    Internet Engineering Task Force (IETF)                           D. Katz
    Request for Comments: 8563                              Juniper Networks
    Category: Standards Track                                        D. Ward
    ISSN: 2070-1721                                            Cisco Systems
                                                          S. Pallagatti, Ed.
                                                                      VMware
                                                              G. Mirsky, Ed.
                                                                   ZTE Corp.
                                                                  April 2019

        Bidirectional Forwarding Detection (BFD) Multipoint Active Tails

    Abstract

       This document describes active tail extensions to the Bidirectional
       Forwarding Detection (BFD) protocol for multipoint networks.

    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/rfc8563.

    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.

    Katz, et al.                 Standards Track                    [Page 1]

    RFC 8563               BFD Multipoint Active Tails            April 2019

    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
       2.  Terminology and Acronyms  . . . . . . . . . . . . . . . . . .   3
       3.  Keywords  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
       4.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
       5.  Operational Scenarios . . . . . . . . . . . . . . . . . . . .   5
         5.1.  No Head Notification  . . . . . . . . . . . . . . . . . .   5
         5.2.  Head Notification . . . . . . . . . . . . . . . . . . . .   5
           5.2.1.  Head Notification without Polling . . . . . . . . . .   5
           5.2.2.  Head Notification and Tail Solicitation with
                   Multipoint Polling  . . . . . . . . . . . . . . . . .   6
           5.2.3.  Head Notification with Composite Polling  . . . . . .   6
       6.  Protocol Details  . . . . . . . . . . . . . . . . . . . . . .   7
         6.1.  Multipoint Client Session . . . . . . . . . . . . . . . .   8
         6.2.  Multipoint Client Session Failure . . . . . . . . . . . .   8
         6.3.  State Variables . . . . . . . . . . . . . . . . . . . . .   8
           6.3.1.  New State Variables . . . . . . . . . . . . . . . . .   8
           6.3.2.  New State Variable Value  . . . . . . . . . . . . . .   9
           6.3.3.  State Variable Initialization and Maintenance . . . .  10
         6.4.  Controlling Multipoint BFD Options  . . . . . . . . . . .  11
         6.5.  State Machine . . . . . . . . . . . . . . . . . . . . . .  11
         6.6.  Session Establishment . . . . . . . . . . . . . . . . . .  12
         6.7.  Discriminators and Packet Demultiplexing  . . . . . . . .  12
         6.8.  Controlling Tail Packet Transmission  . . . . . . . . . .  12
         6.9.  Soliciting the Tails  . . . . . . . . . . . . . . . . . .  13
         6.10. Verifying Connectivity to Specific Tails  . . . . . . . .  13
         6.11. Detection Times . . . . . . . . . . . . . . . . . . . . .  14
         6.12. MultipointClient Down/AdminDown Sessions  . . . . . . . .  15
         6.13. Base BFD for Multipoint Networks Specification Text
               Replacement . . . . . . . . . . . . . . . . . . . . . . .  15
           6.13.1.  Reception of BFD Control Packets . . . . . . . . . .  15
           6.13.2.  Demultiplexing BFD Control Packets . . . . . . . . .  16
           6.13.3.  Transmitting BFD Control Packets . . . . . . . . . .  16
       7.  Assumptions . . . . . . . . . . . . . . . . . . . . . . . . .  17
       8.  Operational Considerations  . . . . . . . . . . . . . . . . .  18
       9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
       10. Security Considerations . . . . . . . . . . . . . . . . . . .  18
       11. Normative References  . . . . . . . . . . . . . . . . . . . .  19
       Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  19
       Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  19
       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

    Katz, et al.                 Standards Track                    [Page 2]

    RFC 8563               BFD Multipoint Active Tails            April 2019

    1.  Introduction

       This application of BFD is an extension to Multipoint BFD [RFC8562],
       which allows tails to notify the head of the lack of multipoint
       connectivity.  As a further option, heads can request a notification
       from the tails by means of a polling mechanism.  Notification to the
       head can be enabled for all tails, or for only a subset of the tails.

       The goal of this application is for the head to have reasonably rapid
       knowledge of tails that have lost connectivity from the head.

       Since scaling is a primary concern (particularly state explosion
       toward the head), it is required that the head be in control of all
       timing aspects of the mechanism and that BFD packets from the tails
       to the head not be synchronized.

       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 path between
       the sender and the receiver.

       This document effectively modifies and adds to Sections 5.12 and 5.13
       of the base BFD multipoint networks specification [RFC8562].

    2.  Terminology and Acronyms

       BFD: Bidirectional Forwarding Detection

       c-poll: Composite Poll

       m-poll: Multipoint Poll

    3.  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.

    Katz, et al.                 Standards Track                    [Page 3]

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    4.  Overview

       A head may wish to be alerted of the tails' connectivity (or lack
       thereof), and there are a number of options to achieve that.  First,
       if all that is needed is a best-effort failure notification, as
       discussed in Section 5.2.1, the tails can send unsolicited unicast
       BFD Control packets to the head when the path fails, as described in
       Section 6.4.

       If the head wishes to know of the active tails on the multipoint
       path, it may send a multipoint BFD Control packet with the Poll (P)
       bit set, which will induce the tails to return a unicast BFD Control
       packet with the Final (F) bit set (see a detailed description in
       Section 5.2.2).  The head can then create BFD session state for each
       of the tails that have multipoint connectivity.  If the head sends
       such a packet on occasion, it can keep track of which tails answer,
       thus providing a more deterministic mechanism for detecting which
       tails fail to respond (implying a loss of multipoint connectivity).
       In this document, this method is referred to as the Multipoint Poll
       (m-poll).

       If the head wishes the definite indication of the tails'
       connectivity, it may do all of the above, but if it detects that a
       tail did not answer the previous multipoint poll, it may initiate a
       Demand mode Poll Sequence as a unicast to that tail (see a detailed
       description in Section 5.2.3).  This covers the case where either the
       multipoint poll or the single reply is also lost in transit.  If
       desired, the head may Poll one or more tails proactively to track the
       tails' connectivity.  In this document, the method that combines the
       use of multipoint and unicast polling of tails by the head is
       referred to as the Composite Poll (c-poll).

       If the awareness of the state of some nodes is more important for the
       head, in the sense that the head needs to detect the lack of
       multipoint connectivity to a subset of tails at a different rate, the
       head may transmit unicast BFD Polls to that subset of tails.  In this
       case, the timing may be independent on a tail-by-tail basis.

       Individual tails may be configured so that they never send BFD
       Control packets to the head.  Such tails will never be known to the
       head but will still be able to detect multipoint path failures from
       the head.

    Katz, et al.                 Standards Track                    [Page 4]

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    5.  Operational Scenarios

       It is worth analyzing how this protocol reacts to various scenarios.
       There are three path components present: namely, the multipoint path,
       the forward unicast path (from the head to a particular tail), and
       the reverse unicast path (from a tail to the head).  There are also
       four options as to how the head is notified about failures from the
       tail.  For the different modes described below, the setting of new
       state variables are given even if these are only introduced later in
       the document (see Section 6.3).

    5.1.  No Head Notification

       In this scenario, only the multipoint path is used and none of the
       others matter.  A failure in the multipoint path will result in the
       tail noticing the failure within a Detection Time, and the head will
       remain ignorant of the tail state.  This mode emulates the behavior
       described in [RFC8562].  In this mode, bfd.SessionType is
       MultipointTail, and the variable bfd.SilentTail (see Section 6.3.1)
       MUST be set to 1.  If bfd.SessionType is MultipointHead or
       MultipointClient, bfd.ReportTailDown MUST be set to zero.  The head
       MAY set bfd.RequiredMinRxInterval to zero and thus suppress tails
       sending any BFD Control packets.

    5.2.  Head Notification

       In these scenarios, the tail sends unsolicited or solicited BFD
       packets in response to the detection of a multipoint path failure.
       All these scenarios have common settings:

       o  if bfd.SessionType is MultipointTail, the variable bfd.SilentTail
          (see Section 6.3.1) MUST be set to zero;

       o  if bfd.SessionType is MultipointHead or MultipointClient,
          bfd.ReportTailDown MUST be set to 1;

       o  the head MUST set bfd.RequiredMinRxInterval to nonzero and thus
          allow tails to send BFD Control packets.

    5.2.1.  Head Notification without Polling

       In this scenario, the tail sends unsolicited BFD packets in response
       to the detection of a multipoint path failure.  It uses the reverse
       unicast path, but not the forward unicast path.

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       If the multipoint path fails but the reverse unicast path stays up,
       the tail will detect the failure within a Detection Time, and the
       head will know about it within one reverse packet time (since the
       notification is delayed).

       If both the multipoint path and the reverse unicast paths fail, the
       tail will detect the failure, but the head will remain unaware of it.

    5.2.2.  Head Notification and Tail Solicitation with Multipoint Polling

       In this scenario, the head sends occasional multipoint Polls in
       addition to (or in lieu of) non-Poll multipoint BFD Control packets,
       expecting the tails to reply with Final.  This also uses the reverse
       unicast path, but not the forward unicast path.

       If the multipoint path fails but the reverse unicast path stays up,
       the tail will detect the failure within a Detection Time, and the
       head will know about it within one reverse packet time (the
       notification is delayed to avoid synchronization of the tails).

       If both the multipoint path and the reverse unicast paths fail, the
       tail will detect the failure, but the head will remain unaware of
       this fact.

       If the reverse unicast path fails but the multipoint path stays up,
       the head will see the BFD session fail, but the state of the
       multipoint path will be unknown to the head.  The tail will continue
       to receive multipoint data traffic.

       If either the multipoint Poll or the unicast reply is lost in
       transit, the head will see the BFD session fail, but the state of the
       multipoint path will be unknown to the head.  The tail will continue
       to receive multipoint data traffic.

    5.2.3.  Head Notification with Composite Polling

       In this scenario, the head sends occasional multipoint Polls in
       addition to (or in lieu of) non-Poll multipoint BFD Control packets,
       expecting the tails to reply with Final.  If a tail that had
       previously replied to a multipoint Poll fails to reply (or if the
       head simply wishes to verify tail connectivity), the head issues a
       unicast Poll Sequence to the tail.  This scenario makes use of all
       three paths.  In this mode for bfd.SessionType of MultipointTail,
       variable bfd.SilentTail (see Section 6.3.1) MUST be set to zero.

       If the multipoint path fails but the two unicast paths stay up, the
       tail will detect the failure within a Detection Time, and the head
       will know about it within one reverse packet time (since the

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       notification is delayed).  Note that the reverse packet time may be
       smaller in this case if the head has previously issued a unicast Poll
       (since the tail will not delay transmission of the notification in
       this case).

       If both the multipoint path and the reverse unicast paths fail
       (regardless of the state of the forward unicast path), the tail will
       detect the failure, but the head will remain unaware of this fact.
       The head will detect a BFD session failure to the tail but cannot
       make a determination about the state of the tail's multipoint
       connectivity.

       If the forward unicast path fails but the reverse unicast path stays
       up, the head will detect a BFD session failure to the tail if it
       happens to send a unicast Poll sequence but cannot make a
       determination about the state of the tail's multipoint connectivity.
       If the multipoint path to the tail fails prior to any unicast Poll
       being sent, the tail will detect the failure within a Detection Time,
       and the head will know about it within one reverse packet time (since
       the notification is delayed).

       If the multipoint path stays up but the reverse unicast path fails,
       the head will see the particular MultipointClient session fail if it
       happens to send a Poll Sequence, but the state of the multipoint path
       will be unknown to the head.  The tail will continue to receive
       multipoint data traffic.

       If the multipoint path and the reverse unicast path both stay up but
       the forward unicast path fails, neither side will notice this failure
       as long as a unicast Poll Sequence is never sent by the head.  If the
       head sends a unicast Poll Sequence, the head will detect the failure
       in the forward unicast path.  The state of the multipoint path will
       be determined by the multipoint Poll.  The tail will continue to
       receive multipoint data traffic.

    6.  Protocol Details

       This section describes the operation of the BFD Multipoint active
       tail in detail.  This section modifies Section 4 of [RFC8562] as
       follows:

       o  Section 6.3 introduces new state variables and modifies the usage
          of a few existing ones;

       o  Section 6.13 replaces the corresponding sections in the base BFD
          for multipoint networks specification.

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    6.1.  Multipoint Client Session

       If the head is keeping track of some or all of the tails, it has a
       session of type MultipointClient per tail that it cares about.  All
       of the MultipointClient sessions for tails on a particular multipoint
       path are associated with the MultipointHead session to which the
       clients are listening.  A BFD Poll Sequence may be sent over a
       MultipointClient session to a tail if the head wishes to verify
       connectivity.  These sessions receive any BFD Control packets sent by
       the tails and MUST NOT transmit periodic BFD Control packets other
       than Poll Sequences (since periodic transmission is always done by
       the MultipointHead session).  Note that the settings of all BFD
       variables in a MultipointClient session for a particular tail
       override the corresponding settings in the MultipointHead session.

    6.2.  Multipoint Client Session Failure

       If a MultipointClient session receives a BFD Control packet from the
       tail with state Down or AdminDown, the head reliably knows that the
       tail has lost multipoint connectivity.  If the Detection Time expires
       on a MultipointClient session, it is ambiguous as to whether the
       multipoint connectivity failed or whether there was a unicast path
       problem in one direction or the other, so the head does not reliably
       know the tail's state.

    6.3.  State Variables

       BFD Multipoint active tail introduces new state variables and
       modifies the usage of a few existing ones defined in Section 5.4 of
       [RFC8562].

    6.3.1.  New State Variables

       A few state variables are added in support of multipoint BFD active
       tail.

          bfd.SilentTail

             If zero, a tail may send packets to the head according to other
             parts of this specification.  Setting this to 1 allows tails to
             be provisioned to always be silent, even when the head is
             soliciting traffic from the tails.  This can be useful, for
             example, in deployments of a large number of tails when the
             head wishes to track the state of a subset of them.  This
             variable MUST be initialized based on configuration.  The
             default value MUST be 1.

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             This variable is only pertinent when bfd.SessionType is
             MultipointTail and SHOULD NOT be modified after the
             MultipointTail session has been created.

          bfd.ReportTailDown

             Set to 1 if the head wishes tails to notify the head, via
             periodic BFD Control packets, when they see the BFD session
             fail.  If zero, the tail will never send periodic BFD Control
             packets, and the head will not be notified of session failures
             by the tails.  This variable MUST be initialized based on
             configuration.  The default value MUST be zero.

             This variable is only pertinent when bfd.SessionType is
             MultipointHead or MultipointClient.

          bfd.UnicastRcvd

             Set to 1 if a tail has received a unicast BFD Control packet
             from the head while being in Up state.  This variable MUST be
             set to zero if the session transitions from Up state to some
             other state.

             This variable MUST be initialized to zero.

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

    6.3.2.  New State Variable Value

       A new state variable value being added to:

       bfd.SessionType

          The type of this session as defined in [RFC7880].  A new value
          introduced is:

             MultipointClient: A session on the head that tracks the state
             of an individual tail, when desirable.

          This variable MUST be initialized to the appropriate type when the
          session is created, according to the rules in Section 5.4 of
          [RFC8562].

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    6.3.3.  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.
       The values of some of these variables relate to those of the same
       variables of a MultipointHead session (see Section 5.4.2 of
       [RFC8562]).

          bfd.LocalDiscr

             For session type MultipointClient, this variable MUST always
             match the value of bfd.LocalDiscr in the associated
             MultipointHead session.

          bfd.DesiredMinTxInterval

             For session type MultipointClient, this variable MUST always
             match the value of bfd.DesiredMinTxInterval in the associated
             MultipointHead session.

          bfd.RequiredMinRxInterval

             It MAY be set to zero at the head BFD system to suppress
             traffic from the tails.  Setting it to zero in the
             MultipointHead session suppresses traffic from all tails; the
             setting in a MultipointClient session suppresses traffic from a
             single tail.

          bfd.DemandMode

             This variable MUST be initialized to 1 for session types
             MultipointClient.

          bfd.DetectMult

             For session type MultipointClient, this variable MUST always
             match the value of bfd.DetectMult in the associated
             MultipointHead session.

    Katz, et al.                 Standards Track                   [Page 10]

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    6.4.  Controlling Multipoint BFD Options

       The state variables defined above are used to choose which
       operational options are active.

       The most basic form of the BFD operation in multipoint networks is
       explained in [RFC8562].  In this scenario, BFD Control packets flow
       only from the head, and no tracking of tail state at the head is
       desired.  That can be accomplished by setting bfd.ReportTailDown to
       zero in the MultipointHead session (Section 5.1).

       If the head wishes to know of active tails, it sends multipoint Polls
       as needed.  Previously known tails that don't respond to the Polls
       will be detected (as per Section 5.2.2).

       If the head wishes to request a notification from the tails when they
       lose connectivity, it sets bfd.ReportTailDown to 1 in either the
       MultipointHead session (if such notification is desired from all
       tails) or the MultipointClient session (if notification is desired
       from a particular tail).  Note that the setting of this variable in a
       MultipointClient session for a particular tail overrides the setting
       in the MultipointHead session.

       If the head wishes to verify the state of a tail on an ongoing basis,
       it sends a Poll Sequence from the MultipointClient session associated
       with that tail as needed.  This has the effect of eliminating the
       initial delay, as described in Section 6.13.3, that the tail would
       otherwise insert prior to transmission of the packet; thus, the head
       may have notification of the session failure more quickly when
       comparing with use of m-poll.

       If a tail wishes to operate silently (sending no BFD Control packets
       to the head), it sets bfd.SilentTail to 1 in the MultipointTail
       session.  This allows a tail to be silent independent of the settings
       on the head.

    6.5.  State Machine

       Though the state transitions for the state machine, as defined in
       Section 5.5 of [RFC8562], for a session type MultipointHead are only
       administratively driven, the state machine for a session of type
       MultipointClient is the same, and the diagram is applicable.

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    6.6.  Session Establishment

       If BFD Control packets are received at the head, they are
       demultiplexed to sessions of type MultipointClient, which represent
       the set of tails that the head is interested in tracking.  These
       sessions will typically also be established dynamically based on the
       receipt of BFD Control packets.  The head has broad latitude in
       choosing which tails to track, if any, without affecting the basic
       operation of the protocol.  The head directly controls whether or not
       tails are allowed to send BFD Control packets back to the head by
       setting bfd.RequiredMinRxInterval to zero in a MultipointHead or a
       MultipointClient session.

    6.7.  Discriminators and Packet Demultiplexing

       When the tails send BFD Control packets to the head from the
       MultipointTail session, the contents of Your Discriminator (the
       discriminator received from the head) will not be sufficient for the
       head to demultiplex the packet, since the same value will be received
       from all tails on the multicast tree.  In this case, the head MUST
       demultiplex packets based on the source address and the value of Your
       Discriminator, which together uniquely identify the tail and the
       multipoint path.

       When the head sends unicast BFD Control packets to a tail from a
       MultipointClient session, the value of Your Discriminator will be
       valid, and the tail MUST demultiplex the packet based solely on Your
       Discriminator.

    6.8.  Controlling Tail Packet Transmission

       As the fan-in from the tails to the head may be very large, it is
       critical that the flow of BFD Control packets from the tails is
       controlled.

       The head always operates in Demand mode.  This means that no tail
       will send an asynchronous BFD Control packet as long as the session
       is Up.

       The value of Required Min Rx Interval received by a tail in a unicast
       BFD Control packet, if any, always takes precedence over the value
       received in multipoint BFD Control packets.  This allows the packet
       rate from individual tails to be controlled separately as desired by
       sending a BFD Control packet from the corresponding MultipointClient
       session.  This also eliminates the random delay, as discussed in
       Section 6.13.3, prior to transmission from the tail that would
       otherwise be inserted, reducing the latency of reporting a failure to
       the head.

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       If the head wishes to suppress traffic from the tails when they
       detect a session failure, it MAY set bfd.RequiredMinRxInterval to
       zero, which is a reserved value that indicates that the sender wishes
       to receive no periodic traffic.  This can be set in the
       MultipointHead session (suppressing traffic from all tails), or it
       can be set in a MultipointClient session (suppressing traffic from
       only a single tail).

       Any tail may be provisioned to never send *any* BFD Control packets
       to the head by setting bfd.SilentTail to 1.  This provides a
       mechanism by which only a subset of tails reports their session
       status to the head.

    6.9.  Soliciting the Tails

       If the head wishes to know of the active tails, the MultipointHead
       session can send a BFD Control packet as specified in Section 6.13.3,
       with the Poll (P) bit set to 1.  This will cause all of the tails to
       reply with a unicast BFD Control Packet, randomized across one packet
       interval.

       The decision as to when to send a multipoint Poll is outside the
       scope of this specification.  However, it MUST NOT be sent more often
       than the regular multipoint BFD Control packet.  Since the tail will
       treat a multipoint Poll like any other multipoint BFD Control packet,
       Polls may be sent in lieu of non-Poll packets.

       Soliciting the tails also starts the Detection Timer for each of the
       associated MultipointClient sessions, which will cause those sessions
       to time out if the associated tails do not respond.

       Note that for this mechanism to work properly, the Detection Time
       (which is equal to bfd.DesiredMinTxInterval) MUST be greater than the
       round-trip time of BFD Control packets from the head to the tail (via
       the multipoint path) and back (via a unicast path).  See Section 6.11
       for more details.

    6.10.  Verifying Connectivity to Specific Tails

       If the head wishes to verify connectivity to a specific tail, the
       corresponding MultipointClient session can send a BFD Poll Sequence
       to said tail.  This might be done in reaction to the expiration of
       the Detection Timer (the tail didn't respond to a multipoint Poll),
       or it might be done on a proactive basis.

       The interval between transmitted packets in the Poll Sequence MUST be
       calculated as specified in the base BFD specification [RFC5880] (the
       greater of bfd.DesiredMinTxInterval and bfd.RemoteMinRxInterval).

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       The value transmitted in Required Min RX Interval will be used by the
       tail (rather than the value received in any multipoint packet) when
       it transmits BFD Control packets to the head to notify it of a
       session failure, and the transmitted packets will not be delayed.
       This value can potentially be set much lower than in the multipoint
       case, in order to speed up a notification to the head, since the
       value will be used only by the single tail.  This value (and the lack
       of delay) are "sticky", in that once the tail receives it, it will
       continue to use it indefinitely.  Therefore, if the head no longer
       wishes to single out the tail, it SHOULD reset the timer to the
       default by sending a Poll Sequence with the same value of Required
       Min Rx Interval as is carried in the multipoint packets, or it MAY
       reset the tail session by sending a Poll Sequence with state
       AdminDown (after the completion of which the session will come back
       up).

       Note that a failure of the head to receive a response to a Poll
       Sequence does not necessarily mean that the tail has lost multipoint
       connectivity, though a reply to a Poll Sequence does reliably
       indicate connectivity or lack thereof (by virtue of the tail's state
       not being Up in the BFD Control packet).

    6.11.  Detection Times

       MultipointClient sessions at the head are always in the Demand mode,
       and as such only care about Detection Time in two cases.  First, if a
       Poll Sequence is being sent on a MultipointClient session, the
       Detection Time on this session is calculated according to the base
       BFD specification [RFC5880], that is, the transmission interval
       multiplied by bfd.DetectMult.  Second, when a multipoint Poll is sent
       to solicit tail replies, the Detection Time on all associated
       MultipointClient sessions that aren't currently sending Poll
       Sequences is set to a value greater than or equal to
       bfd.RequiredMinRxInterval (one packet time).  This value can be made
       arbitrarily large in order to ensure that the Detection Time is
       greater than the round-trip time of a BFD Control packet between the
       head and the tail with no ill effects, other than delaying the
       detection of unresponsive tails.  Note that a Detection Time
       expiration on a MultipointClient session at the head, while
       indicating a BFD session failure, cannot be construed to mean that
       the tail is not hearing multipoint packets from the head.

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    6.12.  MultipointClient Down/AdminDown Sessions

       If the MultipointHead session is in Down/AdminDown state (which only
       happens administratively), all associated MultipointClient sessions
       SHOULD be destroyed as they are superfluous.

       If a MultipointClient session goes down due to the receipt of an
       unsolicited BFD Control packet from the tail with state Down or
       AdminDown (not in response to a Poll), and tail connectivity
       verification is not being done, the session MAY be destroyed.  If
       verification is desired, the session SHOULD send a Poll Sequence and
       the session SHOULD be maintained.

       If the tail replies to a Poll Sequence with state Down or AdminDown,
       it means that the tail session is definitely down.  In this case, the
       session MAY be destroyed.

       If the Detection Time expires on a MultipointClient session (meaning
       that the tail did not reply to a Poll Sequence), the session MAY be
       destroyed.

    6.13.  Base BFD for Multipoint Networks Specification Text Replacement

       The following sections are meant to extend the corresponding sections
       in the base BFD for multipoint networks specification [RFC8562].

    6.13.1.  Reception of BFD Control Packets

       The following procedure modifies parts of Section 5.13.1 of
       [RFC8562].

       When a BFD Control packet is received, the procedure defined in
       Section 5.13.1 of [RFC8562] 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.  In addition to that, if tail
       tracking is desired by the head, the following procedure MUST be
       applied.

          If bfd.SessionType is MultipointTail

             If bfd.UnicastRcvd is zero or the Multipoint (M) bit is clear,
             set bfd.RemoteMinRxInterval to the value of Required Min RX
             Interval.

             If the Multipoint (M) bit is clear, set bfd.UnicastRcvd to 1.

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          Else (not MultipointTail)

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

          If the Poll (P) bit is set, and bfd.SilentTail is zero, send a BFD
          Control packet to the remote system with the Poll (P) bit clear
          and the Final (F) bit set (see Section 6.13.3).

    6.13.2.  Demultiplexing BFD Control Packets

       This section is part of the addition to Section 5.13.2 of [RFC8562],
       separated for clarity.

          If the 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.

                If bfd.SessionType is MultipointHead:

                   Find a MultipointClient session grouped to this
                   MultipointHead session, based on the source address and
                   the value of Your Discriminator.  If a session is found
                   and is not MultipointClient, the packet MUST be
                   discarded.  If no session is found, a new session of type
                   MultipointClient MAY be created, or the packet MAY be
                   discarded.  This choice is outside the scope of this
                   specification.

                   If bfd.SessionType is not MultipointClient, the packet
                   MUST be discarded.

    6.13.3.  Transmitting BFD Control Packets

       A system MUST NOT periodically transmit BFD Control packets if
       bfd.SessionType is MultipointClient and a Poll Sequence is not being
       transmitted.

       If the bfd.SessionType value is MultipointTail and the periodic
       transmission of BFD Control packets is just starting (due to Demand
       mode not being active on the remote system), the first packet to be
       transmitted MUST be delayed by a random amount of time between zero
       and (0.9 * bfd.RemoteMinRxInterval).

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       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, without respect to the
       transmission timer or any other transmission limitations, the session
       state, and 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.  If the
       Multipoint (M) bit is set in the received packet, the packet
       transmission MUST be delayed by a random amount of time between zero
       and (0.9 * bfd.RemoteMinRxInterval).  Otherwise, the packet MUST be
       transmitted as soon as practicable.

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

       Content of the transmitted packet MUST be as explained in
       Section 5.13.3 of [RFC8562].

    7.  Assumptions

       If the head notification is to be used, it is assumed that a
       multipoint BFD packet encapsulation contains enough information so
       that a tail can address a unicast BFD packet to the head.

       If the head notification is to be used, it is assumed that there is
       bidirectional unicast communication available (at the same protocol
       layer within which BFD is being run) between the tail and head.

       For the head to know reliably that a tail has lost multipoint
       connectivity, the unicast paths in both directions between that tail
       and the head must remain operational when the multipoint path fails.
       It is thus desirable that unicast paths not share fate with the
       multipoint path to the extent possible if the head wants more
       definite knowledge of the tail state.

       Since the normal BFD three-way handshake is not used in this
       application, a tail transitioning from state Up to Down and back to
       Up again may not be reliably detected at the head.

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    8.  Operational Considerations

       Section 7 of [RFC5880] includes the requirements for implementation
       of a congestion control mechanism when BFD is used across multiple
       hops and a mechanism that uses congestion detection to reduce the
       amount of BFD packets the system generates.  These requirements are
       also applicable to this specification.  When this specification is
       used in the mode with no head notifications by tails, as discussed in
       Section 5.1, the head MUST limit the packet transmission rate to no
       higher than one BFD packet per second (see Section 5.13.3 of
       [RFC8562]).  When the BFD uses one of the notifications by the tails
       to the head mechanisms described in Section 5.2, Min RX Interval can
       be used by the tail to control the packet transmission rate of the
       head.  The exact mechanism of processing changes in the Min RX
       Interval value in the received from the tail response to multicast or
       the unicast Poll BFD packet is outside the scope of this document.

       As noted in Section 7 of [RFC5880], "any mechanism that increases the
       transmit or receive intervals will increase the Detection Time for
       the session".

    9.  IANA Considerations

       This document has no IANA actions.

    10.  Security Considerations

       The same security considerations as those described in [RFC5880] and
       [RFC8562] apply to this document.

       Additionally, implementations that create MultpointClient sessions
       dynamically upon receipt of a BFD Control packet from a tail MUST
       implement protective measures to prevent a number of MultipointClient
       sessions from being created and growing out of control.  Below are
       some points to be considered in such implementations.

          When the number of MultipointClient sessions exceeds the number of
          expected tails, the implementation should generate an alarm to
          users to indicate the anomaly.

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

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

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    11.  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>.

       [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>.

       [RFC8562]  Katz, D., Ward, D., Pallagatti, S., Ed., and G. Mirsky,
                  Ed., "Bidirectional Forwarding Detection (BFD) for
                  Multipoint Networks", RFC 8562, DOI 10.17487/RFC8562,
                  April 2019, <https://www.rfc-editor.org/info/rfc8562>.

    Acknowledgments

       The authors would like to thank Nobo Akiya, Vengada Prasad Govindan,
       Jeff Haas, Wim Henderickx, 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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