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Network Working Group                                 Y. Rekhter, EditorRequest for Comments: 1266        T.J. Watson Research Center, IBM Corp.                                                            October 1991Experience with the BGP Protocol1. 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 inSection 3.0, as well as   Requirements for Draft Standard specified inSection 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 19914. Documentation.   BGP is an inter-autonomous system routing protocol designed for the   TCP/IP internets.  Version 1 of the BGP protocol was published inRFC1105. Since then BGP Versions 2 and 3 have been developed. Version 2   was documented inRFC 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 19916. 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:      - 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.      - "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.      - 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 ofRFC 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 theBGP 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:      - real versus perceived problems      - offer potential solutions to real problems      - perspective on the convergence problem      - 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 reliableBGP 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 dueBGP 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 theBGP 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 theBGP 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 insection 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.eduBGP Working Group                                               [Page 9]