Skip to main content

Experience with the BGP-4 Protocol

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 4277.
Authors Danny R. McPherson , Keyur Patel
Last updated 2018-12-20 (Latest revision 2004-09-15)
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state (None)
Document shepherd (None)
IESG IESG state RFC 4277 (Informational)
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Alex D. Zinin
Send notices to,
INTERNET-DRAFT                               Danny McPherson
                                              Arbor Networks
                                                 Keyur Patel
                                               Cisco Systems
Category                                       Informational
Expires: March 2005                           September 2004

                   Experience with the BGP-4 Protocol

Status of this Document

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

   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-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This document is an individual submission. Comments are solicited and
   should be addressed to the author(s).

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.

McPherson, Patel                                                [Page 1]
INTERNET-DRAFT             Expires: March 2005            September 2004


   The purpose of this memo is to document how the requirements for
   advancing a routing protocol from Draft Standard to full Standard
   have been satisfied by Border Gateway Protocol version 4 (BGP-4).

   This report satisfies the requirement for "the second report", as
   described in Section 6.0 of RFC 1264.  In order to fulfill the
   requirement, this report augments RFC 1773 and describes additional
   knowledge and understanding gained in the time between when the
   protocol was made a Draft Standard and when it was submitted for

McPherson, Patel                                                [Page 2]
INTERNET-DRAFT             Expires: March 2005            September 2004

                           Table of Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .   4
   2. BGP-4 Overview . . . . . . . . . . . . . . . . . . . . . . . .   4
    2.1. A Border Gateway Protocol . . . . . . . . . . . . . . . . .   4
   3. Management Information Base (MIB). . . . . . . . . . . . . . .   5
   4. Implementation Information . . . . . . . . . . . . . . . . . .   5
   5. Operational Experience . . . . . . . . . . . . . . . . . . . .   5
   6. TCP Awareness. . . . . . . . . . . . . . . . . . . . . . . . .   6
   7. Metrics. . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
    7.1. MULTI_EXIT_DISC (MED) . . . . . . . . . . . . . . . . . . .   7
     7.1.1. MEDs and Potatoes. . . . . . . . . . . . . . . . . . . .   8
     7.1.2. Sending MEDs to BGP Peers. . . . . . . . . . . . . . . .   8
     7.1.3. MED of Zero Versus No MED. . . . . . . . . . . . . . . .   9
     7.1.4. MEDs and Temporal Route Selection. . . . . . . . . . . .   9
   8. Local Preference . . . . . . . . . . . . . . . . . . . . . . .   9
   9. Internal BGP In Large Autonomous Systems . . . . . . . . . . .  10
   10. Internet Dynamics . . . . . . . . . . . . . . . . . . . . . .  11
   11. BGP Routing Information Bases (RIBs). . . . . . . . . . . . .  12
   12. Update Packing. . . . . . . . . . . . . . . . . . . . . . . .  12
   13. Limit Rate Updates. . . . . . . . . . . . . . . . . . . . . .  13
    13.1. Consideration of TCP Characteristics . . . . . . . . . . .  14
   14. Ordering of Path Attributes . . . . . . . . . . . . . . . . .  14
   15. AS_SET Sorting. . . . . . . . . . . . . . . . . . . . . . . .  15
   16. Control over Version Negotiation. . . . . . . . . . . . . . .  15
   17. Security Considerations . . . . . . . . . . . . . . . . . . .  15
    17.1. TCP MD5 Signature Option . . . . . . . . . . . . . . . . .  16
    17.2. BGP Over IPSEC . . . . . . . . . . . . . . . . . . . . . .  16
    17.3. Miscellaneous. . . . . . . . . . . . . . . . . . . . . . .  17
   18. PTOMAINE and GROW . . . . . . . . . . . . . . . . . . . . . .  17
   19. Internet Routing Registries (IRRs). . . . . . . . . . . . . .  17
   20. Regional Internet Registries (RIRs) and IRRs, A
   Bit of History. . . . . . . . . . . . . . . . . . . . . . . . . .  18
   21. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . .  19
   22. References. . . . . . . . . . . . . . . . . . . . . . . . . .  20
    22.1. Normative References . . . . . . . . . . . . . . . . . . .  20
    22.2. Informative References . . . . . . . . . . . . . . . . . .  21
   23. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . .  21

McPherson, Patel                                                [Page 3]
INTERNET-DRAFT             Expires: March 2005            September 2004

1.  Introduction

   The purpose of this memo is to document how the requirements for
   advancing a routing protocol from Draft Standard to full Standard
   have been satisfied by Border Gateway Protocol version 4 (BGP-4).

   This report satisfies the requirement for "the second report", as
   described in Section 6.0 of RFC 1264.  In order to fulfill the
   requirement, this report augments RFC 1773 and describes additional
   knowledge and understanding gained in the time between when the
   protocol was made a Draft Standard and when it was submitted for

2.  BGP-4 Overview

   BGP is an inter-autonomous system routing protocol designed for
   TCP/IP internets.  The primary function of a BGP speaking system is
   to exchange network reachability information with other BGP systems.
   This network reachability information includes information on the
   list of Autonomous Systems (ASs) that reachability information
   traverses.  This information is sufficient to construct a graph of AS
   connectivity for this reachability from which routing loops may be
   pruned and some policy decisions at the AS level may be enforced.

   The initial version of the BGP protocol was published in RFC 1105.
   Since then BGP Versions 2, 3, and 4 have been developed and are
   specified in [RFC 1163], [RFC 1267], and [RFC 1771], respectively.
   Changes since BGP-4 went to Draft Standard [RFC 1771] are listed in
   Appendix N of [BGP4].

2.1.  A Border Gateway Protocol

   The Initial Version of BGP protocol was published in [RFC 1105].  BGP
   version 2 is defined in [RFC 1163].  BGP version 3 is defined in [RFC
   1267].  BGP version 4 is defined in [RFC 1771] and [BGP4].
   Appendices A, B, C, and D of [BGP4] provide summaries of the changes
   between each iteration of the BGP specification.

McPherson, Patel                                  Section 2.1.  [Page 4]
INTERNET-DRAFT             Expires: March 2005            September 2004

3.  Management Information Base (MIB)

   The BGP-4 Management Information Base (MIB) has been published [BGP-
   MIB].  The MIB was updated from previous versions documented in [RFC
   1657] and [RFC 1269], respectively.

   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.

4.  Implementation Information

   There are numerous independent interoperable implementations of BGP
   currently available.  Although the previous version of this report
   provided an overview of the implementations currently used in the
   operational Internet, at this time it has been suggested that a
   separate BGP Implementation Report [BGP-IMPL] be generated.

   It should be noted that implementation experience with Cisco's BGP-4
   implementation was documented as part of [RFC 1656].

   For all additional implementation information please reference [BGP-

5.  Operational Experience

   This section discusses operational experience with BGP and BGP-4.

   BGP has been used in the production environment since 1989, BGP-4
   since 1993.  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 Kbps to 10 Gbps.  In terms of the actual routers that
   run BGP, it ranges from a relatively slow performance general purpose
   CPUs to very high performance RISC network processors, and includes

McPherson, Patel                                    Section 5.  [Page 5]
INTERNET-DRAFT             Expires: March 2005            September 2004

   both special purpose routers and the general purpose workstations
   running various UNIX derivatives and other operating systems.

   In terms of the actual topologies it varies from very sparse to quite
   dense.  The requirement for full-mesh IBGP topologies has been
   largely remedied by BGP Route Reflection, Autonomous System
   Confederations for BGP, and often some mix of the two.  BGP Route
   Reflection was initially defined in [RFC 1966] and subsequently
   updated in [RFC 2796].  Autonomous System Confederations for BGP were
   initially defined in [RFC 1965] and subsequently updated in [RFC

   At the time of this writing BGP-4 is used as an inter-autonomous
   system routing protocol between all Internet-attached autonomous
   systems, with nearly 15k active autonomous systems in the global
   Internet routing table.

   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
   protocol distinction between sites historically considered
   "backbones" versus "regional" or "edge" networks.

   The full set of exterior routes that is carried by BGP is well over
   134,000 aggregate entries, representing several times that number of
   connected networks.  The number of active paths in some service
   provider core routers exceeds 2.5 million.  Native AS path lengths
   are as long as 10 for some routes, and "padded" path lengths of 25 or
   more autonomous systems exist.

6.  TCP Awareness

   BGP employs TCP [RFC 793] as it's Transport Layer protocol.  As such,
   all characteristics inherent to TCP are inherited by BGP.

   For example, due to TCP's behavior, bandwidth capabilities may not be
   realized due to TCP's slow start algorithms, and slow-start restarts
   of connections, etc..

7.  Metrics

   This section discusses different metrics used within the BGP

McPherson, Patel                                    Section 7.  [Page 6]
INTERNET-DRAFT             Expires: March 2005            September 2004

   protocol. BGP has a separate metric parameter for IBGP and EBGP. This
   allows policy based metrics to overwrite the distance based metrics;
   allowing each autonomous systems to define their independent policies
   in Intra-AS as well as Inter-AS. BGP Multi Exit Discriminator (MED)
   is used as a metric by EBGP peers (i.e., inter-domain) while Local
   Preference (LOCAL_PREF) is used by IBGP peers (i.e., intra-domain).


   BGP version 4 re-defined the old INTER-AS metric as a MULTI_EXIT_
   DISC (MED).  This value may be used in the tie-breaking process when
   selecting a preferred path to a given address space, and provides BGP
   speakers with the capability to convey to a peer AS the optimal entry
   point into the local AS.

   Although the MED was meant to only be used when comparing paths
   received from different external peers in the same AS, many
   implementations provide the capability to compare MEDs between
   different autonomous systems as well.

   Though this may seem a fine idea for some configurations, care must
   be taken when comparing MEDs between different autonomous systems.
   BGP speakers often derive MED values by obtaining the IGP metric
   associated with reaching a given BGP NEXT_HOP within the local AS.
   This allows MEDs to reasonably reflect IGP topologies when
   advertising routes to peers.  While this is fine when comparing MEDs
   between multiple paths learned from a single adjacent AS, it can
   result in potentially bad decisions when comparing MEDs between
   different automomous systems.  This is most typically the case when
   the autonomous systems use different mechanisms to derive IGP
   metrics, BGP MEDs, or perhaps even use different IGP procotols with
   vastly contrasting metric spaces.

   Another MED deployment consideration involves the impact of
   aggregation of BGP routing information on MEDs.  Aggregates are often
   generated from multiple locations in an AS in order to accommodate
   stability, redundancy and other network design goals.  When MEDs are
   derived from IGP metrics associated with said aggregates the MED
   value advertised to peers can result in very suboptimal routing.

   The MED was purposely designed to be a "weak" metric that would only
   be used late in the best-path decision process.  The BGP working
   group was concerned 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 to ensure

McPherson, Patel                                  Section 7.1.  [Page 7]
INTERNET-DRAFT             Expires: March 2005            September 2004

   that peers could not "shed" or "absorb" traffic for networks that
   they advertise.

7.1.1.  MEDs and Potatoes

   In a situation where traffic flows between a pair of destinations,
   each connected to two transit networks, each of the transit networks
   has the choice of either sending the traffic to the closest peering
   to other transit provider or passing traffic to the peering which
   advertises the least cost through the other provider.  The former
   method is called "hot potato routing" because like a hot potato held
   in bare hands, whoever has it tries to get rid of it quickly.  Hot
   potato routing is accomplished by not passing the EGBP learned MED
   into IBGP.  This minimizes transit traffic for the provider routing
   the traffic.  Far less common is "cold potato routing" where the
   transit provider uses their own transit capacity to get the traffic
   to the point in the adjacent transit provider advertised as being
   closest to the destination.  Cold potato routing is accomplished by
   passing the EBGP learned MED into IBGP.

   If one transit provider uses hot potato routing and another uses cold
   potato, traffic between the two tends to be symetric.  Depending on
   the business relationships, if one provider has more capacity or a
   significantly less congested transit network, then that provider may
   use cold potato routing.  An example of widespread use of cold potato
   routing was the NSF funded NSFNET backbone and NSF funded regional
   networks in the mid 1990s.

   In some cases a provider may use hot potato routing for some
   destinations for a given peer AS and cold potato routing for others.
   An example of this is the different treatment of commercial and
   research traffic in the NSFNET in the mid 1990s.  Then again, this
   might best be described as 'mashed potato routing', a term which
   reflects the complexity of router configurations in use at the time.

   Seemingly more intuitive references that fall outside the vegetable
   kingdom refer to cold potato routing as "best exit routing", and hot
   potato routing as "closest exit routing".

7.1.2.  Sending MEDs to BGP Peers

   [BGP4] allows MEDs received from any EBGP peers by a BGP speaker to

McPherson, Patel                                Section 7.1.2.  [Page 8]
INTERNET-DRAFT             Expires: March 2005            September 2004

   be passed to its IBGP peers.  Although advertising MEDs to IBGP peers
   is not a required behavior, it is a common default.  MEDs received
   from EBGP peers by a BGP speaker SHOULD NOT be sent to other EBGP

   Note that many implementations provide a mechanism to derive MED
   values from IGP metrics in order to allow BGP MED information to
   reflect the IGP topologies and metrics of the network when
   propagating information to adjacent autonomous systems.

7.1.3.  MED of Zero Versus No MED

   [BGP4] requires that an implementation must provide a mechanism that
   allows for MED to be removed.  Previously, implementations did not
   consider a missing MED value to be the same as a MED of zero.  [BGP4]
   now requires that no MED value be equal to a value of zero.

   Note that many implementations provide a mechanism to explicitly
   define a missing MED value as "worst" or less preferable than zero or
   larger values.

7.1.4.  MEDs and Temporal Route Selection

   Some implementations have hooks to apply temporal behavior in MED-
   based best path selection.  That is, all other things being equal up
   to MED consideration, preference would be applied to the "oldest"
   path, without preferring the lower MED value.  The reasoning for this
   is that "older" paths are presumably more stable, and thus more
   preferable.  However, temporal behavior in route selection results in
   non-deterministic behavior, and as such, may often be undesirable.

8.  Local Preference

   The LOCAL_PREF attribute was added so a network operator could easily
   configure a policy that overrode the standard best path determination
   mechanism without independently configuring local preference policy
   on each router.

   One shortcoming in the BGP-4 specification was a suggestion for a

McPherson, Patel                                    Section 8.  [Page 9]
INTERNET-DRAFT             Expires: March 2005            September 2004

   default value of LOCAL_PREF to be assumed if none was provided.
   Defaults of 0 or the maximum value each have range limitations, so a
   common default would aid in the interoperation of multi-vendor
   routers in the same AS (since LOCAL_PREF is a local administration
   attribute, there is no interoperability drawback across AS

   [BGP4] requires that LOCAL_PREF be sent to IBGP Peers and must not be
   sent to EBGP Peers.  Although no default value for LOCAL_PREF is
   defined, the common default value is 100.

   Another area where more exploration is required is a method whereby
   an originating 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.

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

   In a topology where the two transit service providers connect to a
   third provider,  the real decision is performed by the third provider
   and there is no mechanism for indicating a preference should the
   third provider wish to respect that preference.

   A general purpose suggestion that has been brought up is the
   possibility of carrying an optional vector corresponding to the AS_
   PATH where each transit AS may indicate a preference value for a
   given route.  Cooperating autonomous systems may then chose traffic
   based upon comparison of "interesting" portions of this vector
   according to routing policy.

   While protecting a given autonoumous systems routing policy is of
   paramount concern, avoiding extensive hand configuration of routing
   policies needs to be examined more carefully in future BGP-like

9.  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

McPherson, Patel                                   Section 9.  [Page 10]
INTERNET-DRAFT             Expires: March 2005            September 2004

   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.

   Note that the number of BGP peers that can be fully meshed depends on
   a number of factors, to include number of prefixes in the routing
   system, number of unique path, stability of the system, and perhaps
   most importantly, implementation efficiency.  As a result, although
   it's difficult to define "a large number of peers", there is always
   some practical limit.

   In a large AS, this leads to a full mesh of TCP connections (n *
   (n-1)) and some method of configuring and maintaining those
   connections.  BGP does not specify how this information is to be
   propagated, so alternatives, such as injecting BGP routing
   information into the local IGP have been attempted, though it turned
   out to be a non-practical alternative (to say the least).

   Several alternatives to a full mesh IBGP have been defined, to
   include BGP Route Reflection [RFC 2796] and AS Confederations for BGP
   [RFC 3065], in order to alleviate the the need for "full mesh" IBGP.

10.  Internet Dynamics

   As discussed in [BGP4-ANALYSIS], the driving force in CPU and
   bandwidth utilization is the dynamic nature of routing in the
   Internet.  As the Internet has grown, the frequency of route changes
   per second has increased.

   We automatically get some level of damping when more specific NLRI is
   aggregated into larger blocks, however, this isn't sufficient.  In
   Appendix F of [BGP4] are descriptions of damping techniques that
   should be applied to advertisements.  In future specifications of
   BGP-like protocols, damping methods should be considered for
   mandatory inclusion in compliant implementations.

   BGP Route Flap Damping is defined in [RFC 2439].  BGP Route Flap
   Damping defines a mechanism to help reduce the amount of routing
   information passed between BGP peers, and subsequently, the load on
   these peers, without adversely affecting route convergence time for
   relatively stable routes.

   None of the current implementations of BGP Route Flap Damping store
   route history by unique NRLI and AS Path although it is listed as
   mandatory in RFC 2439.  A potential result of failure to consider

McPherson, Patel                                  Section 10.  [Page 11]
INTERNET-DRAFT             Expires: March 2005            September 2004

   each AS Path separately is an overly aggressive suppression of
   destinations in a densely meshed network, with the most severe
   consequence being suppression of a destination after a single
   failure.  Because the top tier autonomous systems in the Internet are
   densely meshed, these adverse consequences are observed.

   Route changes are announced using BGP UPDATE messages. The greatest
   overhead in advertising UPDATE messages happens whenever route
   changes to be announced are inefficiently packed.  As discussed in a
   later section, announcing routing changes sharing common attributes
   in a single BGP UPDATE message helps save considerable bandwidth and
   lower processing overhead.

   Persistent BGP errors may cause BGP peers to flap persistently if
   peer dampening is not implemented. This would result in significant
   CPU utilization. Implementors may find it useful to implement peer
   dampening to avoid such persistent peer flapping [BGP4].

11.  BGP Routing Information Bases (RIBs)

   [BGP4] states "Any local policy which results in routes being added
   to an Adj-RIB-Out without also being added to the local BGP speaker's
   forwarding table, is outside the scope of this document".

   However, several well-known implementations do not confirm that Loc-
   RIB entries were used to populate the forwarding table before
   installing them in the Adj-RIB-Out.  The most common occurrence of
   this is when routes for a given prefix are presented by more than one
   protocol and the preferences for the BGP learned route is lower than
   that of another protocol.  As such, the route learned via the other
   protocol is used to populate the forwarding table.

   It may be desirable for an implementation to provide a knob that
   permits advertisement of "inactive" BGP routes.

   It may be also desirable for an implementation to provide a knob that
   allows a BGP speaker to advertise BGP routes that were not selected
   by decision process.

12.  Update Packing

   Multiple unfeasible routes can be advertised in a single BGP Update

McPherson, Patel                                  Section 12.  [Page 12]
INTERNET-DRAFT             Expires: March 2005            September 2004

   message.  In addition, one or more feasible routes can be advertised
   in a single Update message so long as all prefixes share a common
   attribute set.

   The BGP4 protocol permits advertisement of multiple prefixes with a
   common set of path attributes to be advertised in a single update
   message, this is commonly referred to as "update packing".  When
   possible, update packing is recommended as it provides a mechanism
   for more efficient behavior in a number of areas, to include:

    o Reduction in system overhead due to generation or receipt of
      fewer Update messages.

    o Reduction in network overhead as a result of less packets
      and lower bandwidth consumption.

    o Allows you to process path attributes and look for matching
      sets in your AS_PATH database (if you have one) less
      frequently.  Consistent ordering of the path attributes
      allows for ease of matching in the database as you don't have
      different representations of the same data.

   The BGP protocol suggests that withdrawal information should be
   packed in the begining of Update message, followed by information
   about more or less specific reachable routes in a single UPDATE
   message. This helps alleviate excessive route flapping in BGP.

13.  Limit Rate Updates

   The BGP protocol defines different mechanisms to rate limit Update
   advertisement. The BGP protocol defines MinRouteAdvertisementInterval
   parameter that determines the minimum time that must be elapse
   between the advertisement of routes to a particular destination from
   a single BGP speaker. This value is set on a per BGP peer basis.

   Due to the fact that BGP relies on TCP as the Transport protocol, TCP
   can prevent transmission of data due to empty windows.  As a result,
   multiple Updates may be spaced closer together than orginally queued.
   Although this is not a common occurrence, implementations should be
   aware of this.

McPherson, Patel                                  Section 13.  [Page 13]
INTERNET-DRAFT             Expires: March 2005            September 2004

13.1.  Consideration of TCP Characteristics

   If a TCP receiver is processing input more slowly than the sender or
   if the TCP connection rate is the limiting factor, a form of
   backpressure is observed by the TCP sending application.  When the
   TCP buffer fills, the sending application will either block on the
   write or receive an error on the write.  Common errors in either
   early implementations or an occasional naive new implementation are
   to either set options to block on the write or set options for non-
   blocking writes and then treat the errors due to a full buffer as

   Having recognized that full write buffers are to be expected
   additional implementation pitfalls exist.  The application should not
   attempt to store the TCP stream within the application itself.  If
   the receiver or the TCP connection is persistently slow, then the
   buffer can grow until memory is exhausted.  A BGP implementation is
   required to send changes to all peers for which the TCP connection is
   not blocked and is required to remember to send those changes to the
   remaining peers when the connection becomes unblocked.

   If the preferred route for a given NLRI changes multiple times while
   writes to one or more peers is blocked, only the most recent best
   route needs to be sent.  In this way BGP is work conserving.  In
   times of extremely high route change, a higher volume of route change
   is sent to those peers which are able to process it more quickly and
   a lower volume of route change is sent to those peers not able to
   process the changes as quickly.

   For implentations which handle differing peer capacity to absorb
   route change well, if the majority of route change is contributed by
   a subset of unstable NRLI, the only impact on relatively stable NRLI
   which make an isolated route change is a slower convergence for which
   convergence time remains bounded regardless of the amount of

14.  Ordering of Path Attributes

   The BGP protocol suggests that BGP speakers sending multiple prefixes
   per an UPDATE message should sort and order path attributes according
   to Type Codes. This would help their peers to quickly identify sets
   of attributes from different update messages which are semantically

McPherson, Patel                                  Section 14.  [Page 14]
INTERNET-DRAFT             Expires: March 2005            September 2004

   Implementers may find it useful to order path attributes according to
   Type Code so that sets of attributes with identical semantics can be
   more quickly identified.

15.  AS_SET Sorting

   AS_SETs are commonly used in BGP route aggregation. They reduce the
   size of AS_PATH information by listing AS numbers only once
   regardless of any number of times it might appear in process of
   aggregation. AS_SETs are usually sorted in increasing order to
   facilitate efficient lookups of AS numbers within them. This
   optimization is entirely optional.

16.  Control over Version Negotiation

   Because pre-BGP-4 route aggregation can't be supported by earlier
   version of BGP, an implementation that supports versions in addition
   to BGP-4 should provide the version support on a per-peer basis.  At
   the time of this writing all BGP speakers on the Internet are thought
   to be running BGP version 4.

17.  Security Considerations

   BGP a provides flexible and extendable mechanism for authentication
   and security.  The mechanism allows to support schemes with various
   degree of complexity.  BGP sessions are authenticated based on the IP
   address of a peer.  In addition, all BGP sessions are authenticated
   based on the autonomous system number advertised by a peer.

   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.

McPherson, Patel                                  Section 17.  [Page 15]
INTERNET-DRAFT             Expires: March 2005            September 2004

17.1.  TCP MD5 Signature Option

   [RFC 2385] defines a way in which the TCP MD5 signature option can be
   used to validate information transmitted between two peers.  This
   method prevents any third party from injecting information (e.g., a
   TCP Reset) into the datastream, or modifying the routing information
   carried between two BGP peers.

   TCP MD5 is not ubiquitously deployed at the moment, especially in
   inter- domain scenarios, largely because of key distribution issues.
   Most key distribution mechanisms are considered to be too "heavy" at
   this point.

   It was naively assumed by many for some time that in order to inject
   a data segement or reset a TCP transport connection between two BGP
   peers an attacker must correctly guess the exact TCP sequence number
   (of course, in addition to source and destination ports and IP
   addresses).  However, it has recently been observed and openly
   discussed that the malicous data only needs to fall within the TCP
   receive window, which may be quite large, thereby significantly
   lowering the complexity of such an attack.

   As such, it is recommended that the MD5 TCP Signature Option be
   employed to protect BGP from session resets and malicious data

17.2.  BGP Over IPSEC

   BGP can run over IPSEC, either in a tunnel, or in transport mode,
   where the TCP portion of the IP packet is encrypted.  This not only
   prevents random insertion of information into the data stream between
   two BGP peers, it also prevents an attacker from learning the data
   which is being exchanged between the peers.

   IPSEC does, however, offer several options for exchanging session
   keys, which may be useful on inter-domain configurations.  These
   options are being explored in many deployments, although no
   definitive solution has been reached on the issue of key exchange for
   BGP in IPSEC.

   It should be noted that 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.

McPherson, Patel                                Section 17.2.  [Page 16]
INTERNET-DRAFT             Expires: March 2005            September 2004

17.3.  Miscellaneous

   Another issue any routing protocol faces is providing evidence of the
   validity and authority of the routing information carried within the
   routing system.  This is currently the focus of several efforts at
   the moment, including efforts to define the threats which can be used
   against this routing information in BGP [draft-murphy, attack tree],
   and efforts at developing a means to provide validation and authority
   for routing information carried within BGP [SBGP] [soBGP].

   In addition, the Routing Protocol Security Requirements (RPSEC)
   working group has been chartered within the Routing Area of the IETF
   in order to discuss and assist in addressing issues surrounding
   routing protocol security.  It is the intent that this work within
   RPSEC will result in feedback to BGPv4 and future enhancements to the
   protocol where appropriate.


   The Prefix Taxonomy (PTOMAINE) working group, recently replaced by
   the Global Routing Operations (GROW) working group, is chartered to
   consider and measure the problem of routing table growth, the effects
   of the interactions between interior and exterior routing protocols,
   and the effect of address allocation policies and practices on the
   global routing system.  Finally, where appropriate, GROW will also
   document the operational aspects of measurement, policy, security and
   VPN infrastructures.

   One such item GROW is currently studying is the effects of route
   aggregation and the inability to aggregate over multiple provider
   boundaries due to inadequate provider coordination.

   It is the intent that this work within GROW will result in feedback
   to BGPv4 and future enhancements to the protocol as necessary.

19.  Internet Routing Registries (IRRs)

   Many organizations register their routing policy and prefix
   origination in the various distributed databases of the Internet
   Routing Registry.  These databases provide access to the information
   using the RPSL language as defined in [RFC 2622].  While registered

McPherson, Patel                                  Section 19.  [Page 17]
INTERNET-DRAFT             Expires: March 2005            September 2004

   information may be maintained and correct for certain providers, the
   lack of timely or correct data in the various IRR databases has
   prevented wide-spread use of this resource.

20.  Regional Internet Registries (RIRs) and IRRs, A Bit of History

   The NSFNET program used EGP and then BGP to provide external routing
   information.  It was the NSF policy of offering differing pricing and
   providing a different level of support to the Research and Education
   (RE) networks and the Commercial (CO) networks that led to BGP's
   initial policy requirements.  CO networks were not able to use the
   NSFNET backbone to reach other CO networks, in addition to being
   charged more.  The rationale was that commercial users of the NSFNET
   with business with research entities should subsidize the RE
   community.  Recognition that the Internet was evolving away from a
   hierarchical network to a mesh of peers led to changes from EGP and
   BGP-1 that eliminated any assumptions of hierarchy.

   Enforcement of NSF policy was accomplished through maintenance of the
   NSF Policy Routing Database (PRDB).  The PRDB not only contained each
   networks designation as CO or RE, but also contained a list of the
   preferred exit points to the NSFNET to reach each network.  This was
   the basis for setting what would later be called BGP LOCAL_PREF on
   the NSFNET.  Tools provided with the PRDB generated complete router
   configurations for the NSFNET.

   Use of the PRDB had the fortunate consequence of greatly improving
   reliability of the NSFNET relative to peer networks of the time and
   offering more optimal routing for those networks sufficiently
   knowledgeable and willing to keep their entries current.

   With the decommission of the NSFNET Backbone Network Service in 1995,
   it was recognized that the PRDB should be made less single provider
   centric and its legacy contents plus any further updates made
   available to any provider willing to make use of it.  The European
   networking community had long seen the PRDB as too US centric.
   Through Reseaux IP Europeens (RIPE) the Europeans had created an open
   format in RIPE-181 and had been maintaining an open database used for
   address and AS registry more than policy.  The initial conversion of
   the PRDB was to RIPE-181 format and tools were converted to make use
   of this format.  The collection of databases was termed the Internet
   Routing Registry, with the RIPE database and US NSF funded Routing
   Arbitrator (RA) being the inital components of the IRR.

   A need to extend RIPE-181 was recognized and RIPE agreed to allow the

McPherson, Patel                                  Section 20.  [Page 18]
INTERNET-DRAFT             Expires: March 2005            September 2004

   extensions to be defined within the IETF in the RPS WG.  The result
   was the RPSL language.  Other work products of the RPS WG provided an
   authentication framework and means to widely distribute the database
   in a controlled manner and synchronize the many repositories.  Freely
   available tools were provided primarily by RIPE, Merit, and ISI, the
   most comprehensive set from ISI.  The efforts of the IRR participants
   has been severely hampered by providers unwilling to keep information
   in the IRR up to date.  The larger of these providers have been
   vocal, claiming that the database entry, simple as it may be, are an
   administrative burden and some acknowledge that doing so provides a
   advantage to competitors that use the IRR.  The result has been an
   erosion of the usefulness of the IRR and an increase in vulnerability
   of the Internet to routing based attack or accidental injection of
   faulty routing information.

   There have been numerous cases of accidental disruption of Internet
   routing which were avoided by providers using the IRR but highly
   detrimental to non-users.  As filters have had to be relaxed due to
   the erosion of the IRR to less complete coverage, these types of
   disruptions have continued to occur very infrequently, but have had
   increasingly widespread impact.

21.  Acknowledgements

   We would like to thank Paul Traina and Yakov Rekhter for authoring
   previous versions of this document and providing valuable input on
   this update as well.  We would also like to explicitly acknowledge
   Curtis Villamizar for providing both text and thorough reviews.
   Thanks to Russ White, Jeffrey Haas, Sean Mentzer, Mitchell Erblich
   and Jude Ballard for supplying their usual keen eye.

   Finally, we'd like to think the IDR WG for general and specific input
   that contributed to this document.

McPherson, Patel                                  Section 21.  [Page 19]
INTERNET-DRAFT             Expires: March 2005            September 2004

22.  References

22.1.  Normative References

   [RFC 1519] Fuller, V., Li. T., Yu J., and K. Varadhan, "Classless
              Inter-Domain Routing (CIDR): an Address Assignment and
              Aggregation Strategy", RFC 1519, September 1993.

   [RFC 1966] Bates, T., Chandra, R., "BGP Route Reflection: An
              alternative to full mesh IBGP", RFC 1966, June 1996.

   [RFC 2385] Heffernan, A., "Protection of BGP Sessions via the TCP
              MD5 Signature Option", RFC 2385, August 1998.

   [RFC 2439] Villamizar, C. and Chandra, R., "BGP Route Flap Damping",
              RFC 2439, November 1998.

   [RFC 2796] Bates, T., Chandra, R., and Chen, E, "Route Reflection -
              An Alternative to Full Mesh IBGP", RFC 2796, April 2000.

   [RFC 3065] Traina, P., McPherson, D., and Scudder, J, "Autonomous
              System Confederations for BGP", RFC 3065, Febuary 2001.

   [RFC 3345] McPherson, D., Gill, V., Walton, D., and Retana, A, "BGP
              Persistent Route Oscillation Condition", RFC 3345,
              August 2002.

   [BGP4-ANALYSIS] "BGP-4 Protocol Analysis", Internet-Draft, Work in

   [BGP4-IMPL] "BGP 4 Implementation Report ", Internet-Draft, Work
              in Progress.

   [BGP4] Rekhter, Y., T. Li., and Hares. S, Editors, "A Border
          Gateway Protocol 4 (BGP-4)", BGP Draft, Work in Progress.

   [RFC 1657]  Willis, S., Burruss, J., Chu, J., " Definitions of
               Managed Objects for the Fourth Version of the Border
               Gateway Protocol (BGP-4) using SMIv2", RFC 1657, July

   [SBGP]  "Secure BGP", Internet-Draft, Work in Progress.

   [soBGP] "Secure Origin BGP", Internet-Draft, Work in Progress.

   [RFC 793] Postel, J., "Transmission Control Protocol", RFC 793,
             September 1981.

McPherson, Patel                                Section 22.1.  [Page 20]
INTERNET-DRAFT             Expires: March 2005            September 2004

22.2.  Informative References

   [RFC 1105] Lougheed, K., and Rekhter, Y, "Border Gateway Protocol
              BGP", RFC 1105, June 1989.

   [RFC 1163] Lougheed, K., and Rekhter, Y, "Border Gateway Protocol
              BGP", RFC 1105, June 1990.

   [RFC 1264] Hinden, R., "Internet Routing Protocol Standardization
              Criteria", RFC 1264, October 1991.

   [RFC 1267] Lougheed, K., and Rekhter, Y, "Border Gateway Protocol 3
              (BGP-3)", RFC 1105, October 1991.

   [RFC 1269] Willis, S., and Burruss, J., "Definitions of Managed
              Objects for the Border Gateway Protocol (Version 3)",
              RFC 1269, October 1991.

   [RFC 1656] Traina, P., "BGP-4 Protocol Document Roadmap and
              Implementation Experience", RFC 1656, July 1994.

   [RFC 1771] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4
              (BGP-4)", RFC 1771, March 1995.

   [RFC 1772] Rekhter, Y., and P. Gross, Editors, "Application of the
              Border Gateway Protocol in the Internet", RFC 1772, March

   [RFC 1773] Traina, P., "Experience with the BGP-4 protocol", RFC
              1773, March 1995.

   [RFC 2622] C. Alaettinoglu et al., "Routing Policy Specification
              Language", RFC 2622, June 1999.

23.  Authors' Addresses

McPherson, Patel                                  Section 23.  [Page 21]
INTERNET-DRAFT             Expires: March 2005            September 2004

   Danny McPherson
   Arbor Networks

   Keyur Patel
   Cisco Systems

Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at

Disclaimer of Validity

   This document and the information contained herein are provided on an

Copyright Statement

   Copyright (C) The Internet Society (2004). This document

McPherson, Patel                                  Section 23.  [Page 22]
INTERNET-DRAFT             Expires: March 2005            September 2004

   is subject to the rights, licenses and restrictions contained in
   BCP 78, and except as set forth therein, the authors retain all
   their rights.


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

McPherson, Patel                                  Section 23.  [Page 23]