Internet Engineering Task Force J. Scudder
Internet-Draft Juniper Networks
Intended status: Standards Track R. Chandra
Expires: May 22, 2009 Sonoa Systems
November 18, 2008
Capabilities Advertisement with BGP-4
draft-ietf-idr-rfc3392bis-01.txt
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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.
1. Introduction
The base BGP-4 specification [RFC4271] requires that when a BGP
speaker receives an OPEN message with one or more unrecognized
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Optional Parameters, the speaker must terminate BGP peering. This
complicates introduction of new capabilities in BGP.
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.
If a BGP speaker that supports a certain capability requires that
this capability be used on a peering but 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). The Error Subcode in the message
is set to Unsupported Capability. The message SHOULD contain the
capability (capabilities) that causes the speaker to send the
message. The decision to send the message and terminate peering is
local to the speaker. If terminated, such peering SHOULD NOT be re-
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established automatically. An example of when this procedure might
be followed is if a BGP speaker is attempting to establish an IPv6
peering but determines that its peer does not support Multiprotocol
Extensions for BGP-4 [RFC4760].
If a BGP speaker receives from its peer a capability which it does
not itself support or recognize, it SHOULD ignore that capability.
In particular, the Unsupported Capability NOTIFICATION message MUST
NOT be generated in response to reception of a capability which 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 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 field that unambiguously
identifies individual capabilities.
Capability Length:
Capability Length is a one octet field 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
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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.
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 which contains multiple Capabilities Optional
Parameters, each of which contains one or more capabilities TLVs.
The set of capabilities should be processed the same in either case,
whether it is enumerated within a single Capabilities Optional
Parameter of the OPEN message, or split across multiple.
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 SHOULD list the set of capabilities that cause the speaker to
send the message. Each such capability is encoded the same way as it
would be encoded in the OPEN message.
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 which it does not understand; such capabilities
SHOULD 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
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Consensus" 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].
7. Security Considerations
This extension to BGP does not change the underlying security issues
inherent in the existing BGP [RFC4272].
8. Acknowledgements
The authors would like to thank members of the IDR Working Group for
their review and comments.
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.
Appendix A. Comparison with RFC 2842
In addition to several minor editorial changes, this document also
clarifies how to handle multiple instances of the same capability.
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Appendix B. Comparison with RFC 3392
In addition to minor editorial changes and updated references, this
document also clarifies the use of the Unsupported Optional Parameter
NOTIFICATION message and clarifies behavior when the Capabilities
parameter is included in the OPEN message multiple times.
Authors' Addresses
John G. Scudder
Juniper Networks
Email: jgs@juniper.net
Ravi Chandra
Sonoa Systems
Email: rchandra@sonoasystems.com
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