Advancing the NSFNET routing architecture

Network Working Group                                         H-W. Braun
Request for Comments: 1222                San Diego Supercomputer Center
                                                              Y. Rekhter
                                         IBM T.J. Watson Research Center
                                                                May 1991


               Advancing the NSFNET Routing Architecture

Status of this Memo

   This RFC suggests improvements in the NSFNET routing architecture to
   accommodate a more flexible interface to the Backbone clients.  This
   memo provides information for the Internet community.  It does not
   specify an Internet standard.  Distribution of this memo is
   unlimited.

Introduction

   This memo describes the history of NSFNET Backbone routing and
   outlines two suggested phases for further evolution of the Backbone's
   routing interface.  The intent is to provide a more flexible
   interface for NSFNET Backbone service subscribers, by providing an
   attachment option that is simpler and lower-cost than the current
   one.

Acknowledgements

   The authors would like to thank Scott Brim (Cornell University),
   Bilal Chinoy (Merit), Elise Gerich (Merit), Paul Love (SDSC), Steve
   Wolff (NSF), Bob Braden (ISI), and Joyce K. Reynolds (ISI) for their
   review and constructive comments.

1. NSFNET Phase 1 Routing Architecture

   In the first phase of the NSFNET Backbone, a 56Kbps infrastructure
   utilized routers based on Fuzzball software [2].  The Phase 1
   Backbone used the Hello Protocol for interior routing.  At the
   periphery of the Backbone, the client networks were typically
   connected by using a gatedaemon ("gated") interface to translate
   between the Backbone's Hello Protocol and the interior gateway
   protocol (IGP) of the mid-level network.

   Mid-level networks primarily used the Routing Information Protocol
   (RIP) [3] for their IGP.  The gatedaemon system acted as an interface
   between the Hello and RIP environments.  The overall appearance was
   that the Backbone, mid-level networks, and the campus networks formed
   a single routing system in which information was freely exchanged.

   Network metrics were translated among the three network levels
   (backbone, mid-level networks, and campuses).

   With the development of the gatedaemon, sites were able to introduce
   filtering based on IP network numbers.  This process was controlled
   by the staff at each individual site.

   Once specific network routes were learned, the infrastructure
   forwarded metric changes throughout the interconnected network. The
   end-result was that a metric fluctuation on one end of the
   interconnected network could permeate all the way to the other end,
   crossing multiple network administrations.  The frequency of metric
   fluctuations within the Backbone itself was further increased when
   event-driven updates (e.g., metric changes) were introduced.  Later,
   damping of the event driven updates lessened their frequency, but the
   overall routing environment still appeared to be quite unstable.

   Given that only limited tools and protocols were available to
   engineer the flow of dynamic routing information, it was fairly easy
   for routing loops to form.  This was amplified as the topology became
   more fully connected without insulation of routing components from
   each other.

   All six nodes of the Phase 1 Backbone were located at client sites,
   specifically NSF funded supercomputer centers.


2. NSFNET Phase 2 Routing Architecture

   The routing architecture for the second phase of the NSFNET Backbone,
   implemented on T1 (1.5Mbps) lines, focused on the lessons learned in
   the first NSFNET phase.  This resulted in a strong decoupling of the
   IGP environments of the backbone network and its attached clients
   [5].  Specifically, each of the administrative entities was able to
   use its own IGP in any way appropriate for the specific network.  The
   interface between the backbone network and its attached client was
   built by means of exterior routing, initially via the Exterior
   Gateway Protocol (EGP) [1,4].

   EGP improved provided routing isolation in two ways.  First, EGP
   signals only up/down transitions for individual network numbers, not
   the fluctuations of metrics (with the exception of metric acceptance
   of local relevance to a single Nodal Switching System (NSS) only for
   inbound routing information, in the case of multiple EGP peers at a
   NSS).  Second, it allowed engineering of the dynamic distribution of
   routing information.  That is, primary, secondary, etc., paths can be
   determined, as long as dynamic externally learned routing information
   is available.  This allows creation of a spanning tree routing
   topology, satisfying the constraints of EGP.

   The pre-engineering of routes is accomplished by means of a routing
   configuration database that is centrally controlled and created, with
   a subsequent distribution of individual configuration information to
   all the NSFNET Backbone nodes.  A computer controlled central system
   ensures the correctness of the database prior to its distribution to
   the nodes.

   All nodes of the 1.5Mbps NSFNET Backbone (currently fourteen) are
   located at client sites, such as NSF funded supercomputer centers and
   mid-level network attachment points.

3. T3 Phase of the NSFNET Backbone

   The T3 (45Mbps) phase of the NSFNET Backbone is implemented by means
   of a new architectural model, in which the principal communication
   nodes (core nodes) are co-located with major phone company switching
   facilities.  Those co-located nodes then form a two-dimensional
   networking infrastructure "cloud".  Individual sites are connected
   via exterior nodes (E-NSS) and typically have a single T3 access line
   to a core node (C-NSS).  That is, an exterior node is physically at
   the service subscriber site.

   With respect to routing, this structure is invisible to client sites,
   as the routing interface uses the same techniques as the T1 NSFNET
   Backbone.  The two backbones will remain independent infrastructures,
   overlaying each other and interconnected by exterior routing, and the
   T1 Backbone will eventually be phased out as a separate network.

4. A Near-term Routing Alternative

   The experience with the T1/T3 NSFNET routing demonstrated clear
   advantages of this routing architecture in which the whole
   infrastructure is strongly compartmentalized.  Previous experience
   also showed that the architecture imposes certain obligations upon
   the attached client networks.  Among them is the requirement that a
   service subscriber must deploy its own routing protocol peer,
   participating in the IGP of the service subscriber and connected via
   a common subnet to the subscriber-site NSFNET node.  The router and
   the NSFNET Backbone exchange routing information via an EGP or BGP
   [7] session.

   The drawbacks imposed by this requirement will become more obvious
   with the transition to the new architecture that is employed by the
   T3 phase of the NSFNET Backbone.  This will allow rapid expansion to
   many and smaller sites for which a very simple routing interface may
   be needed.

   We strongly believe that separating the routing of the service
   subscriber from the NSFNET Backbone routing via some kind of EGP is
   the correct routing architecture.  However, it should not be
   necessary to translate this architecture into a requirement for each
   service subscriber to install and maintain additional equipment, or
   for the subscriber to deal with more complicated routing
   environments.  In other words, while maintaining that the concept of
   routing isolation is correct, we view the present implementation of
   the concept as more restrictive than necessary.

   An alternative implementation of this concept may be realized by
   separating the requirement for an EGP/BGP session, as the mechanism
   for exchanging routing information between the service subscriber
   network and the backbone, from the actual equipment that has to be
   deployed and maintained to support such a requirement.  The only
   essential requirement for routing isolation is the presence of two
   logical routing entities.  The first logical entity participates in
   the service subscriber's IGP, the second logical entity participates
   in the NSFNET Backbone IGP, and the two logical entities exchange
   information with each other by means of inter-domain mechanisms.  We
   suggest that these two logical entities could exist within a single
   physical entity.

   In terms of implementation, this would be no different from a
   gatedaemon system interfacing with the previous 56Kbps NSFNET
   Backbone from the regional clients, except that we want to continue
   the strong routing and administrative control that decouple the two
   IGP domains.  Retaining an inter-domain mechanism (e.g., BGP) to
   connect the two IGP domains within the single physical entity allows
   the use of a well defined and understood interface.  At the same
   time, care must be taken in the implementation that the two daemons
   will not simultaneously interact with the system kernel in unwanted
   ways.

   The possibility of interfacing two IGP domains within a single router
   has also been noted in [8].  For the NSFNET Backbone case, we propose
   in addition to retain strong firewalls between the IGP domains.  The
   IGP information would need to be tagged with exterior domain
   information at its entry into the other IGP.  It would also be
   important to allow distributed control of the configuration.  The
   NSFNET Backbone organization and the provider of the attached client
   network are each responsible for the integrity of their own routing
   information.

   An example implementation might be a single routing engine that
   executed two instances of routing daemons.  In the NSFNET Backbone
   case, one of the daemons would participate in the service
   subscriber's IGP, and the other would participate in the NSFNET
   Backbone IGP.  These two instances could converse with each other by
   running EGP/BGP via a local loopback mechanism or internal IPC.  In
   the NSFNET Backbone implementation, the NSFNET T1 E-PSP or T3 E-NSS
   are UNIX machines, so the local loopback interface (lo0) of the UNIX
   operating system may be used.

   Putting both entities into the same physical machine means that the
   E-PSP/E-NSS would participate in the regional IGP on its exterior
   interface.  We would still envision the Ethernet attachment to be the
   demarcation point for the administrative control and operational
   responsibility.  However, the regional client could provide the
   configuration information for the routing daemon that interfaced to
   the regional IGP, allowing the regional to continue to exercise
   control over the introduction of routing information into its IGP.

5. Long-Term Alternatives

   As technology employed by the NSFNET Backbone evolves, one may
   envision the demarcation line between the Backbone and the service
   subscribers moving in the direction of the "C-NSS cloud", so that the
   NSFNET IGP will be confined to the C-NSS, while the E-NSS will be a
   full participant in the IGP of the service subscriber.

   Clearly, one of the major prerequisites for such an evolution is the
   ability for operational management of the physical medium connecting
   a C-NSS with an E-NSS by two different administrative entities (i.e.,
   the NSFNET Backbone provider as well as the service subscriber).  It
   will also have to be manageable enough to be comparable in ease of
   use to an Ethernet interface, as a well-defined demarcation point.

   The evolution of the Point-to-Point Protocol, as well as a
   significantly enhanced capability for managing serial lines via
   standard network management protocols, will clearly help.  This may
   not be the complete answer, as a variety of equipment is used on
   serial lines, making it difficult to isolate a hardware problem.
   Similar issues may arise for future demarcation interfaces to
   Internet infrastructure (e.g., SMDS interfaces).

   In summary, there is an opportunity to simplify the management,
   administration, and exchange of routing information by collapsing the
   number of physical entities involved.

6. References

   [1] Mills, D., "Exterior Gateway Protocol Formal Specification", RFC
       904, BBN, April 1984.

   [2] Mills, D., and H-W. Braun, "The NSFNET Backbone Network", SIGCOMM

       1987, August 1987.

   [3] Hedrick, C., "Routing Information Protocol", RFC 1058, Rutgers
       University, June 1988.

   [4] Rekhter, Y., "EGP and Policy Based Routing in the New NSFNET
       Backbone", RFC 1092, IBM T.J. Watson Research Center, February
       1989.

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

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

   [7] Lougheed, K., and Y. Rekhter, "A Border Gateway Protocol (BGP)",
       RFC 1163, cisco Systems, IBM T.J. Watson Research Center, June
       1990.

   [8] Almquist, P., "Requirements for Internet IP Routers", to be
       published as a RFC.

7.  Security Considerations

   Security issues are not discussed in this memo.

8. Authors' Addresses

   Hans-Werner Braun
   San Diego Supercomputer Center
   P.O. Box 85608
   La Jolla, CA 92186-9784

   Phone: (619) 534-5035
   Fax:   (619) 534-5113

   EMail: HWB@SDSC.EDU

   Yakov Rekhter
   T.J. Watson Research Center
   IBM Corporation
   P.O. Box 218
   Yorktown Heights, NY  10598

   Phone: (914) 945-3896

   EMail: Yakov@Watson.IBM.COM