Network Working Group                                           D. Katz
Internet Draft                                         Juniper Networks
                                                                D. Ward
                                                          Cisco Systems
Expires: December, 2006                                      June, 2006

                       Generic Application of BFD

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Internet Draft         Generic Application of BFD             June, 2006


   This document describes the generic application of the Bidirectional
   Forwarding Detection (BFD) protocol.  Comments on this draft should
   be directed to

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 protocol [BFD] provides a
   liveness detection mechanism that can be utilized by other network
   components for which their integral liveness mechanisms are either
   too slow, inappropriate, or nonexistent.  Other drafts have detailed
   the use of BFD with specific encapsulations ([BFD-1HOP], [BFD-MULTI],
   [BFD-MPLS]).  As the utility of BFD has become understood, there have
   been calls to specify BFD interactions with a growing list of network
   functions.  Rather than producing a long series of short documents on
   the application of BFD, it seemed worthwhile to describe the
   interactions between BFD and other network functions in a broad way.

   This document describes the generic application of BFD.  Specific
   protocol applications are provided for illustrative purposes.

2. Overview

   The Bidirectional Forwarding Detection (BFD) specification defines a
   protocol with simple and specific semantics.  Its sole purpose is to
   verify connectivity between a pair of systems, for a particular data
   protocol across a path (which may be of any technology, length, or
   OSI layer).  The promptness of the detection of a path failure can be
   controlled by trading off protocol overhead and system load with
   detection times.

   BFD is *not* intended to directly provide control protocol liveness
   information; those protocols have their own means and vagaries.
   Rather, control protocols can use the services provided by BFD to
   inform their operation.  BFD can be viewed as a service provided by

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   the layer in which it is running.

   The service interface with BFD is straightforward.  The application
   supplies session parameters (neighbor address, time parameters,
   protocol options), and BFD provides the session state, of which the
   most interesting transitions are to and from the Up state.  The
   application is expected to bootstrap the BFD session, as BFD has no
   discovery mechanism.

   An implementation SHOULD establish only a single BFD session per data
   protocol path, regardless of the number of applications that wish to
   utilize it.  There is no additional value in having multiple BFD
   sessions to the same endpoints.  If multiple applications request
   different session parameters, it is a local issue as to how to
   resolve the parameter conflicts.  BFD in turn will notify all
   applications bound to a session when a session state change occurs.

   BFD should be viewed as having an advisory role to the protocol or
   protocols or other network functions with which it is interacting,
   which will then use their own mechanisms to effect any state
   transitions.  The interaction is very much at arm's length, which
   keeps things simple and decoupled.  In particular, BFD explicitly
   does not carry application-specific information, partly for
   architectural reasons, and partly because BFD may have curious and
   unpredictable latency characteristics and as such makes a poor
   transport mechanism.

   It is important to remember that the interaction between BFD and its
   client applications has essentially no interoperability issues,
   because BFD is acting in an advisory nature (similar to hardware
   signaling the loss of light on a fiber optic circuit, for example)
   and existing mechanisms in the client applications are used in
   reaction to BFD events.  In fact, BFD may interact with only one of a
   pair of systems for a particular client application without any ill

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3. Control Protocol Interactions

   Very common client applications of BFD are control protocols, such as
   routing protocols.  The object when BFD interacts with a control
   protocol is to advise the control protocol of the connectivity of the
   data protocol.  In the case of routing protocols, for example, this
   allows the connectivity failure to trigger the rerouting of traffic
   around the failed path more quickly than the native detection

3.1. Session Establishment

   BFD sessions are typically bootstrapped by the control protocol,
   using the mechanism (discovery, configuration) used by the control
   protocol to find neighbors.  In most cases it is not desirable to
   preclude the control protocol from establishing an adjacency if the
   BFD session cannot be established (usually because the neighbor does
   not support BFD.)

   If the control protocol carries signaling that indicates the
   willingness of each system to establish a BFD session, the lack of a
   BFD session MAY be used to block establishment of a control protocol

   If it appears that the neighboring system does not support BFD
   (because the control protocol adjacency was established but no BFD
   Control packets have been received from the neighbor) a system MAY
   increase the interval between transmitted BFD Control packets beyond
   the minimum specified in [BFD].  This will have negligible impact on
   BFD session establishment if the neighbor decides to run BFD after
   all, since BFD Control packets will be sent on an event-driven basis
   once the first packet is seen from the neighbor.

   The setting of BFD's various timing parameters and modes are not
   subject to standardization.  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.  It is generally useful
   to choose the parameters resulting in the shortest detection time;  a
   particular client application can always apply hysteresis to the
   notifications from BFD if it desires longer detection times.

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3.2. Reaction to BFD Session State Changes

   The mechanism by which the control protocol reacts to a path failure
   signaled by BFD depends on the capabilities of the protocol.

3.2.1. Control Protocols with a Single Data Protocol

   A control protocol that is tightly bound to a single failing data
   protocol SHOULD take action to ensure that data traffic is no longer
   directed to the failing path.  Note that this should not be
   interpreted as BFD replacing the control protocol liveness mechanism,
   if any, as the control protocol may rely on mechanisms not verified
   by BFD (multicast, for instance) so BFD most likely cannot detect all
   failures that would impact the control protocol.  However, a control
   protocol MAY choose to use BFD session state information to more
   rapidly detect an impending control protocol failure, particularly if
   the control protocol operates in band (over the data protocol.)

   Therefore, when a BFD session transitions from Up to Down, action
   SHOULD be taken in the control protocol to signal the lack of
   connectivity for the data protocol over which BFD is running.  If the
   control protocol has an explicit mechanism for announcing path state,
   a system SHOULD use that mechanism rather than impacting the
   connectivity of the control protocol, particularly if the control
   protocol operates out of band from the failed data protocol.
   However, if such a mechanism is not available, a control protocol
   timeout SHOULD be emulated for the associated neighbor.

3.2.2. Control Protocols with Multiple Data Protocols

   Slightly different mechanisms are used if the control protocol
   supports the routing of multiple data protocols, depending on whether
   the control protocol supports separate topologies for each data
   protocol. Shared Topologies

   With a shared topology, if one of the data protocols fails (as
   signaled by the associated BFD session), it is necessary to consider
   the path to have failed for all data protocols.  Otherwise, there is
   no way for the control protocol to turn away traffic for the failed
   data protocol (and such traffic would be black holed indefinitely.)

   Therefore, when a BFD session transitions from Up to Down, action
   SHOULD be taken in the control protocol to signal the lack of

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   connectivity for all data protocols sharing the topology.  If this
   cannot be signaled otherwise, a control protocol timeout SHOULD be
   emulated for the associated neighbor. Independent Topologies

   With individual routing topologies for each data protocol, only the
   failed data protocol needs to be rerouted around the failed path.

   Therefore, when a BFD session transitions from Up to Down, action
   SHOULD be taken in the control protocol to signal the lack of
   connectivity for the data protocol over which BFD is running.
   Generally this can be done without impacting the connectivity of
   other data protocols (since otherwise it is very difficult to support
   separate topologies for multiple data protocols.)

3.2.3. Partial BFD Deployments

   Note that it is possible in some failure scenarios for the network to
   be in a state such that the control protocol comes up, but the BFD
   session cannot be established, and, more particularly, data cannot be
   forwarded.  To avoid this situation, it would be beneficial to not
   allow the control protocol to establish a neighbor adjacency.
   However, this would preclude the operation of the control protocol in
   an environment in which not all systems support BFD.

   Therefore, if a BFD session is not in Up state (possibly because the
   remote system does not support BFD), it is OPTIONAL to preclude the
   establishment of a control protocol neighbor adjacency.  The choice
   of whether to do so SHOULD be controlled by means outside the scope
   of this specification, such as configuration or other mechanisms.  If
   a neighbor adjacency is established for the control protocol but the
   corresponding BFD session is not in Up state (implying that the
   neighbor does not support BFD) implementations MAY raise the BFD
   transmit and receive intervals beyond the minimum of one second
   specified in [BFD] in order to minimize extraneous traffic.

3.3. Interactions with Graceful Restart Mechanisms

   A number of control protocols support Graceful Restart mechanisms.
   These mechanisms are designed to allow a control protocol to restart
   without perturbing network connectivity state (lest it appear that
   the system and/or all of its links had failed.)  They are predicated
   on the existence of a separate forwarding plane that does not
   necessarily share fate with the control plane in which the routing

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   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 in a control protocol is NOT viable (the
   forwarding plane is not operating.)

   If the control protocol has a Graceful Restart mechanism, BFD may be
   used in conjunction with this mechanism.  The interaction between BFD
   and the control protocol depends on the capabilities of the control
   protocol, and whether or not BFD shares fate with the control plane.
   In particular, it may be desirable for a BFD session failure to abort
   the Graceful Restart process and allow the failure to be visible to
   the network.

3.3.1. BFD Fate Independent of the Control Plane

   If BFD is implemented in the forwarding plane and does not share fate
   with the control plane on either system (the "C" bit is set in the
   BFD Control packets in both directions), control protocol restarts
   should not affect the BFD Session.  In this case, a BFD session
   failure implies that data can no longer be forwarded, so any Graceful
   Restart in progress at the time of the BFD session failure SHOULD be
   aborted in order to avoid black holes, and a topology change SHOULD
   be signaled in the control protocol.

3.3.2. BFD Shares Fate with the Control Plane

   If BFD shares fate with the control plane on either system (the "C"
   bit is clear in either direction), a BFD session failure cannot be
   disentangled from other events taking place in the control plane.  In
   many cases, the BFD session will fail as a side effect of the restart
   taking place.  As such, it would be best to avoid aborting any
   Graceful Restart taking place, if possible (since otherwise BFD and
   Graceful Restart cannot coexist.)

   There is some risk in doing so, since a simultaneous failure or
   restart of the forwarding plane will not be detected, but this is
   always an issue when BFD shares fate with the control plane.

Katz, Ward                                                      [Page 7]

Internet Draft         Generic Application of BFD             June, 2006 Control Protocols with Planned Restart Signaling

   Some control protocols can signal a planned restart prior to the
   restart taking place.  In this case, if a BFD session failure occurs
   during the restart, such a planned restart SHOULD NOT be aborted and
   the session failure SHOULD NOT result in a topology change being
   signaled in the control protocol. Control Protocols Without Planned Restart Signaling

   Control protocols that cannot signal a planned restart depend on the
   recently restarted system to signal the Graceful Restart prior to the
   control protocol adjacency timeout.  In most cases, whether the
   restart is planned or unplanned, it is likely that the BFD session
   will time out prior to the onset of Graceful Restart, in which case a
   topology change SHOULD be signaled in the control protocol as
   specified in section 3.2.

   However, if the restart is in fact planned, an implementation MAY
   adjust the BFD session timing parameters prior to restarting in such
   a way that the detection time in each direction is longer than the
   restart period of the control protocol, providing the restarting
   system the same opportunity to enter Graceful Restart as it would
   have without BFD.  The restarting system SHOULD NOT send any BFD
   Control packets until there is a high likelihood that its neighbors
   know a Graceful Restart is taking place, as the first BFD Control
   packet will cause the BFD session to fail.

3.4. Interactions with Multiple Control Protocols

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

   If hierarchical or dependent layers of control protocols are in use
   (say, OSPF and IBGP), it may not be useful for more than one of them
   to interact with BFD.  In this example, because IBGP is dependent on
   OSPF for its routing information, the faster failure detection
   relayed to IBGP may actually be detrimental.  The cost of a peer
   state transition is high in BGP, and OSPF will naturally heal the
   path through the network if it were to receive the failure detection.

   In general, it is best for the protocol at the lowest point in the
   hierarchy to interact with BFD, and then to use existing interactions
   between the control protocols to effect changes as necessary.  This
   will provide the fastest possible failure detection and recovery in a

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4. Interactions With Non-Protocol Functions

   BFD session status may be used to affect other system functions that
   are not protocol-based (for example, static routes.)  If the path to
   a remote system fails, it may be desirable to avoid passing traffic
   to that remote system, so the local system may wish to take internal
   measures to accomplish this (such as withdrawing a static route and
   withdrawing that route from routing protocols.)

   Bootstrapping of the BFD session in the non-protocol case is likely
   to be derived from configuration information.

   There is no need to exchange endpoints or discriminator values via
   any mechanism other than configuration (via Operational Support
   Systems or any other means) as the endpoints must be known and
   configured by the same means.

5. Data Protocols and Demultiplexing

   BFD is intended to protect a single "data protocol" and is
   encapsulated within that protocol.  A pair of systems may have
   multiple BFD sessions over the same topology if they support (and are
   encapsulated by) different protocols.  For example, if two systems
   have IPv4 and IPv6 running across the same link between them, these
   are considered two separate paths and require two separate BFD

   This same technique is used for more fine-grained paths.  For
   example, if multiple differentiated services [DIFFSERV] are being
   operated on over IPv4, an independent BFD session may be run for each
   service level.  The BFD Control packets must be marked in the same
   way as the data packets, partly to ensure as much fate sharing as
   possible between BFD and data traffic, and also to demultiplex the
   initial packet if the discriminator values have not been exchanged.

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6. Other Application Issues

   BFD can provide liveness detection for OAM-like functions in
   tunneling and pseudowire protocols.  Running BFD inside the tunnel is
   recommended, as it exercises more aspects of the path.  One way to
   accommodate this is to address BFD packets based on the tunnel
   endpoints, assuming that they are numbered.

   If a planned outage is to take place on a path over which BFD is run,
   it is preferable to take down the BFD session by going into ADMIN
   DOWN state prior to the outage.

7. Interoperability Issues

   The BFD protocol itself is designed so that it will always
   interoperate at a basic level;  asynchronous mode is mandatory and is
   always available, and other modes and functions are negotiated at run
   time.  Since the service provided by BFD is identical regardless of
   the variants used, the particular choice of BFD options has no
   bearing on interoperability.

   The interaction between BFD and other protocols and control functions
   is very loosely coupled.  The actions taken are based on existing
   mechanisms in those protocols and functions, so interoperability
   problems are very unlikely unless BFD is applied in contradictory
   ways (such as a BFD session failure causing one implementation to go
   down and another implementation to come up.)  In fact, BFD may be
   advising one system for a particular control function but not the
   other;  the only impact of this would be potentially asymmetric
   control protocol failure detection.

8. Specific Protocol Interactions (Non-Normative)

   As noted above, there are no interoperability concerns regarding
   interactions between BFD and control protocols.  However, there is
   enough concern and confusion in this area so that it is worthwhile to
   provide examples of interactions with specific protocols.

   Since the interactions do not affect interoperability, they are non-

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8.1. BFD Interactions with OSPFv2, OSPFv3, and IS-IS

   The two versions of OSPF ([OSPFv2] and [OSPFv3]), as well as IS-IS
   [ISIS], all suffer from an architectural limitation, namely that
   their Hello protocols are limited in the granularity of their failure
   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 supplementing the Hello protocols used in each

8.1.1. Session Establishment

   The most obvious choice for triggering BFD session establishment with
   these protocols would be to use the discovery mechanism inherent in
   the Hello protocols in OSPF and IS-IS to bootstrap the establishment
   of the BFD session.  Any BFD sessions established to support OSPF and
   IS-IS across a single IP hop must operate in accordance with

8.1.2. Reaction to BFD State Changes

   The basic mechanisms are covered in section 3 above.  At this time,
   OSPFv2 and OSPFv3 carry routing information for a single data
   protocol (IPv4 and IPv6, respectively) so when it is desired to
   signal a topology change after a BFD session failure, this should be
   done by tearing down the corresponding OSPF neighbor.

   ISIS may be used to support only one data protocol, or multiple data
   protocols.  [ISIS] specifies a common topology for multiple data
   protocols, but work is underway to support multiple topologies.  If
   multiple data protocols are advertised in the ISIS Hello, and
   independent topologies are in use, the failing data protocol should
   no longer be advertised in ISIS Hello packets in order to signal a
   lack of connectivity for that protocol.  Otherwise, a failing BFD
   session should be signaled by simulating an ISIS adjacency failure.

   OSPF has a planned restart signaling mechanism, whereas ISIS does
   not.  The appropriate mechanisms outlined in section 3.3 should be

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8.1.3. OSPF Virtual Links

   If it is desired to use BFD for failure detction of OSPF Virtual
   Links, the mechanism described in [BFD-MULTI] MUST be used, since
   OSPF Virtual Links may traverse an arbitrary number of hops.  BFD
   Authentication SHOULD be used and is strongly encouraged.

8.2. Interactions with BGP

   BFD may be useful with EBGP sessions [BGP] in order to more rapidly
   trigger topology changes in the face of path failure.  As noted in
   section 3.4, it is generally unwise for IBGP sessions to interact
   with BFD if the underlying IGP is already doing so.

   EBGP sessions being advised by BFD may establish either a one hop
   [BFD-1HOP] or a multihop [BFD-MULTIHOP] session, depending on whether
   the neighbor is immediately adjacent or not.  The BFD session should
   be established to the BGP neighbor (as opposed to any other Next Hop
   advertised in BGP.)

   [BGP-GRACE] describes a Graceful Restart mechanism for BGP.  If
   Graceful Restart is not taking place on an EBGP session, and the
   corresponding BFD session fails, the EBGP session should be torn down
   in accordance with section 3.2.  If Graceful Restart is taking place,
   the basic procedures in section 3.3 applies.  BGP Graceful Restart
   does not signal planned restarts, so section applies.  If
   Graceful Restart is aborted due to the rules described in section
   3.3, the "receiving speaker" should act as if the "restart timer"
   expired (as described in [BGP-GRACE].)

8.3. Interactions with RIP

   The RIP protocol [RIP] is somewhat unique in that, at least as
   specified, neighbor adjacency state is not stored per se.  Rather,
   installed routes contain a next hop address, which in most cases is
   the address of the advertising neighbor (but may not be.)

   In the case of RIP, when the BFD session associated with a neighbor
   fails, an expiration of the "timeout" timer for each route installed
   from the neighbor (for which the neighbor is the next hop) should be

   Note that if a BFD session fails, and a route is received from that
   neighbor with a next hop address that is not the address of the
   neighbor itself, the route will linger until it naturally times out
   (after 180 seconds.  However, if an implementation keeps track of all

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   of the routes received from each neighbor, all of the routes from the
   neighbor corresponding to the failed BFD session should be timed out,
   regardless of the next hop specified therein, and thereby avoiding
   the lingering route problem.

Normative References

   [BFD] Katz, D., and Ward, D., "Bidirectional Forwarding Detection",
       draft-ietf-bfd-base-05.txt, June, 2006.

   [BFD-1HOP] Katz, D., and Ward, D., "BFD for IPv4 and IPv6 (Single
       Hop)", draft-ietf-bfd-v4v6-1hop-05.txt, June, 2006.

   [BFD-MPLS] Aggarwal, R., and Kompella, K., "BFD for MPLS LSPs",
       draft-ietf-bfd-mpls-03.txt, June, 2006.

   [BFD-MULTI] Katz, D., and Ward, D., "BFD for Multihop Paths", draft-
       ietf-bfd-multihop-04.txt, June, 2006.

   [BGP] Rekhter, Y., Li, T. et al, "A Border Gateway Protocol 4
       (BGP-4)", RFC 4271, January, 2006.

   [BGP-GRACE] Sangli, S., Rekhter, Y., et al, "Graceful Restart
       Mechanism for BGP", draft-ietf-idr-restart-12.txt, June, 2006.

   [DIFFSERV] Nichols, K. et al, "Definition of the Differentiated
       Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC
       2474, December, 1998.

   [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", RFC 3847, July 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.

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

   [RIP] Malkin, G., "RIP Version 2", RFC 2453, November, 1998.

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Security Considerations

   This specification does not raise any additional security issues
   beyond those of the specifications referred to in the list of
   normative references.

IANA Considerations

   This document has no actions for IANA.

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

Changes from the previous draft

   Control protocol-specific interactions were moved from [BFD-1HOP] to
   this document, and brief mentions of BGP and RIP were added.

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