BGP capabilities: Capabilities Advertisement with BGP-4
How two BGP routers tell each other which optional features they support, without breaking the session.
Chandra, R. and J. Scudder, "Capabilities Advertisement with BGP-4", RFC 3392, DOI 10.17487/RFC3392, November 2002, <https://www.rfc-editor.org/info/rfc3392>.
@misc{rfc3392,
series = {Request for Comments},
number = 3392,
howpublished = {RFC 3392},
publisher = {RFC Editor},
doi = {10.17487/RFC3392},
url = {https://www.rfc-editor.org/info/rfc3392},
author = {R. Chandra and J. Scudder},
title = {{Capabilities Advertisement with BGP-4}},
pagetotal = 6,
year = 2002,
month = nov,
} RFC text
Contents
Network Working Group R. Chandra
Request for Comments: 3392 Redback Networks
Obsoletes: 2842 J. Scudder
Category: Standards Track Cisco Systems
November 2002
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) The Internet Society (2002). All Rights Reserved.
Abstract
This document defines a new 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 2842.
1. Introduction
Currently BGP-4 requires that when a BGP speaker receives an OPEN
message with one or more unrecognized Optional Parameters, the
speaker must terminate BGP peering. This complicates introduction of
new capabilities in BGP.
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 RFC 2119 [RFC2119].
3. Overview of Operations
When a BGP speaker [BGP-4] 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.
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.
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 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-established automatically.
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
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 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.
5. Extensions to Error Handling
This document defines 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.
6. IANA Considerations
This document defines a Capability Optional Parameter along with an
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
Consensus" policy defined in RFC 2434. Capability Code values 64
through 127 are to be assigned by IANA, using the "First Come First
Served" policy defined in RFC 2434. Capability Code values 128
through 255 are for "Private Use" as defined in RFC 2434.
7. Security Considerations
This extension to BGP does not change the underlying security issues
inherent in the existing BGP [Heffernan].
8. Acknowledgements
The authors would like to thank members of the IDR Working Group for
their review and comments.
9. Comparison with RFC 2842
In addition to several minor editorial changes, this document also
clarifies how to handle multiple instances of the same capability.
10. References
[BGP-4] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
[Heffernan] Heffernan, A., "Protection of BGP Sessions via the TCP
MD5 Signature Option", RFC 2385, August 1998.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11. Authors' Addresses
Ravi Chandra
Redback Networks Inc.
350, Holger Way
San Jose, CA 95134
EMail: rchandra@redback.com
John G. Scudder
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
EMail: jgs@cisco.com
12. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
§1The Problem With Unknown Parameters
BGP-4 sessions start with an OPEN message. Under the base rules, a BGP speaker that receives an OPEN message with one or more unrecognized Optional Parameters must terminate BGP peering. That makes it hard to introduce new capabilities, because any new parameter could cause old peers to drop the session.
RFC 3392 defines a new Optional Parameter called Capabilities. It is expected to make new capabilities easy to introduce by advertising them gracefully, without requiring that BGP peering be terminated.
Before this, a router that heard a word it did not know would hang up. Now routers can mention extra skills without anyone hanging up.
§3How Capability Advertisement Works
A speaker that supports capabilities advertisement MAY include the Capabilities Optional Parameter in the OPEN message it sends. The parameter lists what the speaker supports. A speaker learns what its peer supports by reading the list in the OPEN message it receives from that peer.
A speaker may use a capability with its peer only after it has determined, in this way, that the peer supports it.
If the peer does not support capabilities advertisement at all, it answers the OPEN with a NOTIFICATION whose Error Subcode is Unsupported Optional Parameter. The speaker SHOULD then try to re-establish the connection without sending the Capabilities parameter.
If a speaker needs a capability that its peer lacks, it MAY send a NOTIFICATION with the Error Subcode Unsupported Capability and end peering. That decision is local to the speaker, and such a peering SHOULD NOT be re-established automatically.
Each router shows its list of skills. If the other one cannot read lists, they start over without the list. If a skill is truly needed and missing, a router may say goodbye.
- OPEN + CapabilitiesLists the capabilities the speaker supports.
- NOTIFICATIONError Subcode: Unsupported Optional Parameter.
- OPEN (no Capabilities)Speaker SHOULD retry without the parameter.
§4The Capabilities Optional Parameter
The Capabilities Optional Parameter has Parameter Type 2. It carries one or more triples of Capability Code, Capability Length and Capability Value. The code and length are each one octet. The code identifies the capability, the length gives the size of the value in octets, and the value is variable and interpreted according to the code.
Speakers SHOULD NOT include more than one instance of a capability with the same code, length and value, although extra copies do not change the meaning. A speaker MAY include several instances with the same code and non-zero length but different values. How those are processed is specific to the capability and MUST be described in the document that introduces it.
Each skill on the list is written as a name tag, a note saying how long the details are, and then the details themselves.
§5The Unsupported Capability Error
The RFC adds a new NOTIFICATION Error Subcode, Unsupported Capability, with the value 7. The Data field of the NOTIFICATION SHOULD list the capabilities that caused the speaker to send it, each encoded the same way as in the OPEN message.
When a router hangs up because a needed skill is missing, it can say which skill was the problem.
§6Capability Code Registry
IANA maintains the registry of Capability Code values. The RFC sets these ranges:
0is reserved.1through63are assigned by IANA under the IETF Consensus policy.64through127are assigned under First Come First Served.128through255are for Private Use.
There is an official list of skill numbers. Some numbers need a big group to agree, some go to whoever asks first, and some are free for private use.
§7Security and Changes From RFC 2842
The RFC says this extension does not change the underlying security issues inherent in existing BGP, and points to the TCP MD5 signature option in RFC 2385 as a related protection.
Compared with RFC 2842, the document makes several minor editorial changes and clarifies how to handle multiple instances of the same capability.
Adding a skill list does not make the routers any safer or less safe than before. The update mostly tidies the wording and explains what to do with repeated entries.