Movatterモバイル変換

Network Working Group                                         J. RekhterRequest for Comments: 1092                  T. J. Watson Research Center                                                           February 1989EGP and Policy Based Routing in the New NSFNET BackboneStatus 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 agreementRekhter                                                         [Page 2]

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 holdRekhter                                                         [Page 3]

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 backboneRekhter                                                         [Page 4]

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: YAKOV@IBM.COMRekhter                                                         [Page 5]