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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

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.


 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.

IESG [Page 1] RFC 1371 Choosing a "Common IGP" October 1992

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"
 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)

IESG [Page 2] RFC 1371 Choosing a "Common IGP" October 1992


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

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.

IESG [Page 3] RFC 1371 Choosing a "Common IGP" October 1992

 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
 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
 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

IESG [Page 4] RFC 1371 Choosing a "Common IGP" October 1992

 (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
 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].

IESG [Page 5] RFC 1371 Choosing a "Common IGP" October 1992

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

IESG [Page 6] RFC 1371 Choosing a "Common IGP" October 1992

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

IESG [Page 7] RFC 1371 Choosing a "Common IGP" October 1992

 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)", RFC 1267, cisco Systems, T.J. Watson Research Center, IBM
     Corp., October 1991.
 [3] Mills, D., "Exterior Gateway Protocol Formal Specification", STD
     18, RFC 904, UDEL, April 1984.
 [4] Moy, J., "OSPF Specification", RFC 1131 (Superceded by [7]),
     Proteon, October 1989.
 [5] Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and Dual
     Environments", RFC 1195, DEC, December 1990.
 [6] Hinden, R., "Criteria for Standardizing Internet Routing
     Protocols", RFC 1264, BBN, October 1991.
 [7] Moy, J., "OSPF Version 2", RFC 1247, Proteon, July 1991.
 [8] Baker, F., and R. Coltun, "OSPF Version 2 Management Information
     Base", RFC 1253, ACC, Computer Science Center, August 1991.
 [9] Moy, J., "Experience with the OSPF Protocol", RFC 1246, Proteon,
     July 1991.
[10] Moy, J., "OSPF Protocol Analysis", RFC 1245, Proteon, July 1991.
[11] Internet Architecture Board, "Applicability Statement for OSPF",
     RFC 1370, IAB, October 1992.

IESG [Page 8] RFC 1371 Choosing a "Common IGP" 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

IESG [Page 9]

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