Network Working Group                                         T. Li, Ed.
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                        R. Fernando, Ed.
Expires: July 8, 2007                             Juniper Networks, Inc.
                                                           J. Abley, Ed.
                                                         January 4, 2007

                    The AS_PATHLIMIT Path Attribute

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

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   This document describes the 'AS path limit' (AS_PATHLIMIT) path
   attribute for BGP.  This is an optional, transitive path attribute
   that is designed to help limit the distribution of routing
   information in the Internet.

   By default, prefixes advertised into the BGP graph are distributed
   freely, and if not blocked by policy will propagate globally.  This
   is harmful to the scalability of the routing subsystem since
   information that only has a local effect on routing will cause state
   creation throughout the default-free zone.  This attribute can be
   attached to a particular path to limit its scope to a subset of the

Table of Contents

   1.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Inter-Domain Traffic Engineering . . . . . . . . . . . . . . .  5
     3.1.  Traffic Engineering on a Diet  . . . . . . . . . . . . . .  6
     3.2.  AS_PATHLIMIT as Control  . . . . . . . . . . . . . . . . .  7
   4.  Anycast Service Distribution . . . . . . . . . . . . . . . . .  8
   5.  The AS_PATHLIMIT Attribute . . . . . . . . . . . . . . . . . .  9
     5.1.  Operations . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.2.  Proxy Control  . . . . . . . . . . . . . . . . . . . . . . 10
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
   Intellectual Property and Copyright Statements . . . . . . . . . . 16

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1.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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2.  Introduction

   A prefix that is injected into BGP [RFC4271] will propagate
   throughout the graph of all BGP speakers unless it is explicitly
   blocked by policy configuration.  This behavior is necessary for the
   correct operation of BGP, but has some unfortunate interactions with
   current operational procedures.  Currently, it is beneficial in some
   cases to inject longer prefixes into BGP to control the flow of
   traffic headed towards a particular destination.  These longer
   prefixes may be advertised in addition to an aggregate, even when the
   aggregate advertisement is sufficient for basic reachability.  This
   particular application is known as "inter-domain traffic engineering"
   and is a well-known phenomenon that is contributing to growth in the
   size of the global routing table [RFC3221].  The mechanism proposed
   here allows the propagation of those longer prefixes to be limited,
   allowing some traffic engineering problems to be solved without such
   global implications.

   Another application of this mechanism is concerned with the
   distribution of services across the Internet using anycast.  Allowing
   an anycast address advertisement to be limited to a subset of ASes in
   the network can help control the scope of the anycast service area.

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3.  Inter-Domain Traffic Engineering

   To perform traffic engineering, a multi-homed site advertises its
   prefix to all of its neighbours and then also advertises more
   specific prefixes to a subset of its neighbours.  The longest match
   lookup algorithm then causes traffic for the more specific prefixes
   to prefer the subset of neighbours with the more specific prefix.

   Figure 1 shows an example of traffic engineering and its impact on
   the network.  The multi-homed site (A) has a primary provider (C) and
   a secondary provider (B).  It has a prefix, Y, that provides
   reachability to all of A, and advertises this to both B and C. In
   addition, due to the internal topology of end-site A, it wishes that
   all incoming traffic to subset X of its site enter through provider
   B. To accomplish this, A advertises the more specific prefix, X, to
   provider B. Longest match again causes traffic to prefer X over Y if
   the destination of the traffic is within X.

   Assuming that there are no policy boundaries involved, BGP will
   propagate both of these prefixes X and Y throughout the entire AS-
   level topology.  This includes distant providers such as H, F and G.
   Unfortunately, this adds to the amount of overhead in the routing
   subsystem.  The problem to be solved is to reduce this overhead and
   thereby improve the scalability of the routing of the Internet.

        ,--------------.   ,--------------.   ,--------------.
        |    Tier 2    +---+    Tier 2    +---+    Tier 3    |
        |  Provider H  |   |  Provider E  |   |  Provider F  |
        `--------------'   `-+---------+--+   `--------------'
                            /          |
                           /           |
       ,------------------+---.   ,----+---------.   ,-------------.
       |       Tier 1         +---+    Tier 1    |   |   Tier 1    |
       |  Primary Provider C  |   |  Provider D  +---+ Provider G  |
       `--------+-----------+-'   `-------+------'   `-------------'
                |            \            |
                |Y            \           |
       ,--------+------.     ,-+----------+-----------.
       |  Multi-homed  +-----+         Tier 2         |
       |     site A    |Y,X  |  Secondary Provider B  |
       `---------------'     `------------------------'

   The longer prefix X traverse a core and then coincides with the less-
   specific, covering prefix Y.

                                 Figure 1

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3.1.  Traffic Engineering on a Diet

   What is needed is one or more mechanisms that an AS can use to
   distribute its more specific routing information to a subset of the
   network that exceeds its immediate neighbouring ASes and yet is also
   significantly less than the global BGP graph.  The solution space for
   this is unbounded, as the limits that a source AS may wish to apply
   to its more specific routes could be a fairly complicated
   manifestation of its routing policies.  One can imagine a policy that
   restricts more specifics to ASes that only have prime AS numbers, for

   We already have one mechanism for performing this type of function.
   The BGP NO_EXPORT community string attribute [RFC1997] can be
   attached to more specific prefixes.  This will cause the more
   specifics not to be advertised past the immediate neighbouring AS.
   This is effective at helping to prevent more specific prefixes from
   becoming global, but it is extremely limited in that the more
   specific prefixes can only propagate to adjacent ASes.

   Some ASes have created a further mechanism wherein a prefix that is
   given a particular community will have NO_EXPORT attached to that
   prefix when the prefix is propagated to a specific AS.  This is not a
   generally deployed mechanism, but is used by some ASes as another
   means of scope control.

   Referring again to our example, A can advertise X with NO_EXPORT to
   provider B. However, this will cause provider B not to advertise X to
   the remainder of the network, and providers C, D, and G will not have
   the longer prefixes and will thus send all of A's traffic via
   provider C. This is not what A hoped to accomplish with advertising a
   longer prefix and demonstrates why this NO_EXPORT mechanism is not
   sufficiently flexible.

   Instead of attempting to provide an infinitely flexible and
   complicated mechanism for controlling the distribution of prefixes,
   we propose a single, coarse, scope control mechanism.  This coarse
   mechanism will provide a limited amount of control at a very low cost
   and address most of the evils associated with performing traffic
   engineering through route distribution.

   We observe that traffic engineering via longer prefixes is only
   effective when the longer prefixes have a different next hop from the
   less specific prefix.  Thus, past the point where the next hops
   become identical, the longer prefixes provide no value whatsoever.
   We also observe that most traffic ends up traversing a subset of the
   network operated by a relatively small number of large market-
   dominant providers, joined by settlement-free interconnects.  If one

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   looks one AS hop past this subset of the network, it is likely that
   the longer prefixes and the site aggregate are using the same next
   hop, and thus the longer prefixes have stopped providing value.

   We can see this clearly in our example.  Provider F sees that both
   prefix X and prefix Y will lead all traffic through provider E. There
   is no point in F carrying and propagating the more specific prefix X.
   Similarly, providers G and H need not carry prefix X.

3.2.  AS_PATHLIMIT as Control

   To accomplish this, we propose to add information that will limit the
   radius of propagation of more specific prefixes.  If we attach a
   count of the ASes that may be traversed by the more specific prefix,
   we gain much of the control that we hope to achieve.  We propose the
   creation of a new path attribute that will carry an upper bound on
   the number of ASes found the AS_PATH attribute.  This new path
   attribute will be called the 'path limit' or AS_PATHLIMIT.  For
   example, if prefix X is advertised with path limit 1, then only
   provider B has the information and we get an effect that is identical
   to NO_EXPORT.  If prefix X is advertised with path limit 2, then only
   B, C and D will carry it.  This is an interesting compromise as
   traffic for X will now flow consistently through provider B, as

   However, this is not identical to fully distributing X. Consider, for
   example that provider E in this circumstance will not receive prefix
   X and is likely to prefer provider C for all A destinations.  This
   causes traffic for X to flow from E to C to B. If provider E did have
   prefix X, it may choose to prefer provider D instead, resulting in a
   different path.  This second result can be achieved by increasing the
   path limit to 3, but this has the unfortunate effect that provider G
   would also receive prefix X.

   Thus, AS_PATHLIMIT is an extremely lightweight mechanism, and
   achieves a great deal of control.  It is easy to imagine more
   complicated control mechanisms, such IDRP [ISO.10747.1993]
   distribution lists, but we currently feel that the complexity of such
   a mechanism is simply not warranted.

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4.  Anycast Service Distribution

   A growing number of services are being distributed using anycast, by
   advertising a route which covers one or more addresses for a service
   which is provided autonomously at multiple locations.

   For some services, it is useful to restrict the peak possible service
   load, to avoid overloading local connectivity or service
   infrastructure capabilities; it may be a better failure mode for
   service to be retained only for a small community of surrounding
   networks than for a single node to fail under a global load of

   Although to some degree this policy can be accomplished through
   negotiation and judicious use of NO_EXPORT without AS_PATHLIMIT, the
   AS_PATHLIMIT attribute provides a more flexible and reliable

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5.  The AS_PATHLIMIT Attribute

   The AS_PATHLIMIT attribute is a transitive optional BGP path
   attribute, with Type Code 21.  The AS_PATHLIMIT attribute has a fixed
   length of 5 octets.  The first octet is an unsigned number that is
   the upper bound on the number of ASes in the AS_PATH attribute of the
   associated paths.  One octet suffices because the TTL field of the IP
   header ensures that only one octet's worth of ASes can ever be
   traversed.  The second thru fifth octets are the AS number of the AS
   that attached the AS_PATHLIMIT attribute to the NLRI.

5.1.  Operations

   A BGP speaker attaching the AS_PATHLIMIT attribute to an NLRI MUST
   encode its AS number in the second thru fifth octets.  The encoding
   is described in [I-D.ietf-idr-as4bytes].  This information is
   intended to aid debugging in the case where the AS_PATHLIMIT
   attribute is added by an AS other than the originator of the NLRI.

   A BGP speaker sending a route with an associated AS_PATHLIMIT
   attribute to an EBGP neighbour MUST examine the value of the
   attribute and the associated AS_PATH to be advertised.  If the number
   of ASes found in the AS_PATH exceeds the AS_PATHLIMIT value, then the
   route SHOULD NOT be sent.

   For the purposes of this attribute, private AS numbers [RFC1930] and
   confederation AS members [RFC3065] found in the AS_PATH are not
   counted.  AS numbers found within an AS_SET are not counted and an
   entire AS_SET is counted as a single AS.  Each instance of an AS
   number that appears multiple times in an AS_PATH is counted.

   If the AS_PATHLIMIT attribute is attached to a prefix by a private
   AS, then when the prefix is advertised outside of the parent AS, the
   AS number contained in the AS_PATHLIMIT attribute should be replaced
   by the AS number of the parent AS.

   Similarly, if the AS_PATHLIMIT attribute is attached to a prefix by a
   member of a confederation, then when the prefix is advertised outside
   of the confederation boundary, then the AS number of the
   confederation member inside of the AS_PATHLIMIT attribute should be
   replaced by the confederation's AS number.

   A BGP speaker receiving a route with an associated AS_PATHLIMIT
   attribute from an EBGP neighbour MUST examine the value of the
   attribute.  If the number of ASes in the AS_PATH exceeds the value of
   the AS_PATHLIMIT attribute, then the route MUST be ignored without
   further processing.

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   When a BGP speaker propagates a route with an associated AS_PATHLIMIT
   attribute, which it has learned from another BGP speaker's UPDATE
   message, it MUST NOT modify the route's AS_PATHLIMIT attribute.  It
   may remove the AS_PATHLIMIT in its entirety.  It may also attach a
   new AS_PATHLIMIT attribute that encodes its own AS number.

   To ensure loop prevention, BGP requires that all aggregate routes
   with AS paths that omit any AS number from the AS_PATHs being
   aggregated to be originated with the ATOMIC_AGGREGATE attribute.  To
   help ensure compliance with this, sites that choose to advertise the
   AS_PATHLIMIT path attribute SHOULD advertise the ATOMIC_AGGREGATE on
   all less specific covering prefixes as well as the more specific

5.2.  Proxy Control

   An AS may attach the AS_PATHLIMIT attribute to a route that it has
   received from another AS.  This is a form of proxy aggregation and
   may result in routing behaviors that the origin of the route did not
   intend.  Further, if the overlapping prefixes are not advertised with
   the ATOMIC_AGGREGATE attribute, adding the AS_PATHLIMIT attribute may
   cause defective implementations to advertise incorrect paths.  Before
   adding the AS_PATHLIMIT attribute an AS must carefully consider the
   risks and consequences outlined here.

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6.  Security Considerations

   This new BGP attribute creates no new security issues.  For it to be
   used, it must be attached to a BGP route.  If the router is forging a
   route, then this attribute limits the extent of the damage caused by
   the forgery.  This may be used by attackers to limit the scope and
   thus the visibility of their attacks.  Presently, the same approach
   can be applied with the use of the NO_EXPORT community, but just as
   the AS_PATHLIMIT attribute gives network operators more granularity
   in the distribution of prefixes, it also gives attackers more
   granularity in their attacks.  If a router fraudulently attaches the
   AS_PATHLIMIT attribute to a route, then it could have just as easily
   have used normal policy mechanisms to filter out the route
   completely.  Thus, the AS_PATHLIMIT attribute does not enable new
   attacks, but it does give an attacker the ability to create more
   subtle attacks that only affect a subset of the entire network.

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7.  IANA Considerations

   This document has no actions for IANA.  IANA has already allocated a
   code point for the AS_PATHLIMIT attribute under the Early IANA
   Allocation process.

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8.  Acknowledgements

   The editors would like to acknowledge that they are not the original
   initiators of this concept.  Over the years, many similar proposals
   have come our way, and we had hoped that self-discipline would cause
   this type of mechanism to be unnecessary.  We were overly optimistic.

   The names of those who originally proposed this are now lost to the
   mists of time.  This should rightfully be their document.  We would
   like to thank them for the opportunity to steward their concept to

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9.  References

9.1.  Normative References

              Vohra, Q. and E. Chen, "BGP Support for Four-octet AS
              Number Space", draft-ietf-idr-as4bytes-12 (work in
              progress), November 2005.

   [RFC1930]  Hawkinson, J. and T. Bates, "Guidelines for creation,
              selection, and registration of an Autonomous System (AS)",
              BCP 6, RFC 1930, March 1996.

   [RFC1997]  Chandrasekeran, R., Traina, P., and T. Li, "BGP
              Communities Attribute", RFC 1997, August 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3065]  Traina, P., McPherson, D., and J. Scudder, "Autonomous
              System Confederations for BGP", RFC 3065, February 2001.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

9.2.  Informative References

              International Organization for Standardization,
              "Information Processing Systems - Telecommunications and
              Information Exchange between Systems - Protocol for
              Exchange of Inter-domain Routeing Information among
              Intermediate Systems to Support Forwarding of ISO 8473
              PDUs", ISO Standard 10747, 1993.

   [RFC3221]  Huston, G., "Commentary on Inter-Domain Routing in the
              Internet", RFC 3221, December 2001.

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Authors' Addresses

   T. Li (editor)
   Cisco Systems, Inc.
   425 East Tasman Drive
   San Jose, CA  95134

   Phone: +1 408 525 1254

   R. Fernando (editor)
   Juniper Networks, Inc.
   1194 North Mathilda Avenue
   Sunnyvale, CA  94089-1206

   Phone: +1 888 586 4737

   J. Abley (editor)
   Afilias Canada, Inc.
   4141 Yonge Street, Suite 204
   Toronto, ON  M2P 2A8

   Phone: +1 416 673 4176

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Full Copyright Statement

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