





Internet Engineering Task Force (IETF)                           D. Katz
Request for Comments: 5883                                       D. Ward
Category: Standards Track                               Juniper Networks
ISSN: 2070-1721                                                June 2010


      Bidirectional Forwarding Detection (BFD) for Multihop Paths

Abstract

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

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5883.

Copyright Notice

   Copyright (c) 2010 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
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   publication of this document.  Please review these documents
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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.









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RFC 5883                 BFD for Multihop Paths                June 2010


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.

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [KEYWORDS].

2.  Applicability

   Please note that BFD is intended as an Operations, Administration,
   and Maintenance (OAM) mechanism for connectivity check and connection
   verification.  It is applicable for network-based services (e.g.
   router-to-router, subscriber-to-gateway, LSP/circuit endpoints, and
   service appliance failure detection).  In these scenarios it is
   required that the operator correctly provision the rates at which BFD
   is transmitted to avoid congestion (e.g link, I/O, CPU) and false
   failure detection.  It is not applicable for application-to-
   application failure detection across the Internet because it does not
   have sufficient capability to do necessary congestion detection and
   avoidance and therefore cannot prevent congestion collapse.  Host-to-
   host or application-to-application deployment across the Internet
   will require the encapsulation of BFD within a transport that
   provides "TCP-friendly" [TFRC] behavior.

3.  Issues

   There are three 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 Time to Live
   (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 second, 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



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RFC 5883                 BFD for Multihop Paths                June 2010


   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.

   [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).

   Finally, the Echo function MUST NOT be used over multiple hops.
   Intermediate hops would route the packets back to the sender, and
   connectivity through the entire path would not be possible to verify.

4.  Demultiplexing Packets

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

4.1.  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].

4.2.  Out-of-Band Discriminator Signaling

   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].




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RFC 5883                 BFD for Multihop Paths                June 2010


   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 the perspective of BFD implementation 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.

4.3.  Unidirectional Links

   Unidirectional links are classified as multihop paths because the
   return path (which should 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 unidirectional links, one in each direction, are
   to carry a single BFD session, this can be done using the single-hop
   approach.)

   Either of the two methods outlined earlier may be used in the
   unidirectional link case, but a more general solution can be found
   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 it 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.

5.  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.







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RFC 5883                 BFD for Multihop Paths                June 2010


6.  Authentication

   By their nature, multihop paths expose BFD to spoofing.  As the
   number of hops increases, the exposure to attack grows.  As such,
   implementations of BFD SHOULD utilize cryptographic authentication
   over multihop paths to help mitigate denial-of-service attacks.

7.  IANA Considerations

   Port 4784 has been assigned by IANA for use with BFD Multihop
   Control.

8.  Security Considerations

   As the number of hops increases, BFD becomes further exposed to
   attack.  The use of strong forms of authentication is strongly
   encouraged.

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

9.  References

9.1.  Normative References

   [BFD]      Katz, D. and D. Ward, "Bidirectional Forwarding
              Detection", RFC 5880, June 2010.

   [BFD-1HOP] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, June
              2010.

   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

9.2.  Informative References

   [BFD-MPLS] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
              "Bidirectional Forwarding Detection (BFD) for MPLS Label
              Switched Paths (LSPs)", RFC 5884, June 2010.

   [OSPFv2]   Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [OSPFv3]   Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, July 2008.






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RFC 5883                 BFD for Multihop Paths                June 2010


   [TFRC]     Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification", RFC
              5348, September 2008.

Authors' Addresses

   Dave Katz
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089-1206
   USA

   Phone: +1-408-745-2000
   EMail: dkatz@juniper.net


   Dave Ward
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089-1206
   USA

   Phone: +1-408-745-2000
   EMail: dward@juniper.net



























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