Internet Engineering Task Force                             Geoff Huston
Internet Draft                                                   Telstra
Document: draft-ietf-ptomaine-nopeer-01.txt draft-ietf-ptomaine-nopeer-02.txt                February 2003
Status: proposed as Informational                   Expires: August 2003

               NOPEER community for BGP route scope control

Status of this Memo

     This document is an Internet-Draft and is in full conformance with
     all provisions of Section 10 of RFC2026 [1].

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     This document proposes describes the use of a scope control BGP community.
     This proposed well-known advisory transitive community is intended
    to allow allows an origin AS to
     specify the extent to which a specific route should be externally
     propagated. In particular this community,
    termed here as 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 proposal memo does not attempt to address all such motivations of scope
     control, and addresses in particular the situation of both
    multi-homing 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 draft proposes memo describes the use of a
     transitive BGP route attribute that is intended to allow 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

2. Proposal
    The proposal is to define NOPEER Attribute

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

     The intended semantics of this attribute is to allow an AS to interpret the
     presence of this community as an advisory qualification to re advertisement
     readvertisement of a route prefix, permitting an AS not to re advertise
     readvertise the route prefix to all external bilateral peer
     neighbour AS's. It is consistent with the intended 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 writing (April
    2002) publication of
     this memo (February 2003) the BGP forwarding table contains over 100,000
     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 40% 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 and also
     contain entries for the prefixes and 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
     and both exist in the routing table. In total, these
     more specific route entries occupy some 52% 51% of the routing table[3],
     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 specifics 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

     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 13% 14% of the number of FIB entries [3].

     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

     The approach proposed 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

     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

    Adoption of this proposal would imply the request to

     The IANA for the
    registration of should register NOPEER as a new BGP well-known transitive
     community field from
    IANA. field.

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

    This proposed

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

    This proposal

     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

     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.


     [1] "The Internet Standards Process -- Revision 3", S. Bradner, RFC
         2026, October 1996.

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

     [3] Analysis of BGP table data -

Author's Address

         Geoff Huston