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RFC 1773 - Experience with the BGP-4 protocol


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Network Working Group                                          P. Traina
Request for Comments: 1773                                 cisco Systems
Obsoletes: 1656                                               March 1995
Category: Informational

                   Experience with the BGP-4 protocol

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Introduction

   The purpose of this memo is to document how the requirements for
   advancing a routing protocol to Draft Standard have been satisfied by
   Border Gateway Protocol version 4 (BGP-4).  This report documents
   experience with BGP.  This is the second of two reports on the BGP
   protocol.  As required by the Internet Architecure Board (IAB) and
   the Internet Engineering Steering Group (IESG), the first report will
   present a performance analysis of the BGP protocol.

   The remaining sections of this memo document how BGP satisfies
   General Requirements specified in Section 3.0, as well as
   Requirements for Draft Standard specified in Section 5.0 of the
   "Internet Routing Protocol Standardization Criteria" document [1].

   This report is based on the initial work of Peter Lothberg (Ebone),
   Andrew Partan (Alternet), and several others.  Details of their work
   were presented at the Twenty-fifth IETF meeting and are available
   from the IETF proceedings.

   Please send comments to iwg@ans.net.

Acknowledgments

   The BGP protocol has been developed by the IDR (formerly BGP) Working
   Group of the Internet Engineering Task Force.  I would like to
   express deepest thanks to Yakov Rekhter and Sue Hares, co-chairs of
   the IDR working group.  I'd also like to explicitly thank Yakov
   Rekhter and Tony Li for the review of this document as well as
   constructive and valuable comments.

Documentation

   BGP is an inter-autonomous system routing protocol designed for
   TCP/IP internets.  Version 1 of the BGP protocol was published in RFC
   1105. Since then BGP Versions 2, 3, and 4 have been developed.
   Version 2 was documented in RFC 1163. Version 3 is documented in RFC
   1267.  The changes between versions 1, 2 and 3 are explained in
   Appendix 2 of [2].  All of the functionality that was present in the
   previous versions is present in version 4.

   BGP version 2 removed from the protocol the concept of "up", "down",
   and "horizontal" relations between autonomous systems that were
   present in version 1.  BGP version 2 introduced the concept of path
   attributes.  In addition, BGP version 2 clarified parts of the
   protocol that were "under-specified".

   BGP version 3 lifted some of the restrictions on the use of the
   NEXT_HOP path attribute, and added the BGP Identifier field to the
   BGP OPEN message.  It also clarifies the procedure for distributing
   BGP routes between the BGP speakers within an autonomous system.

   BGP version 4 redefines the (previously class-based) network layer
   reachability portion of the updates to specify prefixes of arbitrary
   length in order to represent multiple classful networks in a single
   entry as discussed in [5].  BGP version 4 has also modified the AS-
   PATH attribute so that sets of autonomous systems, as well as
   individual ASs may be described.  In addition, BGP version for has
   redescribed the INTER-AS METRIC attribute as the MULTI-EXIT
   DISCRIMINATOR and added new LOCAL-PREFERENCE and AGGREGATOR
   attributes.

   Possible applications of BGP in the Internet are documented in [3].

   The BGP protocol was developed by the IDR Working Group of the
   Internet Engineering Task Force. This Working Group has a mailing
   list, iwg@ans.net, where discussions of protocol features and
   operation are held. The IDR Working Group meets regularly during the
   quarterly Internet Engineering Task Force conferences. Reports of
   these meetings are published in the IETF's Proceedings.

MIB

   A BGP-4 Management Information Base has been published [4].  The MIB
   was written by Steve Willis (Wellfleet), John Burruss (Wellfleet),
   and John Chu (IBM).

   Apart from a few system variables, the BGP MIB is broken into two
   tables: the BGP Peer Table and the BGP Received Path Attribute Table.

   The Peer Table reflects information about BGP peer connections, such
   as their state and current activity. The Received Path Attribute
   Table contains all attributes received from all peers before local
   routing policy has been applied. The actual attributes used in
   determining a route are a subset of the received attribute table.

Security Considerations

   BGP provides flexible and extendible mechanism for authentication and
   security.  The mechanism allows to support schemes with various
   degree of complexity.  All BGP sessions are authenticated based on
   the BGP Identifier of a peer.  In addition, all BGP sessions are
   authenticated based on the autonomous system number advertised by a
   peer.  As part of the BGP authentication mechanism, the protocol
   allows to carry encrypted digital signature in every BGP message.
   All authentication failures result in sending the NOTIFICATION
   messages and immediate termination of the BGP connection.

   Since BGP runs over TCP and IP, BGP's authentication scheme may be
   augmented by any authentication or security mechanism provided by
   either TCP or IP.

   However, since BGP runs over TCP and IP, BGP is vulnerable to the
   same denial of service or authentication attacks that are present in
   any other TCP based protocol.

Implementations

   There are multiple independent interoperable implementations of BGP
   currently available.  This section gives a brief overview of the
   implementations that are currently used in the operational Internet.
   They are:

         - cisco Systems
         - gated consortium
         - 3COM
         - Bay Networks (Wellfleet)
         - Proteon

   To facilitate efficient BGP implementations, and avoid commonly made
   mistakes, the implementation experience with BGP-4 in with cisco's
   implementation was documented as part of RFC 1656 [4].

   Implementors are strongly encouraged to follow the implementation
   suggestions outlined in that document and in the appendix of [2].

   Experience with implementing BGP-4 showed that the protocol is
   relatively simple to implement. On the average BGP-4 implementation
   takes about 2 man/months effort, not including any restructuring that
   may be needed to support CIDR.

   Note that, as required by the IAB/IESG for Draft Standard status,
   there are multiple interoperable completely independent
   implementations.

Operational experience

   This section discusses operational experience with BGP and BGP-4.

   BGP has been used in the production environment since 1989, BGP-4
   since 1993.  This use involves at least two of the implementations
   listed above.  Production use of BGP includes utilization of all
   significant features of the protocol.  The present production
   environment, where BGP is used as the inter-autonomous system routing
   protocol, is highly heterogeneous.  In terms of the link bandwidth it
   varies from 28 Kbits/sec to 150 Mbits/sec.  In terms of the actual
   routes that run BGP it ranges from a relatively slow performance
   PC/RT to a very high performance RISC based CPUs, and includes both
   the special purpose routers and the general purpose workstations
   running UNIX.

   In terms of the actual topologies it varies from a very sparse
   (spanning tree of ICM) to a quite dense (NSFNET backbone).

   At the time of this writing BGP-4 is used as an inter-autonomous
   system routing protocol between ALL significant autonomous systems,
   including, but by all means not limited to: Alternet, ANS, Ebone,
   ICM, IIJ, MCI, NSFNET, and Sprint.  The smallest know backbone
   consists of one router, whereas the largest contains nearly 90 BGP
   speakers.  All together, there are several hundred known BGP speaking
   routers.

   BGP is used both for the exchange of routing information between a
   transit and a stub autonomous system, and for the exchange of routing
   information between multiple transit autonomous systems.  There is no
   distinction between sites historically considered backbones vs
   "regional" networks.

   Within most transit networks, BGP is used as the exclusive carrier of
   the exterior routing information.  At the time of this writing within
   a few sites use BGP in conjunction with an interior routing protocol
   to carry exterior routing information.

   The full set of exterior routes that is carried by BGP is well over
   20,000 aggregate entries representing several times that number of
   connected networks.

   Operational experience described above involved multi-vendor
   deployment (cisco, and "gated").

   Specific details of the operational experience with BGP in Alternet,
   ICM and Ebone were presented at the Twenty-fifth IETF meeting
   (Toronto, Canada) by Peter Lothberg (Ebone), Andrew Partan (Alternet)
   and Paul Traina (cisco).

   Operational experience with BGP exercised all basic features of the
   protocol, including authentication, routing loop suppression and the
   new features of BGP-4, enhanced metrics and route aggregation.

   Bandwidth consumed by BGP has been measured at the interconnection
   points between CA*Net and T1 NSFNET Backbone. The results of these
   measurements were presented by Dennis Ferguson during the Twenty-
   first IETF, and are available from the IETF Proceedings. These
   results showed clear superiority of BGP as compared with EGP in the
   area of bandwidth consumed by the protocol. Observations on the
   CA*Net by Dennis Ferguson, and on the T1 NSFNET Backbone by Susan
   Hares confirmed clear superiority of the BGP protocol family as
   compared with EGP in the area of CPU requirements.

Migration to BGP version 4

   On multiple occasions some members of IETF expressed concern about
   the migration path from classful protocols to classless protocols
   such as BGP-4.

   BGP-4 was rushed into production use on the Internet because of the
   exponential growth of routing tables and the increase of memory and
   CPU utilization required by BGP.  As such,  migration issues that
   normally would have stalled deployment were cast aside in favor of
   pragmatic and intelligent deployment of BGP-4 by network operators.

   There was much discussion about creating "route exploders" which
   would enumerate individual class-based networks of CIDR allocations
   to BGP-3 speaking routers,  however a cursory examination showed that
   this would vastly hasten the requirement for more CPU and memory
   resources for these older implementations.  There would be no way
   internal to BGP to differentiate between known used networks and the
   unused portions of the CIDR allocation.

   The migration path chosen by the majority of the operators was known
   as "CIDR, default, or die!"

   To test BGP-4 operation, a virtual "shadow" Internet was created by
   linking Alternet, Ebone, ICM, and cisco over GRE based tunnels.
   Experimentation was done with actual live routing information by
   establishing BGP version 3 connections with the production networks
   at those sites.  This allowed extensive regression testing before
   deploying BGP-4 on production equipment.

   After testing on the shadow network, BGP-4 implementations were
   deployed on the production equipment at those sites.  BGP-4 capable
   routers negotiated BGP-4 connections and interoperated with other
   sites by speaking BGP-3.  Several test aggregate routes were injected
   into this network in addition to class-based networks for
   compatibility with BGP-3 speakers.

   At this point, the shadow-Internet was re-chartered as an
   "operational experience" network.  tunnel connections were
   established with most major transit service operators so that
   operators could gain some understanding of how the introduction of
   aggregate networks would affect routing.

   After being satisfied with the initial deployment of BGP-4, a number
   of sites chose to withdraw their class-based advertisements and rely
   only on their CIDR aggregate advertisements.  This provided
   motivation for transit providers who had not migrated to either do
   so, accept a default route, or lose connectivity to several popular
   destinations.

Metrics

   BGP version 4 re-defined the old INTER-AS metric as a MULTI-EXIT-
   DISCRIMINATOR.  This value may be used in the tie breaking process
   when selecting a preferred path to a given address space.  The MED is
   meant to only be used when comparing paths received from different
   external peers in the same AS to indicate the preference of the
   originating AS.

   The MED was purposely designed to be a "weak" metric that would only
   be used late in the best-path decision process.  The BGP working
   group was concerned that any metric specified by a remote operator
   would only affect routing in a local AS if no other preference was
   specified.  A paramount goal of the design of the MED was insure that
   peers could not "shed" or "absorb" traffic for networks that they
   advertise.

   The LOCAL-PREFERENCE attribute was added so a local operator could
   easily configure a policy that overrode the standard best path
   determination mechanism without configuring local preference on each
   router.

   One shortcoming in the BGP4 specification was a suggestion for a
   default value of LOCAL-PREF to be assumed if none was provided.
   Defaults of 0 or the maximum value each have range limitations, so a
   common default would aid in the interoperation of multi-vendor
   routers in the same AS (since LOCAL-PREF is a local administration
   knob, there is no interoperability drawback across AS boundaries).

   Another area where more exploration is required is a method whereby
   an originating AS may influence the best path selection process.  For
   example, a dual-connected site may select one AS as a primary transit
   service provider and have one as a backup.

                    /---- transit B ----\
        end-customer                     transit A----
                    \---- transit C ----/

   In a topology where the two transit service providers connect to a
   third provider,  the real decision is performed by the third provider
   and there is no mechanism for indicating a preference should the
   third provider wish to respect that preference.

   A general purpose suggestion that has been brought up is the
   possibility of carrying an optional vector corresponding to the AS-
   PATH where each transit AS may indicate a preference value for a
   given route.  Cooperating ASs may then chose traffic based upon
   comparison of "interesting" portions of this vector according to
   routing policy.

   While protecting a given ASs routing policy is of paramount concern,
   avoiding extensive hand configuration of routing policies needs to be
   examined more carefully in future BGP-like protocols.

Internal BGP in large autonomous systems

   While not strictly a protocol issue, one other concern has been
   raised by network operators who need to maintain autonomous systems
   with a large number of peers.  Each speaker peering with an external
   router is responsible for propagating reachability and path
   information to all other transit and border routers within that AS.
   This is typically done by establishing internal BGP connections to
   all transit and border routers in the local AS.

   In a large AS, this leads to an n^2 mesh of TCP connections and some
   method of configuring and maintaining those connections.  BGP does
   not specify how this information is to be propagated,  so
   alternatives, such as injecting BGP attribute information into the
   local IGP have been suggested.  Also, there is effort underway to
   develop internal BGP "route reflectors" or a reliable multicast

   transport of IBGP information which would reduce configuration,
   memory and CPU requirements of conveying information to all other
   internal BGP peers.

Internet Dynamics

   As discussed in [7], the driving force in CPU and bandwidth
   utilization is the dynamic nature of routing in the Internet.  As the
   net has grown, the number of changes per second has increased.  We
   automatically get some level of damping when more specific NLRI is
   aggregated into larger blocks, however this isn't sufficient.  In
   Appendix 6 of [2] are descriptions of dampening techniques that
   should be applied to advertisements.  In future specifications of
   BGP-like protocols,  damping methods should be considered for
   mandatory inclusion in compliant implementations.

Acknowledgments

   The BGP-4 protocol has been developed by the IDR/BGP Working Group of
   the Internet Engineering Task Force.  I would like to express thanks
   to Yakov Rekhter for providing RFC 1266.  I'd also like to explicitly
   thank Yakov Rekhter and Tony Li for their review of this document as
   well as their constructive and valuable comments.

Author's Address

   Paul Traina
   cisco Systems, Inc.
   170 W. Tasman Dr.
   San Jose, CA 95134

   EMail: pst@cisco.com

References

   [1] Hinden, R., "Internet Routing Protocol Standardization Criteria",
       RFC 1264, BBN, October 1991.

   [2] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
       RFC 1771, T.J. Watson Research Center, IBM Corp., cisco Systems,
       March 1995.

   [3] Rekhter, Y., and P. Gross, Editors, "Application of the Border
       Gateway Protocol in the Internet", RFC 1772, T.J. Watson Research
       Center, IBM Corp., MCI, March 1995.

   [4] Willis, S., Burruss, J., and J. Chu, "Definitions of Managed
       Objects for the Fourth Version of the Border Gateway Protocol
       (BGP-4) using SMIv2", RFC 1657, Wellfleet Communications Inc.,
       IBM Corp., July 1994.

   [5] Fuller V., Li. T., Yu J., and K. Varadhan, "Classless Inter-
       Domain Routing (CIDR): an Address Assignment and Aggregation
       Strategy", RFC 1519, BARRNet, cisco, MERIT, OARnet, September
       1993.

   [6] Traina P., "BGP-4 Protocol Document Roadmap and Implementation
       Experience", RFC 1656, cisco Systems, July 1994.

   [7] Traina P., "BGP Version 4 Protocol Analysis", RFC 1774, cisco
       Systems, March 1995.

 

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