Network Working Group                                         T. Li, Ed.
Internet-Draft                                    Portola Networks, Inc.
Expires: June 25, 2006                                  R. Fernando, Ed.
                                                            Amoora, Inc.
                                                           J. Abley, Ed.
                                             Internet Systems Consortium
                                                       December 22, 2005

                     The AS_HOPCOUNT Path Attribute

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document describes the AS hopcount 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 mesh are distributed

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   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_HOPCOUNT as Control . . . . . . . . . . . . . . . . . .  7
     3.3.  AS_HOPCOUNT and NO_EXPORT  . . . . . . . . . . . . . . . .  7
   4.  Anycast Service Distribution . . . . . . . . . . . . . . . . .  9
   5.  The AS_HOPCOUNT Attribute  . . . . . . . . . . . . . . . . . . 10
     5.1.  Operations . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.2.  Proxy Control  . . . . . . . . . . . . . . . . . . . . . . 11
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   9.  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 [RFC1771] will propagate
   throughout the mesh 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 neighbors and then also advertises more specific
   prefixes to a subset of its neighbors.  The longest match lookup
   algorithm then causes traffic for the more specific prefixes to
   prefer the subset of neighbors with the more specific.

   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 A and X 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 neighboring ASes and yet is also
   significantly less than the global BGP mesh.  The solution space for
   this is fully 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 neighboring 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.

   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 control mechanism.  This coarse mechanism
   will provide a limited amount of control but 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
   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

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   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_HOPCOUNT 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.  For example, if
   prefix X is advertised with hopcount 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 hopcount 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 desired.

   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
   hopcount to 3, but this has the unfortunate effect that provider G
   would also receive prefix X.

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


   Further control can be achieved by considering the implications of
   using both AS_HOPCOUNT and NO_EXPORT simultaneously.  Since NO_EXPORT
   is widely deployed, understood by almost all implementations, and
   since AS_HOPCOUNT is not deployed, we can make use of the overlap in
   their semantics to provide a powerful transition mechanism.

   Systems that receive NLRI with only the AS_HOPCOUNT attribute but
   which do not implement AS_HOPCOUNT will ignore the attribute.  This
   will provide the current, existing behavior and the NLRI will
   propagate according to normal BGP rules.

   Systems that receive NLRI with both an AS_HOPCOUNT and NO_EXPORT and
   which do implement AS_HOPCOUNT will ignore the NO_EXPORT community
   and propagate the NLRI.

   Systems that receive NLRI with both an AS_HOPCOUNT and NO_EXPORT but

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   which do not implement AS_HOPCOUNT will recognize and operate
   according to NO_EXPORT semantics.  This will cause them not to
   forward the NLRI to other ASes.

   Thus, an AS that chooses to attach the AS_HOPCOUNT attribute can
   control how their NLRI will be processed by other ASes.  If the NLRI
   should be dropped by ASes that do not support AS_HOPCOUNT, then
   NO_EXPORT can be attached.  If the NLRI should propagate by default,
   then NO_EXPORT should not be attached.

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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_HOPCOUNT, the
   AS_HOPCOUNT attribute provides a more flexible and reliable

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5.  The AS_HOPCOUNT Attribute

   The AS_HOPCOUNT attribute is a transitive optional BGP path
   attribute, with Type Code XXXX.  The AS_HOPCOUNT attribute has a
   fixed length of 5 octets.  The first octet is an unsigned number that
   is the hopcount of the associated paths.  The second thru fifth
   octets are the AS number of the AS that attached the AS_HOPCOUNT
   attribute to the NLRI.

5.1.  Operations

   A BGP speaker attaching the AS_HOPCOUNT attribute to an NLRI MUST
   encode its AS number in the second thru fifth octets.  The encoding
   is described in [4B AS].  This information is intended to aid
   debugging in the case where the AS_HOPCOUNT attribute is added by an
   AS other than the originator of the NLRI.

   A BGP speaker receiving a route with an associated AS_HOPCOUNT
   attribute from an EBGP neighbor MUST examine the value of the
   attribute.  If the attribute value is zero, the path MUST be ignored
   without further processing.  If the attribute value is non-zero, then
   the BGP speaker MAY process the path.

   When a BGP speaker propagates a route with an associated AS_HOPCOUNT
   attribute, which it has learned from another BGP speaker's UPDATE
   message, it MUST modify the route's AS_HOPCOUNT attribute based on
   the location of the BGP speaker to which the route will be sent:

   a.  When a given BGP speaker advertises the route to an internal
       peer, the advertising speaker SHALL NOT modify the AS_HOPCOUNT
       attribute associated with the route.

   b.  If the BGP speaker chooses to advertise the route to an external
       peer, then the BGP speaker MUST advertise an AS_HOPCOUNT
       attribute of one less than the value received.

   In the context of a confederation [RFC3065], all peers outside of the
   BGP speaker's Member-AS are considered external peers.

   If a BGP speaker receives a route with both the AS_HOPCOUNT attribute
   and the NO_EXPORT community string attribute, then the normal
   semantics of NO_EXPORT do not apply and the route should be processed
   as if NO_EXPORT was not present.

   BGP requires that a BGP speaker that advertises a less specific
   prefix, but not a more specific prefix that it is using, must
   advertise the less specific prefix with the ATOMIC_AGGREGATE
   attribute.  BGP speakers that do not advertise a more specific prefix

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   based on the AS_HOPCOUNT must comply with this rule and advertise the
   less specific prefixes with the ATOMIC_AGGREGATE attribute.  To help
   ensure compliance with this, sites that choose to advertise the
   AS_HOPCOUNT path attribute should advertise the ATOMIC_AGGREGATE
   attribute on all less specific covering prefixes.

5.2.  Proxy Control

   An AS may attach the AS_HOPCOUNT attribute to a path that it has
   received from another system.  This is a form of proxy aggregation
   and may result in routing behaviors that the origin of the path did
   not intend.  Further, if the overlapping prefixes are not advertised
   with the ATOMIC_AGGREGATE attribute, adding the AS_HOPCOUNT attribute
   may cause defective implementations to advertise incorrect paths.
   Before adding the AS_HOPCOUNT 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.  If a router attaches this attribute to a route, then it
   could have just as easily have used normal policy mechanisms to
   filter out the route.

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7.  IANA Considerations

   IANA is hereby requested to allocate a code point from the BGP path
   attribute Type Code space for the AS_HOPCOUNT path attribute.  Please
   replace 'XXXX' in the text above with the newly allocated code point

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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

9.  References

   [4B AS]    Vohra, Q. and E. Chen, "BGP support for Four-octet AS
              Number Space", Sept. 2005, <

   [IDRP]     ISO/IEC, "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", IS 10747, 1993, <http://

   [RFC1771]  Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
              (BGP-4)", RFC 1771, March 1995.

   [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.

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

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Authors' Addresses

   T. Li (editor)
   Portola Networks, Inc.


   R. Fernando (editor)
   Amoora, Inc.
   1463 Cedarmeadow Ct.
   San Jose, CA  95131


   J. Abley (editor)
   Internet Systems Consortium
   950 Charter Street
   Redwood City, CA  94023

   Phone: +1 650 423 1317

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