7
Network Working Group J. Scudder
8
Request for Comments: 5492 Juniper Networks
9
Obsoletes: 3392 R. Chandra
10
Category: Standards Track Sonoa Systems
14
Capabilities Advertisement with BGP-4
18
This document specifies an Internet standards track protocol for the
19
Internet community, and requests discussion and suggestions for
20
improvements. Please refer to the current edition of the "Internet
21
Official Protocol Standards" (STD 1) for the standardization state
22
and status of this protocol. Distribution of this memo is unlimited.
26
Copyright (c) 2009 IETF Trust and the persons identified as the
27
document authors. All rights reserved.
29
This document is subject to BCP 78 and the IETF Trust's Legal
30
Provisions Relating to IETF Documents in effect on the date of
31
publication of this document (http://trustee.ietf.org/license-info).
32
Please review these documents carefully, as they describe your rights
33
and restrictions with respect to this document.
35
This document may contain material from IETF Documents or IETF
36
Contributions published or made publicly available before November
37
10, 2008. The person(s) controlling the copyright in some of this
38
material may not have granted the IETF Trust the right to allow
39
modifications of such material outside the IETF Standards Process.
40
Without obtaining an adequate license from the person(s) controlling
41
the copyright in such materials, this document may not be modified
42
outside the IETF Standards Process, and derivative works of it may
43
not be created outside the IETF Standards Process, except to format
44
it for publication as an RFC or to translate it into languages other
49
This document defines an Optional Parameter, called Capabilities,
50
that is expected to facilitate the introduction of new capabilities
51
in the Border Gateway Protocol (BGP) by providing graceful capability
52
advertisement without requiring that BGP peering be terminated.
54
This document obsoletes RFC 3392.
58
Scudder & Chandra Standards Track [Page 1]
60
RFC 5492 Capabilities Advertisement February 2009
65
The base BGP-4 specification [RFC4271] requires that when a BGP
66
speaker receives an OPEN message with one or more unrecognized
67
Optional Parameters, the speaker must terminate the BGP peering.
68
This complicates the introduction of new capabilities in BGP.
70
This specification defines an Optional Parameter and processing rules
71
that allow BGP speakers to communicate capabilities in an OPEN
72
message. A pair of BGP speakers that supports this specification can
73
establish the peering even when presented with unrecognized
74
capabilities, so long as all capabilities required to support the
75
peering are supported.
77
2. Requirements Language
79
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
80
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
81
document are to be interpreted as described in RFC 2119 [RFC2119].
83
3. Overview of Operations
85
When a BGP speaker [RFC4271] that supports capabilities advertisement
86
sends an OPEN message to its BGP peer, the message MAY include an
87
Optional Parameter, called Capabilities. The parameter lists the
88
capabilities supported by the speaker.
90
A BGP speaker determines the capabilities supported by its peer by
91
examining the list of capabilities present in the Capabilities
92
Optional Parameter carried by the OPEN message that the speaker
93
receives from the peer.
95
A BGP speaker that supports a particular capability may use this
96
capability with its peer after the speaker determines (as described
97
above) that the peer supports this capability. Simply put, a given
98
capability can be used on a peering if that capability has been
99
advertised by both peers. If either peer has not advertised it, the
100
capability cannot be used.
102
A BGP speaker determines that its peer doesn't support capabilities
103
advertisement if, in response to an OPEN message that carries the
104
Capabilities Optional Parameter, the speaker receives a NOTIFICATION
105
message with the Error Subcode set to Unsupported Optional Parameter.
106
(This is a consequence of the base BGP-4 specification [RFC4271] and
107
not a new requirement.) In this case, the speaker SHOULD attempt to
108
re-establish a BGP connection with the peer without sending to the
109
peer the Capabilities Optional Parameter.
114
Scudder & Chandra Standards Track [Page 2]
116
RFC 5492 Capabilities Advertisement February 2009
119
If a BGP speaker that supports a certain capability determines that
120
its peer doesn't support this capability, the speaker MAY send a
121
NOTIFICATION message to the peer and terminate peering (see Section
122
"Extensions to Error Handling" for more details). For example, a BGP
123
speaker may need to terminate peering if it established peering to
124
exchange IPv6 routes and determines that its peer does not support
125
Multiprotocol Extensions for BGP-4 [RFC4760]. The Error Subcode in
126
the NOTIFICATION message is then set to Unsupported Capability. The
127
message MUST contain the capability or capabilities that cause the
128
speaker to send the message. The decision to send the message and
129
terminate the peering is local to the speaker. If terminated, such
130
peering SHOULD NOT be re-established automatically.
132
If a BGP speaker receives from its peer a capability that it does not
133
itself support or recognize, it MUST ignore that capability. In
134
particular, the Unsupported Capability NOTIFICATION message MUST NOT
135
be generated and the BGP session MUST NOT be terminated in response
136
to reception of a capability that is not supported by the local
139
4. Capabilities Optional Parameter (Parameter Type 2):
141
This is an Optional Parameter that is used by a BGP speaker to convey
142
to its BGP peer the list of capabilities supported by the speaker.
143
The encoding of BGP Optional Parameters is specified in Section 4.2
144
of [RFC4271]. The parameter type of the Capabilities Optional
147
The parameter contains one or more triples <Capability Code,
148
Capability Length, Capability Value>, where each triple is encoded as
151
+------------------------------+
152
| Capability Code (1 octet) |
153
+------------------------------+
154
| Capability Length (1 octet) |
155
+------------------------------+
156
| Capability Value (variable) |
158
+------------------------------+
160
The use and meaning of these fields are as follows:
164
Capability Code is a one-octet unsigned binary integer that
165
unambiguously identifies individual capabilities.
170
Scudder & Chandra Standards Track [Page 3]
172
RFC 5492 Capabilities Advertisement February 2009
177
Capability Length is a one-octet unsigned binary integer that
178
contains the length of the Capability Value field in octets.
182
Capability Value is a variable-length field that is interpreted
183
according to the value of the Capability Code field.
185
BGP speakers SHOULD NOT include more than one instance of a
186
capability with the same Capability Code, Capability Length, and
187
Capability Value. Note, however, that processing of multiple
188
instances of such capability does not require special handling, as
189
additional instances do not change the meaning of the announced
190
capability; thus, a BGP speaker MUST be prepared to accept such
193
BGP speakers MAY include more than one instance of a capability (as
194
identified by the Capability Code) with non-zero Capability Length
195
field, but with different Capability Value and either the same or
196
different Capability Length. Processing of these capability
197
instances is specific to the Capability Code and MUST be described in
198
the document introducing the new capability.
200
The Capabilities Optional Parameter (OPEN Optional Parameter Type 2)
201
SHOULD only be included in the OPEN message once. If the BGP speaker
202
wishes to include multiple capabilities in the OPEN message, it
203
SHOULD do so as discussed above -- by listing all those capabilities
204
as TLVs within a single Capabilities Optional Parameter. However,
205
for backward compatibility, a BGP speaker MUST be prepared to receive
206
an OPEN message that contains multiple Capabilities Optional
207
Parameters, each of which contains one or more capabilities TLVs.
208
The set of capabilities should be processed in the same way in either
209
case, whether it is enumerated within a single Capabilities Optional
210
Parameter of the OPEN message or split across multiple Capabilities
213
5. Extensions to Error Handling
215
This document defines a new Error Subcode, Unsupported Capability.
216
The value of this Subcode is 7. The Data field in the NOTIFICATION
217
message MUST list the set of capabilities that causes the speaker to
218
send the message. Each such capability is encoded in the same way as
219
it would be encoded in the OPEN message.
226
Scudder & Chandra Standards Track [Page 4]
228
RFC 5492 Capabilities Advertisement February 2009
231
As explained in the "Overview of Operations" section, the Unsupported
232
Capability NOTIFICATION is a way for a BGP speaker to complain that
233
its peer does not support a required capability without which the
234
peering cannot proceed. It MUST NOT be used when a BGP speaker
235
receives a capability that it does not understand; such capabilities
238
6. IANA Considerations
240
This document defines a Capability Optional Parameter along with a
241
Capability Code field. IANA maintains the registry for Capability
242
Code values. Capability Code value 0 is reserved. Capability Code
243
values 1 through 63 are to be assigned by IANA using the "IETF
244
Review" policy defined in [RFC5226]. Capability Code values 64
245
through 127 are to be assigned by IANA using the "First Come First
246
Served" policy defined in [RFC5226]. Capability Code values 128
247
through 255 are for "Private Use" as defined in [RFC5226].
249
IANA created and maintains a registry for OPEN message Optional
250
Parameters called "BGP OPEN Optional Parameter Types". Optional
251
Parameters are identified by the Parameter Type, which is a one-octet
252
unsigned integer. Values (0 reserved, 1-255) are to be allocated
253
according to the "IETF Review" policy as defined in [RFC5226].
255
The registry has been populated with the two Parameter Type codes
256
that are currently defined:
258
o Parameter Type 1: Authentication (deprecated) [RFC4271] [RFC5492]
260
o Parameter Type 2: Capabilities [RFC5492]
262
7. Security Considerations
264
This extension to BGP does not change the underlying security issues
265
inherent in the existing BGP [RFC4272].
269
The authors would like to thank members of the IDR Working Group and
270
the IESG and its Directorates for their review and comments.
282
Scudder & Chandra Standards Track [Page 5]
284
RFC 5492 Capabilities Advertisement February 2009
289
9.1. Normative References
291
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
292
Requirement Levels", BCP 14, RFC 2119, March 1997.
294
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
295
Protocol 4 (BGP-4)", RFC 4271, January 2006.
297
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
298
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
301
9.2. Informative References
303
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
304
RFC 4272, January 2006.
306
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
307
"Multiprotocol Extensions for BGP-4", RFC 4760,
338
Scudder & Chandra Standards Track [Page 6]
340
RFC 5492 Capabilities Advertisement February 2009
343
Appendix A. Comparison between RFC 2842 and RFC 3392
345
In addition to several minor editorial changes, RFC 3392 also
346
clarified how to handle multiple instances of the same capability.
348
Appendix B. Comparison between RFC 3392 and This Document
350
This document makes minor editorial changes and updated references,
351
clarifies the use of the Unsupported Optional Parameter NOTIFICATION
352
message, clarifies behavior when the Capabilities Parameter is
353
included in the OPEN message multiple times, and clarifies
354
requirements by changing a number of SHOULDs to MUSTs.
361
EMail: jgs@juniper.net
367
EMail: rchandra@sonoasystems.com
394
Scudder & Chandra Standards Track [Page 7]
added
removed
dev/RFC/rfc5492.txt
