rfc1265.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

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 [e-mail].

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]