ARCH:7]:

o Application Layer
The Application Layer is the top layer of the Internet protocol
suite. The Internet suite does not further subdivide the
Application Layer, although some application layer protocols do
contain some internal sub-layering. The application layer of
the Internet suite essentially combines the functions of the
top two layers - Presentation and Application - of the OSI
Reference Model [ARCH:8]. The Application Layer in the
Internet protocol suite also includes some of the function
relegated to the Session Layer in the OSI Reference Model.

We distinguish two categories of application layer protocols:
user protocols that provide service directly to users, and
support protocols that provide common system functions. The
most common Internet user protocols are:
- Telnet (remote login)
- FTP (file transfer)
- SMTP (electronic mail delivery)

There are a number of other standardized user protocols and
many private user protocols.

Support protocols, used for host name mapping, booting, and
management, include SNMP, BOOTP, TFTP, the Domain Name System
(DNS) protocol, and a variety of routing protocols.

Application Layer protocols relevant to routers are discussed
in chapters 7, 8, and 9 of this memo.

o Transport Layer
The Transport Layer provides end-to-end communication services.
This layer is roughly equivalent to the Transport Layer in the
OSI Reference Model, except that it also incorporates some of
OSI's Session Layer establishment and destruction functions.

There are two primary Transport Layer protocols at present:
- Transmission Control Protocol (TCP)
- User Datagram Protocol (UDP)

TCP is a reliable connection-oriented transport service that
provides end-to-end reliability, resequencing, and flow
control. UDP is a connectionless (datagram) transport service.
Other transport protocols have been developed by the research
community, and the set of official Internet transport protocols
may be expanded in the future.

Transport Layer protocols relevant to routers are discussed in
Chapter 6.

o Internet Layer
All Internet transport protocols use the Internet Protocol (IP)
to carry data from source host to destination host. IP is a
connectionless or datagram internetwork service, providing no
end-to-end delivery guarantees. IP datagrams may arrive at the
destination host damaged, duplicated, out of order, or not at
all. The layers above IP are responsible for reliable delivery
service when it is required. The IP protocol includes
provision for addressing, type-of-service specification,
fragmentation and reassembly, and security.

The datagram or connectionless nature of IP is a fundamental
and characteristic feature of the Internet architecture.

The Internet Control Message Protocol (ICMP) is a control
protocol that is considered to be an integral part of IP,
although it is architecturally layered upon IP, i.e., it uses
IP to carry its data end-to-end. ICMP provides error
reporting, congestion reporting, and first-hop router
redirection.

The Internet Group Management Protocol (IGMP) is an Internet
layer protocol used for establishing dynamic host groups for IP
multicasting.

The Internet layer protocols IP, ICMP, and IGMP are discussed
in chapter 4.

o Link Layer
To communicate on its directly-connected network, a host must
implement the communication protocol used to interface to that
network. We call this a Link Layer layer protocol.

Some older Internet documents refer to this layer as the
Network Layer, but it is not the same as the Network Layer in
the OSI Reference Model.

This layer contains everything below the Internet Layer.

Protocols in this Layer are generally outside the scope of
Internet standardization; the Internet (intentionally) uses
existing standards whenever possible. Thus, Internet Link
Layer standards usually address only address resolution and
rules for transmitting IP packets over specific Link Layer
protocols. Internet Link Layer standards are discussed in
chapter 3.

2.2.2 Networks

The constituent networks of the Internet system are required to
provide only packet (connectionless) transport. According to the
IP service specification, datagrams can be delivered out of order,
be lost or duplicated, and/or contain errors.

For reasonable performance of the protocols that use IP (e.g.,
TCP), the loss rate of the network should be very low. In
networks providing connection-oriented service, the extra
reliability provided by virtual circuits enhances the end-end
robustness of the system, but is not necessary for Internet
operation.

Constituent networks may generally be divided into two classes:

o Local-Area Networks (LANs)
LANs may have a variety of designs. In general, a LAN will
cover a small geographical area (e.g., a single building or
plant site) and provide high bandwidth with low delays. LANs
may be passive (similar to Ethernet) or they may be active
(such as ATM).

o Wide-Area Networks (WANs)
Geographically-dispersed hosts and LANs are interconnected by
wide-area networks, also called long-haul networks. These
networks may have a complex internal structure of lines and
packet-switches, or they may be as simple as point-to-point
lines.

2.2.3 Routers

In the Internet model, constituent networks are connected together
by IP datagram forwarders which are called routers or IP routers.
In this document, every use of the term router is equivalent to IP
router. Many older Internet documents refer to routers as
gateways.

Historically, routers have been realized with packet-switching
software executing on a general-purpose CPU. However, as custom
hardware development becomes cheaper and as higher throughput is
required, but special-purpose hardware is becoming increasingly
common. This specification applies to routers regardless of how
they are implemented.

A router is connected to two or more networks, appearing to each
of these networks as a connected host. Thus, it has (at least)
one physical interface and (at least) one IP address on each of
the connected networks (this ignores the concept of un-numbered
links, which is discussed in section [2.2.7]). Forwarding an IP
datagram generally requires the router to choose the address of
the next-hop router or (for the final hop) the destination host.
This choice, called routing, depends upon a routing database
within the router. The routing database is also sometimes known
as a routing table or forwarding table.

The routing database should be maintained dynamically to reflect
the current topology of the Internet system. A router normally
accomplishes this by participating in distributed routing and
reachability algorithms with other routers.

Routers provide datagram transport only, and they seek to minimize
the state information necessary to sustain this service in the
interest of routing flexibility and robustness.

Packet switching devices may also operate at the Link Layer; such
devices are usually called bridges. Network segments which are
connected by bridges share the same IP network number, i.e., they
logically form a single IP network. These other devices are
outside of the scope of this document.

Another variation on the simple model of networks connected with
routers sometimes occurs: a set of routers may be interconnected
with only serial lines, to form a network in which the packet
switching is performed at the Internetwork (IP) Layer rather than
the Link Layer.

2.2.4 Autonomous Systems

For technical, managerial, and sometimes political reasons, the
routers of the Internet system are grouped into collections called
autonomous systems. The routers included in a single autonomous
system (AS) are expected to:

o Be under the control of a single operations and maintenance
(O&M) organization;

o Employ common routing protocols among themselves, to
dynamically maintain their routing databases.

A number of different dynamic routing protocols have been
developed (see Section [7.2]); the routing protocol within a
single AS is generically called an interior gateway protocol or
IGP.

An IP datagram may have to traverse the routers of two or more ASs
to reach its destination, and the ASs must provide each other with
topology information to allow such forwarding. An exterior
gateway protocol (generally BGP or EGP) is used for this purpose.

2.2.5 Addresses and Subnets

An IP datagram carries 32-bit source and destination addresses,
each of which is partitioned into two parts - a constituent
network number and a host number on that network. Symbolically:

IP-address ::= { <Network-number>, <Host-number> }

To finally deliver the datagram, the last router in its path must
map the Host-number (or rest) part of an IP address into the
physical address of a host connection to the constituent network.

This simple notion has been extended by the concept of subnets,
which were introduced in order to allow arbitrary complexity of
interconnected LAN structures within an organization, while
insulating the Internet system against explosive growth in network
numbers and routing complexity. Subnets essentially provide a
multi-level hierarchical routing structure for the Internet
system. The subnet extension, described in [INTERNET:2], is now a
required part of the Internet architecture. The basic idea is to
partition the <Host-number> field into two parts: a subnet number,
and a true host number on that subnet:

IP-address ::=

{ <Network-number>, <Subnet-number>, <Host-number> }

The interconnected physical networks within an organization will
be given the same network number but different subnet numbers.
The distinction between the subnets of such a subnetted network is
normally not visible outside of that network. Thus, routing in
the rest of the Internet will be based only upon the <Network-
number> part of the IP destination address; routers outside the
network will combine <Subnet-number> and <Host-number> together to
form an uninterpreted rest part of the 32-bit IP address. Within
the subnetted network, the routers must route on the basis of an
extended network number:

{ <Network-number>, <Subnet-number> }

Under certain circumstances, it may be desirable to support
subnets of a particular network being interconnected only via a
path which is not part of the subnetted network. Even though many
IGP's and no EGP's currently support this configuration
effectively, routers need to be able to support this configuration
of subnetting (see Section [4.2.3.4]). In general, routers should
not make assumptions about what are subnets and what are not, but
simply ignore the concept of Class in networks, and treat each
route as a { network, mask }-tuple.

DISCUSSION:
It is becoming clear that as the Internet grows larger and
larger, the traditional uses of Class A, B, and C networks will
be modified in order to achieve better use of IP's 32-bit
address space. Classless Interdomain Routing (CIDR)
[INTERNET:15] is a method currently being deployed in the
Internet backbones to achieve this added efficiency. CIDR
depends on the ability of assigning and routing to networks
that are not based on Class A, B, or C networks. Thus, routers
should always treat a route as a network with a mask.

Furthermore, for similar reasons, a subnetted network need not
have a consistent subnet mask through all parts of the network.
For example, one subnet may use an 8 bit subnet mask, another 10
bit, and another 6 bit. Routers need to be able to support this
type of configuration (see Section [4.2.3.4]).

The bit positions containing this extended network number are
indicated by a 32-bit mask called the subnet mask; it is
recommended but not required that the <Subnet-number> bits be
contiguous and fall between the <Network-number> and the <Host-
number> fields. No subnet should be assigned the value zero or -1

(all one bits).

Although the inventors of the subnet mechanism probably expected
that each piece of an organization's network would have only a
single subnet number, in practice it has often proven necessary or
useful to have several subnets share a single physical cable.

There are special considerations for the router when a connected
network provides a broadcast or multicast capability; these will
be discussed later.

2.2.6 IP Multicasting

IP multicasting is an extension of Link Layer multicast to IP
internets. Using IP multicasts, a single datagram can be
addressed to multiple hosts. This collection of hosts is called a
multicast group. Each multicast group is represented as a Class D
IP address. An IP datagram sent to the group is to be delivered
to each group member with the same best-effort delivery as that
provided for unicast IP traffic. The sender of the datagram does
not itself need to be a member of the destination group.

The semantics of IP multicast group membership are defined in
[INTERNET:4]. That document describes how hosts and routers join
and leave multicast groups. It also defines a protocol, the
Internet Group Management Protocol (IGMP), that monitors IP
multicast group membership.

Forwarding of IP multicast datagrams is accomplished either
through static routing information or via a multicast routing
protocol. Devices that forward IP multicast datagrams are called
multicast routers. They may or may not also forward IP unicasts.
In general, multicast datagrams are forwarded on the basis of both
their source and destination addresses. Forwarding of IP
multicast packets is described in more detail in Section [5.2.1].
Appendix D discusses multicast routing protocols.

2.2.7 Unnumbered Lines and Networks and Subnets

Traditionally, each network interface on an IP host or router has
its own IP address. Over the years, people have observed that
this can cause inefficient use of the scarce IP address space,
since it forces allocation of an IP network number, or at least a
subnet number, to every point-to-point link.

To solve this problem, a number of people have proposed and
implemented the concept of unnumbered serial lines. An unnumbered

serial line does not have any IP network or subnet number
associated with it. As a consequence, the network interfaces
connected to an unnumbered serial line do not have IP addresses.

Because the IP architecture has traditionally assumed that all
interfaces had IP addresses, these unnumbered interfaces cause
some interesting dilemmas. For example, some IP options (e.g.
Record Route) specify that a router must insert the interface
address into the option, but an unnumbered interface has no IP
address. Even more fundamental (as we shall see in chapter 5) is
that routes contain the IP address of the next hop router. A
router expects that that IP address will be on an IP (sub)net that
the router is connected to. That assumption is of course violated
if the only connection is an unnumbered serial line.

To get around these difficulties, two schemes have been invented.
The first scheme says that two routers connected by an unnumbered
serial line aren't really two routers at all, but rather two
half-routers which together make up a single (virtual) router.
The unnumbered serial line is essentially considered to be an
internal bus in the virtual router. The two halves of the virtual
router must coordinate their activities in such a way that they
act exactly like a single router.

This scheme fits in well with the IP architecture, but suffers
from two important drawbacks. The first is that, although it
handles the common case of a single unnumbered serial line, it is
not readily extensible to handle the case of a mesh of routers and
unnumbered serial lines. The second drawback is that the
interactions between the half routers are necessarily complex and
are not standardized, effectively precluding the connection of
equipment from different vendors using unnumbered serial lines.

Because of these drawbacks, this memo has adopted an alternative
scheme, which has been invented multiple times but which is
probably originally attributable to Phil Karn. In this scheme, a
router which has unnumbered serial lines also has a special IP
address, called a router-id in this memo. The router-id is one of
the router's IP addresses (a router is required to have at least
one IP address). This router-id is used as if it is the IP
address of all unnumbered interfaces.

2.2.8 Notable Oddities

2.2.8.1 Embedded Routers

A router may be a stand-alone computer system, dedicated to its
IP router functions. Alternatively, it is possible to embed
router functions within a host operating system which supports
connections to two or more networks. The best-known example of
an operating system with embedded router code is the Berkeley
BSD system. The embedded router feature seems to make
internetting easy, but it has a number of hidden pitfalls:

(1) If a host has only a single constituent-network interface,
it should not act as a router.

For example, hosts with embedded router code that
gratuitously forward broadcast packets or datagrams on the
same net often cause packet avalanches.

(2) If a (multihomed) host acts as a router, it must implement
ALL the relevant router requirements contained in this
document.

For example, the routing protocol issues and the router
control and monitoring problems are as hard and important
for embedded routers as for stand-alone routers.

Since Internet router requirements and specifications may
change independently of operating system changes, an
administration that operates an embedded router in the
Internet is strongly advised to have the ability to
maintain and update the router code (e.g., this might
require router code source).

(3) Once a host runs embedded router code, it becomes part of
the Internet system. Thus, errors in software or
configuration can hinder communication between other
hosts. As a consequence, the host administrator must lose
some autonomy.

In many circumstances, a host administrator will need to
disable router code embedded in the operating system, and
any embedded router code must be organized so that it can
be easily disabled.

(4) If a host running embedded router code is concurrently

used for other services, the O&M (Operation and
Maintenance) requirements for the two modes of use may be
in serious conflict.

For example, router O&M will in many cases be performed
remotely by an operations center; this may require
privileged system access which the host administrator
would not normally want to distribute.

2.2.8.2 Transparent Routers

There are two basic models for interconnecting local-area
networks and wide-area (or long-haul) networks in the Internet.
In the first, the local-area network is assigned a network
number and all routers in the Internet must know how to route
to that network. In the second, the local-area network shares
(a small part of) the address space of the wide-area network.
Routers that support this second model are called address
sharing routers or transparent routers. The focus of this memo
is on routers that support the first model, but this is not
intended to exclude the use of transparent routers.

The basic idea of a transparent router is that the hosts on the
local-area network behind such a router share the address space
of the wide-area network in front of the router. In certain
situations this is a very useful approach and the limitations
do not present significant drawbacks.

The words in front and behind indicate one of the limitations
of this approach: this model of interconnection is suitable
only for a geographically (and topologically) limited stub
environment. It requires that there be some form of logical
addressing in the network level addressing of the wide-area
network. All of the IP addresses in the local environment map
to a few (usually one) physical address in the wide-area
network. This mapping occurs in a way consistent with the { IP
address <-> network address } mapping used throughout the
wide-area network.

Multihoming is possible on one wide-area network, but may
present routing problems if the interfaces are geographically
or topologically separated. Multihoming on two (or more)
wide-area networks is a problem due to the confusion of
addresses.

The behavior that hosts see from other hosts in what is
apparently the same network may differ if the transparent

router cannot fully emulate the normal wide-area network
service. For example, the ARPANET used a Link Layer protocol
that provided a Destination Dead indication in response to an
attempt to send to a host which was powered off. However, if
there were a transparent router between the ARPANET and an
Ethernet, a host on the ARPANET would not receive a Destination
Dead indication if it sent a datagram to a host that was
powered off and was connected to the ARPANET via the
transparent router instead of directly.

2.3 Router Characteristics

An Internet router performs the following functions:

(1) Conforms to specific Internet protocols specified in this
document, including the Internet Protocol (IP), Internet Control
Message Protocol (ICMP), and others as necessary.

(2) Interfaces to two or more packet networks. For each connected
network the router must implement the functions required by that
network. These functions typically include:

o Encapsulating and decapsulating the IP datagrams with the
connected network framing (e.g., an Ethernet header and
checksum),

o Sending and receiving IP datagrams up to the maximum size
supported by that network, this size is the network's Maximum
Transmission Unit or MTU,

o Translating the IP destination address into an appropriate
network-level address for the connected network (e.g., an
Ethernet hardware address), if needed, and

o Responding to the network flow control and error indication,
if any.

See chapter 3 (Link Layer).

(3) Receives and forwards Internet datagrams. Important issues in
this process are buffer management, congestion control, and
fairness.

o Recognizes various error conditions and generates ICMP error
and information messages as required.

o Drops datagrams whose time-to-live fields have reached zero.

o Fragments datagrams when necessary to fit into the MTU of the
next network.

See chapter 4 (Internet Layer - Protocols) and chapter 5
(Internet Layer - Forwarding) for more information.

(4) Chooses a next-hop destination for each IP datagram, based on
the information in its routing database. See chapter 5
(Internet Layer - Forwarding) for more information.

(5) (Usually) supports an interior gateway protocol (IGP) to carry
out distributed routing and reachability algorithms with the
other routers in the same autonomous system. In addition, some
routers will need to support an exterior gateway protocol (EGP)
to exchange topological information with other autonomous
systems. See chapter 7 (Application Layer - Routing Protocols)
for more information.

(6) Provides network management and system support facilities,
including loading, debugging, status reporting, exception
reporting and control. See chapter 8 (Application Layer -
Network Management Protocols) and chapter 10 (Operation and
Maintenance) for more information.

A router vendor will have many choices on power, complexity, and
features for a particular router product. It may be helpful to
observe that the Internet system is neither homogeneous nor fully-
connected. For reasons of technology and geography it is growing
into a global interconnect system plus a fringe of LANs around the
edge. More and more these fringe LANs are becoming richly
interconnected, thus making them less out on the fringe and more
demanding on router requirements.

o The global interconnect system is comprised of a number of wide-
area networks to which are attached routers of several Autonomous
Systems (AS); there are relatively few hosts connected directly to
the system.

o Most hosts are connected to LANs. Many organizations have
clusters of LANs interconnected by local routers. Each such
cluster is connected by routers at one or more points into the
global interconnect system. If it is connected at only one point,
a LAN is known as a stub network.

Routers in the global interconnect system generally require:

o Advanced Routing and Forwarding Algorithms

These routers need routing algorithms which are highly dynamic and
also offer type-of-service routing. Congestion is still not a
completely resolved issue (see Section [5.3.6]). Improvements in
these areas are expected, as the research community is actively
working on these issues.

o High Availability

These routers need to be highly reliable, providing 24 hours a
day, 7 days a week service. Equipment and software faults can
have a wide-spread (sometimes global) effect. In case of failure,
they must recover quickly. In any environment, a router must be
highly robust and able to operate, possibly in a degraded state,
under conditions of extreme congestion or failure of network
resources.

o Advanced O&M Features

Internet routers normally operate in an unattended mode. They
will typically be operated remotely from a centralized monitoring
center. They need to provide sophisticated means for monitoring
and measuring traffic and other events and for diagnosing faults.

o High Performance

Long-haul lines in the Internet today are most frequently 56 Kbps,
DS1 (1.4Mbps), and DS3 (45Mbps) speeds. LANs are typically
Ethernet (10Mbps) and, to a lesser degree, FDDI (100Mbps).
However, network media technology is constantly advancing and even
higher speeds are likely in the future. Full-duplex operation is
provided at all of these speeds.

The requirements for routers used in the LAN fringe (e.g., campus
networks) depend greatly on the demands of the local networks. These
may be high or medium-performance devices, probably competitively
procured from several different vendors and operated by an internal
organization (e.g., a campus computing center). The design of these
routers should emphasize low average latency and good burst
performance, together with delay and type-of-service sensitive
resource management. In this environment there may be less formal O&M
but it will not be less important. The need for the routing
mechanism to be highly dynamic will become more important as networks
become more complex and interconnected. Users will demand more out
of their local connections because of the speed of the global
interconnects.

As networks have grown, and as more networks have become old enough

that they are phasing out older equipment, it has become increasingly
imperative that routers interoperate with routers from other vendors.

Even though the Internet system is not fully interconnected, many
parts of the system need to have redundant connectivity. Rich
connectivity allows reliable service despite failures of
communication lines and routers, and it can also improve service by
shortening Internet paths and by providing additional capacity.
Unfortunately, this richer topology can make it much more difficult
to choose the best path to a particular destination.

2.4 Architectural Assumptions

The current Internet architecture is based on a set of assumptions
about the communication system. The assumptions most relevant to
routers are as follows:

o The Internet is a network of networks.

Each host is directly connected to some particular network(s); its
connection to the Internet is only conceptual. Two hosts on the
same network communicate with each other using the same set of
protocols that they would use to communicate with hosts on distant
networks.

o Routers don't keep connection state information.

To improve the robustness of the communication system, routers are
designed to be stateless, forwarding each IP packet independently
of other packets. As a result, redundant paths can be exploited
to provide robust service in spite of failures of intervening
routers and networks.

All state information required for end-to-end flow control and
reliability is implemented in the hosts, in the transport layer or
in application programs. All connection control information is
thus co-located with the end points of the communication, so it
will be lost only if an end point fails. Routers effect flow
control only indirectly, by dropping packets or increasing network
delay.

Note that future protocol developments may well end up putting
some more state into routers. This is especially likely for
resource reservation and flows.

o Routing complexity should be in the routers.

Routing is a complex and difficult problem, and ought to be
performed by the routers, not the hosts. An important objective
is to insulate host software from changes caused by the inevitable
evolution of the Internet routing architecture.

o The system must tolerate wide network variation.

A basic objective of the Internet design is to tolerate a wide
range of network characteristics - e.g., bandwidth, delay, packet
loss, packet reordering, and maximum packet size. Another
objective is robustness against failure of individual networks,
routers, and hosts, using whatever bandwidth is still available.
Finally, the goal is full open system interconnection: an Internet
router must be able to interoperate robustly and effectively with
any other router or Internet host, across diverse Internet paths.

Sometimes implementors have designed for less ambitious goals.
For example, the LAN environment is typically much more benign
than the Internet as a whole; LANs have low packet loss and delay
and do not reorder packets. Some vendors have fielded
implementations that are adequate for a simple LAN environment,
but work badly for general interoperation. The vendor justifies
such a product as being economical within the restricted LAN
market. However, isolated LANs seldom stay isolated for long;
they are soon connected to each other, to organization-wide
internets, and eventually to the global Internet system. In the
end, neither the customer nor the vendor is served by incomplete
or substandard routers.

The requirements spelled out in this document are designed for a
full-function router. It is intended that fully compliant routers
will be usable in almost any part of the Internet.

3. LINK LAYER

Although [INTRO:1] covers Link Layer standards (IP over foo, ARP,
etc.), this document anticipates that Link-Layer material will be
covered in a separate Link Layer Requirements document. A Link-Layer
requirements document would be applicable to both hosts and routers.
Thus, this document will not obsolete the parts of [INTRO:1] that deal
with link-layer issues.

3.1 INTRODUCTION

Routers have essentially the same Link Layer protocol requirements as
other sorts of Internet systems. These requirements are given in
chapter 3 of Requirements for Internet Gateways [INTRO:1]. A router
MUST comply with its requirements and SHOULD comply with its
recommendations. Since some of the material in that document has
become somewhat dated, some additional requirements and explanations
are included below.

DISCUSSION:
It is expected that the Internet community will produce a
Requirements for Internet Link Layer standard which will supersede
both this chapter and chapter 3 of [INTRO:1].

3.2 LINK/INTERNET LAYER INTERFACE

Although this document does not attempt to specify the interface
between the Link Layer and the upper layers, it is worth noting here
that other parts of this document, particularly chapter 5, require
various sorts of information to be passed across this layer boundary.

This section uses the following definitions:

o Source physical address

The source physical address is the Link Layer address of the host
or router from which the packet was received.

o Destination physical address

The destination physical address is the Link Layer address to
which the packet was sent.

The information that must pass from the Link Layer to the
Internetwork Layer for each received packet is:

(1) The IP packet [5.2.2],

(2) The length of the data portion (i.e., not including the Link-
Layer framing) of the Link Layer frame [5.2.2],

(3) The identity of the physical interface from which the IP packet
was received [5.2.3], and

(4) The classification of the packet's destination physical address
as a Link Layer unicast, broadcast, or multicast [4.3.2],
[5.3.4].

In addition, the Link Layer also should provide:

(5) The source physical address.

The information that must pass from the Internetwork Layer to the
Link Layer for each transmitted packet is:

(1) The IP packet [5.2.1]

(2) The length of the IP packet [5.2.1]

(3) The destination physical interface [5.2.1]

(4) The next hop IP address [5.2.1]

In addition, the Internetwork Layer also should provide:

(5) The Link Layer priority value [5.3.3.2]

The Link Layer must also notify the Internetwork Layer if the packet
to be transmitted causes a Link Layer precedence-related error
[5.3.3.3].

3.3 SPECIFIC ISSUES

3.3.1 Trailer Encapsulation

Routers which can connect to 10Mb Ethernets MAY be able to receive
and forward Ethernet packets encapsulated using the trailer
encapsulation described in [LINK:1]. However, a router SHOULD NOT
originate trailer encapsulated packets. A router MUST NOT
originate trailer encapsulated packets without first verifying,
using the mechanism described in section 2.3.1 of [INTRO:2], that
the immediate destination of the packet is willing and able to

accept trailer-encapsulated packets. A router SHOULD NOT agree
(using these same mechanisms) to accept trailer-encapsulated
packets.

3.3.2 Address Resolution Protocol - ARP

Routers which implement ARP MUST be compliant and SHOULD be
unconditionally compliant with the requirements in section 2.3.2
of [INTRO:2].

The link layer MUST NOT report a Destination Unreachable error to
IP solely because there is no ARP cache entry for a destination.

A router MUST not believe any ARP reply which claims that the Link
Layer address of another host or router is a broadcast or
multicast address.

3.3.3 Ethernet and 802.3 Coexistence

Routers which can connect to 10Mb Ethernets MUST be compliant and
SHOULD be unconditionally compliant with the requirements of
Section [2.3.3] of [INTRO:2].

3.3.4 Maximum Transmission Unit - MTU

The MTU of each logical interface MUST be configurable.

Many Link Layer protocols define a maximum frame size that may be
sent. In such cases, a router MUST NOT allow an MTU to be set
which would allow sending of frames larger than those allowed by
the Link Layer protocol. However, a router SHOULD be willing to
receive a packet as large as the maximum frame size even if that
is larger than the MTU.

DISCUSSION:
Note that this is a stricter requirement than imposed on hosts
by [INTRO:2], which requires that the MTU of each physical
interface be configurable.

If a network is using an MTU smaller than the maximum frame
size for the Link Layer, a router may receive packets larger
than the MTU from hosts which are in the process of
initializing themselves, or which have been misconfigured.

In general, the Robustness Principle indicates that these
packets should be successfully received, if at all possible.

3.3.5 Point-to-Point Protocol - PPP

Contrary to [INTRO:1], the Internet does have a standard serial
line protocol: the Point-to-Point Protocol (PPP), defined in
[LINK:2], [LINK:3], [LINK:4], and [LINK:5].

A serial line interface is any interface which is designed to send
data over a telephone, leased, dedicated or direct line (either 2
or 4 wire) using a standardized modem or bit serial interface
(such as RS-232, RS-449 or V.35), using either synchronous or
asynchronous clocking.

A general purpose serial interface is a serial line interface
which is not solely for use as an access line to a network for
which an alternative IP link layer specification exists (such as
X.25 or Frame Relay).

Routers which contain such general purpose serial interfaces MUST
implement PPP.

PPP MUST be supported on all general purpose serial interfaces on
a router. The router MAY allow the line to be configured to use
serial line protocols other than PPP, all general purpose serial
interfaces MUST default to using PPP.

3.3.5.1 Introduction

This section provides guidelines to router implementors so that
they can ensure interoperability with other routers using PPP
over either synchronous or asynchronous links.

It is critical that an implementor understand the semantics of
the option negotiation mechanism. Options are a means for a
local device to indicate to a remote peer what the local device
will *accept* from the remote peer, not what it wishes to send.
It is up to the remote peer to decide what is most convenient
to send within the confines of the set of options that the
local device has stated that it can accept. Therefore it is
perfectly acceptable and normal for a remote peer to ACK all
the options indicated in an LCP Configuration Request (CR) even
if the remote peer does not support any of those options.
Again, the options are simply a mechanism for either device to
indicate to its peer what it will accept, not necessarily what
it will send.

3.3.5.2 Link Control Protocol (LCP) Options

The PPP Link Control Protocol (LCP) offers a number of options
that may be negotiated. These options include (among others)
address and control field compression, protocol field
compression, asynchronous character map, Maximum Receive Unit
(MRU), Link Quality Monitoring (LQM), magic number (for
loopback detection), Password Authentication Protocol (PAP),
Challenge Handshake Authentication Protocol (CHAP), and the
32-bit Frame Check Sequence (FCS).

A router MAY do address/control field compression on either
synchronous or asynchronous links. A router MAY do protocol
field compression on either synchronous or asynchronous links.
A router MAY indicate that it can accept these compressions,
but MUST be able to accept uncompressed PPP header information
even if it has indicated a willingness to receive compressed
PPP headers.

DISCUSSION:
These options control the appearance of the PPP header.
Normally the PPP header consists of the address field (one
byte containing the value 0xff), the control field (one byte
containing the value 0x03), and the two-byte protocol field
that identifies the contents of the data area of the frame.
If a system negotiates address and control field compression
it indicates to its peer that it will accept PPP frames that
have or do not have these fields at the front of the header.
It does not indicate that it will be sending frames with
these fields removed. The protocol field may also be
compressed from two to one byte in most cases.

IMPLEMENTATION:
Some hardware does not deal well with variable length header
information. In those cases it makes most sense for the
remote peer to send the full PPP header. Implementations
may ensure this by not sending the address/control field and
protocol field compression options to the remote peer. Even
if the remote peer has indicated an ability to receive
compressed headers there is no requirement for the local
router to send compressed headers.

A router MUST negotiate the Async Control Character Map (ACCM)
for asynchronous PPP links, but SHOULD NOT negotiate the ACCM
for synchronous links. If a router receives an attempt to
negotiate the ACCM over a synchronous link, it MUST ACKnowledge

the option and then ignore it.

DISCUSSION:
There are implementations that offer both sync and async
modes of operation and may use the same code to implement
the option negotiation. In this situation it is possible
that one end or the other may send the ACCM option on a
synchronous link.

A router SHOULD properly negotiate the maximum receive unit
(MRU). Even if a system negotiates an MRU smaller than 1,500
bytes, it MUST be able to receive a 1,500 byte frame.

A router SHOULD negotiate and enable the link quality
monitoring (LQM) option.

DISCUSSION:
This memo does not specify a policy for deciding whether the
link's quality is adequate. However, it is important (see
Section [3.3.6]) that a router disable failed links.

A router SHOULD implement and negotiate the magic number option
for loopback detection.

A router MAY support the authentication options (PAP - password
authentication protocol, and/or CHAP - challenge handshake
authentication protocol).

A router MUST support 16-bit CRC frame check sequence (FCS) and
MAY support the 32-bit CRC.

3.3.5.3 IP Control Protocol (ICP) Options

A router MAY offer to perform IP address negotiation. A router
MUST accept a refusal (REJect) to perform IP address
negotiation from the peer.

A router SHOULD NOT perform Van Jacobson header compression of
TCP/IP packets if the link speed is in excess of 64 Kbps.
Below that speed the router MAY perform Van Jacobson (VJ)
header compression. At link speeds of 19,200 bps or less the
router SHOULD perform VJ header compression.

3.3.6 Interface Testing

A router MUST have a mechanism to allow routing software to
determine whether a physical interface is available to send
packets or not. A router SHOULD have a mechanism to allow routing
software to judge the quality of a physical interface. A router
MUST have a mechanism for informing the routing software when a
physical interface becomes available or unavailable to send
packets because of administrative action. A router MUST have a
mechanism for informing the routing software when it detects a
Link level interface has become available or unavailable, for any
reason.

DISCUSSION:
It is crucial that routers have workable mechanisms for
determining that their network connections are functioning
properly, since failure to do so (or failure to take the proper
actions when a problem is detected) can lead to black holes.

The mechanisms available for detecting problems with network
connections vary considerably, depending on the Link Layer
protocols in use and also in some cases on the interface
hardware chosen by the router manufacturer. The intent is to
maximize the capability to detect failures within the Link-
Layer constraints.

4. INTERNET LAYER - PROTOCOLS

4.1 INTRODUCTION

This chapter and chapter 5 discuss the protocols used at the Internet
Layer: IP, ICMP, and IGMP. Since forwarding is obviously a crucial
topic in a document discussing routers, chapter 5 limits itself to
the ASPects of the protocols which directly relate to forwarding.
The current chapter contains the remainder of the discussion of the
Internet Layer protocols.

4.2 INTERNET PROTOCOL - IP

4.2.1 INTRODUCTION

Routers MUST implement the IP protocol, as defined by
[INTERNET:1]. They MUST also implement its mandatory extensions:
subnets (defined in [INTERNET:2]), and IP broadcast (defined in
[INTERNET:3]).

A router MUST be compliant, and SHOULD be unconditionally
compliant, with the requirements of sections 3.2.1 and 3.3 of
[INTRO:2], except that:

o Section 3.2.1.1 may be ignored, since it duplicates
requirements found in this memo.

o Section 3.2.1.2 may be ignored, since it duplicates
requirements found in this memo.

o Section 3.2.1.3 should be ignored, since it is superseded by
Section [4.2.2.11] of this memo.

o Section 3.2.1.4 may be ignored, since it duplicates
requirements found in this memo.

o Section 3.2.1.6 should be ignored, since it is superseded by
Section [4.2.2.4] of this memo.

o Section 3.2.1.8 should be ignored, since it is superseded by
Section [4.2.2.1] of this memo.

In the following, the action specified in certain cases is to
silently discard a received datagram. This means that the
datagram will be discarded without further processing and that the

router will not send any ICMP error message (see Section [4.3]) as
a result. However, for diagnosis of problems a router SHOULD
provide the capability of logging the error (see Section [1.3.3]),
including the contents of the silently-discarded datagram, and
SHOULD record the event in a statistics counter.

4.2.2 PROTOCOL WALK-THROUGH

RFC791 is [INTERNET:1], the specification for the Internet
Protocol.

4.2.2.1 Options: RFC-791 Section 3.2

In datagrams received by the router itself, the IP layer MUST
interpret those IP options that it understands and preserve the
rest unchanged for use by higher layer protocols.

Higher layer protocols may require the ability to set IP
options in datagrams they send or examine IP options in
datagrams they receive. Later sections of this document
discuss specific IP option support required by higher layer
protocols.

DISCUSSION:
Neither this memo nor [INTRO:2] define the order in which a
receiver must process multiple options in the same IP
header. Hosts and routers originating datagrams containing
multiple options must be aware that this introduces an
ambiguity in the meaning of certain options when combined
with a source-route option.

Here are the requirements for specific IP options:

(a) Security Option

Some environments require the Security option in every
packet originated or received. Routers SHOULD IMPLEMENT
the revised security option described in [INTERNET:5].

DISCUSSION:
Note that the security options described in
[INTERNET:1] and RFC1038 ([INTERNET:16]) are obsolete.

(b) Stream Identifier Option

This option is obsolete; routers SHOULD NOT place this
option in a datagram that the router originates. This

option MUST be ignored in datagrams received by the
router.

(c) Source Route Options

A router MUST be able to act as the final destination of a
source route. If a router receives a packet containing a
completed source route (i.e., the pointer points beyond
the last field and the destination address in the IP
header addresses the router), the packet has reached its
final destination; the option as received (the recorded
route) MUST be passed up to the transport layer (or to
ICMP message processing).

In order to respond correctly to source-routed datagrams
it receives, a router MUST provide a means whereby
transport protocols and applications can reverse the
source route in a received datagram and insert the
reversed source route into datagrams they originate (see
Section 4 of [INTRO:2] for details).

Some applications in the router MAY require that the user
be able to enter a source route.

A router MUST NOT originate a datagram containing multiple
source route options. What a router should do if asked to
forward a packet containing multiple source route options
is described in Section [5.2.4.1].

When a source route option is created, it MUST be
correctly formed even if it is being created by reversing
a recorded route that erroneously includes the source host
(see case (B) in the discussion below).

DISCUSSION:
Suppose a source routed datagram is to be routed from
source S to destination D via routers G1, G2, ... Gn.
Source S constructs a datagram with G1's IP address as
its destination address, and a source route option to
get the datagram the rest of the way to its
destination. However, there is an ambiguity in the
specification over whether the source route option in a
datagram sent out by S should be (A) or (B):

(A): {>>G2, G3, ... Gn, D} <--- CORRECT

(B): {S, >>G2, G3, ... Gn, D} <---- WRONG

(where >> represents the pointer). If (A) is sent, the
datagram received at D will contain the option: {G1,
G2, ... Gn >>}, with S and D as the IP source and
destination addresses. If (B) were sent, the datagram
received at D would again contain S and D as the same
IP source and destination addresses, but the option
would be: {S, G1, ...Gn >>}; i.e., the originating host
would be the first hop in the route.

(d) Record Route Option

Routers MAY support the Record Route option in datagrams
originated by the router.

(e) Timestamp Option

Routers MAY support the timestamp option in datagrams
originated by the router. The following rules apply:

o When originating a datagram containing a Timestamp
Option, a router MUST record a timestamp in the option
if

- Its Internet address fields are not pre-specified or
- Its first pre-specified address is the IP address of
the logical interface over which the datagram is
being sent (or the router's router-id if the
datagram is being sent over an unnumbered
interface).

o If the router itself receives a datagram containing a
Timestamp Option, the router MUST insert the current
timestamp into the Timestamp Option (if there is space
in the option to do so) before passing the option to
the transport layer or to ICMP for processing.

o A timestamp value MUST follow the rules given in
Section [3.2.2.8] of [INTRO:2].

IMPLEMENTATION:
To maximize the utility of the timestamps contained in
the timestamp option, it is suggested that the
timestamp inserted be, as nearly as practical, the time
at which the packet arrived at the router. For
datagrams originated by the router, the timestamp
inserted should be, as nearly as practical, the time at
which the datagram was passed to the Link Layer for

transmission.

4.2.2.2 Addresses in Options: RFC-791 Section 3.1

When a router inserts its address into a Record Route, Strict
Source and Record Route, Loose Source and Record Route, or
Timestamp, it MUST use the IP address of the logical interface
on which the packet is being sent. Where this rule cannot be
obeyed because the output interface has no IP address (i.e., is
an unnumbered interface), the router MUST instead insert its
router-id. The router's router-id is one of the router's IP
addresses. Which of the router's addresses is used as the
router-id MUST NOT change (even across reboots) unless changed
by the network manager or unless the configuration of the
router is changed such that the IP address used as the router-
id ceases to be one of the router's IP addresses. Routers with
multiple unnumbered interfaces MAY have multiple router-id's.
Each unnumbered interface MUST be associated with a particular
router-id. This association MUST NOT change (even across
reboots) without reconfiguration of the router.

DISCUSSION:
This specification does not allow for routers which do not
have at least one IP address. We do not view this as a
serious limitation, since a router needs an IP address to
meet the manageability requirements of Chapter [8] even if
the router is connected only to point-to-point links.

IMPLEMENTATION:
One possible method of choosing the router-id that fulfills
this requirement is to use the numerically smallest (or
greatest) IP address (treating the address as a 32-bit
integer) that is assigned to the router.

4.2.2.3 Unused IP Header Bits: RFC-791 Section 3.1

The IP header contains two reserved bits: one in the Type of
Service byte and the other in the Flags field. A router MUST
NOT set either of these bits to one in datagrams originated by
the router. A router MUST NOT drop (refuse to receive or
forward) a packet merely because one or more of these reserved
bits has a non-zero value.

DISCUSSION:
Future revisions to the IP protocol may make use of these
unused bits. These rules are intended to ensure that these
revisions can be deployed without having to simultaneously
upgrade all routers in the Internet.

4.2.2.4 Type of Service: RFC-791 Section 3.1

The Type-of-Service byte in the IP header is divided into three
sections: the Precedence field (high-order 3 bits), a field
that is customarily called Type of Service or TOS (next 4
bits), and a reserved bit (the low order bit).

Rules governing the reserved bit were described in Section
[4.2.2.3].

A more extensive discussion of the TOS field and its use can be
found in [ROUTE:11].

The description of the IP Precedence field is superseded by
Section [5.3.3]. RFC-795, Service Mappings, is obsolete and
SHOULD NOT be implemented.

4.2.2.5 Header Checksum: RFC-791 Section 3.1

As stated in Section [5.2.2], a router MUST verify the IP
checksum of any packet which is received. The router MUST NOT
provide a means to disable this checksum verification.

IMPLEMENTATION:
A more extensive description of the IP checksum, including
extensive implementation hints, can be found in [INTERNET:6]
and [INTERNET:7].

4.2.2.6 Unrecognized Header Options: RFC-791 Section 3.1

A router MUST ignore IP options which it does not recognize. A
corollary of this requirement is that a router MUST implement
the End of Option List option and the No Operation option,
since neither contains an explicit length.

DISCUSSION:
All future IP options will include an explicit length.

4.2.2.7 Fragmentation: RFC-791 Section 3.2

Fragmentation, as described in [INTERNET:1], MUST be supported
by a router.

When a router fragments an IP datagram, it SHOULD minimize the
number of fragments. When a router fragments an IP datagram,
it MUST send the fragments in order. A fragmentation method
which may generate one IP fragment which is significantly
smaller than the other MAY cause the first IP fragment to be
the smaller one.

DISCUSSION:
There are several fragmentation techniques in common use in
the Internet. One involves splitting the IP datagram into
IP fragments with the first being MTU sized, and the others
being approximately the same size, smaller than the MTU.
The reason for this is twofold. The first IP fragment in
the sequence will be the effective MTU of the current path
between the hosts, and the following IP fragments are sized
to hopefully minimize the further fragmentation of the IP
datagram. Another technique is to split the IP datagram
into MTU sized IP fragments, with the last fragment being
the only one smaller, as per page 26 of [INTERNET:1].

A common trick used by some implementations of TCP/IP is to
fragment an IP datagram into IP fragments that are no larger
than 576 bytes when the IP datagram is to travel through a
router. In general, this allows the resulting IP fragments
to pass the rest of the path without further fragmentation.
This would, though, create more of a load on the destination
host, since it would have a larger number of IP fragments to
reassemble into one IP datagram. It would also not be
efficient on networks where the MTU only changes once, and
stays much larger than 576 bytes (such as an 802.5 network
with a MTU of 2048 or an Ethernet network with an MTU of
1536).

One other fragmentation technique discussed was splitting
the IP datagram into approximately equal sized IP fragments,
with the size being smaller than the next hop network's MTU.
This is intended to minimize the number of fragments that
would result from additional fragmentation further down the

path.

In most cases, routers should try and create situations that
will generate the lowest number of IP fragments possible.

Work with slow machines leads us to believe that if it is
necessary to send small packets in a fragmentation scheme,
sending the small IP fragment first maximizes the chance of
a host with a slow interface of receiving all the fragments.

4.2.2.8 Reassembly: RFC-791 Section 3.2

As specified in Section 3.3.2 of [INTRO:2], a router MUST
support reassembly of datagrams which it delivers to itself.

4.2.2.9 Time to Live: RFC-791 Section 3.2

Time to Live (TTL) handling for packets originated or received
by the router is governed by [INTRO:2]. Note in particular
that a router MUST NOT check the TTL of a packet except when
forwarding it.

4.2.2.10 Multi-subnet Broadcasts: RFC-922

All-subnets broadcasts (called multi-subnet broadcasts in
[INTERNET:3]) have been deprecated. See Section [5.3.5.3].

4.2.2.11 Addressing: RFC-791 Section 3.2

There are now five classes of IP addresses: Class A through
Class E. Class D addresses are used for IP multicasting
[INTERNET:4], while Class E addresses are reserved for
experimental use.

A multicast (Class D) address is a 28-bit logical address that
stands for a group of hosts, and may be either permanent or
transient. Permanent multicast addresses are allocated by the
Internet Assigned Number Authority [INTRO:7], while transient
addresses may be allocated dynamically to transient groups.
Group membership is determined dynamically using IGMP
[INTERNET:4].

We now summarize the important special cases for Unicast (that
is class A, B, and C) IP addresses, using the following
notation for an IP address:

{ <Network-number>, <Host-number> }

or

{ <Network-number>, <Subnet-number>, <Host-number> }

and the notation -1 for a field that contains all 1 bits and
the notation 0 for a field that contains all 0 bits. This
notation is not intended to imply that the 1-bits in a subnet
mask need be contiguous.

(a) { 0, 0 }

This host on this network. It MUST NOT be used as a
source address by routers, except the router MAY use this
as a source address as part of an initialization procedure
(e.g., if the router is using BOOTP to load its
configuration information).

Incoming datagrams with a source address of { 0, 0 } which
are received for local delivery (see Section [5.2.3]),
MUST be accepted if the router implements the associated
protocol and that protocol clearly defines appropriate
action to be taken. Otherwise, a router MUST silently
discard any locally-delivered datagram whose source
address is { 0, 0 }.

DISCUSSION:
Some protocols define specific actions to take in
response to a received datagram whose source address is
{ 0, 0 }. Two examples are BOOTP and ICMP Mask
Request. The proper operation of these protocols often
depends on the ability to receive datagrams whose
source address is { 0, 0 }. For most protocols,
however, it is best to ignore datagrams having a source
address of { 0, 0 } since they were probably generated
by a misconfigured host or router. Thus, if a router
knows how to deal with a given datagram having a { 0, 0
} source address, the router MUST accept it.
Otherwise, the router MUST discard it.

See also Section [4.2.3.1] for a non-standard use of { 0,
0 }.

(b) { 0, <Host-number> }

Specified host on this network. It MUST NOT be sent by

routers except that the router MAY uses this as a source
address as part of an initialization procedure by which
the it learns its own IP address.

(c) { -1, -1 }

Limited broadcast. It MUST NOT be used as a source
address.

A datagram with this destination address will be received
by every host and router on the connected physical
network, but will not be forwarded outside that network.

(d) { <Network-number>, -1 }

Network Directed Broadcast - a broadcast directed to the
specified network. It MUST NOT be used as a source
address. A router MAY originate Network Directed
Broadcast packets. A router MUST receive Network Directed
Broadcast packets; however a router MAY have a
configuration option to prevent reception of these
packets. Such an option MUST default to allowing
reception.

(e) { <Network-number>, <Subnet-number>, -1 }

Subnetwork Directed Broadcast - a broadcast sent to the
specified subnet. It MUST NOT be used as a source
address. A router MAY originate Network Directed
Broadcast packets. A router MUST receive Network Directed
Broadcast packets; however a router MAY have a
configuration option to prevent reception of these
packets. Such an option MUST default to allowing
reception.

(f) { <Network-number>, -1, -1 }

All Subnets Directed Broadcast - a broadcast sent to all
subnets of the specified subnetted network. It MUST NOT
be used as a source address. A router MAY originate
Network Directed Broadcast packets. A router MUST receive
Network Directed Broadcast packets; however a router MAY
have a configuration option to prevent reception of these
packets. Such an option MUST default to allowing
reception.

(g) { 127, <any> }

Internal host loopback address. Addresses of this form
MUST NOT appear outside a host.

The <Network-number> is administratively assigned so that its
value will be unique in the entire world.

IP addresses are not permitted to have the value 0 or -1 for
any of the <Host-number>, <Network-number>, or <Subnet-number>
fields (except in the special cases listed above). This
implies that each of these fields will be at least two bits
long.

For further discussion of broadcast addresses, see Section
[4.2.3.1].

Since (as described in Section [4.2.1]) a router must support
the subnet extensions to IP, there will be a subnet mask of the
form: { -1, -1, 0 } associated with each of the host's local IP
addresses; see Sections [4.3.3.9], [5.2.4.2], and [10.2.2].

When a router originates any datagram, the IP source address
MUST be one of its own IP addresses (but not a broadcast or
multicast address). The only exception is during
initialization.

For most purposes, a datagram addressed to a broadcast or
multicast destination is processed as if it had been addressed
to one of the router's IP addresses; that is to say:

o A router MUST receive and process normally any packets with
a broadcast destination address.

o A router MUST receive and process normally any packets sent
to a multicast destination address which the router is
interested in.

The term specific-destination address means the equivalent
local IP address of the host. The specific-destination address
is defined to be the destination address in the IP header
unless the header contains a broadcast or multicast address, in
which case the specific-destination is an IP address assigned
to the physical interface on which the datagram arrived.

A router MUST silently discard any received datagram containing
an IP source address that is invalid by the rules of this

section. This validation could be done either by the IP layer
or by each protocol in the transport layer.

DISCUSSION:
A misaddressed datagram might be caused by a Link Layer
broadcast of a unicast datagram or by another router or host
that is confused or misconfigured.

4.2.3 SPECIFIC ISSUES

4.2.3.1 IP Broadcast Addresses

For historical reasons, there are a number of IP addresses
(some standard and some not) which are used to indicate that an
IP packet is an IP broadcast. A router

(1) MUST treat as IP broadcasts packets addressed to
255.255.255.255, { <Network-number>, -1 }, { <Network-
number>, <Subnet-number>, -1 }, and { <Network-number>,
-1, -1 }.

(2) SHOULD silently discard on receipt (i.e., don't even
deliver to applications in the router) any packet
addressed to 0.0.0.0, { <Network-number>, 0 }, {
<Network-number>, <Subnet-number>, 0 }, or { <Network-
number>, 0, 0 }; if these packets are not silently
discarded, they MUST be treated as IP broadcasts (see
Section [5.3.5]). There MAY be a configuration option to
allow receipt of these packets. This option SHOULD
default to discarding them.

(3) SHOULD (by default) use the limited broadcast address
(255.255.255.255) when originating an IP broadcast
destined for a connected network or subnet (except when
sending an ICMP Address Mask Reply, as discussed in
Section [4.3.3.9]). A router MUST receive limited
broadcasts.

(4) SHOULD NOT originate datagrams addressed to 0.0.0.0, {
<Network-number>, 0 }, { <Network-number>, <Subnet-
number>, 0 }, or { <Network-number>, 0, 0 }. There MAY be
a configuration option to allow generation of these
packets (instead of using the relevant 1s format
broadcast). This option SHOULD default to not generating
them.

DISCUSSION:
In the second bullet, the router obviously cannot recognize
addresses of the form { <Network-number>, <Subnet-number>, 0
} if the router does not know how the particular network is
subnetted. In that case, the rules of the second bullet do
not apply because, from the point of view of the router, the
packet is not an IP broadcast packet.

4.2.3.2 IP Multicasting

An IP router SHOULD satisfy the Host Requirements with respect
to IP multicasting, as specified in Section 3.3.7 of [INTRO:2].
An IP router SHOULD support local IP multicasting on all
connected networks for which a mapping from Class D IP
addresses to link-layer addresses has been specified (see the
various IP-over-xxx specifications), and on all connected
point-to-point links. Support for local IP multicasting
includes originating multicast datagrams, joining multicast
groups and receiving multicast datagrams, and leaving multicast
groups. This implies support for all of [INTERNET:4] including
IGMP (see Section [4.4]).

DISCUSSION:
Although [INTERNET:4] is entitled Host Extensions for IP
Multicasting, it applies to all IP systems, both hosts and
routers. In particular, since routers may join multicast
groups, it is correct for them to perform the host part of
IGMP, reporting their group memberships to any multicast
routers that may be present on their attached networks
(whether or not they themselves are multicast routers).

Some router protocols may specifically require support for
IP multicasting (e.g., OSPF [ROUTE:1]), or may recommend it
(e.g., ICMP Router Discovery [INTERNET:13]).

4.2.3.3 Path MTU Discovery

In order to eliminate fragmentation or minimize it, it is
desirable to know what is the path MTU along the path from the
source to destination. The path MTU is the minimum of the MTUs
of each hop in the path. [INTERNET:14] describes a technique
for dynamically discovering the maximum transmission unit (MTU)
of an arbitrary internet path. For a path that passes through
a router that does not support [INTERNET:14], this technique
might not discover the correct Path MTU, but it will always

choose a Path MTU as accurate as, and in many cases more
accurate than, the Path MTU that would be chosen by older
techniques or the current practice.

When a router is originating an IP datagram, it SHOULD use the
scheme described in [INTERNET:14] to limit the datagram's size.
If the router's route to the datagram's destination was learned
from a routing protocol that provides Path MTU information, the
scheme described in [INTERNET:14] is still used, but the Path
MTU information from the routing protocol SHOULD be used as the
initial guess as to the Path MTU and also as an upper bound on
the Path MTU.

4.2.3.4 Subnetting

Under certain circumstances, it may be desirable to support
subnets of a particular network being interconnected only via a
path which is not part of the subnetted network. This is known
as discontiguous subnetwork support.

Routers MUST support discontiguous subnetworks.

IMPLEMENTATION:
In general, a router should not make assumptions about what
are subnets and what are not, but simply ignore the concept
of Class in networks, and treat each route as a { network,
mask }-tuple.

DISCUSSION:
The Internet has been growing at a tremendous rate of late.
This has been placing severe strains on the IP addressing
technology. A major factor in this strain is the strict IP
Address class boundaries. These make it difficult to
efficiently size network numbers to their networks and
aggregate several network numbers into a single route
advertisement. By eliminating the strict class boundaries
of the IP address and treating each route as a {network
number, mask}-tuple these strains may be greatly reduced.

The technology for currently doing this is Classless
Interdomain Routing (CIDR) [INTERNET:15].

Furthermore, for similar reasons, a subnetted network need not
have a consistent subnet mask through all parts of the network.
For example, one subnet may use an 8 bit subnet mask, another
10 bit, and another 6 bit. This is known as variable subnet-

masks.

Routers MUST support variable subnet-masks.

4.3 INTERNET CONTROL MESSAGE PROTOCOL - ICMP

4.3.1 INTRODUCTION

ICMP is an auxiliary protocol, which provides routing, diagnostic
and and error functionality for IP. It is described in
[INTERNET:8]. A router MUST support ICMP.

ICMP messages are grouped in two classes which are discussed in
the following sections:

ICMP error messages:

Destination Unreachable Section 4.3.3.1
Redirect Section 4.3.3.2
Source Quench Section 4.3.3.3
Time Exceeded Section 4.3.3.4
Parameter Problem Section 4.3.3.5

ICMP query messages:
Echo Section 4.3.3.6
Information Section 4.3.3.7
Timestamp Section 4.3.3.8
Address Mask Section 4.3.3.9
Router Discovery Section 4.3.3.10

General ICMP requirements and discussion are in the next section.

4.3.2 GENERAL ISSUES

4.3.2.1 Unknown Message Types

If an ICMP message of unknown type is received, it MUST be
passed to the ICMP user interface (if the router has one) or
silently discarded (if the router doesn't have one).

4.3.2.2 ICMP Message TTL

When originating an ICMP message, the router MUST initialize
the TTL. The TTL for ICMP responses must not be taken from the
packet which triggered the response.

4.3.2.3 Original Message Header

Every ICMP error message includes the Internet header and at
least the first 8 data bytes of the datagram that triggered the
error. More than 8 bytes MAY be sent, but the resulting ICMP
datagram SHOULD have a length of less than or equal to 576
bytes. The returned IP header (and user data) MUST be
identical to that which was received, except that the router is
not required to undo any modifications to the IP header that
are normally performed in forwarding that were performed before
the error was detected (e.g., decrementing the TTL, updating
options). Note that the requirements of Section [4.3.3.5]
supersede this requirement in some cases (i.e., for a Parameter
Problem message, if the problem is in a modified field, the
router must undo the modification). See Section [4.3.3.5])

4.3.2.4 ICMP Message Source Address

Except where this document specifies otherwise, the IP source
address in an ICMP message originated by the router MUST be one
of the IP addresses associated with the physical interface over
which the ICMP message is transmitted. If the interface has no
IP addresses associated with it, the router's router-id (see
Section [5.2.5]) is used instead.

4.3.2.5 TOS and Precedence

ICMP error messages SHOULD have their TOS bits set to the same
value as the TOS bits in the packet which provoked the sending
of the ICMP error message, unless setting them to that value
would cause the ICMP error message to be immediately discarded
because it could not be routed to its destination. Otherwise,
ICMP error messages MUST be sent with a normal (i.e. zero) TOS.
An ICMP reply message SHOULD have its TOS bits set to the same
value as the TOS bits in the ICMP request that provoked the
reply.

EDITOR'S COMMENTS:
The following paragraph originally read:

ICMP error messages MUST have their IP Precedence field

set to the same value as the IP Precedence field in the
packet which provoked the sending of the ICMP error
message, except that the precedence value MUST be 6
(INTERNETWORK CONTROL) or 7 (NETWORK CONTROL), SHOULD be
7, and MAY be settable for the following types of ICMP
error messages: Unreachable, Redirect, Time Exceeded, and
Parameter Problem.

I believe that the following paragraph is equivalent and
easier for humans to parse (Source Quench is the only other
ICMP Error message). Other interpretations of the original
are sought.

ICMP Source Quench error messages MUST have their IP Precedence
field set to the same value as the IP Precedence field in the
packet which provoked the sending of the ICMP Source Quench
message. All other ICMP error messages (Destination
Unreachable, Redirect, Time Exceeded, and Parameter Problem)
MUST have their precedence value set to 6 (INTERNETWORK
CONTROL) or 7 (NETWORK CONTROL), SHOULD be 7. The IP
Precedence value for these error messages MAY be settable.

An ICMP reply message MUST have its IP Precedence field set to
the same value as the IP Precedence field in the ICMP request
that provoked the reply.

4.3.2.6 Source Route

If the packet which provokes the sending of an ICMP error
message contains a source route option, the ICMP error message
SHOULD also contain a source route option of the same type
(strict or loose), created by reversing the portion before the
pointer of the route recorded in the source route option of the
original packet UNLESS the ICMP error message is an ICMP
Parameter Problem complaining about a source route option in
the original packet.

DISCUSSION:
In environments which use the U.S. Department of Defense
security option (defined in [INTERNET:5]), ICMP messages may
need to include a security option. Detailed information on
this topic should be available from the Defense
Communications Agency.

4.3.2.7 When Not to Send ICMP Errors

An ICMP error message MUST NOT be sent as the result of
receiving:

o An ICMP error message, or

o A packet which fails the IP header validation tests
described in Section [5.2.2] (except where that section
specifically permits the sending of an ICMP error message),
or

o A packet destined to an IP broadcast or IP multicast
address, or

o A packet sent as a Link Layer broadcast or multicast, or

o A packet whose source address has a network number of zero
or is an invalid source address (as defined in Section
[5.3.7]), or

o Any fragment of a datagram other then the first fragment
(i.e., a packet for which the fragment offset in the IP
header is nonzero).

Furthermore, an ICMP error message MUST NOT be sent in any case
where this memo states that a packet is to be silently
discarded.

NOTE: THESE RESTRICTIONS TAKE PRECEDENCE OVER ANY REQUIREMENT
ELSEWHERE IN THIS DOCUMENT FOR SENDING ICMP ERROR MESSAGES.

DISCUSSION:
These rules aim to prevent the broadcast storms that have
resulted from routers or hosts returning ICMP error messages
in response to broadcast packets. For example, a broadcast
UDP packet to a non-existent port could trigger a flood of
ICMP Destination Unreachable datagrams from all devices that
do not have a client for that destination port. On a large
Ethernet, the resulting collisions can render the network
useless for a second or more.

Every packet that is broadcast on the connected network
should have a valid IP broadcast address as its IP
destination (see Section [5.3.4] and [INTRO:2]). However,
some devices violate this rule. To be certain to detect
broadcast packets, therefore, routers are required to check

for a link-layer broadcast as well as an IP-layer address.

IMPLEMENTATION:
This requires that the link layer inform the IP layer when a
link-layer broadcast packet has been received; see Section
[3.1].

4.3.2.8 Rate Limiting

A router which sends ICMP Source Quench messages MUST be able
to limit the rate at which the messages can be generated. A
router SHOULD also be able to limit the rate at which it sends
other sorts of ICMP error messages (Destination Unreachable,
Redirect, Time Exceeded, Parameter Problem). The rate limit
parameters SHOULD be settable as part of the configuration of
the router. How the limits are applied (e.g., per router or
per interface) is left to the implementor's discretion.

DISCUSSION:
Two problems for a router sending ICMP error message are:
(1) The consumption of bandwidth on the reverse path, and
(2) The use of router resources (e.g., memory, CPU time)

To help solve these problems a router can limit the
frequency with which it generates ICMP error messages. For
similar reasons, a router may limit the frequency at which
some other sorts of messages, such as ICMP Echo Replies, are
generated.

IMPLEMENTATION:
Various mechanisms have been used or proposed for limiting
the rate at which ICMP messages are sent:

(1) Count-based - for example, send an ICMP error message
for every N dropped packets overall or per given source
host. This mechanism might be appropriate for ICMP
Source Quench, but probably not for other types of ICMP
messages.

(2) Timer-based - for example, send an ICMP error message
to a given source host or overall at most once per T
milliseconds.

(3) Bandwidth-based - for example, limit the rate at which

ICMP messages are sent over a particular interface to
some fraction of the attached network's bandwidth.

4.3.3 SPECIFIC ISSUES

4.3.3.1 Destination Unreachable

If a route can not forward a packet because it has no routes at
all to the destination network specified in the packet then the
router MUST generate a Destination Unreachable, Code 0 (Network
Unreachable) ICMP message. If the router does have routes to
the destination network specified in the packet but the TOS
specified for the routes is neither the default TOS (0000) nor
the TOS of the packet that the router is attempting to route,
then the router MUST generate a Destination Unreachable, Code
11 (Network Unreachable for TOS) ICMP message.

If a packet is to be forwarded to a host on a network that is
directly connected to the router (i.e., the router is the
last-hop router) and the router has ascertained that there is
no path to the destination host then the router MUST generate a
Destination Unreachable, Code 1 (Host Unreachable) ICMP
message. If a packet is to be forwarded to a host that is on a
network that is directly connected to the router and the router
cannot forward the packet because because no route to the
destination has a TOS that is either equal to the TOS requested
in the packet or is the default TOS (0000) then the router MUST
generate a Destination Unreachable, Code 12 (Host Unreachable
for TOS) ICMP message.

DISCUSSION:
The intent is that a router generates the "generic"
host/network unreachable if it has no path at all (including
default routes) to the destination. If the router has one
or more paths to the destination, but none of those paths
have an acceptable TOS, then the router generates the
"unreachable for TOS" message.

4.3.3.2 Redirect

The ICMP Redirect message is generated to inform a host on the
same subnet that the router used by the host to route certain
packets should be changed.

Contrary to section 3.2.2.2 of [INTRO:2], a router MAY ignore
ICMP Redirects when choosing a path for a packet originated by
the router if the router is running a routing protocol or if
forwarding is enabled on the router and on the interface over
which the packet is being sent.

4.3.3.3 Source Quench

A router SHOULD NOT originate ICMP Source Quench messages. As
specified in Section [4.3.2], a router which does originate
Source Quench messages MUST be able to limit the rate at which
they are generated.

DISCUSSION:
Research seems to suggest that Source Quench consumes
network bandwidth but is an ineffective (and unfair)
antidote to congestion. See, for example, [INTERNET:9] and
[INTERNET:10]. Section [5.3.6] discusses the current
thinking on how routers ought to deal with overload and
network congestion.

A router MAY ignore any ICMP Source Quench messages it
receives.

DISCUSSION:
A router itself may receive a Source Quench as the result of
originating a packet sent to another router or host. Such
datagrams might be, e.g., an EGP update sent to another
router, or a telnet stream sent to a host. A mechanism has
been proposed ([INTERNET:11], [INTERNET:12]) to make the IP
layer respond directly to Source Quench by controlling the
rate at which packets are sent, however, this proposal is
currently experimental and not currently recommended.

4.3.3.4 Time Exceeded

When a router is forwarding a packet and the TTL field of the
packet is reduced to 0, the requirements of section [5.2.3.8]
apply.

When the router is reassembling a packet that is destined for
the router, it MUST fulfill requirements of [INTRO:2], section
[3.3.2] apply.

When the router receives (i.e., is destined for the router) a
Time Exceeded message, it MUST comply with section 3.2.2.4 of

[INTRO:2].

4.3.3.5 Parameter Problem

A router MUST generate a Parameter Problem message for any
error not specifically covered by another ICMP message. The IP
header field or IP option including the byte indicated by the
pointer field MUST be included unchanged in the IP header
returned with this ICMP message. Section [4.3.2] defines an
exception to this requirement.

A new variant of the Parameter Problem message was defined in
[INTRO:2]:
Code 1 = required option is missing.

DISCUSSION:
This variant is currently in use in the military community
for a missing security option.

4.3.3.6 Echo Request/Reply

A router MUST implement an ICMP Echo server function that
receives Echo Requests and sends corresponding Echo Replies. A
router MUST be prepared to receive, reassemble and echo an ICMP
Echo Request datagram at least as large as the maximum of 576
and the MTUs of all the connected networks.

The Echo server function MAY choose not to respond to ICMP echo
requests addressed to IP broadcast or IP multicast addresses.

A router SHOULD have a configuration option which, if enabled,
causes the router to silently ignore all ICMP echo requests; if
provided, this option MUST default to allowing responses.

DISCUSSION:
The neutral provision about responding to broadcast and
multicast Echo Requests results from the conclusions reached
in section [3.2.2.6] of [INTRO:2].

As stated in Section [10.3.3], a router MUST also implement an
user/application-layer interface for sending an Echo Request
and receiving an Echo Reply, for diagnostic purposes. All ICMP
Echo Reply messages MUST be passed to this interface.

The IP source address in an ICMP Echo Reply MUST be the same as
the specific-destination address of the corresponding ICMP Echo

Request message.

Data received in an ICMP Echo Request MUST be entirely included
in the resulting Echo Reply.

If a Record Route and/or Timestamp option is received in an
ICMP Echo Request, this option (these options) SHOULD be
updated to include the current router and included in the IP
header of the Echo Reply message, without truncation. Thus,
the recorded route will be for the entire round trip.

If a Source Route option is received in an ICMP Echo Request,
the return route MUST be reversed and used as a Source Route
option for the Echo Reply message.

4.3.3.7 Information Request/Reply

A router SHOULD NOT originate or respond to these messages.

DISCUSSION:
The Information Request/Reply pair was intended to support
self-configuring systems such as diskless workstations, to
allow them to discover their IP network numbers at boot
time. However, these messages are now obsolete. The RARP
and BOOTP protocols provide better mechanisms for a host to
discover its own IP address.

4.3.3.8 Timestamp and Timestamp Reply

A router MAY implement Timestamp and Timestamp Reply. If they
are implemented then:

o The ICMP Timestamp server function MUST return a Timestamp
Reply to every Timestamp message that is received. It
SHOULD be designed for minimum variability in delay.

o An ICMP Timestamp Request message to an IP broadcast or IP
multicast address MAY be silently discarded.

o The IP source address in an ICMP Timestamp Reply MUST be the
same as the specific-destination address of the
corresponding Timestamp Request message.

o If a Source Route option is received in an ICMP Timestamp
Request, the return route MUST be reversed and used as a
Source Route option for the Timestamp Reply message.

o If a Record Route and/or Timestamp option is received in a
Timestamp Request, this (these) option(s) SHOULD be updated
to include the current router and included in the IP header
of the Timestamp Reply message.

o If the router provides an application-layer interface for
sending Timestamp Request messages then incoming Timestamp
Reply messages MUST be passed up to the ICMP user interface.

The preferred form for a timestamp value (the standard value)
is milliseconds since midnight, Universal Time. However, it
may be difficult to provide this value with millisecond
resolution. For example, many systems use clocks that update
only at line frequency, 50 or 60 times per second. Therefore,
some latitude is allowed in a standard value:

(a) A standard value MUST be updated at least 16 times per
second (i.e., at most the six low-order bits of the value
may be undefined).

(b) The accuracy of a standard value MUST approximate that of
operator-set CPU clocks, i.e., correct within a few
minutes.

IMPLEMENTATION:
To meet the second condition, a router may need to query
some time server when the router is booted or restarted. It
is recommended that the UDP Time Server Protocol be used for
this purpose. A more advanced implementation would use the
Network Time Protocol (NTP) to achieve nearly millisecond
clock synchronization; however, this is not required.

4.3.3.9 Address Mask Request/Reply

A router MUST implement support for receiving ICMP Address Mask
Request messages and responding with ICMP Address Mask Reply
messages. These messages are defined in [INTERNET:2].

A router SHOULD have a configuration option for each logical
interface specifying whether the router is allowed to answer
Address Mask Requests for that interface; this option MUST
default to allowing responses. A router MUST NOT respond to an
Address Mask Request before the router knows the correct subnet
mask.

A router MUST NOT respond to an Address Mask Request which has

a source address of 0.0.0.0 and which arrives on a physical
interface which has associated with it multiple logical
interfaces and the subnet masks for those interfaces are not
all the same.

A router SHOULD examine all ICMP Address Mask Replies which it
receives to determine whether the information it contains
matches the router's knowledge of the subnet mask. If the ICMP
Address Mask Reply appears to be in error, the router SHOULD
log the subnet mask and the sender's IP address. A router MUST
NOT use the contents of an ICMP Address Mask Reply to determine
the correct subnet mask.

Because hosts may not be able to learn the subnet mask if a
router is down when the host boots up, a router MAY broadcast a
gratuitous ICMP Address Mask Reply on each of its logical
interfaces after it has configured its own subnet masks.
However, this feature can be dangerous in environments which
use variable length subnet masks. Therefore, if this feature
is implemented, gratuitous Address Mask Replies MUST NOT be
broadcast over any logical interface(s) which either:

o Are not configured to send gratuitous Address Mask Replies.
Each logical interface MUST have a configuration parameter
controlling this, and that parameter MUST default to not
sending the gratuitous Address Mask Replies.

o Share the same IP network number and physical interface but
have different subnet masks.

The { <Network-number>, -1, -1 } form (on subnetted networks)
or the { <Network-number>, -1 } form (on non-subnetted
networks) of the IP broadcast address MUST be used for
broadcast Address Mask Replies.

DISCUSSION:
The ability to disable sending Address Mask Replies by
routers is required at a few sites which intentionally lie
to their hosts about the subnet mask. The need for this is
expected to go away as more and more hosts become compliant
with the Host Requirements standards.

The reason for both the second bullet above and the
requirement about which IP broadcast address to use is to
prevent problems when multiple IP networks or subnets are in
use on the same physical network.

4.3.3.10 Router Advertisement and Solicitations

An IP router MUST support the router part of the ICMP Router
Discovery Protocol [INTERNET:13] on all connected networks on
which the router supports either IP multicast or IP broadcast
addressing. The implementation MUST include all of the
configuration variables specified for routers, with the
specified defaults.

DISCUSSION:
Routers are not required to implement the host part of the
ICMP Router Discovery Protocol, but might find it useful for
operation while IP forwarding is disabled (i.e., when
operating as a host).

DISCUSSION:
We note that it is quite common for hosts to use RIP as the
router discovery protocol. Such hosts listen to RIP traffic
and use and use information extracted from that traffic to
discover routers and to make decisions as to which router to
use as a first-hop router for a given destination. While
this behavior is discouraged, it is still common and
implementors should be aware of it.

4.4 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP

IGMP [INTERNET:4] is a protocol used between hosts and multicast
routers on a single physical network to establish hosts' membership
in particular multicast groups. Multicast routers use this
information, in conjunction with a multicast routing protocol, to
support IP multicast forwarding across the Internet.

A router SHOULD implement the host part of IGMP.

5. INTERNET LAYER - FORWARDING

5.1 INTRODUCTION

This section describes the process of forwarding packets.

5.2 FORWARDING WALK-THROUGH

There is no separate specification of the forwarding function in IP.
Instead, forwarding is covered by the protocol specifications for the
internet layer protocols ([INTERNET:1], [INTERNET:2], [INTERNET:3],
[INTERNET:8], and [ROUTE:11]).

5.2.1 Forwarding Algorithm

Since none of the primary protocol documents describe the
forwarding algorithm in any detail, we present it here. This is
just a general outline, and omits important details, such as
handling of congestion, that are dealt with in later sections.

It is not required that an implementation follow exactly the
algorithms given in sections [5.2.1.1], [5.2.1.2], and [5.2.1.3].
Much of the challenge of writing router software is to maximize
the rate at which the router can forward packets while still
achieving the same effect of the algorithm. Details of how to do
that are beyond the scope of this document, in part because they
are heavily dependent on the architecture of the router. Instead,
we merely point out the order dependencies among the steps:

(1) A router MUST verify the IP header, as described in section
[5.2.2], before performing any actions based on the contents
of the header. This allows the router to detect and discard
bad packets before the expenditure of other resources.

(2) Processing of certain IP options requires that the router
insert its IP address into the option. As noted in Section
[5.2.4], the address inserted MUST be the address of the
logical interface on which the packet is sent or the router's
router-id if the packet is sent over an unnumbered interface.
Thus, processing of these options cannot be completed until
after the output interface is chosen.

(3) The router cannot check and decrement the TTL before checking
whether the packet should be delivered to the router itself,
for reasons mentioned in Section [4.2.2.9].

(4) More generally, when a packet is delivered locally to the
router, its IP header MUST NOT be modified in any way (except
that a router may be required to insert a timestamp into any
Timestamp options in the IP header). Thus, before the router
determines whether the packet is to be delivered locally to
the router, it cannot update the IP header in any way that it
is not prepared to undo.

5.2.1.1 General

This section covers the general forwarding algorithm. This
algorithm applies to all forms of packets to be forwarded:
unicast, multicast, and broadcast.

(1) The router receives the IP packet (plus additional
information about it, as described in Section [3.1]) from
the Link Layer.

(2) The router validates the IP header, as described in
Section [5.2.2]. Note that IP reassembly is not done,
except on IP fragments to be queued for local delivery in
step (4).

(3) The router performs most of the processing of any IP
options. As described in Section [5.2.4], some IP options
require additional processing after the routing decision
has been made.

(4) The router examines the destination IP address of the IP
datagram, as described in Section [5.2.3], to determine
how it should continue to process the IP datagram. There
are three possibilities:

o The IP datagram is destined for the router, and should
be queued for local delivery, doing reassembly if
needed.

o The IP datagram is not destined for the router, and
should be queued for forwarding.

o The IP datagram should be queued for forwarding, but (a
copy) must also be queued for local delivery.

5.2.1.2 Unicast

Since the local delivery case is well-covered by [INTRO:2], the
following assumes that the IP datagram was queued for
forwarding. If the destination is an IP unicast address:

(5) The forwarder determines the next hop IP address for the
packet, usually by looking up the packet's destination in
the router's routing table. This procedure is described
in more detail in Section [5.2.4]. This procedure also
decides which network interface should be used to send the
packet.

(6) The forwarder verifies that forwarding the packet is
permitted. The source and destination addresses should be
valid, as described in Section [5.3.7] and Section [5.3.4]
If the router supports administrative constraints on
forwarding, such as those described in Section [5.3.9],
those constraints must be satisfied.

(7) The forwarder decrements (by at least one) and checks the
packet's TTL, as described in Section [5.3.1].

(8) The forwarder performs any IP option processing that could
not be completed in step 3.

(9) The forwarder performs any necessary IP fragmentation, as
described in Section [4.2.2.7]. Since this step occurs
after outbound interface selection (step 5), all fragments
of the same datagram will be transmitted out the same
interface.

(10) The forwarder determines the Link Layer address of the
packet's next hop. The mechanisms for doing this are Link
Layer-dependent (see chapter 3).

(11) The forwarder encapsulates the IP datagram (or each of the
fragments thereof) in an appropriate Link Layer frame and
queues it for output on the interface selected in step 5.

(12) The forwarder sends an ICMP redirect if necessary, as
described in Section [4.3.3.2].

5.2.1.3 Multicast

If the destination is an IP multicast, the following steps are
taken.

Note that the main differences between the forwarding of IP
unicasts and the forwarding of IP multicasts are

o IP multicasts are usually forwarded based on both the
datagram's source and destination IP addresses,

o IP multicast uses an expanding ring search,

o IP multicasts are forwarded as Link Level multicasts, and

o ICMP errors are never sent in response to IP multicast
datagrams.

Note that the forwarding of IP multicasts is still somewhat
experimental. As a result, the algorithm presented below is not
mandatory, and is provided as an example only.

(5a) Based on the IP source and destination addresses found in
the datagram header, the router determines whether the
datagram has been received on the proper interface for
forwarding. If not, the datagram is dropped silently. The
method for determining the proper receiving interface
depends on the multicast routing algorithm(s) in use. In
one of the simplest algorithms, reverse path forwarding
(RPF), the proper interface is the one that would be used
to forward unicasts back to the datagram source.

(6a) Based on the IP source and destination addresses found in
the datagram header, the router determines the datagram's
outgoing interfaces. In order to implement IP multicast's
expanding ring search (see [INTERNET:4]) a minimum TTL
value is specified for each outgoing interface. A copy of
the multicast datagram is forwarded out each outgoing
interface whose minimum TTL value is less than or equal to
the TTL value in the datagram header, by separately
applying the remaining steps on each such interface.

(7a) The router decrements the packet's TTL by one.

(8a) The forwarder performs any IP option processing that could
not be completed in step (3).

(9a) The forwarder performs any necessary IP fragmentation, as
described in Section [4.2.2.7].

(10a) The forwarder determines the Link Layer address to use in
the Link Level encapsulation. The mechanisms for doing
this are Link Layer-dependent. On LANs a Link Level
multicast or broadcast is selected, as an algorithmic
translation of the datagrams' class D destination address.
See the various IP-over-xxx specifications for more
details.

(11a) The forwarder encapsulates the packet (or each of the
fragments thereof) in an appropriate Link Layer frame and
queues it for output on the appropriate interface.

5.2.2 IP Header Validation

Before a router can process any IP packet, it MUST perform a the
following basic validity checks on the packet's IP header to
ensure that the header is meaningful. If the packet fails any of
the following tests, it MUST be silently discarded, and the error
SHOULD be logged.

(1) The packet length reported by the Link Layer must be large
enough to hold the minimum length legal IP datagram (20
bytes).

(2) The IP checksum must be correct.

(3) The IP version number must be 4. If the version number is
not 4 then the packet may well be another version of IP, such
as ST-II.

(4) The IP header length field must be at least 5.

(5) The IP total length field must be at least 4 * IP header
length field.

A router MUST NOT have a configuration option which allows
disabling any of these tests.

If the packet passes the second and third tests, the IP header
length field is at least 4, and both the IP total length field and
the packet length reported by the Link Layer are at least 16 then,
despite the above rule, the router MAY respond with an ICMP
Parameter Problem message, whose pointer points at the IP header
length field (if it failed the fourth test) or the IP total length

field (if it failed the fifth test). However, it still MUST
discard the packet and still SHOULD log the error.

These rules (and this entire document) apply only to version 4 of
the Internet Protocol. These rules should not be construed as
prohibiting routers from supporting other versions of IP.
Furthermore, if a router can truly classify a packet as being some
other version of IP then it ought not treat that packet as an
error packet within the context of this memo.

IMPLEMENTATION:
It is desirable for purposes of error reporting, though not
always entirely possible, to determine why a header was
invalid. There are four possible reasons:

o The Link Layer truncated the IP header

o The datagram is using a version of IP other than the
standard one (version 4).

o The IP header has been corrupted in transit.

o The sender generated an illegal IP header.

It is probably desirable to perform the checks in the order
listed, since we believe that this ordering is most likely to
correctly categorize the cause of the error. For purposes of
error reporting, it may also be desirable to check if a packet
which fails these tests has an IP version number equal to 6.
If it does, the packet is probably an ST-II datagram and should
be treated as such. ST-II is described in [FORWARD:1].

Additionally, the router SHOULD verify that the packet length
reported by the Link Layer is at least as large as the IP total
length recorded in the packet's IP header. If it appears that the
packet has been truncated, the packet MUST be discarded, the error
SHOULD be logged, and the router SHOULD respond with an ICMP
Parameter Problem message whose pointer points at the IP total
length field.

DISCUSSION:
Because any higher layer protocol which concerns itself with
data corruption will detect truncation of the packet data when
it reaches its final destination, it is not absolutely
necessary for routers to perform the check suggested above in
order to maintain protocol correctness. However, by making
this check a router can simplify considerably the task of

determining which hop in the path is truncating the packets.
It will also reduce the expenditure of resources down-stream
from the router in that down-stream systems will not need to
deal with the packet.

Finally, if the destination address in the IP header is not one of
the addresses of the router, the router SHOULD verify that the
packet does not contain a Strict Source and Record Route option.
If a packet fails this test, the router SHOULD log the error and
SHOULD respond with an ICMP Parameter Problem error with the
pointer pointing at the offending packet's IP destination address.

DISCUSSION:
Some people might suggest that the router should respond with a
Bad Source Route message instead of a Parameter Problem
message. However, when a packet fails this test, it usually
indicates a protocol error by the previous hop router, whereas
Bad Source Route would suggest that the source host had
requested a nonexistent or broken path through the network.

5.2.3 Local Delivery Decision

When a router receives an IP packet, it must decide whether the
packet is addressed to the router (and should be delivered
locally) or the packet is addressed to another system (and should
be handled by the forwarder). There is also a hybrid case, where
certain IP broadcasts and IP multicasts are both delivered locally
and forwarded. A router MUST determine which of the these three
cases applies using the following rules:

o An unexpired source route option is one whose pointer value
does not point past the last entry in the source route. If the
packet contains an unexpired source route option, the pointer
in the option is advanced until either the pointer does point
past the last address in the option or else the next address is
not one of the router's own addresses. In the latter (normal)
case, the packet is forwarded (and not delivered locally)
regardless of the rules below.

o The packet is delivered locally and not considered for
forwarding in the following cases:

- The packet's destination address exactly matches one of the
router's IP addresses,

- The packet's destination address is a limited broadcast

address ({-1, -1}), and

- The packet's destination is an IP multicast address which is
limited to a single subnet (such as 224.0.0.1 or 224.0.0.2)
and (at least) one of the logical interfaces associated with
the physical interface on which the packet arrived is a
member of the destination multicast group.

o The packet is passed to the forwarder AND delivered locally in
the following cases:

- The packet's destination address is an IP broadcast address
that addresses at least one of the router's logical
interfaces but does not address any of the logical
interfaces associated with the physical interface on which
the packet arrived

- The packet's destination is an IP multicast address which is
not limited to a single subnetwork (such as 224.0.0.1 and
224.0.0.2 are) and (at least) one of the logical interfaces
associated with the physical interface on which the packet
arrived is a member of the destination multicast group.

o The packet is delivered locally if the packet's destination
address is an IP broadcast address (other than a limited
broadcast address) that addresses at least one of the logical
interfaces associated with the physical interface on which the
packet arrived. The packet is ALSO passed to the forwarder
unless the link on which the packet arrived uses an IP
encapsulation that does not encapsulate broadcasts differently
than unicasts (e.g. by using different Link Layer destination
addresses).

o The packet is passed to the forwarder in all other cases.

DISCUSSION:
The purpose of the requirement in the last sentence of the
fourth bullet is to deal with a directed broadcast to another
net or subnet on the same physical cable. Normally, this works
as expected: the sender sends the broadcast to the router as a
Link Layer unicast. The router notes that it arrived as a
unicast, and therefore must be destined for a different logical
net (or subnet) than the sender sent it on. Therefore, the
router can safely send it as a Link Layer broadcast out the
same (physical) interface over which it arrived. However, if
the router can't tell whether the packet was received as a Link
Layer unicast, the sentence ensures that the router does the

safe but wrong thing rather than the unsafe but right thing.

IMPLEMENTATION:
As described in Section [5.3.4], packets received as Link Layer
broadcasts are generally not forwarded. It may be advantageous
to avoid passing to the forwarder packets it would later
discard because of the rules in that section.

Some Link Layers (either because of the hardware or because of
special code in the drivers) can deliver to the router copies
of all Link Layer broadcasts and multicasts it transmits. Use
of this feature can simplify the implementation of cases where
a packet has to both be passed to the forwarder and delivered
locally, since forwarding the packet will automatically cause
the router to receive a copy of the packet that it can then
deliver locally. One must use care in these circumstances in
order to prevent treating a received loop-back packet as a
normal packet that was received (and then being subject to the
rules of forwarding, etc etc).

Even in the absence of such a Link Layer, it is of course
hardly necessary to make a copy of an entire packet in order to
queue it both for forwarding and for local delivery, though
care must be taken with fragments, since reassembly is
performed on locally delivered packets but not on forwarded
packets. One simple scheme is to associate a flag with each
packet on the router's output queue which indicates whether it
should be queued for local delivery after it has been sent.

5.2.4 Determining the Next Hop Address

When a router is going to forward a packet, it must determine
whether it can send it directly to its destination, or whether it
needs to pass it through another router. If the latter, it needs
to determine which router to use. This section explains how these
determinations are made.

This section makes use of the following definitions:

o LSRR - IP Loose Source and Record Route option

o SSRR - IP Strict Source and Record Route option

o Source Route Option - an LSRR or an SSRR

o Ultimate Destination Address - where the packet is being sent

to: the last address in the source route of a source-routed
packet, or the destination address in the IP header of a non-
source-routed packet

o Adjacent - reachable without going through any IP routers

o Next Hop Address - the IP address of the adjacent host or
router to which the packet should be sent next

o Immediate Destination Address - the ultimate destination
address, except in source routed packets, where it is the next
address specified in the source route

o Immediate Destination - the node, system, router, end-system,
or whatever that is addressed by the Immediate Destination
Address.

5.2.4.1 Immediate Destination Address

If the destination address in the IP header is one of the
addresses of the router and the packet contains a Source Route
Option, the Immediate Destination Address is the address
pointed at by the pointer in that option if the pointer does
not point past the end of the option. Otherwise, the Immediate
Destination Address is the same as the IP destination address
in the IP header.

A router MUST use the Immediate Destination Address, not the
Ultimate Destination Address, when determining how to handle a
packet.

It is an error for more than one source route option to appear
in a datagram. If it receives one, it SHOULD discard the
packet and reply with an ICMP Parameter Problem message whose
pointer points at the beginning of the second source route
option.

5.2.4.2 Local/Remote Decision

After it has been determined that the IP packet needs to be
forwarded in accordance with the rules specified in Section
[5.2.3], the following algorithm MUST be used to determine if
the Immediate Destination is directly accessible (see
[INTERNET:2]):

(1) For each network interface that has not been assigned any
IP address (the unnumbered lines as described in Section

[2.2.7]), compare the router-id of the other end of the
line to the Immediate Destination Address. If they are
exactly equal, the packet can be transmitted through this
interface.

DISCUSSION:
In other words, the router or host at the remote end of
the line is the destination of the packet or is the
next step in the source route of a source routed
packet.

(2) If no network interface has been selected in the first
step, for each IP address assigned to the router:
(a) Apply the subnet mask associated with the address to
this IP address.

IMPLEMENTATION:
The result of this operation will usually have
been computed and saved during initialization.

(b) Apply the same subnet mask to the Immediate
Destination Address of the packet.
(c) Compare the resulting values. If they are equal to
each other, the packet can be transmitted through the
corresponding network interface.

(3) If an interface has still not been selected, the Immediate
Destination is accessible only through some other router.
The selection of the router and the next hop IP address is
described in Section [5.2.4.3].

5.2.4.3 Next Hop Address

EDITOR'S COMMENTS:
Note that this section has been extensively rewritten. The
original document indicated that Phil Almquist wished to
revise this section to conform to his "Ruminations on the
Next Hop" document. I am under the assumption that the
working group generally agreed with this goal; there was an
editor's note from Phil that remained in this document to
that effect, and the RoNH document contains a "mandatory
RRWG algorithm".

So, I have taken said algorithm from RoNH and moved it into
here.

Additional useful or interesting information from RoNH has
been extracted and placed into an appendix to this note.

The router applies the algorithm in the previous section to
determine if the Immediate Destination Address is adjacent. If
so, the next hop address is the same as the Immediate
Destination Address. Otherwise, the packet must be forwarded
through another router to reach its Immediate Destination. The
selection of this router is the topic of this section.

If the packet contains an SSRR, the router MUST discard the
packet and reply with an ICMP Bad Source Route error.
Otherwise, the router looks up the Immediate Destination
Address in its routing table to determine an appropriate next
hop address.

DISCUSSION:
Per the IP specification, a Strict Source Route must specify
a sequence of nodes through which the packet must traverse;
the packet must go from one node of the source route to the
next, traversing intermediate networks only. Thus, if the
router is not adjacent to the next step of the source route,
the source route can not be fulfilled. Therefore, the ICMP
Bad Source Route error.

The goal of the next-hop selection process is to examine the
entries in the router's Forwarding Information Base (FIB) and
select the best route (if there is one) for the packet from
those available in the FIB.

Conceptually, any route lookup algorithm starts out with a set
of candidate routes which consists of the entire contents of
the FIB. The algorithm consists of a series of steps which
discard routes from the set. These steps are referred to as
Pruning Rules. Normally, when the algorithm terminates there
is exactly one route remaining in the set. If the set ever
becomes empty, the packet is discarded because the destination
is unreachable. It is also possible for the algorithm to
terminate when more than one route remains in the set. In this
case, the router may arbitrarily discard all but one of them,
or may perform "load-splitting" by choosing whichever of the
routes has been least recently used.

With the exception of rule 3 (Weak TOS), a router MUST use the
following Pruning Rules when selecting a next hop for a packet.
If a router does consider TOS when making next-hop decisions,
the Rule 3 must be applied in the order indicated below. These

rules MUST be (conceptually) applied to the FIB in the order
that they are presented. (For some historical perspective,
additional pruning rules, and other common algorithms in use,
see Appendix E).

DISCUSSION:
Rule 3 is optional in that Section [5.3.2] says that a
router only SHOULD consider TOS when making forwarding
decisions.

(1) Basic Match
This rule discards any routes to destinations other than
the Immediate Destination Address of the packet. For
example, if a packet's Immediate Destination Address is
36.144.2.5, this step would discard a route to net
128.12.0.0 but would retain any routes to net 36.0.0.0,
any routes to subnet 36.144.0.0, and any default routes.

More precisely, we assume that each route has a
destination attribute, called route.dest, and a
corresponding mask, called route.mask, to specify which
bits of route.dest are significant. The Immediate
Destination Address of the packet being forwarded is
ip.dest. This rule discards all routes from the set of
candidate routes except those for which (route.dest &
route.mask) = (ip.dest & route.mask).

(2) Longest Match
Longest Match is a refinement of Basic Match, described
above. After Basic Match pruning is performed, the
remaining routes are examined to determine the maximum
number of bits set in any of their route.mask attributes.
The step then discards from the set of candidate routes
any routes which have fewer than that maximum number of
bits set in their route.mask attributes.

For example, if a packet's Immediate Destination Address
is 36.144.2.5 and there are {route.dest, route.mask}
pairs of {36.144.2.0, 255.255.255.0}, {36.144.0.5,
255.255.0.255}, {36.144.0.0, 255.255.0.0}, and {36.0.0.0,
255.0.0.0}, then this rule would keep only the first two
pairs; {36.144.2.0, 255.255.255.0} and {36.144.0.5,
255.255.0.255}.

(3) Weak TOS
Each route has a type of service attribute, called
route.tos, whose possible values are assumed to be
identical to those used in the TOS field of the IP header.
Routing protocols which distribute TOS information fill in
route.tos appropriately in routes they add to the FIB;
routes from other routing protocols are treated as if they
have the default TOS (0000). The TOS field in the IP
header of the packet being routed is called ip.tos.

The set of candidate routes is examined to determine if it
contains any routes for which route.tos = ip.tos. If so,
all routes except those for which route.tos = ip.tos are
discarded. If not, all routes except those for which
route.tos = 0000 are discarded from the set of candidate
routes.

Additional discussion of routing based on Weak TOS may be
found in [ROUTE:11].

DISCUSSION:
The effect of this rule is to select only those routes
which have a TOS that matches the TOS requested in the
packet. If no such routes exist then routes with the
default TOS are considered. Routes with a non-default
TOS that is not the TOS requested in the packet are
never used, even if such routes are the only available
routes that go to the packet's destination.

(4) Best Metric
Each route has a metric attribute, called route.metric,
and a routing domain identifier, called route.domain.
Each member of the set of candidate routes is compared
with each other member of the set. If route.domain is
equal for the two routes and route.metric is strictly
inferior for one when compared with the other, then the
one with the inferior metric is discarded from the set.
The determination of inferior is usually by a simple
arithmetic comparison, though some protocols may have
structured metrics requiring more complex comparisons.

(5) Vendor Policy
Vendor Policy is sort of a catch-all to make up for the
fact that the previously listed rules are often inadequate
to chose from among the possible routes. Vendor Policy
pruning rules are extremely vendor-specific. See section
[5.2.4.4].

This algorithm has two distinct disadvantages. Presumably, a
router implementor might develop techniques to deal with these
disadvantages and make them a part of the Vendor Policy pruning
rule.

(1) IS-IS and OSPF route classes are not directly handled.

(2) Path properties other than type of service (e.g. MTU) are
ignored.

It is also worth noting a deficiency in the way that TOS is
supported: routing protocols which support TOS are implicitly
preferred when forwarding packets which have non-zero TOS
values.

The Basic Match and Longest Match pruning rules generalize the
treatment of a number of particular types of routes. These
routes are selected in the following, decreasing, order of
preference:

(1) Host Route: This is a route to a specific end system.

(2) Subnetwork Route: This is a route to a particular subnet
of a network.

(3) Default Subnetwork Route: This is a route to all subnets
of a particular net for which there are not (explicit)
subnet routes.

(4) Network Route: This is a route to a particular network.

(5) Default Network Route (also known as the default route):
This is a route to all networks for which there are no
explicit routes to the net or any of its subnets.

If, after application of the pruning rules, the set of routes
is empty (i.e., no routes were found), the packet MUST be
discarded and an appropriate ICMP error generated (ICMP Bad
Source Route if the Immediate Destination Address came from a
source route option; otherwise, whichever of ICMP Destination
Host Unreachable or Destination Network Unreachable is
appropriate, as described in Section [4.3.3.1]).

5.2.4.4 Administrative Preference

One suggested mechanism for the Vendor Policy Pruning Rule is
to use administrative preference.

Each route has associated with it a preference value, based on
various attributes of the route (specific mechanisms for
assignment of preference values are suggested below). This
preference value is an integer in the range [0..255], with zero
being the most preferred and 254 being the least preferred.
255 is a special value that means that the route should never
be used. The first step in the Vendor Policy pruning rule
discards all but the most preferable routes (and always
discards routes whose preference value is 255).

This policy is not safe in that it can easily be misused to
create routing loops. Since no protocol ensures that the
preferences configured for a router are consistent with the
preferences configured in its neighbors, network managers must
exercise care in configuring preferences.

o Address Match
It is useful to be able to assign a single preference value
to all routes (learned from the same routing domain) to any
of a specified set of destinations, where the set of
destinations is all destinations that match a specified
address/mask pair.

o Route Class
For routing protocols which maintain the distinction, it is
useful to be able to assign a single preference value to all
routes (learned from the same routing domain) which have a
particular route class (intra-area, inter-area, external
with internal metrics, or external with external metrics).

o Interface
It is useful to be able to assign a single preference value
to all routes (learned from a particular routing domain)
that would cause packets to be routed out a particular
logical interface on the router (logical interfaces
generally map one-to-one onto the router's network
interfaces, except that any network interface which has
multiple IP addresses will have multiple logical interfaces
associated with it).

o Source router
It is useful to be able to assign a single preference value

to all routes (learned from the same routing domain) which
were learned from any of a set of routers, where the set of
routers are those whose updates have a source address which
match a specified address/mask pair.

o Originating AS
For routing protocols which provide the information, it is
useful to be able to assign a single preference value to all
routes (learned from a particular routing domain) which
originated in another particular routing domain. For BGP
routes, the originating AS is the first AS listed in the
route's AS_PATH attribute. For OSPF external routes, the
originating AS may be considered to be the low order 16 bits
of the route's external route tag if the tag's Automatic bit
is set and the tag's PathLength is not equal to 3.

o External route tag
It is useful to be able to assign a single preference value
to all OSPF external routes (learned from the same routing
domain) whose external route tags match any of a list of
specified values. Because the external route tag may
contain a structured value, it may be useful to provide the
ability to match particular subfields of the tag.

o AS path
It may be useful to be able to assign a single preference
value to all BGP routes (learned from the same routing
domain) whose AS path "matches" any of a set of specified
values. It is not yet clear exactly what kinds of matches
are most useful. A simple option would be to allow matching
of all routes for which a particular AS number appears (or
alternatively, does not appear) anywhere in the route's
AS_PATH attribute. A more general but somewhat more
difficult alternative would be to allow matching all routes
for which the AS path matches a specified regular
expression.

5.2.4.6 Load Splitting

At the end of the Next-hop selection process, multiple routes
may still remain. A router has several options when this
occurs. It may arbitrarily discard some of the routes. It may
reduce the number of candidate routes by comparing metrics of
routes from routing domains which are not considered
equivalent. It may retain more than one route and employ a
load-splitting mechanism to divide traffic among them. Perhaps
the only thing that can be said about the relative merits of

the options is that load-splitting is useful in some situations
but not in others, so a wise implementor who implements load-
splitting will also provide a way for the network manager to
disable it.

5.2.5 Unused IP Header Bits: RFC-791 Section 3.1

The IP header contains several reserved bits, in the Type of
Service field and in the Flags field. Routers MUST NOT drop
packets merely because one or more of these reserved bits has a
non-zero value.

Routers MUST ignore and MUST pass through unchanged the values of
these reserved bits. If a router fragments a packet, it MUST copy
these bits into each fragment.

DISCUSSION:
Future revisions to the IP protocol may make use of these
unused bits. These rules are intended to ensure that these
revisions can be deployed without having to simultaneously
upgrade all routers in the Internet.

5.2.6 Fragmentation and Reassembly: RFC-791 Section 3.2

As was discussed in Section [4.2.2.7], a router MUST support IP
fragmentation.

A router MUST NOT reassemble any datagram before forwarding it.

DISCUSSION:
A few people have suggested that there might be some topologies
where reassembly of transit datagrams by routers might improve
performance. In general, however, the fact that fragments may
take different paths to the destination precludes safe use of
such a feature.

Nothing in this section should be construed to control or limit
fragmentation or reassembly performed as a link layer function
by the router.

5.2.7 Internet Control Message Protocol - ICMP

General requirements for ICMP were discussed in Section [4.3].
This section discusses ICMP messages which are sent only by
routers.

5.2.7.1 Destination Unreachable

The ICMP Destination Unreachable message is sent by a router in
response to a packet which it cannot forward because the
destination (or next hop) is unreachable or a service is
unavailable

A router MUST be able to generate ICMP Destination Unreachable
messages and SHOULD choose a response code that most closely
matches the reason why the message is being generated.

The following codes are defined in [INTERNET:8] and [INTRO:2]:

0 = Network Unreachable - generated by a router if a
forwarding path (route) to the destination network is not
available;

1 = Host Unreachable - generated by a router if a forwarding
path (route) to the destination host on a directly
connected network is not available;

2 = Protocol Unreachable - generated if the transport protocol
designated in a datagram is not supported in the transport
layer of the final destination;

3 = Port Unreachable - generated if the designated transport
protocol (e.g. UDP) is unable to demultiplex the datagram
in the transport layer of the final destination but has no
protocol mechanism to inform the sender;

4 = Fragmentation Needed and DF Set - generated if a router
needs to fragment a datagram but cannot since the DF flag
is set;

5 = Source Route Failed - generated if a router cannot forward
a packet to the next hop in a source route option;

6 = Destination Network Unknown - This code SHOULD NOT be
generated since it would imply on the part of the router
that the destination network does not exist (net
unreachable code 0 SHOULD be used in place of code 6);

7 = Destination Host Unknown - generated only when a router
can determine (from link layer advice) that the
destination host does not exist;

11 = Network Unreachable For Type Of Service - generated by a
router if a forwarding path (route) to the destination
network with the requested or default TOS is not
available;

12 = Host Unreachable For Type Of Service - generated if a
router cannot forward a packet because its route(s) to the
destination do not match either the TOS requested in the
datagram or the default TOS (0).

The following additional codes are hereby defined:

13 = Communication Administratively Prohibited - generated if a
router cannot forward a packet due to administrative
filtering;

14 = Host Precedence Violation. Sent by the first hop router
to a host to indicate that a requested precedence is not
permitted for the particular combination of
source/destination host or network, upper layer protocol,
and source/destination port;

15 = Precedence cutoff in effect. The network operators have
imposed a minimum level of precedence required for
operation, the datagram was sent with a precedence below
this level;

NOTE: [INTRO:2] defined Code 8 for source host isolated.
Routers SHOULD NOT generate Code 8; whichever of Codes 0
(Network Unreachable) and 1 (Host Unreachable) is appropriate
SHOULD be used instead. [INTRO:2] also defined Code 9 for
communication with destination network administratively
prohibited and Code 10 for communication with destination host
administratively prohibited. These codes were intended for use
by end-to-end encryption devices used by U.S military agencies.
Routers SHOULD use the newly defined Code 13 (Communication
Administratively Prohibited) if they administratively filter
packets.

Routers MAY have a configuration option that causes Code 13
(Communication Administratively Prohibited) messages not to be
generated. When this option is enabled, no ICMP error message
is sent in response to a packet which is dropped because its

forwarding is administratively prohibited.

Similarly, routers MAY have a configuration option that causes
Code 14 (Host Precedence Violation) and Code 15 (Precedence
Cutoff in Effect) messages not to be generated. When this
option is enabled, no ICMP error message is sent in response to
a packet which is dropped because of a precedence violation.

Routers MUST use Host Unreachable or Destination Host Unknown
codes whenever other hosts on the same destination network
might be reachable; otherwise, the source host may erroneously
conclude that all hosts on the network are unreachable, and
that may not be the case.

[INTERNET:14] describes a slight modification the form of
Destination Unreachable messages containing Code 4
(Fragmentation needed and DF set). A router MUST use this
modified form when originating Code 4 Destination Unreachable
messages.

5.2.7.2 Redirect

The ICMP Redirect message is generated to inform a host on the
same subnet that the router used by the host to route certain
packets should be changed.

Routers MUST NOT generate the Redirect for Network or Redirect
for Network and Type of Service messages (Codes 0 and 2)
specified in [INTERNET:8]. Routers MUST be able to generate
the Redirect for Host message (Code 1) and SHOULD be able to
generate the Redirect for Type of Service and Host message
(Code 3) specified in [INTERNET:8].

DISCUSSION:
If the directly-connected network is not subnetted, a router
can normally generate a network Redirect which applies to
all hosts on a specified remote network. Using a network
rather than a host Redirect may economize slightly on
network traffic and on host routing table storage. However,
the savings are not significant, and subnets create an
ambiguity about the subnet mask to be used to interpret a
network Redirect. In a general subnet environment, it is
difficult to specify precisely the cases in which network
Redirects can be used. Therefore, routers must send only
host (or host and type of service) Redirects.

A Code 3 (Redirect for Host and Type of Service) message is

generated when the packet provoking the redirect has a
destination for which the path chosen by the router would
depend (in part) on the TOS requested.

Routers which can generate Code 3 redirects (Host and Type of
Service) MUST have a configuration option (which defaults to
on) to enable Code 1 (Host) redirects to be substituted for
Code 3 redirects. A router MUST send a Code 1 Redirect in
place of a Code 3 Redirect if it has been configured to do so.

If a router is not able to generate Code 3 Redirects then it
MUST generate Code 1 Redirects in situations where a Code 3
Redirect is called for.

Routers MUST NOT generate a Redirect Message unless all of the
following conditions are met:

o The packet is being forwarded out the same physical
interface that it was received from,

o The IP source address in the packet is on the same Logical
IP (sub)network as the next-hop IP address, and

o The packet does not contain an IP source route option.

The source address used in the ICMP Redirect MUST belong to the
same logical (sub)net as the destination address.

A router using a routing protocol (other than static routes)
MUST NOT consider paths learned from ICMP Redirects when
forwarding a packet. If a router is not using a routing
protocol, a router MAY have a configuration which, if set,
allows the router to consider routes learned via ICMP Redirects
when forwarding packets.

DISCUSSION:
ICMP Redirect is a mechanism for routers to convey routing
information to hosts. Routers use other mechanisms to learn
routing information, and therefore have no reason to obey
redirects. Believing a redirect which contradicted the
router's other information would likely create routing
loops.

On the other hand, when a router is not acting as a router,
it MUST comply with the behavior required of a host.

5.2.7.3 Time Exceeded

A router MUST generate a Time Exceeded message Code 0 (In
Transit) when it discards a packet due to an expired TTL field.
A router MAY have a per-interface option to disable origination
of these messages on that interface, but that option MUST
default to allowing the messages to be originated.

5.2.8 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP

IGMP [INTERNET:4] is a protocol used between hosts and multicast
routers on a single physical network to establish hosts'
membership in particular multicast groups. Multicast routers use
this information, in conjunction with a multicast routing
protocol, to support IP multicast forwarding across the Internet.

A router SHOULD implement the multicast router part of IGMP.

5.3 SPECIFIC ISSUES

5.3.1 Time to Live (TTL)

The Time-to-Live (TTL) field of the IP header is defined to be a
timer limiting the lifetime of a datagram. It is an 8-bit field
and the units are seconds. Each router (or other module) that
handles a packet MUST decrement the TTL by at least one, even if
the elapsed time was much less than a second. Since this is very
often the case, the TTL is effectively a hop count limit on how
far a datagram can propagate through the Internet.

When a router forwards a packet, it MUST reduce the TTL by at
least one. If it holds a packet for more than one second, it MAY
decrement the TTL by one for each second.

If the TTL is reduced to zero (or less), the packet MUST be
discarded, and if the destination is not a multicast address the
router MUST send an ICMP Time Exceeded message, Code 0 (TTL
Exceeded in Transit) message to the source. Note that a router
MUST NOT discard an IP unicast or broadcast packet with a non-zero
TTL merely because it can predict that another router on the path
to the packet's final destination will decrement the TTL to zero.
However, a router MAY do so for IP multicasts, in order to more
efficiently implement IP multicast's expanding ring search
algorithm (see [INTERNET:4]).

DISCUSSION:
The IP TTL is used, somewhat schizophrenically, as both a hop
count limit and a time limit. Its hop count function is
critical to ensuring that routing problems can't melt down the
network by causing packets to loop infinitely in the network.
The time limit function is used by transport protocols such as
TCP to ensure reliable data transfer. Many current
implementations treat TTL as a pure hop count, and in parts of
the Internet community there is a strong sentiment that the
time limit function should instead be performed by the
transport protocols that need it.

In this specification, we have reluctantly decided to follow
the strong belief among the router vendors that the time limit
function should be optional. They argued that implementation
of the time limit function is difficult enough that it is
currently not generally done. They further pointed to the lack
of documented cases where this shortcut has caused TCP to
corrupt data (of course, we would expect the problems created
to be rare and difficult to reproduce, so the lack of
documented cases provides little reassurance that there haven't
been a number of undocumented cases).

IP multicast notions such as the expanding ring search may not
work as expected unless the TTL is treated as a pure hop count.
The same thing is somewhat true of traceroute.

ICMP Time Exceeded messages are required because the traceroute
diagnostic tool depends on them.

Thus, the tradeoff is between severely crippling, if not
eliminating, two very useful tools vs. a very rare and
transient data transport problem (which may not occur at all).

5.3.2 Type of Service (TOS)

The Type-of-Service byte in the IP header is divided into three
sections: the Precedence field (high-order 3 bits), a field that
is customarily called Type of Service or "TOS (next 4 bits), and a
reserved bit (the low order bit). Rules governing the reserved
bit were described in Section [4.2.2.3]. The Precedence field
will be discussed in Section [5.3.3]. A more extensive discussion
of the TOS field and its use can be found in [ROUTE:11].

A router SHOULD consider the TOS field in a packet's IP header
when deciding how to forward it. The remainder of this section

describes the rules that apply to routers that conform to this
requirement.

A router MUST maintain a TOS value for each route in its routing
table. Routes learned via a routing protocol which does not
support TOS MUST be assigned a TOS of zero (the default TOS).

To choose a route to a destination, a router MUST use an algorithm
equivalent to the following:

(1) The router locates in its routing table all available routes
to the destination (see Section [5.2.4]).

(2) If there are none, the router drops the packet because the
destination is unreachable. See section [5.2.4].

(3) If one or more of those routes have a TOS that exactly
matches the TOS specified in the packet, the router chooses
the route with the best metric.

(4) Otherwise, the router repeats the above step, except looking
at routes whose TOS is zero.

(5) If no route was chosen above, the router drops the packet
because the destination is unreachable. The router returns
an ICMP Destination Unreachable error specifying the
appropriate code: either Network Unreachable with Type of
Service (code 11) or Host Unreachable with Type of Service
(code 12).

DISCUSSION:
Although TOS has been little used in the past, its use by hosts
is now mandated by the Requirements for Internet Hosts RFCs
([INTRO:2] and [INTRO:3]). Support for TOS in routers may
become a MUST in the future, but is a SHOULD for now until we
get more experience with it and can better judge both its
benefits and its costs.

Various people have proposed that TOS should affect other
aspects of the forwarding function. For example:

(1) A router could place packets which have the Low Delay bit
set ahead of other packets in its output queues.

(2) a router is forced to discard packets, it could try to
avoid discarding those which have the High Reliability bit
set.

These ideas have been explored in more detail in [INTERNET:17]
but we don't yet have enough experience with such schemes to
make requirements in this area.

5.3.3 IP Precedence

This section specifies requirements and guidelines for appropriate
processing of the IP Precedence field in routers. Precedence is a
scheme for allocating resources in the network based on the
relative importance of different traffic flows. The IP
specification defines specific values to be used in this field for
various types of traffic.

The basic mechanisms for precedence processing in a router are
preferential resource allocation, including both precedence-
ordered queue service and precedence-based congestion control, and
selection of Link Layer priority features. The router also
selects the IP precedence for routing, management and control
traffic it originates. For a more extensive discussion of IP
Precedence and its implementation see [FORWARD:6].

Precedence-ordered queue service, as discussed in this section,
includes but is not limited to the queue for the forwarding
process and queues for outgoing links. It is intended that a
router supporting precedence should also use the precedence
indication at whatever points in its processing are concerned with
allocation of finite resources, such as packet buffers or Link
Layer connections. The set of such points is implementation-
dependent.

DISCUSSION:
Although the Precedence field was originally provided for use
in DOD systems where large traffic surges or major damage to
the network are viewed as inherent threats, it has useful
applications for many non-military IP networks. Although the
traffic handling capacity of networks has grown greatly in
recent years, the traffic generating ability of the users has
also grown, and network overload conditions still occur at
times. Since IP-based routing and management protocols have
become more critical to the successful operation of the
Internet, overloads present two additional risks to the
network:

(1) High delays may result in routing protocol packets being
lost. This may cause the routing protocol to falsely
deduce a topology change and propagate this false

information to other routers. Not only can this cause
routes to oscillate, but an extra processing burden may be
placed on other routers.

(2) High delays may interfere with the use of network
management tools to analyze and perhaps correct or relieve
the problem in the network that caused the overload
condition to occur.

Implementation and appropriate use of the Precedence mechanism
alleviates both of these problems.

5.3.3.1 Precedence-Ordered Queue Service

Routers SHOULD implement precedence-ordered queue service.
Precedence-ordered queue service means that when a packet is
selected for output on a (logical) link, the packet of highest
precedence that has been queued for that link is sent. Routers
that implement precedence-ordered queue service MUST also have
a configuration option to suppress precedence-ordered queue
service in the Internet Layer.

Any router MAY implement other policy-based throughput
management procedures that result in other than strict
precedence ordering, but it MUST be configurable to suppress
them (i.e., use strict ordering).

As detailed in Section [5.3.6], routers that implement
precedence-ordered queue service discard low precedence packets
before discarding high precedence packets for congestion
control purposes.

Preemption (interruption of processing or transmission of a
packet) is not envisioned as a function of the Internet Layer.
Some protocols at other layers may provide preemption features.

5.3.3.2 Lower Layer Precedence Mappings

Routers that implement precedence-ordered queueing MUST
IMPLEMENT, and other routers SHOULD IMPLEMENT, Lower Layer
Precedence Mapping.

A router which implements Lower Layer Precedence Mapping:

o MUST be able to map IP Precedence to Link Layer priority
mechanisms for link layers that have such a feature defined.

o MUST have a configuration option to select the Link Layer's
default priority treatment for all IP traffic

o SHOULD be able to configure specific nonstandard mappings of
IP precedence values to Link Layer priority values for each
interface.

DISCUSSION:
Some research questions the workability of the priority
features of some Link Layer protocols, and some networks may
have faulty implementations of the link layer priority
mechanism. It seems prudent to provide an escape mechanism
in case such problems show up in a network.

On the other hand, there are proposals to use novel queueing
strategies to implement special services such as low-delay
service. Special services and queueing strategies to
support them need further research and experimentation
before they are put into widespread use in the Internet.
Since these requirements are intended to encourage (but not
force) the use of precedence features in the hope of
providing better Internet service to all users, routers
supporting precedence-ordered queue service should default
to maintaining strict precedence ordering regardless of the
type of service requested.

Implementors may wish to consider that correct link layer
mapping of IP precedence is required by DOD policy for
TCP/IP systems used on DOD networks.

5.3.3.3 Precedence Handling For All Routers

A router (whether or not it employs precedence-ordered queue
service):

(1) MUST accept and process incoming traffic of all precedence
levels normally, unless it has been administratively
configured to do otherwise.

(2) MAY implement a validation filter to administratively
restrict the use of precedence levels by particular
traffic sources. If provided, this filter MUST NOT filter
out or cut off the following sorts of ICMP error messages:
Destination Unreachable, Redirect, Time Exceeded, and
Parameter Problem. If this filter is provided, the
procedures required for packet filtering by addresses are

required for this filter also.

DISCUSSION:
Precedence filtering should be applicable to specific
source/destination IP Address pairs, specific
protocols, specific ports, and so on.

An ICMP Destination Unreachable message with code 14
SHOULD be sent when a packet is dropped by the validation
filter, unless this has been suppressed by configuration
choice.

(3) MAY implement a cutoff function which allows the router to
be set to refuse or drop traffic with precedence below a
specified level. This function may be activated by
management actions or by some implementation dependent
heuristics, but there MUST be a configuration option to
disable any heuristic mechanism that operates without
human intervention. An ICMP Destination Unreachable
message with code 15 SHOULD be sent when a packet is
dropped by the cutoff function, unless this has been
suppressed by configuration choice.

A router MUST NOT refuse to forward datagrams with IP
precedence of 6 (Internetwork Control) or 7 (Network
Control) solely due to precedence cutoff. However, other
criteria may be used in conjunction with precedence cutoff
to filter high precedence traffic.

DISCUSSION:
Unrestricted precedence cutoff could result in an
unintentional cutoff of routing and control traffic.
In general, host traffic should be restricted to a
value of 5 (CRITIC/ECP) or below although this is not a
requirement and may not be valid in certain systems.

(4) MUST NOT change precedence settings on packets it did not
originate.

(5) SHOULD be able to configure distinct precedence values to
be used for each routing or management protocol supported
(except for those protocols, such as OSPF, which specify
which precedence value must be used).

(6) MAY be able to configure routing or management traffic
precedence values independently for each peer address.

(7) MUST respond appropriately to Link Layer precedence-
related error indications where provided. An ICMP
Destination Unreachable message with code 15 SHOULD be
sent when a packet is dropped because a link cannot accept
it due to a precedence-related condition, unless this has
been suppressed by configuration choice.

DISCUSSION:
The precedence cutoff mechanism described in (3) is
somewhat controversial. Depending on the topological
location of the area affected by the cutoff, transit
traffic may be directed by routing protocols into the
area of the cutoff, where it will be dropped. This is
only a problem if another path which is unaffected by
the cutoff exists between the communicating points.
Proposed ways of avoiding this problem include
providing some minimum bandwidth to all precedence
levels even under overload conditions, or propagating
cutoff information in routing protocols. In the
absence of a widely accepted (and implemented) solution
to this problem, great caution is recommended in
activating cutoff mechanisms in transit networks.

A transport layer relay could legitimately provide the
function prohibited by (4) above. Changing precedence
levels may cause subtle interactions with TCP and
perhaps other protocols; a correct design is a non-
trivial task.

The intent of (5) and (6) (and the discussion of IP
Precedence in ICMP messages in Section [4.3.2]) is that
the IP precedence bits should be appropriately set,
whether or not this router acts upon those bits in any
other way. We expect that in the future specifications
for routing protocols and network management protocols
will specify how the IP Precedence should be set for
messages sent by those protocols.

The appropriate response for (7) depends on the link
layer protocol in use. Typically, the router should
stop trying to send offensive traffic to that
destination for some period of time, and should return
an ICMP Destination Unreachable message with code 15
(service not available for precedence requested) to the
traffic source. It also should not try to reestablish
a preempted Link Layer connection for some period of
time.

5.3.4 Forwarding of Link Layer Broadcasts

The encapsulation of IP packets in most Link Layer protocols
(except PPP) allows a receiver to distinguish broadcasts and
multicasts from unicasts simply by examining the Link Layer
protocol headers (most commonly, the Link Layer destination
address). The rules in this section which refer to Link Layer
broadcasts apply only to Link Layer protocols which allow
broadcasts to be distinguished; likewise, the rules which refer to
Link Layer multicasts apply only to Link Layer protocols which
allow multicasts to be distinguished.

A router MUST NOT forward any packet which the router received as
a Link Layer broadcast (even if the IP destination address is also
some form of broadcast address) unless the packet is an all-
subnets-directed broadcast being forwarded as specified in
[INTERNET:3].

DISCUSSION:
As noted in Section [5.3.5.3], forwarding of all-subnets-
directed broadcasts in accordance with [INTERNET:3] is optional
and is not something that routers do by default.

A router MUST NOT forward any packet which the router received as
a Link Layer multicast unless the packet's destination address is
an IP multicast address.

A router SHOULD silently discard a packet that is received via a
Link Layer broadcast but does not specify an IP multicast or IP
broadcast destination address.

When a router sends a packet as a Link Layer broadcast, the IP
destination address MUST be a legal IP broadcast or IP multicast
address.

5.3.5 Forwarding of Internet Layer Broadcasts

There are two major types of IP broadcast addresses; limited
broadcast and directed broadcast. In addition, there are three
subtypes of directed broadcast; a broadcast directed to a
specified network, a broadcast directed to a specified subnetwork,
and a broadcast directed to all subnets of a specified network.
Classification by a router of a broadcast into one of these
categories depends on the broadcast address and on the router's
understanding (if any) of the subnet structure of the destination
network. The same broadcast will be classified differently by
different routers.

A limited IP broadcast address is defined to be all-ones: { -1, -1
} or 255.255.255.255.

A net-directed broadcast is composed of the network portion of the
IP address with a local part of all-ones, { <Network-number>, -1
}. For example, a Class A net broadcast address is
net.255.255.255, a Class B net broadcast address is
net.net.255.255 and a Class C net broadcast address is
net.net.net.255 where net is a byte of the network address.

An all-subnets-directed broadcast is composed of the network part
of the IP address with a subnet and a host part of all-ones, {
<Network-number>, -1, -1 }. For example, an all-subnets broadcast
on a subnetted class B network is net.net.255.255. A network must
be known to be subnetted and the subnet part must be all-ones
before a broadcast can be classified as all-subnets-directed.

A subnet-directed broadcast address is composed of the network and
subnet part of the IP address with a host part of all-ones, {
<Network-number>, <Subnet-number>, -1 }. For example, a subnet-
directed broadcast to subnet 2 of a class B network might be
net.net.2.255 (if the subnet mask was 255.255.255.0) or
net.net.1.127 (if the subnet mask was 255.255.255.128). A network
must be known to be subnetted and the net and subnet part must not
be all-ones before an IP broadcast can be classified as subnet-
directed.

As was described in Section [4.2.3.1], a router may encounter
certain non-standard IP broadcast addresses:

o 0.0.0.0 is an obsolete form of the limited broadcast address

o { broadcast address.

o { broadcast address.

o { form of a subnet-directed broadcast address.

As was described in that section, packets addressed to any of
these addresses SHOULD be silently discarded, but if they are not,
they MUST be treated in accordance with the same rules that apply
to packets addressed to the non-obsolete forms of the broadcast
addresses described above. These rules are described in the next
few sections.

5.3.5.1 Limited Broadcasts

Limited broadcasts MUST NOT be forwarded. Limited broadcasts
MUST NOT be discarded. Limited broadcasts MAY be sent and
SHOULD be sent instead of directed broadcasts where limited
broadcasts will suffice.

DISCUSSION:
Some routers contain UDP servers which function by resending
the requests (as unicasts or directed broadcasts) to other
servers. This requirement should not be interpreted as
prohibiting such servers. Note, however, that such servers
can easily cause packet looping if misconfigured. Thus,
providers of such servers would probably be well-advised to
document their setup carefully and to consider carefully the
TTL on packets which are sent.

5.3.5.2 Net-directed Broadcasts

A router MUST classify as net-directed broadcasts all valid,
directed broadcasts destined for a remote network or an
attached nonsubnetted network. A router MUST forward net-
directed broadcasts. Net-directed broadcasts MAY be sent.

A router MAY have an option to disable receiving net-directed
broadcasts on an interface and MUST have an option to disable
forwarding net-directed broadcasts. These options MUST default
to permit receiving and forwarding net-directed broadcasts.

DISCUSSION:
There has been some debate about forwarding or not
forwarding directed broadcasts. In this memo we have made
the forwarding decision depend on the router's knowledge of
the subnet mask for the destination network. Forwarding
decisions for subnetted networks should be made by routers
with an understanding of the subnet structure. Therefore,
in general, routers must forward directed broadcasts for
networks they are not attached to and for which they do not
understand the subnet structure. One router may interpret
and handle the same IP broadcast packet differently than
another, depending on its own understanding of the structure
of the destination (sub)network.

5.3.5.3 All-subnets-directed Broadcasts

A router MUST classify as all-subnets-directed broadcasts all
valid directed broadcasts destined for a directly attached
subnetted network which have all-ones in the subnet part of the
address. If the destination network is not subnetted, the
broadcast MUST be treated as a net-directed broadcast.

A router MUST forward an all-subnets-directed broadcast as a
link level broadcast out all physical interfaces connected to
the IP network addressed by the broadcast, except that:

o A router MUST NOT forward an all-subnet-directed broadcast
that was received by the router as a Link Layer broadcast,
unless the router is forwarding the broadcast in accordance
with [INTERNET:3] (see below).

o If a router receives an all-subnets-directed broadcast over
a network which does not indicate via Link Layer framing
whether the frame is a broadcast or a unicast, the packet
MUST NOT be forwarded to any network which likewise does not
indicate whether a frame is a broadcast.

o A router MUST NOT forward an all-subnets-directed broadcast
if the router is configured not to forward such broadcasts.
A router MUST have a configuration option to deny forwarding
of all-subnets-directed broadcasts. The configuration
option MUST default to permit forwarding of all-subnets-
directed broadcasts.

EDITOR'S COMMENTS:
The algorithm presented here is broken. The working group
explicitly desired this algorithm, knowing its failures.

The second bullet, above, prevents All Subnets Directed
Broadcasts from traversing more than one PPP (or other
serial) link in a row. Such a topology is easily conceived.
Suppose that some corporation builds its corporate backbone
out of PPP links, connecting routers at geographically
dispersed locations. Suppose that this corporation has 3
sites (S1, S2, and S3) and there is a router at each site
(R1, R2, and R3). At each site there are also several LANs
connected to the local router. Let there be a PPP link
connecting S1 to S2 and one connecting S2 to S3 (i.e. the
links are R1-R2 and R2-R3). So, if a host on a LAN at S1
sends a All Subnets Directed Broadcast, R1 will forward the
broadcast over the R1-R2 link to R2. R2 will forward the

broadcast to the LAN(s) connected to R2. Since the PPP does
not differentiate broadcast from non-broadcast frames, R2
will NOT forward the broadcast onto the R2-R3 link.
Therefore, the broadcast will not reach S3.

[INTERNET:3] describes an alternative set of rules for
forwarding of all-subnets-directed broadcasts (called multi-
subnet-broadcasts in that document). A router MAY IMPLEMENT
that alternative set of rules, but MUST use the set of rules
described above unless explicitly configured to use the
[INTERNET:3] rules. If routers will do [INTERNET:3]-style
forwarding, then the router MUST have a configuration option
which MUST default to doing the rules presented in this
document.

DISCUSSION:
As far as we know, the rules for multi-subnet broadcasts
described in [INTERNET:3] have never been implemented,
suggesting that either they are too complex or the utility
of multi-subnet broadcasts is low. The rules described in
this section match current practice. In the future, we
expect that IP multicast (see [INTERNET:4]) will be used to
better solve the sorts of problems that multi-subnets
broadcasts were intended to address.

We were also concerned that hosts whose system managers
neglected to configure with a subnet mask could
unintentionally send multi-subnet broadcasts.

A router SHOULD NOT originate all-subnets broadcasts, except as
required by Section [4.3.3.9] when sending ICMP Address Mask
Replies on subnetted networks.

DISCUSSION:
The current intention is to decree that (like 0-filled IP
broadcasts) the notion of the all-subnets broadcast is
obsolete. It should be treated as a directed broadcast to
the first subnet of the net in question that it appears on.

Routers may implement a switch (default off) which if turned
on enables the [INTERNET:3] behavior for all-subnets
broadcasts.

If a router has a configuration option to allow for
forwarding all-subnet broadcasts, it should use a spanning
tree, RPF, or other multicast forwarding algorithm (which
may be computed for other purposes such as bridging or OSPF)

to distribute the all-subnets broadcast efficiently. In
general, it is better to use an IP multicast address rather
than an all-subnets broadcast.

5.3.5.4 Subnet-directed Broadcasts

A router MUST classify as subnet-directed broadcasts all valid
directed broadcasts destined for a directly attached subnetted
network in which the subnet part is not all-ones. If the
destination network is not subnetted, the broadcast MUST be
treated as a net-directed broadcast.

A router MUST forward subnet-directed broadcasts.

A router MUST have a configuration option to prohibit
forwarding of subnet-directed broadcasts. Its default setting
MUST permit forwarding of subnet-directed broadcasts.

A router MAY have a configuration option to prohibit forwarding
of subnet-directed broadcasts from a source on a network on
which the router has an interface. If such an option is
provided, its default setting MUST permit forwarding of
subnet-directed broadcasts.

5.3.6 Congestion Control

Congestion in a network is loosely defined as a condition where
demand for resources (usually bandwidth or CPU time) exceeds
capacity. Congestion avoidance tries to prevent demand from
exceeding capacity, while congestion recovery tries to restore an
operative state. It is possible for a router to contribute to
both of these mechanisms. A great deal of effort has been spent
studying the problem. The reader is encouraged to read
[FORWARD:2] for a survey of the work. Important papers on the
subject include [FORWARD:3], [FORWARD:4], [FORWARD:5], and
[INTERNET:10], among others.

The amount of storage that router should have available to handle
peak instantaneous demand when hosts use reasonable congestion
policies, such as described in [FORWARD:5], is a function of the
product of the bandwidth of the link times the path delay of the
flows using the link, and therefore storage should increase as
this Bandwidth*Delay product increases. The exact function
relating storage capacity to probability of discard is not known.

When a router receives a packet beyond its storage capacity it

must (by definition, not by decree) discard it or some other
packet or packets. Which packet to discard is the subject of much
study but, unfortunately, little agreement so far.

A router MAY discard the packet it has just received; this is the
simplest but not the best policy. It is considered better policy
to randomly pick some transit packet on the queue and discard it
(see [FORWARD:2]). A router MAY use this Random Drop algorithm to
determine which packet to discard.

If a router implements a discard policy (such as Random Drop)
under which it chooses a packet to discard from among a pool of
eligible packets:

o If precedence-ordered queue service (described in Section
[5.3.3.1]) is implemented and enabled, the router MUST NOT
discard a packet whose IP precedence is higher than that of a
packet which is not discarded.

o A router MAY protect packets whose IP headers request the
maximize reliability TOS, except where doing so would be in
violation of the previous rule.

o A router MAY protect fragmented IP packets, on the theory that
dropping a fragment of a datagram may increase congestion by
causing all fragments of the datagram to be retransmitted by
the source.

o To help prevent routing perturbations or disruption of
management functions, the router MAY protect packets used for
routing control, link control, or network management from being
discarded. Dedicated routers (i.e.. routers which are not also
general purpose hosts, terminal servers, etc.) can achieve an
approximation of this rule by protecting packets whose source
or destination is the router itself.

Advanced methods of congestion control include a notion of
fairness, so that the 'user' that is penalized by losing a packet
is the one that contributed the most to the congestion. No matter
what mechanism is implemented to deal with bandwidth congestion
control, it is important that the CPU effort expended be
sufficiently small that the router is not driven into CPU
congestion also.

As described in Section [4.3.3.3], this document recommends that a
router should not send a Source Quench to the sender of the packet
that it is discarding. ICMP Source Quench is a very weak

mechanism, so it is not necessary for a router to send it, and
host software should not use it exclusively as an indicator of
congestion.

5.3.7 Martian Address Filtering

An IP source address is invalid if it is an IP broadcast address
or is not a class A, B, or C address.

An IP destination address is invalid if it is not a class A, B, C,
or D address.

A router SHOULD NOT forward any packet which has an invalid IP
source address or a source address on network 0. A router SHOULD
NOT forward, except over a loopback interface, any packet which
has a source address on network 127. A router MAY have a switch
which allows the network manager to disable these checks. If such
a switch is provided, it MUST default to performing the checks.

A router SHOULD NOT forward any packet which has an invalid IP
destination address or a destination address on network 0. A
router SHOULD NOT forward, except over a loopback interface, any
packet which has a destination address on network 127. A router
MAY have a switch which allows the network manager to disable
these checks. If such a switch is provided, it MUST default to
performing the checks.

If a router discards a packet because of these rules, it SHOULD
log at least the IP source address, the IP destination address,
and, if the problem was with the source address, the physical
interface on which the packet was received and the Link Layer
address of the host or router from which the packet was received.

5.3.8 Source Address Validation

A router SHOULD IMPLEMENT the ability to filter traffic based on a
comparison of the source address of a packet and the forwarding
table for a logical interface on which the packet was received.
If this filtering is enabled, the router MUST silently discard a
packet if the interface on which the packet was received is not
the interface on which a packet would be forwarded to reach the
address contained in the source address. In simpler terms, if a
router wouldn't route a packet containing this address through a
particular interface, it shouldn't believe the address if it
appears as a source address in a packet read from this interface.

If this feature is implemented, it MUST be disabled by default.

DISCUSSION:
This feature can provide useful security improvements in some
situations, but can erroneously discard valid packets in
situations where paths are asymmetric.

5.3.9 Packet Filtering and Access Lists

As a means of providing security and/or limiting traffic through
portions of a network a router SHOULD provide the ability to
selectively forward (or filter) packets. If this capability is
provided, filtering of packets MUST be configurable either to
forward all packets or to selectively forward them based upon the
source and destination addresses. Each source and destination
address SHOULD allow specification of an arbitrary mask.

If supported, a router MUST be configurable to allow one of an

o Include list - specification of a list of address pairs to be
forwarded, or an

o Exclude list - specification of a list of address pairs NOT to
be forwarded.

A router MAY provide a configuration switch which allows a choice
between specifying an include or an exclude list.

A value matching any address (e.g. a keyword any or an address
with a mask of all 0's) MUST be allowed as a source and/or
destination address.

In addition to address pairs, the router MAY allow any combination
of transport and/or application protocol and source and
destination ports to be specified.

The router MUST allow packets to be silently discarded (i.e..
discarded without an ICMP error message being sent).

The router SHOULD allow an appropriate ICMP unreachable message to
be sent when a packet is discarded. The ICMP message SHOULD
specify Communication Administratively Prohibited (code 13) as the
reason for the destination being unreachable.

The router SHOULD allow the sending of ICMP destination
unreachable messages (code 13) to be configured for each
combination of address pairs, protocol types, and ports it allows
to be specified.

The router SHOULD count and SHOULD allow selective logging of
packets not forwarded.

5.3.10 Multicast Routing

An IP router SHOULD support forwarding of IP multicast packets,
based either on static multicast routes or on routes dynamically
determined by a multicast routing protocol (e.g., DVMRP
[ROUTE:9]). A router that forwards IP multicast packets is called
a multicast router.

5.3.11 Controls on Forwarding

For each physical interface, a router SHOULD have a configuration
option which specifies whether forwarding is enabled on that
interface. When forwarding on an interface is disabled, the
router:

o MUST silently discard any packets which are received on that
interface but are not addressed to the router

o MUST NOT send packets out that interface, except for datagrams
originated by the router

o MUST NOT announce via any routing protocols the availability of
paths through the interface

DISCUSSION:
This feature allows the network manager to essentially turn off
an interface but leaves it accessible for network management.

Ideally, this control would apply to logical rather than
physical interfaces, but cannot because there is no known way
for a router to determine which logical interface a packet
arrived on when there is not a one-to-one correspondence
between logical and physical interfaces.

5.3.12 State Changes

During the course of router operation, interfaces may fail or be
manually disabled, or may become available for use by the router.
Similarly, forwarding may be disabled for a particular interface
or for the entire router or may be (re)enabled. While such
transitions are (usually) uncommon, it is important that routers
handle them correctly.

5.3.12.1 When a Router Ceases Forwarding

When a router ceases forwarding it MUST stop advertising all
routes, except for third party routes. It MAY continue to
receive and use routes from other routers in its routing
domains. If the forwarding database is retained, the router
MUST NOT cease timing the routes in the forwarding database.
If routes that have been received from other routers are
remembered, the router MUST NOT cease timing the routes which
it has remembered. It MUST discard any routes whose timers
expire while forwarding is disabled, just as it would do if
forwarding were enabled.

DISCUSSION:
When a router ceases forwarding, it essentially ceases being
a router. It is still a host, and must follow all of the
requirements of Host Requirements [INTRO: 2]. The router
may still be a passive member of one or more routing
domains, however. As such, it is allowed to maintain its
forwarding database by listening to other routers in its
routing domain. It may not, however, advertise any of the
routes in its forwarding database, since it itself is doing
no forwarding. The only exception to this rule is when the
router is advertising a route which uses only some other
router, but which this router has been asked to advertise.

A router MAY send ICMP destination unreachable (host
unreachable) messages to the senders of packets that it is
unable to forward. It SHOULD NOT send ICMP redirect messages.

DISCUSSION:
Note that sending an ICMP destination unreachable (host
unreachable) is a router action. This message should not be
sent by hosts. This exception to the rules for hosts is
allowed so that packets may be rerouted in the shortest
possible time, and so that black holes are avoided.

5.3.12.2 When a Router Starts Forwarding

When a router begins forwarding, it SHOULD expedite the sending
of new routing information to all routers with which it
normally exchanges routing information.

5.3.12.3 When an Interface Fails or is Disabled

If an interface fails or is disabled a router MUST remove and
stop advertising all routes in its forwarding database which
make use of that interface. It MUST disable all static routes
which make use of that interface. If other routes to the same
destination and TOS are learned or remembered by the router,
the router MUST choose the best alternate, and add it to its
forwarding database. The router SHOULD send ICMP destination
unreachable or ICMP redirect messages, as appropriate, in reply
to all packets which it is unable to forward due to the
interface being unavailable.

5.3.12.4 When an Interface is Enabled

If an interface which had not been available becomes available,
a router MUST reenable any static routes which use that
interface. If routes which would use that interface are
learned by the router, then these routes MUST be evaluated
along with all of the other learned routes, and the router MUST
make a decision as to which routes should be placed in the
forwarding database. The implementor is referred to Chapter
[7], Application Layer - Routing Protocols for further
information on how this decision is made.

A router SHOULD expedite the sending of new routing information
to all routers with which it normally exchanges routing
information.

5.3.13 IP Options

Several options, such as Record Route and Timestamp, contain slots
into which a router inserts its address when forwarding the
packet. However, each such option has a finite number of slots,
and therefore a router may find that there is not free slot into
which it can insert its address. No requirement listed below
should be construed as requiring a router to insert its address
into an option that has no remaining slot to insert it into.
Section [5.2.5] discusses how a router must choose which of its
addresses to insert into an option.

5.3.13.1 Unrecognized Options

Unrecognized IP options in forwarded packets MUST be passed
through unchanged.

5.3.13.2 Security Option

Some environments require the Security option in every packet;
such a requirement is outside the scope of this document and
the IP standard specification. Note, however, that the
security options described in [INTERNET:1] and [INTERNET:16]
are obsolete. Routers SHOULD IMPLEMENT the revised security
option described in [INTERNET:5].

5.3.13.3 Stream Identifier Option

This option is obsolete. If the Stream Identifier option is
present in a packet forwarded by the router, the option MUST be
ignored and passed through unchanged.

5.3.13.4 Source Route Options

A router MUST implement support for source route options in
forwarded packets. A router MAY implement a configuration
option which, when enabled, causes all source-routed packets to
be discarded. However, such an option MUST NOT be enabled by
default.

DISCUSSION:
The ability to source route datagrams through the Internet
is important to various network diagnostic tools. However,
in a few rare cases, source routing may be used to bypass
administrative and security controls within a network.
Specifically, those cases where manipulation of routing
tables is used to provide administrative separation in lieu
of other methods such as packet filtering may be vulnerable
through source routed packets.

5.3.13.5 Record Route Option

Routers MUST support the Record Route option in forwarded
packets.

A router MAY provide a configuration option which, if enabled,
will cause the router to ignore (i.e. pass through unchanged)
Record Route options in forwarded packets. If provided, such
an option MUST default to enabling the record-route. This
option does not affect the processing of Record Route options
in datagrams received by the router itself (in particular,
Record Route options in ICMP echo requests will still be
processed in accordance with Section [4.3.3.6]).

DISCUSSION:
There are some people who believe that Record Route is a
security problem because it discloses information about the
topology of the network. Thus, this document allows it to
be disabled.

5.3.13.6 Timestamp Option

Routers MUST support the timestamp option in forwarded packets.
A timestamp value MUST follow the rules given in Section
[3.2.2.8] of [INTRO:2].

If the flags field = 3 (timestamp and prespecified address),
the router MUST add its timestamp if the next prespecified
address matches any of the router's IP addresses. It is not
necessary that the prespecified address be either the address
of the interface on which the packet arrived or the address of
the interface over which it will be sent.

IMPLEMENTATION:
To maximize the utility of the timestamps contained in the
timestamp option, it is suggested that the timestamp
inserted be, as nearly as practical, the time at which the
packet arrived at the router. For datagrams originated by
the router, the timestamp inserted should be, as nearly as
practical, the time at which the datagram was passed to the
network layer for transmission.

A router MAY provide a configuration option which, if enabled,
will cause the router to ignore (i.e. pass through unchanged)
Timestamp options in forwarded datagrams when the flag word is
set to zero (timestamps only) or one (timestamp and registering
IP address). If provided, such an option MUST default to off
(that is, the router does not ignore the timestamp). This
option does not affect the processing of Timestamp options in
datagrams received by the router itself (in particular, a
router will insert timestamps into Timestamp options in
datagrams received by the router, and Timestamp options in ICMP
echo requests will still be processed in accordance with
Section [4.3.3.6]).

DISCUSSION:
Like the Record Route option, the Timestamp option can
reveal information about a network's topology. Some people
consider this to be a security concern.

6. TRANSPORT LAYER

A router is not required to implement any Transport Layer protocols
except those required to support Application Layer protocols supported
by the router. In practice, this means that most routers implement both
the Transmission Control Protocol (TCP) and the User Datagram Protocol
(UDP).

6.1 USER DATAGRAM PROTOCOL - UDP

The User Datagram Protocol (UDP) is specified in [TRANS:1].

A router which implements UDP MUST be compliant, and SHOULD be
unconditionally compliant, with the requirements of section 4.1.3 of
[INTRO:2], except that:

o This specification does not specify the interfaces between the
various protocol layers. Thus, a router need not comply with
sections 4.1.3.2, 4.1.3.3, and 4.1.3.5 of [INTRO:2] (except of
course where compliance is required for proper functioning of
Application Layer protocols supported by the router).

o Contrary to section 4.1.3.4 of [INTRO:2], an application MUST NOT
be able to disable to generation of UDP checksums.

DISCUSSION:
Although a particular application protocol may require that UDP
datagrams it receives must contain a UDP checksum, there is no
general requirement that received UDP datagrams contain UDP
checksums. Of course, if a UDP checksum is present in a received
datagram, the checksum must be verified and the datagram discarded
if the checksum is incorrect.

6.2 TRANSMISSION CONTROL PROTOCOL - TCP

The Transmission Control Protocol (TCP) is specified in [TRANS:2].

A router which implements TCP MUST be compliant, and SHOULD be
unconditionally compliant, with the requirements of section 4.2 of
[INTRO:2], except that:

o This specification does not specify the interfaces between the
various protocol layers. Thus, a router need not comply with the
following requirements of [INTRO:2] (except of course where
compliance is required for proper functioning of Application Layer

protocols supported by the router):

Section 4.2.2.2:
Passing a received PSH flag to the application layer is now
OPTIONAL.

Section 4.2.2.4:
A TCP MUST inform the application layer asynchronously
whenever it receives an Urgent pointer and there was
previously no pending urgent data, or whenever the Urgent
pointer advances in the data stream. There MUST be a way for
the application to learn how much urgent data remains to be
read from the connection, or at least to determine whether or
not more urgent data remains to be read.

Section 4.2.3.5:
An application MUST be able to set the value for R2 for a
particular connection. For example, an interactive
application might set R2 to ``infinity,'' giving the user
control over when to disconnect.

Section 4.2.3.7:
If an application on a multihomed host does not specify the
local IP address when actively opening a TCP connection, then
the TCP MUST ask the IP layer to select a local IP address
before sending the (first) SYN. See the function
GET_SRCADDR() in Section 3.4.

Section 4.2.3.8:
An application MUST be able to specify a source route when it
actively opens a TCP connection, and this MUST take
precedence over a source route received in a datagram.

o For similar reasons, a router need not comply with any of the
requirements of section 4.2.4 of [INTRO:2].

o The requirements of section 4.2.2.6 of [INTRO:2] are amended as
follows: a router which implements the host portion of MTU
discovery (discussed in Section [4.2.3.3] of this memo) uses 536
as the default value of SendMSS only if the path MTU is unknown;
if the path MTU is known, the default value for SendMSS is the
path MTU - 40.

o The requirements of section 4.2.2.6 of [INTRO:2] are amended as
follows: ICMP Destination Unreachable codes 11 and 12 are
additional soft error conditions. Therefore, these message MUST
NOT cause TCP to abort a connection.

DISCUSSION:
It should particularly be noted that a TCP implementation in a
router must conform to the following requirements of [INTRO:2]:

o Providing a configurable TTL. [4.2.2.1]

o Providing an interface to configure keep-alive behavior, if
keep-alives are used at all. [4.2.3.6]

o Providing an error reporting mechanism, and the ability to
manage it. [4.2.4.1]

o Specifying type of service. [4.2.4.2]

The general paradigm applied is that if a particular interface is
visible outside the router, then all requirements for the
interface must be followed. For example, if a router provides a
telnet function, then it will be generating traffic, likely to be
routed in the external networks. Therefore, it must be able to
set the type of service correctly or else the telnet traffic may
not get through.

7. APPLICATION LAYER - ROUTING PROTOCOLS

7.1 INTRODUCTION

An Autonomous System (AS) is defined as a set of routers all
belonging under the same authority and all subject to a consistent
set of routing policies. Interior gateway protocols (IGPs) are used
to distribute routing information inside of an AS (i.e. intra-AS
routing). Exterior gateway protocols are used to exchange routing
information between ASs (i.e. inter-AS routing).

7.1.1 Routing Security Considerations

Routing is one of the few places where the Robustness Principle
(be liberal in what you accept) does not apply. Routers should be
relatively suspicious in accepting routing data from other routing
systems.

A router SHOULD provide the ability to rank routing information
sources from most trustworthy to least trustworthy and to accept
routing information about any particular destination from the most
trustworthy sources first. This was implicit in the original
core/stub autonomous system routing model using EGP and various
interior routing protocols. It is even more important with the
demise of a central, trusted core.

A router SHOULD provide a mechanism to filter out obviously
invalid routes (such as those for net 127).

Routers MUST NOT by default redistribute routing data they do not
themselves use, trust or otherwise consider invalid. In rare
cases, it may be necessary to redistribute suspicious information,
but this should only happen under direct intercession by some
human agency.

In general, routers must be at least a little paranoid about
accepting routing data from anyone, and must be especially careful
when they distribute routing information provided to them by
another party. See below for specific guidelines.

Routers SHOULD IMPLEMENT peer-to-peer authentication for those
routing protocols that support them.

7.1.2 Precedence

Except where the specification for a particular routing protocol
specifies otherwise, a router SHOULD set the IP Precedence value
for IP datagrams carrying routing traffic it originates to 6
(INTERNETWORK CONTROL).

DISCUSSION:
Routing traffic with VERY FEW exceptions should be the highest
precedence traffic on any network. If a system's routing
traffic can't get through, chances are nothing else will.

7.2 INTERIOR GATEWAY PROTOCOLS

7.2.1 INTRODUCTION

An Interior Gateway Protocol (IGP) is used to distribute routing
information between the various routers in a particular AS.
Independent of the algorithm used to implement a particular IGP,
it should perform the following functions:

(1) Respond quickly to changes in the internal topology of an AS

(2) Provide a mechanism such that circuit flapping does not cause
continuous routing updates

(3) Provide quick convergence to loop-free routing

(4) Utilize minimal bandwidth

(5) Provide equal cost routes to enable load-splitting

(6) Provide a means for authentication of routing updates

Current IGPs used in the internet today are characterized as
either being being based on a distance-vector or a link-state
algorithm.

Several IGPs are detailed in this section, including those most
commonly used and some recently developed protocols which may be
widely used in the future. Numerous other protocols intended for
use in intra-AS routing exist in the Internet community.

A router which implements any routing protocol (other than static
routes) MUST IMPLEMENT OSPF (see Section [7.2.2]) and MUST

IMPLEMENT RIP (see Section [7.2.4]). A router MAY implement
additional IGPs.

7.2.2 OPEN SHORTEST PATH FIRST - OSPF

7.2.2.1 Introduction

Shortest Path First (SPF) based routing protocols are a class
of link-state algorithms which are based on the shortest-path
algorithm of Dijkstra. Although SPF based algorithms have been
around since the inception of the ARPANet, it is only recently
that they have achieved popularity both inside both the IP and
the OSI communities. In an SPF based system, each router
obtains an exact replica of the entire topology database via a
process known as flooding. Flooding insures a reliable
transfer of the information. Each individual router then runs
the SPF algorithm on its database to build the IP routing
table. The OSPF routing protocol is an implementation of an
SPF algorithm. The current version, OSPF version 2, is
specified in [ROUTE:1]. Note that RFC-1131, which describes
OSPF version 1, is obsolete.

Note that to comply with Section [8.3] of this memo, a router
which implements OSPF MUST implement the OSPF MIB [MGT:14].

7.2.2.2 Specific Issues

Virtual Links

There is a minor error in the specification that can cause
routing loops when all of the following conditions are
simultaneously true:

(1) A virtual link is configured through a transit area,

(2) Two separate paths exist, each having the same
endpoints, but one utilizing only non-virtual
backbone links, and the other using links in the
transit area, and

(3) The latter path is part of the (underlying physical
representation of the) configured virtual link,
routing loops may occur.

To prevent this, an implementation of OSPF SHOULD invoke

the calculation in Section 16.3 of [ROUTE:1] whenever any
part of the path to the destination is a virtual link (the
specification only says this is necessary when the first
hop is a virtual link).

7.2.2.3 New Version of OSPF

As of this writing (4/4/94) there is a new version of the OSPF
specification that is winding its way through the Internet
standardization process. A prudent implementor will be aware
of this and develop an implementation accordingly.

The new version fixes several errors in the current
specification [ROUTE:1]. For this reason, implementors and
vendors ought to expect to upgrade to the new version
relatively soon. In particular, the following problems exist
in [ROUTE:1] that the new version fixes:

o In [ROUTE:1], certain configurations of virtual links can
lead to incorrect routing and/or routing loops. A fix for
this is specified in the new specification.

o In [ROUTE:1], OSPF external routes to For example, a router
cannot import into an OSPF domain external routes both for
192.2.0.0, 255.255.0.0 and 192.2.0.0, 255.255.255.0. Routes
such as these may become common with the deployment of CIDR
[INTERNET:15]. This has been addressed in the new OSPF
specification.

o In [ROUTE:1], OSPF Network-LSAs originated before a router
changes its OSPF Router ID can confuse the Dijkstra
calculation if the router again becomes Designated Router
for the network. This has been fixed.

7.2.3 INTERMEDIATE SYSTEM TO INTERMEDIATE SYSTEM - DUAL IS-IS

The American National Standards Institute (ANSI) X3S3.3 committee
has defined an intra-domain routing protocol. This protocol is
titled Intermediate System to Intermediate System Routeing
Exchange Protocol.

Its application to an IP network has been defined in [ROUTE:2],
and is referred to as Dual IS-IS (or sometimes as Integrated IS-
IS). IS-IS is based on a link-state (SPF) routing algorithm and
shares all the advantages for this class of protocols.

7.2.4 ROUTING INFORMATION PROTOCOL - RIP

7.2.4.1 Introduction

RIP is specified in [ROUTE:3]. Although RIP is still quite
important in the Internet, it is being replaced in
sophisticated applications by more modern IGPs such as the ones
described above.

Another common use for RIP is as a router discovery protocol.
Section [4.3.3.10] briefly touches upon this subject.

7.2.4.2 Protocol Walk-Through

Dealing with changes in topology: [ROUTE:3], pp. 11

An implementation of RIP MUST provide a means for timing
out routes. Since messages are occasionally lost,
implementations MUST NOT invalidate a route based on a
single missed update.

Implementations MUST by default wait six times the update
interval before invalidating a route. A router MAY have
configuration options to alter this value.

DISCUSSION:
It is important to routing stability that all routers
in a RIP autonomous system use similar timeout value
for invalidating routes, and therefore it is important
that an implementation default to the timeout value
specified in the RIP specification. However, that
timeout value is overly conservative in environments
where packet loss is reasonably rare. In such an
environment, a network manager may wish to be able to
decrease the timeout period in order to promote faster
recovery from failures.

IMPLEMENTATION:
There is a very simple mechanism which a router may use
to meet the requirement to invalidate routes promptly
after they time out. Whenever the router scans the
routing table to see if any routes have timed out, it
also notes the age of the least recently updated route
which has not yet timed out. Subtracting this age from

the timeout period gives the amount of time until the
router again needs to scan the table for timed out
routes.

Split Horizon: [ROUTE:3], pp. 14-15

An implementation of RIP MUST implement split horizon, a
scheme used for avoiding problems caused by including
routes in updates sent to the router from which they were
learned.

An implementation of RIP SHOULD implement Split horizon
with poisoned reverse, a variant of split horizon which
includes routes learned from a router sent to that router,
but sets their metric to infinity. Because of the routing
overhead which may be incurred by implementing split
horizon with poisoned reverse, implementations MAY include
an option to select whether poisoned reverse is in effect.
An implementation SHOULD limit the period of time in which
it sends reverse routes at an infinite metric.

IMPLEMENTATION:
Each of the following algorithms can be used to limit
the period of time for which poisoned reverse is
applied to a route. The first algorithm is more
complex but does a more complete job of limiting
poisoned reverse to only those cases where it is
necessary.

The goal of both algorithms is to ensure that poison
reverse is done for any destination whose route has
changed in the last Route Lifetime (typically 180
seconds), unless it can be sure that the previous route
used the same output interface. The Route Lifetime is
used because that is the amount of time RIP will keep
around an old route before declaring it stale.

The time intervals (and derived variables) used in the
following algorithms are as follows:

Tu The Update Timer; the number of seconds between
RIP updates. This typically defaults to 30
seconds.

Rl The Route Lifetime, in seconds. This is the
amount of time that a route is presumed to be

good, without requiring an update. This typically
defaults to 180 seconds.

Ul The Update Loss; the number of consecutive updates
that have to be lost or fail to mention a route
before RIP deletes the route. Ul is calculated to
be (Rl/Tu)+1. The +1 is to account for the fact
that the first time the ifcounter is decremented
will be less than Tu seconds after it is
initialized. Typically, Ul will be 7: (180/30)+1.

In The value to set ifcounter to when a destination
is newly learned. This value is Ul-4, where the 4
is RIP's garbage collection timer/30

The first algorithm is:

- Associated with each destination is a counter, called
the ifcounter below. Poison reverse is done for any
route whose destination's ifcounter is greater than
zero.

- After a regular (not triggered or in response to a
request) update is sent, all of the non-zero
ifcounters are decremented by one.

- When a route to a destination is created, its
ifcounter is set as follows:

- If the new route is superseding a valid route, and
the old route used a different (logical) output
interface, then the ifcounter is set to Ul.

- If the new route is superseding a stale route, and
the old route used a different (logical) output
interface, then the ifcounter is set to MAX(0, Ul
- INT(seconds that the route has been stale/Ut).

- If there was no previous route to the destination,
the ifcounter is set to In.

- Otherwise, the ifcounter is set to zero

- RIP also maintains a timer, called the resettimer
below. Poison reverse is done on all routes
whenever resettimer has not expired (regardless of

the ifcounter values).

- When RIP is started, restarted, reset, or otherwise
has its routing table cleared, it sets the
resettimer to go off in Rl seconds.

The second algorithm is identical to the first except
that:

- The rules which set the ifcounter to non-zero values
are changed to always set it to Rl/Tu, and

- The resettimer is eliminated.

Triggered updates: [ROUTE:3], pp. 15-16; pp. 29

Triggered updates (also called flash updates) are a
mechanism for immediately notifying a router's
neighbors when the router adds or deletes routes or
changes their metrics. A router MUST send a triggered
update when routes are deleted or their metrics are
increased. A router MAY send a triggered update when
routes are added or their metrics decreased.

Since triggered updates can cause excessive routing
overhead, implementations MUST use the following
mechanism to limit the frequency of triggered updates:

(1) When a router sends a triggered update, it sets a
timer to a random time between one and five
seconds in the future. The router must not
generate additional triggered updates before this
timer expires.

(2) If the router would generate a triggered update
during this interval it sets a flag indicating
that a triggered update is desired. The router
also logs the desired triggered update.

(3) When the triggered update timer expires, the
router checks the triggered update flag. If the
flag is set then the router sends a single
triggered update which includes all of the changes
that were logged. The router then clears the flag
and, since a triggered update was sent, restarts
this algorithm.

(4) The flag is also cleared whenever a regular update
is sent.

Triggered updates SHOULD include all routes that have
changed since the most recent regular (non-triggered)
update. Triggered updates MUST NOT include routes that
have not changed since the most recent regular update.

DISCUSSION:
Sending all routes, whether they have changed
recently or not, is unacceptable in triggered
updates because the tremendous size of many Internet
routing tables could otherwise result in
considerable bandwidth being wasted on triggered
updates.

Use of UDP: [ROUTE:3], pp. 18-19.

RIP packets sent to an IP broadcast address SHOULD have
their initial TTL set to one.

Note that to comply with Section [6.1] of this memo, a
router MUST use UDP checksums in RIP packets which it
originates, MUST discard RIP packets received with
invalid UDP checksums, but MUST not discard received
RIP packets simply because they do not contain UDP
checksums.

Addressing Considerations: [ROUTE:3], pp. 22

A RIP implementation SHOULD support host routes. If it
does not, it MUST (as described on page 27 of
[ROUTE:3]) ignore host routes in received updates. A
router MAY log ignored hosts routes.

The special address 0.0.0.0 is used to describe a
default route. A default route is used as the route of
last resort (i.e. when a route to the specific net does
not exist in the routing table). The router MUST be
able to create a RIP entry for the address 0.0.0.0.

Input Processing - Response: [ROUTE:3], pp. 26

When processing an update, the following validity
checks MUST be performed:

o The response MUST be from UDP port 520.

o The source address MUST be on a directly connected
subnet (or on a directly connected, non-subnetted
network) to be considered valid.

o The source address MUST NOT be one of the router's
addresses.

DISCUSSION:
Some networks, media, and interfaces allow a
sending node to receive packets that it
broadcasts. A router must not accept its own
packets as valid routing updates and process
them. The last requirement prevents a router
from accepting its own routing updates and
processing them (on the assumption that they were
sent by some other router on the network).

An implementation MUST NOT replace an existing route if
the metric received is equal to the existing metric
except in accordance with the following heuristic.

An implementation MAY choose to implement the following
heuristic to deal with the above situation. Normally,
it is useless to change the route to a network from one
router to another if both are advertised at the same
metric. However, the route being advertised by one of
the routers may be in the process of timing out.
Instead of waiting for the route to timeout, the new
route can be used after a specified amount of time has
elapsed. If this heuristic is implemented, it MUST wait
at least halfway to the expiration point before the new
route is installed.

7.2.4.3 Specific Issues

RIP Shutdown

An implementation of RIP SHOULD provide for a graceful
shutdown using the following steps:

(1) Input processing is terminated,

(2) Four updates are generated at random intervals of
between two and four seconds, These updates contain
all routes that were previously announced, but with
some metric changes. Routes that were being

announced at a metric of infinity should continue to
use this metric. Routes that had been announced with
a non-infinite metric should be announced with a
metric of 15 (infinity - 1).

DISCUSSION:
The metric used for the above really ought to be
16 (infinity); setting it to 15 is a kludge to
avoid breaking certain old hosts which wiretap the
RIP protocol. Such a host will (erroneously)
abort a TCP connection if it tries to send a
datagram on the connection while the host has no
route to the destination (even if the period when
the host has no route lasts only a few seconds
while RIP chooses an alternate path to the
destination).

RIP Split Horizon and Static Routes

Split horizon SHOULD be applied to static routes by
default. An implementation SHOULD provide a way to
specify, per static route, that split horizon should not
be applied to this route.

7.2.5 GATEWAY TO GATEWAY PROTOCOL - GGP

The Gateway to Gateway protocol is considered obsolete and SHOULD
NOT be implemented.

7.3 EXTERIOR GATEWAY PROTOCOLS

7.3.1 INTRODUCTION

Exterior Gateway Protocols are utilized for inter-Autonomous
System routing to exchange reachability information for a set of
networks internal to a particular autonomous system to a
neighboring autonomous system.

The area of inter-AS routing is a current topic of research inside
the Internet Engineering Task Force. The Exterior Gateway
Protocol (EGP) described in Section [7.3.3] has traditionally been
the inter-AS protocol of choice. The Border Gateway Protocol
(BGP) eliminates many of the restrictions and limitations of EGP,
and is therefore growing rapidly in popularity. A router is not
required to implement any inter-AS routing protocol. However, if
a router does implement EGP it also MUST IMPLEMENT BGP.

Although it was not designed as an exterior gateway protocol, RIP
(described in Section [7.2.4]) is sometimes used for inter-AS
routing.

7.3.2 BORDER GATEWAY PROTOCOL - BGP

7.3.2.1 Introduction

The Border Gateway Protocol (BGP) is an inter-AS routing
protocol which exchanges network reachability information with
other BGP speakers. The information for a network includes the
complete list of ASs that traffic must transit to reach that
network. This information can then be used to insure loop-free
paths. This information is sufficient to construct a graph of
AS connectivity from which routing loops may be pruned and some
policy decisions at the AS level may be enforced.

BGP is defined by [ROUTE:4]. [ROUTE:5] specifies the proper
usage of BGP in the Internet, and provides some useful
implementation hints and guidelines. [ROUTE:12] and [ROUTE:13]
provide additional useful information.

To comply with Section [8.3] of this memo, a router which
implements BGP MUST also implement the BGP MIB [MGT:15].

To characterize the set of policy decisions that can be
enforced using BGP, one must focus on the rule that an AS
advertises to its neighbor ASs only those routes that it itself
uses. This rule reflects the hop-by-hop routing paradigm
generally used throughout the current Internet. Note that some
policies cannot be supported by the hop-by-hop routing paradigm
and thus require techniques such as source routing to enforce.
For example, BGP does not enable one AS to send traffic to a
neighbor AS intending that that traffic take a different route
from that taken by traffic originating in the neighbor AS. On
the other hand, BGP can support any policy conforming to the
hop-by-hop routing paradigm.

Implementors of BGP are strongly encouraged to follow the
recommendations outlined in Section 6 of [ROUTE:5].

7.3.2.2 Protocol Walk-through

While BGP provides support for quite complex routing policies
(as an example see Section 4.2 in [ROUTE:5]), it is not
required for all BGP implementors to support such policies. At

a minimum, however, a BGP implementation:

(1) SHOULD allow an AS to control announcements of the BGP
learned routes to adjacent AS's. Implementations SHOULD
support such control with at least the granularity of a
single network. Implementations SHOULD also support such
control with the granularity of an autonomous system,
where the autonomous system may be either the autonomous
system that originated the route, or the autonomous system
that advertised the route to the local system (adjacent
autonomous system).

(2) SHOULD allow an AS to prefer a particular path to a
destination (when more than one path is available). Such
function SHOULD be implemented by allowing system
administrator to assign weights to Autonomous Systems, and
making route selection process to select a route with the
lowest weight (where weight of a route is defined as a sum
of weights of all AS's in the AS_PATH path attribute
associated with that route).

(3) SHOULD allow an AS to ignore routes with certain AS's in
the AS_PATH path attribute. Such function can be
implemented by using technique outlined in (2), and by
assigning infinity as weights for such AS's. The route
selection process must ignore routes that have weight
equal to infinity.

7.3.3 EXTERIOR GATEWAY PROTOCOL - EGP

7.3.3.1 Introduction

The Exterior Gateway Protocol (EGP) specifies an EGP which is
used to exchange reachability information between routers of
the same or differing autonomous systems. EGP is not considered
a routing protocol since there is no standard interpretation
(i.e. metric) for the distance fields in the EGP update
message, so distances are comparable only among routers of the
same AS. It is however designed to provide high-quality
reachability information, both about neighbor routers and about
routes to non-neighbor routers.

EGP is defined by [ROUTE:6]. An implementor almost certainly
wants to read [ROUTE:7] and [ROUTE:8] as well, for they contain
useful explanations and background material.

DISCUSSION:
The present EGP specification has serious limitations, most
importantly a restriction which limits routers to
advertising only those networks which are reachable from
within the router's autonomous system. This restriction
against propagating third party EGP information is to
prevent long-lived routing loops. This effectively limits
EGP to a two-level hierarchy.

RFC-975 is not a part of the EGP specification, and should
be ignored.

7.3.3.2 Protocol Walk-through

Indirect Neighbors: RFC-888, pp. 26

An implementation of EGP MUST include indirect neighbor
support.

Polling Intervals: RFC-904, pp. 10

The interval between Hello command retransmissions and the
interval between Poll retransmissions SHOULD be configurable
but there MUST be a minimum value defined.

The interval at which an implementation will respond to
Hello commands and Poll commands SHOULD be configurable but
there MUST be a minimum value defined.

Network Reachability: RFC-904, pp. 15

An implementation MUST default to not providing the external
list of routers in other autonomous systems; only the
internal list of routers together with the nets which are
reachable via those routers should be included in an Update
Response/Indication packet. However, an implementation MAY
elect to provide a configuration option enabling the
external list to be provided. An implementation MUST NOT
include in the external list routers which were learned via
the external list provided by a router in another autonomous
system. An implementation MUST NOT send a network back to
the autonomous system from which it is learned, i.e. it MUST
do split-horizon on an autonomous system level.

If more than 255 internal or 255 external routers need to be

specified in a Network Reachability update, the networks
reachable from routers that can not be listed MUST be merged
into the list for one of the listed routers. Which of the
listed routers is chosen for this purpose SHOULD be user
configurable, but SHOULD default to the source address of
the EGP update being generated.

An EGP update contains a series of blocks of network
numbers, where each block contains a list of network numbers
reachable at a particular distance via a particular router.
If more than 255 networks are reachable at a particular
distance via a particular router, they are split into
multiple blocks (all of which have the same distance).
Similarly, if more than 255 blocks are required to list the
networks reachable via a particular router, the router's
address is listed as many times as necessary to include all
of the blocks in the update.

Unsolicited Updates: RFC-904, pp. 16

If a network is shared with the peer, an implementation MUST
send an unsolicited update upon entry to the Up state
assuming that the source network is the shared network.

Neighbor Reachability: RFC-904, pp. 6, 13-15

The table on page 6 which describes the values of j and k
(the neighbor up and down thresholds) is incorrect. It is
reproduced correctly here:

Name Active Passive Description
-----------------------------------------------
j 3 1 neighbor-up threshold
k 1 0 neighbor-down threshold

The value for k in passive mode also specified incorrectly
in RFC-904, pp. 14 The values in parenthesis should read:

(j = 1, k = 0, and T3/T1 = 4)

As an optimization, an implementation can refrain from
sending a Hello command when a Poll is due. If an
implementation does so, it SHOULD provide a user
configurable option to disable this optimization.

Abort timer: RFC-904, pp. 6, 12, 13

An EGP implementation MUST include support for the abort
timer (as documented in section 4.1.4 of RFC-904). An
implementation SHOULD use the abort timer in the Idle state
to automatically issue a Start event to restart the protocol
machine. Recommended values are P4 for a critical error
(Administratively prohibited, Protocol Violation and
Parameter Problem) and P5 for all others. The abort timer
SHOULD NOT be started when a Stop event was manually
initiated (such as via a network management protocol).

Cease command received in Idle state: RFC-904, pp. 13

When the EGP state machine is in the Idle state, it MUST
reply to Cease commands with a Cease-ack response.

Hello Polling Mode: RFC-904, pp. 11

An EGP implementation MUST include support for both active
and passive polling modes.

Neighbor Acquisition Messages: RFC-904, pp. 18

As noted the Hello and Poll Intervals should only be present
in Request and Confirm messages. Therefore the length of an
EGP Neighbor Acquisition Message is 14 bytes for a Request
or Confirm message and 10 bytes for a Refuse, Cease or
Cease-ack message. Implementations MUST NOT send 14 bytes
for Refuse, Cease or Cease-ack messages but MUST allow for
implementations that send 14 bytes for these messages.

Sequence Numbers: RFC-904, pp. 10

Response or indication packets received with a sequence
number not equal to S MUST be discarded. The send sequence
number S MUST be incremented just before the time a Poll
command is sent and at no other times.

7.3.4 INTER-AS ROUTING WITHOUT AN EXTERIOR PROTOCOL

It is possible to exchange routing information between two
autonomous systems or routing domains without using a standard
exterior routing protocol between two separate, standard interior
routing protocols. The most common way of doing this is to run
both interior protocols independently in one of the border routers
with an exchange of route information between the two processes.

As with the exchange of information from an EGP to an IGP, without

appropriate controls these exchanges of routing information
between two IGPs in a single router are subject to creation of
routing loops.

7.4 STATIC ROUTING

Static routing provides a means of explicitly defining the next hop
from a router for a particular destination. A router SHOULD provide
a means for defining a static route to a destination, where the
destination is defined by an address and an address mask. The
mechanism SHOULD also allow for a metric to be specified for each
static route.

A router which supports a dynamic routing protocol MUST allow static
routes to be defined with any metric valid for the routing protocol
used. The router MUST provide the ability for the user to specify a
list of static routes which may or may not be propagated via the
routing protocol. In addition, a router SHOULD support the following
additional information if it supports a routing protocol that could
make use of the information. They are:

o TOS,

o Subnet mask, or

o A metric specific to a given routing protocol that can import the
route.

DISCUSSION:
We intend that one needs to support only the things useful to the
given routing protocol. The need for TOS should not require the
vendor to implement the other parts if they are not used.

Whether a router prefers a static route over a dynamic route (or vice
versa) or whether the associated metrics are used to choose between
conflicting static and dynamic routes SHOULD be configurable for each
static route.

A router MUST allow a metric to be assigned to a static route for
each routing domain that it supports. Each such metric MUST be
explicitly assigned to a specific routing domain. For example:

route 36.0.0.0 255.0.0.0 via 192.19.200.3 rip metric 3

route 36.21.0.0 255.255.0.0 via 192.19.200.4 ospf inter-area
metric 27

route 36.22.0.0 255.255.0.0 via 192.19.200.5 egp 123 metric 99

route 36.23.0.0 255.255.0.0 via 192.19.200.6 igrp 47 metric 1 2
3 4 5

DISCUSSION:
It has been suggested that, ideally, static routes should have
preference values rather than metrics (since metrics can only be
compared with metrics of other routes in the same routing domain,
the metric of a static route could only be compared with metrics
of other static routes). This is contrary to some current
implementations, where static routes really do have metrics, and
those metrics are used to determine whether a particular dynamic
route overrides the static route to the same destination. Thus,
this document uses the term metric rather than preference.

This technique essentially makes the static route into a RIP
route, or an OSPF route (or whatever, depending on the domain of
the metric). Thus, the route lookup algorithm of that domain
applies. However, this is NOT route leaking, in that coercing a
static route into a dynamic routing domain does not authorize the
router to redistribute the route into the dynamic routing domain.

For static routes not put into a specific routing domain, the
route lookup algorithm is:

(1) Basic match

(2) Longest match

(3) Weak TOS (if TOS supported)

(4) Best metric (where metric are implementation-defined)

The last step may not be necessary, but it's useful in the case
where you want to have a primary static route over one interface
and a secondary static route over an alternate interface, with
failover to the alternate path if the interface for the primary
route fails.

7.5 FILTERING OF ROUTING INFORMATION

Each router within a network makes forwarding decisions based upon
information contained within its forwarding database. In a simple
network the contents of the database may be statically configured.
As the network grows more complex, the need for dynamic updating of
the forwarding database becomes critical to the efficient operation
of the network.

If the data flow through a network is to be as efficient as possible,
it is necessary to provide a mechanism for controlling the
propagation of the information a router uses to build its forwarding
database. This control takes the form of choosing which sources of
routing information should be trusted and selecting which pieces of
the information to believe. The resulting forwarding database is a
filtered version of the available routing information.

In addition to efficiency, controlling the propagation of routing
information can reduce instability by preventing the spread of
incorrect or bad routing information.

In some cases local policy may require that complete routing
information not be widely propagated.

These filtering requirements apply only to non-SPF-based protocols
(and therefore not at all to routers which don't implement any
distance vector protocols).

7.5.1 Route Validation

A router SHOULD log as an error any routing update advertising a
route to network zero, subnet zero, or subnet -1, unless the
routing protocol from which the update was received uses those
values to encode special routes (such as default routes).

7.5.2 Basic Route Filtering

Filtering of routing information allows control of paths used by a
router to forward packets it receives. A router should be
selective in which sources of routing information it listens to
and what routes it believes. Therefore, a router MUST provide the
ability to specify:

o On which logical interfaces routing information will be
accepted and which routes will be accepted from each logical
interface.

o Whether all routes or only a default route is advertised on a
logical interface.

Some routing protocols do not recognize logical interfaces as a
source of routing information. In such cases the router MUST
provide the ability to specify

o from which other routers routing information will be accepted.

For example, assume a router connecting one or more leaf networks
to the main portion or backbone of a larger network. Since each
of the leaf networks has only one path in and out, the router can
simply send a default route to them. It advertises the leaf
networks to the main network.

7.5.3 Advanced Route Filtering

As the topology of a network grows more complex, the need for more
complex route filtering arises. Therefore, a router SHOULD
provide the ability to specify independently for each routing
protocol:

o Which logical interfaces or routers routing information
(routes) will be accepted from and which routes will be
believed from each other router or logical interface,

o Which routes will be sent via which logical interface(s), and

o Which routers routing information will be sent to, if this is
supported by the routing protocol in use.

In many situations it is desirable to assign a reliability
ordering to routing information received from another router
instead of the simple believe or don't believe choice listed in
the first bullet above. A router MAY provide the ability to
specify:

o A reliability or preference to be assigned to each route
received. A route with higher reliability will be chosen over
one with lower reliability regardless of the routing metric
associated with each route.

If a router supports assignment of preferences, the router MUST
NOT propagate any routes it does not prefer as first party
information. If the routing protocol being used to propagate the
routes does not support distinguishing between first and third
party information, the router MUST NOT propagate any routes it

does not prefer.

DISCUSSION:
For example, assume a router receives a route to network C from
router R and a route to the same network from router S. If
router R is considered more reliable than router S traffic
destined for network C will be forwarded to router R regardless
of the route received from router S.

Routing information for routes which the router does not use
(router S in the above example) MUST NOT be passed to any other
router.

7.6 INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE

Routers MUST be able to exchange routing information between separate
IP interior routing protocols, if independent IP routing processes
can run in the same router. Routers MUST provide some mechanism for
avoiding routing loops when routers are configured for bi-directional
exchange of routing information between two separate interior routing
processes. Routers MUST provide some priority mechanism for choosing
routes from among independent routing processes. Routers SHOULD
provide administrative control of IGP-IGP exchange when used across
administrative boundaries.

Routers SHOULD provide some mechanism for translating or transforming
metrics on a per network basis. Routers (or routing protocols) MAY
allow for global preference of exterior routes imported into an IGP.

DISCUSSION:
Different IGPs use different metrics, requiring some translation
technique when introducing information from one protocol into
another protocol with a different form of metric. Some IGPs can
run multiple instances within the same router or set of routers.
In this case metric information can be preserved exactly or
translated.

There are at least two techniques for translation between
different routing processes. The static (or reachability)
approach uses the existence of a route advertisement in one IGP to
generate a route advertisement in the other IGP with a given
metric. The translation or tabular approach uses the metric in
one IGP to create a metric in the other IGP through use of either
a function (such as adding a constant) or a table lookup.

Bi-directional exchange of routing information is dangerous
without control mechanisms to limit feedback. This is the same

problem that distance vector routing protocols must address with
the split horizon technique and that EGP addresses with the
third-party rule. Routing loops can be avoided explicitly through
use of tables or lists of permitted/denied routes or implicitly
through use of a split horizon rule, a no-third-party rule, or a
route tagging mechanism. Vendors are encouraged to use implicit
techniques where possible to make administration easier for
network operators.

8. APPLICATION LAYER - NETWORK MANAGEMENT PROTOCOLS

Note that this chapter supersedes any requirements stated in section 6.3
of [INTRO:3].

8.1 The Simple Network Management Protocol - SNMP

8.1.1 SNMP Protocol Elements

Routers MUST be manageable by SNMP [MGT:3]. The SNMP MUST operate
using UDP/IP as its transport and network protocols. Others MAY
be supported (e.g., see [MGT:25, MGT:26, MGT:27, and MGT:28]).
SNMP management operations MUST operate as if the SNMP was
implemented on the router itself. Specifically, management
operations MUST be effected by sending SNMP management requests to
any of the IP addresses assigned to any of the router's
interfaces. The actual management operation may be performed
either by the router or by a proxy for the router.

DISCUSSION:
This wording is intended to allow management either by proxy,
where the proxy device responds to SNMP packets which have one
of the router's IP addresses in the packets destination address
field, or the SNMP is implemented directly in the router itself
and receives packets and responds to them in the proper manner.

It is important that management operations can be sent to one
of the router's IP Addresses. In diagnosing network problems
the only thing identifying the router that is available may be
one of the router's IP address; obtained perhaps by looking
through another router's routing table.

All SNMP operations (get, get-next, get-response, set, and trap)
MUST be implemented.

Routers MUST provide a mechanism for rate-limiting the generation
of SNMP trap messages. Routers MAY provide this mechanism via the
algorithms for asynchronous alert management described in [MGT:5].

DISCUSSION:
Although there is general agreement about the need to rate-
limit traps, there is not yet consensus on how this is best
achieved. The reference cited is considered experimental.

8.2 Community Table

For the purposes of this specification, we assume that there is an
abstract `community table' in the router. This table contains
several entries, each entry for a specific community and containing
the parameters necessary to completely define the attributes of that
community. The actual implementation method of the abstract
community table is, of course, implementation specific.

A router's community table MUST allow for at least one entry and
SHOULD allow for at least two entries.

DISCUSSION:
A community table with zero capacity is useless. It means that
the router will not recognize any communities and, therefore, all
SNMP operations will be rejected.

Therefore, one entry is the minimal useful size of the table.
Having two entries allows one entry to be limited to read-only
access while the other would have write capabilities.

Routers MUST allow the user to manually (i.e., without using SNMP)
examine, add, delete and change entries in the SNMP community table.
The user MUST be able to set the community name. The user MUST be
able to configure communities as read-only (i.e., they do not allow
SETs) or read-write (i.e., they do allow SETs).

The user MUST be able to define at least one IP address to which
traps are sent for each community. These addresses MUST be definable
on a per-community basis. Traps MUST be enablable or disablable on a
per-community basis.

A router SHOULD provide the ability to specify a list of valid
network managers for any particular community. If enabled, a router
MUST validate the source address of the SNMP datagram against the
list and MUST discard the datagram if its address does not appear.
If the datagram is discarded the router MUST take all actions
appropriate to an SNMP authentication failure.

DISCUSSION:
This is a rather limited authentication system, but coupled with
various forms of packet filtering may provide some small measure
of increased security.

The community table MUST be saved in non-volatile storage.

The initial state of the community table SHOULD contain one entry,

with the community name string public and read-only access. The
default state of this entry MUST NOT send traps. If it is
implemented, then this entry MUST remain in the community table until
the administrator changes it or deletes it.

DISCUSSION:
By default, traps are not sent to this community. Trap PDUs are
sent to unicast IP addresses. This address must be configured into
the router in some manner. Before the configuration occurs, there
is no such address, so to whom should the trap be sent? Therefore
trap sending to the public community defaults to be disabled. This
can, of course, be changed by an administrative operation once the
router is operational.

8.3 Standard MIBS

All MIBS relevant to a router's configuration are to be implemented.
To wit:

o The System, Interface, IP, ICMP, and UDP groups of MIB-II [MGT:2]
MUST be implemented.

o The Interface Extensions MIB [MGT:18] MUST be implemented.

o The IP Forwarding Table MIB [MGT:20] MUST be implemented.

o If the router implements TCP (e.g. for Telnet) then the TCP group
of MIB-II [MGT:2] MUST be implemented.

o If the router implements EGP then the EGP group of MIB-II [MGT:2]
MUST be implemented.

o If the router supports OSPF then the OSPF MIB [MGT:14] MUST be
implemented.

o If the router supports BGP then the BGP MIB [MGT:15] MUST be
implemented.

o If the router has Ethernet, 802.3, or StarLan interfaces then the
Ethernet-Like MIB [MGT:6] MUST be implemented.

o If the router has 802.4 interfaces then the 802.4 MIB [MGT:7] MAY
be implemented.

o If the router has 802.5 interfaces then the 802.5 MIB [MGT:8] MUST
be implemented.

o If the router has FDDI interfaces that implement ANSI SMT 7.3 then
the FDDI MIB [MGT:9] MUST be implemented.

o If the router has FDDI interfaces that implement ANSI SMT 6.2 then
the FDDI MIB [MGT:29] MUST be implemented.

o If the router has RS-232 interfaces then the RS-232 [MGT:10] MIB
MUST be implemented.

o If the router has T1/DS1 interfaces then the T1/DS1 MIB [MGT:16]
MUST be implemented.

o If the router has T3/DS3 interfaces then the T3/DS3 MIB [MGT:17]
MUST be implemented.

o If the router has SMDS interfaces then the SMDS Interface Protocol
MIB [MGT:19] MUST be implemented.

o If the router supports PPP over any of its interfaces then the PPP
MIBs [MGT:11], [MGT:12], and [MGT:13] MUST be implemented.

o If the router supports RIP Version 2 then the RIP Version 2 MIB
[MGT:21] MUST be implemented.

o If the router supports X.25 over any of its interfaces then the
X.25 MIBs [MGT:22, MGT:23 and MGT:24] MUST be implemented.

8.4 Vendor Specific MIBS

The Internet Standard and Experimental MIBs do not cover the entire
range of statistical, state, configuration and control information
that may be available in a network element. This information is,
never the less, extremely useful. Vendors of routers (and other
network devices) generally have developed MIB extensions that cover
this information. These MIB extensions are called Vendor Specific
MIBs.

The Vendor Specific MIB for the router MUST provide access to all
statistical, state, configuration, and control information that is
not available through the Standard and Experimental MIBs that have
been implemented. This information MUST be available for both
monitoring and control operations.

DISCUSSION:
The intent of this requirement is to provide the ability to do
anything on the router via SNMP that can be done via a console. A
certain minimal amount of configuration is necessary before SNMP
can operate (e.g., the router must have an IP address). This
initial configuration can not be done via SNMP. However, once the
initial configuration is done, full capabilities ought to be
available via network management.

The vendor SHOULD make available the specifications for all Vendor
Specific MIB variables. These specifications MUST conform to the SMI
[MGT:1] and the descriptions MUST be in the form specified in
[MGT:4].

DISCUSSION:
Making the Vendor Specific MIB available to the user is necessary.
Without this information the users would not be able to configure
their network management systems to be able to access the Vendor
Specific parameters. These parameters would then be useless.

The format of the MIB specification is also specified. Parsers
which read MIB specifications and generate the needed tables for
the network management station are available. These parsers
generally understand only the standard MIB specification format.

8.5 Saving Changes

Parameters altered by SNMP MAY be saved to non-volatile storage.

DISCUSSION:
Reasons why this requirement is a MAY:

o The exact physical nature of non-volatile storage is not
specified in this document. Hence, parameters may be saved in
NVRAM/EEPROM, local floppy or hard disk, or in some TFTP file
server or BOOTP server, etc. Suppose that that this information
is in a file that is retrieved via TFTP. In that case, a change
made to a configuration parameter on the router would need to
be propagated back to the file server holding the configuration
file. Alternatively, the SNMP operation would need to be
directed to the file server, and then the change somehow
propagated to the router. The answer to this problem does not
seem obvious.

This also places more requirements on the host holding the
configuration information than just having an available tftp

server, so much more that its probably unsafe for a vendor to
assume that any potential customer will have a suitable host
available.

o The timing of committing changed parameters to non-volatile
storage is still an issue for debate. Some prefer to commit all
changes immediately. Others prefer to commit changes to non-
volatile storage only upon an explicit command.

9. APPLICATION LAYER - MISCELLANEOUS PROTOCOLS

For all additional application protocols that a router implements, the
router MUST be compliant and SHOULD be unconditionally compliant with
the relevant requirements of [INTRO:3].

9.1 BOOTP

9.1.1 Introduction

The Bootstrap Protocol (BOOTP) is a UDP/IP-based protocol which
allows a booting host to configure itself dynamically and without
user supervision. BOOTP provides a means to notify a host of its
assigned IP address, the IP address of a boot server host, and the
name of a file to be loaded into memory and executed ([APPL:1]).
Other configuration information such as the local subnet mask, the
local time offset, the addresses of default routers, and the
addresses of various Internet servers can also be communicated to
a host using BOOTP ([APPL:2]).

9.1.2 BOOTP Relay Agents

In many cases, BOOTP clients and their associated BOOTP server(s)
do not reside on the same IP network or subnet. In such cases, a
third-party agent is required to transfer BOOTP messages between
clients and servers. Such an agent was originally referred to as
a BOOTP forwarding agent. However, in order to avoid confusion
with the IP forwarding function of a router, the name BOOTP relay
agent has been adopted instead.

DISCUSSION:
A BOOTP relay agent performs a task which is distinct from a
router's normal IP forwarding function. While a router
normally switches IP datagrams between networks more-or-less
transparently, a BOOTP relay agent may more properly be thought
to receive BOOTP messages as a final destination and then
generate new BOOTP messages as a result. One should resist the
notion of simply forwarding a BOOTP message straight through
like a regular packet.

This relay-agent functionality is most conveniently located in the
routers which interconnect the clients and servers (although it
may alternatively be located in a host which is directly connected
to the client subnet).

A router MAY provide BOOTP relay-agent capability. If it does, it

MUST conform to the specifications in [APPL:3].

Section [5.2.3] discussed the circumstances under which a packet
is delivered locally (to the router). All locally delivered UDP
messages whose UDP destination port number is BOOTPS (67) are
considered for special processing by the router's logical BOOTP
relay agent.

Sections [4.2.2.11] and [5.3.7] discussed invalid IP source
addresses. According to these rules, a router must not forward
any received datagram whose IP source address is 0.0.0.0.
However, routers which support a BOOTP relay agent MUST accept for
local delivery to the relay agent BOOTREQUEST messages whose IP
source address is 0.0.0.0.

10. OPERATIONS AND MAINTENANCE

This chapter supersedes any requirements stated in section 6.2 of
[INTRO:3].

Facilities to support operation and maintenance (O&M) activities form an
essential part of any router implementation. Although these functions
do not seem to relate directly to interoperability, they are essential
to the network manager who must make the router interoperate and must
track down problems when it doesn't. This chapter also includes some
discussion of router initialization and of facilities to assist network
managers in securing and accounting for their networks.

10.1 Introduction

The following kinds of activities are included under router O&M:

o Diagnosing hardware problems in the router's processor, in its
network interfaces, or in its connected networks, modems, or
communication lines.

o Installing new hardware

o Installing new software.

o Restarting or rebooting the router after a crash.

o Configuring (or reconfiguring) the router.

o Detecting and diagnosing Internet problems such as congestion,
routing loops, bad IP addresses, black holes, packet avalanches,
and misbehaved hosts.

o Changing network topology, either temporarily (e.g., to bypass a
communication line problem) or permanently.

o Monitoring the status and performance of the routers and the
connected networks.

o Collecting traffic statistics for use in (Inter-)network planning.

o Coordinating the above activities with appropriate vendors and
telecommunications specialists.

Routers and their connected communication lines are often operated as
a system by a centralized O&M organization. This organization may
maintain a (Inter-)network operation center, or NOC, to carry out its

O&M functions. It is essential that routers support remote control
and monitoring from such a NOC through an Internet path, since
routers might not be connected to the same network as their NOC.
Since a network failure may temporarily preclude network access, many
NOCs insist that routers be accessible for network management via an
alternative means, often dialup modems attached to console ports on
the routers.

Since an IP packet traversing an internet will often use routers
under the control of more than one NOC, Internet problem diagnosis
will often involve cooperation of personnel of more than one NOC. In
some cases, the same router may need to be monitored by more than one
NOC, but only if necessary, because excessive monitoring could impact
a router's performance.

The tools available for monitoring at a NOC may cover a wide range of
sophistication. Current implementations include multi-window, dynamic
displays of the entire router system. The use of AI techniques for
automatic problem diagnosis is proposed for the future.

Router O&M facilities discussed here are only a part of the large and
difficult problem of Internet management. These problems encompass
not only multiple management organizations, but also multiple
protocol layers. For example, at the current stage of evolution of
the Internet architecture, there is a strong coupling between host
TCP implementations and eventual IP-level congestion in the router
system [OPER:1]. Therefore, diagnosis of congestion problems will
sometimes require the monitoring of TCP statistics in hosts. There
are currently a number of R&D efforts in progress in the area of
Internet management and more specifically router O&M. These R&D
efforts have already produced standards for router O&M. This is also
an area in which vendor creativity can make a significant
contribution.

10.2 Router Initialization

10.2.1 Minimum Router Configuration

There exists a minimum set of conditions that must be satisfied
before a router may forward packets. A router MUST NOT enable
forwarding on any physical interface unless either:

(1) The router knows the IP address and associated subnet mask of
at least one logical interface associated with that physical
interface, or

(2) The router knows that the interface is an unnumbered
interface and also knows its router-id.

These parameters MUST be explicitly configured:

o A router MUST NOT use factory-configured default values for its
IP addresses, subnet masks, or router-id, and

o A router MUST NOT assume that an unconfigured interface is an
unnumbered interface.

DISCUSSION:
There have been instances in which routers have been shipped
with vendor-installed default addresses for interfaces. In a
few cases, this has resulted in routers advertising these
default addresses into active networks.

10.2.2 Address and Address Mask Initialization

A router MUST allow its IP addresses and their subnet masks to be
statically configured and saved in permanent storage.

A router MAY obtain its IP addresses and their corresponding
subnet masks dynamically as a side effect of the system
initialization process (see Section 10.2.3]);

If the dynamic method is provided, the choice of method to be used
in a particular router MUST be configurable.

As was described in Section [4.2.2.11], IP addresses are not
permitted to have the value 0 or -1 for any of the <Host-number>,
<Network-number>, or <Subnet-number> fields. Therefore, a router
SHOULD NOT allow an IP address or subnet mask to be set to a value
which would make any of the the three fields above have the value
zero or -1.

DISCUSSION:
It is possible using variable length subnet masks to create
situations in which routing is ambiguous (i.e., two routes with
different but equally-specific subnet masks match a particular
destination address). We suspect that a router could, when
setting a subnet mask, check whether the mask would cause
routing to be ambiguous, and that implementors might be able to
decrease their customer support costs by having routers
prohibit or log such erroneous configurations. However, at
this time we do not require routers to make such checks because

we know of no published method for accurately making this
check.

A router SHOULD make the following checks on any subnet mask it
installs:

o The mask is not all 1-bits.

o The bits which correspond to the network number part of the
address are all set to 1.

DISCUSSION:
The masks associated with routes are also sometimes called
subnet masks, this test should not be applied to them.

10.2.3 Network Booting using BOOTP and TFTP

There has been a lot of discussion on how routers can and should
be booted from the network. In general, these discussions have
centered around BOOTP and TFTP. Currently, there are routers that
boot with TFTP from the network. There is no reason that BOOTP
could not be used for locating the server that the boot image
should be loaded from.

In general, BOOTP is a protocol used to boot end systems, and
requires some stretching to accommodate its use with routers. If
a router is using BOOTP to locate the current boot host, it should
send a BOOTP Request with its hardware address for its first
interface, or, if it has been previously configured otherwise,
with either another interface's hardware address, or another
number to put in the hardware address field of the BOOTP packet.
This is to allow routers without hardware addresses (like sync
line only routers) to use BOOTP for bootload discovery. TFTP can
then be used to retrieve the image found in the BOOTP Reply. If
there are no configured interfaces or numbers to use, a router MAY
cycle through the interface hardware addresses it has until a
match is found by the BOOTP server.

A router SHOULD IMPLEMENT the ability to store parameters learned
via BOOTP into local stable storage. A router MAY implement the
ability to store a system image loaded over the network into local
stable storage.

A router MAY have a facility to allow a remote user to request
that the router get a new boot image. Differentiation should be

made between getting the new boot image from one of three
locations: the one included in the request, from the last boot
image server, and using BOOTP to locate a server.

10.3 Operation and Maintenance

10.3.1 Introduction

There is a range of possible models for performing O&M functions
on a router. At one extreme is the local-only model, under which
the O&M functions can only be executed locally (e.g., from a
terminal plugged into the router machine). At the other extreme,
the fully-remote model allows only an absolute minimum of
functions to be performed locally (e.g., forcing a boot), with
most O&M being done remotely from the NOC. There are intermediate
models, such as one in which NOC personnel can log into the router
as a host, using the Telnet protocol, to perform functions which
can also be invoked locally. The local-only model may be adequate
in a few router installations, but in general remote operation
from a NOC will be required, and therefore remote O&M provisions
are required for most routers.

Remote O&M functions may be exercised through a control agent
(program). In the direct approach, the router would support
remote O&M functions directly from the NOC using standard Internet
protocols (e.g., SNMP, UDP or TCP); in the indirect approach, the
control agent would support these protocols and control the router
itself using proprietary protocols. The direct approach is
preferred, although either approach is acceptable. The use of
specialized host hardware and/or software requiring significant
additional investment is discouraged; nevertheless, some vendors
may elect to provide the control agent as an integrated part of
the network in which the routers are a part. If this is the case,
it is required that a means be available to operate the control
agent from a remote site using Internet protocols and paths and
with equivalent functionality with respect to a local agent
terminal.

It is desirable that a control agent and any other NOC software
tools which a vendor provides operate as user programs in a
standard operating system. The use of the standard Internet
protocols UDP and TCP for communicating with the routers should
facilitate this.

Remote router monitoring and (especially) remote router control
present important access control problems which must be addressed.

Care must also be taken to ensure control of the use of router
resources for these functions. It is not desirable to let router
monitoring take more than some limited fraction of the router CPU
time, for example. On the other hand, O&M functions must receive
priority so they can be exercised when the router is congested,
since often that is when O&M is most needed.

10.3.2 Out Of Band Access

Routers MUST support Out-Of-Band (OOB) access. OOB access SHOULD
provide the same functionality as in-band access.

DISCUSSION:
This Out-Of-Band access will allow the NOC a way to access
isolated routers during times when network access is not
available.

Out-Of-Band access is an important management tool for the
network administrator. It allows the access of equipment
independent of the network connections. There are many ways to
achieve this access. Whichever one is used it is important
that the access is independent of the network connections. An
example of Out-Of-Band access would be a serial port connected
to a modem that provides dial up access to the router.

It is important that the OOB access provides the same
functionality as in-band access. In-band access, or accessing
equipment through the existing network connection, is limiting,
because most of the time, administrators need to reach
equipment to figure out why it is unreachable. In band access
is still very important for configuring a router, and for
troubleshooting more subtle problems.

10.3.2 Router O&M Functions

10.3.2.1 Maintenance - Hardware Diagnosis

Each router SHOULD operate as a stand-alone device for the
purposes of local hardware maintenance. Means SHOULD be
available to run diagnostic programs at the router site using
only on-site tools. A router SHOULD be able to run diagnostics
in case of a fault. For suggested hardware and software
diagnostics see Section [10.3.3].

10.3.2.2 Control - Dumping and Rebooting

A router MUST include both in-band and out-of-band mechanisms
to allow the network manager to reload, stop, and restart the
router. A router SHOULD also contain a mechanism (such as a
watchdog timer) which will reboot the router automatically if
it hangs due to a software or hardware fault.

A router SHOULD IMPLEMENT a mechanism for dumping the contents
of a router's memory (and/or other state useful for vendor
debugging after a crash), and either saving them on a stable
storage device local to the router or saving them on another
host via an up-line dump mechanism such as TFTP (see [OPER:2],
[INTRO:3]).

10.3.2.3 Control - Configuring the Router

Every router has configuration parameters which may need to be
set. It SHOULD be possible to update the parameters without
rebooting the router; at worst, a restart MAY be required.
There may be cases when it is not possible to change parameters
without rebooting the router (for instance, changing the IP
address of an interface). In these cases, care should be taken
to minimize disruption to the router and the surrounding
network.

There SHOULD be a way to configure the router over the network
either manually or automatically. A router SHOULD be able to
upload or download its parameters from a host or another
router, and these parameters SHOULD be convertible into some
sort of text format for making changes and then back to the
form the router can read. A router SHOULD have some sort of
stable storage for its configuration. A router SHOULD NOT
believe protocols such as RARP, ICMP Address Mask Reply, and
MAY not believe BOOTP.

DISCUSSION:
It is necessary to note here that in the future RARP, ICMP
Address Mask Reply, BOOTP and other mechanisms may be needed
to allow a router to auto-configure. Although routers may
in the future be able to configure automatically, the intent
here is to discourage this practice in a production
environment until such time as auto-configuration has been
tested more thoroughly. The intent is NOT to discourage
auto-configuration all together. In cases where a router is
expected to get its configuration automatically it may be
wise to allow the router to believe these things as it comes

up and then ignore them after it has gotten its
configuration.

10.3.2.4 Netbooting of System Software

A router SHOULD keep its system image in local non-volatile
storage such as PROM, NVRAM, or disk. It MAY also be able to
load its system software over the network from a host or
another router.

A router which can keep its system image in local non-volatile
storage MAY be configurable to boot its system image over the
network. A router which offers this option SHOULD be
configurable to boot the system image in its non-volatile local
storage if it is unable to boot its system image over the
network.

DISCUSSION:
It is important that the router be able to come up and run
on its own. NVRAM may be a particular solution for routers
used in large networks, since changing PROMs can be quite
time consuming for a network manager responsible for
numerous or geographically dispersed routers. It is
important to be able to netboot the system image because
there should be an easy way for a router to get a bug fix or
new feature more quickly than getting PROMS installed. Also
if the router has NVRAM instead of PROMs, it will netboot
the image and then put it in NVRAM.

A router MAY also be able to distinguish between different
configurations based on which software it is running. If
configuration commands change from one software version to
another, it would be helpful if the router could use the
configuration that was compatible with the software.

10.3.2.5 Detecting and responding to misconfiguration

There MUST be mechanisms for detecting and responding to
misconfigurations. If a command is executed incorrectly, the
router SHOULD give an error message. The router SHOULD NOT
accept a poorly formed command as if it were correct.

DISCUSSION:
There are cases where it is not possible to detect errors:
the command is correctly formed, but incorrect with respect
to the network. This may be detected by the router, but may
not be possible.

Another form of misconfiguration is misconfiguration of the
network to which the router is attached. A router MAY detect
misconfigurations in the network. The router MAY log these
findings to a file, either on the router or a host, so that the
network manager will see that there are possible problems on
the network.

DISCUSSION:
Examples of such misconfigurations might be another router
with the same address as the one in question or a router
with the wrong subnet mask. If a router detects such
problems it is probably not the best idea for the router to
try to fix the situation. That could cause more harm than
good.

10.3.2.6 Minimizing Disruption

Changing the configuration of a router SHOULD have minimal
affect on the network. Routing tables SHOULD NOT be
unnecessarily flushed when a simple change is made to the
router. If a router is running several routing protocols,
stopping one routing protocol SHOULD NOT disrupt other routing
protocols, except in the case where one network is learned by
more than one routing protocol.

DISCUSSION:
It is the goal of a network manager to run a network so that
users of the network get the best connectivity possible.
Reloading a router for simple configuration changes can
cause disruptions in routing and ultimately cause
disruptions to the network and its users. If routing tables
are unnecessarily flushed, for instance, the default route
will be lost as well as specific routes to sites within the
network. This sort of disruption will cause significant
downtime for the users. It is the purpose of this section to
point out that whenever possible, these disruptions should
be avoided.

10.3.2.7 Control - Troubleshooting Problems

(1) A router MUST provide in-band network access, but (except
as required by Section [8.2]) for security considerations
this access SHOULD be disabled by default. Vendors MUST
document the default state of any in-band access.

DISCUSSION:
In-band access primarily refers to access via the
normal network protocols which may or may not affect
the permanent operational state of the router. This
includes, but is not limited to Telnet/RLOGIN console
access and SNMP operations.

This was a point of contention between the operational
out of the box and secure out of the box contingents.
Any automagic access to the router may introduce
insecurities, but it may be more important for the
customer to have a router which is accessible over the
network as soon as it is plugged in. At least one
vendor supplies routers without any external console
access and depends on being able to access the router
via the network to complete its configuration.

Basically, it is the vendors call whether or not in-
band access is enabled by default; but it is also the
vendors responsibility to make its customers aware of
possible insecurities.

(2) A router MUST provide the ability to initiate an ICMP
echo. The following options SHOULD be implemented:

o Choice of data patterns

o Choice of packet size

o Record route

and the following additional options MAY be implemented:

o Loose source route

o Strict source route

o Timestamps

(3) A router SHOULD provide the ability to initiate a
traceroute. If traceroute is provided, then the 3rd party
traceroute SHOULD be implemented.

Each of the above three facilities (if implemented) SHOULD have
access restrictions placed on it to prevent its abuse by
unauthorized persons.

10.4 Security Considerations

10.4.1 Auditing and Audit Trails

Auditing and billing are the bane of the network operator, but are
the two features most requested by those in charge of network
security and those who are responsible for paying the bills. In
the context of security, auditing is desirable if it helps you
keep your network working and protects your resources from abuse,
without costing you more than those resources are worth.

(1) Configuration Changes

Router SHOULD provide a method for auditing a configuration
change of a router, even if it's something as simple as
recording the operator's initials and time of change.

DISCUSSION:
Having the ability to track who made changes and when is
highly desirable, especially if your packets suddenly
start getting routed through Alaska on their way across
town.

(2) Packet Accounting

Vendors should strongly consider providing a system for
tracking traffic levels between pairs of hosts or networks.
A mechanism for limiting the collection of this information
to specific pairs of hosts or networks is also strongly
encouraged.

DISCUSSION:
A host traffic matrix as described above can give the
network operator a glimpse of traffic trends not apparent
from other statistics. It can also identify hosts or
networks which are probing the structure of the attached
networks - e.g., a single external host which tries to
send packets to every IP address in the network address

range for a connected network.

(3) Security Auditing

Routers MUST provide a method for auditing security related
failures or violations to include:

o Authorization Failures: bad passwords, invalid SNMP
communities, invalid authorization tokens,

o Violations of Policy Controls: Prohibited Source Routes,
Filtered Destinations, and

o Authorization Approvals: good passwords - Telnet in-band
access, console access.

Routers MUST provide a method of limiting or disabling such
auditing but auditing SHOULD be on by default. Possible
methods for auditing include listing violations to a console
if present, logging or counting them internally, or logging
them to a remote security server via the SNMP trap mechanism
or the Unix logging mechanism as appropriate. A router MUST
implement at least one of these reporting mechanisms - it MAY
implement more than one.

10.4.2 Configuration Control

A vendor has a responsibility to use good configuration control
practices in the creation of the software/firmware loads for their
routers. In particular, if a vendor makes updates and loads
available for retrieval over the Internet, the vendor should also
provide a way for the customer to confirm the load is a valid one,
perhaps by the verification of a checksum over the load.

DISCUSSION:
Many vendors currently provide short notice updates of their
software products via the Internet. This a good trend and
should be encouraged, but provides a point of vulnerability in
the configuration control process.

If a vendor provides the ability for the customer to change the
configuration parameters of a router remotely, for example via a
Telnet session, the ability to do so SHOULD be configurable and
SHOULD default to off. The router SHOULD require a password or
other valid authentication before permitting remote
reconfiguration.

DISCUSSION:
Allowing your properly identified network operator to twiddle
with your routers is necessary; allowing anyone else to do so
is foolhardy.

A router MUST NOT have undocumented back door access and master
passwords. A vendor MUST ensure any such access added for
purposes of debugging or product development are deleted before
the product is distributed to its customers.

DISCUSSION:
A vendor has a responsibility to its customers to ensure they
are aware of the vulnerabilities present in its code by
intention - e.g. in-band access. Trap doors, back doors and
master passwords intentional or unintentional can turn a
relatively secure router into a major problem on an operational
network. The supposed operational benefits are not matched by
the potential problems.

11. REFERENCES

Implementors should be aware that Internet protocol standards are
occasionally updated. These references are current as of this writing,
but a cautious implementor will always check a recent version of the RFC
index to ensure that an RFChas not been updated or superseded by
another, more recent RFC. Reference [INTRO:6] explains various ways to
obtain a current RFCindex.

APPL:1.
B. Croft and J. Gilmore, Bootstrap Protocol (BOOTP), Request For
Comments (RFC) 951, Stanford and SUN Microsystems, September 1985.

APPL:2.
S. Alexander and R. Droms, DHCP Options and BOOTP Vendor
Extensions, Request For Comments (RFC) 1533, Lachman Technology,
Inc., Bucknell University, October 1993.

APPL:3.
W. Wimer, Clarifications and Extensions for the Bootstrap Protocol,
Request For Comments (RFC) 1542, Carnegie Mellon University,
October 1993.

ARCH:1.
DDN Protocol Handbook, NIC-50004, NIC-50005, NIC-50006 (three
volumes), DDN Network Information Center, SRI International, Menlo
Park, California, USA, December 1985.

ARCH:2.
V. Cerf and R. Kahn, A Protocol for Packet Network
Intercommunication," IEEE Transactions on Communication, May 1974.
Also included in [ARCH:1].

ARCH:3.
J. Postel, C. Sunshine, and D. Cohen, The ARPA Internet Protocol,"
Computer Networks, vol. 5, no. 4, July 1981. Also included in
[ARCH:1].

ARCH:4.
B. Leiner, J. Postel, R. Cole, and D. Mills, The DARPA Internet
Protocol Suite, Proceedings of INFOCOM '85, IEEE, Washington, DC,
March 1985. Also in: IEEE Communications Magazine, March 1985.
Also available from the Information Sciences Institute, University
of Southern California as Technical Report ISI-RS-85-153.

ARCH:5.
D. Comer, Internetworking With TCP/IP Volume 1: Principles,
Protocols, and Architecture, Prentice Hall, Englewood Cliffs, NJ,
1991.

ARCH:6.
W. Stallings, Handbook of Computer-Communications Standards Volume
3: The TCP/IP Protocol Suite, Macmillan, New York, NY, 1990.

ARCH:7.
J. Postel, Internet Official Protocol Standards, Request For
Comments (RFC) 1610, STD 1, USC/Information Sciences Institute,
July 1994.

ARCH:8.
Information processing systems - Open Systems Interconnection -
Basic Reference Model, ISO 7489, International Standards
Organization, 1984.

FORWARD:1.
IETF CIP Working Group (C. Topolcic, Editor), Experimental Internet
Stream Protocol, Version 2 (ST-II), Request For Comments (RFC)
1190, CIP Working Group, October 1990.

FORWARD:2.
A. Mankin and K. Ramakrishnan, Editors, Gateway Congestion Control
Survey, Request For Comments (RFC) 1254, MITRE, Digital Equipment
Corporation, August 1991.

FORWARD:3.
J. Nagle, On Packet Switches with Infinite Storage, IEEE
Transactions on Communications, vol. COM-35, no. 4, April 1987.

FORWARD:4.
R. Jain, K. Ramakrishnan, and D. Chiu, Congestion Avoidance in
Computer Networks With a Connectionless Network Layer, Technical
Report DEC-TR-506, Digital Equipment Corporation.

FORWARD:5.
V. Jacobson, Congestion Avoidance and Control, Proceedings of
SIGCOMM '88, Association for Computing Machinery, August 1988.

FORWARD:6.
W. Barns, Precedence and Priority Access Implementation for
Department of Defense Data Networks, Technical Report MTR-91W00029,
The Mitre Corporation, McLean, Virginia, USA, July 1991.

INTERNET:1.
J. Postel, Internet Protocol, Request For Comments (RFC) 791, STD
5, USC/Information Sciences Institute, September 1981.

INTERNET:2.
J. Mogul and J. Postel, Internet Standard Subnetting Procedure,
Request For Comments (RFC) 950, STD 5, USC/Information Sciences
Institute, August 1985.

INTERNET:3.
J. Mogul, Broadcasting Internet Datagrams in the Presence of
Subnets, Request For Comments (RFC) 922, STD 5, Stanford, October
1984.

INTERNET:4.
S. Deering, Host Extensions for IP Multicasting, Request For
Comments (RFC) 1112, STD 5, Stanford University, August 1989.

INTERNET:5.
S. Kent, U.S. Department of Defense Security Options for the
Internet Protocol, Request for Comments (RFC) 1108, BBN
Communications, November 1991.

INTERNET:6.
R. Braden, D. Borman, and C. Partridge, Computing the Internet
Checksum, Request For Comments (RFC) 1071, USC/Information Sciences
Institute, Cray Researc, BBN, September 1988.

INTERNET:7.
T. Mallory and A. Kullberg, Incremental Updating of the Internet
Checksum, Request For Comments (RFC) 1141, BBN, January 1990.

INTERNET:8.
J. Postel, Internet Control Message Protocol, Request For Comments
(RFC) 792, STD 5, USC/Information Sciences Institute, September
1981.

INTERNET:9.
A. Mankin, G. Hollingsworth, G. Reichlen, K. Thompson, R. Wilder,
and R. Zahavi, Evaluation of Internet Performance - FY89, Technical
Report MTR-89W00216, MITRE Corporation, February, 1990.

INTERNET:10.
G. Finn, A Connectionless Congestion Control Algorithm, Computer
Communications Review, vol. 19, no. 5, Association for Computing
Machinery, October 1989.

INTERNET:11.
W. Prue, J. Postel, The Source Quench Introduced Delay (SQuID),
Request For Comments (RFC) 1016, USC/Information Sciences
Institute, August 1987.

INTERNET:12.
A. McKenzie, Some comments on SQuID, Request For Comments (RFC)
1018, BBN, August 1987.

INTERNET:13.
S. Deering, ICMP Router Discovery Messages, Request For Comments
(RFC) 1256, Xerox PARC, September 1991.

INTERNET:14.
J. Mogul and S. Deering, Path MTU Discovery, Request For Comments
(RFC) 1191, DECWRL, Stanford University, November 1990.

INTERNET:15
V. Fuller, T. Li, J. Yi, and K. Varadhan, Classless Inter-Domain
Routing (CIDR): an Address Assignment and Aggregation Strategy
Request For Comments (RFC) 1519, BARRNet, cisco, Merit, OARnet,
September 1993.

INTERNET:16
M. St. Johns, Draft Revised IP Security Option, Request for
Comments 1038, IETF, January 1988.

INTERNET:17
W. Prue and J. Postel, Queuing Algorithm to Provide Type-of-service
For IP Links, Request for Comments 1046, USC/Information Sciences
Institute, February 1988.

INTRO:1.
R. Braden and J. Postel, Requirements for Internet Gateways,
Request For Comments (RFC) 1009, STD 4, USC/Information Sciences
Institute, June 1987.

INTRO:2.
Internet Engineering Task Force (R. Braden, Editor), Requirements
for Internet Hosts - Communication Layers, Request For Comments
(RFC) 1122, STD 3, USC/Information Sciences Institute, October
1989.

INTRO:3.
Internet Engineering Task Force (R. Braden, Editor), Requirements
for Internet Hosts - Application and Support, Request For Comments
(RFC) 1123, STD 3, USC/Information Sciences Institute, October
1989.

INTRO:4.
D. Clark, Modularity and Efficiency in Protocol Implementations,
Request For Comments (RFC) 817, MIT, July 1982.

INTRO:5.
D. Clark, The Structuring of Systems Using Upcalls, Proceedings of
10th ACM SOSP, December 1985.

INTRO:6.
O. Jacobsen and J. Postel, Protocol Document Order Information,
Request For Comments (RFC) 980, SRI, USC/Information Sciences
Institute, March 1986.

INTRO:7.
J. Reynolds and J. Postel, Assigned Numbers, Request For Comments
(RFC) 1700, STD 2, USC/Information Sciences Institute, October
1994. This document is periodically updated and reissued with a
new number. It is wise to verify occasionally that the version you
have is still current.

INTRO:8.
DoD Trusted Computer System Evaluation Criteria, DoD publication
5200.28-STD, U.S. Department of Defense, December 1985.

INTRO:9
G. Malkin and T. LaQuey Parker, Internet Users' Glossary, Request
for Comments (RFC) 1392 (also FYI 0018), Xylogics, Inc., UTexas,
January 1993.

LINK:1.
S. Leffler and M. Karels, Trailer Encapsulations, Request For
Comments (RFC) 893, U. C. Berkeley, April 1984.

LINK:2
W. Simpson, The Point-to-Point Protocol (PPP) for the Transmission
of Multi-protocol Datagrams over Point-to-Point Links, Daydreamer,
Request For Comments (RFC) 1331, May 1992.

LINK:3
G. McGregor, The PPP Internet Protocol Control Protocol (IPCP),
Request For Comments (RFC) 1332, Merit, May 1992.

LINK:4
B. Lloyd, W. Simpson, PPP Authentication Protocols, Request For
Comments (RFC) 1334, Daydreamer, May 1992.

LINK:5
W. Simpson, PPP Link Quality Monitoring, Daydreamer, Request For
Comments (RFC) 1333, May 1992.

MGT:1.
M. Rose and K. McCloghrie, Structure and Identification of
Management Information of TCP/IP-based Internets, Request For
Comments (RFC) 1155, STD 16, Performance Systems International,
Hughes LAN Systems, May 1990.

MGT:2.
K. McCloghrie and M. Rose (Editors), Management Information Base of
TCP/IP-Based Internets: MIB-II, Request For Comments (RFC) 1213,
STD 16, Hughes LAN Systems, Performance Systems International,
March 1991.

MGT:3.
J. Case, M. Fedor, M. Schoffstall, and J. Davin, Simple Network
Management Protocol, Request For Comments (RFC) 1157, STD 15, SNMP
Research, Performance Systems International, MIT Laboratory for
Computer Science, May 1990.

MGT:4.
M. Rose and K. McCloghrie (Editors), Towards Concise MIB
Definitions, Request For Comments (RFC) 1212, STD 16, Performance
Systems International, Hughes LAN Systems, March 1991.

MGT:5.
L. Steinberg, Techniques for Managing Asynchronously Generated
Alerts, Request for Comments (RFC) 1224, IBM, May 1991.

MGT:6.
F. Kastenholz, Definitions of Managed Objects for the Ethernet-like
Interface Types, Request for Comments (RFC) 1398, FTP Software
January 1993.

MGT:7.
R. Fox and K. McCloghrie, IEEE 802.4 Token Bus MIB, Request for
Comments (RFC) 1230, Hughes LAN Systems, Synoptics, Inc., May 1991.

MGT:8.
K. McCloghrie, R. Fox and E. Decker, IEEE 802.5 Token Ring MIB,
Request for Comments (RFC) 1231, Hughes LAN Systems, Synoptics,
Inc., cisco Systems, Inc., February 1993.

MGT:9.
J. Case and A. Rijsinghani, FDDI Management Information Base,
Request for Comments (RFC) 1512, SNMP Research, Digital Equipment
Corporation, September 1993.

MGT:10.
B. Stewart, Definitions of Managed Objects for RS-232-like Hardware
Devices, Request for Comments (RFC) 1317, Xyplex, Inc., April 1992.

MGT:11.
F. Kastenholz, Definitions of Managed Objects for the Link Control
Protocol of the Point-to-Point Protocol, Request For Comments (RFC)
1471, FTP Software, June 1992.

MGT:12.
F. Kastenholz, The Definitions of Managed Objects for the Security
Protocols of the Point-to-Point Protocol, Request For Comments
(RFC) 1472, FTP Software, June 1992.

MGT:13.
F. Kastenholz, The Definitions of Managed Objects for the IP
Network Control Protocol of the Point-to-Point Protocol, Request
For Comments (RFC) 1473, FTP Software, June 1992.

MGT:14.
F. Baker and R. Coltun, OSPF Version 2 Management Information Base,
Request For Comments (RFC) 1253, ACC, Computer Science Center,
August 1991.

MGT:15.
S. Willis and J. Burruss, Definitions of Managed Objects for the
Border Gateway Protocol (Version 3), Request For Comments (RFC)
1269, Wellfleet Communications Inc., October 1991.

MGT:16.
F. Baker, J. Watt, Definitions of Managed Objects for the DS1 and
E1 Interface Types, Request For Comments (RFC) 1406, Advanced
Computer Communications, Newbridge Networks Corporation, January

1993.

MGT:17.
T. Cox and K. Tesink, Definitions of Managed Objects for the DS3/E3
Interface Types, Request For Comments (RFC) 1407, Bell
Communications Research, January 1993.

MGT:18.
K. McCloghrie, Extensions to the Generic-Interface MIB, Request For
Comments (RFC) 1229, Hughes LAN Systems, August 1992.

MGT:19.
T. Cox and K. Tesink, Definitions of Managed Objects for the SIP
Interface Type, Request For Comments (RFC) 1304, Bell
Communications Research, February 1992.

MGT:20
F. Baker, IP Forwarding Table MIB, Request For Comments (RFC) 1354,
ACC, July 1992.

MGT:21.
G. Malkin and F. Baker, RIP Version 2 MIB Extension, Request For
Comments (RFC) 1389, Xylogics, Inc., Advanced Computer
Communications, January 1993.

MGT:22.
D. Throop, SNMP MIB Extension for the X.25 Packet Layer, Request
For Comments (RFC) 1382, Data General Corporation, November 1992.

MGT:23.
D. Throop and F. Baker, SNMP MIB Extension for X.25 LAPB, Request
For Comments (RFC) 1381, Data General Corporation, Advanced
Computer Communications, November 1992.

MGT:24.
D. Throop and F. Baker, SNMP MIB Extension for MultiProtocol
Interconnect over X.25, Request For Comments (RFC) 1461, Data
General Corporation, May 1993.

MGT:25.
M. Rose, SNMP over OSI, Request For Comments (RFC) 1418, Dover
Beach Consulting, Inc., March 1993.

MGT:26.
G. Minshall and M. Ritter, SNMP over AppleTalk, Request For
Comments (RFC) 1419, Novell, Inc., Apple Computer, Inc., March
1993.

MGT:27.
S. Bostock, SNMP over IPX, Request For Comments (RFC) 1420, Novell,
Inc., March 1993.

MGT:28.
M. Schoffstall, C. Davin, M. Fedor, J. Case, SNMP over Ethernet,
Request For Comments (RFC) 1089, Rensselaer Polytechnic Institute,
MIT Laboratory for Computer Science, NYSERNet, Inc., University of
Tennessee at Knoxville, February 1989.

MGT:29.
J. Case, FDDI Management Information Base, Request For Comments
(RFC) 1285, SNMP Research, Incorporated, January 1992.

OPER:1.
J. Nagle, Congestion Control in IP/TCP Internetworks, Request For
Comments (RFC) 896, FACC, January 1984.

OPER:2.
K.R. Sollins, TFTP Protocol (revision 2), Request For Comments
(RFC) 1350, MIT, July 1992.

ROUTE:1.
J. Moy, OSPF Version 2, Request For Comments (RFC) 1247, Proteon,
July 1991.

ROUTE:2.
R. Callon, Use of OSI IS-IS for Routing in TCP/IP and Dual
Environments, Request For Comments (RFC) 1195, DEC, December 1990.

ROUTE:3.
C. L. Hedrick, Routing Information Protocol, Request For Comments
(RFC) 1058, Rutgers University, June 1988.

ROUTE:4.
K. Lougheed and Y. Rekhter, A Border Gateway Protocol 3 (BGP-3),
Request For Comments (RFC) 1267, cisco, T.J. Watson Research
Center, IBM Corp., October 1991.

ROUTE:5.
Y. Rekhter and P. Gross Application of the Border Gateway Protocol
in the Internet, Request For Comments (RFC) 1268, T.J. Watson
Research Center, IBM Corp., ANS, October 1991.

ROUTE:6.
D. Mills, Exterior Gateway Protocol Formal Specification, Request
For Comments (RFC) 904, UDEL, April 1984.

ROUTE:7.
E. Rosen, Exterior Gateway Protocol (EGP), Request For Comments
(RFC) 827, BBN, October 1982.

ROUTE:8.
L. Seamonson and E. Rosen, "STUB" Exterior Gateway Protocol,
Request For Comments (RFC) 888, BBN, January 1984.

ROUTE:9.
D. Waitzman, C. Partridge, and S. Deering, Distance Vector
Multicast Routing Protocol, Request For Comments (RFC) 1075, BBN,
Stanford, November 1988.

ROUTE:10.
S. Deering, Multicast Routing in Internetworks and Extended LANs,
Proceedings of SIGCOMM '88, Association for Computing Machinery,
August 1988.

ROUTE:11.
P. Almquist, Type of Service in the Internet Protocol Suite,
Request for Comments (RFC) 1349, Consultant, July 1992.

ROUTE:12.
Y. Rekhter, Experience with the BGP Protocol, Request For Comments
(RFC) 1266, T.J. Watson Research Center, IBM Corp., October 1991.

ROUTE:13.
Y. Rekhter, BGP Protocol Analysis, Request For Comments (RFC) 1265,
T.J. Watson Research Center, IBM Corp., October 1991.

TRANS:1.
J. Postel, User Datagram Protocol, Request For Comments (RFC) 768,
STD 6, USC/Information Sciences Institute, August 1980.

TRANS:2.
J. Postel, Transmission Control Protocol, Request For Comments
(RFC) 793, STD 7, T.J. Watson Research Center, IBM Corp., September
1981.

APPENDIX A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS

Subject to restrictions given below, a host MAY be able to act as an
intermediate hop in a source route, forwarding a source-routed datagram
to the next specified hop.

However, in performing this router-like function, the host MUST obey all
the relevant rules for a router forwarding source-routed datagrams
[INTRO:2]. This includes the following specific provisions:

(A) TTL
The TTL field MUST be decremented and the datagram perhaps
discarded as specified for a router in [INTRO:2].

(B) ICMP Destination Unreachable
A host MUST be able to generate Destination Unreachable messages
with the following codes:
4 (Fragmentation Required but DF Set) when a source-routed datagram
cannot be fragmented to fit into the target network;
5 (Source Route Failed) when a source-routed datagram cannot be
forwarded, e.g., because of a routing problem or because the next
hop of a strict source route is not on a connected network.

(C) IP Source Address
A source-routed datagram being forwarded MAY (and normally will)
have a source address that is not one of the IP addresses of the
forwarding host.

(D) Record Route Option
A host that is forwarding a source-routed datagram containing a
Record Route option MUST update that option, if it has room.

(E) Timestamp Option
A host that is forwarding a source-routed datagram containing a
Timestamp Option MUST add the current timestamp to that option,
according to the rules for this option.

To define the rules restricting host forwarding of source-routed
datagrams, we use the term local source-routing if the next hop will be
through the same physical interface through which the datagram arrived;
otherwise, it is non-local source-routing.

A host is permitted to perform local source-routing without restriction.

A host that supports non-local source-routing MUST have a configurable
switch to disable forwarding, and this switch MUST default to disabled.

The host MUST satisfy all router requirements for configurable policy
filters [INTRO:2] restricting non-local forwarding.

If a host receives a datagram with an incomplete source route but does
not forward it for some reason, the host SHOULD return an ICMP
Destination Unreachable (code 5, Source Route Failed) message, unless
the datagram was itself an ICMP error message.

APPENDIX B. GLOSSARY

This Appendix defines specific terms used in this memo. It also defines
some general purpose terms that may be of interest. See also [INTRO:9]
for a more general set of definitions.

AS
Autonomous System A collection of routers under a single
administrative authority using a common Interior Gateway Protocol
for routing packets.

Connected Network
A network to which a router is interfaced is often known as the
local network or the subnetwork relative to that router. However,
these terms can cause confusion, and therefore we use the term
Connected Network in this memo.

Connected (Sub)Network
A Connected (Sub)Network is an IP subnetwork to which a router is
interfaced, or a connected network if the connected network is not
subnetted. See also Connected Network.

Datagram
The unit transmitted between a pair of internet modules. data,
called datagrams, from sources to destinations. The Internet
Protocol does not provide a reliable communication facility. There
are no acknowledgments either end-to-end or hop-by-hop. There is
no error no retransmissions. There is no flow control. See IP.

Default Route
A routing table entry which is used to direct any data addressed to
any network numbers not explicitly listed in the routing table.

EGP
Exterior Gateway Protocol A protocol which distributes routing
information to the gateways (routers) which connect autonomous
systems. See IGP.

EGP-2
Exterior Gateway Protocol version 2 This is an EGP routing protocol
developed to handle traffic between AS's in the Internet.

Forwarder
The logical entity within a router that is responsible for
switching packets among the router's interfaces. The Forwarder
also makes the decisions to queue a packet for local delivery, to

queue a packet for transmission out another interface, or both.

Forwarding
Forwarding is the process a router goes through for each packet
received by the router. The packet may be consumed by the router,
it may be output on one or more interfaces of the router, or both.
Forwarding includes the process of deciding what to do with the
packet as well as queuing it up for (possible) output or internal
consumption.

Fragment
An IP datagram which represents a portion of a higher layer's
packet which was too large to be sent in its entirety over the
output network.

IGP
Interior Gateway Protocol A protocol which distributes routing
information with an Autonomous System (AS). See EGP.

Interface IP Address
The IP Address and subnet mask that is assigned to a specific
interface of a router.

Internet Address
An assigned number which identifies a host in an internet. It has
two or three parts: network number, optional subnet number, and
host number.

IP
Internet Protocol The network layer protocol for the Internet. It
is a packet switching, datagram protocol defined in RFC791. IP
does not provide a reliable communications facility; that is, there
are no end-to-end of hop-by-hop acknowledgments.

IP Datagram
An IP Datagram is the unit of end-to-end transmission in the
Internet Protocol. An IP Datagram consists of an IP header
followed by all of higher-layer data (such as TCP, UDP, ICMP, and
the like). An IP Datagram is an IP header followed by a message.

An IP Datagram is a complete IP end-to-end transmission unit. An
IP Datagram is composed of one or more IP Fragments.

In this memo, the unqualified term Datagram should be understood to
refer to an IP Datagram.

IP Fragment
An IP Fragment is a component of an IP Datagram. An IP Fragment
consists of an IP header followed by all or part of the higher-
layer of the original IP Datagram.

One or more IP Fragments comprises a single IP Datagram.

In this memo, the unqualified term Fragment should be understood to
refer to an IP Fragment.

IP Packet
An IP Datagram or an IP Fragment.

In this memo, the unqualified term Packet should generally be
understood to refer to an IP Packet.

Logical [network] interface
We define a logical [network] interface to be a logical path,
distinguished by a unique IP address, to a connected network.

Martian Filtering
A packet which contains an invalid source or destination address is
considered to be martian and discarded.

MTU (Maximum Transmission Unit)
The size of the largest packet that can be transmitted or received
through a logical interface. This size includes the IP header but
does not include the size of any Link Layer headers or framing.

Multicast
A packet which is destined for multiple hosts. See broadcast.

Multicast Address
A special type of address which is recognized by multiple hosts.

A Multicast Address is sometimes known as a Functional Address or a
Group Address.

Originate
Packets can be transmitted by a router for one of two reasons: 1)
the packet was received and is being forwarded or 2) the router
itself created the packet for transmission (such as route
advertisements). Packets that the router creates for transmission
are said to originate at the router.

Packet
A packet is the unit of data passed across the interface between

the Internet Layer and the Link Layer. It includes an IP header
and data. A packet may be a complete IP datagram or a fragment of
an IP datagram.

Path
The sequence of routers and (sub-)networks which a packet traverses
from a particular router to a particular destination host. Note
that a path is uni-directional; it is not unusual to have different
paths in the two directions between a given host pair.

Physical Network
A Physical Network is a network (or a piece of an internet) which
is contiguous at the Link Layer. Its internal structure (if any)
is transparent to the Internet Layer.

In this memo, several media components that are connected together
via devices such as bridges or repeaters are considered to be a
single Physical Network since such devices are transparent to the
IP.

Physical Network Interface
This is a physical interface to a Connected Network and has a
(possibly unique) Link-Layer address. Multiple Physical Network
Interfaces on a single router may share the same Link-Layer
address, but the address must be unique for different routers on
the same Physical Network.

router
A special-purpose dedicated computer that attaches several networks
together. Routers switch packets between these networks in a
process known as forwarding. This process may be repeated several
times on a single packet by multiple routers until the packet can
be delivered to the final destination - switching the packet from
router to router to router... until the packet gets to its
destination.

RPF
Reverse Path Forwarding A method used to deduce the next hops for
broadcast and multicast packets.

serial line
A physical medium which we cannot define, but we recognize one when
we see one. See the U.S. Supreme Court's definitions on
pornography.

Silently Discard
This memo specifies several cases where a router is to Silently

Discard a received packet (or datagram). This means that the
router should discard the packet without further processing, and
that the router will not send any ICMP error message (see Section
[4.3.2]) as a result. However, for diagnosis of problems, the
router should provide the capability of logging the error (see
Section [1.3.3]), including the contents of the silently-discarded
packet, and should record the event in a statistics counter.

Silently Ignore
A router is said to Silently Ignore an error or condition if it
takes no action other than possibly generating an error report in
an error log or via some network management protocol, and
discarding, or ignoring, the source of the error. In particular,
the router does NOT generate an ICMP error message.

Specific-destination address
This is defined to be the destination address in the IP header
unless the header contains an IP broadcast or IP multicast address,
in which case the specific-destination is an IP address assigned to
the physical interface on which the packet arrived.

subnet
A portion of a network, which may be a physically independent
network, which shares a network address with other portions of the
network and is distinguished by a subnet number. A subnet is to a
network what a network is to an internet.

subnet number
A part of the internet address which designates a subnet. It is
ignored for the purposes internet routing, but is used for intranet
routing.

TOS
Type Of Service A field in the IP header which represents the
degree of reliability expected from the network layer by the
transport layer or application.

TTL
Time To Live A field in the IP header which represents how long a
packet is considered valid. It is a combination hop count and
timer value.

APPENDIX C. FUTURE DIRECTIONS

This appendix lists work that future revisions of this document may wish
to address.

In the preparation of Router Requirements, we stumbled across several
other architectural issues. Each of these is dealt with somewhat in the
document, but still ought to be classified as an open issue in the IP
architecture.

Most of the he topics presented here generally indicate areas where the
technology is still relatively new and it is not appropriate to develop
specific requirements since the community is still gaining operational
experience.

Other topics represent areas of ongoing research and indicate areas that
the prudent developer would closely monitor.

(1) SNMP Version 2

(2) Additional SNMP MIBs

(3) IDPR

(4) CIPSO

(5) IP Next Generation research

(6) More detailed requirements for next-hop selection

(7) More detailed requirements for leaking routes between routing
protocols

(8) Router system security

(9) Routing protocol security

(10) Internetwork Protocol layer security. There has been extensive
work refining the security of IP since the original work writing
this document. This security work should be included in here.

(11) Route caching

(12) Load Splitting

(13) Sending fragments along different paths

(14) Variable width subnet masks (i.e., not all subnets of a particular
net use the same subnet mask). Routers are required (MUST) support
them, but are not required to detect ambiguous configurations.

(15) Multiple logical (sub)nets on the same wire. Router Requirements
does not require support for this. We made some attempt to
identify pieces of the architecture (e.g. forwarding of directed
broadcasts and issuing of Redirects) where the wording of the rules
has to be done carefully to make the right thing happen, and tried
to clearly distinguish logical interfaces from physical interfaces.
However, we did not study this issue in detail, and we are not at
all confident that all of the rules in the document are correct in
the presence of multiple logical (sub)nets on the same wire.

(15) Congestion control and resource management. On the advice of the
IETF's experts (Mankin and Ramakrishnan) we deprecated (SHOULD NOT)
Source Quench and said little else concrete (Section 5.3.6).

(16) Developing a Link-Layer requirements document that would be common
for both routers and hosts.

(17) Developing a common PPP LQM algorithm.

(18) Investigate of other information (above and beyond section [3.2])
that passes between the layers, such as physical network MTU,
mappings of IP precedence to Link Layer priority values, etc.

(19) Should the Link Layer notify IP if address resolution failed (just
like it notifies IP when there is a Link Layer priority value
problem)?

(20) Should all routers be required to implement a DNS resolver?

(21) Should a human user be able to use a host name anywhere you can use
an IP address when configuring the router? Even in ping and
traceroute?

(22) Almquist's draft ruminations on the next hop and ruminations on
route leaking need to be reviewed, brought up to date, and
published.

(23) Investigation is needed to determine if a redirect message for
precedence is needed or not. If not, are the type-of-service
redirects acceptable?

(24) RIPv2 and RIP+CIDR and variable length subnet masks.

(25) BGP-4 CIDR is going to be important, and everyone is betting on
BGP-4. We can't avoid mentioning it. Probably need to describe the
differences between BGP-3 and BGP-4, and explore upgrade issues...

(26) Loose Source Route Mobile IP and some multicasting may require
this. Perhaps it should be elevated to a SHOULD (per Fred Baker's
Suggestion).

APPENDIX D. Multicast Routing Protocols

Multicasting is a relatively new technology within the Internet Protocol
family. It is not widely deployed or commonly in use yet. Its
importance, however, is expected to grow over the coming years.

This Appendix describes some of the technologies being investigated for
routing multicasts through the Internet.

A diligent implementor will keep abreast of developments in this area in
order to properly develop multicast facilities.

This Appendix does not specify any standards or requirements.

D.1 Introduction

Multicast routing protocols enable the forwarding of IP multicast
datagrams throughout a TCP/IP internet. Generally these algorithms
forward the datagram based on its source and destination addresses.
Additionally, the datagram may need to be forwarded to several
multicast group members, at times requiring the datagram to be
replicated and sent out multiple interfaces.

The state of multicast routing protocols is less developed than the
protocols available for the forwarding of IP unicasts. Two multicast
routing protocols have been documented for TCP/IP; both are currently
considered to be experimental. Both also use the IGMP protocol
(discussed in Section [4.4]) to monitor multicast group membership.

D.2 Distance Vector Multicast Routing Protocol - DVMRP

DVMRP, documented in [ROUTE:9], is based on Distance Vector or
Bellman-Ford technology. It routes multicast datagrams only, and does
so within a single Autonomous System. DVMRP is an implementation of
the Truncated Reverse Path Broadcasting algorithm described in
[ROUTE:10]. In addition, it specifies the tunneling of IP multicasts
through non-multicast-routing-capable IP domains.

D.3 Multicast Extensions to OSPF - MOSPF

MOSPF, currently under development, is a backward-compatible addition
to OSPF that allows the forwarding of both IP multicasts and unicasts
within an Autonomous System. MOSPF routers can be mixed with OSPF
routers within a routing domain, and they will interoperate in the
forwarding of unicasts. OSPF is a link-state or SPF-based protocol.
By adding link state advertisements that pinpoint group membership,
MOSPF routers can calculate the path of a multicast datagram as a
tree rooted at the datagram source. Those branches that do not
contain group members can then be discarded, eliminating unnecessary
datagram forwarding hops.

APPENDIX E Additional Next-Hop Selection Algorithms

Section [5.2.4.3] specifies an algorithm that routers ought to use when
selecting a next-hop for a packet.

This appendix provides historical perspective for the next-hop selection
problem. It also presents several additional pruning rules and next-hop
selection algorithms that might be found in the Internet.

This appendix presents material drawn from an earlier, unpublished, work
by Philip Almquist; Ruminations on the Next Hop.

This Appendix does not specify any standards or requirements.

E.1. Some Historical Perspective

It is useful to briefly review the history of the topic, beginning
with what is sometimes called the "classic model" of how a router
makes routing decisions. This model predates IP. In this model, a
router speaks some single routing protocol such as RIP. The protocol
completely determines the contents of the router's FIB. The route
lookup algorithm is trivial: the router looks in the FIB for a route
whose destination attribute exactly matches the network number
portion of the destination address in the packet. If one is found,
it is used; if none is found, the destination is unreachable.
Because the routing protocol keeps at most one route to each
destination, the problem of what to do when there are multiple routes
which match the same destination cannot arise.

Over the years, this classic model has been augmented in small ways.
With the advent of default routes, subnets, and host routes, it
became possible to have more than one routing table entry which in
some sense matched the destination. This was easily resolved by a
consensus that there was a hierarchy of routes: host routes should be
preferred over subnet routes, subnet routes over net routes, and net
routes over default routes.

With the advent of variable length subnet masks, the general approach
remained the same although its description became a little more
complicated. We now say that each route has a bit mask associated
with it. If a particular bit in a route's bit mask is set, the
corresponding bit in the route's destination attribute is
significant. A route cannot be used to route a packet unless each
significant bit in the route's destination attribute matches the
corresponding bit in the packet's destination address, and routes
with more bits set in their masks are preferred over routes which
have fewer bits set in their masks. This is simply a generalization

of the hierarchy of routes described above, and will be referred to
for the rest of this memo as choosing a route by preferring longest
match.

Another way the classic model has been augmented is through a small
amount of relaxation of the notion that a routing protocol has
complete control over the contents of the routing table. First,
static routes were introduced. For the first time, it was possible
to simultaneously have two routes (one dynamic and one static) to the
same destination. When this happened, a router had to have a policy
(in some cases configurable, and in other cases chosen by the author
of the router's software) which determined whether the static route
or the dynamic route was preferred. However, this policy was only
used as a tie-breaker when longest match didn't uniquely determine
which route to use. Thus, for example, a static default route would
never be preferred over a dynamic net route even if the policy
preferred static routes over dynamic routes.

The classic model had to be further augmented when inter-domain
routing protocols were invented. Traditional routing protocols came
to be called "interior gateway protocols" (IGPs), and at each
Internet site there was a strange new beast called an "exterior
gateway", a router which spoke EGP to several "BBN Core Gateways"
(the routers which made up the Internet backbone at the time) at the
same time as it spoke its IGP to the other routers at its site. Both
protocols wanted to determine the contents of the router's routing
table. Theoretically, this could result in a router having three
routes (EGP, IGP, and static) to the same destination. Because of
the Internet topology at the time, it was resolved with little debate
that routers would be best served by a policy of preferring IGP
routes over EGP routes. However, the sanctity of longest match
remained unquestioned: a default route learned from the IGP would
never be preferred over a net route from learned EGP.

Although the Internet topology, and consequently routing in the
Internet, have evolved considerably since then, this slightly
augmented version of the classic model has survived pretty much
intact to this day in the Internet (except that BGP has replaced
EGP). Conceptually (and often in implementation) each router has a
routing table and one or more routing protocol processes. Each of
these processes can add any entry that it pleases, and can delete or
modify any entry that it has created. When routing a packet, the
router picks the best route using longest match, augmented with a
policy mechanism to break ties. Although this augmented classic model
has served us well, it has a number of shortcomings:

o It ignores (although it could be augmented to consider) path

characteristics such as quality of service and MTU.

o It doesn't support routing protocols (such as OSPF and Integrated
IS-IS) that require route lookup algorithms different than pure
longest match.

o There has not been a firm consensus on what the tie-breaking
mechanism ought to be. Tie-breaking mechanisms have often been
found to be difficult if not impossible to configure in such a way
that the router will always pick what the network manger considers
to be the "correct" route.

E.2. Additional Pruning Rules

Section [5.2.4.3] defined several pruning rules to use to select
routes from the FIB. There are other rules that could also be used.

o OSPF Route Class
Routing protocols which have areas or make a distinction between
internal and external routes divide their routes into classes,
where classes are rank-ordered in terms of preference. A route is
always chosen from the most preferred class unless none is
available, in which case one is chosen from the second most
preferred class, and so on. In OSPF, the classes (in order from
most preferred to least preferred) are intra-area, inter-area,
type 1 external (external routes with internal metrics), and type
2 external. As an additional wrinkle, a router is configured to
know what addresses ought to be accessible via intra-area routes,
and will not use inter- area or external routes to reach these
destinations even when no intra-area route is available.

More precisely, we assume that each route has a class attribute,
called route.class, which is assigned by the routing protocol.
The set of candidate routes is examined to determine if it
contains any for which route.class = intra-area. If so, all
routes except those for which route.class = intra-area are
discarded. Otherwise, router checks whether the packet's
destination falls within the address ranges configured for the
local area. If so, the entire set of candidate routes is deleted.
Otherwise, the set of candidate routes is examined to determine if
it contains any for which route.class = inter-area. If so, all
routes except those for which route.class = inter-area are
discarded. Otherwise, the set of candidate routes is examined to
determine if it contains any for which route.class = type 1
external. If so, all routes except those for which route.class =
type 1 external are discarded.

o IS-IS Route Class
IS-IS route classes work identically to OSPF's. However, the set
of classes defined by Integrated IS-IS is different, such that
there isn't a one-to-one mapping between IS-IS route classes and
OSPF route classes. The route classes used by Integrated IS-IS are
(in order from most preferred to least preferred) intra-area,
inter-area, and external.

The Integrated IS-IS internal class is equivalent to the OSPF
internal class. Likewise, the Integrated IS-IS external class is
equivalent to OSPF's type 2 external class. However, Integrated
IS-IS does not make a distinction between inter-area routes and
external routes with internal metrics - both are considered to be
inter-area routes. Thus, OSPF prefers true inter-area routes over
external routes with internal metrics, whereas Integrated IS-IS
gives the two types of routes equal preference.

o IDPR Policy
A specific case of Policy. The IETF's Inter-domain Policy Routing
Working Group is devising a routing protocol called Inter-Domain
Policy Routing (IDPR) to support true policy-based routing in the
Internet. Packets with certain combinations of header attributes
(such as specific combinations of source and destination addresses
or special IDPR source route options) are required to use routes
provided by the IDPR protocol. Thus, unlike other Policy pruning
rules, IDPR Policy would have to be applied before any other
pruning rules except Basic Match.

Specifically, IDPR Policy examines the packet being forwarded to
ascertain if its attributes require that it be forwarded using
policy-based routes. If so, IDPR Policy deletes all routes not
provided by the IDPR protocol.

E.3 Some Route Lookup Algorithms

This section examines several route lookup algorithms that are in use
or have been proposed. Each is described by giving the sequence of
pruning rules it uses. The strengths and weaknesses of each
algorithm are presented

E.3.1 The Revised Classic Algorithm

The Revised Classic Algorithm is the form of the traditional
algorithm which was discussed in Section [E.1]. The steps of this
algorithm are:
1. Basic match
2. Longest match
3. Best metric
4. Policy

Some implementations omit the Policy step, since it is needed only
when routes may have metrics that are not comparable (because they
were learned from different routing domains).

The advantages of this algorithm are:

(1) It is widely implemented.

(2) Except for the Policy step (which an implementor can choose
to make arbitrarily complex) the algorithm is simple both to
understand and to implement.

Its disadvantages are:

(1) It does not handle IS-IS or OSPF route classes, and therefore
cannot be used for Integrated IS-IS or OSPF.

(2) It does not handle TOS or other path attributes.

(3) The policy mechanisms are not standardized in any way, and
are therefore are often implementation-specific. This causes
extra work for implementors (who must invent appropriate
policy mechanisms) and for users (who must learn how to use
the mechanisms. This lack of a standardized mechanism also
makes it difficult to build consistent configurations for
routers from different vendors. This presents a significant
practical deterrent to multi-vendor interoperability.

(4) The proprietary policy mechanisms currently provided by
vendors are often inadequate in complex parts of the
Internet.

(5) The algorithm has not been written down in any generally
available document or standard. It is, in effect, a part of
the Internet Folklore.

E.3.2 The Variant Router Requirements Algorithm

Some Router Requirements Working Group members have proposed a
slight variant of the algorithm described in the Section
[5.2.4.3]. In this variant, matching the type of service
requested is considered to be more important, rather than less
important, than matching as much of the destination address as
possible. For example, this algorithm would prefer a default
route which had the correct type of service over a network route
which had the default type of service, whereas the algorithm in
[5.2.4.3] would make the opposite choice.

The steps of the algorithm are:
1. Basic match
2. Weak TOS
3. Longest match
4. Best metric
5. Policy

Debate between the proponents of this algorithm and the regular
Router Requirements Algorithm suggests that each side can show
cases where its algorithm leads to simpler, more intuitive routing
than the other's algorithm does. In general, this variant has the
same set of advantages and disadvantages that the algorithm
specified in [5.2.4.3] does, except that pruning on Weak TOS
before pruning on Longest Match makes this algorithm less
compatible with OSPF and Integrated IS-IS than the standard Router
Requirements Algorithm.

E.3.3 The OSPF Algorithm

OSPF uses an algorithm which is virtually identical to the Router
Requirements Algorithm except for one crucial difference: OSPF
considers OSPF route classes.

The algorithm is:
1. Basic match
2. OSPF route class
3. Longest match
4. Weak TOS
5. Best metric
6. Policy

Type of service support is not always present. If it is not
present then, of course, the fourth step would be omitted

This algorithm has some advantages over the Revised Classic

Algorithm:

(1) It supports type of service routing.

(2) Its rules are written down, rather than merely being a part
of the Internet folklore.

(3) It (obviously) works with OSPF.

However, this algorithm also retains some of the disadvantages of
the Revised Classic Algorithm:

(1) Path properties other than type of service (e.g. MTU) are
ignored.

(2) As in the Revised Classic Algorithm, the details (or even the
existence) of the Policy step are left to the discretion of
the implementor.

The OSPF Algorithm also has a further disadvantage (which is not
shared by the Revised Classic Algorithm). OSPF internal (intra-
area or inter-area) routes are always considered to be superior to
routes learned from other routing protocols, even in cases where
the OSPF route matches fewer bits of the destination address.
This is a policy decision that is inappropriate in some networks.

Finally, it is worth noting that the OSPF Algorithm's TOS support
suffers from a deficiency in that routing protocols which support
TOS are implicitly preferred when forwarding packets which have
non-zero TOS values. This may not be appropriate in some cases.

E.3.4 The Integrated IS-IS Algorithm

Integrated IS-IS uses an algorithm which is similar to but not
quite identical to the OSPF Algorithm. Integrated IS-IS uses a
different set of route classes, and also differs slightly in its
handling of type of service. The algorithm is:
1. Basic Match
2. IS-IS Route Classes
3. Longest Match
4. Weak TOS
5. Best Metric
6. Policy

Although Integrated IS-IS uses Weak TOS, the protocol is only
capable of carrying routes for a small specific subset of the
possible values for the TOS field in the IP header. Packets

containing other values in the TOS field are routed using the
default TOS.

Type of service support is optional; if disabled, the fourth step
would be omitted. As in OSPF, the specification does not include
the Policy step.

This algorithm has some advantages over the Revised Classic
Algorithm:
(1) It supports type of service routing.
(2) Its rules are written down, rather than merely being a part
of the Internet folklore.
(3) It (obviously) works with Integrated IS-IS.

However, this algorithm also retains some of the disadvantages of
the Revised Classic Algorithm:
(1) Path properties other than type of service (e.g. MTU) are
ignored.
(2) As in the Revised Classic Algorithm, the details (or even the
existence) of the Policy step are left to the discretion of
the implementor.
(3) It doesn't work with OSPF because of the differences between
IS-IS route classes and OSPF route classes. Also, because
IS-IS supports only a subset of the possible TOS values, some
obvious implementations of the Integrated IS-IS algorithm
would not support OSPF's interpretation of TOS.

The Integrated IS-IS Algorithm also has a further disadvantage
(which is not shared by the Revised Classic Algorithm): IS-IS
internal (intra-area or inter-area) routes are always considered
to be superior to routes learned from other routing protocols,
even in cases where the IS-IS route matches fewer bits of the
destination address and doesn't provide the requested type of
service. This is a policy decision that may not be appropriate in
all cases.

Finally, it is worth noting that the Integrated IS-IS Algorithm's
TOS support suffers from the same deficiency noted for the OSPF
Algorithm.

Security Considerations

Although the focus of this document is interoperability rather than
security, there are obviously many sections of this document which have
some ramifications on network security.

Security means different things to different people. Security from a
router's point of view is anything that helps to keep its own networks
operational and in addition helps to keep the Internet as a whole
healthy. For the purposes of this document, the security services we
are concerned with are denial of service, integrity, and authentication
as it applies to the first two. Privacy as a security service is
important, but only peripherally a concern of a router - at least as of
the date of this document.

In several places in this document there are sections entitled ...
Security Considerations. These sections discuss specific considerations
that apply to the general topic under discussion.

Rarely does this document say do this and your router/network will be
secure. More likely, it says this is a good idea and if you do it, it
*may* improve the security of the Internet and your local system in
general.

Unfortunately, this is the state-of-the-art AT THIS TIME. Few if any of
the network protocols a router is concerned with have reasonable,
built-in security features. Industry and the protocol designers have
been and are continuing to struggle with these issues. There is
progress, but only small baby steps such as the peer-to-peer
authentication available in the BGP and OSPF routing protocols.

In particular, this document notes the current research into developing
and enhancing network security. Specific areas of research,
development, and engineering that are underway as of this writing
(December 1993) are in IP Security, SNMP Security, and common
authentication technologies.

Notwithstanding all of the above, there are things both vendors and
users can do to improve the security of their router. Vendors should
get a copy of Trusted Computer System Interpretation [INTRO:8]. Even if
a vendor decides not to submit their device for formal verification
under these guidelines, the publication provides Excellent guidance on
general security design and practices for computing devices.

Acknowledgments

O that we now had here
But one ten thousand of those men in England
That do no work to-day!

What's he that wishes so?
My cousin Westmoreland? No, my fair cousin:
If we are mark'd to die, we are enow
To do our country loss; and if to live,
The fewer men, the greater share of honour.
God's will! I pray thee, wish not one man more.
By Jove, I am not covetous for gold,
Nor care I who doth feed upon my cost;
It yearns me not if men my garments wear;
Such outward things dwell not in my desires:
But if it be a sin to covet honour,
I am the most offending soul alive.
No, faith, my coz, wish not a man from England:
God's peace! I would not lose so great an honour
As one man more, methinks, would share from me
For the best hope I have. O, do not wish one more!
Rather proclaim it, Westmoreland, through my host,
That he which hath no stomach to this fight,
Let him depart; his passport shall be made
And crowns for convoy put into his purse:
We would not die in that man's company
That fears his fellowship to die with us.
This day is called the feast of Crispian:
He that outlives this day, and comes safe home,
Will stand a tip-toe when the day is named,
And rouse him at the name of Crispian.
He that shall live this day, and see old age,
Will yearly on the vigil feast his neighbours,
And say 'To-morrow is Saint Crispian:'
Then will he strip his sleeve and show his scars.
And say 'These wounds I had on Crispin's day.'
Old men forget: yet all shall be forgot,
But he'll remember with advantages
What feats he did that day: then shall our names.
Familiar in his mouth as household words
Harry the king, Bedford and Exeter,
Warwick and Talbot, Salisbury and Gloucester,
Be in their flowing cups freshly remember'd.
This story shall the good man teach his son;
And Crispin Crispian shall ne'er go by,

From this day to the ending of the world,
But we in it shall be remember'd;
We few, we happy few, we band of brothers;
For he to-day that sheds his blood with me
Shall be my brother; be he ne'er so vile,
This day shall gentle his condition:
And gentlemen in England now a-bed
Shall think themselves accursed they were not here,
And hold their manhoods cheap whiles any speaks
That fought with us upon Saint Crispin's day.

This memo is a product of the IETF's Router Requirements Working Group.
A memo such as this one is of necessity the work of many more people
than could be listed here. A wide variety of vendors, network managers,
and other experts from the Internet community graciously contributed
their time and wisdom to improve the quality of this memo. The editor
wishes to extend sincere thanks to all of them.

The current editor also wishes to single out and extend his heartfelt
gratitude and appreciation to the original editor of this document;
Philip Almquist. Without Philip's work, both as the original editor and
as the Chair of the working group, this document would not have been
produced.

Philip Almquist, Jeffrey Burgan, Frank Kastenholz, and Cathy Wittbrodt
each wrote major chapters of this memo. Others who made major
contributions to the document included Bill Barns, Steve Deering, Kent
England, Jim Forster, Martin Gross, Jeff Honig, Steve Knowles, Yoni
Malachi, Michael Reilly, and Walt Wimer.

Additional text came from Art Berggreen, John Cavanaugh, Ross Callon,
John Lekashman, Brian Lloyd, Gary Malkin, Milo Medin, John Moy, Craig
Partridge, Stephanie Price, Yakov Rekhter, Steve Senum, Richard Smith,
Frank Solensky, Rich Woundy, and others who have been inadvertently
overlooked.

Some of the text in this memo has been (shamelessly) plagiarized from
earlier documents, most notably RFC-1122 by Bob Braden and the Host
Requirements Working Group, and RFC-1009 by Bob Braden and Jon Postel.
The work of these earlier authors is gratefully acknowledged.

Jim Forster was a co-chair of the Router Requirements Working Group
during its early meetings, and was instrumental in getting the group off
to a good start. Jon Postel, Bob Braden, and Walt Prue also contributed
to the success by providing a wealth of good advice prior to the group's
first meeting. Later on, Phill Gross, Vint Cerf, and Noel Chiappa all
provided valuable advice and support.

Mike St. Johns coordinated the Working Group's interactions with the
security community, and Frank Kastenholz coordinated the Working Group's
interactions with the network management area. Allison Mankin and K.K.
Ramakrishnan provided expertise on the issues of congestion control and
resource allocation.

Many more people than could possibly be listed or credited here
participated in the deliberations of the Router Requirements Working
Group, either through electronic mail or by attending meetings.
However, the efforts of Ross Callon and Vince Fuller in sorting out the
difficult issues of route choice and route leaking are especially
acknowledged.

The previous editor, Philip Almquist, wishes to extend his thanks and
appreciation to his former employers, Stanford University and BARRNet,
for allowing him to spend a large fraction (probably far more than they
ever imagined when he started on this) of his time working on this
project.

The current editor wishes to thank his employer, FTP Software, for
allowing him to spend the time necessary to finish this document.

Editor's Address

The address of the current editor of this document is
Frank J. Kastenholz
FTP Software
2 High Street
North Andover, MA, 01845-2620
USA

Phone: +1 508-685-4000

EMail: kasten@ftp.com