Network Working Group                                           D. Katz
Internet Draft                                         Juniper Networks
                                                                D. Ward
                                                          Cisco Systems
Category: Informational                                       May, 2004
Expires: November, 2004


                   BFD for IPv4 and IPv6 (Single Hop)
                  draft-katz-ward-bfd-v4v6-1hop-01.txt


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   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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Abstract

   This document describes the use of the Bidirectional Forwarding
   Detection protocol over IPv4 and IPv6 for single IP hops.  It further
   describes the use of BFD with OSPFv2, OSPFv3, and IS-IS.  Comments on
   this draft should be directed to rtg-bfd@ietf.org.



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].



1. Introduction

   One very desirable application for BFD [BFD] is to track IPv4 and
   IPv6 connectivity between directly-connected systems.  This could be
   used to supplant the detection mechanisms in IS-IS and OSPF, or to
   monitor router-host connectivity, among other applications.

   This document describes the particulars necessary to use BFD in this
   environment, and describes how BFD can be used in conjunction OSPFv2
   [OSPFv2], OSPFv3 [OSPFv3], and IS-IS [ISIS].



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 can be
   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
   path in both directions.

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








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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

4.1. BFD for IPv4

   In the case of IPv4, BFD Control packets MUST be transmitted in UDP
   packets with destination port 3784, within an IPv4 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 packet.  The setting of the UDP source port
   is outside the scope of this specification.  The 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 Redirect messages (in particular, the source address MUST NOT be
   part of the subnet bound to the interface over which the BFD Echo
   packet is being transmitted.)


4.2. BFD for IPv6

   In the case of IPv6, BFD Control packets MUST be transmitted in UDP
   packets with destination port 3784, within an 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



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   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 IPv6 packet.  The setting of the UDP source port
   is outside the scope of this specification.  The source and
   destination addresses MUST both be associated with the local system.
   The destination address MUST be chosen in such a way as to cause the
   remote system to forward the packet back to the local system.



5. TTL/Hop Count Issues

   All BFD Control packets for sessions operating according to this
   specification MUST be sent with a TTL or Hop Count value of 255.  All
   received BFD Control packets that are demultiplexed to sessions
   operating according to this specification MUST be discarded if the
   received TTL or Hop Count is not equal to 255.



6. Addressing Issues

   On a subnetted 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 an addressed but unsubnetted point-to-point link, BFD Control
   packets MUST be transmitted with source and destination addresses
   that match the addresses configured on that link.

   On an unnumbered 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
   My Discriminator field (as is always the case.)  This allows the
   source address to change if necessary.  Note that the TTL/Hop Count
   check described in section 5 precludes the BFD packets from having
   come from any source other than the immediate neighbor.




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7. BFD for use with OSPFv2, OSPFv3, and IS-IS

   The two versions of OSPF, as well as IS-IS, all suffer from an
   architectural limitation, namely that their Hello protocols are
   limited in the granularity of failure their detection times.  In
   particular, OSPF has a minimum detection time of two seconds, and IS-
   IS has a minimum detection time of one second.

   BFD MAY be used to achieve arbitrarily small detection times for
   these protocols by supplanting the Hello protocols used in each case.


7.1. Session Establishment

   The mechanism by which a BFD session is established in this
   environment is outside the scope of this specification.  An obvious
   choice would be to use the discovery mechanism inherent in the Hello
   protocols in OSPF and IS-IS to bootstrap the establishment of a BFD
   session.

   Any BFD sessions established to support OSPF and IS-IS across a
   single IP hop MUST operate in accordance with the rest of this
   document.

   If multiple routing protocols wish to establish BFD sessions with the
   same remote system for the same data protocol, all MUST share a
   single BFD session.


7.2. Session Parameters

   The setting of the various timing parameters and modes in this
   application are outside the scope of this specification.

   Note that all protocols sharing a session will operate using the same
   parameters.  The mechanism for choosing the parameters among those
   desired by the various protocols are outside the scope of this
   specification.


7.3. Interactions with OSPF and IS-IS without Graceful Restart

   When a BFD session transitions from Up to Failing, action SHOULD be
   taken in the routing protocol to signal the lack of connectivity for
   the data protocol (IPv4 or IPv6) over which BFD is running.  If only
   one data protocol is being advertised in the routing protocol Hello,
   or if multiple protocols are being advertised but the protocols must
   share a common topology, a Hello protocol timeout SHOULD be emulated



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   for the associated OSPF neighbors and/or IS-IS adjacencies.

   If multiple data protocols are advertised in the routing protocol
   Hello, and the routing protocol supports different topologies for
   each data protocol, the failing data protocol SHOULD no longer be
   advertised in Hello packets in order to signal a lack of connectivity
   for that protocol.

   If a BFD session never comes up (possibly because the remote system
   does not support BFD), this MUST NOT preclude the establishment of an
   OSPF neighbor or an IS-IS adjacency.


7.4. Interactions with OSPF and IS-IS with Graceful Restart

   The Graceful Restart functions in OSPF [OSPF-GRACE] and IS-IS [ISIS-
   GRACE] are predicated on the existence of a separate forwarding plane
   that does not necessarily share fate with the control plane in which
   the routing protocols operate.  In particular, the assumption is that
   the forwarding plane can continue to function while the protocols
   restart and sort things out.

   BFD implementations announce via the Control Plane Independent (C)
   bit whether or not BFD shares fate with the control plane.  This
   information is used to determine the actions to be taken in
   conjunction with Graceful Restart.

   If BFD does not share its fate with the control plane on either
   system, it can be used to determine whether Graceful Restart is NOT
   viable (the forwarding plane is not operating.)  In this situation,
   if a BFD session fails while graceful restart is taking place, and
   BFD is independent of the control plane on the local system, and the
   remote system has been transmitting BFD Control packets with the C
   bit set, the graceful restart SHOULD be aborted and the topology
   change made visible to the network as outlined in section 7.3.

   If BFD shares its fate with the control plane on either system
   (either the local system shares fate with the control plane, or the
   remote system is transmitting BFD packets with the C bit set to
   zero), it is not useful during graceful restart, as the BFD session
   is likely to fail regardless of the state of the forwarding plane.
   In this situation, if a BFD session fails while graceful restart is
   taking place (or if the BFD session failure triggers a graceful
   restart event), the graceful restart SHOULD be allowed to complete
   and the topology change should not be made visible to the network as
   outlined in section 7.3.





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7.5. OSPF Virtual Links

   The use of BFD to support failure detection of OSPF Virtual Links is
   for further study and is not yet supported, pending the full
   specification of multihop BFD.



Authors' Addresses

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

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



Changes from the previous draft

   The changes in this draft from the previous version are primarily
   editorial.  Some language has been clarified, a description of how to
   handle multiprotocol routing has been added, and a section on the
   interaction with graceful restart has also been added.  The two
   versions of this draft are believed to be fully interoperable.



Normative References

   [BFD] Katz, D., and Ward, D., "Bidirectional Forwarding Detection",
       draft-katz-ward-bfd-02.txt, May, 2004.

   [ISIS] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and dual
       environments", RFC 1195, December 1990.

   [ISIS-GRACE] Shand, M., and Ginsberg, L., "Restart signaling for IS-
       IS", draft-ietf-isis-restart-05.txt, January 2004.

   [KEYWORD] Bradner, S., "Key words for use in RFCs to Indicate



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       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.

   [OSPF-GRACE] Moy, J., et al, "Graceful OSPF Restart", RFC 3623,
       November 2003.



Security Considerations

   In this application, the use of TTL=255 on transmit and receive is
   viewed as supplying equivalent security characteristics to other
   protocols used in the infrastructure, as it is not trivially
   spoofable.  Other security mechanisms are for further study.



Full Copyright Notice

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

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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."




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Acknowledgement

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















































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