path: root/doc/rfc/rfc8562.txt

Internet Engineering Task Force (IETF)                           D. Katz
Request for Comments: 8562                              Juniper Networks
Updates: 5880                                                    D. Ward
Category: Standards Track                                  Cisco Systems
ISSN: 2070-1721                                       S. Pallagatti, Ed.
                                                                  VMware
                                                          G. Mirsky, Ed.
                                                               ZTE Corp.
                                                              April 2019


    Bidirectional Forwarding Detection (BFD) for Multipoint Networks

Abstract

   This document describes extensions to the Bidirectional Forwarding
   Detection (BFD) protocol for its use in multipoint and multicast
   networks.

   This document updates RFC 5880.

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 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8562.

















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Copyright Notice

   Copyright (c) 2019 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
   (https://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  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Keywords  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Protocol Details  . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Multipoint BFD Control Packets  . . . . . . . . . . . . .   6
     5.2.  Session Model . . . . . . . . . . . . . . . . . . . . . .   6
     5.3.  Session-Failure Semantics . . . . . . . . . . . . . . . .   6
     5.4.  State Variables . . . . . . . . . . . . . . . . . . . . .   6
       5.4.1.  New State Variable Values . . . . . . . . . . . . . .   6
       5.4.2.  State Variable Initialization and Maintenance . . . .   7
     5.5.  State Machine . . . . . . . . . . . . . . . . . . . . . .   7
     5.6.  Session Establishment . . . . . . . . . . . . . . . . . .   8
     5.7.  Discriminators and Packet Demultiplexing  . . . . . . . .   8
     5.8.  Packet Consumption on Tails . . . . . . . . . . . . . . .   9
     5.9.  Bringing Up and Shutting Down Multipoint BFD Service  . .   9
     5.10. Timer Manipulation  . . . . . . . . . . . . . . . . . . .  10
     5.11. Detection Times . . . . . . . . . . . . . . . . . . . . .  10
     5.12. State Maintenance for Down/AdminDown Sessions . . . . . .  11
       5.12.1.  MultipointHead Sessions  . . . . . . . . . . . . . .  11
       5.12.2.  MultipointTail Sessions  . . . . . . . . . . . . . .  11
     5.13. Base Specification Text Replacement . . . . . . . . . . .  11
       5.13.1.  Reception of BFD Control Packets . . . . . . . . . .  12
       5.13.2.  Demultiplexing BFD Control Packets . . . . . . . . .  15
       5.13.3.  Transmitting BFD Control Packets . . . . . . . . . .  16
   6.  Congestion Considerations . . . . . . . . . . . . . . . . . .  19
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  21
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  22
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  22
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23















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1.  Introduction

   The Bidirectional Forwarding Detection (BFD) protocol [RFC5880]
   specifies a method for verifying unicast connectivity between a pair
   of systems.  This document updates [RFC5880] by defining a new method
   for using BFD.  This new method provides verification of multipoint
   or multicast connectivity between a multipoint sender (the "head")
   and a set of one or more multipoint receivers (the "tails").

   As multipoint transmissions are inherently unidirectional, this
   mechanism purports only to verify this unidirectional connectivity.
   Although this seems in conflict with the "Bidirectional" in BFD, the
   protocol is capable of supporting this use case.  Use of BFD in
   Demand mode allows a tail to monitor the availability of a multipoint
   path even without the existence of some kind of a return path to the
   head.  As an option, if a return path from a tail to the head exists,
   the tail may notify the head of the lack of multipoint connectivity.
   Details of tail notification to the head are outside the scope of
   this document and are discussed in [RFC8563].

   This application of BFD allows for the tails to detect a lack of
   connectivity from the head.  For some applications, such detection of
   the failure at the tail is useful, for example, the use of multipoint
   BFD to enable fast failure detection and faster failover in multicast
   VPN as described in [MVPN-FAILOVER].  Due to its unidirectional
   nature, virtually all options and timing parameters are controlled by
   the head.

   Throughout this document, the term "multipoint" is defined as a
   mechanism by which one or more systems receive packets sent by a
   single sender.  This specifically includes such things as IP
   multicast and point-to-multipoint MPLS.

   The term "connectivity" in this document is not being used in the
   context of connectivity verification in a transport network but as an
   alternative to "continuity", i.e., the existence of a forwarding path
   between the sender and the receiver.

   This document effectively updates and extends the base BFD
   specification [RFC5880].

2.  Keywords

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.



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3.  Goals

   The primary goal of this mechanism is to allow tails to rapidly
   detect the fact that multipoint connectivity from the head has
   failed.

   Another goal is for the mechanism to work on any multicast
   technology.

   A further goal is to support multiple, overlapping point-to-
   multipoint paths, as well as multipoint-to-multipoint paths, and to
   allow point-to-point BFD sessions to operate simultaneously among the
   systems participating in multipoint BFD.

   It is not a goal for this protocol to verify point-to-point
   bidirectional connectivity between the head and any tail.  This can
   be done independently (and with no penalty in protocol overhead) by
   using point-to-point BFD.

4.  Overview

   The heart of this protocol is the periodic transmission of BFD
   Control packets along a multipoint path, from the head to all tails
   on the path.  The contents of the BFD packets provide the means for
   the tails to calculate the Detection Time for path failure.  If no
   BFD Control packets are received by a tail for a Detection Time, the
   tail declares that the path has failed.  For some applications, this
   is the only mechanism necessary; the head can remain ignorant of the
   status of connectivity to the tails.

   The head of a multipoint BFD session may wish to be alerted to the
   tails' connectivity (or lack thereof).  Details of how the head keeps
   track of tails and how tails alert their connectivity to the head are
   outside the scope of this document and are discussed in [RFC8563].

   Although this document describes a single head and a set of tails
   spanned by a single multipoint path, the protocol is capable of
   supporting (and discriminating between) more than one multipoint path
   at both heads and tails, as described in Sections 5.7 and 5.13.2.
   Furthermore, the same head and tail may share multiple multipoint
   paths, and a multipoint path may have multiple heads.

5.  Protocol Details

   This section describes the operation of Multipoint BFD in detail.






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5.1.  Multipoint BFD Control Packets

   Multipoint BFD Control packets (packets sent by the head over a
   multipoint path) are explicitly marked as such, via the setting of
   the Multipoint (M) bit [RFC5880].  This means that multipoint BFD
   does not depend on the recipient of a packet to know whether the
   packet was received over a multipoint path.  This can be useful in
   scenarios where this information may not be available to the
   recipient.

5.2.  Session Model

   Multipoint BFD is modeled as a set of sessions of different types.
   The elements of procedure differ slightly for each type.

   The head has a session of type MultipointHead, as defined in
   Section 5.4.1, that is bound to a multipoint path.  Multipoint BFD
   Control packets are sent by this session over the multipoint path,
   and no BFD Control packets are received by it.

   Each tail has a session of type MultipointTail, as defined in
   Section 5.4.1, associated with a multipoint path.  These sessions
   receive BFD Control packets from the head over the multipoint path.

5.3.  Session-Failure Semantics

   The semantics of session failure is subtle enough to warrant further
   explanation.

   MultipointHead sessions cannot fail (since they are controlled
   administratively).

   If a MultipointTail session fails, it means that the tail definitely
   has lost contact with the head (or the head has been administratively
   disabled), and the tail may use mechanisms other than BFD, e.g.,
   logging or NETCONF [RFC6241], to send a notification to the user.

5.4.  State Variables

   Multipoint BFD introduces some new state variables and modifies the
   usage of a few existing ones.

5.4.1.  New State Variable Values

   A number of new values of the state variable bfd.SessionType are
   added to the base BFD [RFC5880] and base Seamless Bidirectional
   Forwarding Detection (S-BFD) [RFC7880] specifications in support of
   multipoint BFD.



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      bfd.SessionType

         The type of this session as defined in [RFC7880].  Newly added
         values are:

            PointToPoint: Classic point-to-point BFD, as described in
            [RFC5880].

            MultipointHead: A session on the head responsible for the
            periodic transmission of multipoint BFD Control packets
            along the multipoint path.

            MultipointTail: A multipoint session on a tail.

         This variable MUST be initialized to the appropriate type when
         the session is created.

5.4.2.  State Variable Initialization and Maintenance

   Some state variables defined in Section 6.8.1 of [RFC5880] need to be
   initialized or manipulated differently depending on the session type.

      bfd.RequiredMinRxInterval

         This variable MUST be initialized to zero for session type
         MultipointHead.

      bfd.DemandMode

         This variable MUST be initialized to 1 for session type
         MultipointHead and MUST be initialized to zero for session type
         MultipointTail.

5.5.  State Machine

   There are slight differences in how the BFD state machine works in
   the multipoint application.  In particular, since there is a many-to-
   one mapping, three-way handshakes for session establishment and
   teardown are neither possible nor appropriate.  As such, there is no
   Init state.  Sessions of type MultipointHead MUST NOT send BFD
   Control packets with the State field being set to INIT, and those
   packets MUST be ignored on receipt.

   The following diagram provides an overview of the state machine for
   session type MultipointTail.  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.




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

   Sessions of type MultipointHead never receive packets and have no
   Detection Timer; as such, all state transitions are administratively
   driven.

5.6.  Session Establishment

   Unlike point-to-point BFD, multipoint BFD provides a form of the
   discovery mechanism that enables tails to discover the head.  The
   minimum amount of a priori information required both on the head and
   tails is the binding to the multipoint path over which BFD is
   running.  The head transmits multipoint BFD packets on that path, and
   the tails listen for BFD packets on that path.  All other information
   can be determined dynamically.

   A session of type MultipointHead is created for each multipoint path
   over which the head wishes to run BFD.  This session runs in the
   Active role, per Section 6.1 of [RFC5880].  Except when
   administratively terminating BFD service, this session is always in
   state Up and always operates in Demand mode.  No received packets are
   ever demultiplexed to the MultipointHead session.  In this sense, it
   is a degenerate form of a session.

   Sessions on the tail MAY be established dynamically, based on the
   receipt of a multipoint BFD Control packet from the head, and are of
   type MultipointTail.  Tail sessions always take the Passive role, per
   Section 6.1 of [RFC5880].

5.7.  Discriminators and Packet Demultiplexing

   The use of discriminators is somewhat different in multipoint BFD
   than in point-to-point BFD.

   The head sends multipoint BFD Control packets over the multipoint
   path via the MultipointHead session with My Discriminator set to a
   value bound to the multipoint path and with Your Discriminator set to
   zero.

   IP and MPLS multipoint tails MUST demultiplex BFD packets based on a
   combination of the source address, My Discriminator, and the identity
   of the multipoint path that the multipoint BFD Control packet was
   received from.  Together they uniquely identify the head of the



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   multipoint path.  Bootstrapping a BFD session to multipoint MPLS
   Label Switched Path (LSP) may use the control plane, e.g., as
   described in [MVPN-FAILOVER], and is outside the scope of this
   document.

   Note that, unlike point-to-point sessions, the My Discriminator value
   on the MultipointHead session MUST NOT be changed during the life of
   a session.  This is a side effect of the more complex demultiplexing
   scheme.

5.8.  Packet Consumption on Tails

   BFD packets received on tails for an IP multicast group MUST be
   consumed by tails and MUST NOT be forwarded to receivers.  Nodes with
   the BFD session of type MultipointTail MUST identify packets received
   on an IP multipoint path as a BFD Control packet if the destination
   UDP port value equals 3784.

   For multipoint LSPs, when IP/UDP encapsulation of BFD Control packets
   is used, MultipointTail MUST expect destination UDP port 3784.  The
   destination IP address of a BFD Control packet MUST be in the
   127.0.0.0/8 range for IPv4 or in the 0:0:0:0:0:FFFF:7F00:0/104 range
   for IPv6.  The use of these destination addresses is consistent with
   the explanations and usage in [RFC8029].  Packets identified as BFD
   packets MUST be consumed by MultipointTail and demultiplexed as
   described in Section 5.13.2.  Use of other types of encapsulation of
   the BFD control message over multipoint LSP is outside the scope of
   this document.

5.9.  Bringing Up and Shutting Down Multipoint BFD Service

   Because there is no three-way handshake in multipoint BFD, a newly
   started head (that does not have any previous state information
   available) SHOULD start with bfd.SessionState set to Down, and
   bfd.RequiredMinRxInterval MUST be set to zero in the MultipointHead
   session.  The session SHOULD remain in this state for a time equal to
   (bfd.DesiredMinTxInterval * bfd.DetectMult).  This will ensure that
   all MultipointTail sessions are reset (so long as the restarted head
   is using the same or a larger value of bfd.DesiredMinTxInterval than
   it did previously).

   Multipoint BFD service is brought up by administratively setting
   bfd.SessionState to Up in the MultipointHead session.








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   The head of a multipoint BFD session may wish to shut down its BFD
   service in a controlled fashion.  This is desirable because the tails
   need not wait for a Detection Time prior to declaring the multipoint
   session to be down (and taking whatever action is necessary in that
   case).

   To shut down a multipoint session in a controlled fashion, the head
   MUST administratively set bfd.SessionState in the MultipointHead
   session to either Down or AdminDown and SHOULD set
   bfd.RequiredMinRxInterval to zero.  The session SHOULD send BFD
   Control packets in this state for a period equal to
   (bfd.DesiredMinTxInterval * bfd.DetectMult).  Alternatively, the head
   MAY stop transmitting BFD Control packets and not send any more BFD
   Control packets with the new state (Down or AdminDown).  Tails will
   declare the multipoint session down only after the Detection Time
   interval runs out.

5.10.  Timer Manipulation

   Because of the one-to-many mapping, a session of type MultipointHead
   SHOULD NOT initiate a Poll Sequence in conjunction with timer value
   changes.  However, to indicate a change in the packets, a
   MultipointHead session MUST send packets with the P bit set.  A
   MultipointTail session MUST NOT reply if the packet has the M and P
   bits set and bfd.RequiredMinRxInterval set to zero.  Because the Poll
   Sequence is not used, the tail cannot negotiate down MultpointHead's
   transmit interval.  If the value of Desired Min TX Interval in the
   BFD Control packet received by MultipointTail is too high (that
   determination may change in time based on the current environment),
   it must be handled by the implementation and may be controlled by
   local policy, e.g., close the MultipointTail session.

   The MultipointHead, when changing the transmit interval to a higher
   value, MUST send BFD Control packets with the P bit set at the old
   transmit interval before using the higher value in order to avoid
   false detection timeouts at the tails.  A MultipointHead session MAY
   also wait some amount of time before making the changes to the
   transmit interval (through configuration).

   Change in the value of bfd.RequiredMinRxInterval is outside the scope
   of this document and is discussed in [RFC8563].

5.11.  Detection Times

   Multipoint BFD is inherently asymmetric.  As such, each session type
   has a different approach to Detection Times.





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   Since MultipointHead sessions never receive packets, they do not
   calculate a Detection Time.

   MultipointTail sessions cannot influence the transmission rate of the
   MultipointHead session using the Required Min Rx Interval field
   because of its one-to-many nature.  As such, the Detection Time
   calculation for a MultipointTail session does not use
   bfd.RequiredMinRxInterval.  The Detection Time is calculated as the
   product of the last received values of Desired Min TX Interval and
   Detect Mult.

   The value of bfd.DetectMult may be changed at any time on any session
   type.

5.12.  State Maintenance for Down/AdminDown Sessions

   The length of time the session state is kept after the session goes
   down determines how long the session will continue to send BFD
   Control packets (since no packets can be sent after the session is
   destroyed).

5.12.1.  MultipointHead Sessions

   When a MultipointHead session transitions to states Down or
   AdminDown, the state SHOULD be maintained for a period equal to
   (bfd.DesiredMinTxInterval * bfd.DetectMult) to ensure that the tails
   more quickly detect the session going down (by continuing to transmit
   BFD Control packets with the new state).

5.12.2.  MultipointTail Sessions

   MultipointTail sessions MAY be destroyed immediately upon leaving Up
   state, since the tail will transmit no packets.

   Otherwise, MultipointTail sessions SHOULD be maintained as long as
   BFD Control packets are being received by it (which by definition
   will indicate that the head is not Up).

5.13.  Base Specification Text Replacement

   The following sections are meant to replace the corresponding
   sections in the base specification [RFC5880] to support BFD for
   multipoint networks while not changing processing for point-to-point
   BFD.







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5.13.1.  Reception of BFD Control Packets

   The following procedure replaces Section 6.8.6 of [RFC5880] entirely.

   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.

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

      Demultiplex the packet to a session according to Section 5.13.2.
      The result is either a session of the proper type, or the packet
      is discarded (and packet processing MUST cease).

      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 of [RFC5880], 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.

      If bfd.SessionType is PointToPoint, update the transmit interval
      as described in Section 6.8.2 of [RFC5880].

      If bfd.SessionType is PointToPoint, update the Detection Time as
      described in Section 6.8.4 of [RFC5880].

      Else

         If bfd.SessionType is MultipointTail, then update the Detection
         Time as the product of the last received values of Desired Min
         TX Interval and Detect Mult, as described in Section 5.11 of
         this specification.

      If bfd.SessionState is AdminDown

         Discard the packet

      If the 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 bfd.SessionType is PointToPoint

               If received State is Down

                  Set bfd.SessionState to Init

               Else if received State is Init

                  Set bfd.SessionState to Up






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            Else (bfd.SessionType is not PointToPoint)

               If received State is Up

                  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
      [RFC5880], 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 5.13.3).

      If bfd.RemoteDemandMode is zero, 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 5.13.3).

      If the Poll (P) bit is set, and bfd.SessionType is PointToPoint,
      send a BFD Control packet to the remote system with the Poll (P)
      bit clear, and the Final (F) bit set (see Section 5.13.3).

      If the packet was not discarded, it has been received for purposes
      of the Detection Time expiration rules in Section 6.8.4 of
      [RFC5880].











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5.13.2.  Demultiplexing BFD Control Packets

   This section is part of the replacement for Section 6.8.6 of
   [RFC5880]; it is separated for clarity.

      If the Multipoint (M) bit is set

         If the Your Discriminator field is nonzero, the packet MUST be
         discarded.

         Select a session based on the source address, My Discriminator,
         and the identity of the multipoint path on which the multipoint
         BFD Control packet was received.

         If a session is found, and bfd.SessionType is not
         MultipointTail, the packet MUST be discarded.

         Else

            If a session is not found, a new session of type
            MultipointTail MAY be created, or the packet MAY be
            discarded.  This choice can be controlled by the local
            policy, e.g., by setting a maximum number of MultipointTail
            sessions.  Use of the local policy and the exact mechanism
            of it are outside the scope of this specification.

      Else (Multipoint (M) bit is clear)

         If the Your Discriminator field is nonzero

            Select a session based on the value of Your Discriminator.
            If no session is found, the packet MUST be discarded.

         Else (Your Discriminator is zero)

            If the State field is not Down or AdminDown, the packet MUST
            be discarded.

            Otherwise, 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 found, and bfd.SessionType is not
            PointToPoint, the packet MUST be discarded.




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            If a matching session is not found, a new session of type
            PointToPoint MAY be created, or the packet MAY be discarded.
            This choice MAY be controlled by a local policy and is
            outside the scope of this specification.

         If the State field is Init and bfd.SessionType is not
         PointToPoint, the packet MUST be discarded.

5.13.3.  Transmitting BFD Control Packets

   The following procedure replaces Section 6.8.7 of [RFC5880] entirely.

   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.

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

   A system MUST NOT transmit any BFD Control packets if bfd.SessionType
   is MultipointTail.

   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.

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






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   If bfd.SessionType is MultipointHead, the transmit interval MUST be
   set to bfd.DesiredMinTxInterval (this should happen automatically, as
   bfd.RemoteMinRxInterval will be zero).

   If bfd.SessionType is not MultipointHead, 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 of [RFC5880] for
   details on transmit timers.

   A system MUST NOT set the Demand (D) bit if bfd.SessionType is
   MultipointTail.

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

   If bfd.SessionType is PointToPoint or MultipointHead, 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 (P) and Final (F) 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 is a
         session of type MultipointHead soliciting the identities of the
         tails, or zero if not.







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      Final (F)

         Set to 1 if the local system is responding to a BFD Control
         packet received with the Poll (P) bit set, or zero 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 in this session
         (bfd.AuthType is nonzero), or zero if not.

      Demand (D)

         Set to bfd.DemandMode if bfd.SessionState is Up and
         bfd.RemoteSessionState is Up.  Set to 1 if bfd.SessionType is
         MultipointHead.  Otherwise, it is set to zero.

      Multipoint (M)

         Set to 1 if bfd.SessionType is MultipointHead.  Otherwise, it
         is set to zero.

      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 zero if bfd.SessionType is MultipointHead or
         MultipointTail.  Otherwise, 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 of
         [RFC5880] if authentication is in use (bfd.AuthType is
         nonzero).  Otherwise, this section is not present.

6.  Congestion Considerations

   As a foreword, although congestion can occur because of a number of
   factors, it should be noted that high transmission rates are by
   themselves subject to creating congestion either along the path or at
   the tail end(s).  As such, as stated in [RFC5883]:

      it is required that the operator correctly provision the rates at
      which BFD is transmitted to avoid congestion (e.g link, I/O, CPU)
      and false failure detection.

   Use of BFD in multipoint networks, as specified in this document,
   over multiple hops requires consideration of the mechanisms to react
   to network congestion.  Requirements stated in Section 7 of the BFD
   base specification [RFC5880] equally apply to BFD in multipoint
   networks and are repeated here:

      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 mechanism to control the load of BFD traffic MAY use BFD's
   configuration interface to control BFD state variable
   bfd.DesiredMinTxInterval.  However, such a control loop does not form
   part of the BFD protocol itself, and its specification is thus
   outside the scope of this document.





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   Additional considerations apply to BFD in multipoint networks, as
   specified in this document.  Indeed, because a tail does not transmit
   any BFD Control packets to the head of the BFD session, such a head
   node has no BFD-based mechanism and thus is not aware of the state of
   the session at the tail.  In the absence of any other mechanism, the
   head of the session could thus continue to send packets towards the
   tail(s) even though a link failure has happened.  In such a scenario,
   when it is required for the head of the session to be aware of the
   state of the tail of the session, it is RECOMMENDED to implement the
   extension described in [RFC8563].

7.  IANA Considerations

   This document has no IANA actions.

8.  Security Considerations

   The same security considerations as those described in [RFC5880]
   apply to this document.  Additionally, implementations that create
   MultpointTail sessions dynamically upon receipt of multipoint BFD
   Control packets MUST implement protective measures to prevent an
   infinite number of MultipointTail sessions from being created.  Below
   are some points to consider in such implementations.

      If a multipoint BFD Control packet did not arrive on a multicast
      path (e.g., on the expected interface, with the expected MPLS
      label, etc.), a MultipointTail session should not be created.

      If redundant streams are expected for a given multicast stream,
      the implementations should not create more MultipointTail sessions
      than the number of streams.  Additionally, when the number of
      MultipointTail sessions exceeds the number of expected streams,
      the implementation should generate an alarm to users to indicate
      the anomaly.

      The implementation should have a reasonable upper bound on the
      number of MultipointHead sessions that can be created, with the
      upper bound potentially being computed based on the load these
      would generate.

      The implementation should have a reasonable upper bound on the
      number of MultipointTail sessions that can be created, with the
      upper bound potentially being computed based on the number of
      multicast streams that the system is expecting.

   If authentication is in use, the head and all tails may be configured
   to have a common authentication key in order for the tails to
   validate multipoint BFD Control packets.



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   Shared keys in multipoint scenarios allow any tail to spoof the head
   from the viewpoint of any other tail.  For this reason, using shared
   keys to authenticate BFD Control packets in multipoint scenarios is a
   significant security exposure unless all tails can be trusted not to
   spoof the head.  Otherwise, asymmetric message authentication would
   be needed, e.g., protocols that use Timed Efficient Stream Loss-
   Tolerant Authentication (TESLA) as described in [RFC4082].
   Applicability of the asymmetric message authentication to BFD for
   multipoint networks is outside the scope of this specification and is
   for further study.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
              <https://www.rfc-editor.org/info/rfc5880>.

   [RFC7880]  Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and
              S. Pallagatti, "Seamless Bidirectional Forwarding
              Detection (S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July
              2016, <https://www.rfc-editor.org/info/rfc7880>.

   [RFC8029]  Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
              Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
              Switched (MPLS) Data-Plane Failures", RFC 8029,
              DOI 10.17487/RFC8029, March 2017,
              <https://www.rfc-editor.org/info/rfc8029>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.













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9.2.  Informative References

   [MVPN-FAILOVER]
              Morin, T., Ed., Kebler, R., Ed., and G. Mirsky, Ed.,
              "Multicast VPN fast upstream failover", Work in Progress,
              draft-ietf-bess-mvpn-fast-failover-05, February 2019.

   [RFC4082]  Perrig, A., Song, D., Canetti, R., Tygar, J., and
              B. Briscoe, "Timed Efficient Stream Loss-Tolerant
              Authentication (TESLA): Multicast Source Authentication
              Transform Introduction", RFC 4082, DOI 10.17487/RFC4082,
              June 2005, <https://www.rfc-editor.org/info/rfc4082>.

   [RFC5883]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883,
              June 2010, <https://www.rfc-editor.org/info/rfc5883>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC8563]  Katz, D., Ward, D., Pallagatti, S., Ed., and G. Mirsky,
              Ed., "Bidirectional Forwarding Detection (BFD) Multipoint
              Active Tails", RFC 8563, DOI 10.17487/RFC8563, April 2019,
              <https://www.rfc-editor.org/info/rfc8563>.

Acknowledgments

   The authors would like to thank Nobo Akiya, Vengada Prasad Govindan,
   Jeff Haas, Wim Henderickx, Gregory Mirsky, and Mingui Zhang who have
   greatly contributed to this document.

Contributors

   Rahul Aggarwal of Juniper Networks and George Swallow of Cisco
   Systems provided the initial idea for this specification and
   contributed to its development.













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Authors' Addresses

   Dave Katz
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, California  94089-1206
   United States of America

   Email: dkatz@juniper.net


   Dave Ward
   Cisco Systems
   170 West Tasman Dr.
   San Jose, California  95134
   United States of America

   Email: wardd@cisco.com


   Santosh Pallagatti (editor)
   VMware

   Email: santosh.pallagatti@gmail.com


   Greg Mirsky (editor)
   ZTE Corp.

   Email: gregimirsky@gmail.com





















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