Ptomaine                                                       G. Huston
Internet-Draft                                                   Telstra
Expires: November 11, 2003                                  May 13, 2003

               NOPEER community for BGP route scope control

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    Copyright (C) The Internet Society (2003). All Rights Reserved.


    This document describes the use of a scope control 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

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    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

    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

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2. NOPEER Attribute

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

    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

    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 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

    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

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    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 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

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    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 should register 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

    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

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    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


    Normative References:

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

    INformative References:

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

Author's Address

    Geoff Huston

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