GENWiki

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.

Braun & Rekhter [Page 1]  RFC 1222 Advancing the NSFNET Routing Architecture May 1991

 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

Braun & Rekhter [Page 2]  RFC 1222 Advancing the NSFNET Routing Architecture May 1991

 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.

Braun & Rekhter [Page 3]  RFC 1222 Advancing the NSFNET Routing Architecture May 1991

 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

Braun & Rekhter [Page 4]  RFC 1222 Advancing the NSFNET Routing Architecture May 1991

 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

Braun & Rekhter [Page 5]  RFC 1222 Advancing the NSFNET Routing Architecture May 1991

     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

Braun & Rekhter [Page 6]