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Network Working Group J. Rekhter Request for Comments 1074 T.J. Watson Research Center

                                           IBM Corporation
                                           October 1988

      The NSFNET Backbone SPF based Interior Gateway Protocol

Status of this Memo

 This memo is an implementation description of the standard ANSI IS-IS
 and ISO ES-IS routing protocols within the NSFNET backbone network.
 Distribution of this memo is unlimited.

Acknowledgements

 I would like to express my thanks to Hans-Werner Braun (MERIT) for
 his contribution to this document.

1. Overview

 This document provides an overview of the NSFNET Backbone routing
 with specific emphasis on the intra-backbone routing.

 By the end of 1987, the American National Standardization Institute
 (ANSI) forwarded a specification for an Intermediate System to
 Intermediate System routing protocol to the International
 Standardization Organizations (ISO) for the adaptation as an
 international standard.  This ANSI IS-IS protocol is used as the
 interior gateway protocol (IGP) of the NSFNET backbone.  Documented
 here is an implementation description which also includes further
 definitions that were necessary for the integration into an Internet
 Protocol (IP) environment.  Therefore, it should be viewed as a
 continuation of the specifications of the ANSI IS-IS protocol [1] and
 the ISO standard End System to Intermediate System (ES-IS) protocol
 [2].  While the ANSI IS-IS protocol suffices as an IGP, additional
 methods are used to orchestrate routing between the backbone and the
 attached mid-level networks; most notably the Exterior Gateway
 Protocol (EGP).  Further information about the overall NSFNET routing
 as well as some future aspects can be found in [3], [4], [5] and [6].

2. A brief overview of the NSFNET backbone

 The NSFNET backbone is a wide area network which currently connects
 thirteen sites within the continental United States.  All connections
 are permanent point-to-point links at T1 speed (1.544Mbps).  These T1
 links may contain multiple logical links at sub-T1 and up to the full
 T1 speed.  The result is a hybrid circuit/packet switching network
 able to contain a connectivity-richer logical topology than the

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 underlying physical topology would allow by itself.  Each site has a
 Nodal Switching Subsystem (NSS) which is responsible for packet
 switching.  Each NSS is a RISC technology based multiprocessor system
 using IBM RT/PC processors which operate a modified version of a
 4.3BSD kernel.  For the purpose of routing, each NSS is considered as
 a single entity which has connections to both other NSS (via the
 logical network infrastructure) and to regional networks (via local
 area network attachments; typically an Ethernet).

 The routing protocol which is used for the inter-NSS routing within
 the NSFNET backbone is an adaptation of the ANSI IS-IS routing
 protocol [1].  The routing protocol which is used between the
 backbone and the attached mid-level networks is the Exterior Gateway
 Protocol (EGP) [3].  The information exchange between the backbone
 and its connected EGP peers is subject to policy based routing
 restrictions which are maintained in the Policy Based Routing
 Database [4,5].

3. An overview of the ANSI IS-IS routing document

 The ANSI IS-IS routing protocol specifies a two level hierarchical
 routing where Level 1 routing deals with routing within an area,
 while Level 2 routing deals with routing between different areas.

 This routing protocol belongs to a class of so called "Link State"
 protocols where each node maintains a complete topology of the whole
 network.  The route computation is based on a modified version of
 Dijkstra's Shortest Path First (SPF) algorithm.

 Both Level 1 and Level 2 routing use two types of Protocol Data Units
 (PDU):

      The Level 1 Router Link PDU lists IS neighbors.  The Level 1 End
      System PDU lists ES neighbors.

      The Level 2 Router Link PDU lists neighbor Level 2 routes.  The
      Level 2 End System PDU lists address prefixes for systems in
      other Routing Domains.

 The ANSI IS-IS document separates subnetwork independent functions
 from the subnetwork dependent functions.  Subnetwork independent
 functions include dissemination of Router Link and End System Link
 PDU's and the Routing Algorithm.  The subnetwork dependent functions
 cover different types of subnets such as X.25, permanent point-to-
 point links and LANs.

 The IS-IS Protocol is designed to interoperate with the End System to
 Intermediate System (ES-IS) routing exchange protocol [2].  The ES-IS

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 protocol is used to determine connectivity and network layer
 addresses.  This information is used to construct the Router Link
 PDUs.

4. How the ANSI IS-IS protocol is adapted for the NSFNET backbone

  routing

 The NSFNET backbone implements a subset of the ANSI IS-IS protocol.
 With respect to subnetwork independent functions, it only supports
 Level 2 routing.  With respect to subnetwork dependent functions, it
 only supports general topology subnetworks with permanent point-to-
 point links.  Since the ANSI IS-IS protocol is designed for ISO
 Network Service Access Point (NSAP) addresses, there is a need to
 encapsulate IP addresses into NSAP addresses.

 For this, the Initial Domain Part (IDP) is unused.  The Domain
 Specific Part (DSP) includes nine bytes which are partitioned as
 follows:

      2 bytes - administrative domain

      2 bytes - empty

      4 bytes - IP address

      1 byte  - empty

 In the ANSI IS-IS protocol, each router has its own identifier (ID)
 which is 6 bytes long.  For the NSFNET implementation, the first 2
 bytes of the ID are empty and the last four bytes include the IP
 address of a particular router.

 The NSFNET backbone PDUs (both IS-IS and IS-ES) are transmitted as a
 protocol on top of IP, with "85" being the assigned protocol number
 for this purpose.  The IS-IS PDUs are distinguished from the IS-ES
 PDUs by the Protocol Discriminator Field within the PDUs.  The IP
 fragmentation/reassembly mechanism provides support for transmission
 of up to 64 kilobytes in a single IP packet.  Within the backbone, it
 is highly unlikely that the size of IS-IS PDUs will exceed this
 limit.  Therefore, no IS-IS fragmentation/reassembly is implemented
 for this environment.  This is different from the ISO framework where
 the ISIS is located directly on top of the Data Link Layer.

 For the purpose of the NSFNET Backbone routing, each Autonomous
 System (AS) is treated as a separate Administrative Domain (AD).  The
 list of administrative domains (as obtained via EGP and filtered
 through the Policy Based Routing Database) which are connected
 directly to a particular NSS is distributed in the set of the

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 partitionAreaAddresses part of the Level 2 Router Links PDU.  Each
 area address is 5 bytes long and consists of 3 empty bytes (IDP)
 followed by 2 bytes of the Administrative Domain.

 The reachability information obtained from regional networks via EGP
 is distributed within the backbone by End System PDUs.  In order to
 support multi-domain topologies, the ANSI IS-IS protocol allows for a
 set of Address Prefixes to be entered by the System Management at the
 boundary IS.  In the NSFNET Backbone, these Address Prefixes are
 obtained via the Exterior Gateway Protocol.  For each network listed
 in EGP NR packets which is received from an EGP peer, the network and
 administrative domain number of the EGP peer are encapsulated into
 NSAP addresses (as described above).  A complete NSAP address is used
 as an address prefix in the reachable address prefix neighbor part of
 the End System PDU.  The cost field in the reachable address prefix
 neighbor part of the End System PDU is derived from the Policy Based
 Routing Database maintained in each NSS.

 At each NSS, the reachability information obtained from other nodes
 (via their End System PDU's) is passed on to the mid-level network
 EGP peers, following the appropriate processing and filtering
 according to the Policy Based Routing Database.

 The Network Entity Title (NET) (which is used in the IS-ES protocol)
 is eleven bytes long and is constructed by first encapsulating an IP
 address into a NSAP address, then taking the first 11 bytes of this
 address as a NET.

5. Current timer parameters

 The following timer parameters are currently implemented:

      Hello Interval (IS-ES Hello):  10 seconds

      Hold Time (ES-IS protocol):    40 seconds

      Other timer parameters for the IS-IS protocol are taken from  the
      section 6.3.7 of [1].

6. References

   [1]  "Intermediate System to Intermediate System Intra-Domain
        Routing Exchange Protocol", ANSI X3S3.3/87-150R, 1987-10-29.

   [2]  "End System to Intermediate System Routing Exchange Protocol
        for use in conjunction with the Protocol for providing the
        Connectionless-Mode Network Service (ISO8473)", ISO
        JTC1/SC6/N4802R, 1988-03-26.

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   [3]  Mills, D., "Exterior Gateway Formal Specification", RFC 904,
        University of Delaware, April 1984.

   [4]  Rekhter, J., "EGP and Policy Based Routing in the New NSFNET
        Backbone", IBM, March 1988.

   [5]  Braun, H-W., "The NSFNET Routing Architecture", Merit Computer
        Network, University of Michigan, April 1988.

   [6]  Braun, H-W., "NSFNET Inter Autonomous System Routing", Merit
        Computer Network, University of Michigan, September 1988.

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