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Versions: (draft-katz-ward-bfd-multipoint) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 RFC 8562

Internet Engineering Task Force                                  D. Katz
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                                 D. Ward
Expires: June 22, 2018                                     Cisco Systems
                                                      S. Pallagatti, Ed.
                                                  Individual contributor
                                                          G. Mirsky, Ed.
                                                               ZTE Corp.
                                                       December 19, 2017


                      BFD for Multipoint Networks
                      draft-ietf-bfd-multipoint-12

Abstract

   This document describes extensions to the Bidirectional Forwarding
   Detection (BFD) protocol for its use in multipoint and multicast
   networks.  Comments on this draft should be directed to rtg-
   bfd@ietf.org.

Requirements Language

   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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on June 22, 2018.






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Copyright Notice

   Copyright (c) 2017 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Protocol Details  . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Multipoint BFD Control Packets  . . . . . . . . . . . . .   4
     4.2.  Session Model . . . . . . . . . . . . . . . . . . . . . .   5
     4.3.  Session Failure Semantics . . . . . . . . . . . . . . . .   5
     4.4.  State Variables . . . . . . . . . . . . . . . . . . . . .   5
       4.4.1.  New State Variables . . . . . . . . . . . . . . . . .   5
       4.4.2.  State Variable Initialization and Maintenance . . . .   6
     4.5.  State Machine . . . . . . . . . . . . . . . . . . . . . .   6
     4.6.  Session Establishment . . . . . . . . . . . . . . . . . .   7
     4.7.  Discriminators and Packet Demultiplexing  . . . . . . . .   7
     4.8.  Packet consumption on tails . . . . . . . . . . . . . . .   8
     4.9.  Bringing Up and Shutting Down Multipoint BFD Service  . .   8
     4.10. Timer Manipulation  . . . . . . . . . . . . . . . . . . .   9
     4.11. Detection Times . . . . . . . . . . . . . . . . . . . . .   9
     4.12. State Maintenance for Down/AdminDown Sessions . . . . . .   9
       4.12.1.  MultipointHead Sessions  . . . . . . . . . . . . . .  10
       4.12.2.  MultipointTail Sessions  . . . . . . . . . . . . . .  10
     4.13. Base Specification Text Replacement . . . . . . . . . . .  10
       4.13.1.  Reception of BFD Control Packets . . . . . . . . . .  10
       4.13.2.  Demultiplexing BFD Control Packets . . . . . . . . .  13
       4.13.3.  Transmitting BFD Control Packets . . . . . . . . . .  14
   5.  Assumptions . . . . . . . . . . . . . . . . . . . . . . . . .  16
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  17
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18



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1.  Introduction

   The Bidirectional Forwarding Detection protocol [RFC5880] specifies a
   method for verifying unicast connectivity between a pair of systems.
   This document defines a method for using BFD to provide 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 supporting this use case.

   This application of BFD allows for the tails to detect a lack of
   connectivity from the head.  Due to unidirectional nature, virtually
   all options and timing parameters are controlled by the head.

   As an option, the tail may notify the head of the lack of multipoint
   connectivity.  Details of tail notification to head are outside the
   scope of this document.

   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.

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

   This document effectively modifies and adds to the base BFD
   specification.  It is the intention of the authors to fold these
   extensions into the base specification at the appropriate time.

2.  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 or
   multipoint medium.

   A further goal is to support multiple, overlapping multipoint paths,
   as well as multipoint paths with multiple heads, and to allow point-




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   to-point BFD sessions to operate simultaneously among the systems
   participating in Multipoint BFD.

   A final goal is to integrate multipoint operation into the base
   specification in such a way as to make it relatively easy to support
   both multipoint and point-to-point operation in a single
   implementation.

   It is a non-goal for this protocol to verify point-to-point bi-
   directional connectivity between the head and any tails.  This can be
   done independently (and with no penalty in protocol overhead) by
   using point-to-point BFD.

3.  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 tree.  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 the path to have failed.  For some applications this is
   the only mechanism necessary; the head can remain ignorant of the
   tails.

   Head may wish to be alerted to the tails' connectivity (or lack
   thereof).  Details of how head keeps track of tails and how tails
   alert it's connectivity to head are outside scope of this document.

   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.  Furthermore, the same head and tail may
   share multiple multipoint paths, and a multipoint path may have
   multiple heads.

4.  Protocol Details

   This section describes the operation of Multipoint BFD in detail.

4.1.  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 M bit.  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.




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4.2.  Session Model

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

   Point-to-point sessions, as described in [RFC5880], are of type
   PointToPoint.

   The head has a session of type MultipointHead 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 associated with a
   multipoint path.  These sessions receive BFD Control packets from the
   head over multipoint path.

4.3.  Session Failure Semantics

   The semantics of session failure are 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 should take appropriate action.

4.4.  State Variables

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

4.4.1.  New State Variables

   A number of state variables and their values are added to the base
   BFD [RFC5880] and base S-BFD [RFC7880] specifications in support of
   Multipoint BFD.

      bfd.SessionType

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

            PointToPoint: Classic point-to-point BFD.





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            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, according to the rules in Section 4.13

      bfd.SilentTail

         Always set to 1, a tail will never transmit any BFD Control
         packets to the head under any circumstances.  Setting to 0 is
         outside the scope of this document.

         This variable is only pertinent when bfd.SessionType is
         MultipointTail.

4.4.2.  State Variable Initialization and Maintenance

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

      bfd.RequiredMinRxInterval

         This variable MUST be set to 0 for session type MultipointHead.

      bfd.DemandMode

         This variable MUST be initialized to 1 for session type
         MultipointHead and MUST be initialized to 0 for session type
         MultipointTail.

4.5.  State Machine

   The BFD state machine works slightly differently 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.
   Session of type MultipointHead MUST NOT send BFD control packets with
   the State field being set to INIT, and 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, and as such all state transitions are
   administratively driven.

4.6.  Session Establishment

   Unlike Point-to-point BFD, Multipoint BFD provides a form of
   discovery mechanism for 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 tree, and the tails
   listen for BFD packets on that tree.  All other information MAY 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.  Except when 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.

4.7.  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 MultipointHead
   session with My Discr set to a value bound to the multipoint path,
   and with Your Discr 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 tree which the Multipoint BFD Control packet was
   received from.  Together they uniquely identify the head of the
   multipoint path.  Bootstrapping BFD session to a multipoint LSP in




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   case of penultimate hop popping is outside the scope of this
   document.

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

4.8.  Packet consumption on tails

   BFD packets received on tails for a multicast group MUST be consumed
   by tails and MUST NOT be forwarded to receivers.  Session of type
   MultipointTail MUST identify the packet as BFD with the help of
   destination UDP port number "3784" on IP multipoint path.

   For multipoint LSP, when IP/UDP encapsulation of BFD control packet
   used, MultipointTail MUST use destination UDP port "3784" and
   "127.0.0.0/8" range for IPv4 or "0:0:0:0:0:FFFF:7F00:0/104" range for
   IPv6.  Packet identified as BFD packet MUST be consumed by
   MultipointTail and demultiplex as described in Section 4.13.2.  Use
   of other types of encapsulation for multipoint LSP is outside the
   scope of this document.

4.9.  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 with
   bfd.RequiredMinRxInterval 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 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.

   A head may wish to shut down its BFD service in a controlled fashion.
   This is desirable because the tails need not wait 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 a 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).



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   The semantic difference between Down and AdminDown state is for
   further discussion.

4.10.  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 change in packet MultipointHead session
   MUST send packet with P bit set.  MultipointTail session MUST NOT
   reply if packet has M, P bit set and bfd.RequiredMinRxInterval set to
   0.

   The MultipointHead, when changing the transmit interval to higher
   value, MUST send BFD control packets with P bit set at the old
   transmit interval before using the higher value in order to avoid
   false detection timeouts at the tails.  MultipointHead 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.

4.11.  Detection Times

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

   Since the MultipointHead session never receives packets, it does 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 in the calculation.  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.

4.12.  State Maintenance for Down/AdminDown Sessions

   The length of time 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.)




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4.12.1.  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.)

4.12.2.  MultipointTail Sessions

   MultipointTail sessions MAY be destroyed immediately upon leaving Up
   state, since 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.)

4.13.  Base Specification Text Replacement

   The following sections are meant to replace the corresponding
   sections in the base specification.

4.13.1.  Reception of BFD Control Packets

   The following procedure replaces section 6.8.6 of [RFC5880].

   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.






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      Demultiplex the packet to a session according to Section 4.13.2
      below.  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, 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.

      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 [RFC5880] section 6.8.2.

      If bfd.SessionType is PointToPoint, update the Detection Time as
      described in [RFC5880] section 6.8.4.  Otherwise, update the
      Detection Time as described in Section 4.11 above.

      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



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      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

            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 4.13.3.)

      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 see Section 4.13.3.)





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

4.13.2.  Demultiplexing BFD Control Packets

   This section is part of the replacement for [RFC5880] section 6.8.6,
   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 as based on source address, My Discriminator
         and the identity of the multipoint tree which the Multipoint
         BFD Control packet was received.  If a session is found, and
         bfd.SessionType is not MultipointTail, the packet MUST be
         discarded.  If a session is not found, a new session of type
         MultipointTail MAY be created, or the packet MAY be discarded.
         This choice is outside the scope of this specification.

      Else (Multipoint 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.

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





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

4.13.3.  Transmitting BFD Control Packets

   The following procedure replaces section 6.8.7 of [RFC5880].

   BFD Control packets MUST be transmitted periodically at the rate
   determined according to [RFC5880] section 6.8.2, except as specified
   in this section.

   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.SilentTail
   is 1.

   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.

   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 [RFC5880] sections 6.8.2 and 6.8.3 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
   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 and Final bits) in order to more rapidly communicate a change in
   state.



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   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 0 if not.

      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.  Set to 1 if bfd.SessionType is
         MultipointHead.  Otherwise it is set to 0.

      Multipoint (M)

         Set to 1 if bfd.SessionType is MultipointHead.  Otherwise it is
         set to 0.




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      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.

      Required Min RX Interval

         Set to bfd.RequiredMinRxInterval.

      Required Min Echo RX Interval

         Set to 0 if bfd.SessionType is MultipointHead or
         MultipointTail.

      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.

5.  Assumptions

   If authentication is in use, all tails must be configured to have a
   common authentication key in order to receive the multipoint BFD
   Control packets.

6.  IANA Considerations

   This document has no actions for IANA.






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7.  Security Considerations

   Implementations that create MultpointTail sessions dynamically upon
   receipt of Multipoint BFD Control packets MUST implement protective
   measures to prevent infinite number of MultipointTail sessions being
   created.  Below are listed some points to be considered in such
   implementations.

      If a Multipoint BFD Control packet did not arrive on a multicast
      tree (ex: on expected interface, with expected MPLS label, etc),
      then a MultipointTail session should not be created.

      If redundant streams are expected for a given multicast stream,
      then 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, then the implementation should generate an alarm to users
      to indicate the anomaly.

      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.

8.  Contributors

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

9.  Acknowledgements

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

10.  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>.





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   [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>.

   [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>.

Authors' Addresses

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

   Email: dkatz@juniper.net


   Dave Ward
   Cisco Systems
   170 West Tasman Dr.
   San Jose, California  95134
   USA

   Email: wardd@cisco.com


   Santosh Pallagatti (editor)
   Individual contributor

   Email: santosh.pallagatti@gmail.com


   Greg Mirsky (editor)
   ZTE Corp.

   Email: gregimirsky@gmail.com












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