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Versions: 00 draft-ietf-bfd-multipoint-active-tail

Internet Engineering Task Force                                  D. Katz
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                                 D. Ward
Expires: August 6, 2015                                    Cisco Systems
                                                      S. Pallagatti, Ed.
                                                        Juniper Networks
                                                        February 2, 2015


                      BFD Multipoint Active Tails.
            draft-spallagatti-bfd-multipoint-active-tail-00

Abstract

   This document describes active tail extensions to the Bidirectional
   Forwarding Detection (BFD) protocol for 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", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

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 http://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 August 6, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Protocol Details  . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Multipoint Client Session . . . . . . . . . . . . . . . .   4
     3.2.  Multipoint Client Session Failure . . . . . . . . . . . .   4
     3.3.  State Variables . . . . . . . . . . . . . . . . . . . . .   5
       3.3.1.  New State Variables . . . . . . . . . . . . . . . . .   5
       3.3.2.  State Variable Initialization and Maintenance . . . .   6
     3.4.  Controlling Multipoint BFD Options  . . . . . . . . . . .   7
     3.5.  State Machine . . . . . . . . . . . . . . . . . . . . . .   7
     3.6.  Session Establishment . . . . . . . . . . . . . . . . . .   7
     3.7.  Discriminators and Packet Demultiplexing  . . . . . . . .   8
     3.8.  Controlling Tail Packet Transmission  . . . . . . . . . .   8
     3.9.  Soliciting the Tails  . . . . . . . . . . . . . . . . . .   9
     3.10. Verifying Connectivity to Specific Tails  . . . . . . . .   9
     3.11. Detection Times . . . . . . . . . . . . . . . . . . . . .  10
     3.12. MultipointClient Down/AdminDown Sessions  . . . . . . . .  10
     3.13. Base Specification Text Replacement . . . . . . . . . . .  11
       3.13.1.  Reception of BFD Control Packets . . . . . . . . . .  11
       3.13.2.  Demultiplexing BFD Control Packets . . . . . . . . .  11
       3.13.3.  Transmitting BFD Control Packets . . . . . . . . . .  12
   4.  Assumptions . . . . . . . . . . . . . . . . . . . . . . . . .  13
   5.  Operational Scenarios . . . . . . . . . . . . . . . . . . . .  13
     5.1.  No Head Notification  . . . . . . . . . . . . . . . . . .  13
     5.2.  Unreliable Head Notification  . . . . . . . . . . . . . .  14
     5.3.  Semi-reliable Head Notification and Tail Solicitation . .  14
     5.4.  Reliable Head Notification  . . . . . . . . . . . . . . .  14
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  16
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16







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1.  Introduction

   This application of BFD is an extension to Multipoint BFD
   [I-D.ietf-bfd-multipoint] which allows tail to unreliably notify the
   head of the lack of multipoint connectivity.  As a further option,
   this notification can be made reliable.  Notification to the head can
   be enabled for all tails, or for only a subset of the tails.

   Multipoint BFD base document [I-D.ietf-bfd-multipoint] describes
   procedures to verify only the head-to-tail connectivity over the
   multipoint path.  Although it may use unicast paths in both
   directions, Multipoint BFD does not verify those paths (and in fact
   it is preferable if unicast paths share as little fate with the
   multipoint path as is feasible.)

   Goal of this application is for the head to reasonably rapidly have
   knowledge of tails that have lost connectivity from the head.

   Since scaling is a primary concern (particularly state implosion
   toward the head), it is a 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.

   This document effectively modifies and adds to the base BFD
   specification and base BFD multipoint document.

2.  Overview

   Head may wish to be alerted to the tails' connectivity (or lack
   thereof), there are a number of options.  First, if all that is
   needed is an unreliable failure notification, the head can direct the
   tails to transmit unicast BFD Control packets back to the head when
   the path fails.

   If the head wishes to know the identity of the tails on the
   multipoint path, it may solicit membership by sending 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.  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 somewhat reliable mechanism for detecting which tails
   fail to respond (implying a loss of multipoint connectivity.)

   If the head wishes a reliable 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 the tail.  This covers the case where



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   either the multipoint poll or the single reply thereto is lost in
   transit.  If desired, the head may Poll one or more tails proactively
   to track the tails' connectivity.

   If some tails are more equal than others, 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, even when the head wishes notification
   of path failure from the tail.  Such tails will never be known to the
   head, but will still be able to detect multipoint path failures from
   the head.

3.  Protocol Details

   This section describes the operation of BFD Multipoint active tail in
   detail.  This section is update to section 4 of
   [I-D.ietf-bfd-multipoint]

3.1.  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 grouped with the MultipointHead session to which the clients
   are listening.  A BFD Poll Sequence may be sent over such a session
   to a tail if the head wishes to verify connectivity.  These sessions
   receive any BFD Control packets sent by the tails, and never transmit
   periodic BFD Control packets other than Poll Sequences (since
   periodic transmission is always done by the MultipointHead session.)

3.2.  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 state.








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3.3.  State Variables

   BFD Multipoint active tail introduces new state variables and
   modifies the usage of a few existing ones defined in section 4.4 of
   [I-D.ietf-bfd-multipoint].

3.3.1.  New State Variables

   Few state variables are added and modified support of Multipoint BFD
   active tail.

      bfd.SessionType

         The type of this session as defined in
         [I-D.ietf-bfd-multipoint].  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 4.13
         of [I-D.ietf-bfd-multipoint].

      bfd.SilentTail

         If 0, 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.

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

      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 0, 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.

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




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      bfd.UnicastRcvd

         Set to 1 if a tail receives a unicast BFD Control packet from
         the head.  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.

3.3.2.  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 (see section 4.4.2 of [I-D.ietf-bfd-multipoint]).

      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 should be noted that for sessions of type MultipointTail,
         this variable only affects the rate of unicast Polls sent by
         the head; the rate of multipoint packets is necessarily
         unaffected by it.

      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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3.4.  Controlling Multipoint BFD Options

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

   The most basic form of operation as explained in
   [I-D.ietf-bfd-multipoint], in which BFD Control packets flow only
   from the head and no tracking is desired of tail state at the head,
   is accomplished by setting bfd.ReportTailDown to 0 in the
   MultipointHead session.

   If the head wishes to know the identity of the tails, it sends
   multipoint Polls as needed.  Previously known tails that don't
   respond to the Polls will be detected.

   If the head wishes to be notified by 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 in 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.

   If the head wants to more quickly be alerted to a session failure
   from a particular tail, it sends a BFD Control packet from the
   MultipointClient session associated with that tail.  This has the
   effect of eliminating the initial delay that the tail would otherwise
   insert prior to transmission of the packet.

   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.

3.5.  State Machine

   State machine for session of type MultipointClient is same as defined
   in section 4.5 of [I-D.ietf-bfd-multipoint].

3.6.  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



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   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.

3.7.  Discriminators and Packet Demultiplexing

   When the tails send BFD Control packets to the head from the
   MultipointTail session, the contents of Your Discr (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
   Discr, 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 Discr will be valid, and
   the tail MUST demultiplex the packet based solely on Your Discr.

3.8.  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 prior to transmission
   from the tail that would otherwise be inserted, reducing the latency
   of reporting a failure to the head.

   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.)




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   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 report their session status
   to the head.

3.9.  Soliciting the Tails

   If the head wishes to know the identities of the tails, the
   MultipointHead session MAY send a BFD Control packet as specified in
   Section 3.13.3, 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 never 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
   associated MultipointClient session, 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 3.11
   for more details.

3.10.  Verifying Connectivity to Specific Tails

   If the head wishes to verify connectivity to a specific tail, the
   corresponding MultipointClient session MAY 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 specification (the greater of
   bfd.DesiredMinTxInterval and bfd.RemoteMinRxInterval.)

   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 notifying 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 notification to the head, since the value
   will be used only by the single tail.  This value (and the lack of



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   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.)

3.11.  Detection Times

   MultipointClient sessions at the head are always in 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
   specification, 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 BFD round trip time
   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.

3.12.  MultipointClient Down/AdminDown Sessions

   If the MultipointHead session is going down (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.





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   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.

3.13.  Base Specification Text Replacement

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

3.13.1.  Reception of BFD Control Packets

   The following procedure replaces section 6.8.6 of [RFC5880].

   When a BFD Control packet is received, procedure defined in section
   4.13.1 of [I-D.ietf-bfd-multipoint] 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 head below procedure MUST be
   applied.

      If bfd.SessionType is MultipointTail

         If bfd.UnicastRcvd is 0 or the M bit is clear, set
         bfd.RemoteMinRxInterval to the value of Required Min RX
         Interval.

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

      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 3.13.3.)

3.13.2.  Demultiplexing BFD Control Packets

   This section is part of the replacement for [RFC5880] section 6.8.6
   and addition to section 4.13.2 of [I-D.ietf-bfd-multipoint],
   separated for clarity.

      If Multipoint (M) bit is clear



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         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 MulticastHead

               Find a MultipointClient session grouped to this
               MulticastHead session, based on the source address and
               the value of Your Discriminator.  If a session is found
               and is not MulticastClient, 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 MulticastClient, the packet
               MUST be discarded.

3.13.3.  Transmitting BFD Control Packets

   The following procedure replaces section 6.8.7 of [RFC5880].

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

   If bfd.SessionType is MulticastTail and 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).

   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, without
   respect to the session state, and without respect to whether Demand
   mode is active on either system.  A system MAY limit the rate at
   which such packets are transmitted.  If rate limiting is in effect,
   the advertised value of Desired Min TX Interval MUST be greater than
   or equal to the interval between transmitted packets imposed by the
   rate limiting function.  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.





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   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.

   Contents of transmitted packet MUST be as explained in section 4.13.3
   of [I-D.ietf-bfd-multipoint].

4.  Assumptions

   If 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 head notification is to be used, it is assumed that is 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 reliable
   knowledge of 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.

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 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.

5.1.  No Head Notification

   Since the only path used in this scenario is the multipoint path,
   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.








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5.2.  Unreliable Head Notification

   In this scenario, the tail sends back unsolicicted BFD packets in
   response to the detection of a multipoint path failure.  It 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 (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.

5.3.  Semi-reliable Head Notification and Tail Solicitation

   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.

5.4.  Reliable Head Notification

   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




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   unicast Poll Sequence to the tail.  This scenario makes use of all
   three paths.

   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
   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 BFD 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 so long as a
   unicast Poll Sequence is never sent by the head.  If the head sends a
   unicast Poll Sequence, 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.

6.  IANA Considerations

   This document has no actions for IANA.

7.  Security Considerations

   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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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.  Normative References

   [I-D.ietf-bfd-multipoint]
              Katz, D., Ward, D., and J. Networks, "BFD for Multipoint
              Networks", draft-ietf-bfd-multipoint-05 (work in
              progress), January 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010.

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)
   Juniper Networks
   Embassy Business Park
   Bangalore, KA  560093
   India

   Email: santoshpk@juniper.net





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