RFC1092

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,

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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

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RFC 1092 IP EGP and Policy Based Routing February 1989

  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

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RFC 1092 IP EGP and Policy Based Routing February 1989

  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

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RFC 1092 IP EGP and Policy Based Routing February 1989

  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: [email protected]

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