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

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

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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

   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

   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
   NSFNET towards the DDN, with two further gateways standing by as hot
   backups.  The metric for network announcements from the DDN to the
   NSFNET was coordinated by the Merit/NSFNET project.

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2.2  Outbound announcement -- Routes announced from the NSFNET to the

   Each NSS involved with NSFNET/DDN routing had an EGP peer relation
   with the NSFNET/ARPANET gateway.  Via EGP it announced a certain set
   of NSFNET connected networks, again, as controlled by the distributed
   policy routing database, to its peer.  The NSFNET/ARPANET gateway
   then redistributed the networks it had learned from the NSS to the
   DDN via a separate EGP session.  Each of the NSFNET/ARPANET gateways
   used a separate Autonomous System number to communicate EGP
   information with the DDN.  Also these Autonomous System numbers were
   not the same as the NSFNET backbone uses to communicate with directly
   attached client networks.  The NSFNET/ARPANET gateways used the
   Autonomous System number of the local network.  The metrics for
   announcing network numbers to the DDN Mailbridges were set according
   to the requests of the mid-level network of which the specific
   individual network was a client.  Mid-level network also influenced
   the specific NSFNET/ARPANET gateway used, including primary/secondary
   selection.  These primary/secondary selections among the
   NSFNET/ARPANET gateways allowed for redundancy, while the preference
   of network announcements was modulated by the metric used for the
   announcements to the DDN from the NSFNET/ARPANET gateways.  Some of
   the selection decisions were based on reliability of a specific
   gateway or congestion expected in a specific PSN that connected to
   the NSFNET/ARPANET gateway.

2.3  Administrative aspects

   From an administrative point of view, the NSFNET/ARPANET gateways
   were administered by the institution to which the gateway belonged.
   This has never been a real problem due to the excellent cooperation
   received from all the involved sites.

3.  NSFNET/DDN routing via attached Mailbridges

   During the first half of 1989 a new means of interconnectivity
   between the NSFNET and the DDN was designed and implemented.
   Ethernet adapters were installed in two of the Mailbridges, which
   previously just connected the MILNET and the ARPANET, allowing a
   direct interface to NSFNET nodes.  Of these two Mailbridges one is
   located on the west coast at NASA-Ames located at Moffett Field, CA,
   and the other one is located on the east coast at Mitre in Reston,
   VA.  With this direct interconnection it became possible for the
   NSFNET to exchange routing information directly with the DDN route
   servers, without a gateway operated by a mid-level network in the
   middle.  This also eliminated the need to traverse the ARPANET in
   order to reach MILNET sites.  It furthermore allows the Defense
   Communication Agency as well as the National Science Foundation to

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   exercise control over the interconnection on a need basis, e.g., the
   connectivity can now be easily disabled from either site at times of
   tighter network security concerns.

3.1  Inbound announcement -- Routes announced from the DDN to the

   The routing setup for the direct Mailbridge connections is somewhat
   different, as compared to the previously used NSFNET/ARPANET
   gateways.  Instead of a single NSFNET/ARPANET gateway carrying all
   the traffic from the DDN to the NSFNET at any moment, the
   distribution of network numbers is now split between the two
   Mailbridges.  This results in a distributed load, with specific
   network numbers always preferring a particular Mailbridge under
   normal operating circumstances.  In the case of an outage at one of
   the Mailbridge connections, the other one fully takes over the load
   for all the involved network numbers.  For this setup, the two DDN
   links are known as two different Autonomous System numbers by the
   NSFNET.  The routes learned via the NASA-Ames Mailbridges are part of
   the Autonomous System 164 which is also the Autonomous System number
   which the Mailbridges are using by themselves during the EGP session.
   In the case of the EGP sessions with the Mitre Mailbridge, the DDN-
   internal Autonomous System number of 164 is overwritten with a
   different Autonomous System number (in this case 184) and the routes
   learned via the Mitre Mailbridge will therefore become part of
   Autonomous System 184 within the NSFNET.

   The NSFNET-inbound routing is controlled by the distributed policy
   routing database.  In particular, the network number is verified
   against a list of legitimate networks, and a metric is associated
   with an authorized network number for a particular site.  For
   example, both NSSs in Palo Alto and College Park learn net 10 (the
   ARPANET network number) from the Mailbridges they are connected to
   and have EGP sessions.  The Palo Alto NSS will accept Net 10 with a
   metric of 10, while the College Park NSS will accept the same network
   number with a metric of 12.  Therefore, traffic destinated to net 10
   will prefer the path via the Palo Alto NSS and the NASA-Ames
   Mailbridge.  If the connection via the NASA-Ames Mailbridge is not
   functioning, the traffic will be re-routed via the Mailbridge link at
   Mitre.  Each of the two NSS accepts half of the network routes via
   EGP from its co- located Mailbridge at a lower metric and the other
   half at a higher metric.  The half with the lower metric at the Palo
   Alto NSS will be the same set which uses a higher metric at the
   College Park NSS and vice versa.

   There are at least three different possibilities about how the NSFNET
   could select a path to a DDN network via a specific Mailbridge, i.e.,
   the one at NASA-Ames versus the one at Mitre:

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      1.  Assign a primary path for all DDN networks to a single
          Mailbridge and use the other purely as a backup path.

      2.  Distribute the DDN networks randomly across the two

      3.  Let the DDN administration inform the NSFNET which networks
          on the DDN are closer to a specific Mailbridge so that the
          particular Mailbridge would accept these networks at a lower
          metric.  The second Mailbridge would then function as a backup
          path.  From a NSFNET point of view, this would mean treating the
          DDN like other NSFNET peer networks such as the NASA Science
          network (NSN) or DOE's Energy Science Network (ESNET).

   We are currently using alternative (2) as an interim solution.  At
   this time, the DDN administration is having discussions with NSFNET
   about moving to alternative (3), which would allow them control over
   how the DDN networks would be treated in the NSFNET.

3.2  Outbound announcement -- Routes announced from the NSFNET to the

   The selection of metrics for announcements of NSFNET networks to the
   DDN is controlled by the NSFNET.  The criteria for the metric
   decisions is based on distances between the NSS, which introduces a
   specific network into the NSFNET, and either one of the NSSs that has
   a co-located Mailbridge.  In this context, the distance translates
   into the hop count between NSSs in the NSFNET backbone.  For example,
   the Princeton NSS is currently one hop away from the NSS co-located
   with the Mitre Mailbridge, but is three hops away from the NSS with
   the NASA-Ames Mailbridge.  Therefore, in the case of networks with
   primary paths via the Princeton NSS, the Mitre Mailbridge will
   receive the announcements for those networks at a lower metric than
   the NASA-Ames Mailbridge.  This means that the traffic from the DDN
   to networks connected to the Princeton NSS will be routed through the
   Mailbridge at Mitre to the College Park NSS and then through the
   Princeton NSS to its final destination.  This will guarantee that
   traffic entering the NSFNET from the DDN will take the shortest path
   to its NSFNET destination under normal operating conditions.

3.3  Administrative aspects

   Any of the networks connected via the NSFNET can be provided with the
   connectivity to the DDN via the NSFNET upon request from the mid-
   level network through which the specific network is connected.

   For networks that do not have a DDN connection other than via NSFNET,
   the NSFNET will announce the nets via one of the Mailbridges with a

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   low metric to create a primary path (e.g., metric "1") and via the
   second Mailbridge as a secondary path (e.g., metric "3").  For
   networks that have their own DDN connection and wish to use the
   NSFNET as a backup connection to DDN, the NSFNET will announce those
   networks via the two Mailbridges at higher metrics.

   The mid-level networks need to make a specific request if they want
   client networks to be announced to the DDN via the NSFNET. Those
   requests need to state whether this would be a primary connection for
   the specific networks.  If the request is for a fallback connection,
   it needs to state the existing metrics in use for announcements of
   the network to the DDN.

4.  Shortcomings of the current NSFNET/DDN interconnection routing

   The current setup makes full use of the two Mailbridges that connect
   to the NSFNET directly, with regard to redundancy and load sharing.
   However, with regard to performance optimization, such as packet
   propagation delay between source/destination pairs located on
   disjoint peer networks, there are some shortcomings.  These
   shortcomings are not easy to overcome because of the limitations of
   the current architecture.  However, it is a worthwhile topic for
   discussion to aid future improvements.

   To make the discussion easier, the following assumptions and
   terminology will be used:

      The NSFNET is viewed as a cloud and so is the DDN.  The two have
      two connections, one at east coast and one at west coast.

      mb-east -- the Mailbridge at Mitre

      mb-west -- the Mailbridge at Ames

      NSS-east -- the NSS egp peer with mb-east

      NSS-west -- the NSS egp peer with mb-west

      DDN.east-net -- networks connected to DDN and physically closer to

      DDN.west-net -- networks connected to DDN and physically closer to

      NSF.east-net -- networks connected to NSFNET and physically closer
                      to NSS-east

      NSF.west-net -- networks connected to NSFNET and physically closer

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                      to NSS-west

   The traffic between NSFNET<->DDN will fall into the following

      a) NSF.east-net <-> DDN.east-net or
         NSF.west-net <-> DDN.west-net

      b) NSF.east-net <-> DDN.west-net or
         NSF.west-net <-> DDN.east-net

   The ideal traffic path for a) and b) should be as follows:

   For traffic pattern a)




   For traffic pattern b)





        -*-> is used to indicate traffic transcontinentally traversing
        the NSFNET backbone

        -**-> is used to indicate traffic transcontinentally traversing
        the DDN backbone

        The traffic for a) will transcontinentally traverse NEITHER the
        NSFNET backbone, NOR the DDN backbone.

        The traffic for b) will transcontinentally traverse NSFNET once
        and DDN once and only once for each.

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   For the current set up,

   The traffic path for pattern a) would have chances to
   transcontinentally traverse both NSFNET and DDN.

   The traffic path for pattern b) would have chances to
   transcontinentally traverse the DDN in both directions.

   To achieve the ideal traffic path it requires the NSFNET to implement
   (3) as stated above, i.e., to treat the DDN like other NSFNET peer or
   mid-level networks.  As mentioned before, discussions between NSFNET
   and DDN people are underway and the DDN is considering providing the
   NSFNET with the required information to accomplish the outlined goals
   in the near future.

   At such time as this is accomplished, it will reduce the likelihood
   of packet traffic unnecessarily traversing national backbones.

   One of the best ways to optimize the traffic between two peer
   networks, not necessary limited to the NSFNET and the DDN, is to try
   to avoid letting traffic traverse a backbone with a comparatively
   slower speed and/or a backbone with a significantly larger diameter
   network.  For example, in the case of traffic between the NSFNET and
   the DDN, the NSFNET has a T1 backbone and a maximum diameter of three
   hops, while the DDN is a relatively slow network running largely at
   56Kbps.  In this case the overall performance would be better if
   traffic would traverse the DDN as little as possible, i.e., whenever
   the traffic has to reach a destination network outside of the DDN, it
   should find the closest Mailbridge to exit the DDN.

   The current architecture employed for DDN routing is not able to
   accomplish this.  Firstly, the technology is designed based on a core
   model.  It does not expect a single network to be announced by
   multiple places.  An example for multiple announcements could be two
   NSSs announcing a single network number to the two Mailbridges at
   their different locations.  Secondly, the way all the existing
   Mailbridges exchange routing information among themselves is done via
   a protocol similar to EGP, and the information is then distributed
   via EGP to the DDN-external networks.  In this case the physical
   distance information and locations of network numbers is lost and
   neither the Mailbridges nor the external gateways will be able to do
   path optimization based on physical distance and/or propagation
   delay.  This is not easy to change, as in the DDN the link level
   routing information is decoupled from the IP level routing, i.e., the
   IP level routing has no information about topology of the physical
   infrastructure.  Thus, even if an external gateway to a DDN network
   were to learn a particular network route from two Mailbridges, it
   would not be able to favor one over the other DDN exit point based on

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RFC 1133         Routing between the NSFNET and the DDN    November 1989

   the distance to the respective Mailbridge.

5.  Conclusions

   While recent changes in the interconnection architecture between the
   NSFNET and DDN peer networks have resulted in significant performance
   and reliability improvements, there are still possibilities for
   further improvements and rationalization of this inter-peer network
   routing.  However, to accomplish this it would most likely require
   significant architectural changes within the DDN.

6.  References

  [1]  Rekhter, Y., "EGP and Policy Based Routing in the New NSFNET
       Backbone", RFC 1092, IBM Research, February 1989.

  [2]  Braun, H-W., "The NSFNET Routing Architecture", RFC 1093,
       Merit/NSFNET Project, February 1989.

  [3]  Collins, M., and R. Nitzan, "ESNET Routing", DRAFT Version 1.0,
       LLNL, May 1989.

  [4]  Braun, H-W., "Models of Policy Based Routing," RFC 1104,
       Merit/NSFNET Project, February 1989.

Security Considerations

   Security issues are not addressed in this memo.

Authors' Addresses

   Jessica (Jie Yun) Yu
   Merit Computer Network
   1075 Beal Avenue
   Ann Arbor, Michigan 48109

   Telephone:      313 936-2655
   Fax:            313 747-3745

   Hans-Werner Braun
   Merit Computer Network
   1075 Beal Avenue
   Ann Arbor, Michigan 48109

   Telephone:      313 763-4897
   Fax:            313 747-3745

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