RFC1222 - Advancing the NSFNET routing architecture

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　　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 RFCsuggests 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", RFC1058, Rutgers
University, June 1988.

[4] Rekhter, Y., "EGP and Policy Based Routing in the New NSFNET
Backbone", RFC1092, IBM T.J. Watson Research Center, February
1989.

[5] Braun, H-W., "The NSFNET Routing Architecture", RFC1093,
Merit/NSFNET, February 1989.

[6] Braun, H-W., "Models of Policy Based Routing", RFC1104,
Merit/NSFNET, June 1989.

[7] Lougheed, K., and Y. Rekhter, "A Border Gateway Protocol (BGP)",
RFC1163, 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

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