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Inter-Domain Routing Working Group                           Paul Traina
INTERNET DRAFT                                             cisco Systems
<draft-ietf-idr-bgp4-op-experience-02.txt>                  May 13, 1996


             Operational Experience with the BGP-4 protocol

                          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.

   This document is an Internet Draft. Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
   and its Working Groups. Note that other groups may also distribute
   working documents as Internet Drafts.

   Internet Drafts are draft documents valid for a maximum of six
   months. Internet Drafts may be updated, replaced, or obsoleted by
   other documents at any time. It is not appropriate to use Internet
   Drafts as reference material or to cite them other than as a "working
   draft" or "work in progress".


Introduction

   The purpose of this memo is to document how the requirements for
   advancing a routing protocol to Full Standard have been satisfied by
   Border Gateway Protocol version 4 (BGP-4).  This report documents
   experience with BGP.  It is the second of two reports on the BGP
   protocol.

   The remaining sections of this memo document how BGP satisfies
   General Requirements specified in Section 3.0, as well as the
   Requirements for Full Standard as specified in Section 6.0 of the
   "Internet Routing Protocol Standardization Criteria" document [1].

   Please send comments to bgp@ans.net.


Documentation

   BGP is an inter-autonomous system routing protocol designed for
   TCP/IP networks.  Version 1 of the BGP protocol was published in RFC
   1105.  Since then BGP Versions 2, 3, and 4 have been developed.
   Version 2 was documented in RFC 1163. Version 3 is documented in RFC
   1267.  The changes between versions 1, 2 and 3 are explained in



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   Appendix 2 of [2].  All of the functionality that was present in the
   previous versions is present in version 4.

   BGP version 2 removed from the protocol the concept of "up", "down",
   and "horizontal" relations between autonomous systems that were
   present in version 1.  BGP version 2 introduced the concept of path
   attributes.  In addition, BGP version 2 clarified parts of the
   protocol that were "under-specified".

   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.

   BGP version 4 redefines the (previously class-based) network layer
   reachability portion of the updates to specify prefixes of arbitrary
   length in order to represent multiple classful networks in a single
   entry as discussed in [5].  BGP version 4 has also modified the AS-
   PATH attribute so that sets of autonomous systems, as well as
   individual ASs may be described.  In addition, BGP version 4 has re-
   described the INTER-AS METRIC attribute as the MULTI-EXIT
   DISCRIMINATOR and added new LOCAL-PREFERENCE and AGGREGATOR
   attributes.

   Possible applications of BGP in the Internet are documented in [3].

   The BGP protocol was developed by the IDR Working Group of the
   Internet Engineering Task Force.  This Working Group has a mailing
   list, bgp@ans.net, where discussions of protocol features and
   operation are held.  The IDR Working Group meets regularly during the
   quarterly Internet Engineering Task Force conferences.  Reports of
   these meetings are published in the IETF's Proceedings.


MIB

   A BGP-4 Management Information Base has been published [4].  The MIB
   was written by Steve Willis (Bay), John Burruss (Bay), and John Chu
   (IBM).

   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.




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Security Considerations

   BGP provides flexible and extendible mechanism for authentication and
   security.  The mechanism allows the support of 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 the carriage of an encrypted digital signature in every BGP
   message.  All authentication failures result in the sending of a
   NOTIFICATION message 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.

   However, since BGP runs over TCP and IP, BGP is vulnerable to the
   same denial of service or authentication attacks that are present in
   any other TCP based protocol.

   One method for improving the security of TCP connections for use with
   BGP has been documented in [7].


Operational experience

   This section discusses operational experience with BGP-4, which has
   involved the use of several independent implementations of BGP.

   BGP has been used in the Internet since 1989, BGP-4 since 1993.  This
   use has involved at least three independant implementations.
   Production use of BGP has included 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.

   This environment includes link bandwidths which vary from from 28
   Kbits/sec to 150 Mbits/sec.

   Routers which run BGP range from relatively low-performanced IBM
   PC/RTs to those equiped with high performance RISC based CPUs, and
   includes both the special purpose routers and the general purpose
   workstations running UNIX.

   Topologies in the production environment vary from the very sparse
   (e.g. the spanning tree of the ICM network) to quite dense (e.g.
   Sprintlink, Alternet, and MCI backbones).




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   At the time of this writing BGP-4 is used as an inter-autonomous
   system routing protocol between all significant autonomous systems,
   including, but by all means not limited to: Alternet, ANS, Ebone,
   ICM, IIJ, MCI, and Sprint.  The smallest know backbone consists of
   one BGP speaker, whereas the largest contains nearly 120 BGP
   speakers.  All together, there are several thousand known BGP
   speaking routers.

   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.  There is no
   distinction between sites historically considered backbones vs those
   considered "local" networks.

   Within most transit networks, BGP is used as the exclusive carrier of
   exterior routing information.  At the time of this writing, few sites
   propogate all exterior routing information into their interior
   routing protocols.

   The full set of exterior routes that is carried by BGP in the
   production Internet is well over 30,000 distinct classless prefixes
   representing several times that number of connected networks.

   Operational experience with BGP-4 has exercised all basic features of
   the protocol, including authentication, routing loop suppression and
   the new features of BGP-4: enhanced metrics and route aggregation.

   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 Twenty-
   first IETF, and are available from the IETF Proceedings.  These
   results showed clear superiority of BGP over EGP when protocol
   bandwidth consumption is compared.  Observations on the CA*Net by
   Dennis Ferguson, and on the T1 NSFNET Backbone by Susan Hares
   confirmed clear superiority of the BGP protocol family as compared
   with EGP in the area of CPU requirements.


Migration to BGP version 4

   On multiple occasions some members of IETF expressed concern about
   the migration path from classful protocols to classless protocols
   such as BGP-4.

   BGP-4 was rushed into production use on the Internet because of the
   exponential growth of routing tables and the increase of memory and
   CPU utilization required by BGP.  As such,  migration issues that
   normally would have stalled deployment were cast aside in favor of



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   pragmatic and intelligent deployment of BGP-4 by network operators.

   There was much discussion about creating "prefix exploders" which
   would enumerate individual class-based networks of CIDR allocations
   to BGP-3 speaking routers,  however a cursory examination showed that
   this would vastly hasten the requirement for more CPU and memory
   resources for these older implementations.  There would be no way
   internal to BGP to differentiate between known used destinations and
   the unused portions of advertised CIDR allocations.

   The migration path chosen by the operators was known as "CIDR,
   default, or die."

   To test BGP-4 operation, a virtual "shadow" Internet was created by
   linking Alternet, Ebone, ICM, and cisco over GRE based tunnels.
   Experimentation was done with actual live routing information by
   establishing BGP version 3 connections with the production networks
   at those sites.  This allowed extensive regression testing before
   deploying BGP-4 on production equipment.

   After testing using the shadow network, BGP-4 implementations were
   deployed on production transit networks at those sites.  BGP-4
   capable routers negotiated BGP-4 connections and inter-operated with
   other sites by speaking BGP-3.  Several test aggregate routes were
   injected into this network in addition to classful destinations for
   compatibility with BGP-3 speakers.

   At this point, the shadow-Internet was re-chartered as an
   "operational experience" network.  Tunnel connections were
   established with most major transit service operators so that
   operators could gain some understanding of how the introduction of
   aggregate destinations would affect routing.

   After being satisfied with the initial deployment of BGP-4,  a number
   of sites chose to withdraw their class-based advertisements and rely
   only on their CIDR aggregate advertisements.  This supplied
   motivation for transit providers who had not migrated to either do
   so, accept a default route, or lose connectivity to several popular
   destinations.

   Currently, BGP-4 is the default choice for carrying exterior routing
   information in the production Internet.









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Metrics

   BGP version 4 re-defined the INTER-AS metric as a MULTI-EXIT-
   DISCRIMINATOR.  This value may be used in the tie breaking process
   when selecting a preferred path to a given address space.  The "MED"
   is intended to be used only when comparing paths received from
   different external peers in the same AS to indicate the preference of
   the originating AS.  The MED was purposely designed to be a "weak"
   metric that would only be used late in the best-path decision
   process.

   The IDR working wanted to insure that any metric specified by a
   remote operator would only affect routing in a local AS if no other
   preference was specified.  A paramount goal of the design of the MED
   was insure that neighboring autonomous systems could not "shed" or
   "absorb" traffic for destinations that they advertise.

   The LOCAL-PREFERENCE attribute was added so a local operator could
   easily configure a policy that overrode the standard best path
   determination mechanism without requiring the manual configuration on
   every router in the AS.

   One shortcoming in the BGP-4 specification was a suggestion for a
   default value of LOCAL-PREFERENCE to be assumed if none was provided.
   Defaults of 0 or the maximum value each have range limitations, so a
   common default would have aided in the interoperation of different
   BGP implementations in the same AS (since LOCAL-PREFERENCE is a local
   administration knob, there is no interoperability drawback across AS
   boundaries).

   Another area where more exploration is required is a method whereby
   an originating or remote AS may influence the best path selection
   process.  For example, a dual-connected site may select one AS as a
   primary transit service provider and have one as a backup.

   In a topology where the multiple transit service providers connect to
   additional autonomous systems,  there is no formal mechanism for
   indicating a path selection preference should a remote autonomous
   system wish to respect that preference.

   In BGP implementations where the total length of the sequence
   portions of the AS path attribute may be used as part of the path
   selection criteria,  one practice in use today is to prepend
   additional copies of the originator's autonomous system number to the
   AS path.






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                             /--- transit A ---\
                            /                   \
                end-customer                     transit C----
                   109      \                   /
                             \--- transit B ---/

   Using the example above, if the "end customer" advertises routes
   originating in its autonomous system as having an AS path of "109" to
   transit A, and a path of "109 109" to transit B,  transit provider C
   may be influenced by the difference in AS sequence lengths and prefer
   the path via transit A.

   There has been some discussion of the creation of an optional
   transitive attribute which would represent a sequence of (AS,
   preference) entries to indicate a preference value for a given path.
   Cooperating ASs would chose traffic based upon comparison of
   "interesting" portions of this sequence according to local routing
   policy.

   Additional suggestions have been made suggesting a less flexible
   "destination provider selection" attribute to indicate desired
   preferences.

   While protecting a given autonomous system's routing policy is of
   paramount concern, avoiding extensive hand configuration of routing
   policies needs to be examined more carefully in future protocol
   varients.


Internal BGP in large autonomous systems

   While not strictly a protocol issue, one other concern has been
   raised by network operators who need to maintain autonomous systems
   with a large number of peers.  Each speaker peering with an external
   router is responsible for propagating reachability and path
   information to all other transit and border routers within that AS.
   This is typically done by establishing internal BGP connections to
   all transit and border routers in the local AS.

   This practice leads to an O(n^2) mesh of TCP connections and requires
   some method of configuring and maintaining those connections.  BGP
   does not regulate how this information is to be propagated,  so
   alternatives, such as injecting BGP attribute information into the
   local IGP have been suggested.  Also, internal BGP "route
   reflectors", and "autonomous system confederation" mechanisms have
   been implemented and demonstrate a significant improvement in
   configuration, memory and CPU requirements necessary to convey
   information to all other BGP peers in an autonomous system.



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Internet Dynamics

   As discussed in [7], the driving force in CPU and bandwidth
   utilization is the dynamic nature of routing in the Internet.  As the
   net has grown, the number of changes per second has increased.  We
   receive some level of damping when more specific reachability
   information is aggregated into larger blocks, however this isn't
   sufficient.

   At least one current implementation of BGP provides route update
   dampening that includes routing hysteresis.  This allows fast
   convergence for routes that flap relatively infrequently while
   suppressing instabilities caused by frequently flapping paths.
   Operational experience in the Internet shows that large-scale
   deployment of this dampening technique has proven to be highly
   beneficial for the stability of the routing system.


Acknowledgments

   The BGP-4 protocol has been developed by the IDR/BGP Working Group of
   the Internet Engineering Task Force.  I would like to express thanks
   to Yakov Rekhter for providing RFC 1266 from which this document is
   based.  I'd like to thank Yakov Rekhter, John Hawkinson, and Vince
   Fuller for the review of this document as well as constructive and
   valuable comments.  This report is based on the initial work of Peter
   Lothberg (STUPI), Andrew Partan (UUNET), and several others.


Author's  Address:

   Paul Traina
   cisco Systems, Inc.
   170 W. Tasman Dr.
   San Jose, CA 95134
   pst@cisco.com


References

[1] RFC1264
    Hinden, R., "Internet Routing Protocol Standardization Criteria",
    October 1991.

[2] draft-ietf-idr-bgp4-02.txt
    Rekhter, Y., and Li, T., "A Border Gateway Protocol 4 (BGP-4)",
    January 1996.




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[3] RFC1772
    Rekhter, Y., and P. Gross, Editors, "Application of the Border
    Gateway Protocol in the Internet", March 1995.

[4] RFC1657
    S. Willis, J. Burruss, J. Chu, "Definitions of Managed Objects for
    the Fourth Version of the Border Gateway Protocol (BGP-4) using
    SMIv2", July 1994.

[5] RFC1519
    Fuller V.; Li. T; Yu J.; Varadhan, K., "Classless Inter-Domain
    Routing (CIDR): an Address Assignment and Aggregation Strategy",
    September 1993.

[6] RFC1773
    Traina P., "Experience with the BGP-4 protocol." March 1995.

[7] RFC1774
    Traina P., "BGP Version 4 Protocol Analysis", March 1995.
































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