rfc1371.txt

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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.








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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)



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   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.



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   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



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   (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