





Internet Engineering Task Force (IETF)                           D. Katz
Request for Comments: 5880                                       D. Ward
Category: Standards Track                               Juniper Networks
ISSN: 2070-1721                                                June 2010


                Bidirectional Forwarding Detection (BFD)

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.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5880.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.







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Table of Contents

   1. Introduction ....................................................3
      1.1. Conventions Used in This Document ..........................4
   2. Design ..........................................................4
   3. Protocol Overview ...............................................5
      3.1. Addressing and Session Establishment .......................5
      3.2. Operating Modes ............................................5
   4. BFD Control Packet Format .......................................7
      4.1. Generic BFD Control Packet Format ..........................7
      4.2. Simple Password Authentication Section Format .............11
      4.3. Keyed MD5 and Meticulous Keyed MD5 Authentication
           Section Format ............................................11
      4.4. Keyed SHA1 and Meticulous Keyed SHA1
           Authentication Section Format .............................13
   5. BFD Echo Packet Format .........................................14
   6. Elements of Procedure ..........................................14
      6.1. Overview ..................................................14
      6.2. BFD State Machine .........................................16
      6.3. Demultiplexing and the Discriminator Fields ...............17
      6.4. The Echo Function and Asymmetry ...........................18
      6.5. The Poll Sequence .........................................19
      6.6. Demand Mode ...............................................19
      6.7. Authentication ............................................21
           6.7.1. Enabling and Disabling Authentication ..............21
           6.7.2. Simple Password Authentication .....................22
           6.7.3. Keyed MD5 and Meticulous Keyed MD5 Authentication ..23
           6.7.4. Keyed SHA1 and Meticulous Keyed SHA1
                  Authentication .....................................25
      6.8. Functional Specifics ......................................27
           6.8.1. State Variables ....................................27
           6.8.2. Timer Negotiation ..................................30
           6.8.3. Timer Manipulation .................................31
           6.8.4. Calculating the Detection Time .....................32
           6.8.5. Detecting Failures with the Echo Function ..........33
           6.8.6. Reception of BFD Control Packets ...................33
           6.8.7. Transmitting BFD Control Packets ...................36
           6.8.8. Reception of BFD Echo Packets ......................39
           6.8.9. Transmission of BFD Echo Packets ...................39
           6.8.10. Min Rx Interval Change ............................40
           6.8.11. Min Tx Interval Change ............................40
           6.8.12. Detect Multiplier Change ..........................40
           6.8.13. Enabling or Disabling The Echo Function ...........40
           6.8.14. Enabling or Disabling Demand Mode .................40
           6.8.15. Forwarding Plane Reset ............................41
           6.8.16. Administrative Control ............................41
           6.8.17. Concatenated Paths ................................41
           6.8.18. Holding Down Sessions .............................42



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   7. Operational Considerations .....................................43
   8. IANA Considerations ............................................44
   9. Security Considerations ........................................45
   10. References ....................................................46
      10.1. Normative References .....................................46
      10.2. Informative References ...................................47
   Appendix A. Backward Compatibility (Non-Normative) ................48
   Appendix B. Contributors ..........................................48
   Appendix C. Acknowledgments .......................................49

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.  Detection can
   come fairly quickly in certain circumstances when data link hardware
   comes into play (such as Synchronous Optical Network (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 time to detect failures ("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 Bidirectional Forwarding Detection (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.

   An additional goal is to provide a single mechanism that can be used
   for liveness detection over any media, at any protocol layer, with a
   wide range of Detection Times and overhead, to avoid a proliferation
   of different methods.

   This document specifies the details of the base protocol.  The use of
   some mechanisms are application dependent and are specified in a
   separate series of application documents.  These issues are so noted.





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   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.

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [KEYWORDS].

2.  Design

   BFD is designed to detect failures in communication with a forwarding
   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 forwarding 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 may
   also be implemented in the control engine, though doing so may
   preclude the detection of some kinds of failures.

   BFD operates on top of any data protocol (network layer, link layer,
   tunnels, etc.)  being forwarded between two systems.  It is always
   run in a unicast, point-to-point mode.  BFD packets are carried as
   the payload of whatever encapsulating protocol is appropriate for the
   medium and network.  BFD may be running at multiple layers in a
   system.  The context of the operation of any particular BFD session
   is bound to its encapsulation.

   BFD can provide failure detection on any kind of path between
   systems, including direct physical links, virtual circuits, tunnels,
   MPLS Label Switched Paths (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 a lesser number of paths are available
   in the other direction (multiple parallel unidirectional links or
   MPLS LSPs, for example).

   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.








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   BFD can be abstracted as a simple service.  The service primitives
   provided by BFD are to create, destroy, and modify a session, given
   the destination address and other parameters.  BFD in return provides
   a signal to its clients indicating when the BFD session goes up or
   down.

3.  Protocol Overview

   BFD is a simple 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.  Under some conditions,
   systems may negotiate not to send periodic BFD packets in order to
   reduce overhead.

   A path is only declared to be operational when two-way communication
   has been established between systems, though this does not preclude
   the use of unidirectional links.

   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
   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.

3.1.  Addressing and Session Establishment

   A BFD session is established based on the needs of the application
   that will be making use of it.  It is up to the application to
   determine the need for BFD, and the addresses to use -- there is no
   discovery mechanism in BFD.  For example, an OSPF [OSPF]
   implementation may request a BFD session to be established to a
   neighbor discovered using the OSPF Hello protocol.

3.2.  Operating Modes

   BFD has two operating modes that may be selected, as well as an
   additional function that can be used in combination with the two
   modes.




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   The primary mode is known as Asynchronous mode.  In this mode, the
   systems periodically send 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 Demand mode.  In this mode, it is assumed
   that a system has an independent way of verifying that it has
   connectivity to the other system.  Once a BFD session is established,
   such a system may ask the other system to stop sending BFD Control
   packets, except when the system feels the need to verify connectivity
   explicitly, in which case a short sequence of BFD Control packets is
   exchanged, and then the far system quiesces.  Demand mode may operate
   independently in each direction, or simultaneously.

   An adjunct to both modes is the Echo function.  When the Echo
   function is active, a stream of BFD Echo packets is transmitted in
   such a way as to have the other system loop them back through its
   forwarding path.  If a number of packets of the echoed data stream
   are not received, the session is declared to be down.  The Echo
   function may be used with either Asynchronous or Demand mode.  Since
   the Echo function is handling the task of detection, the rate of
   periodic transmission of Control packets may be reduced (in the case
   of Asynchronous mode) or eliminated completely (in the case of Demand
   mode).

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

   The Echo function has the advantage of truly testing only the
   forwarding path on the remote system.  This 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.

   The Echo function may be enabled individually in each direction.  It
   is enabled in a particular direction only when the system that loops
   the Echo packets back signals that it will allow it, and when the
   system that sends the Echo packets decides it wishes to.

   Demand mode is useful in situations where the overhead of a periodic
   protocol might prove onerous, such as a system with a very large
   number of BFD sessions.  It is also useful when the Echo function is
   being used symmetrically.  Demand mode has the disadvantage that
   Detection Times are essentially driven by the heuristics of the
   system implementation and are not known to the BFD protocol.  Demand




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   mode may not be used when the path round-trip time is greater than
   the desired Detection Time, or the protocol will fail to work
   properly.  See section 6.6 for more details.

4.  BFD Control Packet Format

4.1.  Generic BFD Control Packet Format

   BFD Control packets are sent in an encapsulation appropriate to the
   environment.  The specific encapsulation is outside of the scope of
   this specification.  See the appropriate application document for
   encapsulation details.

   The BFD Control packet has a Mandatory Section and an optional
   Authentication Section.  The format of the Authentication Section, if
   present, is dependent on the type of authentication in use.

   The Mandatory Section 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Vers |  Diag   |Sta|P|F|C|A|D|M|  Detect Mult  |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       My Discriminator                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Your Discriminator                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Desired Min TX Interval                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Required Min RX Interval                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Required Min Echo RX Interval                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   An optional Authentication Section MAY be present:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Auth Type   |   Auth Len    |    Authentication Data...     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Version (Vers)

      The version number of the protocol.  This document defines
      protocol version 1.



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   Diagnostic (Diag)

      A diagnostic code specifying the local system's reason for the
      last change in session state.  Values are:

         0 -- No Diagnostic
         1 -- Control Detection Time Expired
         2 -- Echo Function Failed
         3 -- Neighbor Signaled Session Down
         4 -- Forwarding Plane Reset
         5 -- Path Down
         6 -- Concatenated Path Down
         7 -- Administratively Down
         8 -- Reverse Concatenated Path Down
      9-31 -- Reserved for future use

      This field allows remote systems to determine the reason that the
      previous session failed, for example.

   State (Sta)

      The current BFD session state as seen by the transmitting system.
      Values are:

         0 -- AdminDown
         1 -- Down
         2 -- Init
         3 -- Up

   Poll (P)

      If set, the transmitting system is requesting verification of
      connectivity, or of a parameter change, and is expecting a packet
      with the Final (F) bit in reply.  If clear, the transmitting
      system is not requesting verification.

   Final (F)

      If set, the transmitting system is responding to a received BFD
      Control packet that had the Poll (P) bit set.  If clear, the
      transmitting system is not responding to a Poll.










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   Control Plane Independent (C)

      If set, the transmitting system's BFD implementation does not
      share fate with its control plane (in other words, BFD is
      implemented in the forwarding plane and can continue to function
      through disruptions in the control plane).  If clear, the
      transmitting system's BFD implementation shares fate with its
      control plane.

      The use of this bit is application dependent and is outside the
      scope of this specification.  See specific application
      specifications for details.

   Authentication Present (A)

      If set, the Authentication Section is present and the session is
      to be authenticated (see section 6.7 for details).

   Demand (D)

      If set, Demand mode is active in the transmitting system (the
      system wishes to operate in Demand mode, knows that the session is
      Up in both directions, and is directing the remote system to cease
      the periodic transmission of BFD Control packets).  If clear,
      Demand mode is not active in the transmitting system.

   Multipoint (M)

      This bit is reserved for future point-to-multipoint extensions to
      BFD.  It MUST be zero on both transmit and receipt.

   Detect Mult

      Detection time multiplier.  The negotiated transmit interval,
      multiplied by this value, provides the Detection Time for the
      receiving system in Asynchronous mode.

   Length

      Length of the BFD Control packet, in bytes.

   My Discriminator

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





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   Your Discriminator

      The discriminator received from the corresponding remote system.
      This field reflects back the received value of My Discriminator,
      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,
      less any jitter applied (see section 6.8.2).  The value zero is
      reserved.

   Required Min RX Interval

      This is the minimum interval, in microseconds, between received
      BFD Control packets that this system is capable of supporting,
      less any jitter applied by the sender (see section 6.8.2).  If
      this value is zero, the transmitting system does not want the
      remote system to send any periodic BFD Control packets.

   Required Min Echo RX Interval

      This is the minimum interval, in microseconds, between received
      BFD Echo packets that this system is capable of supporting, less
      any jitter applied by the sender (see section 6.8.9).  If this
      value is zero, the transmitting system does not support the
      receipt of BFD Echo packets.

   Auth Type

      The authentication type in use, if the Authentication Present (A)
      bit is set.

         0 - Reserved
         1 - Simple Password
         2 - Keyed MD5
         3 - Meticulous Keyed MD5
         4 - Keyed SHA1
         5 - Meticulous Keyed SHA1
     6-255 - Reserved for future use

   Auth Len

      The length, in bytes, of the authentication section, including the
      Auth Type and Auth Len fields.





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4.2.  Simple Password Authentication Section Format

   If the Authentication Present (A) bit is set in the header, and the
   Authentication Type field contains 1 (Simple Password), the
   Authentication Section 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Auth Type   |   Auth Len    |  Auth Key ID  |  Password...  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Auth Type

      The Authentication Type, which in this case is 1 (Simple
      Password).

   Auth Len

      The length of the Authentication Section, in bytes.  For Simple
      Password authentication, the length is equal to the password
      length plus three.

   Auth Key ID

      The authentication key ID in use for this packet.  This allows
      multiple keys to be active simultaneously.

   Password

      The simple password in use on this session.  The password is a
      binary string, and MUST be from 1 to 16 bytes in length.  The
      password MUST be encoded and configured according to section
      6.7.2.

4.3.  Keyed MD5 and Meticulous Keyed MD5 Authentication Section Format

   The use of MD5-based authentication is strongly discouraged.
   However, it is documented here for compatibility with existing
   implementations.

   If the Authentication Present (A) bit is set in the header, and the
   Authentication Type field contains 2 (Keyed MD5) or 3 (Meticulous
   Keyed MD5), the Authentication Section has the following format:





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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Auth Type   |   Auth Len    |  Auth Key ID  |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sequence Number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Auth Key/Digest...                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Auth Type

      The Authentication Type, which in this case is 2 (Keyed MD5) or 3
      (Meticulous Keyed MD5).

   Auth Len

      The length of the Authentication Section, in bytes.  For Keyed MD5
      and Meticulous Keyed MD5 authentication, the length is 24.

   Auth Key ID

      The authentication key ID in use for this packet.  This allows
      multiple keys to be active simultaneously.

   Reserved

      This byte MUST be set to zero on transmit, and ignored on receipt.

   Sequence Number

      The sequence number for this packet.  For Keyed MD5
      Authentication, this value is incremented occasionally.  For
      Meticulous Keyed MD5 Authentication, this value is incremented for
      each successive packet transmitted for a session.  This provides
      protection against replay attacks.

   Auth Key/Digest

      This field carries the 16-byte MD5 digest for the packet.  When
      the digest is calculated, the shared MD5 key is stored in this
      field, padded to 16 bytes with trailing zero bytes if needed.  The
      shared key MUST be encoded and configured to section 6.7.3.






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4.4.  Keyed SHA1 and Meticulous Keyed SHA1 Authentication Section Format

   If the Authentication Present (A) bit is set in the header, and the
   Authentication Type field contains 4 (Keyed SHA1) or 5 (Meticulous
   Keyed SHA1), the Authentication Section 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Auth Type   |   Auth Len    |  Auth Key ID  |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sequence Number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Auth Key/Hash...                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Auth Type

      The Authentication Type, which in this case is 4 (Keyed SHA1) or 5
      (Meticulous Keyed SHA1).

   Auth Len

      The length of the Authentication Section, in bytes.  For Keyed
      SHA1 and Meticulous Keyed SHA1 authentication, the length is 28.

   Auth Key ID

      The authentication key ID in use for this packet.  This allows
      multiple keys to be active simultaneously.

   Reserved

      This byte MUST be set to zero on transmit and ignored on receipt.

   Sequence Number

      The sequence number for this packet.  For Keyed SHA1
      Authentication, this value is incremented occasionally.  For
      Meticulous Keyed SHA1 Authentication, this value is incremented
      for each successive packet transmitted for a session.  This
      provides protection against replay attacks.







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   Auth Key/Hash

      This field carries the 20-byte SHA1 hash for the packet.  When the
      hash is calculated, the shared SHA1 key is stored in this field,
      padded to a length of 20 bytes with trailing zero bytes if needed.
      The shared key MUST be encoded and configured to section 6.7.4.

5.  BFD Echo Packet Format

   BFD Echo packets are sent in an encapsulation appropriate to the
   environment.  See the appropriate application documents for the
   specifics of particular environments.

   The payload of a BFD Echo packet is a local matter, since only the
   sending system ever processes the content.  The only requirement is
   that sufficient information is included to demultiplex the received
   packet to the correct BFD session after it is looped back to the
   sender.  The contents are otherwise outside the scope of this
   specification.

   Some form of authentication SHOULD be included, since Echo packets
   may be spoofed.

6.  Elements of Procedure

   This section discusses the normative requirements of the protocol in
   order to achieve interoperability.  It is important for implementors
   to enforce only the requirements specified in this section, as
   misguided pedantry has been proven by experience to affect
   interoperability adversely.

   Remember that all references of the form "bfd.Xx" refer to internal
   state variables (defined in section 6.8.1), whereas all references to
   "the Xxx field" refer to fields in the protocol packets themselves
   (defined in section 4).

6.1.  Overview

   A system may take either an Active role or a Passive role in session
   initialization.  A system taking the Active role MUST send BFD
   Control packets for a particular session, regardless of whether it
   has received any BFD packets for that session.  A system taking the
   Passive role MUST NOT begin sending BFD packets for a particular
   session until it has received a BFD packet for that session, and thus
   has learned the remote system's discriminator value.  At least one
   system MUST take the Active role (possibly both).  The role that a
   system takes is specific to the application of BFD, and is outside
   the scope of this specification.



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   A session begins with the periodic, slow transmission of BFD Control
   packets.  When bidirectional communication is achieved, the BFD
   session becomes Up.

   Once the BFD session is Up, a system can choose to start the Echo
   function if it desires and the other system signals that it will
   allow it.  The rate of transmission of Control packets is typically
   kept low when the Echo function is active.

   If the Echo function is not active, the transmission rate of Control
   packets may be increased to a level necessary to achieve the
   Detection Time requirements for the session.

   Once the session is Up, a system may signal that it has entered
   Demand mode, and the transmission of BFD Control packets by the
   remote system ceases.  Other means of implying connectivity are used
   to keep the session alive.  If either system wishes to verify
   bidirectional connectivity, it can initiate a short exchange of BFD
   Control packets (a "Poll Sequence"; see section 6.5) to do so.

   If Demand mode is not active, and no Control packets are received in
   the calculated Detection Time (see section 6.8.4), the session is
   declared Down.  This is signaled to the remote end via the State
   (Sta) field in outgoing packets.

   If sufficient Echo packets are lost, the session is declared Down in
   the same manner.  See section 6.8.5.

   If Demand mode is active and no appropriate Control packets are
   received in response to a Poll Sequence, the session is declared Down
   in the same manner.  See section 6.6.

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

   Once a session has been declared Down, it cannot come back up until
   the remote end first signals that it is down (by leaving the Up
   state), thus implementing a three-way handshake.

   A session MAY be kept administratively down by entering the AdminDown
   state and sending an explanatory diagnostic code in the Diagnostic
   field.








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6.2.  BFD State Machine

   The BFD state machine is quite straightforward.  There are three
   states through which a session normally proceeds: two for
   establishing a session (Init and Up) and one for tearing down a
   session (Down).  This allows a three-way handshake for both session
   establishment and session teardown (assuring that both systems are
   aware of all session state changes).  A fourth state (AdminDown)
   exists so that a session can be administratively put down
   indefinitely.

   Each system communicates its session state in the State (Sta) field
   in the BFD Control packet, and that received state, in combination
   with the local session state, drives the state machine.

   Down state means that the session is down (or has just been created).
   A session remains in Down state until the remote system indicates
   that it agrees that the session is down by sending a BFD Control
   packet with the State field set to anything other than Up.  If that
   packet signals Down state, the session advances to Init state; if
   that packet signals Init state, the session advances to Up state.
   Semantically, Down state indicates that the forwarding path is
   unavailable, and that appropriate actions should be taken by the
   applications monitoring the state of the BFD session.  A system MAY
   hold a session in Down state indefinitely (by simply refusing to
   advance the session state).  This may be done for operational or
   administrative reasons, among others.

   Init state means that the remote system is communicating, and the
   local system desires to bring the session up, but the remote system
   does not yet realize it.  A session will remain in Init state until
   either a BFD Control Packet is received that is signaling Init or Up
   state (in which case the session advances to Up state) or the
   Detection Time expires, meaning that communication with the remote
   system has been lost (in which case the session advances to Down
   state).

   Up state means that the BFD session has successfully been
   established, and implies that connectivity between the systems is
   working.  The session will remain in the Up state until either
   connectivity fails or the session is taken down administratively.  If
   either the remote system signals Down state or the Detection Time
   expires, the session advances to Down state.








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   AdminDown state means that the session is being held administratively
   down.  This causes the remote system to enter Down state, and remain
   there until the local system exits AdminDown state.  AdminDown state
   has no semantic implications for the availability of the forwarding
   path.

   The following diagram provides an overview of the state machine.
   Transitions involving AdminDown state are deleted for clarity (but
   are fully specified in sections 6.8.6 and 6.8.16).  The notation on
   each arc represents the state of the remote system (as received in
   the State field in the BFD Control packet) or indicates the
   expiration of the Detection Timer.

                             +--+
                             |  | UP, ADMIN DOWN, TIMER
                             |  V
                     DOWN  +------+  INIT
              +------------|      |------------+
              |            | DOWN |            |
              |  +-------->|      |<--------+  |
              |  |         +------+         |  |
              |  |                          |  |
              |  |               ADMIN DOWN,|  |
              |  |ADMIN DOWN,          DOWN,|  |
              |  |TIMER                TIMER|  |
              V  |                          |  V
            +------+                      +------+
       +----|      |                      |      |----+
   DOWN|    | INIT |--------------------->|  UP  |    |INIT, UP
       +--->|      | INIT, UP             |      |<---+
            +------+                      +------+

6.3.  Demultiplexing and the Discriminator Fields

   Since multiple BFD sessions may be running between two systems, there
   needs to be a mechanism for demultiplexing received BFD packets to
   the proper session.

   Each system MUST choose an opaque discriminator value that identifies
   each session, and which MUST be unique among all BFD sessions on the
   system.  The local discriminator is sent in the My Discriminator
   field in the BFD Control packet, and is echoed back in the Your
   Discriminator field of packets sent from the remote end.








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   Once the remote end echoes back the local discriminator, all further
   received packets are demultiplexed based on the Your Discriminator
   field only (which means that, among other things, the source address
   field can change or the interface over which the packets are received
   can change, but the packets will still be associated with the proper
   session).

   The method of demultiplexing the initial packets (in which Your
   Discriminator is zero) is application dependent, and is thus outside
   the scope of this specification.

   Note that it is permissible for a system to change its discriminator
   during a session without affecting the session state, since only that
   system uses its discriminator for demultiplexing purposes (by having
   the other system reflect it back).  The implications on an
   implementation for changing the discriminator value is outside the
   scope of this specification.

6.4.  The Echo Function and Asymmetry

   The Echo function can be run independently in each direction between
   a pair of systems.  For whatever reason, a system may advertise that
   it is willing to receive (and loop back) Echo packets, but may not
   wish to ever send any.  The fact that a system is sending Echo
   packets is not directly signaled to the system looping them back.

   When a system is using the Echo function, it is advantageous to
   choose a sedate reception rate for Control packets, since liveness
   detection is being handled by the Echo packets.  This can be
   controlled by manipulating the Required Min RX Interval field (see
   section 6.8.3).

   If the Echo function is only being run in one direction, the system
   not running the Echo function will more likely wish to receive fairly
   rapid Control packets in order to achieve its desired Detection Time.
   Since BFD allows independent transmission rates in each direction,
   this is easily accomplished.

   A system SHOULD otherwise advertise the lowest value of Required Min
   RX Interval and Required Min Echo RX Interval that it can under the
   circumstances, to give the other system more freedom in choosing its
   transmission rate.  Note that a system is committing to be able to
   receive both streams of packets at the rate it advertises, so this
   should be taken into account when choosing the values to advertise.







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6.5.  The Poll Sequence

   A Poll Sequence is an exchange of BFD Control packets that is used in
   some circumstances to ensure that the remote system is aware of
   parameter changes.  It is also used in Demand mode (see section 6.6)
   to validate bidirectional connectivity.

   A Poll Sequence consists of a system sending periodic BFD Control
   packets with the Poll (P) bit set.  When the other system receives a
   Poll, it immediately transmits a BFD Control packet with the Final
   (F) bit set, independent of any periodic BFD Control packets it may
   be sending (see section 6.8.7).  When the system sending the Poll
   sequence receives a packet with Final, the Poll Sequence is
   terminated, and any subsequent BFD Control packets are sent with the
   Poll bit cleared.  A BFD Control packet MUST NOT have both the Poll
   (P) and Final (F) bits set.

   If periodic BFD Control packets are already being sent (the remote
   system is not in Demand mode), the Poll Sequence MUST be performed by
   setting the Poll (P) bit on those scheduled periodic transmissions;
   additional packets MUST NOT be sent.

   After a Poll Sequence is terminated, the system requesting the Poll
   Sequence will cease the periodic transmission of BFD Control packets
   if the remote end is in Demand mode; otherwise, it will return to the
   periodic transmission of BFD Control packets with the Poll (P) bit
   clear.

   Typically, the entire sequence consists of a single packet in each
   direction, though packet losses or relatively long packet latencies
   may result in multiple Poll packets to be sent before the sequence
   terminates.

6.6.  Demand Mode

   Demand mode is requested independently in each direction by virtue of
   a system setting the Demand (D) bit in its BFD Control packets.  The
   system receiving the Demand bit ceases the periodic transmission of
   BFD Control packets.  If both systems are operating in Demand mode,
   no periodic BFD Control packets will flow in either direction.

   Demand mode requires that some other mechanism is used to imply
   continuing connectivity between the two systems.  The mechanism used
   does not have to be the same in both directions, and is outside of
   the scope of this specification.  One possible mechanism is the
   receipt of traffic from the remote system; another is the use of the
   Echo function.




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   When a system in Demand mode wishes to verify bidirectional
   connectivity, it initiates a Poll Sequence (see section 6.5).  If no
   response is received to a Poll, the Poll is repeated until the
   Detection Time expires, at which point the session is declared to be
   Down.  Note that if Demand mode is operating only on the local
   system, the Poll Sequence is performed by simply setting the Poll (P)
   bit in regular periodic BFD Control packets, as required by section
   6.5.

   The Detection Time in Demand mode is calculated differently than in
   Asynchronous mode; it is based on the transmit rate of the local
   system, rather than the transmit rate of the remote system.  This
   ensures that the Poll Sequence mechanism works properly.  See section
   6.8.4 for more details.

   Note that the Poll mechanism will always fail unless the negotiated
   Detection Time is greater than the round-trip time between the two
   systems.  Enforcement of this constraint is outside the scope of this
   specification.

   Demand mode MAY be enabled or disabled at any time, independently in
   each direction, by setting or clearing the Demand (D) bit in the BFD
   Control packet, without affecting the BFD session state.  Note that
   the Demand bit MUST NOT be set unless both systems perceive the
   session to be Up (the local system thinks the session is Up, and the
   remote system last reported Up state in the State (Sta) field of the
   BFD Control packet).

   When the transmitted value of the Demand (D) bit is to be changed,
   the transmitting system MUST initiate a Poll Sequence in conjunction
   with changing the bit in order to ensure that both systems are aware
   of the change.

   If Demand mode is active on either or both systems, a Poll Sequence
   MUST be initiated whenever the contents of the next BFD Control
   packet to be sent would be different than the contents of the
   previous packet, with the exception of the Poll (P) and Final (F)
   bits.  This ensures that parameter changes are transmitted to the
   remote system and that the remote system acknowledges these changes.

   Because the underlying detection mechanism is unspecified, and may
   differ between the two systems, the overall Detection Time
   characteristics of the path will not be fully known to either system.
   The total Detection Time for a particular system is the sum of the
   time prior to the initiation of the Poll Sequence, plus the
   calculated Detection Time.





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   Note that if Demand mode is enabled in only one direction, continuous
   bidirectional connectivity verification is lost (only connectivity in
   the direction from the system in Demand mode to the other system will
   be verified).  Resolving the issue of one system requesting Demand
   mode while the other requires continuous bidirectional connectivity
   verification is outside the scope of this specification.

6.7.  Authentication

   An optional Authentication Section MAY be present in the BFD Control
   packet.  In its generic form, the purpose of the Authentication
   Section is to carry all necessary information, based on the
   authentication type in use, to allow the receiving system to
   determine the validity of the received packet.  The exact mechanism
   depends on the authentication type in use, but in general the
   transmitting system will put information in the Authentication

   Section that vouches for the packet's validity, and the receiving
   system will examine the Authentication Section and either accept the
   packet for further processing or discard it.

   The same authentication type, and any keys or other necessary
   information, obviously must be in use by the two systems.  The
   negotiation of authentication type, key exchange, etc., are all
   outside the scope of this specification and are expected to be
   performed by means outside of the protocol.

   Note that in the subsections below, to "accept" a packet means only
   that the packet has passed authentication; it may in fact be
   discarded for other reasons as described in the general packet
   reception rules described in section 6.8.6.

   Implementations supporting authentication MUST support both types of
   SHA1 authentication.  Other forms of authentication are optional.

6.7.1.  Enabling and Disabling Authentication

   It may be desirable to enable or disable authentication on a session
   without disturbing the session state.  The exact mechanism for doing
   so is outside the scope of this specification.  However, it is useful
   to point out some issues in supporting this mechanism.

   In a simple implementation, a BFD session will fail when
   authentication is either turned on or turned off, because the packet
   acceptance rules essentially require the local and remote machines to






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   do so in a more or less synchronized fashion (within the Detection
   Time) -- a packet with authentication will only be accepted if
   authentication is "in use" (and likewise packets without
   authentication).

   One possible approach is to build an implementation such that
   authentication is configured, but not considered "in use" until the
   first packet containing a matching authentication section is received
   (providing the necessary synchronization).  Likewise, authentication
   could be configured off, but still considered "in use" until the
   receipt of the first packet without the authentication section.

   In order to avoid security risks, implementations using this method
   SHOULD only allow the authentication state to be changed at most once
   without some form of intervention (so that authentication cannot be
   turned on and off repeatedly simply based on the receipt of BFD
   Control packets from remote systems).  Unless it is desired to enable
   or disable authentication, an implementation SHOULD NOT allow the
   authentication state to change based on the receipt of BFD Control
   packets.

6.7.2.  Simple Password Authentication

   The most straightforward (and weakest) form of authentication is
   Simple Password Authentication.  In this method of authentication,
   one or more Passwords (with corresponding Key IDs) are configured in
   each system and one of these Password/ID pairs is carried in each BFD
   Control packet.  The receiving system accepts the packet if the
   Password and Key ID matches one of the Password/ID pairs configured
   in that system.

   Transmission Using Simple Password Authentication

      The currently selected password and Key ID for the session MUST be
      stored in the Authentication Section of each outgoing BFD Control
      packet.  The Auth Type field MUST be set to 1 (Simple Password).
      The Auth Len field MUST be set to the proper length (4 to 19
      bytes).

      The password is a binary string, and MUST be 1 to 16 bytes in
      length.  For interoperability, the management interface by which
      the password is configured MUST accept ASCII strings, and SHOULD
      also allow for the configuration of any arbitrary binary string in
      hexadecimal form.  Other configuration methods MAY be supported.







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   Reception Using Simple Password Authentication

      If the received BFD Control packet does not contain an
      Authentication Section, or the Auth Type is not 1 (Simple
      Password), then the received packet MUST be discarded.

      If the Auth Key ID field does not match the ID of a configured
      password, the received packet MUST be discarded.

      If the Auth Len field is not equal to the length of the password
      selected by the key ID, plus three, the packet MUST be discarded.

      If the Password field does not match the password selected by the
      key ID, the packet MUST be discarded.

      Otherwise, the packet MUST be accepted.

6.7.3.  Keyed MD5 and Meticulous Keyed MD5 Authentication

   The Keyed MD5 and Meticulous Keyed MD5 Authentication mechanisms are
   very similar to those used in other protocols.  In these methods of
   authentication, one or more secret keys (with corresponding key IDs)
   are configured in each system.  One of the keys is included in an MD5
   [MD5] digest calculated over the outgoing BFD Control packet, but the
   Key itself is not carried in the packet.  To help avoid replay
   attacks, a sequence number is also carried in each packet.  For Keyed
   MD5, the sequence number is occasionally incremented.  For Meticulous
   Keyed MD5, the sequence number is incremented on every packet.

   The receiving system accepts the packet if the key ID matches one of
   the configured Keys, an MD5 digest including the selected key matches
   that carried in the packet, and the sequence number is greater than
   or equal to the last sequence number received (for Keyed MD5), or
   strictly greater than the last sequence number received (for
   Meticulous Keyed MD5).

   Transmission Using Keyed MD5 and Meticulous Keyed MD5 Authentication

      The Auth Type field MUST be set to 2 (Keyed MD5) or 3 (Meticulous
      Keyed MD5).  The Auth Len field MUST be set to 24.  The Auth Key
      ID field MUST be set to the ID of the current authentication key.
      The Sequence Number field MUST be set to bfd.XmitAuthSeq.

      The authentication key value is a binary string of up to 16 bytes,
      and MUST be placed into the Auth Key/Digest field, padded with
      trailing zero bytes as necessary.  For interoperability, the
      management interface by which the key is configured MUST accept




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      ASCII strings, and SHOULD also allow for the configuration of any
      arbitrary binary string in hexadecimal form.  Other configuration
      methods MAY be supported.

      An MD5 digest MUST be calculated over the entire BFD Control
      packet.  The resulting digest MUST be stored in the Auth
      Key/Digest field prior to transmission (replacing the secret key,
      which MUST NOT be carried in the packet).

      For Keyed MD5, bfd.XmitAuthSeq MAY be incremented in a circular
      fashion (when treated as an unsigned 32-bit value).
      bfd.XmitAuthSeq SHOULD be incremented when the session state
      changes, or when the transmitted BFD Control packet carries
      different contents than the previously transmitted packet.  The
      decision as to when to increment bfd.XmitAuthSeq is outside the
      scope of this specification.  See the section titled "Security
      Considerations" below for a discussion.

      For Meticulous Keyed MD5, bfd.XmitAuthSeq MUST be incremented in a
      circular fashion (when treated as an unsigned 32-bit value).

   Receipt Using Keyed MD5 and Meticulous Keyed MD5 Authentication

      If the received BFD Control packet does not contain an
      Authentication Section, or the Auth Type is not correct (2 for
      Keyed MD5 or 3 for Meticulous Keyed MD5), then the received packet
      MUST be discarded.

      If the Auth Key ID field does not match the ID of a configured
      authentication key, the received packet MUST be discarded.

      If the Auth Len field is not equal to 24, the packet MUST be
      discarded.

      If bfd.AuthSeqKnown is 1, examine the Sequence Number field.  For
      Keyed MD5, if the sequence number lies outside of the range of
      bfd.RcvAuthSeq to bfd.RcvAuthSeq+(3*Detect Mult) inclusive (when
      treated as an unsigned 32-bit circular number space), the received
      packet MUST be discarded.  For Meticulous Keyed MD5, if the
      sequence number lies outside of the range of bfd.RcvAuthSeq+1 to
      bfd.RcvAuthSeq+(3*Detect Mult) inclusive (when treated as an
      unsigned 32-bit circular number space) the received packet MUST be
      discarded.

      Otherwise (bfd.AuthSeqKnown is 0), bfd.AuthSeqKnown MUST be set to
      1, and bfd.RcvAuthSeq MUST be set to the value of the received
      Sequence Number field.




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      Replace the contents of the Auth Key/Digest field with the
      authentication key selected by the received Auth Key ID field.  If
      the MD5 digest of the entire BFD Control packet is equal to the
      received value of the Auth Key/Digest field, the received packet
      MUST be accepted.  Otherwise (the digest does not match the Auth
      Key/Digest field), the received packet MUST be discarded.

6.7.4.  Keyed SHA1 and Meticulous Keyed SHA1 Authentication

   The Keyed SHA1 and Meticulous Keyed SHA1 Authentication mechanisms
   are very similar to those used in other protocols.  In these methods
   of authentication, one or more secret keys (with corresponding key
   IDs) are configured in each system.  One of the keys is included in a
   SHA1 [SHA1] hash calculated over the outgoing BFD Control packet, but
   the key itself is not carried in the packet.  To help avoid replay
   attacks, a sequence number is also carried in each packet.  For Keyed
   SHA1, the sequence number is occasionally incremented.  For
   Meticulous Keyed SHA1, the sequence number is incremented on every
   packet.

   The receiving system accepts the packet if the key ID matches one of
   the configured keys, a SHA1 hash including the selected key matches
   that carried in the packet, and if the sequence number is greater
   than or equal to the last sequence number received (for Keyed SHA1),
   or strictly greater than the last sequence number received (for
   Meticulous Keyed SHA1).

   Transmission Using Keyed SHA1 and Meticulous Keyed SHA1
      Authentication

      The Auth Type field MUST be set to 4 (Keyed SHA1) or 5 (Meticulous
      Keyed SHA1).  The Auth Len field MUST be set to 28.  The Auth Key
      ID field MUST be set to the ID of the current authentication key.
      The Sequence Number field MUST be set to bfd.XmitAuthSeq.

      The authentication key value is a binary string of up to 20 bytes,
      and MUST be placed into the Auth Key/Hash field, padding with
      trailing zero bytes as necessary.  For interoperability, the
      management interface by which the key is configured MUST accept
      ASCII strings, and SHOULD also allow for the configuration of any
      arbitrary binary string in hexadecimal form.  Other configuration
      methods MAY be supported.

      A SHA1 hash MUST be calculated over the entire BFD control packet.
      The resulting hash MUST be stored in the Auth Key/Hash field prior
      to transmission (replacing the secret key, which MUST NOT be
      carried in the packet).




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      For Keyed SHA1, bfd.XmitAuthSeq MAY be incremented in a circular
      fashion (when treated as an unsigned 32-bit value).
      bfd.XmitAuthSeq SHOULD be incremented when the session state
      changes, or when the transmitted BFD Control packet carries
      different contents than the previously transmitted packet.  The
      decision as to when to increment bfd.XmitAuthSeq is outside the
      scope of this specification.  See the section titled "Security
      Considerations" below for a discussion.

      For Meticulous Keyed SHA1, bfd.XmitAuthSeq MUST be incremented in
      a circular fashion (when treated as an unsigned 32-bit value).

   Receipt Using Keyed SHA1 and Meticulous Keyed SHA1 Authentication

      If the received BFD Control packet does not contain an
      Authentication Section, or the Auth Type is not correct (4 for
      Keyed SHA1 or 5 for Meticulous Keyed SHA1), then the received
      packet MUST be discarded.

      If the Auth Key ID field does not match the ID of a configured
      authentication key, the received packet MUST be discarded.

      If the Auth Len field is not equal to 28, the packet MUST be
      discarded.

      If bfd.AuthSeqKnown is 1, examine the Sequence Number field.  For
      Keyed SHA1, if the sequence number lies outside of the range of
      bfd.RcvAuthSeq to bfd.RcvAuthSeq+(3*Detect Mult) inclusive (when
      treated as an unsigned 32-bit circular number space), the received
      packet MUST be discarded.  For Meticulous Keyed SHA1, if the
      sequence number lies outside of the range of bfd.RcvAuthSeq+1 to
      bfd.RcvAuthSeq+(3*Detect Mult) inclusive (when treated as an
      unsigned 32-bit circular number space, the received packet MUST be
      discarded.

      Otherwise (bfd.AuthSeqKnown is 0), bfd.AuthSeqKnown MUST be set to
      1, bfd.RcvAuthSeq MUST be set to the value of the received
      Sequence Number field, and the received packet MUST be accepted.

      Replace the contents of the Auth Key/Hash field with the
      authentication key selected by the received Auth Key ID field.  If
      the SHA1 hash of the entire BFD Control packet is equal to the
      received value of the Auth Key/Hash field, the received packet
      MUST be accepted.  Otherwise (the hash does not match the Auth
      Key/Hash field), the received packet MUST be discarded.






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6.8.  Functional Specifics

   The following section of this specification is normative.  The means
   by which this specification is achieved is outside the scope of this
   specification.

   When a system is said to have "the Echo function active" it means
   that the system is sending BFD Echo packets, implying that the
   session is Up and the other system has signaled its willingness to
   loop back Echo packets.

   When the local system is said to have "Demand mode active," it means
   that bfd.DemandMode is 1 in the local system (see section 6.8.1), the
   session is Up, and the remote system is signaling that the session is
   in state Up.

   When the remote system is said to have "Demand mode active," it means
   that bfd.RemoteDemandMode is 1 (the remote system set the Demand (D)
   bit in the last received BFD Control packet), the session is Up, and
   the remote system is signaling that the session is in state Up.

6.8.1.  State Variables

   A minimum amount of information about a session needs to be tracked
   in order to achieve the elements of procedure described here.  The
   following is a set of state variables that are helpful in describing
   the mechanisms of BFD.  Any means of tracking this state may be used
   so long as the protocol behaves as described.

   When the text refers to initializing a state variable, this takes
   place only at the time that the session (and the corresponding state
   variables) is created.  The state variables are subsequently
   manipulated by the state machine and are never reinitialized, even if
   the session fails and is reestablished.

   Once session state is created, and at least one BFD Control packet is
   received from the remote end, it MUST be preserved for at least one
   Detection Time (see section 6.8.4) subsequent to the receipt of the
   last BFD Control packet, regardless of the session state.  This
   preserves timing parameters in case the session flaps.  A system MAY
   preserve session state longer than this.  The preservation or
   destruction of session state when no BFD Control packets for this
   session have been received from the remote system is outside the
   scope of this specification.







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   All state variables in this specification are of the form "bfd.Xx"
   and should not be confused with fields carried in the protocol
   packets, which are always spelled out to match the names in section
   4.

   bfd.SessionState

      The perceived state of the session (Init, Up, Down, or AdminDown).
      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 variable MUST be initialized
      to Down.

   bfd.RemoteSessionState

      The session state last reported by the remote system in the State
      (Sta) field of the BFD Control packet.  This variable MUST be
      initialized to Down.

   bfd.LocalDiscr

      The local discriminator for this BFD session, used to uniquely
      identify it.  It MUST be unique across all BFD sessions on this
      system, and nonzero.  It SHOULD be set to a random (but still
      unique) value to improve security.  The value is otherwise outside
      the scope of this specification.

   bfd.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.  If a
      period of a Detection Time passes without the receipt of a valid,
      authenticated BFD packet from the remote system, this variable
      MUST be set to zero.

   bfd.LocalDiag

      The diagnostic code specifying the reason for the most recent
      change in the local session state.  This MUST be initialized to
      zero (No Diagnostic).









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   bfd.DesiredMinTxInterval

      The minimum interval, in microseconds, between transmitted BFD
      Control packets that this system would like to use at the current
      time, less any jitter applied (see section 6.8.2).  The actual
      interval is negotiated between the two systems.  This MUST be
      initialized to a value of at least one second (1,000,000
      microseconds) according to the rules described in section 6.8.3.
      The setting of this variable is otherwise outside the scope of
      this specification.

   bfd.RequiredMinRxInterval

      The minimum interval, in microseconds, between received BFD
      Control packets that this system requires, less any jitter applied
      by the sender (see section 6.8.2).  The setting of this variable
      is outside the scope of this specification.  A value of zero means
      that this system does not want to receive any periodic BFD Control
      packets.  See section 6.8.18 for details.

   bfd.RemoteMinRxInterval

      The last value of Required Min RX Interval received from the
      remote system in a BFD Control packet.  This variable MUST be
      initialized to 1.

   bfd.DemandMode

      Set to 1 if the local system wishes to use Demand mode, or 0 if
      not.

   bfd.RemoteDemandMode

      Set to 1 if the remote system wishes to use Demand mode, or 0 if
      not.  This is the value of the Demand (D) bit in the last received
      BFD Control packet.  This variable MUST be initialized to zero.

   bfd.DetectMult

      The desired Detection Time multiplier for BFD Control packets on
      the local system.  The negotiated Control packet transmission
      interval, multiplied by this variable, will be the Detection Time
      for this session (as seen by the remote system).  This variable
      MUST be a nonzero integer, and is otherwise outside the scope of
      this specification.  See section 6.8.4 for further information.






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   bfd.AuthType

      The authentication type in use for this session, as defined in
      section 4.1, or zero if no authentication is in use.

   bfd.RcvAuthSeq

      A 32-bit unsigned integer containing the last sequence number for
      Keyed MD5 or SHA1 Authentication that was received.  The initial
      value is unimportant.

   bfd.XmitAuthSeq

      A 32-bit unsigned integer containing the next sequence number for
      Keyed MD5 or SHA1 Authentication to be transmitted.  This variable
      MUST be initialized to a random 32-bit value.

   bfd.AuthSeqKnown

      Set to 1 if the next sequence number for Keyed MD5 or SHA1
      authentication expected to be received is known, or 0 if it is not
      known.  This variable MUST be initialized to zero.

      This variable MUST be set to zero after no packets have been
      received on this session for at least twice the Detection Time.
      This ensures that the sequence number can be resynchronized if the
      remote system restarts.

6.8.2.  Timer Negotiation

   The time values used to determine BFD packet transmission intervals
   and the session Detection Time are continuously negotiated, and thus
   may be changed at any time.  The negotiation and time values are
   independent in each direction for each session.

   Each system reports in the BFD Control packet how rapidly it would
   like to transmit BFD packets, as well as how rapidly it is prepared
   to receive them.  This allows either system to unilaterally determine
   the maximum packet rate (minimum interval) in both directions.

   See section 6.8.7 for the details of packet transmission timing and
   negotiation.









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6.8.3.  Timer Manipulation

   The time values used to determine BFD packet transmission intervals
   and the session Detection Time may be modified at any time without
   affecting the state of the session.  When the timer parameters are
   changed for any reason, the requirements of this section apply.

   If either bfd.DesiredMinTxInterval is changed or
   bfd.RequiredMinRxInterval is changed, a Poll Sequence MUST be
   initiated (see section 6.5).  If the timing is such that a system
   receiving a Poll Sequence wishes to change the parameters described
   in this paragraph, the new parameter values MAY be carried in packets
   with the Final (F) bit set, even if the Poll Sequence has not yet
   been sent.

   If bfd.DesiredMinTxInterval is increased and bfd.SessionState is Up,
   the actual transmission interval used MUST NOT change until the Poll
   Sequence described above has terminated.  This is to ensure that the
   remote system updates its Detection Time before the transmission
   interval increases.

   If bfd.RequiredMinRxInterval is reduced and bfd.SessionState is Up,
   the previous value of bfd.RequiredMinRxInterval MUST be used when
   calculating the Detection Time for the remote system until the Poll
   Sequence described above has terminated.  This is to ensure that the
   remote system is transmitting packets at the higher rate (and those
   packets are being received) prior to the Detection Time being
   reduced.

   When bfd.SessionState is not Up, the system MUST set
   bfd.DesiredMinTxInterval to a value of not less than one second
   (1,000,000 microseconds).  This is intended to ensure that the
   bandwidth consumed by BFD sessions that are not Up is negligible,
   particularly in the case where a neighbor may not be running BFD.

   If the local system reduces its transmit interval due to
   bfd.RemoteMinRxInterval being reduced (the remote system has
   advertised a reduced value in Required Min RX Interval), and the
   remote system is not in Demand mode, the local system MUST honor the
   new interval immediately.  In other words, the local system cannot
   wait longer than the new interval between the previous packet
   transmission and the next one.  If this interval has already passed
   since the last transmission (because the new interval is
   significantly shorter), the local system MUST send the next periodic
   BFD Control packet as soon as practicable.






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   When the Echo function is active, a system SHOULD set
   bfd.RequiredMinRxInterval to a value of not less than one second
   (1,000,000 microseconds).  This is intended to keep received BFD
   Control traffic at a negligible level, since the actual detection
   function is being performed using BFD Echo packets.

   In any case other than those explicitly called out above, timing
   parameter changes MUST be effected immediately (changing the
   transmission rate and/or the Detection Time).

   Note that the Poll Sequence mechanism is ambiguous if more than one
   parameter change is made that would require its use, and those
   multiple changes are spread across multiple packets (since the
   semantics of the returning Final are unclear).  Therefore, if
   multiple changes are made that require the use of a Poll Sequence,
   there are three choices: 1) they MUST be communicated in a single BFD
   Control packet (so the semantics of the Final reply are clear), or 2)
   sufficient time must have transpired since the Poll Sequence was
   completed to disambiguate the situation (at least a round trip time
   since the last Poll was transmitted) prior to the initiation of
   another Poll Sequence, or 3) an additional BFD Control packet with
   the Final (F) bit *clear* MUST be received after the Poll Sequence
   has completed prior to the initiation of another Poll Sequence (this
   option is not available when Demand mode is active).

6.8.4.  Calculating the Detection Time

   The Detection Time (the period of time without receiving BFD packets
   after which the session is determined to have failed) is not carried
   explicitly in the protocol.  Rather, it is calculated independently
   in each direction by the receiving system based on the negotiated
   transmit interval and the detection multiplier.  Note that there may
   be different Detection Times in each direction.

   The calculation of the Detection Time is slightly different when in
   Demand mode versus Asynchronous mode.

   In Asynchronous mode, the Detection Time calculated in the local
   system is equal to the value of Detect Mult received from the remote
   system, multiplied by the agreed transmit interval of the remote
   system (the greater of bfd.RequiredMinRxInterval and the last
   received Desired Min TX Interval).  The Detect Mult value is (roughly
   speaking, due to jitter) the number of packets that have to be missed
   in a row to declare the session to be down.







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   If Demand mode is not active, and a period of time equal to the
   Detection Time passes without receiving a BFD Control packet from the
   remote system, and bfd.SessionState is Init or Up, the session has
   gone down -- the local system MUST set bfd.SessionState to Down and
   bfd.LocalDiag to 1 (Control Detection Time Expired).

   In Demand mode, the Detection Time calculated in the local system is
   equal to bfd.DetectMult, multiplied by the agreed transmit interval
   of the local system (the greater of bfd.DesiredMinTxInterval and
   bfd.RemoteMinRxInterval).  bfd.DetectMult is (roughly speaking, due
   to jitter) the number of packets that have to be missed in a row to
   declare the session to be down.

   If Demand mode is active, and a period of time equal to the Detection
   Time passes after the initiation of a Poll Sequence (the transmission
   of the first BFD Control packet with the Poll bit set), the session
   has gone down -- the local system MUST set bfd.SessionState to Down,
   and bfd.LocalDiag to 1 (Control Detection Time Expired).

   (Note that a packet is considered to have been received, for the
   purposes of Detection Time expiration, only if it has not been
   "discarded" according to the rules of section 6.8.6).

6.8.5.  Detecting Failures with the Echo Function

   When the Echo function is active and a sufficient number of Echo
   packets have not arrived as they should, the session has gone down --
   the local system MUST set bfd.SessionState to Down and bfd.LocalDiag
   to 2 (Echo Function Failed).

   The means by which the Echo function failures are detected is outside
   of the scope of this specification.  Any means that will detect a
   communication failure are acceptable.

6.8.6.  Reception of BFD Control Packets

   When a BFD Control packet is received, the following procedure MUST
   be followed, in the order specified.  If the packet is discarded
   according to these rules, processing of the packet MUST cease at that
   point.

      If the version number is not correct (1), the packet MUST be
      discarded.

      If the Length field is less than the minimum correct value (24 if
      the A bit is clear, or 26 if the A bit is set), the packet MUST be
      discarded.




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

      If the Detect Mult field is zero, the packet MUST be discarded.

      If the Multipoint (M) bit is nonzero, the packet MUST be
      discarded.

      If the My Discriminator field is zero, the packet MUST be
      discarded.

      If the Your Discriminator field is nonzero, it MUST be used to
      select the session with which this BFD packet is associated.  If
      no session is found, the packet MUST be discarded.

      If the Your Discriminator field is zero and the State field is not
      Down or AdminDown, the packet MUST be discarded.

      If the Your Discriminator field is zero, the session MUST be
      selected based on some combination of other fields, possibly
      including source addressing information, the My Discriminator
      field, and the interface over which the packet was received.  The
      exact method of selection is application specific and is thus
      outside the scope of this specification.  If a matching session is
      not found, a new session MAY be created, or the packet MAY be
      discarded.  This choice is outside the scope of this
      specification.

      If the A bit is set and no authentication is in use (bfd.AuthType
      is zero), the packet MUST be discarded.

      If the A bit is clear and authentication is in use (bfd.AuthType
      is nonzero), the packet MUST be discarded.

      If the A bit is set, the packet MUST be authenticated under the
      rules of section 6.7, based on the authentication type in use
      (bfd.AuthType).  This may cause the packet to be discarded.

      Set bfd.RemoteDiscr to the value of My Discriminator.

      Set bfd.RemoteState to the value of the State (Sta) field.

      Set bfd.RemoteDemandMode to the value of the Demand (D) bit.

      Set bfd.RemoteMinRxInterval to the value of Required Min RX
      Interval.





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      If the Required Min Echo RX Interval field is zero, the
      transmission of Echo packets, if any, MUST cease.

      If a Poll Sequence is being transmitted by the local system and
      the Final (F) bit in the received packet is set, the Poll Sequence
      MUST be terminated.

      Update the transmit interval as described in section 6.8.2.

      Update the Detection Time as described in section 6.8.4.

      If bfd.SessionState is AdminDown

          Discard the packet

      If received state is AdminDown
          If bfd.SessionState is not Down
              Set bfd.LocalDiag to 3 (Neighbor signaled
                  session down)
              Set bfd.SessionState to Down

      Else

          If bfd.SessionState is Down
              If received State is Down
                  Set bfd.SessionState to Init
              Else if received State is Init
                  Set bfd.SessionState to Up

          Else if bfd.SessionState is Init
              If received State is Init or Up
                  Set bfd.SessionState to Up

          Else (bfd.SessionState is Up)
              If received State is Down
                  Set bfd.LocalDiag to 3 (Neighbor signaled
                      session down)
                  Set bfd.SessionState to Down

      Check to see if Demand mode should become active or not (see
      section 6.6).

      If bfd.RemoteDemandMode is 1, bfd.SessionState is Up, and
      bfd.RemoteSessionState is Up, Demand mode is active on the remote
      system and the local system MUST cease the periodic transmission
      of BFD Control packets (see section 6.8.7).





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      If bfd.RemoteDemandMode is 0, or bfd.SessionState is not Up, or
      bfd.RemoteSessionState is not Up, Demand mode is not active on the
      remote system and the local system MUST send periodic BFD Control
      packets (see section 6.8.7).

      If the Poll (P) bit is set, send a BFD Control packet to the
      remote system with the Poll (P) bit clear, and the Final (F) bit
      set (see section 6.8.7).

      If the packet was not discarded, it has been received for purposes
      of the Detection Time expiration rules in section 6.8.4.

6.8.7.  Transmitting BFD Control Packets

   With the exceptions listed in the remainder of this section, a system
   MUST NOT transmit BFD Control packets at an interval less than the
   larger of bfd.DesiredMinTxInterval and bfd.RemoteMinRxInterval, less
   applied jitter (see below).  In other words, the system reporting the
   slower rate determines the transmission rate.

   The periodic transmission of BFD Control packets MUST be jittered on
   a per-packet basis by up to 25%, that is, the interval MUST be
   reduced by a random value of 0 to 25%, in order to avoid self-
   synchronization with other systems on the same subnetwork.  Thus, the
   average interval between packets will be roughly 12.5% less than that
   negotiated.

   If bfd.DetectMult is equal to 1, the interval between transmitted BFD
   Control packets MUST be no more than 90% of the negotiated
   transmission interval, and MUST be no less than 75% of the negotiated
   transmission interval.  This is to ensure that, on the remote system,
   the calculated Detection Time does not pass prior to the receipt of
   the next BFD Control packet.

   The transmit interval MUST be recalculated whenever
   bfd.DesiredMinTxInterval changes, or whenever bfd.RemoteMinRxInterval
   changes, and is equal to the greater of those two values.  See
   sections 6.8.2 and 6.8.3 for details on transmit timers.

   A system MUST NOT transmit BFD Control packets if bfd.RemoteDiscr is
   zero and the system is taking the Passive role.

   A system MUST NOT periodically transmit BFD Control packets if
   bfd.RemoteMinRxInterval is zero.







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   A system MUST NOT periodically transmit BFD Control packets if Demand
   mode is active on the remote system (bfd.RemoteDemandMode is 1,
   bfd.SessionState is Up, and bfd.RemoteSessionState is Up) and a Poll
   Sequence is not being transmitted.

   If a BFD Control packet is received with the Poll (P) bit set to 1,
   the receiving system MUST transmit a BFD Control packet with the Poll
   (P) bit clear and the Final (F) bit set as soon as practicable,
   without respect to the transmission timer or any other transmission
   limitations, without respect to the session state, and without
   respect to whether Demand mode is active on either system.  A system
   MAY limit the rate at which such packets are transmitted.  If rate
   limiting is in effect, the advertised value of Desired Min TX
   Interval MUST be greater than or equal to the interval between
   transmitted packets imposed by the rate limiting function.

   A system MUST NOT set the Demand (D) bit unless bfd.DemandMode is 1,
   bfd.SessionState is Up, and bfd.RemoteSessionState is Up.

   A BFD Control packet SHOULD be transmitted during the interval
   between periodic Control packet transmissions when the contents of
   that packet would differ from that in the previously transmitted
   packet (other than the Poll and Final bits) in order to more rapidly
   communicate a change in state.

   The contents of transmitted BFD Control packets MUST be set as
   follows:

   Version

      Set to the current version number (1).

   Diagnostic (Diag)

      Set to bfd.LocalDiag.

   State (Sta)

      Set to the value indicated by bfd.SessionState.

   Poll (P)

      Set to 1 if the local system is sending a Poll Sequence, or 0 if
      not.







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   Final (F)

      Set to 1 if the local system is responding to a Control packet
      received with the Poll (P) bit set, or 0 if not.

   Control Plane Independent (C)

      Set to 1 if the local system's BFD implementation is independent
      of the control plane (it can continue to function through a
      disruption of the control plane).

   Authentication Present (A)

      Set to 1 if authentication is in use on this session (bfd.AuthType
      is nonzero), or 0 if not.

   Demand (D)

      Set to bfd.DemandMode if bfd.SessionState is Up and
      bfd.RemoteSessionState is Up.  Otherwise, it is set to 0.

   Multipoint (M)

      Set to 0.

   Detect Mult

      Set to bfd.DetectMult.

   Length

      Set to the appropriate length, based on the fixed header length
      (24) plus any Authentication Section.

   My Discriminator

      Set to bfd.LocalDiscr.

   Your Discriminator

      Set to bfd.RemoteDiscr.

   Desired Min TX Interval

      Set to bfd.DesiredMinTxInterval.






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   Required Min RX Interval

      Set to bfd.RequiredMinRxInterval.

   Required Min Echo RX Interval

      Set to the minimum required Echo packet receive interval for this
      session.  If this field is set to zero, the local system is
      unwilling or unable to loop back BFD Echo packets to the remote
      system, and the remote system will not send Echo packets.

   Authentication Section

      Included and set according to the rules in section 6.7 if
      authentication is in use (bfd.AuthType is nonzero).  Otherwise,
      this section is not present.

6.8.8.  Reception of BFD Echo Packets

   A received BFD Echo packet MUST be demultiplexed to the appropriate
   session for processing.  A means of detecting missing Echo packets
   MUST be implemented, which most likely involves processing of the
   Echo packets that are received.  The processing of received Echo
   packets is otherwise outside the scope of this specification.

6.8.9.  Transmission of BFD Echo Packets

   BFD Echo packets MUST NOT be transmitted when bfd.SessionState is not
   Up.  BFD Echo packets MUST NOT be transmitted unless the last BFD
   Control packet received from the remote system contains a nonzero
   value in Required Min Echo RX Interval.

   BFD Echo packets MAY be transmitted when bfd.SessionState is Up.  The
   interval between transmitted BFD Echo packets MUST NOT be less than
   the value advertised by the remote system in Required Min Echo RX
   Interval, except as follows:

      A 25% jitter MAY be applied to the rate of transmission, such that
      the actual interval MAY be between 75% and 100% of the advertised
      value.  A single BFD Echo packet MAY be transmitted between
      normally scheduled Echo transmission intervals.

   The transmission of BFD Echo packets is otherwise outside the scope
   of this specification.







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6.8.10.  Min Rx Interval Change

   When it is desired to change the rate at which BFD Control packets
   arrive from the remote system, bfd.RequiredMinRxInterval can be
   changed at any time to any value.  The new value will be transmitted
   in the next outgoing Control packet, and the remote system will
   adjust accordingly.  See section 6.8.3 for further requirements.

6.8.11.  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), bfd.DesiredMinTxInterval can be changed at
   any time to any value.  The rules in section 6.8.3 apply.

6.8.12.  Detect Multiplier Change

   When it is desired to change the detect multiplier, the value of
   bfd.DetectMult can be changed to any nonzero value.  The new value
   will be transmitted with the next BFD Control packet, and the use of
   a Poll Sequence is not necessary.  See section 6.6 for additional
   requirements.

6.8.13.  Enabling or Disabling The Echo Function

   If it is desired to start or stop the transmission of BFD Echo
   packets, this MAY be done at any time (subject to the transmission
   requirements detailed in section 6.8.9).

   If it is desired to enable or disable the looping back of received
   BFD Echo packets, this MAY be done at any time by changing the value
   of Required Min Echo RX Interval to zero or nonzero in outgoing BFD
   Control packets.

6.8.14.  Enabling or Disabling Demand Mode

   If it is desired to start or stop Demand mode, this MAY be done at
   any time by setting bfd.DemandMode to the proper value.  Demand mode
   will subsequently become active under the rules described in section
   6.6.

   If Demand mode is no longer active on the remote system, the local
   system MUST begin transmitting periodic BFD Control packets as
   described in section 6.8.7.







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6.8.15.  Forwarding Plane Reset

   When the forwarding plane in the local system is reset for some
   reason, such that the remote system can no longer rely on the local
   forwarding state, the local system MUST set bfd.LocalDiag to 4
   (Forwarding Plane Reset), and set bfd.SessionState to Down.

6.8.16.  Administrative Control

   There may be circumstances where it is desirable to administratively
   enable or disable a BFD session.  When this is desired, the following
   procedure MUST be followed:

      If enabling session
         Set bfd.SessionState to Down

      Else
         Set bfd.SessionState to AdminDown
         Set bfd.LocalDiag to an appropriate value
         Cease the transmission of BFD Echo packets

   If signaling is received from outside BFD that the underlying path
   has failed, an implementation MAY administratively disable the
   session with the diagnostic Path Down.

   Other scenarios MAY use the diagnostic Administratively Down.

   BFD Control packets SHOULD be transmitted for at least a Detection
   Time after transitioning to AdminDown state in order to ensure that
   the remote system is aware of the state change.  BFD Control packets
   MAY be transmitted indefinitely after transitioning to AdminDown
   state in order to maintain session state in each system (see section
   6.8.18 below).

6.8.17.  Concatenated Paths

   If the path being monitored by BFD is concatenated with other paths
   (connected end-to-end in series), it may be desirable to propagate
   the indication of a failure of one of those paths across the BFD
   session (providing an interworking function for liveness monitoring
   between BFD and other technologies).

   Two diagnostic codes are defined for this purpose: Concatenated Path
   Down and Reverse Concatenated Path Down.  The first propagates
   forward path failures (in which the concatenated path fails in the
   direction toward the interworking system), and the second propagates





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   reverse path failures (in which the concatenated path fails in the
   direction away from the interworking system, assuming a bidirectional
   link).

   A system MAY signal one of these failure states by simply setting
   bfd.LocalDiag to the appropriate diagnostic code.  Note that the BFD
   session is not taken down.  If Demand mode is not active on the
   remote system, no other action is necessary, as the diagnostic code
   will be carried via the periodic transmission of BFD Control packets.
   If Demand mode is active on the remote system (the local system is
   not transmitting periodic BFD Control packets), a Poll Sequence MUST
   be initiated to ensure that the diagnostic code is transmitted.  Note
   that if the BFD session subsequently fails, the diagnostic code will
   be overwritten with a code detailing the cause of the failure.  It is
   up to the interworking agent to perform the above procedure again,
   once the BFD session reaches Up state, if the propagation of the
   concatenated path failure is to resume.

6.8.18.  Holding Down Sessions

   A system MAY choose to prevent a BFD session from being established.
   One possible reason might be to manage the rate at which sessions are
   established.  This can be done by holding the session in Down or
   AdminDown state, as appropriate.

   There are two related mechanisms that are available to help with this
   task.  First, a system is REQUIRED to maintain session state
   (including timing parameters), even when a session is down, until a
   Detection Time has passed without the receipt of any BFD Control
   packets.  This means that a system may take down a session and
   transmit an arbitrarily large value in the Required Min RX Interval
   field to control the rate at which it receives packets.

   Additionally, a system MAY transmit a value of zero for Required Min
   RX Interval to indicate that the remote system should send no packets
   whatsoever.

   So long as the local system continues to transmit BFD Control
   packets, the remote system is obligated to obey the value carried in
   Required Min RX Interval.  If the remote system does not receive any
   BFD Control packets for a Detection Time, it SHOULD reset
   bfd.RemoteMinRxInterval to its initial value of 1 (per section 6.8.1,
   since it is no longer required to maintain previous session state)
   and then can transmit at its own rate.







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7.  Operational Considerations

   BFD is likely to be deployed as a critical part of network
   infrastructure.  As such, care should be taken to avoid disruption.

   Obviously, any mechanism that blocks BFD packets, such as firewalls
   or other policy processes, will cause BFD to fail.

   Mechanisms that control packet scheduling, such as policers, traffic
   shapers, priority queueing, etc., have the potential of impacting BFD
   operations if the Detection Time is similar in scale to the scheduled
   packet transmit or receive rate.  The delivery of BFD packets is
   time-critical, relative to the magnitude of the Detection Time, so
   this may need to be taken into account in implementation and
   deployment, particularly when very short Detection Times are to be
   used.

   When BFD is used across multiple hops, a congestion control mechanism
   MUST be implemented, and when congestion is detected, the BFD
   implementation MUST reduce the amount of traffic it generates.  The
   exact mechanism used is outside the scope of this specification, and
   the requirements of this mechanism may differ depending on how BFD is
   deployed, and how it interacts with other parts of the system (for
   example, exponential backoff may not be appropriate in cases where
   routing protocols are interacting closely with BFD).

   Note that "congestion" is not only a traffic phenomenon, but also a
   computational one.  It is possible for systems with a large number of
   BFD sessions and/or very short packet intervals to become CPU-bound.
   As such, a congestion control algorithm SHOULD be used even across
   single hops in order to avoid the possibility of catastrophic system
   collapse, as such failures have been seen repeatedly in other
   periodic Hello-based protocols.

   The mechanisms for detecting congestion are outside the scope of this
   specification, but may include the detection of lost BFD Control
   packets (by virtue of holes in the authentication sequence number
   space, or by BFD session failure) or other means.

   The mechanisms for reducing BFD's traffic load are the control of the
   local and remote packet transmission rate via the Min RX Interval and
   Min TX Interval fields.

   Note that any mechanism that increases the transmit or receive
   intervals will increase the Detection Time for the session.






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   It is worth noting that a single BFD session does not consume a large
   amount of bandwidth.  An aggressive session that achieves a detection
   time of 50 milliseconds, by using a transmit interval of 16.7
   milliseconds and a detect multiplier of 3, will generate 60 packets
   per second.  The maximum length of each packet on the wire is on the
   order of 100 bytes, for a total of around 48 kilobits per second of
   bandwidth consumption in each direction.

8.  IANA Considerations

   This document defines two registries administered by IANA.  The first
   is titled "BFD Diagnostic Codes" (see section 4.1).  Initial values
   for the BFD Diagnostic Code registry are given below.  Further
   assignments are to be made through Expert Review
   [IANA-CONSIDERATIONS].  Assignments consist of a BFD Diagnostic Code
   name and its associated value.

      Value    BFD Diagnostic Code Name
      -----    ------------------------
       0       No Diagnostic
       1       Control Detection Time Expired
       2       Echo Function Failed
       3       Neighbor Signaled Session Down
       4       Forwarding Plane Reset
       5       Path Down
       6       Concatenated Path Down
       7       Administratively Down
       8       Reverse Concatenated Path Down
       9-31    Unassigned

   The second registry is titled "BFD Authentication Types" (see section
   4.1).  Initial values for the BFD Authentication Type registry are
   given below.  Further assignments are to be made through Expert
   Review [IANA-CONSIDERATIONS].  Assignments consist of a BFD
   Authentication Type Code name and its associated value.

      Value    BFD Authentication Type Name
      -----    ----------------------------
       0       Reserved
       1       Simple Password
       2       Keyed MD5
       3       Meticulous Keyed MD5
       4       Keyed SHA1
       5       Meticulous Keyed SHA1
       6-255   Unassigned






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9.  Security Considerations

   As BFD may be tied into the stability of the network infrastructure
   (such as routing protocols), the effects of an attack on a BFD
   session may be very serious: a link may be falsely declared to be
   down, or falsely declared to be up; in either case, the effect is
   denial of service.

   An attacker who is in complete control of the link between the
   systems can easily drop all BFD packets but forward everything else
   (causing the link to be falsely declared down), or forward only the
   BFD packets but nothing else (causing the link to be falsely declared
   up).  This attack cannot be prevented by BFD.

   To mitigate threats from less capable attackers, BFD specifies two
   mechanisms to prevent spoofing of BFD Control packets.  The
   Generalized TTL Security Mechanism [GTSM] uses the time to live (TTL)
   or Hop Count to prevent off-link attackers from spoofing packets.
   The Authentication Section authenticates the BFD Control packets.
   These mechanisms are described in more detail below.

   When a BFD session is directly connected across a single link
   (physical, or a secure tunnel such as IPsec), the TTL or Hop Count
   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).  See [GTSM] for more information on this technique.  If
   BFD is run across multiple hops or an insecure tunnel (such as
   Generic Routing Encapsulation (GRE)), the Authentication Section
   SHOULD be utilized.

   The level of security provided by the Authentication Section varies
   based on the authentication type used.  Simple Password
   authentication is obviously only as secure as the secrecy of the
   passwords used, and should be considered only if the BFD session is
   guaranteed to be run over an infrastructure not subject to packet
   interception.  Its chief advantage is that it minimizes the
   computational effort required for authentication.

   Keyed MD5 Authentication is much stronger than Simple Password
   Authentication since the keys cannot be discerned by intercepting
   packets.  It is vulnerable to replay attacks in between increments of
   the sequence number.  The sequence number can be incremented as
   seldom (or as often) as desired, trading off resistance to replay
   attacks with the computational effort required for authentication.

   Meticulous Keyed MD5 authentication is stronger yet, as it requires
   the sequence number to be incremented for every packet.  Replay
   attack vulnerability is reduced due to the requirement that the



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   sequence number must be incremented on every packet, the window size
   of acceptable packets is small, and the initial sequence number is
   randomized.  There is still a window of attack at the beginning of
   the session while the sequence number is being determined.  This
   authentication scheme requires an MD5 calculation on every packet
   transmitted and received.

   Using SHA1 is believed to have stronger security properties than MD5.
   All comments about MD5 in this section also apply to SHA1.

   Both Keyed MD5/SHA1 and Meticulous Keyed MD5/SHA1 use the "secret
   suffix" construction (also called "append only") in which the shared
   secret key is appended to the data before calculating the hash,
   instead of the more common Hashed Message Authentication Code (HMAC)
   construction [HMAC].  This construction is believed to be appropriate
   for BFD, but designers of any additional authentication mechanisms
   for BFD are encouraged to read [HMAC] and its references.

   If both systems randomize their Local Discriminator values at the
   beginning of a session, replay attacks may be further mitigated,
   regardless of the authentication type in use.  Since the Local
   Discriminator may be changed at any time during a session, this
   mechanism may also help mitigate attacks.

   The security implications of the use of BFD Echo packets are
   dependent on how those packets are defined, since their structure is
   local to the transmitting system and outside the scope of this
   specification.  However, since Echo packets are defined and processed
   only by the transmitting system, the use of cryptographic
   authentication does not guarantee that the other system is actually
   alive; an attacker could loop the Echo packets back (without knowing
   any secret keys) and cause the link to be falsely declared to be up.
   This can be mitigated by using a suitable interval for BFD Control
   packets.  [GTSM] could be applied to BFD Echo packets, though the
   TTL/Hop Count will be decremented by 1 in the course of echoing the
   packet, so spoofing is possible from one hop away.

10.  References

10.1.  Normative References

   [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.




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   [MD5]      Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

   [SHA1]     Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, September 2001.

10.2.  Informative References

   [HMAC]     Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104, February
              1997.

   [IANA-CONSIDERATIONS]
              Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [OSPF]     Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

































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Appendix A.  Backward Compatibility (Non-Normative)

   Although version 0 of this protocol (as defined in early versions of
   the Internet-Draft that became this RFC) is unlikely to have been
   deployed widely, some implementors may wish to have a backward
   compatibility mechanism.  Note that any mechanism may be potentially
   used that does not alter the protocol definition, so interoperability
   should not be an issue.

   The suggested mechanism described here has the property that it will
   converge on version 1 if both systems implement it, even if one
   system is upgraded from version 0 within a Detection Time.  It will
   interoperate with a system that implements only one version (or is
   configured to support only one version).  A system should obviously
   not perform this function if it is configured to or is only capable
   of using a single version.

   A BFD session will enter a "negotiation holddown" if it is configured
   for automatic versioning and either has just started up, or the
   session has been manually cleared.  The session is set to AdminDown
   state and version 1.  During the holddown period, which lasts for one
   Detection Time, the system sends BFD Control packets as usual, but
   ignores received packets.  After the holddown time is complete, the
   state transitions to Down and normal operation resumes.

   When a system is not in holddown, if it doing automatic versioning
   and is currently using version 1, if any version 0 packet is received
   for the session, it switches immediately to version 0.  If it is
   currently using version 0 and a version 1 packet is received that
   indicates that the neighbor is in state AdminDown, it switches to
   version 1.  If using version 0 and a version 1 packet is received
   indicating a state other than AdminDown, the packet is ignored (per
   spec).

   If the version being used is changed, the session goes down as
   appropriate for the new version (Down state for version 1 or Failing
   state for version 0).

Appendix B.  Contributors

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









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Appendix C.  Acknowledgments

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

   Demand mode was inspired by "A Traffic-Based Method of Detecting Dead
   Internet Key Exchange (IKE) Peers", by G. Huang, et al.

   The authors would also like to thank Mike Shand, John Scudder,
   Stewart Bryant, Pekka Savola, Richard Spencer, and Pasi Eronen for
   their substantive input.

   The authors would also like to thank Owen Wheeler for hosting
   teleconferences between the authors of this specification and
   multiple vendors in order address implementation and clarity issues.

Authors' Addresses

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

   Phone: +1-408-745-2000
   EMail: dkatz@juniper.net


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

   Phone: +1-408-745-2000
   EMail: dward@juniper.net















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