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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
+
+
+
+Rekhter                                                         [Page 1]
+
+RFC 1074             NSFNET Backbone SPF based IGP          October 1988
+
+
+   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
+
+
+
+Rekhter                                                         [Page 2]
+
+RFC 1074             NSFNET Backbone SPF based IGP          October 1988
+
+
+   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
+
+
+
+Rekhter                                                         [Page 3]
+
+RFC 1074             NSFNET Backbone SPF based IGP          October 1988
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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.
+
+
+
+Rekhter                                                         [Page 4]
+
+RFC 1074             NSFNET Backbone SPF based IGP          October 1988
+
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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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+Rekhter                                                         [Page 5]
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