Extended Optional Parameters Length for BGP OPEN Message
draft-ietf-idr-ext-opt-param-07

Document Type Active Internet-Draft (idr WG)
Last updated 2019-09-12 (latest revision 2019-08-01)
Replaces draft-chen-bgp-ext-opt-param
Stream IETF
Intended RFC status (None)
Formats plain text xml pdf htmlized bibtex
Reviews
Stream WG state WG Consensus: Waiting for Write-Up
Doc Shepherd Follow-up Underway, Other - see Comment Log
Document shepherd No shepherd assigned
IESG IESG state I-D Exists
Consensus Boilerplate Unknown
Telechat date
Responsible AD (None)
Send notices to (None)
Internet Engineering Task Force                                  E. Chen
Internet-Draft                                             Cisco Systems
Updates: 4271 (if approved)                                   J. Scudder
Intended status: Standards Track                        Juniper Networks
Expires: February 1, 2020                                  July 31, 2019

        Extended Optional Parameters Length for BGP OPEN Message
                    draft-ietf-idr-ext-opt-param-07

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 concern about
   this limitation.

   In this document we update RFC 4271 by extending the BGP OPEN length
   field in a backward-compatible manner.  The Parameter Length field of
   individual Optional Parameters is also extended.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 1, 2020.

Copyright Notice

   Copyright (c) 2019 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

Chen & Scudder          Expires February 1, 2020                [Page 1]
Internet-Draft     Extended Optional Parameters Length         July 2019

   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.

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.

   In this document we update [RFC4271] by extending the BGP OPEN length
   field in a backward-compatible manner.  The Parameter Length field of
   individual Optional Parameters is also extended.  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 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", "MAY", and "OPTIONAL" in this
   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, an
   implementation MUST be prepared to accept an OPEN message that uses
   the encoding of this specification for Optional Parameters of any
   length.

   If the length of Optional Parameters is greater than 255, the
   extended encoding defined here MUST be used.  The (non-extended)
   length field MUST be set to 255.  The subsequent octet (which would
   be the first Optional Parameter Type in the non-extended format) MUST
   be set to 255 as well.  The subsequent 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

Chen & Scudder          Expires February 1, 2020                [Page 2]
Internet-Draft     Extended Optional Parameters Length         July 2019

   the given fields, there is no change to the BGP OPEN message defined
   in [RFC4271].

   When receiving an OPEN, a BGP speaker determines the extended
   encoding is in use if the first Optional Parameter Type field is 255.
   In this case, the BGP speaker MUST use the Extended Optional
   Parameters Length field in lieu of the [RFC4271] encoding to
   determine the length of Optional Parameters contained in the message.

   Accordingly, when the length of Optional Parameters in the BGP OPEN
   message is greater than 255, the OPEN message format is modified as
   follows, using the first Optional Parameter Type field to indicate
   the use of the extended format:

       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)                    |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The non-extended Optional Parameters Length field MUST be set to 255
   on transmission, 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 field.

   The subsequent one-octet field, that in the non-extended format would
   be the first Optional Parameter Type field, 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.

Chen & Scudder          Expires February 1, 2020                [Page 3]
Internet-Draft     Extended Optional Parameters Length         July 2019

   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)         ~
       |                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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.

   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.

   The choice to mandate that when the extended encoding is in use, the
   (non-extended) Optional Parameters Length field must be 255 was made
   for backward compatibility with implementations of earlier versions
   of this specification.  In any event the value 0 MUST NOT be used in
   this field since the presence of that value could have the effect of
   causing a message parser to never inspect the following octet.

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

Chen & Scudder          Expires February 1, 2020                [Page 4]
Internet-Draft     Extended Optional Parameters Length         July 2019

   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
   [RFC4271].

   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.

   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, any value SHOULD be silently accepted.  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 issues
   inherent in the existing BGP [RFC4272].

Chen & Scudder          Expires February 1, 2020                [Page 5]
Internet-Draft     Extended Optional Parameters Length         July 2019

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,
              <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>.

7.2.  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>.

Authors' Addresses

   Enke Chen
   Cisco Systems

   Email: enkechen@cisco.com

   John Scudder
   Juniper Networks

   Email: jgs@juniper.net

Chen & Scudder          Expires February 1, 2020                [Page 6]