Extended Message support for BGP
draft-ietf-idr-bgp-extended-messages-31

Network Working Group                                            R. Bush
Internet-Draft                                              IIJ & Arrcus
Updates: 4271 (if approved)                                     K. Patel
Intended status: Standards Track                            Arrcus, Inc.
Expires: January 1, 2020                                         D. Ward
                                                           Cisco Systems
                                                           June 30, 2019

                    Extended Message support for BGP
                draft-ietf-idr-bgp-extended-messages-31

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 January 1, 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
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   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

   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, see Section 4, 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.

   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 a backward compatibility

   A BGP speaker that is capable of sending and 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 its peer only if both peers have
   negotiated the Extended Message Capability with each other.

   An implementation that advertises support for BGP Extended Messages
   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.

   If a BGP message with a Length lgreater than 4,096 octets is received
   by a BGP listener who has not advertised the Extended Message
   Capability, the listener MUST treat this as a malformed message, and
   MUST generate a NOTIFICATION with the Error Subcode set to Bad
   Message Length (see [RFC4271] Sec 6.1).

   A BGP announcement will (policy, best path, etc., allowing) propagate
   throughout the BGP speaking Internet; and hence to BGP speakers which
   may not have the Extended Message capability.  Therefore, an
   announcement in an Extended Message where the size of the attribute

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   set plus the NLRI can not be decomposed to 4,096 octets or less may
   cause lack of reachability.

   A speaker capable of BGP Extended Messages having a mixture of peers
   some of which have not exchanged the BGP Extended Message capability,
   may receive an announcement from one of its capable peers that would
   (due to the new AS on the path, new added attributes, etc.) produce
   an ongoing announcement that would be over 4,096 octets.  When
   propagating that update onward to a neighbor with which it has not
   negotiated the BGP Extended Message capability, the sender SHOULD try
   to reduce the outgoing message size by downgrading BGPsec to BGP4,
   decomposing a multi-NLRI update producing multiple updates with fewer
   NLRI per update, removing attributes eligible under the attribute
   discard approach of [RFC7606], etc.  If the resulting message would
   still be over the 4,096 octet limit, the sender SHOULD treat-as-
   withdraw per [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 4,096 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
   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 treat it
   similarly.

   The inconsistency between the local and remote BGP speakers MUST be
   flagged to the network operator through standard operational

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   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 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:  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
   unintentional.

   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
   verify.

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   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 act to reduce
   the outgoing message, see Section 4, and in doing so produce a
   downgrade attack, e.g. convert BGPsec to BGP4.

   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 passed by an extended
   message speaker.

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

   [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>.

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

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <http://www.rfc-editor.org/info/rfc5116>.

   [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>.

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   [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>.

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,
              <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
   IIJ & Arrcus
   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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