Network Working Group P. Gross, Editor
Request for Comments: 1371 IETF/IESG Chair
October 1992

Choosing a "Common IGP" for the IP Internet
(The IESG's Recommendation to the IAB)

Status of this Memo

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

Special Note

This document was originally prepared as an Internet Engineering
Steering Group (IESG) recommendation to the Internet Architecture
Board (IAB) in mid-summer 1991, reaching the current version by the
date shown above. Although the document is now somewhat dated (e.g.,
CIDR and RIP II are not mentioned), the IESG felt it was important to
publish this along with the recent OSPF Applicability Statement [11]
to help establish context and motivation.

Abstract

This memo presents motivation, rationale and other surrounding
background information leading to the IESG's recommendation to the
IAB for a single "common IGP" for the IP portions of the Internet.

In this memo, the term "common IGP" is defined, the need for a common
IGP is eXPlained, the relation of this issue to other ongoing
Internet Engineering Task Force (IETF) routing protocol development
is provided, and the relation of this issue to the goal for multi-
protocol integration in the Internet is explored.

Finally, a specific IGP is recommended as the "common IGP" for IP
portions of the Internet -- the Open Shortest Path First (OSPF)
routing protocol.

The goal of this recommendation is for all vendors of Internet IP
routers to make OSPF available as one of the IGP's provided with
their routers.

Table of Contents

1. Background .................................................... 2
2. Multiple Internet Standard Routing Protocols Possible ......... 3
3. A Common IGP .................................................. 3
4. Impact of Multi-protocol Topology and Integrated IP/CLNP Routing 3
5. Commitment to Both IP and CLNP ................................ 5
6. Some History .................................................. 5
7. IESG Recommendations .......................................... 6
7.1 Regarding the Common IGP for the IP Internet ................. 6
7.2 Regarding Integrated IP/CLNP Routing ......................... 7
7.3 Limits of the Common IGP Recommendation ...................... 7
8. References .................................................... 8
9. Security Considerations ....................................... 9
10. Author's Address ............................................. 9

1. Background

There is a pressing need for a high functionality non-proprietary
"common" Interior Gateway Protocol (IGP) for the TCP/IP protocol
family. An IGP is the routing protocol used within a single
administrative domain (commonly referred to as an "Autonomous System"
(AS).

By "common", we simply mean a protocol that is ubiquitously available
from all router vendors (as in "in common"). Users and network
operators have expressed a strong need for routers from different
vendors to have the capablity to interoperate within an AS through
use of a common IGP.

Note: Routing between AS's is handled by a different type of routing
protocol, called an "Exterior Gateway Protocol" ("an EGP", of which
the Border Gateway Protocol [2] and "The Exterior Gateway Protocol"
[3] are examples.) The issues of routing between AS's using "an" EGP
is not considered in this memo.

There are two IGPs in the Internet standards track capable of routing
IP traffic -- Open Shortest Path First (OSPF) [4] and Integrated IS-
IS [5] (based on the OSI IS-IS). These two protocols are both modern
"link state" routing protocols, based on the Dijkstra algorithm.
There has been substantial interaction and cooperation among the
engineers involved in each effort, and the protocols share some
similar features.

However, there are a number of technical design differences. Most
noteably, OSPF has been designed solely for support of the Internet
Protocol (IP), while Integrated IS-IS has been designed to support
both IP and the OSI Connectionless Network Layer Protocol (CLNP)

simultaneously.

2. Multiple Internet Standard Routing Protocols Possible

The Internet architecture makes a distinction between "Interior
Gateway Protocols (IGPs)" and "Exterior Gateway Protocols (EGPs)".
IGPs are routing protocols used within an Autonomous System (AS), and
EGPs are routing protocols used between different AS's.

Therefore, the Internet architecture supports the use and
standardization of multiple IGP routing protocols. For example, it
is perfectly reasonable for one standard routing protocol to be used
within one AS; while a second standard routing protocol is used
within a second AS; at the same time that a non-standard proprietary
routing protocol is used within a third AS.

The primary purpose for making standards is to allow
interoperability. Setting a protocol standard in the Internet says,
in effect, "if you wish to use this protocol, you should do it as
specified in the standard so that you can interoperate with others
who also wish to use this protocol." It is important to understand
that simply specifying a standard does not, by itself, designate a
requirement to use the standard. It is merely meant to allow
interoperability among those who choose to follow the standard.

Therefore, it is reasonable for both OSPF and Integrated IS-IS to be
progressed through the Internet Standards process as appropriate
(based on the criteria specified in [6]). In addition, it is
possible that other IGPs may be developed and standardized in the
future.

3. A Common IGP

Although the Internet architecture allows for multiple standard IGP
routing protocols, interoperability of router prodUCts from different
vendors within a single AS would be greatly facilitated if a single
"common" IGP were available from all router vendors. Designating a
single common IGP would have the goal of enabling multi-vendor router
interoperation with a modern high functionality routing protocol.

However, designating a common IGP does not mandate the use of that
IGP, nor would it be meant to discourage the use of other IGPs in
situations where there may be sound technical reasons to do so.

4. Impact of Multi-protocol Topology and Integrated IP/CLNP Routing

There are topology considerations which will affect the designation
of a "common" Internet IGP.

The Internet requires support for a wide variety of protocol suites.
If we consider only IP and OSI CLNP, then the Internet is expected to
contain:

1. Pure IP AS's (in which IP is used but OSI CLNP is not used);

2. Pure CLNP AS's (in which CLNP is used but IP is not used);

3. Dual IP/CLNP ASs, with a common topology (i.e., all links and
routers in the AS support IP and CLNP, and a single common
topology is used for both protocol suites);

4. Dual, overlapping IP/CLNP ASs with differing topologies (i.e.,
some links are dual, while some are IP-only and some are
CLNP-only, resulting in different topologies for IP routing and
CLNP routing).

For (1), (i.e., a pure IP environment) any IGP capable of routing IP
traffic could be used (e.g., OSPF or Integrated IS-IS).

For (2), (i.e., a pure CLNP environment) any IGP capable of routing
CLNP traffic could be used (e.g., OSI IS-IS or Integrated IS-IS).

For (3), (i.e., routing environments in which both IP and CLNP are
present in a common topology) there are two possibilities for managing
routing:

1. Separate routing protocols could be used for each supported
protocol suite. For example, OSPF may be used for calculating
routes for IP traffic and OSI IS-IS may be used for calculating
routes for OSI traffic. Or Integrated IS-IS could be used for
calculating routes for IP traffic and OSI IS-IS could be used
for calculating routes for CLNP traffic.

This approach of using separate routing protocols and management
for each supported protocol family has come to be known as "Ships
in the Night" because the two routing protocols share the
hardware/software resources of the router without ever actually
interacting on a protocol level.

2. "Integrated routing" could be used, in which a single routing
protocol is used for both IP and CLNP. At this time, Integrated
IS-IS is the only choice for "integrated routing".

For (4), (i.e., routing environments in which both IP and CLNP are
present but in an overlapping different topology) separate routing
protocols are required for the IP and CLNP environments (i.e., "Ships
in the Night"). This is equivalent to two separates cases of (1) and

(2), but it is pointed out here as a separate case for completeness.

5. Commitment to both IP and CLNP

The IAB/IETF are committed to a timely introduction of OSI into the
Internet. In recognition of this commitment, the IETF has an entire
area devoted to OSI integration.

However, while this introduction is taking place, it is essential
that existing services based on IP be continued. Furthermore, IESG
also feels that even after more widespread introduction of CLNP, IP
and CLNP will continue to coexist in the Internet for quite some
time. This view is consistent with the IAB goal of a multi-protocol
Internet.

Therefore, the IESG has a strong commitment to the continued support
for IP throughout the Internet. Maintenance of this IP support
requires selection of a common IGP suitable for support of IP, and
requires that this selection be based on operational experience.

6. Some History

In February 1990, the IESG recommended that the question of
designating a "common" IGP be postponed until more information was
available from each protocol. More than a year has now passed since
the IESG's recommendation. There have been significant advancements
in specification, implementation, and operational experience with
each protocol. It is now reasonable to re-open the consideration of
designating a "common IGP".

At the March 1991 meeting of the IETF, the IETF Routing Area Director
presented a set of criteria for the advancement of routing protocols
through the Internet standards process [6]. More information
regarding the IAB Internet Standards process can be found in [1].

Also, at the March 1991 meeting of the IETF, the OSPF Working Group
requested that OSPF be considered for advancement to Draft Internet
Standard. The OSPF WG submitted four documents to the IETF to
support its request:

o a revised protocol specification to update [4];

o an SNMP Management Information Base (MIB);

o two technical reports giving a technical analysis and operational
experience with OSPF. These reports follow the format recommended
in [6].

These four documents have now been published as [7, 8, 9, 10]
respectively.

In summary for OSPF:

o all features of OSPF have tested (although not all features have
been used in operation),

o OSPF has been shown to operate well in several operational
networks containing between 10 and 30 routers,

o interoperation among routers from multiple vendors has been
demonstrated at organized "bakeoffs".

In May 1991, the IAB approved the IETF/IESG recommendation to advance
OSPF to Draft Internet Standard.

Integrated IS-IS, as specified in [5], is currently a Proposed
Internet Standard. In July 1991, the status of Integrated IS-IS is
as follows:

o There are several separate implementations of integrated
IS-IS under development,

o Integrated IS-IS has worked well in several multi-area operational
networks, one containing between 20 and 30 routers,

o These recent operational results have not yet been fully
documented. Documentation, showing satisfaction of the criteria
given in [6] for advancing routing protocols, will be submitted
to the IESG when Integrated IS-IS is submitted for Draft Internet
Standard status.

7. IESG Recommendations

7.1 Regarding the Common IGP for the IP Internet

Based on the available operational experience and the pressing need
for a high functionality IGP for the IP protocol family, the IESG
recommends that OSPF be designated as the common IGP for the IP
portions of the Internet. To help ensure that this IGP is available
to all users, the IESG recommends that the IETF Router Requirements
Working Group specify OSPF as "MUST IMPLEMENT" in the document
"Requirements for Internet IP Routers".

7.2 Regarding Integrated Routing

As mentioned above, the IESG is commited to multiprotocol
environments, and expects usage of OSI CLNP to increase in the
Internet over time.

However, at this time, the IESG is not prepared to take a position
regarding the preference of either "Ships in the Night" or Integrated
routing for such mixed routing environments. At this time, the
"Ships in the Night" approach is most widely used in the Internet.
Integrated routing has the potential advantage of reducing resource
utilization. However, additional operational experience is needed
before any potential advantages can be fully evaluated.

Therefore, the IESG wishes to encourage implementation of Integrated
IS-IS so that a reasonable position can be determined based on
operational experience. All implementers of Integrated IS-IS are
encouraged to coordinate their activity with the IETF IS-IS Working
Group, which is actively collecting information on such experience.

7.3 Limits of the Recommendation

It is useful to recognize the limits of this recommendation. This
recommendation does not take a position on any of the following
issues:

1. What IGP (if any) users should run inside an AS. Users are free to
run any IGP they wish inside an AS.

2. What IGP is technically superior, or has greater operational
utility.

3. What IGP any vendor should recommend to its users for any specific
environment.

4. What IGP should be used within a CLNP-only environment.

Again, this recommendation is meant to designate one modern high
functionality IGP that should be implemented by all vendors of
routers for the IP portion of the Internet. This will enable routers
from vendors who follow this recommendation to interoperate within a
single IP Autonomous System.

It is not our intent to discourage the use of other routing protocols
in situations where there may be sound technical reasons to do so.
Therefore, developers of Internet routers are free to implement, and
network operators are free to use, other Internet standard routing
protocols, or proprietary non-Internet-standard routing protocols, as

they wish.

8. References

[1] Internet Activities Board, "The Internet Standards Process", RFC
1310, IAB, March 1992.

[2] Lougheed, K., and Y. Rekhter, "A Border Gateway Protocol 3 (BGP-
3)", RFC1267, cisco Systems, T.J. Watson Research Center, IBM
Corp., October 1991.

[3] Mills, D., "Exterior Gateway Protocol Formal Specification", STD
18, RFC904, UDEL, April 1984.

[4] Moy, J., "OSPF Specification", RFC1131 (Superceded by [7]),
Proteon, October 1989.

[5] Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and Dual
Environments", RFC1195, DEC, December 1990.

[6] Hinden, R., "Criteria for Standardizing Internet Routing
Protocols", RFC1264, BBN, October 1991.

[7] Moy, J., "OSPF Version 2", RFC1247, Proteon, July 1991.

[8] Baker, F., and R. Coltun, "OSPF Version 2 Management Information
Base", RFC1253, ACC, Computer Science Center, August 1991.

[9] Moy, J., "Experience with the OSPF Protocol", RFC1246, Proteon,
July 1991.

[10] Moy, J., "OSPF Protocol Analysis", RFC1245, Proteon, July 1991.

[11] Internet Architecture Board, "Applicability Statement for OSPF",
RFC1370, IAB, October 1992.

9. Security Considerations

Security issues are not discussed in this memo.

10. Author's Address

Phillip Gross, IESG Chair
Advanced Network & Services
100 Clearbrook Road
Elmsford, NY

Phone: 914-789-5300
EMail: pgross@ans.net