Network Working Group D. Katz
Request for Comments: 1377 cisco
November 1992

The PPP OSI Network Layer Control Protocol (OSINLCP)

Status of this Memo

This RFCspecifies an IAB standards track protocol for the Internet
community, and requests discussion and suggestions for improvements.
Please refer to the current edition of the "IAB Official Protocol
Standards" for the standardization state and status of this protocol.
Distribution of this memo is unlimited.

Abstract

The Point-to-Point Protocol (PPP) [1] provides a standard method of
encapsulating Network Layer protocol information over point-to-point
links. PPP also defines an extensible Link Control Protocol, and
proposes a family of Network Control Protocols (NCPs) for
establishing and configuring different network-layer protocols.

This document defines the NCP for establishing and configuring OSI
Network Layer Protocols.

This memo is the prodUCt of the Point-to-Point Protocol Working Group
of the Internet Engineering Task Force (IETF). Comments on this memo
should be submitted to the ietf-ppp@ucdavis.edu mailing list.

Table of Contents

1. Introduction .......................................... 2
1.1 OSI Network Layer Protocols over PPP .................. 2
2. A PPP Network Control Protocol (NCP) for OSI .......... 5
2.1 Sending OSI NPDUs ..................................... 6
2.2 NPDU Alignment ........................................ 6
2.3 Network Layer Addressing Information .................. 6
3. OSINLCP Configuration Options ......................... 7
3.1 Align-NPDU ............................................ 7
REFERENCES ................................................... 9
ACKNOWLEDGEMENTS ............................................. 9
SECURITY CONSIDERATIONS ...................................... 10
CHAIR'S ADDRESS .............................................. 10
AUTHOR'S ADDRESS ............................................. 10

1. Introduction

PPP has three main components:

1. A method for encapsulating datagrams over serial links.

2. A Link Control Protocol (LCP) for establishing, configuring,
and testing the data-link connection.

3. A family of Network Control Protocols (NCPs) for establishing
and configuring different network-layer protocols.

In order to establish communications over a point-to-point link, each
end of the PPP link must first send LCP packets to configure and test
the data link. After the link has been established and optional
facilities have been negotiated as needed by the LCP, PPP must send
NCP packets to choose and configure one or more network-layer
protocols. Once each of the chosen network-layer protocols has been
configured, datagrams from each network-layer protocol can be sent
over the link.

The link will remain configured for communications until eXPlicit LCP
or NCP packets close the link down, or until some external event
occurs (an inactivity timer expires or network administrator
intervention).

1.1. OSI Network Layer Protocols over PPP

A number of protocols have been defined for the Network Layer of OSI,
including the Connectionless Network Layer Protocol (CLNP, ISO 8473)
[3], the End System to Intermediate System routing protocol (ES-IS,
ISO 9542) [4], the Intermediate System to Intermediate System routing
protocol (IS-IS, ISO 10589) [5], and the Inter-Domain Routeing
Protocol (IDRP, CD 10747) [6]. Generally, these protocols were
designed to run over non-reliable data link protocols such as PPP.

Network Layer Protocol Identifier (NLPID)

OSI Network Layer protocols can be discriminated according to the
first octet in each Network Protocol Data Unit (NPDU, that is,
packet), known as the Network Layer Protocol Identifier (NLPID),
which is defined in ISO/TR 9577 [7]. This allows the various
protocols to be run over a common data link without any
discriminator below the network layer.

Inactive Network Layer Protocol

ISO/TR 9577 reserves a NLPID value of zero to represent the
"Inactive Network Layer Protocol", as defined in ISO 8473. The
inactive network layer protocol MUST NOT be used over PPP. This
assures that whichever OSI network layer protocol is used will
have a non-zero NLPID value.

Connection-Oriented Network Protocol

The OSI Connection-Oriented Network Protocol (ISO 8208) [8],
effectively the Packet Layer of CCITT X.25, is intended to be run
over a reliable data link, such as IEEE 802.2 type II or LAPB.
Therefore, the unreliable data link service provided by PPP is not
appropriate for use with ISO 8208.

ConnectionLess Network Protocol (CLNP)

The ConnectionLess Network Protocol offers a simple non-reliable
datagram service very similar to IP, and is designed to run over a
non-reliable data link service, such as provided by PPP.

End-System to Intermediate-System Protocol (ES-IS)

ES Hellos and IS Hellos are retransmitted on a periodic timer-
driven basis (based on expiration of the "Configuration Timer").
The resulting ES and IS configuration information is invalidated
on a timer driven basis, based on expiration of the "Holding
Timer" for each piece of information. The value of a Holding
Timer is set by the source of the information, and transmitted in
the Holding Time field of the appropriate ES-IS packet. ISO 9542
recommends that the holding time field is set to approximately
twice the Configuration Timer parameter, such that even if every
other Hello packet is lost the configuration information will be
retained (implying that the Holding Timer is actually set to
slightly more than twice the Configuration Timer).

Generally, the recommendation in ISO 9542 is sufficient for PPP
links. For very unreliable links, it may be necessary to set the
Holding Timer to be slightly more than three times the
Configuration Timer to ensure that loss of configuration
information is an unusual event.

Redirect information is not transmitted on point-to-point links,
but may be transmitted on general topology subnetworks, which in
turn may make use of PPP. Redirect information is sent on a
event-driven basis (based on a CLNP packet being forwarded by a
router out the incoming interface), but redirect information is

invalidated on a timer-driven basis. Loss of a single redirect
may result in a subsequent data packet being sent to the same
incorrect router, which will re-issue the redirect. This operates
in the same manner as ICMP redirects for IP packets, and does not
pose any problem for operation over PPP links.

Intermediate-System to Intermediate-System Protocol (IS-IS)

IS-IS allows for broadcast links (typically LANs), point-to-point
links (such as PPP), and general topology links (such as X.25
networks) which are modelled as a collection of point-to-point
links.

There are four types of IS-IS packets: IS-IS Hello Packets, Link
State Packets (LSPs), Complete Sequence Number Packets (CSNPs),
and Partial Sequence Number Packets (PSNPs).

IS-IS Hello messages are transmitted periodically on point-to-
point links (based on expiration of the "ISISHello" timer).
Routers expect to receive IS-IS Hello packets periodically.
Specifically, the IS-IS Hello packet specifies a "Holding Time".
If no subsequent IS-IS Hello is received over the corresponding
link for the specified time period, then the neighboring router is
assumed to have been disconnected or to be down. It is highly
undesireable for links to "flap" up and down unnecessarily, which
implies that the holding time needs to be large enough that a link
is very unlikely to be declared down due to a failure to receive
an IS-IS Hello. This implies that running IS-IS over unreliable
data links requires the Holding time to be greater than "k" times
the ISISHello timer, where k is chosen such that the loss of k
consecutive IS-IS Hello's is rare. If the quality of the link is
poor, then the Holding Time will need to be increased or the
"ISISHello" time decreased.

LSPs are acknowledged by the IS-IS protocol (via use of partial
sequence number packets). A lost LSP will be recovered from with
no problem provided that PPP links are treated the same way as
other point-to-point links. On those rare occasions where a
partial sequence number packet is lost, this might result in the
retransmission of a link state packet over a single link, but will
not impact the correct operation of the routing algorithm.

CSNPs are sent upon link startup on a point-to-point link. This
does not need to be changed for PPP. If a CSNP fragment is lost
upon startup it is merely loss of an optimization -- LSPs that did
not need to be transmitted over the link will be transmitted. If
a periodic CSNP fragment is lost it merely means that detection of
low probability database corruption will take longer.

PSNPs function as ACKs. Loss of a PSNP may result in an
unnecessary retransmission of an LSP, but does not prevent correct
operation of the routing protocol.

Inter-Domain Routeing Protocol (IDRP)

IDRP expects to run over datagram links, but requires reliable
exchange of IDRP information. For this reason, IDRP contains
built-in reliability mechanisms which ensure that packets will be
received correctly.

2. A PPP Network Control Protocol (NCP) for OSI

The OSI Network Layer Control Protocol (OSINLCP) is responsible for
configuring, enabling, and disabling the OSI protocol modules on both
ends of the point-to-point link. OSINLCP uses the same packet
exchange machanism as the Link Control Protocol (LCP). OSINLCP
packets may not be exchanged until PPP has reached the Network-Layer
Protocol phase. OSINLCP packets received before this phase is
reached should be silently discarded.

The OSI Network Layer Control Protocol is exactly the same as the
Link Control Protocol [1] with the following exceptions:

Frame Modifications

The packet may utilize any modifications to the basic frame format
which have been negotiated during the Link Establishment phase.

Data Link Layer Protocol Field

Exactly one OSINLCP packet is encapsulated in the Information
field of a PPP Data Link Layer frame where the Protocol field
indicates type hex 8023 (OSI Network Layer Control Protocol).

Code field

Only Codes 1 through 7 (Configure-Request, Configure-Ack,
Configure-Nak, Configure-Reject, Terminate-Request, Terminate-Ack
and Code-Reject) are used. Other Codes should be treated as
unrecognized and should result in Code-Rejects.

Timeouts

OSINLCP packets may not be exchanged until PPP has reached the
Network-Layer Protocol phase. An implementation should be
prepared to wait for Authentication and Link Quality Determination
to finish before timing out waiting for a Configure-Ack or other

response. It is suggested that an implementation give up only
after user intervention or a configurable amount of time.

Configuration Option Types

OSINLCP has one Configuration Option, which is defined below.

2.1. Sending OSI NPDUs

Before any Network Protocol Data Units (NPDUs) may be communicated,
PPP must reach the Network-Layer Protocol phase, and the OSI Network
Layer Control Protocol must reach the Opened state.

Exactly one OSI NPDU is encapsulated in the Information field of a
PPP Data Link Layer frame where the Protocol field indicates type hex
0023 (OSI Network Layer).

The maximum length of an OSI NPDU transmitted over a PPP link is the
same as the maximum length of the Information field of a PPP data
link layer frame. Larger NPDUs must be segmented as necessary. If a
system wishes to avoid segmentation and reassembly, it should use
transport layer mechanisms to discourage others from sending large
PDUs.

2.2. NPDU Alignment

OSI protocols have peculiar alignment problems due to the fact that
they are often encapsulated in data link protocols with odd-length
headers, while PPP defaults to even-length headers. A router
switching an OSI packet may find that the beginning of the packet
falls on an inconvenient memory boundary when the hardware used to
transmit the packet to its next hop requires a particular alignment.
This situation can be addressed by the use of leading zero padding.

When sending, an implementation MAY insert one to three octets of
zero between the PPP header and the OSI NPDU. These zero octets
correspondingly reduce the maximum length of the NPDU that may be
transmitted.

On reception, any such leading zero octets (if present) MUST be
removed. Regardless of whether leading zero padding is used, an
implementation MUST also be able to receive a PPP packet with any
arbitrary alignment of the NPDU.

2.3. Network Layer Addressing Information

OSINLCP does not define a separate configuration option for the
exchange of OSI Network Layer address information. Instead, the ES-

IS protocol, ISO 9542, should be used. This protocol provides a
mechanism for determining the Network Layer address(es) of the
neighbor on the link, as well as determining if the neighbor is an
End System or an Intermediate System.

A draft addendum to ES-IS [9] is being defined in ISO to add support
for dynamic address assignment. This addendum has currently passed
the formal "Committee Draft" (CD) letter ballot.

3. OSINLCP Configuration Options

OSINLCP Configuration Options allow negotiatiation of desirable
Internet Protocol parameters. OSINLCP uses the same Configuration
Option format defined for LCP [1], with a separate set of Options.

The most up-to-date values of the OSINLCP Option Type field are
specified in the most recent "Assigned Numbers" RFC[2]. Current
values are assigned as follows:

1 Align-NPDU

3.1. Align-NPDU

Description

This Configuration Option provides a way for the receiver to
negotiate a particular alignment of the OSI NPDU. Empirical
evidence suggests that the greatest time deficit for re-alignment
exists at the receiver.

The alignment is accomplished through combination of PPP header
compression with leading zero padding (see above). It is
recommended that alignment be entirely through header compression
combinations whenever possible. For example, an alignment of 3
could be achieved by combining uncompressed PPP Address and
Control fields (2 octets) with a compressed PPP Protocol field (1
octet).

This option is negotiated separately in each direction. A
receiver which does not need alignment MUST NOT request the
option. A sender which desires alignment prior to sending SHOULD
Configure-Nak with an appropriate value.

Implementation Note: In a complex environment, there might be
several conflicting needs for alignment. It is recommended
that the receiver request alignment based on the needs of the
highest speed next hop link. Also, greater efficiency might be
oBTained by negotiating upstream the values requested by

downstream PPP links, since those packets will not need a
change in alignment on transit.

The alignment request is advisory, and failure to agree on an
alignment MUST NOT prevent the OSINLCP from reaching the Opened
state. By default, the alignment is done according to the needs
of the sender, and all receivers MUST be capable of accepting
packets with any alignment.

Vernacular: If you don't like this option, you can refuse to
negotiate it, and you can send whatever alignment you want.
However, if you accept the peer's alignment option, then you
MUST transmit packets with the agreed alignment.

A summary of the Align-NPDU Configuration Option format is shown
below. The fields are transmitted from left to right.

0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Length Alignment
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

1

Length

3

Alignment

This field specifies the offset of the beginning of the OSI NPDU
relative to the beginning of the PPP packet header (not including
any leading Flag Sequences).

A value of 1 through 4 requires an offset of that specific length,
modulo 4. For example, a value of 1 would require no padding when
the PPP Address, Control, and Protocol fields are compressed. One
octet of leading zero padding would be necessary when the PPP
header is full sized.

A value of 255 requests an offset of an odd length (1 or 3). A
value of 254 requests an offset of an even length (2 or 4). If
the sender is not capable of dynamically varying the amount of
padding, it MUST NAK with one of the two specific values.

References

[1] Simpson, W., "The Point-to-Point Protocol (PPP)", RFC1331,
Daydreamer, May 1992.

[2] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC1340,
USC/Information Sciences Institute, July 1992.

[3] ISO, "Information processing systems -- Data communications --
Protocol for providing the connectionless-mode network
service", ISO 8473, 1988.

[4] ISO, "Information processing systems -- Telecommunications and
information exchange between systems -- End system to
Intermediate system Routeing exchange protocol for use in
conjunction with the protocol for providing the connectionless-
mode network service (ISO 8473)", ISO 9542, 1988.

[5] ISO, "Information processing systems -- Telecommunications and
information exchange between systems -- Intermediate system to
Intermediate system Intra-Domain routeing exchange protocol for
use in conjunction with the protocol for providing the
connectionless-mode network service (ISO 8473)", ISO 10589,
1990.

[6] ISO, "Protocol for Exchange of Inter-domain Routeing
Information among Intermediate Systems to Support Forwarding of
ISO 8473 PDUs", ISO CD 10747, 1991.

[7] ISO, "Information technology -- Telecommunications and
information exchange between systems -- Protocol identification
in the network layer", ISO/IEC TR9577:1990.

[8] ISO, "Information processing systems -- Data communications --
X.25 packet level protocol for Data terminal equipment", ISO
8208, 1984.

[9] Taylor, E., "Addendum to ISO 9542 (PDAM 1 - Dynamic Discovery
of OSI NSAP Addresses by End Systems)", SC6/N7248.

Acknowledgments

Some of the text in this document is taken from previous documents
produced by the Point-to-Point Protocol Working Group of the Internet
Engineering Task Force (IETF).

Special thanks to Ross Callon (DEC), and Cyndi Jung (3Com), for
contributions of text and design suggestions based on implementation

experience.

Thanks also to Bill Simpson for his editing and formatting efforts,
both for this document and for PPP in general.

Security Considerations

Security issues are not discussed in this memo.

Chair's Address

The working group can be contacted via the current chair:

Brian Lloyd
Lloyd & Associates
3420 Sudbury Road
Cameron Park, California 95682

Phone: (916) 676-1147
EMail: brian@lloyd.com

Author's Address

Questions about this memo can also be directed to:

Dave Katz
cisco Systems, Inc.
1525 O'Brien Dr.
Menlo Park, CA 94025

Phone: (415) 688-8284
EMail: dkatz@cisco.com