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Versions: (RFC 4893) 00 01 02 03 04 05 06 07 RFC 6793

Network Working Group                                           Q. Vohra
Internet Draft                                          Juniper Networks
Obsoletes: 4893 (if approved)                                    E. Chen
Intended Status: Standards Track                           Cisco Systems
Expiration Date: December 21, 2012                         June 20, 2012


               BGP Support for Four-octet AS Number Space
                    draft-ietf-idr-rfc4893bis-07.txt


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Abstract

   The Autonomous System number is encoded as a two-octet entity in the
   base BGP specification. This document describes extensions to BGP to
   carry the Autonomous System numbers as four-octet entities.  This
   document obsoletes RFC 4893.


1. Introduction

   In the base BGP specification [RFC4271] the Autonomous System (AS)
   number is encoded as a two-octet entity.  To prepare for the
   anticipated exhaustion of the two-octet AS numbers, this document
   describes extensions to BGP to carry the AS numbers as four-octet
   entities.

   More specifically, this document defines a BGP capability, "support
   for four-octet AS number capability", to be used by a BGP speaker to
   indicate its support for the four-octet AS numbers.  Two 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 specified in this document allow a gradual transition
   from two-octet AS numbers to four-octet AS numbers.

   This document obsoletes RFC 4893, and a comparison with RFC 4893 is
   provided in Appendix A.


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].














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3. Protocol Extensions

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

   BGP carries the AS numbers 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
   AS numbers in the BGP Communities attribute.

   A NEW BGP speaker uses BGP Capability Advertisements [RFC5492] to
   advertise to its neighbors (either internal or external) that it
   supports four-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 four-octet Autonomous System number capability, also carries
   the AS number (encoded as a four-octet entity) of the speaker in the
   Capability Value field of the Capability Optional Parameter.  The
   Capability Length field of the Capability is set to 4.

   The AS path information exchanged between NEW BGP speakers are
   carried in the existing AS_PATH attribute, except that each AS number
   in the attribute is encoded as a four-octet entity (instead of a two-
   octet entity).  The same applies to the AGGREGATOR attribute - the
   same attribute is used between NEW BGP speakers, except that the AS
   number carried in the attribute is encoded as a four-octet entity.

   The AS_PATH attribute and the AGGREGATOR attribute carried between a
   NEW BGP speaker and an OLD BGP speaker will continue to contain two-
   octet AS numbers.

   To preserve the AS path information with four-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 four-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 four-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 [RFC5065] are declared invalid for the AS4_PATH
   attribute, and MUST NOT be included in the AS4_PATH attribute of an
   UPDATE message.




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   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 four-octet AS number.

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

   To represent four-octet AS numbers (which are not mapped from two-
   octets) as two-octet AS numbers in the AS path information encoded
   with two-octet AS numbers, this document reserves a two-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 and only if the speaker does not
   have a (globally unique) two-octet AS number.


4. Operations


4.1. Interaction Between NEW BGP Speakers

   A BGP speaker that supports four-octet AS numbers SHALL advertise
   this to its peers using the BGP Capability Advertisements.  The AS
   number of the BGP speaker MUST be carried in the capability value
   field of the "support for four-octet AS number capability".

   When a NEW BGP speaker processes an OPEN message from another NEW BGP
   speaker, it MUST use the AS number encoded in the Capability Value
   field of the Capability in lieu of the "My Autonomous System" field
   of the OPEN message.

   A BGP speaker that advertises such capability to a particular peer,
   and receives from that peer the advertisement of such capability MUST
   encode AS numbers as four-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 AS numbers as four-octet entities.

   The new attributes, AS4_PATH and AS4_AGGREGATOR MUST NOT be carried
   in an UPDATE message between NEW BGP speakers.  A NEW BGP speaker
   that receives the AS4_PATH attribute or the AS4_AGGREGATOR attribute
   in an UPDATE message from another NEW BGP speaker MUST discard the
   path attribute and continue processing the UPDATE message.




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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 BGP speaker is
   possible only if the NEW BGP speaker has a two-octet AS number.
   However, this document does not assume that an Autonomous System with
   NEW speakers has to have a globally unique two-octet AS number --
   AS_TRANS MUST be used when the NEW speaker does not have a two-octet
   AS number (even if multiple Autonomous Systems would use it).



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 two-
   octet AS numbers.  The NEW speaker MUST also send the AS path
   information in the AS4_PATH attribute (encoded with four-octet AS
   numbers), except for the case where the entire AS path information is
   composed of two-octet mappable AS numbers only.  In this case, the
   NEW speaker MUST NOT send the AS4_PATH attribute.

   In the AS_PATH attribute encoded with two-octet AS numbers, non-
   mappable four-octet AS numbers are represented by the well-known two-
   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 AS path
   information.  Whenever the AS path information contains the
   AS_CONFED_SEQUENCE or AS_CONFED_SET path segment, the NEW BGP speaker
   MUST exclude such path segments from the AS4_PATH attribute being
   constructed.

   The AS4_PATH attribute, being optional transitive, will be carried
   across a series of OLD BGP speakers without modification and will
   help preserve the non-mappable four-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 a non-
   mappable four-octet AS number, then the speaker MUST use the
   AS4_AGGREGATOR attribute, and set the AS number field in the existing
   AGGREGATOR attribute to the reserved AS number, AS_TRANS.  Note that
   if the AS number is two-octets mappable, then the AS4_AGGREGATOR



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   attribute MUST NOT be sent.


4.2.3. Processing Received Updates

   When a NEW BGP speaker receives an update from an OLD BGP speaker, it
   MUST 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 two-octet AS numbers and OLD BGP speakers only.  In that case,
   if the route carries the AS4_PATH attribute, this attribute would
   have remained unmodified since the route left the last NEW BGP
   speaker.  The trailing AS path information (representing autonomous
   systems with two-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, for example, 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 four-octet AS number (as opposed to a two-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 MUST 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.




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   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 is 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 [RFC4271]
   and [RFC5065] for route selection.

   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 AS number.  Quite clearly this would not work for a NEW
   BGP speaker with a non-mappable four-octet AS number.  Such BGP
   speakers should use the four-octet AS Specific Extended Communities
   [RFC5668] instead.











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6. Error Handling

   This section provides an update to RFC 4271 [RFC4271] with respect to
   the error conditions noted here and their handling.

   Given that the two-octet AS numbers dominate during the transition,
   and are carried in the AS_PATH attribute by an OLD BGP speaker, in
   this document the "attribute discard" approach is chosen to handle a
   malformed AS4_PATH attribute.

   Similarly, as the AS4_AGGREGATOR is just informational, the
   "attribute discard" approach is chosen to handle a malformed
   AS4_AGGREGATOR attribute.

   The AS4_PATH attribute and AS4_AGGREGATOR attribute MUST NOT be
   carried in an UPDATE message between NEW BGP speakers.  A NEW BGP
   speaker that receives the AS4_PATH attribute or the AS4_AGGREGATOR
   attribute in an UPDATE message from another NEW BGP speaker MUST
   discard the path attribute and continue processing the UPDATE
   message.  This case SHOULD be logged locally for analysis.

   In addition, the path segment types AS_CONFED_SEQUENCE and
   AS_CONFED_SET [RFC5065] MUST NOT be carried in the AS4_PATH attribute
   of an UPDATE message.  A NEW BGP speaker that receives these path
   segment types in the AS4_PATH attribute of an UPDATE message from an
   OLD BGP speaker MUST discard these path segments, adjust the relevant
   attribute fields accordingly, and continue processing the UPDATE
   message.  This case SHOULD be logged locally for analysis.

   The AS4_PATH attribute in an UPDATE message SHALL be considered
   malformed under the following conditions:

      - the attribute length is not a multiple of two, or is too small
        (i.e., less than 6) for the attribute to carry at least one
        AS number, or

      - the path segment length in the attribute is either zero, or
        is inconsistent with the attribute length, or

      - the path segment type in the attribute is not one of the
        types defined: AS_SEQUENCE, AS_SET, AS_CONFED_SEQUENCE
        and AS_CONFED_SET.

   A NEW BGP speaker that receives a malformed AS4_PATH attribute in an
   UPDATE message from an OLD BGP speaker MUST discard the attribute,
   and continue processing the UPDATE message.  The error SHOULD be
   logged locally for analysis.




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   The AS4_AGGREGATOR attribute in an UPDATE message SHALL be considered
   malformed if the attribute length is not 8.

   A NEW BGP speaker that receives a malformed AS4_AGGREGATOR attribute
   in an UPDATE message from an OLD BGP speaker MUST discard the
   attribute, and continue processing the UPDATE message.  The error
   SHOULD be logged locally for analysis.


7. Transition

   When an Autonomous System is using a two-octet AS number, then the
   BGP speakers within that Autonomous System MAY be upgraded to support
   the four-octet AS number extensions on a piecemeal basis.  There is
   no requirement for a coordinated upgrade of the four-octet AS number
   capability in this case.  However, if an Autonomous System wishes to
   use a four-octet AS number as its own AS number, then this document
   assumes that an Autonomous System can use a four-octet AS number only
   after all the BGP speakers within that Autonomous System have been
   upgraded to support four-octet AS numbers.

   A non-mappable four-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 four-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 two-
   octet mappable AS number), use of Multi-Exit Discriminators by the
   Autonomous System 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 four-octet AS number (as opposed to a two-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.




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8. IANA Considerations

   [Remove this paragraph before publication: IANA should update the
   reference for the "32-bit Autonomous System Numbers" sub-registry to
   cite this document once an RFC number has been assigned to it,
   replacing the citation of RFC 4893.]

   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 four-octet AS numbers.  The Capability Code 65
   has been assigned by IANA per [RFC5492].

   In addition, this document introduces two 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 four-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
   four-octet AS number.

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


9. 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.

   It is a misconfiguration to assign a non-mappable four-octet AS
   number as the "Member AS Number" in a BGP confederation before all
   the BGP speakers within the confederation have transitioned to
   support four-octet AS numbers.  Such a misconfiguration would weaken
   the AS path loop detection within a confederation.






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10. 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.

   The authors would also like to thank members of the IDR Working Group
   for their review and comments.


11. Normative References

   [RFC4271]   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.

   [RFC5492]   Scudder, J. and R. Chandra, "Capabilities Advertisement
               with BGP-4", RFC 5492, February 2009.

   [RFC5065]   Traina, P., McPherson, D., and J. Scudder, "Autonomous
               System Confederations for BGP", RFC 5065, August 2007.

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

   [RFC5668]   Rekhter, Y., Ramachandra, S., and D. Tappan, "4-Octet AS
               Specific BGP Extended Community", RFC 5668, October 2009.


Appendix A.  Comparison with RFC 4893

   This document includes several clarifications and editorial changes,
   and specifies the error handling for the new attributes.















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12. Authors' Addresses

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

   EMail: quaizar.vohra@gmail.com


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

   EMail: enkechen@cisco.com

































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