Internet Engineering Task Force                                  E. Chen
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                              J. Scudder
Expires: July 17, 2015 September 15, 2016                             Juniper Networks
                                                        January 13, 2015
                                                          March 14, 2016

        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.

   In this document we extend the BGP OPEN length field in a backward-
   compatible manner.  The Parameter Length field of individual Optional
   Parameters is similarly extended.

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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.  As BGP Capabilities [RFC5492] are carried in the Optional
   Parameters field, and new BGP capabilities continue to be introduced,
   the limitation is becoming a concern for BGP development.

   In this document we extend the BGP OPEN length field in a backward-
   compatible manner.  The Parameter Length field of individual Optional
   Parameters is similarly extended.  This is done by using Optional
   Parameters Length of 255 combined with Optional Parameter Type 255 as
   a distinguished value pair, which indicates that an extended Optional
   Parameters Length field follows.  In this case the Parameter Length
   field of the Optional Parameters in the BGP OPEN message is also

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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, if the length of Optional Parameters is greater than 255,
   four octets are used for the length field.  Each of the first two
   octets is set to 255, and the remaining two octets carry the actual
   length.  In addition, the "Parameter Length" field of each Optional
   Parameter is enlarged to two octets.  Other than the larger sizes of
   the given fields, there is no change to the BGP OPEN message defined
   in [RFC4271].

   Accordingly, when the length of Optional Parameters in the BGP OPEN
   message is greater than 255, the OPEN message format is modified to
   be the following:

       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                        |
       |      0xFF     |      0xFF     |  Extended Opt. Parm. Length   |
       |                                                               |
       |             Optional Parameters (variable)                    |
       |                                                               |

   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)         ~
       |                                               |

   In parsing an OPEN message, a BGP speaker MUST use the value of the
   one-octet "Optional Parameters Length" field and the value of the
   octet following it to determine the encoding of the Optional
   Parameters length, as well as the size of the "Parameter Length"
   field of the Optional Parameters.  If both values are 255, then the
   four-octet encoding described above is used for the Optional
   Parameters length.  Otherwise the encoding defined in [RFC4271] is

   This encoding is chosen for backward compatibility reasons -- a BGP
   speaker which has not been upgraded to support this specification may
   legitimately send Optional Parameters whose length equals exactly
   255, thus the Optional Parameters Length field alone is insufficient
   as an indicator.  However, such a speaker would never legitimately
   send an Optional Parameter whose type code is 255, since that value
   has been reserved by this specification.

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 may be 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 above is the most likely error to be sent, it is not
   impossible that the old speaker might detect some other error first,
   such as a length error, depending on the details of the
   implementation.  In no case would the peering be expected to
   establish successfully; the only question is which NOTIFICATION would
   be generated.

   We note that in any case, such a peering could not succeed, since by
   definition the extended length encoding would not be used by the new
   speaker unless the basic encoding was insufficient.

4.  IANA Considerations

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

5.  Security Considerations

   This extension to BGP does not change the underlying security issues.

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 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. 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. 2006,

7.2.  Informative References

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

Authors' Addresses

   Enke Chen
   Cisco Systems


   John Scudder
   Juniper Networks