Network Working Group                                           D. Katz
Internet Draft                                         Juniper Networks
                                                                D. Ward
                                                          Cisco Systems
Expires: December, 2006                                      June, 2006

                         BFD for Multihop Paths

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   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."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

Copyright Notice

   Copyright (C) The Internet Society (2006).  All Rights Reserved.

Katz, Ward                                                      [Page 1]

Internet Draft           BFD for Multihop Paths               June, 2006


   This document describes the use of the Bidirectional Forwarding
   Detection protocol (BFD) over multihop paths, including
   unidirectional links.

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC-2119 [KEYWORDS].

1. Introduction

   The Bidirectional Forwarding Detection (BFD) protocol [BFD] defines a
   method for liveness detection of arbitrary paths between systems.
   The BFD one-hop specification [BFD-1HOP] describes how to use BFD
   across single hops of IPv4 and IPv6.

   BFD can also be useful on arbitrary paths between systems, which may
   span multiple network hops and follow unpredictable paths.
   Furthermore, a pair of systems may have multiple paths between them
   that may overlap.  This document describes methods for using BFD in
   such scenarios.

2. Issues

   There are two primary issues in the use of BFD for multihop paths.
   The first is security and spoofing; [BFD-1HOP] describes a
   lightweight method of avoiding spoofing by requiring a TTL/hop limit
   of 255 on both transmit and receive, but this obviously does not work
   across multiple hops.  The utilization of BFD authentication
   addresses this issue.

   The more subtle issue is that of demultiplexing multiple BFD sessions
   between the same pair of systems to the proper BFD session.  In
   particular, the first BFD packet received for a session may carry a
   Your Discriminator value of zero, resulting in ambiguity as to which
   session the packet should be associated.  Once the discriminator
   values have been exchanged, all further packets are demultiplexed to
   the proper BFD session solely by the contents of the Your
   Discriminator field.

Katz, Ward                                                      [Page 2]

Internet Draft           BFD for Multihop Paths               June, 2006

   [BFD-1HOP] addresses this by requiring that multiple sessions
   traverse independent physical or logical links--the first packet is
   demultiplexed based on the link over which it was received.  In the
   more general case, this scheme cannot work, as two paths over which
   BFD is running may overlap to an arbitrary degree (including the
   first and/or last hop.)

3. Demultiplexing Packets

   There are a number of possibilities for addressing the demultiplexing
   issue which may be used, depending on the application.

3.1. Totally Arbitrary Paths

   It may be desired to use BFD for liveness detection over paths for
   which no part of the route is known (or if known, may not be stable.)
   A straightforward approach to this problem is to limit BFD deployment
   to a single session between a source/destination address pair.
   Multiple sessions between the same pair of systems must have at least
   one endpoint address distinct from one another.

   In this scenario, the initial packet is demultiplexed to the
   appropriate BFD session based on the source/destination address pair
   when Your Discriminator is set to zero.

   This approach is appropriate for general connectivity detection
   between systems over routed paths, and is also useful for OSPF
   Virtual Links [OSPFv2] [OSPFv3].

3.2. Out-of-band Discriminator Signalling

   Another approach to the demultiplexing problem is to signal the
   discriminator values in each direction through an out-of-band
   mechanism prior to establishing the BFD session.  Once learned, the
   discriminators are sent as usual in the BFD Control packets;  no
   packets with Your Discriminator set to zero are ever sent.  This
   method is used by the BFD MPLS specification [BFD-MPLS].

   This approach is advantageous because it allows BFD to be directed by
   other system components that have knowledge of the paths in use, and
   from BFD's perspective it is very simple.

   The disadvantage is that it requires at least some level of BFD-
   specific knowledge in parts of the system outside of BFD.

Katz, Ward                                                      [Page 3]

Internet Draft           BFD for Multihop Paths               June, 2006

3.3. Unidirectional Links

   Unidirectional links are classified as multihop paths because the
   return path (which must exist at some level in order to make the link
   useful) may be arbitrary, and the return paths for BFD sessions
   protecting parallel unidirectional links may overlap or even be
   identical.  (If two unidirection links, one in each direction, are to
   carry a single BFD session, this can be done using the single-hop

   Either of the two methods outlined earlier may be used in the
   Unidirectional link case, but a more general solution can be done
   strictly within BFD and without addressing limitations.

   The approach is similar to the one-hop specification, since the
   unidirectional link is a single hop.  Let's define the two systems as
   the Unidirectional Sender and the Unidirectional Receiver.  In this
   approach the Unidirectional Sender MUST operate in the Active role
   (as defined in the base BFD specification), and the Unidirectional
   Receiver MUST operate in the Passive role.

   In the Passive role, by definition, the Unidirectional Receiver does
   not transmit any BFD Control packets until it learns the
   discriminator value in use by the other system (upon receipt of the
   first BFD Control packet.)  The Unidirectional Receiver demultiplexes
   the first packet to the proper BFD session based on the physical or
   logical link over which was received.  This allows the receiver to
   learn the remote discriminator value, which it then echoes back to
   the sender in its own (arbitrarily routed) BFD Control packet, after
   which time all packets are demultiplexed solely by discriminator.

4. Encapsulation

   The encapsulation of BFD Control packets for multihop application in
   IPv4 and IPv6 is identical to that defined in [BFD-1HOP], except that
   the UDP destination port MUST have a value of 4784.  This can aid in
   the demultiplexing and internal routing of incoming BFD packets.

Katz, Ward                                                      [Page 4]

Internet Draft           BFD for Multihop Paths               June, 2006

5. Authentication

   By their nature, multihop paths expose BFD to spoofing.
   Implementations of BFD SHOULD utilize authentication over multihop
   paths to help mitigate denial-of-service attacks.

Normative References

   [BFD] Katz, D., and Ward, D., "Bidirectional Forwarding Detection",
       draft-ietf-bfd-base-05.txt, June, 2006.

   [BFD-1HOP] Katz, D., and Ward, D., "BFD for IPv4 and IPv6 (Single
       Hop)", draft-ietf-bfd-v4v6-1hop-05.txt, June, 2006.

   [BFD-MPLS] Aggarwal, R., and Kompella, K., "BFD for MPLS LSPs",
       draft-ietf-bfd-mpls-03.txt, June, 2006.

   [KEYWORD] Bradner, S., "Key words for use in RFCs to Indicate
       Requirement Levels", RFC 2119, March 1997.

   [OSPFv2] Moy, J., "OSPF Version 2", RFC 2328, April 1998.

   [OSPFv3] Coltun, R., et al, "OSPF for IPv6", RFC 2740, December 1999.

Security Considerations

   No additional security issues are raised in this document beyond
   those that exist in the referenced BFD documents.

IANA Considerations

   This document has no actions for IANA.

Katz, Ward                                                      [Page 5]

Internet Draft           BFD for Multihop Paths               June, 2006

Authors' Addresses

    Dave Katz
    Juniper Networks
    1194 N. Mathilda Ave.
    Sunnyvale, California 94089-1206 USA
    Phone: +1-408-745-2000

    Dave Ward
    Cisco Systems
    170 W. Tasman Dr.
    San Jose, CA 95134 USA
    Phone: +1-408-526-4000

Changes from the previous draft

   The only substantive change was the addition of the assigned UDP port
   number for use with this protocol.  All other changes are purely
   editorial in nature.

IPR Disclaimer

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-

Katz, Ward                                                      [Page 6]

Internet Draft           BFD for Multihop Paths               June, 2006

Full Copyright Notice

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

   This document expires in December, 2006.

Katz, Ward                                                      [Page 7]