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

                                                                VMware
                                                        G. Mirsky, Ed.
                                                             ZTE Corp.
                                                            April 2019
  Bidirectional Forwarding Detection (BFD) for Multipoint Networks

Abstract

 This document describes extensions to the Bidirectional Forwarding
 Detection (BFD) protocol for its use in multipoint and multicast
 networks.
 This document updates RFC 5880.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8562.

Katz, et al. Standards Track [Page 1] RFC 8562 BFD for Multipoint Networks April 2019

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 2] RFC 8562 BFD for Multipoint Networks April 2019

Table of Contents

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

Katz, et al. Standards Track [Page 3] RFC 8562 BFD for Multipoint Networks April 2019

1. Introduction

 The Bidirectional Forwarding Detection (BFD) protocol [RFC5880]
 specifies a method for verifying unicast connectivity between a pair
 of systems.  This document updates [RFC5880] by defining a new method
 for using BFD.  This new method provides verification of multipoint
 or multicast connectivity between a multipoint sender (the "head")
 and a set of one or more multipoint receivers (the "tails").
 As multipoint transmissions are inherently unidirectional, this
 mechanism purports only to verify this unidirectional connectivity.
 Although this seems in conflict with the "Bidirectional" in BFD, the
 protocol is capable of supporting this use case.  Use of BFD in
 Demand mode allows a tail to monitor the availability of a multipoint
 path even without the existence of some kind of a return path to the
 head.  As an option, if a return path from a tail to the head exists,
 the tail may notify the head of the lack of multipoint connectivity.
 Details of tail notification to the head are outside the scope of
 this document and are discussed in [RFC8563].
 This application of BFD allows for the tails to detect a lack of
 connectivity from the head.  For some applications, such detection of
 the failure at the tail is useful, for example, the use of multipoint
 BFD to enable fast failure detection and faster failover in multicast
 VPN as described in [MVPN-FAILOVER].  Due to its unidirectional
 nature, virtually all options and timing parameters are controlled by
 the head.
 Throughout this document, the term "multipoint" is defined as a
 mechanism by which one or more systems receive packets sent by a
 single sender.  This specifically includes such things as IP
 multicast and point-to-multipoint MPLS.
 The term "connectivity" in this document is not being used in the
 context of connectivity verification in a transport network but as an
 alternative to "continuity", i.e., the existence of a forwarding path
 between the sender and the receiver.
 This document effectively updates and extends the base BFD
 specification [RFC5880].

2. Keywords

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

Katz, et al. Standards Track [Page 4] RFC 8562 BFD for Multipoint Networks April 2019

3. Goals

 The primary goal of this mechanism is to allow tails to rapidly
 detect the fact that multipoint connectivity from the head has
 failed.
 Another goal is for the mechanism to work on any multicast
 technology.
 A further goal is to support multiple, overlapping point-to-
 multipoint paths, as well as multipoint-to-multipoint paths, and to
 allow point-to-point BFD sessions to operate simultaneously among the
 systems participating in multipoint BFD.
 It is not a goal for this protocol to verify point-to-point
 bidirectional connectivity between the head and any tail.  This can
 be done independently (and with no penalty in protocol overhead) by
 using point-to-point BFD.

4. Overview

 The heart of this protocol is the periodic transmission of BFD
 Control packets along a multipoint path, from the head to all tails
 on the path.  The contents of the BFD packets provide the means for
 the tails to calculate the Detection Time for path failure.  If no
 BFD Control packets are received by a tail for a Detection Time, the
 tail declares that the path has failed.  For some applications, this
 is the only mechanism necessary; the head can remain ignorant of the
 status of connectivity to the tails.
 The head of a multipoint BFD session may wish to be alerted to the
 tails' connectivity (or lack thereof).  Details of how the head keeps
 track of tails and how tails alert their connectivity to the head are
 outside the scope of this document and are discussed in [RFC8563].
 Although this document describes a single head and a set of tails
 spanned by a single multipoint path, the protocol is capable of
 supporting (and discriminating between) more than one multipoint path
 at both heads and tails, as described in Sections 5.7 and 5.13.2.
 Furthermore, the same head and tail may share multiple multipoint
 paths, and a multipoint path may have multiple heads.

5. Protocol Details

 This section describes the operation of Multipoint BFD in detail.

Katz, et al. Standards Track [Page 5] RFC 8562 BFD for Multipoint Networks April 2019

5.1. Multipoint BFD Control Packets

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

5.2. Session Model

 Multipoint BFD is modeled as a set of sessions of different types.
 The elements of procedure differ slightly for each type.
 The head has a session of type MultipointHead, as defined in
 Section 5.4.1, that is bound to a multipoint path.  Multipoint BFD
 Control packets are sent by this session over the multipoint path,
 and no BFD Control packets are received by it.
 Each tail has a session of type MultipointTail, as defined in
 Section 5.4.1, associated with a multipoint path.  These sessions
 receive BFD Control packets from the head over the multipoint path.

5.3. Session-Failure Semantics

 The semantics of session failure is subtle enough to warrant further
 explanation.
 MultipointHead sessions cannot fail (since they are controlled
 administratively).
 If a MultipointTail session fails, it means that the tail definitely
 has lost contact with the head (or the head has been administratively
 disabled), and the tail may use mechanisms other than BFD, e.g.,
 logging or NETCONF [RFC6241], to send a notification to the user.

5.4. State Variables

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

5.4.1. New State Variable Values

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

Katz, et al. Standards Track [Page 6] RFC 8562 BFD for Multipoint Networks April 2019

    bfd.SessionType
       The type of this session as defined in [RFC7880].  Newly added
       values are:
          PointToPoint: Classic point-to-point BFD, as described in
          [RFC5880].
          MultipointHead: A session on the head responsible for the
          periodic transmission of multipoint BFD Control packets
          along the multipoint path.
          MultipointTail: A multipoint session on a tail.
       This variable MUST be initialized to the appropriate type when
       the session is created.

5.4.2. State Variable Initialization and Maintenance

 Some state variables defined in Section 6.8.1 of [RFC5880] need to be
 initialized or manipulated differently depending on the session type.
    bfd.RequiredMinRxInterval
       This variable MUST be initialized to zero for session type
       MultipointHead.
    bfd.DemandMode
       This variable MUST be initialized to 1 for session type
       MultipointHead and MUST be initialized to zero for session type
       MultipointTail.

5.5. State Machine

 There are slight differences in how the BFD state machine works in
 the multipoint application.  In particular, since there is a many-to-
 one mapping, three-way handshakes for session establishment and
 teardown are neither possible nor appropriate.  As such, there is no
 Init state.  Sessions of type MultipointHead MUST NOT send BFD
 Control packets with the State field being set to INIT, and those
 packets MUST be ignored on receipt.
 The following diagram provides an overview of the state machine for
 session type MultipointTail.  The notation on each arc represents the
 state of the remote system (as received in the State field in the BFD
 Control packet) or indicates the expiration of the Detection Timer.

Katz, et al. Standards Track [Page 7] RFC 8562 BFD for Multipoint Networks April 2019

                       DOWN, ADMIN DOWN,
                     +------+  TIMER               +------+
                +----|      |<---------------------|      |----+
           DOWN,|    | DOWN |                      |  UP  |    |UP
     ADMIN DOWN,+--->|      |--------------------->|      |<---+
          TIMER      +------+          UP          +------+
 Sessions of type MultipointHead never receive packets and have no
 Detection Timer; as such, all state transitions are administratively
 driven.

5.6. Session Establishment

 Unlike point-to-point BFD, multipoint BFD provides a form of the
 discovery mechanism that enables tails to discover the head.  The
 minimum amount of a priori information required both on the head and
 tails is the binding to the multipoint path over which BFD is
 running.  The head transmits multipoint BFD packets on that path, and
 the tails listen for BFD packets on that path.  All other information
 can be determined dynamically.
 A session of type MultipointHead is created for each multipoint path
 over which the head wishes to run BFD.  This session runs in the
 Active role, per Section 6.1 of [RFC5880].  Except when
 administratively terminating BFD service, this session is always in
 state Up and always operates in Demand mode.  No received packets are
 ever demultiplexed to the MultipointHead session.  In this sense, it
 is a degenerate form of a session.
 Sessions on the tail MAY be established dynamically, based on the
 receipt of a multipoint BFD Control packet from the head, and are of
 type MultipointTail.  Tail sessions always take the Passive role, per
 Section 6.1 of [RFC5880].

5.7. Discriminators and Packet Demultiplexing

 The use of discriminators is somewhat different in multipoint BFD
 than in point-to-point BFD.
 The head sends multipoint BFD Control packets over the multipoint
 path via the MultipointHead session with My Discriminator set to a
 value bound to the multipoint path and with Your Discriminator set to
 zero.
 IP and MPLS multipoint tails MUST demultiplex BFD packets based on a
 combination of the source address, My Discriminator, and the identity
 of the multipoint path that the multipoint BFD Control packet was
 received from.  Together they uniquely identify the head of the

Katz, et al. Standards Track [Page 8] RFC 8562 BFD for Multipoint Networks April 2019

 multipoint path.  Bootstrapping a BFD session to multipoint MPLS
 Label Switched Path (LSP) may use the control plane, e.g., as
 described in [MVPN-FAILOVER], and is outside the scope of this
 document.
 Note that, unlike point-to-point sessions, the My Discriminator value
 on the MultipointHead session MUST NOT be changed during the life of
 a session.  This is a side effect of the more complex demultiplexing
 scheme.

5.8. Packet Consumption on Tails

 BFD packets received on tails for an IP multicast group MUST be
 consumed by tails and MUST NOT be forwarded to receivers.  Nodes with
 the BFD session of type MultipointTail MUST identify packets received
 on an IP multipoint path as a BFD Control packet if the destination
 UDP port value equals 3784.
 For multipoint LSPs, when IP/UDP encapsulation of BFD Control packets
 is used, MultipointTail MUST expect destination UDP port 3784.  The
 destination IP address of a BFD Control packet MUST be in the
 127.0.0.0/8 range for IPv4 or in the 0:0:0:0:0:FFFF:7F00:0/104 range
 for IPv6.  The use of these destination addresses is consistent with
 the explanations and usage in [RFC8029].  Packets identified as BFD
 packets MUST be consumed by MultipointTail and demultiplexed as
 described in Section 5.13.2.  Use of other types of encapsulation of
 the BFD control message over multipoint LSP is outside the scope of
 this document.

5.9. Bringing Up and Shutting Down Multipoint BFD Service

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

Katz, et al. Standards Track [Page 9] RFC 8562 BFD for Multipoint Networks April 2019

 The head of a multipoint BFD session may wish to shut down its BFD
 service in a controlled fashion.  This is desirable because the tails
 need not wait for a Detection Time prior to declaring the multipoint
 session to be down (and taking whatever action is necessary in that
 case).
 To shut down a multipoint session in a controlled fashion, the head
 MUST administratively set bfd.SessionState in the MultipointHead
 session to either Down or AdminDown and SHOULD set
 bfd.RequiredMinRxInterval to zero.  The session SHOULD send BFD
 Control packets in this state for a period equal to
 (bfd.DesiredMinTxInterval * bfd.DetectMult).  Alternatively, the head
 MAY stop transmitting BFD Control packets and not send any more BFD
 Control packets with the new state (Down or AdminDown).  Tails will
 declare the multipoint session down only after the Detection Time
 interval runs out.

5.10. Timer Manipulation

 Because of the one-to-many mapping, a session of type MultipointHead
 SHOULD NOT initiate a Poll Sequence in conjunction with timer value
 changes.  However, to indicate a change in the packets, a
 MultipointHead session MUST send packets with the P bit set.  A
 MultipointTail session MUST NOT reply if the packet has the M and P
 bits set and bfd.RequiredMinRxInterval set to zero.  Because the Poll
 Sequence is not used, the tail cannot negotiate down MultpointHead's
 transmit interval.  If the value of Desired Min TX Interval in the
 BFD Control packet received by MultipointTail is too high (that
 determination may change in time based on the current environment),
 it must be handled by the implementation and may be controlled by
 local policy, e.g., close the MultipointTail session.
 The MultipointHead, when changing the transmit interval to a higher
 value, MUST send BFD Control packets with the P bit set at the old
 transmit interval before using the higher value in order to avoid
 false detection timeouts at the tails.  A MultipointHead session MAY
 also wait some amount of time before making the changes to the
 transmit interval (through configuration).
 Change in the value of bfd.RequiredMinRxInterval is outside the scope
 of this document and is discussed in [RFC8563].

5.11. Detection Times

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

Katz, et al. Standards Track [Page 10] RFC 8562 BFD for Multipoint Networks April 2019

 Since MultipointHead sessions never receive packets, they do not
 calculate a Detection Time.
 MultipointTail sessions cannot influence the transmission rate of the
 MultipointHead session using the Required Min Rx Interval field
 because of its one-to-many nature.  As such, the Detection Time
 calculation for a MultipointTail session does not use
 bfd.RequiredMinRxInterval.  The Detection Time is calculated as the
 product of the last received values of Desired Min TX Interval and
 Detect Mult.
 The value of bfd.DetectMult may be changed at any time on any session
 type.

5.12. State Maintenance for Down/AdminDown Sessions

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

5.12.1. MultipointHead Sessions

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

5.12.2. MultipointTail Sessions

 MultipointTail sessions MAY be destroyed immediately upon leaving Up
 state, since the tail will transmit no packets.
 Otherwise, MultipointTail sessions SHOULD be maintained as long as
 BFD Control packets are being received by it (which by definition
 will indicate that the head is not Up).

5.13. Base Specification Text Replacement

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

Katz, et al. Standards Track [Page 11] RFC 8562 BFD for Multipoint Networks April 2019

5.13.1. Reception of BFD Control Packets

 The following procedure replaces Section 6.8.6 of [RFC5880] entirely.
 When a BFD Control packet is received, the following procedure MUST
 be followed, in the order specified.  If the packet is discarded
 according to these rules, processing of the packet MUST cease at that
 point.
    If the version number is not correct (1), the packet MUST be
    discarded.
    If the Length field is less than the minimum correct value (24 if
    the A bit is clear, or 26 if the A bit is set), the packet MUST be
    discarded.
    If the Length field is greater than the payload of the
    encapsulating protocol, the packet MUST be discarded.
    If the Detect Mult field is zero, the packet MUST be discarded.
    If the My Discriminator field is zero, the packet MUST be
    discarded.
    Demultiplex the packet to a session according to Section 5.13.2.
    The result is either a session of the proper type, or the packet
    is discarded (and packet processing MUST cease).
    If the A bit is set and no authentication is in use (bfd.AuthType
    is zero), the packet MUST be discarded.
    If the A bit is clear and authentication is in use (bfd.AuthType
    is nonzero), the packet MUST be discarded.
    If the A bit is set, the packet MUST be authenticated under the
    rules of Section 6.7 of [RFC5880], based on the authentication
    type in use (bfd.AuthType).  This may cause the packet to be
    discarded.
    Set bfd.RemoteDiscr to the value of My Discriminator.
    Set bfd.RemoteState to the value of the State (Sta) field.
    Set bfd.RemoteDemandMode to the value of the Demand (D) bit.
    Set bfd.RemoteMinRxInterval to the value of Required Min RX
    Interval.

Katz, et al. Standards Track [Page 12] RFC 8562 BFD for Multipoint Networks April 2019

    If the Required Min Echo RX Interval field is zero, the
    transmission of Echo packets, if any, MUST cease.
    If a Poll Sequence is being transmitted by the local system and
    the Final (F) bit in the received packet is set, the Poll Sequence
    MUST be terminated.
    If bfd.SessionType is PointToPoint, update the transmit interval
    as described in Section 6.8.2 of [RFC5880].
    If bfd.SessionType is PointToPoint, update the Detection Time as
    described in Section 6.8.4 of [RFC5880].
    Else
       If bfd.SessionType is MultipointTail, then update the Detection
       Time as the product of the last received values of Desired Min
       TX Interval and Detect Mult, as described in Section 5.11 of
       this specification.
    If bfd.SessionState is AdminDown
       Discard the packet
    If the received State is AdminDown
       If bfd.SessionState is not Down
          Set bfd.LocalDiag to 3 (Neighbor signaled session down)
          Set bfd.SessionState to Down
    Else
       If bfd.SessionState is Down
          If bfd.SessionType is PointToPoint
             If received State is Down
                Set bfd.SessionState to Init
             Else if received State is Init
                Set bfd.SessionState to Up

Katz, et al. Standards Track [Page 13] RFC 8562 BFD for Multipoint Networks April 2019

          Else (bfd.SessionType is not PointToPoint)
             If received State is Up
                Set bfd.SessionState to Up
       Else if bfd.SessionState is Init
          If received State is Init or Up
             Set bfd.SessionState to Up
       Else (bfd.SessionState is Up)
          If received State is Down
             Set bfd.LocalDiag to 3 (Neighbor signaled session down)
             Set bfd.SessionState to Down
    Check to see if Demand mode should become active or not (see
    [RFC5880], Section 6.6).
    If bfd.RemoteDemandMode is 1, bfd.SessionState is Up, and
    bfd.RemoteSessionState is Up, Demand mode is active on the remote
    system and the local system MUST cease the periodic transmission
    of BFD Control packets (see Section 5.13.3).
    If bfd.RemoteDemandMode is zero, bfd.SessionState is not Up, or
    bfd.RemoteSessionState is not Up, Demand mode is not active on the
    remote system and the local system MUST send periodic BFD Control
    packets (see Section 5.13.3).
    If the Poll (P) bit is set, and bfd.SessionType is PointToPoint,
    send a BFD Control packet to the remote system with the Poll (P)
    bit clear, and the Final (F) bit set (see Section 5.13.3).
    If the packet was not discarded, it has been received for purposes
    of the Detection Time expiration rules in Section 6.8.4 of
    [RFC5880].

Katz, et al. Standards Track [Page 14] RFC 8562 BFD for Multipoint Networks April 2019

5.13.2. Demultiplexing BFD Control Packets

 This section is part of the replacement for Section 6.8.6 of
 [RFC5880]; it is separated for clarity.
    If the Multipoint (M) bit is set
       If the Your Discriminator field is nonzero, the packet MUST be
       discarded.
       Select a session based on the source address, My Discriminator,
       and the identity of the multipoint path on which the multipoint
       BFD Control packet was received.
       If a session is found, and bfd.SessionType is not
       MultipointTail, the packet MUST be discarded.
       Else
          If a session is not found, a new session of type
          MultipointTail MAY be created, or the packet MAY be
          discarded.  This choice can be controlled by the local
          policy, e.g., by setting a maximum number of MultipointTail
          sessions.  Use of the local policy and the exact mechanism
          of it are outside the scope of this specification.
    Else (Multipoint (M) bit is clear)
       If the Your Discriminator field is nonzero
          Select a session based on the value of Your Discriminator.
          If no session is found, the packet MUST be discarded.
       Else (Your Discriminator is zero)
          If the State field is not Down or AdminDown, the packet MUST
          be discarded.
          Otherwise, the session MUST be selected based on some
          combination of other fields, possibly including source
          addressing information, the My Discriminator field, and the
          interface over which the packet was received.  The exact
          method of selection is application specific and is thus
          outside the scope of this specification.
          If a matching session is found, and bfd.SessionType is not
          PointToPoint, the packet MUST be discarded.

Katz, et al. Standards Track [Page 15] RFC 8562 BFD for Multipoint Networks April 2019

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

5.13.3. Transmitting BFD Control Packets

 The following procedure replaces Section 6.8.7 of [RFC5880] entirely.
 With the exceptions listed in the remainder of this section, a system
 MUST NOT transmit BFD Control packets at an interval less than the
 larger of bfd.DesiredMinTxInterval and bfd.RemoteMinRxInterval, less
 applied jitter (see below).  In other words, the system reporting the
 slower rate determines the transmission rate.
 The periodic transmission of BFD Control packets MUST be jittered on
 a per-packet basis by up to 25%; that is, the interval MUST be
 reduced by a random value of 0 to 25%, in order to avoid self-
 synchronization with other systems on the same subnetwork.  Thus, the
 average interval between packets will be roughly 12.5% less than that
 negotiated.
 If bfd.DetectMult is equal to 1, the interval between transmitted BFD
 Control packets MUST be no more than 90% of the negotiated
 transmission interval and MUST be no less than 75% of the negotiated
 transmission interval.  This is to ensure that, on the remote system,
 the calculated Detection Time does not pass prior to the receipt of
 the next BFD Control packet.
 A system MUST NOT transmit any BFD Control packets if bfd.RemoteDiscr
 is zero and the system is taking the Passive role.
 A system MUST NOT transmit any BFD Control packets if bfd.SessionType
 is MultipointTail.
 A system MUST NOT periodically transmit BFD Control packets if Demand
 mode is active on the remote system (bfd.RemoteDemandMode is 1,
 bfd.SessionState is Up, and bfd.RemoteSessionState is Up), and a Poll
 Sequence is not being transmitted.
 A system MUST NOT periodically transmit BFD Control packets if
 bfd.RemoteMinRxInterval is zero.

Katz, et al. Standards Track [Page 16] RFC 8562 BFD for Multipoint Networks April 2019

 If bfd.SessionType is MultipointHead, the transmit interval MUST be
 set to bfd.DesiredMinTxInterval (this should happen automatically, as
 bfd.RemoteMinRxInterval will be zero).
 If bfd.SessionType is not MultipointHead, the transmit interval MUST
 be recalculated whenever bfd.DesiredMinTxInterval changes, or
 whenever bfd.RemoteMinRxInterval changes, and is equal to the greater
 of those two values.  See Sections 6.8.2 and 6.8.3 of [RFC5880] for
 details on transmit timers.
 A system MUST NOT set the Demand (D) bit if bfd.SessionType is
 MultipointTail.
 A system MUST NOT set the Demand (D) bit if bfd.SessionType is
 PointToPoint unless bfd.DemandMode is 1, bfd.SessionState is Up, and
 bfd.RemoteSessionState is Up.
 If bfd.SessionType is PointToPoint or MultipointHead, a BFD Control
 packet SHOULD be transmitted during the interval between periodic
 Control packet transmissions when the contents of that packet would
 differ from that in the previously transmitted packet (other than the
 Poll (P) and Final (F) bits) in order to more rapidly communicate a
 change in state.
 The contents of transmitted BFD Control packets MUST be set as
 follows:
    Version
       Set to the current version number (1).
    Diagnostic (Diag)
       Set to bfd.LocalDiag.
    State (Sta)
       Set to the value indicated by bfd.SessionState.
    Poll (P)
       Set to 1 if the local system is sending a Poll Sequence or is a
       session of type MultipointHead soliciting the identities of the
       tails, or zero if not.

Katz, et al. Standards Track [Page 17] RFC 8562 BFD for Multipoint Networks April 2019

    Final (F)
       Set to 1 if the local system is responding to a BFD Control
       packet received with the Poll (P) bit set, or zero if not.
    Control Plane Independent (C)
       Set to 1 if the local system's BFD implementation is
       independent of the control plane (it can continue to function
       through a disruption of the control plane).
    Authentication Present (A)
       Set to 1 if authentication is in use in this session
       (bfd.AuthType is nonzero), or zero if not.
    Demand (D)
       Set to bfd.DemandMode if bfd.SessionState is Up and
       bfd.RemoteSessionState is Up.  Set to 1 if bfd.SessionType is
       MultipointHead.  Otherwise, it is set to zero.
    Multipoint (M)
       Set to 1 if bfd.SessionType is MultipointHead.  Otherwise, it
       is set to zero.
    Detect Mult
       Set to bfd.DetectMult.
    Length
       Set to the appropriate length, based on the fixed header length
       (24) plus any Authentication Section.
    My Discriminator
       Set to bfd.LocalDiscr.
    Your Discriminator
       Set to bfd.RemoteDiscr.
    Desired Min TX Interval
       Set to bfd.DesiredMinTxInterval.

Katz, et al. Standards Track [Page 18] RFC 8562 BFD for Multipoint Networks April 2019

    Required Min RX Interval
       Set to bfd.RequiredMinRxInterval.
    Required Min Echo RX Interval
       Set to zero if bfd.SessionType is MultipointHead or
       MultipointTail.  Otherwise, set to the minimum required Echo
       packet receive interval for this session.  If this field is set
       to zero, the local system is unwilling or unable to loop back
       BFD Echo packets to the remote system, and the remote system
       will not send Echo packets.
    Authentication Section
       Included and set according to the rules in Section 6.7 of
       [RFC5880] if authentication is in use (bfd.AuthType is
       nonzero).  Otherwise, this section is not present.

6. Congestion Considerations

 As a foreword, although congestion can occur because of a number of
 factors, it should be noted that high transmission rates are by
 themselves subject to creating congestion either along the path or at
 the tail end(s).  As such, as stated in [RFC5883]:
    it is required that the operator correctly provision the rates at
    which BFD is transmitted to avoid congestion (e.g link, I/O, CPU)
    and false failure detection.
 Use of BFD in multipoint networks, as specified in this document,
 over multiple hops requires consideration of the mechanisms to react
 to network congestion.  Requirements stated in Section 7 of the BFD
 base specification [RFC5880] equally apply to BFD in multipoint
 networks and are repeated here:
    When BFD is used across multiple hops, a congestion control
    mechanism MUST be implemented, and when congestion is detected,
    the BFD implementation MUST reduce the amount of traffic it
    generates.
 The mechanism to control the load of BFD traffic MAY use BFD's
 configuration interface to control BFD state variable
 bfd.DesiredMinTxInterval.  However, such a control loop does not form
 part of the BFD protocol itself, and its specification is thus
 outside the scope of this document.

Katz, et al. Standards Track [Page 19] RFC 8562 BFD for Multipoint Networks April 2019

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

7. IANA Considerations

 This document has no IANA actions.

8. Security Considerations

 The same security considerations as those described in [RFC5880]
 apply to this document.  Additionally, implementations that create
 MultpointTail sessions dynamically upon receipt of multipoint BFD
 Control packets MUST implement protective measures to prevent an
 infinite number of MultipointTail sessions from being created.  Below
 are some points to consider in such implementations.
    If a multipoint BFD Control packet did not arrive on a multicast
    path (e.g., on the expected interface, with the expected MPLS
    label, etc.), a MultipointTail session should not be created.
    If redundant streams are expected for a given multicast stream,
    the implementations should not create more MultipointTail sessions
    than the number of streams.  Additionally, when the number of
    MultipointTail sessions exceeds the number of expected streams,
    the implementation should generate an alarm to users to indicate
    the anomaly.
    The implementation should have a reasonable upper bound on the
    number of MultipointHead sessions that can be created, with the
    upper bound potentially being computed based on the load these
    would generate.
    The implementation should have a reasonable upper bound on the
    number of MultipointTail sessions that can be created, with the
    upper bound potentially being computed based on the number of
    multicast streams that the system is expecting.
 If authentication is in use, the head and all tails may be configured
 to have a common authentication key in order for the tails to
 validate multipoint BFD Control packets.

Katz, et al. Standards Track [Page 20] RFC 8562 BFD for Multipoint Networks April 2019

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

9. References

9.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <https://www.rfc-editor.org/info/rfc2119>.
 [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
            (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
            <https://www.rfc-editor.org/info/rfc5880>.
 [RFC7880]  Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and
            S. Pallagatti, "Seamless Bidirectional Forwarding
            Detection (S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July
            2016, <https://www.rfc-editor.org/info/rfc7880>.
 [RFC8029]  Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
            Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
            Switched (MPLS) Data-Plane Failures", RFC 8029,
            DOI 10.17487/RFC8029, March 2017,
            <https://www.rfc-editor.org/info/rfc8029>.
 [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
            2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
            May 2017, <https://www.rfc-editor.org/info/rfc8174>.

Katz, et al. Standards Track [Page 21] RFC 8562 BFD for Multipoint Networks April 2019

9.2. Informative References

 [MVPN-FAILOVER]
            Morin, T., Ed., Kebler, R., Ed., and G. Mirsky, Ed.,
            "Multicast VPN fast upstream failover", Work in Progress,
            draft-ietf-bess-mvpn-fast-failover-05, February 2019.
 [RFC4082]  Perrig, A., Song, D., Canetti, R., Tygar, J., and
            B. Briscoe, "Timed Efficient Stream Loss-Tolerant
            Authentication (TESLA): Multicast Source Authentication
            Transform Introduction", RFC 4082, DOI 10.17487/RFC4082,
            June 2005, <https://www.rfc-editor.org/info/rfc4082>.
 [RFC5883]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
            (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883,
            June 2010, <https://www.rfc-editor.org/info/rfc5883>.
 [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
            and A. Bierman, Ed., "Network Configuration Protocol
            (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
            <https://www.rfc-editor.org/info/rfc6241>.
 [RFC8563]  Katz, D., Ward, D., Pallagatti, S., Ed., and G. Mirsky,
            Ed., "Bidirectional Forwarding Detection (BFD) Multipoint
            Active Tails", RFC 8563, DOI 10.17487/RFC8563, April 2019,
            <https://www.rfc-editor.org/info/rfc8563>.

Acknowledgments

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

Contributors

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

Katz, et al. Standards Track [Page 22] RFC 8562 BFD for Multipoint Networks April 2019

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

Katz, et al. Standards Track [Page 23]

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