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.


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:  [phone]
   EMail: [e-mail]

   IETF BGP WG mailing list: [e-mail]
   To be added: [e-mail]





















BGP Working Group                                               [Page 8]