RFC 9072

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


   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

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

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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

   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",
   "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

   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

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

   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,

   [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, <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,

   [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>.


   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