BGP OPEN Extended Length: Extended Optional Parameters Length for BGP OPEN Message
How a BGP OPEN message escapes the 255-octet limit on its optional parameters without breaking older routers.
Chen, E. and J. Scudder, "Extended Optional Parameters Length for BGP OPEN Message", RFC 9072, DOI 10.17487/RFC9072, July 2021, <https://www.rfc-editor.org/info/rfc9072>.
@misc{rfc9072,
series = {Request for Comments},
number = 9072,
howpublished = {RFC 9072},
publisher = {RFC Editor},
doi = {10.17487/RFC9072},
url = {https://www.rfc-editor.org/info/rfc9072},
author = {E. Chen and J. Scudder},
title = {{Extended Optional Parameters Length for BGP OPEN Message}},
pagetotal = 6,
year = 2021,
month = jul,
} RFC text
Contents
Internet Engineering Task Force (IETF) E. Chen
Request for Comments: 9072 Palo Alto Networks
Updates: 4271 J. Scudder
Category: Standards Track Juniper Networks
ISSN: 2070-1721 July 2021
Extended Optional Parameters Length for BGP OPEN Message
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 concerns
about this limitation.
This document updates RFC 4271 by extending, in a backward-compatible
manner, the length of the Optional Parameters in a BGP OPEN message.
The Parameter Length field of individual Optional Parameters is also
extended.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9072.
Copyright Notice
Copyright (c) 2021 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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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
1.1. Requirements Language
2. Update to RFC 4271
3. Backward Compatibility
4. IANA Considerations
5. Security Considerations
6. References
6.1. Normative References
6.2. Informative References
Acknowledgements
Authors' Addresses
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. Since BGP capabilities [RFC5492] are carried in the Optional
Parameters field, and new BGP capabilities continue to be introduced,
the limitation is a concern for BGP development.
This document updates [RFC4271] by extending the length of the
Optional Parameters in BGP OPEN in a backward-compatible manner.
This is done by using Optional Parameter type code 255 as a
distinguished value, which indicates an extended Optional Parameters
Length field follows and that the parsing of the BGP OPEN should be
modified according to these procedures. In this case, the Parameter
Length field of the individual 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", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Update to RFC 4271
This document reserves Optional Parameter type code 255 as the
"Extended Length".
In the event that the length of the Optional Parameters in the BGP
OPEN message does not exceed 255, the encodings of the base BGP
specification [RFC4271] SHOULD be used without alteration.
Configuration MAY override this to force the extended format to be
used in all cases; this might be used, for example, to test that a
peer supports this specification. (In any case, an implementation
MUST accept an OPEN message that uses the encoding of this
specification even if the length of the Optional Parameters is 255 or
less.)
However, if the length of the Optional Parameters in the BGP OPEN
message does exceed 255, the OPEN message MUST be encoded according
to the procedure below.
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) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Extended Encoding OPEN Format
The Non-Extended Optional Parameters Length field (Non-Ext OP Len.)
SHOULD be set to 255 on transmission and, in any event, MUST NOT be
set to 0; it MUST be ignored on receipt once the use of the extended
format is determined positively by inspection of the Non-Extended
Optional Parameters Type (Non-Ext OP Type) field.
The subsequent one-octet field (which would be the first Optional
Parameter Type field in the non-extended format and is called "Non-
Ext OP Type" in the figure above) 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.
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) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Extended Parameters Format
The rules for encoding Optional Parameters are unchanged with respect
to those given in [RFC4271], except that the Parameter Length field
is extended to be a two-octet unsigned integer.
In parsing an OPEN message, if the one-octet Optional Parameters
Length field (labeled "Non-Ext OP Len." in Figure 1) is non-zero, a
BGP speaker MUST use the value of the octet following the one-octet
Optional Parameters Length field (labeled "Non-Ext OP Type" in
Figure 1) to determine both the encoding of the Optional Parameters
length and the size of the Parameter Length field of individual
Optional Parameters. If the value of the "Non-Ext OP Type" field is
255, then the encoding described above is used for the Optional
Parameters length. Otherwise, the encoding defined in [RFC4271] is
used.
3. Backward Compatibility
If a BGP speaker supporting this specification (a "new speaker") is
peering with one that 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 that the old speaker will interpret as
containing an Optional Parameter with type code 255. Since the old
speaker will not recognize that type code by definition, the old
speaker is 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 Optional Parameter type code 255 is used in this
specification as the indication that the extended encoding is in use,
it is not a bona fide Optional Parameter type code in the usual sense
and MUST NOT be used other than as described above. If encountered
other than as the Non-Ext OP Type, it MUST be treated as an
unrecognized Optional Parameter and handled according to [RFC4271],
Section 6.2.
It is not considered an error to receive an OPEN message whose
Extended Optional Parameters Length value is less than or equal to
255. It is not considered a fatal error to receive an OPEN message
whose (non-extended) Optional Parameters Length value is not 255 and
whose first Optional Parameter type code is 255 -- in this case, the
encoding of this specification MUST be used for decoding the message.
4. IANA Considerations
IANA has assigned value 255 as the Extended Length type code in the
"BGP OPEN Optional Parameter Types" registry.
5. Security Considerations
This extension to BGP does not change the underlying security or
confidentiality issues inherent in the existing BGP [RFC4272].
6. References
6.1. 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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
6.2. 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>.
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, Bruno Decraene, John
Heasley, Jakob Heitz, Christer Holmberg, Pradosh Mohapatra, Keyur
Patel, and Hannes Gredler for their valuable comments.
Authors' Addresses
Enke Chen
Palo Alto Networks
Email: enchen@paloaltonetworks.com
John Scudder
Juniper Networks
Email: jgs@juniper.net
AI ELI5
On this page
§1The 255-Octet Problem
In the base BGP specification (RFC 4271), the Optional Parameters Length field of the OPEN message is one octet. That limits the Optional Parameters field to 255 octets.
BGP capabilities (RFC 5492) are carried in that field, and new capabilities keep being introduced. The limit is therefore a concern for BGP development. RFC 9072 updates RFC 4271 to extend the length in a backward-compatible way.
The notes box on the form only holds a small number of lines, and people keep wanting to write more. This document finds a way to make room.
§2The Extended OPEN Encoding
The trick is to reserve Optional Parameter type code 255 as Extended Length. In the extended format, the one-octet Non-Extended Optional Parameters Length (Non-Ext OP Len.) is followed by a one-octet Non-Ext OP Type set to 255, and then a two-octet Extended Optional Parameters Length, an unsigned integer giving the total length of the Optional Parameters in octets. A value of zero means no Optional Parameters are present.
The Non-Ext OP Len. field SHOULD be set to 255 on transmission and MUST NOT be set to 0. It MUST be ignored on receipt once the extended format is positively identified by the 255 in the Non-Ext OP Type field.
Each individual Optional Parameter keeps its rules from RFC 4271, except that its Parameter Length field becomes a two-octet unsigned integer.
The old form has a small box, then a big sticker that says 'use the large box instead', then the size of the large box. Each note inside also gets a bigger size field.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
VERSION | Version | 1 byte | BGP version. | |
MY-AS | My Autonomous System | 2 bytes | Sender's autonomous system number field. | |
HOLD-TIME | Hold Time | 2 bytes | Hold time field of the OPEN message. | |
BGP-ID | BGP Identifier | 4 bytes | Identifier of the BGP speaker. | |
NON-EXT-LEN | Non-Ext OP Len. | 1 byte | SHOULD be 255, MUST NOT be 0; ignored once extended format is detected. | |
NON-EXT-TYPE | Non-Ext OP Type | 1 byte | 255 | MUST be 255 to signal the extended format. |
EXT-LEN | Extended Opt. Parm. Length | 2 bytes | Total length of the Optional Parameters in octets. |
§2.1When to Use It and How to Parse It
If the Optional Parameters do not exceed 255 octets, the RFC 4271 encoding SHOULD be used unchanged. Configuration MAY force the extended format in all cases, for example to test that a peer supports this specification. In any case, an implementation MUST accept an OPEN message that uses the extended encoding even if the parameters are 255 octets or less. If the parameters do exceed 255, the extended encoding MUST be used.
When parsing, if the one-octet Optional Parameters Length is non-zero, the speaker MUST look at the octet that follows it (Non-Ext OP Type). If it is 255, the extended encoding applies to both the overall length and each parameter's length field. Otherwise the RFC 4271 encoding is used.
If your notes fit in the small box, just use it. If they do not, put the sticker on. Whoever reads the form checks for the sticker first.
§3Backward Compatibility
No problem arises between a new speaker and an old one unless the new speaker has to send Optional Parameters longer than 255 octets. In that case the old speaker sees what looks like an Optional Parameter of type 255. It does not recognize that type, so it is expected to close the connection with a NOTIFICATION carrying error code OPEN Message Error and subcode Unsupported Optional Parameters, as RFC 4271 Section 6.2 describes.
Type code 255 is not a real Optional Parameter type in the usual sense and MUST NOT be used other than as described. If it appears anywhere except as the Non-Ext OP Type, it MUST be treated as an unrecognized Optional Parameter. It is not an error to receive an extended OPEN whose Extended Optional Parameters Length is 255 or less. It is also not a fatal error to receive an OPEN whose non-extended length is not 255 but whose first parameter type is 255; the extended encoding MUST then be used to decode it.
An old reader does not know the sticker and says 'I do not understand this form' and hangs up. That only happens when the notes really were too long for the old form.
§4IANA and Security
IANA has assigned value 255 as the Extended Length type code in the 'BGP OPEN Optional Parameter Types' registry.
The security section is a single sentence: the extension does not change the underlying security or confidentiality issues inherent in the existing BGP, which are analyzed in RFC 4272.
The special sticker number was written down in an official list. The change does not make BGP any safer or less safe than before.