Network Working Group                                           D. Katz
Internet Draft                                         Juniper Networks
                                                                D. Ward
                                                          Cisco Systems
Expires: January, 2005                                       July, 2004

                         BFD for Multihop Paths

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   all provisions of Section 10 of RFC2026.

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Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

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Internet Draft           BFD for Multihop Paths               July, 2004


   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; the one-hop spec 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.

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   The one-hop specification 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.

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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 (as an MPLS LSP is in fact a unidirectional
   link), 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. 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.

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Normative References

   [BFD] Katz, D., and Ward, D., "Bidirectional Forwarding Detection",
       draft-ietf-bfd-base-00.txt, July, 2004.

   [BFD-1HOP] Katz, D., and Ward, D., "BFD for IPv4 and IPv6 (Single
       Hop)", draft-ietf-bfd-v4v6-1hop-00.txt, July, 2004.

   [BFD-MPLS] Aggarwal, R., and Kompella, K., "BFD for MPLS LSPs",
       draft-ietf-bfd-mpls-00.txt, July, 2004.

   [GTSM] Gill, V., et al, "The Generalized TTL Security Mechanism
       (GTSM)", RFC 3682, February 2004.

   [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.

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

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