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+Network Working Group                                         J. Rekhter
+Request for Comments: 1092                  T. J. Watson Research Center
+                                                           February 1989
+
+
+        EGP and Policy Based Routing in the New NSFNET Backbone
+
+Status of this Memo
+
+   This memo discusses implementation decisions for routing issues in
+   the NSFNET, especially in the NSFNET Backbone.  Of special concern is
+   the restriction of routing information to advertize the best route as
+   established by a policy decision.  Distribution of this memo is
+   unlimited.
+
+Introduction
+
+   The NSFNET backbone routes packets between the Regionals Networks to
+   which it is connected, (i.e., the packets arriving at a backbone
+   entry node are routed to an exit node).  How they travel through the
+   network is determined by two components:
+
+     the NSFNET backbone routing protocol/algorithm, and
+
+     additional information about the externally connected networks.
+
+   This paper is concerned with how reachability information between the
+   external networks and the NSFNET backbone is exchanged so that
+   packets can be routed to the correct destination by using a
+   reasonable path.
+
+EGP as reachability protocol
+
+   The EGP (Exterior Gateway Protocol) routing method will be used to
+   exchange reachability information between the NSFNET backbone and the
+   regional networks.
+
+   There are several problems with using EGP as a reachability protocol
+   for routing in a meshed environment.  Some EGP components require
+   further definitions for the NSFNET backbone - regional network
+   interactions.  It should be noted that the use of EGP is only viewed
+   as an interim measure until better inter autonomous system protocols
+   are defined and widely deployed for gateways used by regional
+   networks.
+
+   The following is a list of some EGP problems and issues:
+
+      The EGP model assumes an engineered spanning tree topology,
+
+
+
+Rekhter                                                         [Page 1]
+
+RFC 1092            IP EGP and Policy Based Routing        February 1989
+
+
+      however, the NSFNET (due to the presence of backdoor routes) does
+      not fit into this model.  In the NSFNET the same network may be
+      advertized as reachable by more than one regional network.
+      Besides the fact that the overall NSFNET does not fit into a
+      spanning tree model there are serious concerns with the concept
+      of the "core" (central to the EGP) and its obvious deficiencies.
+
+      While EGP is going to isolate intra-Regional routing from the
+      intra-NSFNET-Backbone routing, it does not address the issue of
+      false information which may be supplied by regional networks.
+      EGP by itself does not protect a particular network from unwanted
+      and unsolicited representation by some regional network.  As an
+      example, if network N1 is reachable through regional network R1
+      as well as through regional network R2, EGP has no provisions to
+      specify one of these paths as a primary and one as a secondary,
+      since there is not generally accepted interpretation of EGP
+      metrics today.  Also, there is nothing in EGP which can prevent one
+      or more regional networks from advertizing other networks (in
+      particular, networks which belong to other regional networks) as
+      reachable with zero distance.  This could result in the creation
+      of a "black hole" or at least in suboptimal IP routing.
+
+      EGP by itself has no provisions to guarantee that routes through
+      the NSFNET Backbone will be preferred over routes through the
+      backdoor routers or vice versa.
+
+Policy Based Routing
+
+   Looking at the problems listed above the appearance of the new
+   factors like autonomy and mutual trust becomes obvious.  While trying
+   to achieve the routing functionality required for the new NSFNET
+   backbone we should realize that one of our primary concerns has to be
+   the accommodation of those new factors.
+
+   This means that some kind of a rudimentary Policy Based Routing
+   method becomes imperative.  We would like to emphasize, however, that
+   we are not talking about complete Policy Based Routing, but that we
+   are rather concerned about supporting a minimum subset of a policy
+   functionality to be an initial solution to the above mentioned
+   problems.  This requires support and cooperation between the
+   management of each of the networks connected to the NSFNET backbone.
+
+   We need to support the ability of a particular network N, which
+   belongs to one of the regional networks, to establish a bilateral
+   agreement with one or more regional networks of the type "network N
+   can be reached via one or more regional networks (RN1, RN2, ...
+   RNx)".  This allows each network to select one or more
+   representatives at the regional network level.  Once this agreement
+
+
+
+Rekhter                                                         [Page 2]
+
+RFC 1092            IP EGP and Policy Based Routing        February 1989
+
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+   is established the information will be available to:
+
+     The network which initiated the agreement.
+
+     The management of the regional network(s) with whom this
+     agreement has been established.
+
+     The NSFNET backbone Network Operation Center where it will be
+     entered into the Routing Policy Data Base which will be available
+     through the NSFNET information services.
+
+   Supporting multiple routes to the NSFNET core requires the guarantee
+   that for a certain network N, no regional network other than the
+   one(s) selected by N, will advertize N as reachable, which
+   necessitates that the NSFNET core will ignore unauthorized
+   advertisements for network N.
+
+EGP and Rudimentary Policy Based Routing
+
+   Each network which belongs to the NSFNET will select a specific
+   regional network as its primary representative to the NSFNET core by
+   bilateral agreement with the management of same regional network as
+   well as the NSFNET backbone management.  The same network can
+   furthermore select an arbitrary number of other regional networks as
+   their secondary, tertiary, etc., representative by establishing
+   bilateral agreements with the management of the corresponding
+   regional networks as well as the NSFNET backbone management.
+
+   Reachability information supplied by each regional network will be
+   distributed to all other NSS nodes of the NSFNET Backbone.  We would
+   like to emphasize that we are not going to flood EGP packets
+   internally within the backbone, but to rather use the learned
+   information for the interior gateway protocol, which uses the ANSI
+   IS-IS protocol.
+
+   The implementation allows for a defined regional network to advertize
+   a particular leaf network in the EGP NR packets with a distance of
+   zero.  Secondary representatives may advertize the same network with
+   distance one or higher.  If the path through the primary regional
+   representative is available all secondary paths will be ignored.  If
+   the path through the primary regional representative goes down (which
+   will be discovered via the EGP NR information), the next path with
+   the lowest available EGP metric will be used.
+
+   We will also be able to detect and report unsolicited
+   representations.  This will be done by examining (on a periodic
+   basis) all reachability information obtained via EGP.  The result
+   will be compared against the Routing Policy Data Base which will hold
+
+
+
+Rekhter                                                         [Page 3]
+
+RFC 1092            IP EGP and Policy Based Routing        February 1989
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+   information about all bilateral agreements between networks and their
+   regional representatives.  Any mismatch will cause an alarm to the
+   Network Operations Center.  For example, network N established a
+   bilateral agreement with the regional network R1 electing it as its
+   primary representative. The EGP NR record received from the regional
+   network R5 advertizes the network N as reachable with distance zero.
+   By comparing the Routing Policy Data Base entry for the network N
+   with the EGP NR record a mismatch will be detected and an alarm is
+   forwarded to the Network Operation Center.
+
+   Since the whole scheme is based on a combination of the network
+   number and the autonomous system number, to allow for further
+   verification, it is also important to insure the correctness of the
+   autonomous system numbers as advertized by the regionals networks to
+   the NSFNET core.
+
+   The autonomous system number validation for each regional network
+   will be performed at the NSS which connects the particular leaf
+   network to the NSFNET backbone.  All discrepancies wil be reported to
+   the Network Operations Center.
+
+   The NSFNET backbone will be considered as a separate Autonomous
+   System with its own autonomous system number.
+
+Backbone versus Backdoor Routes
+
+   There are instances where regional networks prefer paths through some
+   backdoor route over paths through the NSFNET backbone.  Therefore,
+   the reachability information advertized by the NSFNET core to the
+   regional networks (via EGP NR records) will always use a fixed metric
+   of 128 for all routes.  This may aid to encourage traffic to flow
+   through backdoors, if desired and available.
+
+   The regional networks can use a variety of techniques to determine
+   how they route traffic for any particular network at their own
+   option.
+
+What do we expect from the Regional Networks
+
+   Each regional network should get its own Autonomous System number.
+
+   The connection between regional networks to NSFNET backbone will be
+   done via EGP.  It is the responsibility of the regional backbone to
+   provide an EGP functionality via the attachment to the E-PSP
+   dedicated to the regional network.
+
+   The EGP functionality may require a translation of network numbers in
+   and out of the regional network.  In any case, the NSFNET backbone
+
+
+
+Rekhter                                                         [Page 4]
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+RFC 1092            IP EGP and Policy Based Routing        February 1989
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+   expects individual network numbers of the leaf networks of the
+   regional network, as long as they should be advertised, and will
+   announce individual networks known to the NSFNET core to the regional
+   network.
+
+   The EGP support should includes the ability to configure EGP metrics
+   from some statically definable configuration table.  If the EGP
+   metrics cannot be defined or if they are not fixed the metric
+   determination will be done by the NSFNET backbone routers, as taken
+   from their databases, themselves.  In that case, it is the
+   responsibility of the regional network to provide the NSFNET backbone
+   management with the metric data to allow for proper use of metrics.
+
+   We also expect each regional network to handle all bilateral
+   agreements with its leaf networks regarding Policy Based Routing and
+   supply a copy of those agreements to the NSFNET backbone management.
+
+Acknowledgements
+
+   I would like to express my thanks to Barry Appelman (T.J. Watson
+   Research Center, IBM Corp.) and Hans-Werner Braun (Merit) for their
+   contributions to this document.
+
+Author's Address
+
+   Jacob Rekhter
+   T.J. Watson Research Center
+   IBM Corporation
+   P.O. Box 218
+   Yorktown Heights, NY 10598
+
+   Phone: (914) 945-3896
+
+   Email: YAKOV@IBM.COM
+
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+Rekhter                                                         [Page 5]
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