Extended Message support for BGP
draft-ietf-idr-bgp-extended-messages-36

Document Type Active Internet-Draft (idr WG)
Last updated 2019-10-08 (latest revision 2019-08-16)
Replaces draft-ymbk-bgp-extended-messages
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Network Working Group                                            R. Bush
Internet-Draft                                              Arrcus & IIJ
Updates: 4271 (if approved)                                     K. Patel
Intended status: Standards Track                            Arrcus, Inc.
Expires: February 17, 2020                                       D. Ward
                                                           Cisco Systems
                                                         August 16, 2019

                    Extended Message support for BGP
                draft-ietf-idr-bgp-extended-messages-36

Abstract

   The BGP specification mandates a maximum BGP message size of 4,096
   octets.  As BGP is extended to support newer AFI/SAFIs and other
   features, there is a need to extend the maximum message size beyond
   4,096 octets.  This document updates the BGP specification RFC4271 by
   extending the maximum message size from 4,096 octets to 65,535 octets
   for all except the OPEN and KEEPALIVE messages.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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.

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 17, 2020.

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

1.  Introduction

   The BGP specification [RFC4271] mandates a maximum BGP message size
   of 4,096 octets.  As BGP is extended to support newer AFI/SAFIs and
   newer capabilities (e.g., BGPsec [RFC8205] and BGP-LS [RFC7752]),
   there is a need to extend the maximum message size beyond 4,096
   octets.  This draft provides an extension to BGP to extend its
   message size limit from 4,096 octets to 65,535 octets for all except
   the OPEN and KEEPALIVE messages.

2.  BGP Extended Message

   A BGP message over 4,096 octets in length is a BGP Extended Message.

   BGP Extended Messages have a maximum message size of 65,535 octets.
   The smallest message that may be sent consists of a BGP KEEPALIVE
   which consists of 19 octets.

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3.  Extended Message Capability for BGP

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

   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 receive and properly handle, see
   Section 4, BGP Extended Messages.

   Peers that wish to use the BGP Extended Message capability MUST
   support Error Handling for BGP UPDATE Messages per [RFC7606].

4.  Operation

   The Extended Message Capability applies to all messages except for
   the OPEN and KEEPALIVE messages.  The former exception is to reduce
   the complexity of providing backward compatibility.

   A BGP speaker that is capable of receiving BGP Extended Messages
   SHOULD advertise the BGP Extended Message Capability to its peers
   using BGP Capabilities Advertisement [RFC5492].  A BGP speaker MAY
   send Extended Messages to a peer only if the Extended Message
   Capability was received from that peer.

   An implementation that advertises the BGP Extended Message capability
   MUST be capable of receiving a message with a Length up to and
   including 65,535 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.

   During the years of incremental deployment, speakers that are capable
   of Extended Messages should not simply pack as many NLRI in a message
   as they can, or otherwise unnecessarily generate UPDATES above the
   4,096 octet pre- Extended Message limit, so as not to require
   downstream routers to decompose for peers that do not support
   Extended Messages.  See Section 8.

   If a BGP message with a Length greater than 4,096 octets is received
   by a BGP listener who has not advertised the Extended Message
   Capability, the listener will generate a NOTIFICATION with the Error
   Subcode set to Bad Message Length ([RFC4271] Sec 6.1).

   A BGP UPDATE will (policy, best path, etc., allowing) typically
   propagate throughout the BGP speaking Internet; and hence to BGP

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   speakers which may not support Extended Messages.  Therefore, an
   announcement in an Extended Message where the size of the attribute
   set plus the NLRI is larger than 4,096 octets may cause lack of
   reachability.

   A BGP speaker that has advertised the BGP Extended Message capability
   to its peers, may receive an UPDATE from one of its peers that
   produces an ongoing announcement that is larger than 4,096 octets.
   When propagating that UPDATE onward to a neighbor which has not
   advertised the BGP Extended Message capability, the speaker SHOULD
   try to reduce the outgoing message size by removing attributes
   eligible under the "attribute discard" approach of [RFC7606].  If the
   message is still too big, then it must not be sent to the neighbor
   ([RFC4271], Section 9.2).  Additionally, if the NLRI was previously
   advertised to that peer, it must be withdrawn from service
   ([RFC4271], Section 9.1.3).

   If an Autonomous System (AS) has multiple internal BGP speakers and
   also has multiple external BGP neighbors, to present a consistent
   external view care must be taken to ensure a consistent view within
   the AS.  In the context of BGP Extended Messages, a consistent view
   can only be guaranteed if all the iBGP speakers advertise the BGP
   Extended Message capability.  If that is not the case, then the
   operator should consider whether the BGP Extended Message capability
   should be advertised to external peers or not.

   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 and any discarded attributes.

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, MUST NOT accept an Extended Message.  A speaker
   MUST NOT implement a more liberal policy accepting BGP Extended
   Messages.

   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 follow the error
   handling procedures of [RFC4271].

   The UPDATE Message Error Handling, as specified in Section 6.3 of
   [RFC4271], is unchanged.  However, if a NOTIFICATION is to be sent to

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   a BGP speaker that has not advertised the BGP Extended Message
   Capability, the size of the message MUST NOT exceed 4,096 octets.

   It is RECOMMENDED that BGP protocol developers and implementers are
   conservative in their application and use of Extended Messages.
   Future protocol specifications MUST describe how to handle peers
   which can only accommodate 4,096 octet messages.

6.  Changes to RFC4271

   [RFC4271] states "The value of the Length field MUST always be at
   least 19 and no greater than 4,096."  This document changes the
   latter number to 65,535 for all except the OPEN and KEEPALIVE
   messages.

   [RFC4271] Sec 6.1, specifies raising an error if the length of a
   message is over 4,096 octets.  For all messages except the OPEN
   message, if the receiver has advertised the BGP Extended Messages
   Capability, this document raises that limit to 65,535.

7.  IANA Considerations

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

   Registry: Capability Codes

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

8.  Security Considerations

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

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

   If a remote speaker 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 act to reduce
   the outgoing message, see Section 4, and in doing so cause an attack
   by discarding attributes its peer may be expecting.  The attributes
   eligible under the "attribute discard" must have no effect on route
   selection or installation [RFC7606].

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   If a remote speaker 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 act to reduce
   the outgoing message, see Section 4, and in doing so allow a
   downgrade attack.  This would only affect the attacker's message,
   where 'downgrade' has questionable meaning.

   If a remote speaker 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.

9.  Acknowledgments

   The authors thank Alvaro Retana for an amazing review, 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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc4271>.

   [RFC5492]  Scudder, J. and R. Chandra, "Capabilities Advertisement
              with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
              2009, <http://www.rfc-editor.org/info/rfc5492>.

   [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,
              <http://www.rfc-editor.org/info/rfc7606>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <http://www.rfc-editor.org/info/rfc8174>.

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10.2.  Informative References

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, DOI 10.17487/RFC4272, January 2006,
              <http://www.rfc-editor.org/info/rfc4272>.

   [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,
              <http://www.rfc-editor.org/info/rfc7752>.

   [RFC8205]  Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
              Specification", RFC 8205, DOI 10.17487/RFC8205, September
              2017, <https://www.rfc-editor.org/info/rfc8205>.

Authors' Addresses

   Randy Bush
   Arrcus & IIJ
   5147 Crystal Springs
   Bainbridge Island, Washington  98110
   US

   Email: randy@psg.com

   Keyur Patel
   Arrcus, Inc.

   Email: keyur@arrcus.com

   Dave Ward
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   US

   Email: dward@cisco.com

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