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RFC 5881

Internet Engineering Task Force (IETF)                           D. Katz
Request for Comments: 5881                                       D. Ward
Category: Standards Track                               Juniper Networks
ISSN: 2070-1721                                                June 2010

                Bidirectional Forwarding Detection (BFD)
                     for IPv4 and IPv6 (Single Hop)


   This document describes the use of the Bidirectional Forwarding
   Detection (BFD) protocol over IPv4 and IPv6 for single IP hops.

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

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
   ( 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.

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RFC 5881           BFD for IPv4 and IPv6 (Single Hop)          June 2010

1.  Introduction

   One very desirable application for Bidirectional Forwarding Detection
   (BFD) [BFD] is to track IPv4 and IPv6 connectivity between directly
   connected systems.  This could be used to supplement the detection
   mechanisms in routing protocols or to monitor router-host
   connectivity, among other applications.

   This document describes the particulars necessary to use BFD in this
   environment.  Interactions between BFD and other protocols and system
   functions are described in the BFD Generic Applications document

1.1.  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].

2.  Applications and Limitations

   This application of BFD can be used by any pair of systems
   communicating via IPv4 and/or IPv6 across a single IP hop that is
   associated with an incoming interface.  This includes, but is not
   limited to, physical media, virtual circuits, and tunnels.

   Each BFD session between a pair of systems MUST traverse a separate
   network-layer path in both directions.  This is necessary for
   demultiplexing to work properly, and also because (by definition)
   multiple sessions would otherwise be protecting the same path.

   If BFD is to be used in conjunction with both IPv4 and IPv6 on a
   particular path, a separate BFD session MUST be established for each
   protocol (and thus encapsulated by that protocol) over that link.

   If the BFD Echo function is used, transmitted packets are immediately
   routed back towards the sender on the interface over which they were
   sent.  This may interact with other mechanisms that are used on the
   two systems that employ BFD.  In particular, ingress filtering
   [BCP38] is incompatible with the way Echo packets need to be sent.
   Implementations that support the Echo function MUST ensure that
   ingress filtering is not used on an interface that employs the Echo
   function or make an exception for ingress filtering Echo packets.

   An implementation of the Echo function also requires Application
   Programming Interfaces (APIs) that may not exist on all systems.  A
   system implementing the Echo function MUST be capable of sending

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RFC 5881           BFD for IPv4 and IPv6 (Single Hop)          June 2010

   packets to its own address, which will typically require bypassing
   the normal forwarding lookup.  This typically requires access to APIs
   that bypass IP-layer functionality.

   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.  Initialization and Demultiplexing

   In this application, there will be only a single BFD session between
   two systems over a given interface (logical or physical) for a
   particular protocol.  The BFD session must be bound to this
   interface.  As such, both sides of a session MUST take the "Active"
   role (sending initial BFD Control packets with a zero value of Your
   Discriminator), and any BFD packet from the remote machine with a
   zero value of Your Discriminator MUST be associated with the session
   bound to the remote system, interface, and protocol.

4.  Encapsulation

   BFD Control packets MUST be transmitted in UDP packets with
   destination port 3784, within an IPv4 or IPv6 packet.  The source
   port MUST be in the range 49152 through 65535.  The same UDP source
   port number MUST be used for all BFD Control packets associated with
   a particular session.  The source port number SHOULD be unique among
   all BFD sessions on the system.  If more than 16384 BFD sessions are
   simultaneously active, UDP source port numbers MAY be reused on
   multiple sessions, but the number of distinct uses of the same UDP
   source port number SHOULD be minimized.  An implementation MAY use
   the UDP port source number to aid in demultiplexing incoming BFD
   Control packets, but ultimately the mechanisms in [BFD] MUST be used
   to demultiplex incoming packets to the proper session.

   BFD Echo packets MUST be transmitted in UDP packets with destination
   UDP port 3785 in an IPv4 or IPv6 packet.  The setting of the UDP
   source port is outside the scope of this specification.  The

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RFC 5881           BFD for IPv4 and IPv6 (Single Hop)          June 2010

   destination address MUST be chosen in such a way as to cause the
   remote system to forward the packet back to the local system.  The
   source address MUST be chosen in such a way as to preclude the remote
   system from generating ICMP or Neighbor Discovery Redirect messages.
   In particular, the source address SHOULD NOT be part of the subnet
   bound to the interface over which the BFD Echo packet is being
   transmitted, and it SHOULD NOT be an IPv6 link-local address, unless
   it is known by other means that the remote system will not send

   BFD Echo packets MUST be transmitted in such a way as to ensure that
   they are received by the remote system.  On multiaccess media, for
   example, this requires that the destination datalink address
   corresponds to the remote system.

   The above requirements may require the bypassing of some common IP
   layer functionality, particularly in host implementations.

5.  TTL/Hop Limit Issues

   If BFD authentication is not in use on a session, all BFD Control
   packets for the session MUST be sent with a Time to Live (TTL) or Hop
   Limit value of 255.  All received BFD Control packets that are
   demultiplexed to the session MUST be discarded if the received TTL or
   Hop Limit is not equal to 255.  A discussion of this mechanism can be
   found in [GTSM].

   If BFD authentication is in use on a session, all BFD Control packets
   MUST be sent with a TTL or Hop Limit value of 255.  All received BFD
   Control packets that are demultiplexed to the session MAY be
   discarded if the received TTL or Hop Limit is not equal to 255.  If
   the TTL/Hop Limit check is made, it MAY be done before any
   cryptographic authentication takes place if this will avoid
   unnecessary calculation that would be detrimental to the receiving

   In the context of this section, "authentication in use" means that
   the system is sending BFD Control packets with the Authentication bit
   set and with the Authentication Section included and that all
   unauthenticated packets demultiplexed to the session are discarded,
   per the BFD base specification.

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RFC 5881           BFD for IPv4 and IPv6 (Single Hop)          June 2010

6.  Addressing Issues

   Implementations MUST ensure that all BFD Control packets are
   transmitted over the one-hop path being protected by BFD.

   On a multiaccess network, BFD Control packets MUST be transmitted
   with source and destination addresses that are part of the subnet
   (addressed from and to interfaces on the subnet).

   On a point-to-point link, the source address of a BFD Control packet
   MUST NOT be used to identify the session.  This means that the
   initial BFD packet MUST be accepted with any source address, and that
   subsequent BFD packets MUST be demultiplexed solely by the Your
   Discriminator field (as is always the case).  This allows the source
   address to change if necessary.  If the received source address
   changes, the local system MUST NOT use that address as the
   destination in outgoing BFD Control packets; rather, it MUST continue
   to use the address configured at session creation.  An implementation
   MAY notify the application that the neighbor's source address has
   changed, so that the application might choose to change the
   destination address or take some other action.  Note that the TTL/Hop
   Limit check described in section 5 (or the use of authentication)
   precludes the BFD packets from having come from any source other than
   the immediate neighbor.

7.  BFD for Use with Tunnels

   A number of mechanisms are available to tunnel IPv4 and IPv6 over
   arbitrary topologies.  If the tunnel mechanism does not decrement the
   TTL or Hop Limit of the network protocol carried within, the
   mechanism described in this document may be used to provide liveness
   detection for the tunnel.  The BFD authentication mechanism SHOULD be
   used and is strongly encouraged.

8.  IANA Considerations

   Ports 3784 and 3875 were assigned by IANA for use with the BFD
   Control and BFD Echo protocols, respectively.

9.  Security Considerations

   In this application, the use of TTL=255 on transmit and receive,
   coupled with an association to an incoming interface, is viewed as
   supplying equivalent security characteristics to other protocols used
   in the infrastructure, as it is not trivially spoofable.  The
   security implications of this mechanism are further discussed in

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RFC 5881           BFD for IPv4 and IPv6 (Single Hop)          June 2010

   The security implications of the use of BFD authentication are
   discussed in [BFD].

   The use of the TTL=255 check simultaneously with BFD authentication
   provides a low overhead mechanism for discarding a class of
   unauthorized packets and may be useful in implementations in which
   cryptographic checksum use is susceptible to denial-of-service
   attacks.  The use or non-use of this mechanism does not impact

10.  References

10.1.  Normative References

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

   [BFD-GENERIC] Katz, D. and D. Ward, "Generic Application of
                 Bidirectional Forwarding Detection (BFD)", RFC 5882,
                 June 2010.

   [GTSM]        Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and
                 C. Pignataro, "The Generalized TTL Security Mechanism
                 (GTSM)", RFC 5082, October 2007.

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

10.2.  Informative References

   [BCP38]       Ferguson, P. and D. Senie, "Network Ingress Filtering:
                 Defeating Denial of Service Attacks which employ IP
                 Source Address Spoofing", BCP 38, RFC 2827, May 2000.

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

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RFC 5881           BFD for IPv4 and IPv6 (Single Hop)          June 2010

Authors' Addresses

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

   Phone: +1-408-745-2000

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

   Phone: +1-408-745-2000

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