Extended Optional Parameters Length for BGP OPEN Message
draft-ietf-idr-ext-opt-param-06
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (idr WG) | |
|---|---|---|---|
| Authors | Enke Chen , John Scudder | ||
| Last updated | 2019-07-30 (Latest revision 2019-07-26) | ||
| Replaces | draft-chen-bgp-ext-opt-param | ||
| Stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-idr-ext-opt-param-06
Internet Engineering Task Force E. Chen
Internet-Draft Cisco Systems
Updates: 4271 (if approved) J. Scudder
Intended status: Standards Track Juniper Networks
Expires: January 27, 2020 July 26, 2019
Extended Optional Parameters Length for BGP OPEN Message
draft-ietf-idr-ext-opt-param-06
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 update RFC 4271 by extending 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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 27, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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 update [RFC4271] by extending 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, an
extended encoding is used. The (non-extended) length field is set to
255, as is the subsequent octet that in the non-extended format would
be the first Optional Parameter Type. The subsequent 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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When receiving an OPEN, a BGP speaker determines the extended
encoding is in use if the first Optional Parameter Type field is 255.
In this case, the BGP speaker MUST ignore the non-extended Optional
Parameters Length field, and must instead rely on the Extended
Optional Parameters Length field.
Accordingly, when the length of Optional Parameters in the BGP OPEN
message is greater than 255, the OPEN message format is modified as
follows, repurposing the Optional Parameters Length field as well as
the first Optional Parameter Type field to indicate the use of the
extended format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Non-Ext OP Len.|Non-Ext OP Type| Extended Opt. Parm. Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Optional Parameters (variable) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The non-extended Optional Parameters Length field MUST be set to 255
on transmission, and MUST be ignored on receipt once the use of the
extended format is determined.
The subsequent one-octet field, that in the non-extended format would
be the first Optional Parameter Type field, MUST be set to 255 on
transmission. On receipt, a value of 255 for this field is the
indication that the extended format is in use.
In this extended encoding, the subsequent two-octet field, termed the
Extended Optional Parameters Length field, is an unsigned integer
indicating the total length of the Optional Parameters field in
octets. If the value of this field is zero, no Optional Parameters
are present (this would never be expected to happen with the extended
encoding, however).
Likewise, in that situation the Optional Parameters encoding is
modified to be the following:
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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) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The rules for encoding Optional Parameters are unchanged with respect
to those given in [RFC4271] other than the extension of the Parameter
Length field to be a two-octet unsigned integer.
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
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
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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.
Although the Optional Parameter Type code 255 is used in this
specification as the indication that the extended encoding is in use,
it is not a bonafide Optional Parameter Type in the usual sense, and
MUST NOT be used other than as described above. If encountered as an
actual Optional Parameter Type code, it MUST be treated as an
unrecognized Optional Parameter and handled according to [RFC4271]
Section 6.2.
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
inherent in the existing BGP [RFC4272].
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 Matthew Bocci, Jakob Heitz, Pradosh
Mohapatra, Keyur Patel and Hannes Gredler for their valuable
comments.
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,
<https://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,
<https://www.rfc-editor.org/info/rfc4271>.
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7.2. Informative References
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<https://www.rfc-editor.org/info/rfc4272>.
[RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement
with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
2009, <https://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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