Routing between the NSFNET and the DDN
RFC 1133

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Network Working Group                                            J. Yu
Request for Comments: 1133                                  H-W. Braun
                                                Merit Computer Network
                                                         November 1989

                 Routing between the NSFNET and the DDN

Status of this Memo

   This document is a case study of the implementation of routing
   between the NSFNET and the DDN components (the MILNET and the
   ARPANET).  We hope that it can be used to expand towards
   interconnection of other Administrative Domains.  We would welcome
   discussion and suggestions about the methods employed for the
   interconnections.  No standards are specified in this memo.
   Distribution of this memo is unlimited.

1.  Definitions for this document

   The NSFNET is the backbone network of the National Science
   Foundation's computer network infrastructure.  It interconnects
   multiple autonomously administered mid-level networks, which in turn
   connect autonomously administered networks of campuses and research
   centers.  The NSFNET connects to multiple peer networks consisting of
   national network infrastructures of other federal agencies.  One of
   these peer networks is the Defense Data Network (DDN) which, for the
   sake of this discussion, should be viewed as the combination of the
   DoD's MILNET and ARPANET component networks, both of which are
   national in scope.

   It should be pointed out that network announcements in one direction
   result in traffic the other direction, e.g., a network announcement
   via a specific interconnection between the NSFNET to the DDN results
   in packet traffic via the same interconnection between the DDN to the
   NSFNET.

2.  NSFNET/DDN routing until mid '89

   Until mid-1989, the NSFNET and the DDN were connected via a few
   intermediate routers which in turn were connected to the ARPANET.
   These routers exchanged network reachability information via the
   Exterior Gateway Protocol (EGP) with the NSFNET nodes as well as with
   the DDN Mailbridges.  In the context of network routing these
   Mailbridges can be viewed as route servers, which exchange external
   network reachability information via EGP while using a proprietary
   protocol to exchange routing information among themselves.
   Currently, there are three Mailbridges at east coast locations and

Yu & Braun                                                      [Page 1]
RFC 1133         Routing between the NSFNET and the DDN    November 1989

   three Mailbridges at west coast locations.  Besides functioning as
   route servers the Mailbridges also provide for connectivity, i.e,
   packet switching, between the ARPANET and the MILNET.

   The intermediate systems between the NSFNET and the ARPANET were
   under separate administrative control, typically by a NSFNET mid-
   level network.

   For a period of time, the traffic between the NSFNET and the DDN was
   carried by three ARPANET gateways.  These ARPANET gateways were under
   the administrative control of a NSFNET mid-level network or local
   site and had direct connections to both a NSFNET NSS and an ARPANET
   PSN.  These routers had simultaneous EGP sessions with a NSFNET NSS
   as well as a DDN Mailbridge.  This resulted in making them function
   as packet switches between the two peer networks.  As network routes
   were established packets were switched between the NSFNET and the
   DDN.

   The NSFNET used three NSFNET/ARPANET gateways which had been provided
   by three different sites for redundancy purposes.  Those three sites
   were initially at Cornell University, the University of Illinois
   (UC), and Merit.  When the ARPANET connections at Cornell University
   and the University of Illinois (UC) were terminated, a similar setup
   was introduced at the Pittsburgh Supercomputer Center and at the John
   von Neumann Supercomputer Center which, together with the Merit
   connection, allowed for continued redundancy.

   As described in RFC1092 and RFC1093, NSFNET routing is controlled by
   a distributed policy routing database that controls the acceptance
   and distribution of routing information.  This control also extends
   to the NSFNET/ARPANET gateways.

2.1  Inbound announcement -- Routes announced from the DDN to the
     NSFNET

   In the case of the three NSFNET/ARPANET gateways, each of the
   associated NSSs accepted the DDN routes at a different metric.  The
   route with the lowest metric then was favored for the traffic towards
   the specific DDN network, but had that specific gateway to the DDN
   experienced problems with loss of routing information, one of the
   redundant gateways would take over and carry the load as a fallback
   path.  Assuming consistent DDN routing information at any of the
   three gateways, as received from the Mailbridges, only a single
   NSFNET/ARPANET gateway was used at a given time for traffic from the