Network Working Group                                           Q. Vohra
Request for Comments: 4893                              Juniper Networks
Category: Standards Track                                        E. Chen
                                                          Cisco Systems
                                                               May 2007


              BGP Support for Four-octet AS Number Space

Status of This Memo

  This document specifies an Internet standards track protocol for the
  Internet community, and requests discussion and suggestions for
  improvements.  Please refer to the current edition of the "Internet
  Official Protocol Standards" (STD 1) for the standardization state
  and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

  Copyright (C) The IETF Trust (2007).

Abstract

  Currently the Autonomous System (AS) number is encoded as a two-octet
  entity in BGP. This document describes extensions to BGP to carry the
  Autonomous System number as a four-octet entity.

1.  Introduction

  Currently the Autonomous System number is encoded as a two-octet
  entity in BGP [BGP].  To prepare for the anticipated exhaustion of
  the two-octet AS numbers, this document describes extensions to BGP
  to carry the Autonomous System number as a four-octet entity.

  More specifically, this document defines a new BGP capability, Four-
  octet AS Number Capability, that can be used by a BGP speaker to
  indicate its support for the four-octet AS numbers.  Two new
  attributes, AS4_PATH and AS4_AGGREGATOR, are introduced that can be
  used to propagate four-octet based AS path information across BGP
  speakers that do not support the four-octet AS numbers.  This
  document also specifies mechanisms for constructing the AS path
  information from the AS_PATH attribute and the AS4_PATH attribute.

  The extensions proposed in this document allow a gradual transition
  from 2-octet AS numbers to 4-octet AS numbers.






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RFC 4893       BGP Support for Four-octet AS Number Space       May 2007


2.  Specification of Requirements

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
  document are to be interpreted as described in [RFC2119].

3.  Protocol Extensions

  For the purpose of this document we define a BGP speaker that does
  not support the new 4-octet AS number extensions as an OLD BGP
  speaker, and a BGP speaker which supports the new 4-octet AS number
  extensions as a NEW BGP speaker.

  BGP carries the Autonomous System number in the "My Autonomous
  System" field of the OPEN message, in the AS_PATH attribute of the
  UPDATE message, and in the AGGREGATOR attribute of the UPDATE
  message.  BGP also carries the Autonomous System number in the BGP
  Communities attribute.

  A NEW BGP speaker uses BGP Capability Advertisements [RFC2842] to
  advertise to its neighbors (either internal or external) that it
  supports 4-octet AS number extensions, as specified in this document.

  The Capability that is used by a BGP speaker to convey to its BGP
  peer the 4-octet Autonomous System number capability, also carries
  the 4-octet Autonomous System number of the speaker in the Capability
  Value field of the Capability Optional Parameter.  The Capability
  Length field of the Capability is set to 4.

  NEW BGP speakers carry AS path information expressed in terms of 4-
  octet Autonomous Systems numbers by using the existing AS_PATH
  attribute, except that each AS number in this attribute is encoded
  not as a 2-octet, but as a 4-octet entity.  The same applies to the
  AGGREGATOR attribute - NEW BGP speakers use the same attribute,
  except that the AS carried in this attribute is encoded as a 4-octet
  entity.

  To preserve AS path information with 4-octet AS numbers across OLD
  BGP speakers, this document defines a new AS path attribute, called
  AS4_PATH.  This is an optional transitive attribute that contains the
  AS path encoded with 4-octet AS numbers.  The AS4_PATH attribute has
  the same semantics as the AS_PATH attribute, except that it is
  optional transitive, and it carries 4-octet AS numbers.

  To prevent the possible propagation of confederation path segments
  outside of a confederation, the path segment types AS_CONFED_SEQUENCE
  and AS_CONFED_SET [RFC3065] are declared invalid for the AS4_PATH
  attribute.



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RFC 4893       BGP Support for Four-octet AS Number Space       May 2007


  Similarly, this document defines a new aggregator attribute called
  AS4_AGGREGATOR, which is optional transitive.  The AS4_AGGREGATOR
  attribute has the same semantics as the AGGREGATOR attribute, except
  that it carries a 4-octet AS number.

  Currently assigned 2-octet Autonomous System numbers are converted
  into 4-octet Autonomous System numbers by setting the two high-order
  octets of the 4-octet field to zero.  Such a 4-octet AS number is
  said to be mappable to a 2-octet AS number.

  To represent 4-octet AS numbers (which are not mapped from 2-octets)
  as 2-octet AS numbers in the AS path information encoded with 2-octet
  AS numbers, this document reserves a 2-octet AS number.  We denote
  this special AS number as AS_TRANS for ease of description in the
  rest of this specification.  This AS number is also placed in the "My
  Autonomous System" field of the OPEN message originated by a NEW BGP
  speaker, if the speaker does not have a (globally unique) 2-octet AS
  number.

4.  Operations

4.1.  Interaction Between NEW BGP Speakers

  A BGP speaker that supports 4-octet Autonomous System numbers SHOULD
  advertise this to its peers using the BGP Capability Advertisements.
  A BGP speaker that advertises such capability to a particular peer,
  and receives from that peer the advertisement of such capability MUST
  encode Autonomous System numbers as 4-octet entities in both the
  AS_PATH and the AGGREGATOR attributes in the updates it sends to the
  peer, and MUST assume that these attributes in the updates received
  from the peer encode Autonomous System numbers as 4-octet entities.

  The new attributes, AS4_PATH and AS4_AGGREGATOR SHOULD NOT be carried
  in the UPDATE messages between NEW BGP peers.  A NEW BGP speaker that
  receives the AS4_PATH and AS4_AGGREGATOR path attributes in an UPDATE
  message from a NEW BGP speaker SHOULD discard these path attributes
  and continue processing the UPDATE message.

4.2.  Interaction Between NEW and OLD BGP Speakers

4.2.1.  BGP Peering

  Note that peering between a NEW BGP speaker and an OLD one is
  possible only if the NEW BGP speaker has a 2-octet AS number.
  However, this document does not assume that an Autonomous System with
  NEW speakers has to have a globally unique 2-octet AS number --
  AS_TRANS could be used instead (even if a multiple Autonomous System
  would use it).



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RFC 4893       BGP Support for Four-octet AS Number Space       May 2007


4.2.2.  Generating Updates

  When communicating with an OLD BGP speaker, a NEW speaker MUST send
  the AS path information in the AS_PATH attribute encoded with 2-octet
  AS numbers.  The NEW speaker MUST also send the AS path information
  in the AS4_PATH attribute (encoded with 4-octet AS numbers), except
  for the case where the entire AS path information is composed of 2-
  octet AS numbers only.  In this case, the NEW speaker SHOULD NOT send
  the AS4_PATH attribute.

  In the AS_PATH attribute encoded with 2-octet AS numbers, non-
  mappable 4-octet AS numbers are represented by the well-known 2-octet
  AS number, AS_TRANS.  This will preserve the path length property of
  the AS path information and also help in updating the AS path
  information received on a NEW BGP speaker from an OLD speaker, as
  explained in the next section.

  The NEW speaker constructs the AS4_PATH attribute from the
  information carried in the AS_PATH attribute.  In the case where the
  AS_PATH attribute contains either AS_CONFED_SEQUENCE or AS_CONFED_SET
  path segments, the NEW speaker, when constructing the AS4_PATH
  attribute from the AS_PATH attribute, MUST exclude such path
  segments.  The AS4_PATH attribute will be carried across a series of
  OLD BGP speakers without modification and will help preserve the
  truly 4-octet AS numbers in the AS path information.

  Similarly, if the NEW speaker has to send the AGGREGATOR attribute,
  and if the aggregating Autonomous System's AS number is truly 4-
  octets, then the speaker constructs the AS4_AGGREGATOR attributes by
  taking the attribute length and attribute value from the AGGREGATOR
  attribute and placing them into the attribute length and attribute
  value of the AS4_AGGREGATOR attribute, and sets the AS number field
  in the existing AGGREGATOR attribute to the reserved AS number,
  AS_TRANS.  Note that if the AS number is 2-octets only, then the
  AS4_AGGREGATOR attribute SHOULD NOT be sent.

4.2.3.  Processing Received Updates

  When a NEW BGP speaker receives an update from an OLD one, it should
  be prepared to receive the AS4_PATH attribute along with the existing
  AS_PATH attribute.  If the AS4_PATH attribute is also received, both
  the attributes will be used to construct the exact AS path
  information, and therefore the information carried by both the
  attributes will be considered for AS path loop detection.

  Note that a route may have traversed a series of autonomous systems
  with 2-octet AS numbers and OLD BGP speakers only.  In that case, if
  the route carries the AS4_PATH attribute, this attribute must have



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RFC 4893       BGP Support for Four-octet AS Number Space       May 2007


  remained unmodified since the route left the last NEW BGP speaker.
  The trailing AS path information (representing autonomous systems
  with 2-octet AS numbers and OLD BGP speakers only) is contained only
  in the current AS_PATH attribute (encoded in the leading part of the
  AS_PATH attribute).

  Under certain conditions, it may not be possible to reconstruct the
  entire AS path information from the AS_PATH and the AS4_PATH
  attributes of a route.  This occurs when two or more routes that
  carry the AS4_PATH attribute are aggregated by an OLD BGP speaker,
  and the AS4_PATH attribute of at least one of these routes carries at
  least one 4-octet AS number (as oppose to a 2-octet AS number that is
  encoded in 4 octets).  Depending on the implementation, either the
  AS4_PATH attribute would be lost during route aggregation, or both
  the AS_PATH attribute and the AS4_PATH attribute would contain valid,
  partial information that cannot be combined seamlessly, resulting in
  incomplete AS path information in these cases.

  A NEW BGP speaker should also be prepared to receive the
  AS4_AGGREGATOR attribute along with the AGGREGATOR attribute from an
  OLD BGP speaker.  When both the attributes are received, if the AS
  number in the AGGREGATOR attribute is not AS_TRANS, then:

     -  the AS4_AGGREGATOR attribute and the AS4_PATH attribute SHALL
        be ignored,

     -  the AGGREGATOR attribute SHALL be taken as the information
        about the aggregating node, and

     -  the AS_PATH attribute SHALL be taken as the AS path
        information.

  Otherwise,

     -  the AGGREGATOR attribute SHALL be ignored,

     -  the AS4_AGGREGATOR attribute SHALL be taken as the information
        about the aggregating node, and

     -  the AS path information would need to be constructed, as in all
        other cases.

  In order to construct the AS path information, it would be necessary
  to first calculate the number of AS numbers in the AS_PATH and
  AS4_PATH attributes using the method specified in Section 9.1.2.2
  [BGP] and [RFC3065] for route selection.





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RFC 4893       BGP Support for Four-octet AS Number Space       May 2007


  If the number of AS numbers in the AS_PATH attribute is less than the
  number of AS numbers in the AS4_PATH attribute, then the AS4_PATH
  attribute SHALL be ignored, and the AS_PATH attribute SHALL be taken
  as the AS path information.

  If the number of AS numbers in the AS_PATH attribute is larger than
  or equal to the number of AS numbers in the AS4_PATH attribute, then
  the AS path information SHALL be constructed by taking as many AS
  numbers and path segments as necessary from the leading part of the
  AS_PATH attribute, and then prepending them to the AS4_PATH attribute
  so that the AS path information has an identical number of AS numbers
  as the AS_PATH attribute.  Note that a valid AS_CONFED_SEQUENCE or
  AS_CONFED_SET path segment SHALL be prepended if it is either the
  leading path segment or adjacent to a path segment that is prepended.

5.  Handling BGP Communities

  As specified in [RFC1997], when the high-order two-octets of the
  community attribute is neither 0x0000 nor 0xffff, these two octets
  encode the Autonomous System number.  Quite clearly this would not
  work for BGP speakers that use 4-octets Autonomous System numbers.
  Such BGP speakers should use the Four-octet AS Specific Extended
  Communities [AS-EXT-COM] instead.

6.  Transition

  The scheme described in this document allows a gradual transition
  from 2-octet AS numbers to 4-octet AS numbers.  One can upgrade one
  Autonomous System or one BGP speaker at a time.

  To simplify transition, this document assumes that an Autonomous
  System could start using a 4-octet AS number only after all the BGP
  speakers within that Autonomous System have been upgraded to support
  4-octet AS numbers.

  An OLD BGP speaker MUST NOT use AS_TRANS as its Autonomous System
  number.

  A non-mappable 4-octet AS number cannot be used as a "Member AS
  Number" of a BGP Confederation until all the BGP speakers within the
  Confederation have transitioned to support 4-octet AS numbers.

  In an environment where an Autonomous System that has OLD BGP
  speakers peers with two or more Autonomous Systems that have NEW BGP
  speakers and use AS_TRANS (rather than having a globally unique AS
  number), use of Multi-Exit Discriminators by the Autonomous System





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RFC 4893       BGP Support for Four-octet AS Number Space       May 2007


  with the OLD speakers may result in a situation where Multi-Exit
  Discriminator will influence route selection among the routes that
  were received from different neighboring Autonomous Systems.

  Under certain conditions, it may not be possible to reconstruct the
  entire AS path information from the AS_PATH and the AS4_PATH
  attributes of a route.  This occurs when two or more routes that
  carry the AS4_PATH attribute are aggregated by an OLD BGP speaker,
  and the AS4_PATH attribute of at least one of these routes carries at
  least one 4-octet AS number (as oppose to a 2-octet AS number that is
  encoded in 4 octets).  When such aggregation results in creating a
  route that is less specific than any of the component routes (route
  whose Network Layer Reachability Information (NLRI) covers NLRI of
  all the component routes), loss of the AS path information does not
  create a risk of a routing loop.  In all other cases, loss of the AS
  path information does create a risk of a routing loop.

7.  IANA Considerations

  This document expands the pool for AS numbers from 0 - 65535 to 0 -
  4294967295.  The AS numbers are managed by the IANA "Autonomous
  System Numbers" registry.  Other than expanding the AS number pool,
  this document does not propose any modifications to the existing
  policies and procedures pertaining to the AS number allocation.

  This document uses a BGP Capability code to indicate that a BGP
  speaker supports the 4-octet AS numbers.  The Capability Code 65 has
  been assigned by IANA per RFC 2842.

  In addition, this document introduces two new BGP optional transitive
  attributes, and their type codes have been assigned by the IANA.  The
  first one is the AS4_PATH attribute, value 17, which preserves the AS
  path information with 4-octet AS numbers across old BGP speakers.
  The second one is the AS4_AGGREGATOR attribute, value 18, which is
  similar in use to the current AGGREGATOR attribute, but it carries a
  4-octet AS number.

  Finally, this document introduces a reserved 2-octet AS number --
  AS_TRANS.  The AS number 23456 has been assigned by the IANA for
  AS_TRANS.











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RFC 4893       BGP Support for Four-octet AS Number Space       May 2007


8.  Security Considerations

  This extension to BGP does not change the underlying security issues
  inherent in the existing BGP, except for the following:

  The inconsistency between the AS_PATH attribute and the AS4_PATH
  attribute can create loss of the AS path information, and potential
  routing loops in certain cases as discussed in the document.  This
  could be exploited by an attacker.

9.  Acknowledgments

  The authors would like to thank Yakov Rekhter, Chaitanya Kodeboyina,
  and Jeffrey Haas for the numerous discussions that went into the
  making of this document.

10.  Normative References

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

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

  [RFC3392]    Chandra, R. and J. Scudder, "Capabilities Advertisement
               with BGP-4", RFC 3392, November 2002.

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

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

11.  Informative References

  [AS-EXT-COM] Rekhter, Y., Ramachandra, S., and D. Tappan, "Four-octet
               AS Specific BGP Extended Community", Work in Progress,
               April 2007.












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RFC 4893       BGP Support for Four-octet AS Number Space       May 2007


Authors' Addresses

  Quaizar Vohra
  Juniper Networks
  1194 N.Mathilda Ave
  Sunnyvale, CA 94089

  EMail: [email protected]


  Enke Chen
  Cisco Systems, Inc.
  170 W. Tasman Dr.
  San Jose, CA 95134

  EMail: [email protected]



































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RFC 4893       BGP Support for Four-octet AS Number Space       May 2007


Full Copyright Statement

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Acknowledgement

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