Internet Engineering Task Force                                  E. Chen
Internet-Draft                                        Palo Alto Networks
Updates: 4271 (if approved)                                   J. Scudder
Intended status: Standards Track                        Juniper Networks
Expires: September 23, 24, 2021                               March 22, 23, 2021

        Extended Optional Parameters Length for BGP OPEN Message


   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.

   This document updates RFC 4271 by extending, in a backward-compatible
   manner, the length of the Optional Parameters in the BGP OPEN.  The
   Parameter Length field of individual Optional Parameters is also

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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, in a backward-
   compatible manner, the length of the Optional Parameters in BGP OPEN.
   This is done by using Optional Parameter Type 255 as a distinguished
   value, that indicates an extended Optional Parameters Length field
   follows.  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",
   "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.  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,  (However,
   an implementation MUST be prepared to accept an OPEN message that uses the encoding
   of this specification for even if the length of Optional Parameters of any
   length. is
   255 or less.)

   However, if the length of 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, and 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 (that 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] other than the extension of the Parameter
   Length field to be a two-octet unsigned integer.

   In parsing an OPEN message, if the one-octet "Optional Parameters
   Length" field is non-zero, a BGP speaker MUST use the value of the
   octet following the one-octet "Optional Parameters Length" field 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 this field is 255, then the encoding
   described above is used for the Optional Parameters length.
   Otherwise the encoding defined in [RFC4271] is used.

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 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

   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.

   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.
   A warning MAY be logged.

4.  IANA Considerations

   IANA is requested to designate type code 255 in the BGP OPEN Optional
   Parameter Types registry as the Extended Length type code.

5.  Security Considerations

   This extension to BGP does not change the underlying security or
   confidentiality 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, Bruno Decraene, John
   Heasley, 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,

   [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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

7.2.  Informative References

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, DOI 10.17487/RFC4272, January 2006,

   [RFC5492]  Scudder, J. and R. Chandra, "Capabilities Advertisement
              with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
              2009, <>.

Authors' Addresses

   Enke Chen
   Palo Alto Networks


   John Scudder
   Juniper Networks