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Network Working Group                                           D. Katz
Internet Draft                                         Juniper Networks
                                                                D. Ward
                                                          Cisco Systems
Category: Informational                                      June, 2003
Expires: December, 2003


                   Bidirectional Forwarding Detection
                       draft-katz-ward-bfd-00.txt


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
              http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
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Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.














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Abstract

   This document describes a protocol intended to detect faults in the
   bidirectional path between two forwarding engines, including
   interfaces, data link(s), and to the extent possible the forwarding
   engines themselves, with potentially very low latency.  It operates
   independently of media, data protocols, and routing protocols.


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

   An increasingly important feature of networking equipment is the
   rapid detection of communication failures between adjacent systems,
   in order to more quickly establish alternative paths.  Currently,
   detection can come fairly quickly in certain circumstances when data
   link hardware comes into play (such as SONET alarms.)  However, there
   are media that do not provide this kind of signaling (such as
   Ethernet), and some media may not detect certain kinds of failures in
   the path, for example, failing interfaces or forwarding engine
   components.

   Networks use relatively slow "Hello" mechanisms, usually in routing
   protocols, to detect failures when there is no hardware signaling to
   help out.  The detection times available in the existing protocols
   are no better than a second, which is far too long for some
   applications and represents a great deal of lost data at gigabit
   rates.  Furthermore, routing protocol Hellos are of no help when
   those routing protocols are not in use, and the semantics of
   detection are subtly different--they detect a failure in the path
   between the two routing protocol engines.

   The goal of BFD is to provide low-overhead, short-duration detection
   of failures in the path between adjacent forwarding engines,
   including the interfaces, data link(s), and to the extent possible
   the forwarding engines themselves.








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2. Design

   BFD is designed to detect failures in communication with a data plane
   next hop.  It is intended to be implemented in some component of the
   forwarding engine of a system, in cases where the forwarding and
   control engines are separated.  This not only binds the protocol more
   to the data plane, but decouples the protocol from the fate of the
   routing protocol engine (making it useful in concert with various
   "graceful restart" mechanisms for those protocols.)

   BFD operates on top of any data protocol being forwarded between two
   systems.  It is always run in a unicast, point-to-point mode.

   BFD can provide failure detection on any kind of path between
   systems, including direct physical links, virtual circuits, tunnels,
   MPLS LSPs, multihop routed paths, and unidirectional links (so long
   as there is some return path, of course.)  Multiple BFD sessions can
   be established between the same pair of systems when multiple paths
   between them are present in at least one direction, even if the same
   path is used in one direction.

   The BFD state machine implements a three-way handshake, both when
   establishing a BFD session and when tearing it down for any reason,
   to ensure that both systems are aware of the state change.



3. Protocol Overview

   BFD is a simple, fixed-field, hello protocol that in many respects is
   similar to the detection components of well-known routing protocols.
   A pair of systems transmit BFD packets periodically over each path
   between the two systems, and if a system stops receiving BFD packets
   for long enough, some component in that particular bidirectional path
   to the neighboring system is assumed to have failed.

   A path is only declared to be operational when two-way communication
   has been established between systems (though this does not
   necessarily mean that a bidirectional link must be used.)

   A separate BFD session is created for each communications path and
   data protocol in use between two systems.

   Each system estimates how quickly it can send and receive BFD packets
   in order to come to an agreement with its neighbor about how rapidly
   detection of failure will take place.  These estimates can be
   modified in real time in order to adapt to unusual situations.  This
   design also allows for fast systems on a shared medium with a slow



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   system to be able to more rapidly detect failures between the fast
   systems while allowing the slow system to participate to the best of
   its ability.

   BFD can operate in two different modes.  The first mode is known as
   Asynchronous mode.  In this mode, each system sends a series of BFD
   Control packets to one another, and if a number of those packets in a
   row are not received by the other system, the session is declared to
   be down.

   The second mode is known as Echo mode.  In echo mode, BFD Control
   packets are sent at a relatively sedate rate, and additionally
   streams of BFD Echo packets are transmitted in each direction in such
   a way as to have the other system loop them back through its
   forwarding path.  If a number of packets in a row of either the
   control stream or the echoed data stream are not received, the
   session is declared to be down.

   Asynchronous mode is advantageous in that it requires half as many
   packets to achieve a particular detection time as does Echo mode.  It
   is also used when Echo mode cannot be supported for some reason.

   Echo mode has the advantage of truly testing only the forwarding path
   on the remote system, which may reduce round-trip jitter and thus
   allow more aggressive detection times, as well as potentially
   detecting some classes of failure that might not otherwise be
   detected.

   Echo mode is enabled only when both systems signal that they are
   willing to do so.



4. Protocol Details

   BFD packets are carried as the payload of whatever encapsulating
   protocol is appropriate for the medium and network.  Note that many
   of the exact mechanisms are implementation dependent and will not
   affect interoperability, and are thus outside the scope of this
   specification.  Those issues are so noted.


4.1. Timer Model

   A timer is an entity that will measure an interval of time and
   provide a notification when that time period expires.  It has two
   states, running and disarmed.  A disarmed timer will not expire,
   whereas a running timer will expire after the specified interval.  A



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   running timer can be restarted prior to its expiration to any
   interval;  it will then not expire until the new interval has passed.

   Some timers may be jittered.  This is a process where a random value
   is subtracted from the interval (expressed as a percentage of the
   interval) when the timer is started.  Jitter is used to avoid the
   self-synchronization of nominally independent timers.


4.2. State

   BFD requires that a set of state elements be maintained for each
   session with neighboring systems.  The creation of this state is
   outside the scope of this specification.  This state description is
   not intended to define implementation;  any equivalent method can be
   used.


   st.SourceAddress

      The source address information used when transmitting BFD Control
      packets for this session, appropriate to the environment.  The
      setting of this value is outside the scope of this specification.


   st.DestinationAddress

      The destination address information used when transmitting BFD
      Control packets for this session, appropriate to the environment.
      The setting of this value is outside the scope of this
      specification.


   st.EchoSourceAddress

      The source address information used when transmitting BFD Echo
      packets for this session, appropriate to the environment, if Echo
      mode is supported.  This address MUST be an address associated
      with the transmitting system, and MAY be part of a subnet other
      than the one over which the packet is being sent (in order to
      avoid the transmission of ICMP Redirects.) The setting of this
      value is otherwise outside the scope of this specification.


   st.EchoDestinationAddress

      The destination address information used when transmitting BFD
      Echo packets for this session, appropriate to the environment.



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      This address MUST be an address associated with the transmitting
      system, SHOULD be an address for which the remote system will
      route packets back on the interface over which they are received,
      and SHOULD be part of the subnet over which the packet is being
      sent (if the link is subnetted.)  The setting of this value is
      otherwise outside the scope of this specification.


   st.LocalDiscr

      The local discriminator for this BFD session, used to uniquely
      identify it.  It MUST be unique on this system, and nonzero.  The
      value is otherwise outside the scope of this specification.


   st.RemoteDiscr

      The remote discriminator for this BFD session.  This is the
      discriminator chosen by the remote system, and is totally opaque
      to the local system.  This MUST be initialized to zero.


   st.RemoteHeard

      This field is set to 1 if the local system is actively receiving
      BFD packets from the remote system, and is set to 0 if the local
      system has not received BFD packets recently (within the detection
      time) or if the local system is attempting to tear down the BFD
      session.  This MUST be initialized to zero.


   st.SessionState

      The perceived state of the session (Init, Up, Failing, or Down.)
      The exact action taken when the session state changes is outside
      the scope of this specification, though it is expected that this
      state change (particularly to and from Up state) is reported to
      other components of the system.  This MUST be initialized to Down.


   st.EchoModeDesired

      A boolean stating whether or this system wishes to use Echo mode.
      The setting of this value is outside the scope of this
      specification.


   st.EchoModeActive



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      A boolean tracking whether or not Echo mode is active.  This MUST
      be initialized to FALSE.


   st.LocalSessionDiagnostic

      A diagnostic code specifying the reason the local session state
      most recently transitioned from Up to some other state.  This MUST
      be initialized to zero.


   st.RemoteSessionDiagnostic

      A diagnostic code specifying the reason the remote session state
      most recently transitioned from Up to some other state.  This MUST
      be initialized to zero.


   st.DesiredMinAsyncTXInterval

      The minimum interval, in microseconds, between transmitted BFD
      Control packets that this system would like to use while operating
      in Asynchronous mode when the session is up.  The actual interval
      is negotiated between the two systems.  This value MUST be
      nonzero, and is otherwise outside the scope of this specification.


   st.DesiredMinSlowTXInterval

      The minimum interval, in microseconds, between transmitted BFD
      Control packets that this system would like to use while operating
      in Echo mode, or when the session is not up.  The actual interval
      is negotiated between the two systems.  This value MUST be
      nonzero, and is otherwise outside the scope of this specification,
      though it is suggested that this value SHOULD be at least one
      second (1,000,000 usec.)


   st.DesiredMinEchoTXInterval

      The minimum interval, in microseconds, between transmitted BFD
      Echo packets that this system would like to use while operating in
      Echo mode.  The actual interval is negotiated between the two
      systems.  If Echo mode is supported, this value MUST be nonzero,
      and is otherwise outside the scope of this specification.


   st.DesiredMinTXInterval



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      The minimum interval, in microseconds, between transmitted BFD
      Control packets that this system would like to use at the current
      time.  The actual interval is negotiated between the two systems.
      This is set to either st.DesiredMinSlowTXInterval or
      st.DesiredMinAsyncTXInterval depending on the session state.  This
      value MUST be initialized to st.DesiredMinSlowTXInterval.


   st.RequiredMinRXInterval

      The minimum interval, in microseconds, between received BFD
      Control packets that this system requires.  The setting of this
      value is outside the scope of this specification.


   st.RequiredMinEchoRXInterval

      The minimum interval, in microseconds, between received BFD Echo
      packets that this system requires.  If this system supports Echo
      mode, this value MUST be nonzero.  If this system does not support
      Echo mode this value MUST be zero.  The setting of this value is
      otherwise outside the scope of this specification.


   st.TxInterval

      The agreed BFD Control packet transmission interval, in
      microseconds, for this session.  This MUST be initialized to
      st.DesiredMinTXInterval.  Note that an independent transmit
      interval may be used in each direction for a single BFD session.


   st.EchoTxInterval

      The agreed BFD Echo packet transmission interval, in microseconds,
      for this session.  This MUST be initialized to zero.  Note that an
      independent transmit interval may be used in each direction for a
      single BFD session.


   st.DetectMult

      The desired detect time multiplier for BFD Control packets.  The
      negotiated Control packet transmission interval, multiplied by
      this value, will be the detection time for this session (as seen
      by the remote system.)  This value MUST be a nonzero integer, and
      is otherwise outside the scope of this specification.




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   st.EchoDetectMult

      The desired detect time multiplier for BFD Echo packets.  The
      negotiated Echo packet transmission interval, multiplied by this
      value, will be the detection time for this session (as seen by the
      local system.)  This value MUST be a nonzero integer, and is
      otherwise outside the scope of this specification.


   st.DetectionTime

      The detection time of the failure of this BFD session by virtue of
      missing BFD Control packets, as seen by the local system, in
      microseconds.  It MUST be initialized to zero.  Note that each
      system determines its own detection time, and the values for each
      system may not be the same.


   st.DetectTimer

      This timer is used to keep track of session liveness by tracking
      the arrival of BFD Control packets.  It MUST be initialized to the
      disarmed state.  When it expires, the session is deemed to have
      failed.


   st.EchoDetectionTime

      The detection time of the failure of this BFD session by virtue of
      missing BFD Echo packets, as seen by the local system, in
      microseconds.  It MUST be initialized to zero.  Note that each
      system determines its own detection time, and the values for each
      system may not be the same.


   st.EchoDetectTimer

      This timer is used to keep track of session liveness by tracking
      the arrival of BFD Echo packets.  It MUST be initialized to the
      disarmed state.  When it expires, the session is deemed to have
      failed.


   st.TransmissionTimer

      This timer triggers the transmission of a BFD Control packet.  It
      MUST be initialized to the running state, with an interval of
      st.DesiredMinSlowTxInterval.  A jitter of 25% SHOULD be applied to



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      this timer.


   st.EchoTransmissionTimer

      This timer triggers the transmission of a BFD Echo packet.  It
      MUST be initialized to the disarmed state.  A jitter of 25% SHOULD
      be applied to this timer.



4.3. BFD Control Packet Format

   BFD Control packets are sent in an encapsulation appropriate to the
   environment.  See "Encapsulation Specifics" below for the specifics
   of particular environments.

   The payload of a BFD Control packet has the following format:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    Version    |H| Diagnostic  |  Detect Mult  |    Length     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           My Discr                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Your Discr                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Desired Min TX Interval                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                   Required Min RX Interval                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 Required Min Echo RX Interval                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Version

      The version number of the protocol.  This document defines
      protocol version 0.


   I Hear You (H)

      This bit is set to 0 if the transmitting system either is not
      receiving BFD packets from the remote system, or is in the process
      of tearing down the BFD session for some reason (see the Elements
      of Procedure below for more details.)



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      Diagnostic

         A diagnostic code specifying the local system's reason for the
         last transition of the session from Up to some other state.
         Values are:

           0 -- No diagnostic
           1 -- Control Detection time expired
           2 -- Echo Detection time expired
           3 -- Neighbor signaled session down
           4 -- Forwarding plane reset


      Detect Mult

         Detect time multiplier.  The negotiated transmission interval,
         multiplied by this value, provides the detection time for the
         transmitting system.


      Length

         Length of the BFD Control packet, in bytes.


      My Discr

         A unique, nonzero discriminator value generated by the
         transmitting system, used to demultiplex multiple BFD sessions
         between the same pair of systems.


      Your Discr

         The discriminator received from the corresponding remote
         system.  This field reflects back the received value of My
         Discr, or is zero if that value is unknown.


      Desired Min TX Interval

         This is the minimum interval, in microseconds, that the local
         system would like to use when transmitting BFD Control packets.


      Required Min RX Interval

         This is the minimum interval, in microseconds, between received



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         BFD Control packets that this system is capable of supporting.


      Required Min Echo RX Interval

         This is the minimum interval, in microseconds, between received
         BFD Echo packets that this system is capable of supporting.  If
         this value is zero, the transmitting system does not support
         BFD Echo packets.


4.4. BFD Echo Packet Format

   BFD Echo packets are sent in an encapsulation appropriate to the
   Environment.  See "Encapsulation Specifics" below for the specifics
   of particular environments.

   The payload of a BFD Echo packet is a local matter, since only the
   sending system ever looks over the content.  The only requirement is
   that sufficient information is included to demultiplex the received
   packet to the correct BFD session.


4.5. Elements of Procedure

4.5.1. Overview

   A session begins with the periodic, slow transmission of BFD Control
   packets.  When bidirectional communication is achieved (by virtue of
   the I Hear You field being nonzero, a three way handshake), the BFD
   session comes up.

   If both systems signal that they can support Echo mode, they continue
   to send Control packets at the slow rate and start transmitting Echo
   packets at the negotiated rate.

   The mechanism for detecting lost Echo packets and determining the
   detection time in Echo mode is outside of the scope of this
   specification.  The only normative aspect of Echo packets is that
   they must not be sent more rapidly than the other system is willing
   to accept them (according to the advertised Required Min Echo Rx
   Interval.)

   One possible mechanism for the handling of Echo mode is described
   herein.  Note that if the round trip time to the remote system is
   greater than st.EchoDetectionTime, the algorithm described will
   falsely declare a session failure when Echo mode is first enabled.  A
   system MAY decide not to negotiate Echo mode when the latency is high



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   relative to the detection time, or it MAY set st.EchoDetectTimer to a
   sufficiently large interval when it is first started (see
   TurnOnEchoMode below), or it MAY choose to use a different mechanism
   altogether (perhaps one that doesn't use timers at all) to determine
   whether Echo packets have not arrived.  The determination of whether
   any of this is necessary is outside the scope of this specification.

   If at least one system does not wish to or cannot support Echo mode,
   the systems instead send Control packets at a higher rate.

   If the session fails, the transmission of Echo packets (if any)
   ceases, and the transmission of Control packets goes back to the slow
   rate.


4.5.2. Reception of BFD Control Packets

   When a BFD Control packet is received, the following procedure MUST
   be followed, in the order specified:

      If the version number is not correct (0), the packet MUST be
      discarded.

      If the length field is less than the correct value (24), the
      packet MUST be discarded.

      If the length field is greater than the payload of the
      encapsulating protocol, the packet MUST be discarded.

      The appropriate BFD state block is selected based on some
      combination of source addressing information, the two
      discriminator fields, and by the interface over which the packet
      was received.  The exact method for looking up a state block is
      outside the scope of this specification.  If a matching session is
      not found, new state may be created, or the packet may be
      discarded.  This choice is outside the scope of this
      specification.

      If the value of st.RemoteDiscr is nonzero, it MUST match the value
      of My Discr.  If it does not, the packet MUST be discarded.

      If the value of Your Discr is nonzero, it MUST match the value of
      st.LocalDiscr.  If it does not, the packet MUST be discarded.

      If the value of st.RemoteDiscr is zero, set it to the value of My
      Discr.

      If st.EchoModeActive is TRUE and the received Required Min Echo RX



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      Interval is zero, execute TurnOffEchoMode.

      Set st.TxInterval to the greater of st.DesiredMinTxInterval and
      the received Required Min Rx Interval.

      Set st.EchoTxInterval to the greater of
      st.DesiredMinEchoTXInterval and the received Required Min Echo Rx
      Interval.

      Set st.EchoDetectionTime to the value of st.EchoTxInterval
      multiplied by the value of st.EchoDetectMult.

      Set st.DetectionTime to the greater of st.RequiredMinRXInterval
      and the received Desired Min TX Interval, multiplied by the
      received Detect Multiplier.

      (Re)start st.DetectTimer with an interval of st.DetectionTime.

      Set st.RemoteSessionDiagnostic to the value of the received
      Diagnostic.

      If st.SessionState is Down
          Set st.RemoteHeard to 1
          If I Hear You is zero
              Set st.SessionState to Init
          Else
              Set st.SessionState to Up
              If st.EchoModeDesired is TRUE and Required Min Echo RX
                Interval is nonzero
                  Execute TurnOnEchoMode

      Else if st.SessionState is Init
          If I Hear You is nonzero
              Set st.SessionState to Up
              If st.EchoModeDesired is TRUE and Required Min Echo RX
                Interval is nonzero
                  Execute TurnOnEchoMode

      Else if st.SessionState is Up
          If I Hear You is zero
              Set st.LocalSessionDiagnostic to 3 (Neighbor signaled
                session down)
              Execute TakeDownSession

      Else if st.SessionState is Failing
          If I Hear You is zero, set st.SessionState to Down





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      TurnOffEchoMode:
          Set st.EchoModeActive to FALSE
          Disarm st.EchoDetectTimer
          Disarm st.EchoTransmissionTimer
          If st.SessionState is Up
              Set st.DesiredMinTxInterval to st.DesiredMinAsyncTXInterval
          Else
              Set st.DesiredMinTxInterval to st.DesiredMinSlowTxInterval


      TurnOnEchoMode:
          Set st.EchoModeActive to TRUE
          Set st.DesiredMinTxInterval to st.DesiredMinSlowTxInterval
          Start st.EchoDetectTimer with an interval of st.EchoDetectionTime.
            st.EchoDetectTimer MUST be started with an interval greater than
            the link round trip time;  if necessary, st.EchoDetectTimer MAY
            be started with a value greater than st.EchoDetectionTime.
          Start st.EchoTransmissionTimer with an interval of st.EchoTxInterval


      TakeDownSession:
          Disarm st.DetectTimer
          Set st.SessionState to Failing
          Set st.RemoteHeard to zero
          Execute TurnOffEchoMode


4.5.3. st.TransmissionTimer Expiration

   When st.TransmissionTimer expires, send a BFD Control packet, and
   restart the timer with an interval of st.TxInterval.  The packet is
   sent with a source address of st.SourceAddress and a destination
   address of st.DestinationAddress.  The fields are set as follows:

   Version

      Set to the current version number (0).


   I Hear You

      Set to the value of st.RemoteHeard.


   Diagnostic

      Set to the value of st.LocalSessionDiagnostic.




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   Detect Mult

      Set to the value of st.DetectMult.


   My Discr

      Set to st.LocalDiscr.


   Your Discr

      If st.SessionState is Init or Up, set to st.RemoteDiscr.
      Otherwise, set to zero.


   Desired Min TX Interval

      Set to st.DesiredMinTXInterval.


   Required Min RX Interval

      Set to st.RequiredMinRXInterval.


   Required Min Echo RX Interval

      Set to st.RequiredMinEchoRXInterval.


4.5.4. Reception of BFD Echo Packets

   The processing of received Echo packets is outside of the scope of
   this specification.  However, when a BFD Echo packet is received, the
   following procedure MAY be followed, in the order specified:

      The appropriate BFD state block is selected based on some
      combination of source addressing information, data placed in the
      payload of the Echo packet, and the interface over which the
      packet was received.  If a matching session is not found, discard
      the packet.

      If st.EchoModeActive is FALSE, discard the packet.

      Restart st.EchoDetectTimer with an interval of
      st.EchoDetectionTime.




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4.5.5. st.EchoTransmissionTimer Expiration

   When st.EchoTransmissionTimer expires, a BFD Echo packet MUST sent,
   and the timer MUST be restarted with an interval of
   st.EchoTxInterval.  The packet is sent with a source address of
   st.EchoSourceAddress and a destination address of
   st.EchoDestinationAddress.  The contents of the packet are outside
   the scope of this specification.


4.5.6. st.DetectTimer Expiration

   When st.DetectTimer expires, set st.LocalSessionDiagnostic to 1
   (Control Detection time expired), set st.RemoteDiscr to zero, and
   execute TakeDownSession.


4.5.7. st.EchoDetectTimer Expiration

   When st.EchoDetectTimer expires, set st.LocalSessionDiagnostic to 2
   (Echo Detection time expired), and execute TakeDownSession.


4.5.8. Min Rx Interval Change

   When it is desired to change the rate at which BFD Control packets
   arrive from the remote system, st.RequiredMinRxInterval can be
   changed at any time to any value.  The new value will be transmitted
   at the next st.TransmissionTimer expiration, and the remote system
   will adjust accordingly.


4.5.9. Min Tx Interval Change

   When it is desired to change the rate at which BFD Control packets
   are transmitted to the remote system (subject to the requirements of
   the neighboring system), st.DesiredMinTxInterval can be changed at
   any time to any value.  The new value will be transmitted at the next
   st.TransmissionTimer expiration.  Note that st.TransmissionTimer
   should not be touched;  it will pick up the new value (if any) at its
   next expiration.  This is necessary when increasing the transmission
   interval to avoid an expiration of the neighbor's detection timer.

   If the first packet containing a new, larger value of the interval is
   dropped, there is a chance that the detect timer will fire on the
   remote system and take down the BFD session.  An implementation MAY
   continue to transmit BFD Control packets at the old, shorter interval
   for up to st.DetectMult packets before using the new, longer



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   interval.


4.5.10. Min Echo RX Interval Change

   When it is desired to change the rate at which BFD Echo packets
   arrive from the remote system, st.RequiredMinEchoRxInterval can be
   changed at any time to any value.  The new value will be transmitted
   at the next st.TransmissionTimer expiration, and the remote system
   will adjust accordingly.


4.5.11. Detect Multiplier Change

   When it is desired to change the detect multiplier, the value of
   st.DetectMult can be changed to any nonzero value.  The new value
   will be transmitted at the next st.TransmissionTimer expiration.


4.5.12. Forwarding Engine Reset

   When the forwarding engine hardware is reset, set
   st.LocalSessionDiagnostic to 4 (Forwarding plane reset), and execute
   TakeDownSession.


4.5.13. Mode Change

   If it is desired to switch between Async mode and Echo mode, this can
   be done at any time (assuming that both systems are capable of
   supporting Echo mode) by changing the value of
   st.RequiredMinEchoRXInterval to zero or nonzero accordingly.  If Echo
   mode is enabled, Echo packets will be sent and the rate of Control
   packets will be reduced, and the opposite will happen if Echo mode is
   disabled.



4.6. Encapsulation Specifics


4.6.1. BFD for IPv4

   In the case of IPv4, BFD Control packets are transmitted with source
   and destination UDP port <TBD1> in an IPv4 packet.  The source and
   destination addresses MUST be associated with the local and remote
   systems, respectively.




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   BFD Echo packets are transmitted with source and destination UDP port
   <TBD2> in an IPv4 packet.  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.


4.6.2. BFD for IPv6

   In the case of IPv6, BFD Control packets are transmitted with source
   and destination UDP port <TBD1> in an IPv6 packet.  The source and
   destination addresses MUST be associated with the local and remote
   systems, respectively.

   BFD Echo packets are transmitted with source and destination UDP port
   <TBD2> in an IPv6 packet.  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.


4.6.3. BFD for IEEE 802 Networks

   BFD can also be used directly on top of the datalink layer in IEEE
   802 networks.  In this case, BFD packets are transmitted in an
   encapsulation appropriate for the particular IEEE 802 media, with
   Ether Type <TBD3>.  The source and destination addresses MUST be
   unicast MAC addresses associated with the local and remote systems,
   respectively.

   BFD Echo packets are transmitted in an encapsulation appropriate for
   the particular IEEE 802 media, with Ether Type <TBD4>.  The source
   and destination addresses MUST both be unicast MAC addresses
   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.

   Note that BFD Echo mode is not likely to be appropriate for use
   directly over the data link layer, since most data link devices are
   not able to forward frames out the interface over which they were
   received.










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Contributors

   Kireeti Kompella and Yakov Rekhter of Juniper Networks were also
   significant contributors to this document.


Acknowledgments

   This document was inspired by (and is intended to replace) the
   Protocol Liveness Protocol draft, written by Kireeti Kompella.

   The authors would also like to thank Mike Shand, John Scudder, and
   Stewart Bryant for their substantive input.


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


Security Considerations

   When BFD is run over network layer protocols, a significant denial-
   of-service risk is created, as BFD packets may be trivial to spoof.
   When the session is directly connected across a single link, the TTL
   MUST be set to the maximum on transmit, and checked to be equal to
   the maximum value on reception (and the packet dropped if this is not
   the case.)  If BFD is run across multiple hops, some alternative
   mechanism MUST be used.  One option would be to ensure that the
   network addresses used for BFD are not routable outside of the
   infrastructure in which BFD is running (and assuming there are no
   users connected within that network.)  Another option would be to
   filter all packets carrying BFD's UDP ports at the edges of the
   network.  Still another option would be to use cryptographic methods,
   though this is not likely to allow for very short detection times.




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IANA Considerations

   Two well-known UDP port numbers need to be assigned to this protocol.



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   copyrights defined in the Internet Standards process must be
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Acknowledgement

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
































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