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Internet Engineering Task Force (IETF) Q. Vohra Request for Comments: 6793 Juniper Networks Obsoletes: 4893 E. Chen Updates: 4271 Cisco Systems Category: Standards Track December 2012 ISSN: 2070-1721

   BGP Support for Four-Octet Autonomous System (AS) Number Space

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 and updates RFC 4271.

Status of This Memo

 This is an Internet Standards Track document.

 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.

 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6793.

Copyright Notice

 Copyright (c) 2012 IETF Trust and the persons identified as the
 document authors.  All rights reserved.

 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Vohra & Chen Standards Track [Page 1] RFC 6793 BGP Support for 4-Octet AS Number Space December 2012

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 code,
 "support for four-octet AS number capability", to be used by a BGP
 speaker to indicate its support for 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 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 updates RFC 4271.  It includes
 several clarifications and editorial changes, and it specifies the
 error handling for the new attributes.

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 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 Capabilities Advertisements [RFC5492] to
 advertise to its neighbors (either internal or external) that it
 supports four-octet AS number extensions, as specified in this
 document.

Vohra & Chen Standards Track [Page 2] RFC 6793 BGP Support for 4-Octet AS Number Space December 2012

 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.  The Capability Length
 field of the capability is set to 4.

 The AS path information exchanged between NEW BGP speakers is 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 BGP 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 and the same encoding 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-related 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.

 Similarly, this document defines a new BGP path attribute called
 AS4_AGGREGATOR, which is optional transitive.  The AS4_AGGREGATOR
 attribute has the same semantics and the same encoding 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.

 This document reserves a two-octet AS number called "AS_TRANS".
 AS_TRANS can be used to represent non-mappable four-octet AS numbers
 as two-octet AS numbers in AS path information that is encoded with
 two-octet AS numbers.  (In this context, four-octet AS numbers that
 are not mapped from two-octet AS numbers are referred to as
 "non-mappable".) We denote this special AS number as AS_TRANS for
 ease of description in the rest of this specification.  This AS

Vohra & Chen Standards Track [Page 3] RFC 6793 BGP Support for 4-Octet AS Number Space December 2012

 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 BGP Capabilities 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 "support for four-octet AS number capability" in lieu of
 the "My Autonomous System" field of the OPEN message.

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

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 BGP speakers has to have a globally unique two-octet AS number --
 AS_TRANS MUST be used when the NEW BGP speaker does not have a
 two-octet AS number (even if multiple Autonomous Systems would
 use it).

Vohra & Chen Standards Track [Page 4] RFC 6793 BGP Support for 4-Octet AS Number Space December 2012

4.2.2. Generating Updates

 When communicating with an OLD BGP speaker, a NEW BGP speaker MUST
 send the AS path information in the AS_PATH attribute encoded with
 two-octet AS numbers.  The NEW BGP speaker MUST also send the AS path
 information in the AS4_PATH attribute (encoded with four-octet AS
 numbers), except for the case where all of the AS path information is
 composed of mappable four-octet AS numbers only.  In this case, the
 NEW BGP 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 BGP
 speaker, as explained in the next section.

 The NEW BGP 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 BGP 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 mappable, then the AS4_AGGREGATOR 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 of the attributes will be used to construct the exact
 AS path information, and therefore the information carried by both of
 the attributes will be considered for AS path loop detection.

Vohra & Chen Standards Track [Page 5] RFC 6793 BGP Support for 4-Octet AS Number Space December 2012

 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 all
 of the 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 of 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.

Vohra & Chen Standards Track [Page 6] RFC 6793 BGP Support for 4-Octet AS Number Space December 2012

 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 of [RFC4271]
 and in [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 a number of AS numbers identical
 to that of 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 is 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 four-octet AS specific extended communities
 [RFC5668] instead.

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.

Vohra & Chen Standards Track [Page 7] RFC 6793 BGP Support for 4-Octet AS Number Space December 2012

 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.

 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.

Vohra & Chen Standards Track [Page 8] RFC 6793 BGP Support for 4-Octet AS Number Space December 2012

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
 mappable AS number), the use of the MULTI_EXIT_DISC attribute
 [RFC4271] by the Autonomous System with the OLD BGP speakers may
 result in a situation where the MULTI_EXIT_DISC attribute 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 all
 of the 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 (routes whose Network Layer Reachability Information (NLRI)
 covers the NLRI of all the component routes), loss of the AS path
 information does not create the risk of a routing loop.  In all other
 cases, loss of the AS path information does create the risk of a
 routing loop.

8. Manageability Considerations

 If the BGP4-MIB [RFC4273] is supported, there are no additional
 manageability concerns that arise from the use of four-octet AS
 numbers, since the InetAutonomousSystemNumber textual convention
 [RFC4001] is defined as Unsigned32.

Vohra & Chen Standards Track [Page 9] RFC 6793 BGP Support for 4-Octet AS Number Space December 2012

 When IP Flow Information Export (IPFIX) [RFC5101] is supported, there
 are no additional manageability concerns that arise from the use of
 four-octet AS numbers.  The bgpSourceAsNumber and
 bgpDestinationAsNumber information elements [IANA-IPFIX] can continue
 to be used, with a new template record, specifying the new length of
 4 bytes.

9. 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 allocation of AS numbers.

 This document uses a BGP capability code to indicate that a BGP
 speaker supports four-octet AS numbers.  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 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, IANA has replaced a reference to RFC 4893 with a reference
 to this document for a reserved two-octet AS number -- AS_TRANS
 (23456).  Also, IANA has replaced a reference to RFC 4893 with a
 reference to this document for the "32-bit Autonomous System Numbers"
 registry.

10. 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 this 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
 AS path loop detection within a Confederation.

Vohra & Chen Standards Track [Page 10] RFC 6793 BGP Support for 4-Octet AS Number Space December 2012

11. 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.

12. References

12.1. Normative References

 [RFC1997]    Chandra, 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.

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

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

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

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

12.2. Informative References

 [IANA-IPFIX] IANA, "IP Flow Information Export (IPFIX) Entities",
              <http://www.iana.org/assignments/ipfix>.

 [RFC4001]    Daniele, M., Haberman, B., Routhier, S., and J.
              Schoenwaelder, "Textual Conventions for Internet Network
              Addresses", RFC 4001, February 2005.

 [RFC4273]    Haas, J., Ed., and S. Hares, Ed., "Definitions of
              Managed Objects for BGP-4", RFC 4273, January 2006.

 [RFC5101]    Claise, B., Ed., "Specification of the IP Flow
              Information Export (IPFIX) Protocol for the Exchange of
              IP Traffic Flow Information", RFC 5101, January 2008.

Vohra & Chen Standards Track [Page 11] RFC 6793 BGP Support for 4-Octet AS Number Space December 2012

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

Vohra & Chen Standards Track [Page 12]