Bidirectional Forwarding Detection (BFD) for Multihop Paths
RFC 5883

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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
   Provisions Relating to IETF Documents
   (http://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.

Katz & Ward                  Standards Track                    [Page 1]
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

Katz & Ward                  Standards Track                    [Page 2]
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