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rfc:rfc5492

Network Working Group J. Scudder Request for Comments: 5492 Juniper Networks Obsoletes: 3392 R. Chandra Category: Standards Track Sonoa Systems

                                                         February 2009
               Capabilities Advertisement with BGP-4

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) 2009 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 in effect on the date of
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 Please review these documents carefully, as they describe your rights
 and restrictions with respect to this document.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Abstract

 This document defines an Optional Parameter, called Capabilities,
 that is expected to facilitate the introduction of new capabilities
 in the Border Gateway Protocol (BGP) by providing graceful capability
 advertisement without requiring that BGP peering be terminated.
 This document obsoletes RFC 3392.

Scudder & Chandra Standards Track [Page 1] RFC 5492 Capabilities Advertisement February 2009

1. Introduction

 The base BGP-4 specification [RFC4271] requires that when a BGP
 speaker receives an OPEN message with one or more unrecognized
 Optional Parameters, the speaker must terminate the BGP peering.
 This complicates the introduction of new capabilities in BGP.
 This specification defines an Optional Parameter and processing rules
 that allow BGP speakers to communicate capabilities in an OPEN
 message.  A pair of BGP speakers that supports this specification can
 establish the peering even when presented with unrecognized
 capabilities, so long as all capabilities required to support the
 peering are supported.

2. Requirements Language

 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 RFC 2119 [RFC2119].

3. Overview of Operations

 When a BGP speaker [RFC4271] that supports capabilities advertisement
 sends an OPEN message to its BGP peer, the message MAY include an
 Optional Parameter, called Capabilities.  The parameter lists the
 capabilities supported by the speaker.
 A BGP speaker determines the capabilities supported by its peer by
 examining the list of capabilities present in the Capabilities
 Optional Parameter carried by the OPEN message that the speaker
 receives from the peer.
 A BGP speaker that supports a particular capability may use this
 capability with its peer after the speaker determines (as described
 above) that the peer supports this capability.  Simply put, a given
 capability can be used on a peering if that capability has been
 advertised by both peers.  If either peer has not advertised it, the
 capability cannot be used.
 A BGP speaker determines that its peer doesn't support capabilities
 advertisement if, in response to an OPEN message that carries the
 Capabilities Optional Parameter, the speaker receives a NOTIFICATION
 message with the Error Subcode set to Unsupported Optional Parameter.
 (This is a consequence of the base BGP-4 specification [RFC4271] and
 not a new requirement.)  In this case, the speaker SHOULD attempt to
 re-establish a BGP connection with the peer without sending to the
 peer the Capabilities Optional Parameter.

Scudder & Chandra Standards Track [Page 2] RFC 5492 Capabilities Advertisement February 2009

 If a BGP speaker that supports a certain capability determines that
 its peer doesn't support this capability, the speaker MAY send a
 NOTIFICATION message to the peer and terminate peering (see Section
 "Extensions to Error Handling" for more details).  For example, a BGP
 speaker may need to terminate peering if it established peering to
 exchange IPv6 routes and determines that its peer does not support
 Multiprotocol Extensions for BGP-4 [RFC4760].  The Error Subcode in
 the NOTIFICATION message is then set to Unsupported Capability.  The
 message MUST contain the capability or capabilities that cause the
 speaker to send the message.  The decision to send the message and
 terminate the peering is local to the speaker.  If terminated, such
 peering SHOULD NOT be re-established automatically.
 If a BGP speaker receives from its peer a capability that it does not
 itself support or recognize, it MUST ignore that capability.  In
 particular, the Unsupported Capability NOTIFICATION message MUST NOT
 be generated and the BGP session MUST NOT be terminated in response
 to reception of a capability that is not supported by the local
 speaker.

4. Capabilities Optional Parameter (Parameter Type 2):

 This is an Optional Parameter that is used by a BGP speaker to convey
 to its BGP peer the list of capabilities supported by the speaker.
 The encoding of BGP Optional Parameters is specified in Section 4.2
 of [RFC4271].  The parameter type of the Capabilities Optional
 Parameter is 2.
 The parameter contains one or more triples <Capability Code,
 Capability Length, Capability Value>, where each triple is encoded as
 shown below:
        +------------------------------+
        | Capability Code (1 octet)    |
        +------------------------------+
        | Capability Length (1 octet)  |
        +------------------------------+
        | Capability Value (variable)  |
        ~                              ~
        +------------------------------+
 The use and meaning of these fields are as follows:
    Capability Code:
       Capability Code is a one-octet unsigned binary integer that
       unambiguously identifies individual capabilities.

Scudder & Chandra Standards Track [Page 3] RFC 5492 Capabilities Advertisement February 2009

    Capability Length:
       Capability Length is a one-octet unsigned binary integer that
       contains the length of the Capability Value field in octets.
    Capability Value:
       Capability Value is a variable-length field that is interpreted
       according to the value of the Capability Code field.
 BGP speakers SHOULD NOT include more than one instance of a
 capability with the same Capability Code, Capability Length, and
 Capability Value.  Note, however, that processing of multiple
 instances of such capability does not require special handling, as
 additional instances do not change the meaning of the announced
 capability; thus, a BGP speaker MUST be prepared to accept such
 multiple instances.
 BGP speakers MAY include more than one instance of a capability (as
 identified by the Capability Code) with non-zero Capability Length
 field, but with different Capability Value and either the same or
 different Capability Length.  Processing of these capability
 instances is specific to the Capability Code and MUST be described in
 the document introducing the new capability.
 The Capabilities Optional Parameter (OPEN Optional Parameter Type 2)
 SHOULD only be included in the OPEN message once.  If the BGP speaker
 wishes to include multiple capabilities in the OPEN message, it
 SHOULD do so as discussed above -- by listing all those capabilities
 as TLVs within a single Capabilities Optional Parameter.  However,
 for backward compatibility, a BGP speaker MUST be prepared to receive
 an OPEN message that contains multiple Capabilities Optional
 Parameters, each of which contains one or more capabilities TLVs.
 The set of capabilities should be processed in the same way in either
 case, whether it is enumerated within a single Capabilities Optional
 Parameter of the OPEN message or split across multiple Capabilities
 Optional Parameters.

5. Extensions to Error Handling

 This document defines a new Error Subcode, Unsupported Capability.
 The value of this Subcode is 7.  The Data field in the NOTIFICATION
 message MUST list the set of capabilities that causes the speaker to
 send the message.  Each such capability is encoded in the same way as
 it would be encoded in the OPEN message.

Scudder & Chandra Standards Track [Page 4] RFC 5492 Capabilities Advertisement February 2009

 As explained in the "Overview of Operations" section, the Unsupported
 Capability NOTIFICATION is a way for a BGP speaker to complain that
 its peer does not support a required capability without which the
 peering cannot proceed.  It MUST NOT be used when a BGP speaker
 receives a capability that it does not understand; such capabilities
 MUST be ignored.

6. IANA Considerations

 This document defines a Capability Optional Parameter along with a
 Capability Code field.  IANA maintains the registry for Capability
 Code values.  Capability Code value 0 is reserved.  Capability Code
 values 1 through 63 are to be assigned by IANA using the "IETF
 Review" policy defined in [RFC5226].  Capability Code values 64
 through 127 are to be assigned by IANA using the "First Come First
 Served" policy defined in [RFC5226].  Capability Code values 128
 through 255 are for "Private Use" as defined in [RFC5226].
 IANA created and maintains a registry for OPEN message Optional
 Parameters called "BGP OPEN Optional Parameter Types".  Optional
 Parameters are identified by the Parameter Type, which is a one-octet
 unsigned integer.  Values (0 reserved, 1-255) are to be allocated
 according to the "IETF Review" policy as defined in [RFC5226].
 The registry has been populated with the two Parameter Type codes
 that are currently defined:
 o  Parameter Type 1: Authentication (deprecated) [RFC4271] [RFC5492]
 o  Parameter Type 2: Capabilities [RFC5492]

7. Security Considerations

 This extension to BGP does not change the underlying security issues
 inherent in the existing BGP [RFC4272].

8. Acknowledgments

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

Scudder & Chandra Standards Track [Page 5] RFC 5492 Capabilities Advertisement February 2009

9. References

9.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
            Protocol 4 (BGP-4)", RFC 4271, January 2006.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.

9.2. Informative References

 [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
            RFC 4272, January 2006.
 [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
            "Multiprotocol Extensions for BGP-4", RFC 4760,
            January 2007.

Scudder & Chandra Standards Track [Page 6] RFC 5492 Capabilities Advertisement February 2009

Appendix A. Comparison between RFC 2842 and RFC 3392

 In addition to several minor editorial changes, RFC 3392 also
 clarified how to handle multiple instances of the same capability.

Appendix B. Comparison between RFC 3392 and This Document

 This document makes minor editorial changes and updated references,
 clarifies the use of the Unsupported Optional Parameter NOTIFICATION
 message, clarifies behavior when the Capabilities Parameter is
 included in the OPEN message multiple times, and clarifies
 requirements by changing a number of SHOULDs to MUSTs.

Authors' Addresses

 John G. Scudder
 Juniper Networks
 EMail: jgs@juniper.net
 Ravi Chandra
 Sonoa Systems
 EMail: rchandra@sonoasystems.com

Scudder & Chandra Standards Track [Page 7]

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