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rfc:rfc1265

Network Working Group Y. Rekhter, Editor Request for Comments: 1265 T.J. Watson Research Center, IBM Corp.

                                                          October 1991
                       BGP Protocol Analysis

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 report is to document how the requirements for
 advancing a routing protocol to Draft Standard have been satisfied by
 the Border Gateway Protocol (BGP). This report summarizes the key
 feature of BGP, and analyzes the protocol with respect to scaling and
 performance. This is the first of two reports on the BGP protocol.
 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 [1]. The
 changes between versions 1, 2 and 3 are explained in Appendix 3 of
 [1].
 Possible applications of BGP in the Internet are documented in [2].
 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 Braden (ISI) and Bob Hinden (BBN) for the review of this document
 as well as their constructive and valuable comments.

4. Key features and algorithms of the BGP protocol.

 This section summarizes the key features and algorithms of the BGP
 protocol. BGP is an inter-autonomous system routing protocol; it is
 designed to be used between multiple autonomous systems. BGP assumes
 that routing within an autonomous system is done by an intra-
 autonomous system routing protocol. BGP does not make any assumptions

BGP Working Group [Page 1] RFC 1265 BGP Protocol Analysis October 1991

 about intra-autonomous system routing protocols employed by the
 various autonomous systems.  Specifically, BGP does not require all
 autonomous systems to run the same intra-autonomous system routing
 protocol.
 BGP is a real inter-autonomous system routing protocol. It imposes no
 constraints on the underlying Internet topology. The information
 exchanged via BGP is sufficient to construct a graph of autonomous
 systems connectivity from which routing loops may be pruned and some
 routing policy decisions at the autonomous system level may be
 enforced.
 The key feature of the protocol is the notion of Path Attributes.
 This feature provides BGP with flexibility and expandability. Path
 attributes are partitioned into well-known and optional. The
 provision for optional attributes allows experimentation that may
 involve a group of BGP routers without affecting the rest of the
 Internet.  New optional attributes can be added to the protocol in
 much the same fashion as new options are added to the Telnet
 protocol, for instance.  One of the most important path attributes is
 the AS-PATH. As reachability information traverses the Internet, this
 information is augmented by the list of autonomous systems that have
 been traversed thusfar, forming the AS-PATH.  The AS-PATH allows
 straightforward suppression of the looping of routing information. In
 addition, the AS-PATH serves as a powerful and versatile mechanism
 for policy-based routing.
 BGP uses an algorithm that cannot be classified as either a pure
 distance vector, or a pure link state. Carrying a complete AS path in
 the AS-PATH attribute allows to reconstruct large portions of the
 overall topology. That makes it similar to the link state algorithms.
 Exchanging only the currently used routes between the peers makes it
 similar to the distance vector algorithms.
 To conserve bandwidth and processing power, BGP uses incremental
 updates, where after the initial exchange of complete routing
 information, a pair of BGP routers exchanges only changes (deltas) to
 that information. Technique of incremental updates requires reliable
 transport between a pair of BGP routers. To achieve this
 functionality BGP uses TCP as its transport.
 BGP is a self-contained protocol. That is, it specifies how routing
 information is exchanged both between BGP speakers in different
 autonomous systems, and between BGP speakers within a single
 autonomous system.
 To allow graceful coexistence with EGP, BGP provides support for
 carrying EGP derived exterior routes. BGP also allows to carry

BGP Working Group [Page 2] RFC 1265 BGP Protocol Analysis October 1991

 statically defined exterior routes.

5. BGP performance characteristics and scalability.

 In this section we'll try to answer the question of how much link
 bandwidth, router memory and router CPU cycles does the BGP protocol
 consume under normal conditions.  We'll also address the scalability
 of BGP, and look at some of its limits.
 BGP does not require all the routers within an autonomous system to
 participate in the BGP protocol. Only the border routers that provide
 connectivity between the local autonomous system and its adjacent
 autonomous systems participate in BGP.  Constraining the set of
 participants is just one way of addressing the scaling issue.

5.1 Link bandwidth and CPU utilization.

 Immediately after the initial BGP connection setup, the peers
 exchange complete set of routing information. If we denote the total
 number of networks in the Internet by N, the mean AS distance of the
 Internet by M (distance at the level of an autonomous system,
 expressed in terms of the number of autonomous systems), the total
 number of autonomous systems in the Internet by A, and assume that
 the networks are uniformly distributed among the autonomous systems,
 then the worst case amount of bandwidth consumed during the initial
 exchange between a pair of BGP speakers is
                      O(N + M * A)
 (provided that an implementation supports multiple networks per
 message as outlined in Appendix 5 of [1]). This information is
 roughly on the order of the number of networks reachable via each
 peer (see also Section 5.2).
 The following table illustrates typical amount of bandwidth consumed
 during the initial exchange between a pair of BGP speakers based on
 the above assumptions (ignoring bandwidth consumed by the BGP
 Header).
       # Networks   Mean AS Distance       # AS's    Bandwidth
       ----------   ----------------       ------    ---------
       2,100        5                      59        9,000 bytes
       4,000        10                     100       18,000 bytes
       10,000       15                     300       49,000 bytes
       100,000      20                     3,000     520,000 bytes
 Note that most of the bandwidth is consumed by the exchange of the
 Network Reachability Information.

BGP Working Group [Page 3] RFC 1265 BGP Protocol Analysis October 1991

 After the initial exchange is completed, the amount of bandwidth and
 CPU cycles consumed by BGP depends only on the stability of the
 Internet. If the Internet is stable, then the only link bandwidth and
 router CPU cycles consumed by BGP are due to the exchange of the BGP
 KEEPALIVE messages. The KEEPALIVE messages are exchanged only between
 peers. The suggested frequency of the exchange is 30 seconds. The
 KEEPALIVE messages are quite short (19 octets), and require virtually
 no processing.  Therefore, the bandwidth consumed by the KEEPALIVE
 messages is about 5 bits/sec.  Operational experience confirms that
 the overhead (in terms of bandwidth and CPU) associated with the
 KEEPALIVE messages should be viewed as negligible.  If the Internet
 is unstable, then only the changes to the reachability information
 (that are caused by the instabilities) are passed between routers
 (via the UPDATE messages). If we denote the number of routing changes
 per second by C, then in the worst case the amount of bandwidth
 consumed by the BGP can be expressed as O(C * M). The greatest
 overhead per UPDATE message occurs when each UPDATE message contains
 only a single network. It should be pointed out that in practice
 routing changes exhibit strong locality with respect to the AS path.
 That is routes that change are likely to have common AS path. In this
 case multiple networks can be grouped into a single UPDATE message,
 thus significantly reducing the amount of bandwidth required (see
 also Appendix 5 of [1]).
 Since in the steady state the link bandwidth and router CPU cycles
 consumed by the BGP protocol are dependent only on the stability of
 the Internet, but are completely independent on the number of
 networks that compose the Internet, it follows that BGP should have
 no scaling problems in the areas of link bandwidth and router CPU
 utilization, as the Internet grows, provided that the overall
 stability of the inter-AS connectivity (connectivity between ASs) of
 the Internet can be controlled. Stability issue could be addressed by
 introducing some form of dampening (e.g., hold downs).  Due to the
 nature of BGP, such dampening should be viewed as a local to an
 autonomous system matter (see also Appendix 5 of [1]). We'd like to
 point out, that regardless of BGP, one should not underestimate the
 significance of the stability in the Internet. Growth of the Internet
 will make the stability issue one of the most crucial one. It is
 important to realize that BGP, by itself, does not introduce any
 instabilities in the Internet. Current observations in the NSFNET
 show that the instabilities are largely due to the ill-behaved
 routing within the autonomous systems that compose the Internet.
 Therefore, while providing BGP with mechanisms to address the
 stability issue, we feel that the right way to handle the issue is to
 address it at the root of the problem, and to come up with intra-
 autonomous routing schemes that exhibit reasonable stability.
 It also may be instructive to compare bandwidth and CPU requirements

BGP Working Group [Page 4] RFC 1265 BGP Protocol Analysis October 1991

 of BGP with EGP. While with BGP the complete information is exchanged
 only at the connection establishment time, with EGP the complete
 information is exchanged periodically (usually every 3 minutes). Note
 that both for BGP and for EGP the amount of information exchanged is
 roughly on the order of the networks reachable via a peer that sends
 the information (see also Section 5.2). Therefore, even if one
 assumes extreme instabilities of BGP, its worst case behavior will be
 the same as the steady state behavior of EGP.
 Operational experience with BGP showed that the incremental updates
 approach employed by BGP presents an enormous improvement both in the
 area of bandwidth and in the CPU utilization, as compared with
 complete periodic updates used by EGP (see also presentation by
 Dennis Ferguson at the Twentieth IETF, March 11-15, 1991, St.Louis).

5.2 Memory requirements.

 To quantify the worst case memory requirements for BGP, denote the
 total number of networks in the Internet by N, the mean AS distance
 of the Internet by M (distance at the level of an autonomous system,
 expressed in terms of the number of autonomous systems), the total
 number of autonomous systems in the Internet by A, and the total
 number of BGP speakers that a system is peering with by K (note that
 K will usually be dominated by the total number of the BGP speakers
 within a single autonomous system). Then the worst case memory
 requirements (MR) can be expressed as
                         MR = O((N + M * A) * K)
 In the current NSFNET Backbone (N = 2110, A = 59, and M = 5) if each
 network is stored as 4 octets, and each autonomous system is stored
 as 2 octets then the overhead of storing the AS path information (in
 addition to the full complement of exterior routes) is less than 7
 percent of the total memory usage.
 It is interesting to point out, that prior to the introduction of BGP
 in the NSFNET Backbone, memory requirements on the NSFNET Backbone
 routers running EGP were on the order of O(N * K). Therefore, the
 extra overhead in memory incurred by the NSFNET routers after the
 introduction of BGP is less than 7 percent.
 Since a mean AS distance grows very slowly with the total number of
 networks (there are about 60 autonomous systems, well over 2,000
 networks known in the NSFNET backbone routers, and the mean AS
 distance of the current Internet is well below 5), for all practical
 purposes the worst case router memory requirements are on the order
 of the total number of networks in the Internet times the number of
 peers the local system is peering with. We expect that the total

BGP Working Group [Page 5] RFC 1265 BGP Protocol Analysis October 1991

 number of networks in the Internet will grow much faster than the
 average number of peers per router. Therefore, scaling with respect
 to the memory requirements is going to be heavily dominated by the
 factor that is linearly proportional to the total number of networks
 in the Internet.
 The following table illustrates typical memory requirements of a
 router running BGP. It is assumed that each network is encoded as 4
 bytes, each AS is encoded as 2 bytes, and each networks is reachable
 via some fraction of all of the peers (# BGP peers/per net).

# Networks Mean AS Distance # AS's # BGP peers/per net Memory Req ———- —————- —— ——————- ———- 2,100 5 59 3 27,000 bytes 4,000 10 100 6 108,000 bytes 10,000 15 300 10 490,000 bytes 100,000 20 3,000 20 1,040,000 bytes

 To put memory requirements of BGP in a proper perspective, let's try
 to put aside for a moment the issue of what information is used to
 construct the forwarding tables in a router, and just focus on the
 forwarding tables themselves. In this case one might ask about the
 limits on these tables.  For instance, given that right now the
 forwarding tables in the NSFNET Backbone routers carry well over
 2,000 entries, one might ask whether it would be possible to have a
 functional router with a table that will have 20,000 entries. Clearly
 the answer to this question is completely independent of BGP. On the
 other hand the answer to the original questions (that was asked with
 respect to BGP) is directly related to the latter question. Very
 interesting comments were given by Paul Tsuchiya in his review of BGP
 in March of 1990 (as part of the BGP review committee appointed by
 Bob Hinden).  In the review he said that, "BGP does not scale well.
 This is not really the fault of BGP. It is the fault of the flat IP
 address space.  Given the flat IP address space, any routing protocol
 must carry network numbers in its updates." To reiterate, BGP limits
 with respect to the memory requirements are directly related to the
 underlying Internet Protocol (IP), and specifically the addressing
 scheme employed by IP. BGP would provide much better scaling in
 environments with more flexible addressing schemes.  It should be
 pointed out that with very minor additions BGP can be extended to
 support hierarchies of autonomous system. Such hierarchies, combined
 with an addressing scheme that would allow more flexible address
 aggregation capabilities, can be utilized by BGP, thus providing
 practically unlimited scaling capabilities of the protocol.

BGP Working Group [Page 6] RFC 1265 BGP Protocol Analysis October 1991

6. Applicability of BGP.

 In this section we'll try to answer the question of what environment
 is BGP well suited, and for what is it not suitable?  Partially this
 question is answered in the Section 2 of [1], where the document
 states the following:
 "To characterize the set of policy decisions that can be enforced
 using BGP, one must focus on the rule that an AS advertises to its
 neighbor ASs only those routes that it itself uses.  This rule
 reflects the "hop-by-hop" routing paradigm generally used throughout
 the current Internet.  Note that some policies cannot be supported by
 the "hop-by-hop" routing paradigm and thus require techniques such as
 source routing to enforce.  For example, BGP does not enable one AS
 to send traffic to a neighbor AS intending that the traffic take a
 different route from that taken by traffic originating in the
 neighbor AS.  On the other hand, BGP can support any policy
 conforming to the "hop-by-hop" routing paradigm.  Since the current
 Internet uses only the "hop-by-hop" routing paradigm and since BGP
 can support any policy that conforms to that paradigm, BGP is highly
 applicable as an inter-AS routing protocol for the current Internet."
 While BGP is well suitable for the current Internet, it is also
 almost a necessity for the current Internet as well.  Operational
 experience with EGP showed that it is highly inadequate for the
 current Internet.  Topological restrictions imposed by EGP are
 unjustifiable from the technical point of view, and unenforceable
 from the practical point of view.  Inability of EGP to efficiently
 handle information exchange between peers is a cause of severe
 routing instabilities in the operational Internet. Finally,
 information provided by BGP is well suitable for enforcing a variety
 of routing policies.
 Rather than trying to predict the future, and overload BGP with a
 variety of functions that may (or may not) be needed, the designers
 of BGP took a different approach. The protocol contains only the
 functionality that is essential, while at the same time provides
 flexible mechanisms within the protocol itself that allow to expand
 its functionality.  Since BGP was designed with flexibility and
 expandability in mind, we think it should be able to address new or
 evolving requirements with relative ease. The existence proof of this
 statement may be found in the way how new features (like repairing a
 partitioned autonomous system with BGP) are already introduced in the
 protocol.
 To summarize, BGP is well suitable as an inter-autonomous system
 routing protocol for the current Internet that is based on IP (RFC
 791) as the Internet Protocol and "hop-by-hop" routing paradigm. It

BGP Working Group [Page 7] RFC 1265 BGP Protocol Analysis October 1991

 is hard to speculate whether BGP will be suitable for other
 environments where internetting is done by other than IP protocols,
 or where the routing paradigm will be different.

References

 [1] 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.
 [2] Rekhter, Y., and P. Gross, Editors, "Application of the Border
     Gateway Protocol in the Internet", RFC 1268, T.J. Watson Research
     Center, IBM Corp., ANS, October 1991.

Security Considerations

 Security issues are not discussed in this memo.

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 8]

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