Network Working Group                                          G. Huston
Request for Comments: 3765                                       Telstra
Category: Informational                                       April 2004

NOPEER Community for Border Gateway Protocol (BGP)
Route Scope Control

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

Abstract

   This document describes the use of a scope control Border Gateway
   Protocol (BGP) community.  This well-known advisory transitive
   community allows an origin AS to specify the extent to which a
   specific route should be externally propagated.  In particular this
   community, NOPEER, allows an origin AS to specify that a route with
   this attribute need not be advertised across bilateral peer
   connections.

1. Introduction

   BGP today has a limited number of commonly defined mechanisms that
   allow a route to be propagated across some subset of the routing
   system.  The NOEXPORT community allows a BGP speaker to specify that
   redistribution should extend only to the neighbouring AS.  Providers
   commonly define a number of communities that allow their neighbours
   to specify how advertised routes should be re-advertised.  Current
   operational practice is that such communities are defined on as AS by
   AS basis, and while they allow an AS to influence the re-
   advertisement behaviour of routes passed from a neighbouring AS, they
   do not allow this scope definition ability to be passed in a
   transitive fashion to a remote AS.

   Advertisement scope specification is of most use in specifying the
   boundary conditions of route propagation.  The specification can take
   on a number of forms, including as AS transit hop count, a set of
   target ASs, the presence of a particular route object, or a
   particular characteristic of the inter-AS connection.

   There are a number of motivations for controlling the scope of
   advertisement of route prefixes, including support of limited transit
   services where advertisements are restricted to certain transit
   providers, and various forms of selective transit in a multi-homed
   environment.

   This memo does not attempt to address all such motivations of scope
   control, and addresses in particular the situation of both multi-
   homing and traffic engineering.  The commonly adopted operational
   technique is that the originating AS advertises an encompassing
   aggregate route to all multi-home neighbours, and also selectively
   advertises a collection of more specific routes.  This implements a
   form of destination-based traffic engineering with some level of fail
   over protection.  The more specific routes typically cease to lever
   any useful traffic engineering outcome beyond a certain radius of
   redistribution, and a means of advising that such routes need not to
   be distributed beyond such a point is of some value in moderating one
   of the factors of continued route table growth.

   Analysis of the BGP routing tables reveals a significant use of the
   technique of advertising more specific prefixes in addition to
   advertising a covering aggregate.  In an effort to ameliorate some of
   the effects of this practice, in terms of overall growth of the BGP
   routing tables in the Internet and the associated burden of global
   propagation of dynamic changes in the reachability of such more
   specific address prefixes, this memo describes the use of a
   transitive BGP route attribute that allows more specific route tables
   entries to be discarded from the BGP tables under appropriate
   conditions.  Specifically, this attribute, NOPEER, allows a remote AS
   not to advertise a route object to a neighbour AS when the two AS's
   are interconnected under the conditions of some form of sender keep
   all arrangement, as distinct from some form of provider / customer
   arrangement.

2. NOPEER Attribute

   This memo defines the use a new well-known bgp transitive community,
   NOPEER.

   The semantics of this attribute is to allow an AS to interpret the
   presence of this community as an advisory qualification to
   readvertisement of a route prefix, permitting an AS not to
   readvertise the route prefix to all external bilateral peer neighbour
   AS's.  It is consistent with these semantics that an AS may filter
   received prefixes that are received across a peering session that the
   receiver regards as a bilateral peer sessions.

3. Motivation

   The size of the BGP routing table has been increasing at an
   accelerating rate since late 1998.  At the time of publication of
   this memo the BGP forwarding table contains over 118,000 entries, and
   the three year growth rate of this table shows a trend rate which can
   be correlated to a compound growth rate of no less than 10% per year
   [2].

   One of the aspects of the current BGP routing table is the widespread
   use of the technique of advertising both an aggregate and a number of
   more specific address prefixes.  For example, the table may contain a
   routing entry for the prefix 10.0.0.0/23 and also contain entries for
   the prefixes 10.0.0.0/24 and 10.0.1.0/24.  In this example the
   specific routes fully cover the aggregate announcement.  Sparse
   coverage of aggregates with more specifics is also observed, where,
   for example, routing entries for 10.0.0.0/8 and 10.0.1.0/24 both
   exist in the routing table.  In total, these more specific route
   entries occupy some 51% of the routing table, so that more than one
   half of the routing table does not add additional address
   reachability information into the routing system, but instead is used
   to impose a finer level of detail on existing reachability
   information.

   There are a number of motivations for having both an aggregate route
   and a number of more specific routes in the routing table, including
   various forms of multi-homed configurations, where there is a
   requirement to specify a different reachability policy for a part of
   the advertised address space.

   One of the observed common requirements in the multi-homed network
   configuration is that of undertaking some form of load balancing of
   incoming traffic across a number of external connections to a number
   of different neighbouring ASs.  If, for example, an AS wishes to use
   a multi-homed configuration for routing-based load balancing and some
   form of mutual fail over between the multiple access connections for
   incoming traffic, then one approach is for the AS to advertise the
   same aggregate address prefix to a number of its upstream transit
   providers, and then advertise a number of more specifics to
   individual upstream providers.  In such a case all of the traffic
   destined to the more specific address prefixes will be received only
   over those connections where the more specific has been advertised.
   If the neighbour BGP peering session of the more specific
   advertisement fails, the more specific will cease to be announced and
   incoming traffic will then be passed to the originating network based
   on the path associated with the advertisement of the encompassing
   aggregate.  In this situation the more specific routes are not
   automatically subsumed by the presence of the aggregate at any remote

   AS.  Both the aggregate and the associated more specific routes are
   redistributed across the entire external BGP routing domain.  In many
   cases, particularly those associated with desire to undertake traffic
   engineering and service resilience, the more specific routes are
   redistributed well beyond the scope where there is any outcomes in
   terms of traffic differentiation.

   To the extent that remote analysis of BGP tables can observe this
   form of configuration, the number of entries in the BGP forwarding
   table where more specific entries share a common origin AS with their
   immediately enclosing aggregates comprise some 20% of the total
   number of FIB entries.  Using a slightly stricter criteria where the
   AS path of the more specific route matches the immediately enclosing
   aggregate, the number of more specific routes comprises some 14% of
   the number of FIB entries.

   One protocol mechanism that could be useful in this context is to
   allow the originator of an advertisement to state some additional
   qualification on the redistribution of the advertisement, allowing a
   remote AS to suppress further redistribution under some originator-
   specified criteria.

   The redistribution qualification condition can be specified either by
   enumeration or by classification.  Enumeration would encompass the
   use of a well-known transitive extended community to specify a list
   of remote AS's where further redistribution is not advised.  The
   weakness of this approach is that the originating AS would need to
   constantly revise this enumerated AS list to reflect the changes in
   inter-AS topology, as, otherwise, the more specific routes would leak
   beyond the intended redistribution scope.  An approach of
   classification allows an originating AS to specify the conditions
   where further redistribution is not advised without having to refer
   to the particular AS's where a match to such conditions are
   anticipated.

   The approach described here to specifying the redistribution boundary
   condition is one based on the type of bilateral inter-AS peering.
   Where one AS can be considered as a customer, and the other AS can be
   considered as a contracted agent of the customer, or provider, then
   the relationship is one where the provider, as an agent of the
   customer, carries the routes and associated policy associated with
   the routes.  Where neither AS can be considered as a customer of the
   other, then the relationship is one of bilateral peering, and neither
   AS can be considered as an agent of the other in redistributing
   policies associated with routes.  This latter arrangement is commonly
   referred to as a "sender keep all peer" relationship, or "peering".

   This peer boundary can be regarded as a logical point where the
   redistribution of additional reachability policy imposed by the
   origin AS on a route is no longer an imposed requirement.

   This approach allows an originator of a prefix to attach a commonly
   defined policy to a route prefix, indicate that a route should be
   re-advertised conditionally, based on the characteristics of the
   inter-AS connection.

4. IANA Considerations

   The IANA has registered NOPEER as a well-known community, as defined
   in [1], as having global significance.

      NOPEER (0xFFFFFF04)

   This is an advisory qualification to readvertisement of a route
   prefix, permitting an AS not to readvertise the route prefix to all
   external bilateral peer neighbour AS's.  It is consistent with these
   semantics that an AS may filter received prefixes that are received
   across a peering session that the receiver regards as a bilateral
   peer sessions

5. Security Considerations

   BGP is an instance of a relaying protocol, where route information is
   received, processed and forwarded.  BGP contains no specific
   mechanisms to prevent the unauthorized modification of the
   information by a forwarding agent, allowing routing information to be
   modified, deleted or false information to be inserted without the
   knowledge of the originator of the routing information or any of the
   recipients.

   The NOPEER community does not alter this overall situation concerning
   the integrity of BGP as a routing system.

   Use of the NOPEER community has the capability to introduce
   additional attack mechanisms into BGP by allowing the potential for
   man-in-the-middle, session-hijacking, or denial of service attacks
   for an address prefix range being launched by a remote AS.

   Unauthorized addition of this community to a route prefix by a
   transit provider where there is no covering aggregate route prefix
   may cause a denial of service attack based on denial of reachability
   to the prefix.  Even in the case that there is a covering aggregate,
   if the more specific route has a different origin AS than the

   aggregate, the addition of this community by a transit AS may cause a
   denial of service attack on the origin AS of the more specific
   prefix.

   BGP is already vulnerable to a denial of service attack based on the
   injection of false routing information.  It is possible to use this
   community to limit the redistribution of a false route entry such
   that its visibility can be limited and detection and rectification of
   the problem can be more difficult under the circumstances of limited
   redistribution.

6. References

6.1. Normative References

[1] Chandrasekeran, R., Traina, P. and T. Li, "BGP Communities
Attribute", RFC 1997, August 1996.

6.2. Informative References

[2] Huston, G., "Commentary on Inter-Domain Routing in the Internet",
RFC 3221, December 2001.

7. Author's Address

   Geoff Huston
   Telstra

   EMail: gih@telstra.net

8. Full Copyright Statement

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