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Network Working Group                                         J. ScudderRequest for Comments: 5492                              Juniper NetworksObsoletes:3392                                               R. ChandraCategory: Standards Track                                  Sonoa Systems                                                           February 2009Capabilities Advertisement with BGP-4Status 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 toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents in effect on the date of   publication of this document (http://trustee.ietf.org/license-info).   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 obsoletesRFC 3392.Scudder & Chandra           Standards Track                     [Page 1]

RFC 5492               Capabilities Advertisement          February 20091.  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 inRFC 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 inSection 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 20099.  References9.1.  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 2009Appendix A.  Comparison betweenRFC 2842 andRFC 3392   In addition to several minor editorial changes,RFC 3392 also   clarified how to handle multiple instances of the same capability.Appendix B.  Comparison betweenRFC 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.comScudder & Chandra           Standards Track                     [Page 7]