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rfc:rfc1266

Network Working Group Y. Rekhter, Editor Request for Comments: 1266 T.J. Watson Research Center, IBM Corp.

                                                          October 1991
                  Experience with the BGP Protocol

1. Status of this Memo.

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

2. 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 (BGP). This report documents experience with
 BGP.  This is the second of two reports on the BGP protocol.  As
 required by the Internet Activities 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 work of Dennis Ferguson (University of
 Toronto), Susan Hares (MERIT/NSFNET), and Jessica Yu (MERIT/NSFNET).
 Details of their work were presented at the Twentieth IETF meeting
 (March 11-15, 1991, St. Louis) and are available from the IETF
 Proceedings.
 Please send comments to iwg@rice.edu.

3. Acknowledgements.

 The BGP protocol has been developed by the IWG/BGP Working Group of
 the Internet Engineering Task Force. We would like to express our
 deepest thanks to Guy Almes (Rice University) who was the previous
 chairman of the IWG Working Group.  We also like to explicitly thank
 Bob Hinden (BBN) for the review of this document as well as his
 constructive and valuable comments.

BGP Working Group [Page 1] RFC 1266 Experience with the BGP Protocol October 1991

4. Documentation.

 BGP is an inter-autonomous system routing protocol designed for the
 TCP/IP internets.  Version 1 of the BGP protocol was published in RFC
 1105. Since then BGP Versions 2 and 3 have been developed. Version 2
 was documented in RFC 1163. Version 3 is documented in [3]. The
 changes between versions 1, 2 and 3 are explained in Appendix 3 of
 [3].  Most of the functionality that was present in the Version 1 is
 present in the Version 2 and 3.  Changes between Version 1 and
 Version 2 affect mostly the format of the BGP messages.  Changes
 between Version 2 and Version 3 are quite minor.
 BGP Version 2 removed from the protocol the concept of "up", "down",
 and "horizontal" relations between autonomous systems that were
 present in the Version 1.  BGP Version 2 introduced the concept of
 path attributes.  In addition, BGP Version 2 clarified parts of the
 protocol that were "underspecified".  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.  Possible applications of BGP in the
 Internet are documented in [2].
 The BGP protocol was developed by the IWG/BGP Working Group of the
 Internet Engineering Task Force. This Working Group has a mailing
 list, iwg@rice.edu, where discussions of protocol features and
 operation are held. The IWG/BGP Working Group meets regularly during
 the quarterly Internet Engineering Task Force conferences. Reports of
 these meetings are published in the IETF's Proceedings.

5. MIB

 A BGP Management Information Base has been published [4].  The MIB
 was written by Steve Willis (swillis@wellfleet.com) and John Burruss
 (jburruss@wellfleet.com).
 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.
 The BGP MIB is quite small. It contains total of 27 objects.

BGP Working Group [Page 2] RFC 1266 Experience with the BGP Protocol October 1991

6. Security architecture.

 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.

7. Implementations.

 There are multiple interoperable implementations of BGP currently
 available. This section gives a brief overview of the three
 completely independent implementations that are currently used in the
 operational Internet. They are:
  1. cisco. This implementation was wholly developed by cisco.

It runs on the proprietary operating system used by the

      cisco routers. Consult Kirk Lougheed (lougheed@cisco.com)
      for more details.
  1. "gated". This implementation was developed wholly by Jeff

Honig (jch@risci.cit.cornell.edu) and Dennis Ferguson

      (dennis@CAnet.CA).  It runs on a variety of operating systems
      (4.3 BSD, AIX, etc...).  It is the only available public domain
      code for BGP. Consult Jeff Honig or Dennis Ferguson for more
      details.
  1. NSFNET. This implementation was developed wholly by Yakov

Rekhter (yakov@watson.ibm.com). It runs on the T1 NSFNET

      Backbone and T3 NSFNET Backbone. Consult Yakov Rekhter for
      more details.
 To facilitate efficient BGP implementations, and avoid commonly made
 mistakes, the implementation experience with BGP in "gated" was
 documented as part of RFC 1164.  Implementors are strongly encouraged
 to follow the implementation suggestions outlined in that document.
 Experience with implementing BGP showed that the protocol is
 relatively simple to implement. On the average BGP implementation
 takes about 1 man/month effort.

BGP Working Group [Page 3] RFC 1266 Experience with the BGP Protocol October 1991

 Note that, as required by the IAB/IESG for Draft Standard status,
 there are multiple interoperable completely independent
 implementations, namely those from cisco, "gated", and IBM.

8. Operational experience.

 This section discusses operational experience with BGP.
 BGP has been used in the production environment since 1989.  This use
 involves all three 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 56 Kbits/sec to 45
 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
 RS/6000, and includes both the special purpose routers (cisco) and
 the general purpose workstations running UNIX. In terms of the actual
 topologies it varies from a very sparse (spanning tree or a ring of
 CA*Net) to a quite dense (T1 or T3 NSFNET Backbones).
 At the time of this writing BGP is used as an inter-autonomous system
 routing protocol between the following autonomous systems: CA*Net, T1
 NSFNET Backbone, T3 NSFNET Backbone, T3 NSFNET Test Network, CICNET,
 MERIT, and PSC. Within CA*Net there are 10 border routers
 participating in BGP. Within T1 NSFNET Backbone there are 20 border
 routers participating in BGP. Within T3 NSFNET Backbone there are 15
 border routers participating in BGP. Within T3 NSFNET Test Network
 there are 7 border routers participating in BGP. Within CICNET there
 are 2 border routers participating in BGP. Within MERIT there is 1
 border router participating in BGP. Within PSC there is 1 router
 participating in BGP. All together there are 56 border routers
 spanning 7 autonomous systems that are running BGP.  Out of these, 49
 border routers that span 6 autonomous systems are part of the
 operational Internet.
 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. It covers
 both the Backbones (CA*Net, T1 NSFNET Backbone, T3 NSFNET Backbone),
 and the Regional Networks (PSC, MERIT).
 Within CA*Net, T3 NSFNET Backbone, and T3 NSFNET Test Network BGP is
 used as the exclusive carrier of the exterior routing information
 both between the autonomous systems that correspond to the above
 networks, and with the autonomous system of each network. At the time
 of this writing within the T1 NSFNET Backbone BGP is used together
 with the NSFNET Backbone Interior Routing Protocol to carry the

BGP Working Group [Page 4] RFC 1266 Experience with the BGP Protocol October 1991

 exterior routing information. T1 NSFNET Backbone is in the process of
 moving toward carrying the exterior routing information exclusively
 by BGP.  The full set of exterior routes that is carried by BGP is
 well over 2,000 networks.
 Operational experience described above involved multi-vendor
 deployment (cisco, "gated", and NSFNET).
 Specific details of the operational experience with BGP in the NSFNET
 were presented at the Twentieth IETF meeting (March 11-15, 1991, St.
 Louis) by Susan Hares (MERIT/NSFNET).  Specific details of the
 operational experience with BGP in the CA*Net were presented at the
 Twentieth IETF meeting (March 11-15, 1991, St. Louis) by Dennis
 Ferguson (University of Toronto).  Both of these presentations are
 available in the IETF Proceedings.
 Operational experience with BGP exercised all basic features of the
 protocol, including the authentication and routing loop suppression.
 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 last 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 BGP as compared with EGP in the area
 of CPU requirements.

9. Using TCP as a transport for BGP.

9.1. Introduction.

 On multiple occasions some members of IETF expressed concern about
 using TCP as a transport protocol for BGP. In this section we examine
 the use of TCP for BGP in terms of:
  1. real versus perceived problems
  2. offer potential solutions to real problems
  3. perspective on the convergence problem
  4. conclusions
 BGP is based on the incremental updates. This is done intentionally
 to conserve the CPU and bandwidth requirements. Extensive operational
 experience with BGP in the Internet showed that indeed the use of the
 incremental updates allows significant saving both in terms of the
 CPU utilization and bandwidth consumption.  However, to operate
 correctly the incremental updates must be exchanged over a reliable

BGP Working Group [Page 5] RFC 1266 Experience with the BGP Protocol October 1991

 transport.  BGP uses TCP as such transport. It had been suggested
 that another transport protocol would be more suitable for BGP.

9.2. Examination of Problems - Real and "perceived".

 Extensive operational experience with BGP in the Internet showed that
 the only real problem that was attributed to BGP in general, and the
 use of TCP as the transport for BGP in particular, was its slow
 convergence in presence of congestion.  This problem was experienced
 in CA*Net. As we mentioned before, CA*Net is composed of 10 routers
 that form a ring. The routers are connected by 56 Kbits/sec links.
 All links are heavily utilized and are often congested.  Experience
 with BGP in CA*Net showed that unless special measures are taken, the
 protocol may exhibit slow convergence when BGP information is passed
 over the slow speed (56 Kbits/sec) congested links. This is because a
 large percentage of packets carrying BGP information are being
 dropped due to congestion.  Therefore, there are three inter-related
 problems: congestion, packet drops, and the resulting slow
 convergence of routing under congestion and packet drops.
 Observe, that any transport protocol used by BGP would have
 difficulty preventing packets from being dropped under congestion,
 since it has no direct control over the routers that drop the
 packets, and the congestion has nothing to do with the BGP traffic.
 Therefore, since BGP is not the cause of congestion, and cannot
 directly influence dropping at the routers, replacing TCP (as the BGP
 transport) with another transport protocol would have no effect on
 packets being dropped due to congestion. We think that once a network
 is congested, packets will be dropped (regardless of whether these
 packets carry BGP or any other information), unless special measures
 outside of BGP in general, and the transport protocol used by BGP in
 particular, are taken.
 If packets carrying routing information are lost, any distributed
 routing protocol will exhibit slow convergence.  If quick convergence
 is viewed as important for a routing within a network, special
 measures to minimize the loss of packets that carry routing
 information must be taken.  The next section suggests some possible
 methods.

9.3. Solutions to the problem.

 Two possible measures could be taken to reduce the drop of BGP
 packets which slows convergence of routing:
    1) alleviate the congestion
    2) reduce the percentage of BGP packets that are dropped due

BGP Working Group [Page 6] RFC 1266 Experience with the BGP Protocol October 1991

       to congestion by marking BGP packets and setting policies to
       routers to try not to drop BGP packets
 Alleviating the network congestion is a subject outside the control
 of BGP, and will not be discussed in this paper.
 Operational experience with BGP in CA*Net shows that reducing the
 percentage of BGP packets dropped due to congestion by marking them,
 and setting policies to routers to try not to drop BGP packets
 completely solves the problem of slow convergence in presence of
 congestion.
 The BGP packets can be marked (explicitly or implicitly) by the
 following three methods:
    a) by means of IP precedence (Internetwork Control)
    b) by using a well-known TCP port number
    c) by identifying packets by just source or destination IP
       address.
 Appendix 4 of the BGP protocol specification, RFC 1163, recommends
 the use of IP precedence (Internetwork Control) because the
 precedence provides a well-defined mechanism to mark BGP packets.
 The method of a well-known TCP port number to identify packets is
 similar to the one that was used by Dave Mills in the NSFNET Phase I.
 Dave Mills identified Telnet traffic by a well known TCP port number,
 and gave it priority over the rest of the traffic.  CA*Net identified
 BGP traffic based on it's source and destination IP address.  Packets
 receive a priority if either the source or the destination IP address
 belongs to CA*Net.
 If packets that carry the routing information are being dropped
 (because of congestion), one also may ask about how does a particular
 routing protocol react to such an event.  In the case of BGP the
 packets are retransmitted using the TCP retransmission mechanism. It
 seems plausible that being more aggressive in terms of the
 retransmission should have positive effect on the convergence.  This
 can be done completely within TCP by adjusting the TCP retransmission
 timers. However, we would like to point out that the change in the
 retransmission strategy should not be viewed as a cure for the
 problem, since the root of the problem lies in the way how packets
 that carry the BGP information are handled within a congested
 network, and not in how frequently the lost packets are
 retransmitted.
 It should also be pointed out that the local system can control the

BGP Working Group [Page 7] RFC 1266 Experience with the BGP Protocol October 1991

 amount of data to be retransmitted (in case of a congestion or
 losses) by adjusting the TCP Window size. That allows to control the
 amount of potentially obsolete data that has to be retransmitted.

9.4. Perspective on the Convergence Problem.

 To put the convergence problem in a proper perspective, we'd like to
 point out that much of the Internet now uses EGP at AS borders,
 ensuring that routing changes cannot be guaranteed to propagate
 between ASes in less than a few minutes. It would take huge amount of
 congestion to slow BGP to this pace. Additionally, the problems of
 EGP in the face of packet loss are well known and far exceed any
 imaginable problem BGP/TCP might ever suffer.  Therefore, the worst
 case behavior of BGP is about the same as the steady case behavior of
 EGP.
 Within an AS the speed of convergence of the AS's IGP in the face of
 congestion is of far greater concern than the propagation speed of
 BGP, and indeed avoiding loss of packets carrying IGP, and a more
 aggressive transport is similarly of much greater importance for an
 IGP than for BGP.
 The issue of BGP convergence is of exaggerated importance to CA*Net
 since CA*Net carries no information about external routes in its IGP.
 CA*Net uses BGP to transfer external routes for use in computing
 internal routes through the CA*Net network.  The reason CA*Net does
 this has nothing to do with BGP. Under more ordinary circumstances an
 IGP carries external routing information for use in computing
 internal routes. CA*Net shows that BGP can work under extreme stress.
 However, it's results should not be taken as the norm since most
 networks will use BGP in a different (and less stressful)
 configuration, where information about external routes will be
 carried by an IGP.

9.5. Conclusion.

 The extensive operational experience with BGP showed that the only
 problem attributed to BGP was the slow convergence problem in
 presence of congestion.  We demonstrated that this problem has
 nothing to do with BGP in general, or with TCP as the BGP transport
 in particular, but is directly related to the way how packets that
 carry routing information are handled within a congested network. The
 document suggests possible ways of solving the problem.  We would
 like to point out that the issue of convergence in presence of
 congested network is important to all distributed routing protocol,
 and not just to BGP.  Therefore, we recommend that every routing
 protocol (whether it is intra-autonomous system or inter-autonomous
 system) should clearly specify how its behavior is affected by the

BGP Working Group [Page 8] RFC 1266 Experience with the BGP Protocol October 1991

 congestion in the networks, and what are the possible mechanisms to
 avoid the negative effect of congestion (if any).

10. Bibliography.

 [1] Hinden, B., "Internet Routing Protocol Standardization Criteria",
     RFC 1264, BBN, October 1991.
 [2] Rekhter, Y., and P. Gross, "Application of the Border Gateway
     Protocol in the Internet", RFC 1268, T.J. Watson Research Center,
     IBM Corp., ANS, October 1991.
 [3] Lougheed, K., and Y. Rekhter, "A Border Gateway Protocol 3 (BGP-
     3)", RFC 1267, cisco Systems, T.J. Watson Research Center, IBM
     Corp., October 1991.
 [4] Willis, S., and J. Burruss, "Definitions of Managed Objects for
     the Border Gateway Protocol (Version 3)", RFC 1269, Wellfleet
     Communications Inc., October 1991.

Security Considerations

 Security issues are discussed in section 6.

Author's Address

 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
 IETF BGP WG mailing list: iwg@rice.edu
 To be added: iwg-request@rice.edu

BGP Working Group [Page 9]

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