Extended Optional Parameters Length for BGP OPEN Message
RFC 9072

"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