Network Working Group                                            R. Bush
Internet-Draft                                 Internet Initiative Japan
Updates: 4271 (if approved)                                     K. Patel
Intended status: Standards Track                            Arrcus, Inc.
Expires: September 11, 27, 2019                                      D. Ward
                                                           Cisco Systems
                                                          March 10, 26, 2019

                    Extended Message support for BGP


   The BGP specification mandates a maximum BGP message size of 4096
   octets.  As BGP is extended to support newer AFI/SAFIs and other
   features, there is a need to extend the maximum message size beyond
   4096 octets.  This document updates the BGP specification RFC4271 by
   providing an extension to BGP to extend its current maximum message
   size from 4096 octets to 65535 octets for all except the OPEN

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   be interpreted as described in [RFC2119] only when they appear in all
   upper case.  They may also appear in lower or mixed case as English
   words, without normative meaning.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 September 11, 27, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( in effect on the date of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  BGP Extended Message  . . . . . . . . . . . . . . . . . . . .   2
   3.  Extended Message Capability for BGP . . . . . . . . . . . . .   3
   4.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Changes to RFC4271  . . . . . . . . . . . . . . . . . . . . .   4
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   6
     10.2.  Informative References . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   The BGP specification [RFC4271] mandates a maximum BGP message size
   of 4096 octets.  As BGP is extended to support newer AFI/SAFIs and
   newer capabilities (e.g., BGPsec, [RFC8205], BGP-LS, [RFC7752]),
   there is a need to extend the maximum message size beyond 4096
   octets.  This draft provides an extension to BGP to extend its
   current message size limit from 4096 octets to 65535 octets for all
   except the OPEN message.

2.  BGP Extended Message

   A BGP message over 4096 octets in length is a BGP Extended Message.

   BGP Extended Messages have maximum message size of 65535 octets.  The
   smallest message that may be sent consists of a BGP header without a
   data portion (19 octets).

3.  Extended Message Capability for BGP

   To advertise the BGP Extended Message Capability to a peer, a BGP
   speaker uses BGP Capabilities Advertisement [RFC5492].  By
   advertising the BGP Extended Message Capability to a peer, a BGP
   speaker conveys that it is able to send, receive, and properly handle
   BGP Extended Messages.

   The BGP Extended Message Capability is a new BGP Capability [RFC5492]
   defined with Capability code 6 and Capability length 0.

   A peer which does not advertise this capability MUST NOT send BGP
   Extended Messages, and BGP Extended Messages MUST NOT be sent to it.

4.  Operation

   A BGP speaker that is capable of sending and receiving BGP Extended
   Messages SHOULD advertise the BGP Extended Message Capability to the
   peer using BGP Capabilities Advertisement [RFC5492].  A BGP speaker
   MAY send Extended Messages to its peer only if it has fully exchanged
   the Extended Message Capability with that peer.

   The Extended Message Capability applies to all messages except for
   the OPEN message.  This exception is made to reduce complexity of
   providing backward compatibility

   An implementation that advertises support for BGP Extended Messages
   MUST be capable of receiving a message with a length up to and
   including 65535 octets.

   Applications generating information which might be encapsulated
   within BGP messages MUST limit the size of their payload to take the
   maximum message size into account.

   If a BGP update with a payload longer than 4096 octets is received by
   a BGP listener who has neither advertised nor agreed to accept BGP
   Extended Messages, the listener MUST treat this as a malformed update
   message, and MUST raise an UPDATE Message Error (see [RFC4271] Sec

   A BGP announcement will, in the normal case, propagate throughout the
   BGP speaking Internet; and there will undoubtedly be BGP speakers
   which do not have the Extended Message capability.  Therefore, having
   an attribute set which can not be decomposed to 4096 octets or less
   in an Extended Message will likely raise errors.

   A BGP speaker with a mixture of peers some of which have negotiated
   BGP Extended Message capability and some which have not, MUST
   o  support [RFC7606], and
   o  "treat as withdraw' withdraw" (see [RFC7606]) a BGP attribue/NLRI attribute/NLRI pair
      (defined as BGP Route) which is too large to be sent to a peer
      which does not support BGP Extended Messages.

   The BGP speaker MAY remove some BGP attributes which are eligible to
   use the Attribute discard approach in [RFC7606].

   In an iBGP mesh, all peers SHOULD support the BGP Extended Message
   Capability and [RFC7606].  Only then is it consistent to deploy with
   eBGP peers.

   During the incremental deployment of BGP Extended Messages and
   [RFC7606] in an iBGP mesh, or with eBGP peers, the operator should
   monitor any routes dropped as "treat as withdraw".

   It is RECOMMENDED that BGP protocol developers and implementers are
   conservative in their application and use of Extended Messages.
   Future protocol specifications will need to describe how to handle
   peers which can only accommodate 4096 octet messages.

5.  Error Handling

   A BGP speaker that has the ability to use Extended Messages but has
   not advertised the BGP Extended Messages capability, presumably due
   to configuration, SHOULD NOT accept an Extended Message.  A speaker
   SHOULD NOT implement a more liberal policy accepting BGP Extended

   A BGP speaker that does not advertise the BGP Extended Messages
   capability might also genuinely not support Extended Messages.  Such
   a speaker will follow the error handling procedures of [RFC4271] if
   it receives an Extended Message.  Similarly, any speaker that treats
   an improper Extended Message as a fatal error, MUST treat it

   The inconsistency between the local and remote BGP speakers MUST be
   flagged to the network operator through standard operational
   interfaces.  The information should include the NLRI and as much
   relevant information as reasonably possible.

6.  Changes to RFC4271

   [RFC4271] states "The value of the Length field MUST always be at
   least 19 and no greater than 4096."  This document changes the latter
   number to 65535 for all except the OPEN message.

   [RFC4271] Sec 6.1, specifies raising an error if the length of a
   message is over 4096 octets.  For all messages except the OPEN
   message, if the receiver has advertised the capability to receive BGP Extended Messages, Messages
   Capability, this document raises that limit to 65535.

7.  IANA Considerations

   The IANA has made an early allocation for this new BGP Extended
   Message Capability referring to this document.

   Registry:  BGP Capability Code

   Value    Description                               Document
   -----    -----------------------------------       -------------
   6        BGP-Extended Message                      [this draft]

8.  Security Considerations

   This extension to BGP does not change BGP's underlying security
   issues; see [RFC4272].

   Section 5 allows a receiver to accept an Extended Message even though
   it had not advertised the capability.  This slippery slope could lead
   to sloppy implementations sending Extended Messages when the receiver
   is not prepared to deal with them, e.g. to peer groups.  At best,
   this will result in errors; at worst, buffer overflows.

   Due to increased memory requirements for buffering, there may be
   increased exposure to resource exhaustion, intentional or

   As this draft requires support for [RFC7606] update error handling,
   it inherits the security considerations of [RFC7606].  BGP peers may
   avoid such issues by using Authenticated Encryption with additional
   Data (AEAD) ciphers [RFC5116] and discard messages that do not

   If a remote attacker is able to craft a large BGP Extended Message to
   send on a path where one or more peers do not support BGP Extended
   Messages, peers which support BGP Extended Messages may incur
   resource load (processing, message resizing, etc.) reformatting the
   large messages.  Worse, ([RFC7606] "treat as withdraw" may
   consistently withdraw announcements causing inconsistent routing.

   BGP routes are filtered by policies set by the operators.
   Implementations may provide policies to filter routes that would
   cause the "treat as withdraw" from being pass passed by an extended
   message speaker.

9.  Acknowledgments

   The authors thank Alvaro Retana, Enke Chen, Susan Hares, John
   Scudder, John Levine, and Job Snijders for their input; and Oliver
   Borchert and Kyehwan Lee for their implementations and testing.

10.  References

10.1.  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,

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

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,

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

   [RFC7606]  Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
              Patel, "Revised Error Handling for BGP UPDATE Messages",
              RFC 7606, DOI 10.17487/RFC7606, August 2015,

10.2.  Informative References

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,

   [RFC8205]  Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
              Specification", RFC 8205, DOI 10.17487/RFC8205, September
              2017, <>.

Authors' Addresses

   Randy Bush
   Internet Initiative Japan
   5147 Crystal Springs
   Bainbridge Island, Washington  98110
   United States of America


   Keyur Patel
   Arrcus, Inc.


   Dave Ward
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   United States of America