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Network Working Group                                         H-W. BraunRequest for Comments: 1222                San Diego Supercomputer Center                                                              Y. Rekhter                                         IBM T.J. Watson Research Center                                                                May 1991Advancing the NSFNET Routing ArchitectureStatus 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 routingBraun & 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 NSFNETBraun & 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",RFC904, BBN, April 1984.   [2] Mills, D., and H-W. Braun, "The NSFNET Backbone Network", SIGCOMMBraun & 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.COMBraun & Rekhter                                                 [Page 6]