





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.





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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









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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.







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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.









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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.













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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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