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.

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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: [email protected]


  John Scudder
  Juniper Networks

  Email: [email protected]

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