| RFC 9072 | Extended Optional Parameters Length | July 2021 |
| Chen & Scudder | Standards Track | [Page] |
The Optional Parameters in the BGP OPEN message as defined in the baseBGP specification are limited to 255 octets due to a one-octet lengthfield.BGP capabilities are carried in this field and may foreseeablyexceed 255 octets in the future, leading to concerns about thislimitation.¶
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.¶
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 athttps://www.rfc-editor.org/info/rfc9072.¶
Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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The Optional Parameters Length field in the BGP OPEN message is definedinthe base BGP specification [RFC4271] as oneoctet, thus limiting the Optional Parameters field in the OPEN messageto 255 octets. Since BGP capabilities[RFC5492] arecarried in the Optional Parameters field, and new BGP capabilitiescontinue to be introduced, the limitation is a concern for BGPdevelopment.¶
This document updates[RFC4271] by extending the length of the Optional Parameters in BGPOPEN in abackward-compatible manner. This is done by using Optional Parameter type code 255 as adistinguished value, which indicates an extended Optional ParametersLength field follows and that the parsing of the BGP OPEN should bemodified according to these procedures. In this case, the Parameter Length field of the individual Optional Parameters in the BGP OPEN message is also extended.¶
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.¶
This document reserves Optional Parameter type code 255 as the "ExtendedLength".¶
In the event that the length of the Optional Parameters in the BGP OPENmessage does not exceed 255, the encodings ofthebase BGP specification [RFC4271]SHOULD be used without alteration.ConfigurationMAY override this to force the extended format to be usedin all cases; this might be used, for example, to test that a peersupports this specification. (In any case, an implementationMUST acceptan OPEN message that uses the encoding of this specification even if thelength 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 messageMUST 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) | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 to0; itMUST be ignored on receipt once the use of the extended format isdetermined positively by inspection of the Non-Extended OptionalParameters Type (Non-Ext OP Type) field.¶
The subsequent one-octet field (which would be the first OptionalParameter Type field in the non-extended format and is called "Non-ExtOP Type" in thefigure above)MUST be set to 255 on transmission. Onreceipt, a value of 255 for this field is the indication that theextended format is in use.¶
In this extended encoding, the subsequent two-octet field, termedthe "Extended Optional Parameters Length field", is an unsignedinteger indicating the total length of the Optional Parameters fieldin octets. If the value of this field is zero, no OptionalParameters are present.¶
Likewise, in that situation, the Optional Parameters encoding is modifiedto 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) ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The rules for encoding Optional Parameters are unchanged with respectto 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 ParametersLength field (labeled "Non-Ext OP Len." inFigure 1) is non-zero, a BGP speakerMUST use the value of the octet following the one-octet OptionalParameters Length field (labeled "Non-Ext OP Type" inFigure 1) to determine both the encoding of the OptionalParameters length and the size of the Parameter Length field ofindividual Optional Parameters. If the value of the "Non-Ext OP Type"field is 255, then the encoding described above is used for the OptionalParameters length. Otherwise, the encoding defined in[RFC4271] is used.¶
If a BGP speaker supporting this specification (a "new speaker") ispeering with one that does not (an "old speaker"), no interoperabilityissues arise unless the new speaker needs to encode Optional Parameterswhose length exceeds 255. In that case, it will transmit an OPENmessage that the old speaker will interpret as containing an OptionalParameter with type code 255. Since the old speaker willnot recognize that type code by definition, the old speaker is expected to closethe connection with a NOTIFICATION with an error code of "OPEN MessageError" and an error subcode of "Unsupported Optional Parameters", accordingtoSection 6.2 of [RFC4271].¶
Although the Optional Parameter type code 255 is used in thisspecification as the indication that the extended encoding is in use, itis not a bona fide Optional Parameter type code in the usual sense andMUST NOT be used other than as described above.If encountered other than as the Non-Ext OP Type, itMUST betreated 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 ExtendedOptional Parameters Length value is less than or equal to 255. It is notconsidered a fatal error to receive an OPEN message whose (non-extended)Optional Parameters Length value is not 255 and whose first OptionalParameter type code is 255 -- in this case, the encoding of thisspecificationMUST be used for decoding the message.¶
IANA has assigned value 255 as the Extended Length type code in the "BGP OPEN OptionalParameter Types" registry.¶
This extension to BGP does not change the underlying security orconfidentiality issuesinherent in the existing BGP[RFC4272].¶
The authors would like to thankYakov Rekhter andSrihari Sangli fordiscussing various options to enlarge the Optional Parameters field. Wewould also like to thankMatthew Bocci,Bruno Decraene,John Heasley,Jakob Heitz,Christer Holmberg,Pradosh Mohapatra,Keyur Patel, andHannes Gredler for theirvaluable comments.¶