Internet Engineering Task Force E. Chen
Internet-Draft Cisco Systems
Intended status: Standards Track J. Scudder
Expires: September 15, 2016 Juniper Networks
March 14, 2016
Extended Optional Parameters Length for BGP OPEN Message
draft-ietf-idr-ext-opt-param-04
Abstract
The Optional Parameters in the BGP OPEN message as defined in the
base BGP specification are limited to 255 octets due to a one-octet
length field. BGP Capabilities are carried in this field and may
foreseeably exceed 255 octets in the future, leading to concern about
this limitation.
In this document we extend the BGP OPEN length field in a backward-
compatible manner. The Parameter Length field of individual Optional
Parameters is similarly extended.
Status of This Memo
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This Internet-Draft will expire on September 15, 2016.
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carefully, as they describe your rights and restrictions with respect
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1. Introduction
The Optional Parameters Length field in the BGP OPEN message is
defined in the base BGP specification [RFC4271] as one octet, thus
limiting the Optional Parameters field in the OPEN message to 255
octets. As BGP Capabilities [RFC5492] are carried in the Optional
Parameters field, and new BGP capabilities continue to be introduced,
the limitation is becoming a concern for BGP development.
In this document we extend the BGP OPEN length field in a backward-
compatible manner. The Parameter Length field of individual Optional
Parameters is similarly extended. This is done by using Optional
Parameters Length of 255 combined with Optional Parameter Type 255 as
a distinguished value pair, which indicates that an extended Optional
Parameters Length field follows. In this case the Parameter Length
field of the Optional Parameters in the BGP OPEN message is also
extended.
1.1. 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].
2. Protocol Extensions
This document reserves Optional Parameter Type code 255 as the
"Extended Length" type code.
In the event that the length of Optional Parameters in the BGP OPEN
message does not exceed 255, the encodings of the base BGP
specification [RFC4271] MUST be used without alteration.
However, if the length of Optional Parameters is greater than 255,
four octets are used for the length field. Each of the first two
octets is set to 255, and the remaining two octets carry the actual
length. In addition, the "Parameter Length" field of each Optional
Parameter is enlarged to two octets. Other than the larger sizes of
the given fields, there is no change to the BGP OPEN message defined
in [RFC4271].
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Accordingly, when the length of Optional Parameters in the BGP OPEN
message is greater than 255, the OPEN message format is modified to
be the following:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| My Autonomous System |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hold Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BGP Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0xFF | 0xFF | Extended Opt. Parm. Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Optional Parameters (variable) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Likewise, in that situation the Optional Parameters encoding is
modified to be the following:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parm. Type | Parameter Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Parameter Value (variable) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In parsing an OPEN message, a BGP speaker MUST use the value of the
one-octet "Optional Parameters Length" field and the value of the
octet following it to determine the encoding of the Optional
Parameters length, as well as the size of the "Parameter Length"
field of the Optional Parameters. If both values are 255, then the
four-octet encoding described above is used for the Optional
Parameters length. Otherwise the encoding defined in [RFC4271] is
used.
This encoding is chosen for backward compatibility reasons -- a BGP
speaker which has not been upgraded to support this specification may
legitimately send Optional Parameters whose length equals exactly
255, thus the Optional Parameters Length field alone is insufficient
as an indicator. However, such a speaker would never legitimately
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send an Optional Parameter whose type code is 255, since that value
has been reserved by this specification.
3. Errors
If a BGP speaker supporting this specification (a "new speaker") is
peering with one which does not (an "old speaker") no
interoperability issues arise unless the new speaker needs to encode
Optional Parameters whose length exceeds 255. In that case, it will
transmit an OPEN message which the old speaker will interpret as
containing an Optional Parameter with type code 255. Since by
definition the old speaker will not recognize that type code, the old
speaker may be expected to close the connection with a NOTIFICATION
with an Error Code of OPEN Message Error and an Error Subcode of
Unsupported Optional Parameters, according to Section 6.2 of
[RFC4271].
Although the above is the most likely error to be sent, it is not
impossible that the old speaker might detect some other error first,
such as a length error, depending on the details of the
implementation. In no case would the peering be expected to
establish successfully; the only question is which NOTIFICATION would
be generated.
We note that in any case, such a peering could not succeed, since by
definition the extended length encoding would not be used by the new
speaker unless the basic encoding was insufficient.
4. IANA Considerations
IANA is requested to designate BGP OPEN Optional Parameter Type code
255 as the Extended Length type code.
5. Security Considerations
This extension to BGP does not change the underlying security issues.
6. Acknowledgements
The authors would like to thank Yakov Rekhter and Srihari Sangli for
discussing various options to enlarge the Optional Parameters field.
We would also like to thank Pradosh Mohapatra, Keyur Patel and Hannes
Gredler for their valuable comments.
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<http://www.rfc-editor.org/info/rfc4271>.
7.2. Informative References
[RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement
with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
2009, <http://www.rfc-editor.org/info/rfc5492>.
Authors' Addresses
Enke Chen
Cisco Systems
Email: enkechen@cisco.com
John Scudder
Juniper Networks
Email: jgs@juniper.net
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