Internet Engineering Task Force                                 N. Akiya
Internet-Draft                                       Big Switch Networks
Updates: 5880 (if approved)                                 C. Pignataro
Intended status: Standards Track                                 D. Ward
Expires: December 21, 2015 August 12, 2016                                   Cisco Systems
                                                               M. Bhatia
                                                          Ionos Networks
                                                           S. Pallagatti
                                                        Juniper Networks
                                                           June 19, 2015
                                                        February 9, 2016

          Seamless Bidirectional Forwarding Detection (S-BFD)
                    draft-ietf-bfd-seamless-base-05
                    draft-ietf-bfd-seamless-base-06

Abstract

   This document defines a simplified mechanism to use Bidirectional
   Forwarding Detection (BFD) with large portions of negotiation aspects
   eliminated, thus providing benefits such as quick provisioning as
   well as improved control and flexibility to network nodes initiating
   the path monitoring.

   This document updates RFC5880.

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 December 21, 2015. August 12, 2016.

Copyright Notice

   Copyright (c) 2015 2016 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
   (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.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Seamless BFD Overview . . . . . . . . . . . . . . . . . . . .   4   5
   4.  S-BFD Discriminators  . . . . . . . . . . . . . . . . . . . .   5   6
     4.1.  S-BFD Discriminator Uniqueness  . . . . . . . . . . . . .   5   6
     4.2.  Discriminator Pools . . . . . . . . . . . . . . . . . . .   6
   5.  Reflector BFD Session . . . . . . . . . . . . . . . . . . . .   7
   6.  State Variables . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  New State Variables . . . . . . . . . . . . . . . . . . .   7
     6.2.  State Variable Initialization and Maintenance . . . . . .   8
   7.  S-BFD Procedures  . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Demultiplexing of S-BFD Control Packet  . . . . . . . . .   8
     7.2.  Initiator  Responder Procedures  . . . . . . . . . . . . . . . . . .   9
       7.2.1.  SBFDInitiator State Machine  Responder Demultiplexing  . . . . . . . . . . . . . .  10   9
       7.2.2.  Transmission of S-BFD Control Packet by SBFDInitiator  10 SBFDReflector   9
       7.2.3.  Additional SBFDReflector Behaviors  . . . . . . . . .  11
     7.3.  Responder  Initiator Procedures  . . . . . . . . . . . . . . . . . .  12  11
       7.3.1.  Responder Demultiplexing  .  SBFDInitiator State Machine . . . . . . . . . . . . .  12
       7.3.2.  Transmission of S-BFD Control Packet by SBFDReflector SBFDInitiator  13
     7.4.  Diagnostic Values . . . . . . . . . . .
       7.3.3.  Additional SBFDInitiator Behaviors  . . . . . . . . .  14
     7.5.  The Poll Sequence  13
     7.4.  Diagnostic Values . . . . . . . . . . . . . . . . . . . .  14
     7.6.  Control Plane Independent (C) . . . . . . . . . . . . . .  15
     7.7.  Additional SBFDInitiator Behaviors  . .
     7.5.  The Poll Sequence . . . . . . . . .  15
     7.8.  Additional SBFDReflector Behaviors . . . . . . . . . . .  15  14
   8.  Scaling Aspect  . . . . . . . . . . . . . . . . . . . . . . .  16  14
   9.  Co-existence with Classical BFD Sessions  . . . . . . . . . .  16  14
   10. S-BFD Echo Function . . . . . . . . . . . . . . . . . . . . .  16  15
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  17  15
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18  16
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18  16
   14. Contributing Authors  . . . . . . . . . . . . . . . . . . . .  18  17
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19  17
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  19  17
     15.2.  Informative References . . . . . . . . . . . . . . . . .  19  17
   Appendix A.  Loop Problem . . . . . . . . . . . . . . . . . . . .  20  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21  19

1.  Introduction

   Bidirectional Forwarding Detection (BFD), [RFC5880] and related
   documents, has efficiently generalized the failure detection
   mechanism for multiple protocols and applications.  There are some
   improvements which can be made to better fit existing technologies.
   There is a possibility of evolving BFD to better fit new
   technologies.  This document focuses on several aspects of BFD in
   order to further improve efficiency, to expand failure detection
   coverage and to allow BFD usage for wider scenarios.  This

   Specifically, this document
   extends BFD to provide solutions defines Seamless Bidirectional Forwarding
   Detection (S-BFD) a simplified mechanism to use Bidirectional
   Forwarding Detection (BFD) with large portions of negotiation aspects
   eliminated, thus providing benefits such as quick provisioning as
   well as improved control and flexibility to network nodes initiating
   the path monitoring.  S-BFD enables cases listed benefiting from the use of
   core BFD technologies in
   [I-D.ietf-bfd-seamless-use-case]. a fashion that leverages existing
   implementations and protocol machinery while providing a rather
   simplified and largely stateless infrastructure for continuity
   testing.

   One key aspect of the mechanism described in this document eliminates
   the time between a network node wanting to perform a continuity test
   and completing the continuity test.  In traditional BFD terms, the
   initial state changes from DOWN to UP are virtually nonexistent.
   Removal of this seam (i.e., time delay) in BFD provides applications
   a smooth and continuous operational experience.  Therefore, "Seamless
   BFD" (S-BFD) has been chosen as the name for this mechanism.

2.  Terminology

   The reader is expected to be familiar with the BFD, BFD [RFC5880], IP
   [RFC0791] [RFC2460] and MPLS [RFC3031] terminologies and protocol
   constructs.  This section describes several new terminologies
   introduced by S-BFD.

   o  Classical BFD - BFD session types based on [RFC5880].

   o  S-BFD - Seamless BFD.

   o  S-BFD control packet - a BFD control packet for the S-BFD
      mechanism.

   o  S-BFD echo packet - a BFD echo packet for the S-BFD mechanism.

   o  S-BFD packet - a BFD control packet or a BFD echo packet.

   o  Entity - a function on a network node that S-BFD mechanism allows
      remote network nodes to perform continuity test to.  An entity can
      be abstract (e.g., reachability) or specific (e.g., IP addresses,
      router-IDs, functions).

   o  SBFDInitiator - an S-BFD session on a network node that performs a
      continuity test to a remote entity by sending S-BFD packets.

   o  SBFDReflector - an S-BFD session on a network node that listens
      for incoming S-BFD control packets to local entities and generates
      response S-BFD control packets.

   o  Reflector BFD session - synonymous with SBFDReflector.

   o  S-BFD discriminator - a BFD discriminator allocated for a local
      entity and is being listened by an SBFDReflector.

   o  BFD discriminator - a BFD discriminator allocated for an
      SBFDInitiator.

   o  Initiator - a network node hosting an SBFDInitiator.

   o  Responder - a network node hosting an SBFDReflector.

   Below figure describes the relationship between S-BFD terminologies.

    +---------------------+                +------------------------+
    |      Initiator      |                |         Responder      |
    | +-----------------+ |                |    +-----------------+ |
    | |  SBFDInitiator  |---S-BFD ctrl pkt----->|  SBFDReflector  | |
    | | +-------------+ |<--S-BFD ctrl pkt------| +-------------+ | |
    | | | BFD discrim | | |                |    | |S-BFD discrim| | |
    | | |             | |---S-BFD echo pkt---+  | |             | | |
    | | +-------------+ | |                | |  | +----------^--+ | |
    | +-----------------+<-------------------+  +------------|----+ |
    |                     |                |                 |      |
    |                     |                |             +---v----+ |
    |                     |                |             | Entity | |
    |                     |                |             +--------+ |
    +---------------------+                +------------------------+

             Figure 1: S-BFD Terminology Relationship

3.  Seamless BFD Overview

   An S-BFD module on each network node allocates one or more S-BFD
   discriminators for local entities, and creates a reflector BFD
   session.  Allocated S-BFD discriminators may be advertised by
   applications (e.g., OSPF/IS-IS).  Required result is that
   applications, on other network nodes, possess the knowledge of the
   mapping from
   S-BFD discriminators allocated by a remote entities node to S-BFD discriminators. remote entities.
   The reflector BFD session is to, upon receiving an S-BFD control
   packet targeted to one of local S-BFD discriminator values, transmit
   a response S-BFD control packet back to the initiator.

   Once above setup is complete, any network nodes, node, having the knowledge
   of the mapping from S-BFD discriminator allocated toby a remote entity node to an S-BFD discriminator, remote
   entity/entities, it can quickly perform a continuity test to the
   remote entity by simply sending S-BFD control packets with
   corresponding S-BFD discriminator value in the "your discriminator"
   field.

   For example:

      <------- IS-IS Network ------->

                +---------+
                |         |
      A---------B---------C---------D
      ^                             ^
      |                             |
   SystemID                      SystemID
     xxx                           yyy
   BFD Discrim                   BFD Discrim
     123                           456

             Figure 2: S-BFD for IS-IS Network

   The

   S-BFD module in a system IS-IS with SystemID xxx (node A) allocates an
   S-BFD discriminator 123, and IS-IS will advertises the S-BFD
   discriminator 123 in an IS-IS TLV.  The
   IS-IS  S-BFD module in a system with IS-
   IS SystemID yyy (node D) allocates an S-BFD discriminator 456, and
   IS-IS advertises the S-BFD discriminator 456 in an IS-IS TLV.  A
   reflector BFD session is created on both network nodes (node A and
   node D).  When network node A wants to check the reachability to
   network node D, node A can send an S-BFD control packet, destined to
   node D, with "your discriminator" field set to 456.  When the
   reflector BFD session on node D receives this S-BFD control packet,
   then response S-BFD control packet is sent back to node A, which
   allows node A to complete the continuity test.

   The use of multiple S-BFD discriminators by a single network node is
   outside the scope of this document.

4.  S-BFD Discriminators

4.1.  S-BFD Discriminator Uniqueness

   One important characteristics of an S-BFD discriminator is that it
   MUST be unique within an administrative domain.  If multiple network
   nodes allocated a same S-BFD discriminator value, then S-BFD control
   packets falsely terminating on a wrong network node can result in a
   reflector BFD session to generate a response back, due to "your
   discriminator" matching.  This is clearly not desirable.  If only IP
   based S-BFD is considered, then it is possible for the reflector BFD
   session to require demultiplexing of incoming S-BFD control packets
   with combination of destination IP address and "your discriminator".
   Then S-BFD discriminator only has to be unique within a local node.
   However, S-BFD is a generic mechanism defined to run on wide range of
   environments: IP, MPLS, etc.  For other transports like MPLS, because
   of the need to use non-routable IP destination address, it is not
   possible for reflector BFD session to demultiplex using IP
   destination address.  With PHP, there may not be any incoming label
   stack to aid in demultiplexing either.  Thus, S-BFD imposes a
   requirement that S-BFD discriminators MUST be unique within an
   administrative domain.

4.2.  Discriminator Pools

   This subsection describes a discriminator pool implementation
   technique to minimize S-BFD discriminator collisions.  The result
   will allow an implementation to better satisfy the S-BFD
   discriminator uniqueness requirement defined in Section 4.1.

   o  SBFDInitiator is to allocate a discriminator from the BFD
      discriminator pool.  If the system also supports classical BFD
      that runs on [RFC5880], then the BFD discriminator pool SHOULD be
      shared by SBFDInitiator sessions and classical BFD sessions.

   o  SBFDReflector is to allocate a discriminator from the S-BFD
      discriminator pool.  The S-BFD discriminator pool SHOULD be a
      separate pool than the BFD discriminator pool.

   Remainder of this subsection describes the reasons for above
   suggestions.

   Locally allocated S-BFD discriminator values for entities, listened
   by SBFDReflector sessions, may be arbitrary allocated or derived from
   values provided by applications.  These values may be protocol IDs
   (e.g., System-ID, Router-ID) or network targets (e.g., IP address).
   To avoid derived S-BFD discriminator values already being assigned to
   other BFD sessions (i.e., SBFDInitiator sessions and classical BFD
   sessions), it is RECOMMENDED that discriminator pool for
   SBFDReflector sessions be separate from other BFD sessions.

   Even when following the separate discriminator pool approach,
   collision is still possible between one S-BFD application to another
   S-BFD application, that may be using different values and algorithms
   to derive S-BFD discriminator values.  If the two applications are
   using S-BFD for a same purpose (e.g., network reachability), then the
   colliding S-BFD discriminator value can be shared.  If the two
   applications are using S-BFD for a different purpose, then the
   collision must be addressed.  How such collisions are addressed is
   outside the scope of this document.

5.  Reflector BFD Session

   Each network node creates one or more reflector BFD sessions.  This
   reflector BFD session is a session which transmits S-BFD control
   packets in response to received S-BFD control packets with "your
   discriminator" having S-BFD discriminators allocated for local
   entities.  Specifically, this reflector BFD session is to have
   following characteristics:

   o  MUST NOT transmit any S-BFD packets based on local timer expiry.

   o  MUST transmit an S-BFD control packet in response to a received
      S-BFD control packet having a valid S-BFD discriminator in the
      "your discriminator" field, unless prohibited by local policies
      (e.g., administrative, security, rate-limiter, etc).

   o  MUST be capable of sending only two states: UP and ADMINDOWN.

   One reflector BFD session may be responsible for handling received
   S-BFD control packets targeted to all locally allocated S-BFD
   discriminators, or few reflector BFD sessions may each be responsible
   for subset of locally allocated S-BFD discriminators.  This policy is
   a local matter, and is outside the scope of this document.

   Note that incoming S-BFD control packets may be IPv4, IPv6 or MPLS
   based.  How such S-BFD control packets reach an appropriate reflector
   BFD session is also a local matter, and is outside the scope of this
   document.

6.  State Variables

   S-BFD introduces new state variables, and modifies the usage of
   existing ones.

6.1.  New State Variables

   A new state variable is added to the base specification in support of
   S-BFD.

   o  bfd.SessionType: This is a variable introduced her and used by
      [I-D.ietf-bfd-multipoint]
      [I-D.ietf-bfd-multipoint], and describes the type of this session.
      Allowable values for S-BFD sessions are:

      *  SBFDInitiator - an S-BFD session on a network node that
         performs a continuity test to a target entity by sending S-BFD
         packets.

      *  SBFDReflector - an S-BFD session on a network node that listens
         for incoming S-BFD control packets to local entities and
         generates response S-BFD control packets.

   bfd.SessionType variable MUST be initialized to the appropriate type
   when an S-BFD session is created.

6.2.  State Variable Initialization and Maintenance

   Some

   A state variables variable defined in section Section 6.8.1 of the BFD base
   specification [RFC5880] need to be
   initialized or manipulated differently depending on the session type.

   o  bfd.DemandMode: This variable MUST be initialized to 1 for session
      type SBFDInitiator, and MUST be initialized to 0 for session type
      SBFDReflector.

7.  S-BFD Procedures

7.1.  Demultiplexing of S-BFD Control Packet

   S-BFD packet MUST be demultiplexed with lower layer information
   (e.g., dedicated destination UDP port, associated channel type).
   Following procedure SHOULD be executed on both initiator and
   reflector.

      If S-BFD packet

         If S-BFD packet is for SBFDReflector

            Packet MUST be looked up to locate a corresponding
            SBFDReflector session based on the value from the "your
            discriminator" field in the table describing S-BFD
            discriminators.

         Else

            Packet MUST be looked up to locate a corresponding
            SBFDInitiator session or classical BFD session based on the
            value from the "your discriminator" field in the table
            describing BFD discriminators.  If no match then received
            packet MUST be discarded.

            If session is SBFDInitiator
               Destination of the packet (i.e., destination IP address)
               SHOULD be validated to be for self.

            Else

               Packet MUST be discarded

      Else

         Procedure described in [RFC5880] MUST be applied.

   More details on S-BFD control packet demultiplexing are described in
   relevant S-BFD data plane documents.

7.2.  Initiator  Responder Procedures

   A network node which receives S-BFD control packets transmitted by an SBFDInitiator MUST set "your
   discriminator" field to an
   initiator is referred as responder.  The responder, upon reception of
   S-BFD discriminator corresponding control packets, is to the
   remote entity.

   Every SBFDInitiator perform necessary relevant validations
   described in [RFC5880].

7.2.1.  Responder Demultiplexing

   S-BFD packet MUST have a locally unique "my be demultiplexed with lower layer information
   (e.g., dedicated destination UDP port, associated channel type).
   Following procedure SHOULD be executed by responder:

      If "your discriminator"
   allocated from the BFD discriminator pool.

   Below ASCII art describes high level concept not one of continuity test using
   S-BFD.  R2 allocates XX as the S-BFD discriminator entry allocated for its network
   reachability purpose, and advertises XX local
      entities

         Packet MUST be discarded.

      Else

         Packet is determined to neighbors.  ASCII art
   shows R1 and R4 performing be handled by a continuity test to R2.

    +--- md=50/yd=XX (ping) ----+
    |                           |
    |+-- md=XX/yd=50 (pong) --+ |
    ||                        | |
    |v                        | v
    R1 ==================== R2[*] ========= R3 ========= R4
                              | ^                        |^
                              | |                        ||
                              | +-- md=60/yd=XX (ping) --+|
                              |                           |
                              +---- md=XX/yd=60 (pong) ---+

   [*] Reflector reflector BFD session on R2.
   === Links connecting network nodes.
   --- S-BFD control packet traversal.

             Figure 3:
         responsible for that S-BFD Continuity Test

7.2.1.  SBFDInitiator State Machine

   An SBFDInitiator may be a persistent discriminator.

         If local policy allows (e.g., administrative, security, rate-
         limiter, etc)

            Chosen reflector BFD session on the initiator with SHOULD transmit a
   timer for S-BFD response BFD
            control packet transmissions (stateful
   SBFDInitiator).  An SBFDInitiator may also be a module, a script or a
   tool on the initiator that transmits one or more using procedures described in Section 7.3.2.

7.2.2.  Transmission of S-BFD Control Packet by SBFDReflector

   Contents of S-BFD control packets "when needed" (stateless SBFDInitiator).  For stateless
   SBFDInitiators, a complete BFD state machine may not sent by an SBFDReflector MUST be applicable.
   For stateful SBFDInitiators, the states and the state machine
   described in
   set as per Section 6.8.7 of [RFC5880].  There are few fields which
   needs to be set differently from [RFC5880] will not function due as follows:

      State (Sta)

         Set to SBFDReflector session
   only sending bfd.SessionState (either UP and or ADMINDOWN states (i.e., SBFDReflector only).
         Clarification of reflector BFD session
   does not send INIT state).  The following diagram provides the
   RECOMMENDED state machine for stateful SBFDInitiators.  The notation
   on each arc represents the state is described in
         Section 7.2.3.

      Demand (D)

         Set to 0.

      Detect Mult

         Value to be copied from "Detection Multiplier" filed of the SBFDInitiator (as
         received in
   the State field in the S-BFD control packet) or indicates the
   expiration BFD packet.

      My Discriminator

         Value be copied from "your discriminator" filed of the Detection Timer.

                       +--+
          ADMIN DOWN,  |  |
          TIMER        |  V
                     +------+   UP                +------+
                     |      |-------------------->|      |----+
                     | DOWN |                     |  UP  |    | UP
                     |      |<--------------------|      |<---+
                     +------+   ADMIN DOWN,       +------+
                                TIMER

             Figure 4: SBFDInitiator FSM

   Note that the above state machine is different from the base received BFD
   specification[RFC5880].  This is because the INIT state is no longer
   applicable for the SBFDInitiator.  Another important difference is
   the transition
         packet.

      Your Discriminator

         Value be copied from "my discriminator" filed of the state machine received BFD
         packet.

      Desired Min TX Interval

         Value be copied from the DOWN state "Desired Min TX Interval" filed of
         received BFD packet.

      Required Min RX Interval

         Set to the UP
   state when a packet with State UP is received by the SBFDInitiator.
   The definitions of the states and the events have the same meaning as bfd.RequiredMinRxInterval, value describing minimum
         interval, in the base BFD specification [RFC5880].

7.2.2.  Transmission of S-BFD microseconds between received SBFD Control Packet by SBFDInitiator

   Contents of
         packets.  Further details are described in Section 7.2.3.

      Required Min Echo RX Interval

         If device supports looping back S-BFD control echo packets sent by an SBFDInitiator MUST be
   set as follows:

      Version

            Set to the current version number (1).

      Diagnostic (Diag)
         MAY be set to appropriate value minimum required Echo packet receive interval for communicating with peer.

      State (Sta)
            this session.

         Else

            Set to the value indicated 0.

7.2.3.  Additional SBFDReflector Behaviors

   o  S-BFD control packets transmitted by local state.

      Poll (P)

         Set to 1 if the local system is sending a Poll Sequence.

      Final (F)

         Set to 1 if the local system is responding SBFDReflector MUST have
      "Required Min RX Interval" set to a Control packet
         received with the Poll (P) bit set, or 0 if not.

      Control Plane Independent (C)

         Set to 1 if the local system's BFD implementation is
         independent of value which expresses, in
      microseconds, the minimum interval between incoming S-BFD control plane (it
      packets this SBFDReflector can handle.  The SBFDReflector can continue to function
         through a disruption of the
      control plane.)

      Authentication Present (A)

         Set to 1 if authentication is in use on this session
         (bfd.AuthType is nonzero), or 0 if not.

      Demand (D)

         MUST be set always.

      Multipoint (M)

         MUST be set to 0.

      Detect Mult

         MUST how fast SBFInitiators will be set sending S-BFD control
      packets to self by ensuring "Required Min RX Interval" indicates a
      value describing locally used multiplier
         value.

      Length

         Set to the appropriate length, based on the fixed header length
         (24) plus any Authentication Section.

      My Discriminator

         Set current load.

   o  If the SBFDReflector wishes to value assigned by communicate to some or all
      SBFDInitiators that monitored local node.

      Your Discriminator

         Set entity is "temporarily out of
      service", then S-BFD control packets with "state" set to value corresponding ADMINDOWN
      are sent to those SBFDInitiators.  The SBFDInitiators, upon
      reception of such packets, MUST NOT conclude loss of reachability
      to corresponding remote entity.

      Desired Min TX Interval entity, and MUST be set back off packet
      transmission interval for the remote entity to an interval no
      faster than 1 second.  If the SBFDReflector is generating a
      response S-BFD control packet for a value describing local desired minimum
         transmit interval.

      Required Min RX Interval entity that is in
      service, then "state" in response BFD control packets MUST be set
      to 0.

      Required Min Echo RX Interval UP.

   o  If an SBFDReflector receives an S-BFD control packet with Demand
      (D) bit cleared, the packet MUST be set to 0. discarded.

7.3.  Responder  Initiator Procedures

   A network node which receives

   S-BFD control packets transmitted by an
   initiator is referred as responder.  The responder, upon reception of
   S-BFD control packets, is SBFDInitiator MUST set "your
   discriminator" field to perform necessary relevant validations
   described in [RFC5880], [RFC5881], [RFC5883], [RFC5884] and
   [RFC5885].

7.3.1.  Responder Demultiplexing an S-BFD packet discriminator corresponding to the
   remote entity.

   Every SBFDInitiator MUST be demultiplexed with lower layer information
   (e.g., dedicated destination UDP port, associated channel type).
   Following procedure SHOULD be executed by responder:

      If "your have a locally unique "my discriminator" not one
   allocated from the BFD discriminator pool.

   Below Figure 3 art describes high level concept of continuity test
   using S-BFD.  R2 allocates XX as the entry allocated for local
      entities

         Packet MUST NOT be considered S-BFD discriminator for this mechanism.

      Else

         Packet is determined its
   network reachability purpose, and advertises XX to be handled by neighbors.  ASCII
   art shows R1 and R4 performing a reflector continuity test to R2.

    +--- md=50/yd=XX (ping) ----+
    |                           |
    |+-- md=XX/yd=50 (pong) --+ |
    ||                        | |
    |v                        | v
    R1 ==================== R2[*] ========= R3 ========= R4
                              | ^                        |^
                              | |                        ||
                              | +-- md=60/yd=XX (ping) --+|
                              |                           |
                              +---- md=XX/yd=60 (pong) ---+

   [*] Reflector BFD session
         responsible for that on R2.
   === Links connecting network nodes.
   --- S-BFD discriminator.

         If local policy allows (e.g., administrative, security, rate-
         limiter, etc)

            Chosen reflector BFD session SHOULD transmit a response BFD control packet using procedures described in Section 7.3.2.

7.3.2.  Transmission of traversal.

             Figure 3: S-BFD Control Packet by SBFDReflector

   Contents of Continuity Test

7.3.1.  SBFDInitiator State Machine

   An SBFDInitiator may be a persistent session on the initiator with a
   timer for S-BFD control packets sent by an SBFDReflector MUST packet transmissions (stateful
   SBFDInitiator).  An SBFDInitiator may also be
   set as follows:

      Version

         Set to a module, a script or a
   tool on the current version number (1).

      Diagnostic (Diag)

         MAY be set to appropriate value for communicating with peer.

      State (Sta)

         MUST be set to UP initiator that transmits one or ADMINDOWN.  Clarification of reflector more S-BFD control
   packets "when needed" (stateless SBFDInitiator).  For stateless
   SBFDInitiators, a complete BFD
         session state is described in Section 7.8.

      Poll (P)

         Set to 1 if the local system is sending a Poll Sequence, or 0
         if not.

      Final (F)

         Set to 1 if the local system is responding to a Control packet
         received with the Poll (P) bit set, or 0 if not.

      Control Plane Independent (C)

         Set to 1 if machine may not be applicable.
   For stateful SBFDInitiators, the local system's BFD implementation is
         independent of states and the control plane (it can continue to state machine
   described in [RFC5880] will not function
         through a disruption of the control plane.)

      Authentication Present (A)

         Set due to 1 if authentication is in use on this SBFDReflector session
         (bfd.AuthType is nonzero), or 0 if not.

      Demand (D)

         MUST be cleared.

      Multipoint (M)

         MUST be set to 0.

      Detect Mult
         MUST be copied from received "Detection Multiplier".

      Length

         Set to
   only sending UP and ADMINDOWN states (i.e., SBFDReflector session
   does not send INIT state).  The following diagram provides the appropriate length, based
   RECOMMENDED state machine for stateful SBFDInitiators.  The notation
   on each arc represents the fixed header length
         (24) plus any Authentication Section.

      My Discriminator

         MUST be copied from received "your discriminator".

      Your Discriminator

         MUST be copied from received "my discriminator".

      Desired Min TX Interval

         MUST be copied from received "Desired Min TX Interval".

      Required Min RX Interval

         MUST be set to a value describing how many incoming control
         packets this reflector BFD session can handle.  Further details
         are described in Section 7.8.

      Required Min Echo RX Interval

         If device supports looping back S-BFD echo packets

            MUST set non-zero value desired by local device.

         Else

            MUST be set to 0.

7.4.  Diagnostic Values

   Diagnostic value in both directions MAY be set to a certain value, to
   attempt to communicate further information to both ends.  However,
   details of such are outside the scope state of this specification.

7.5.  The Poll Sequence

   Poll sequence MAY be used the SBFDInitiator (as received in both directions.  The Poll sequence MUST
   operate
   the State field in accordance with [RFC5880].  An SBFDReflector MAY use the
   Poll sequence to slow down that rate at which S-BFD control packets
   are generated packet) or indicates the
   expiration of the Detection Timer.

                       +--+
          ADMIN DOWN,  |  |
          TIMER        |  V
                     +------+   UP                +------+
                     |      |-------------------->|      |----+
                     | DOWN |                     |  UP  |    | UP
                     |      |<--------------------|      |<---+
                     +------+   ADMIN DOWN,       +------+
                                TIMER

             Figure 4: SBFDInitiator FSM

   Note that the above state machine is different from an SBFDInitiator. the base BFD
   specification[RFC5880].  This is done by the
   SBFDReflector using procedures described in Section 7.8 and setting because the Poll (P) bit in INIT state is no longer
   applicable for the reflected S-BFD control packet.  The
   SBFDInitiator SBFDInitiator.  Another important difference is
   the transition of the state machine from the DOWN state to then send the next S-BFD control UP
   state when a packet with State UP is received by the
   Final (F) bit set.  If an SBFDReflector receives an S-BFD control
   packet with Poll (P) bit set, then SBFDInitiator.
   The definitions of the SBFDReflector MUST respond
   with an S-BFD control packet with Poll (P) bit cleared states and Final (F)
   bit set.

7.6.  Control Plane Independent (C)

   Control plane independent (C) bit for an SBFDInitiator sending S-BFD
   control packets to a reflector BFD session MUST work according to
   [RFC5880].  Reflector BFD session also MUST work according to
   [RFC5880].  Specifically, if reflector BFD session implementation
   does not share fate with control plane, then response S-BFD control
   packets transmitted MUST the events have control plane independent (C) bit set.
   If reflector the same meaning as
   in the base BFD session implementation shares fate with control
   plane, then response specification [RFC5880].

7.3.2.  Transmission of S-BFD Control Packet by SBFDInitiator

   Contents of S-BFD control packets transmitted sent by an SBFDInitiator MUST NOT have
   control plane independent (C) be
   set as per Section 6.8.7 of [RFC5880].  There are few fields which
   needs to be set differently from [RFC5880] as follows:

      Demand (D)

         D bit set.

7.7. is used to identify S-BFD packet originated from
         SBFDInitiator and is always set to 1.

      Your Discriminator

         Set to bfd.RemoteDiscr. bfd.RemoteDiscr is set to discriminator
         value of remote entity.  It MAY be learnt from routing
         protocols or configured locally.

      Required Min RX Interval

         Set to 0.

      Required Min Echo RX Interval

         Set to 0.

7.3.3.  Additional SBFDInitiator Behaviors

   o  If the SBFDInitiator receives a valid S-BFD control packet in
      response to transmitted S-BFD control packet to a remote entity,
      then the SBFDInitiator SHOULD conclude that S-BFD control packet
      reached the intended remote entity.

   o  When a sufficient number of S-BFD packets have not arrived as they
      should, the SBFDInitiator SHOULD declare loss of reachability to
      the remote entity.  The criteria for declaring loss of
      reachability and the action that would be triggered as a result
      are outside the scope of this document.

   o  Relating to above bullet item, it is critical for an
      implementation to understand the latency to/from the reflector BFD
      session on the responder.  In other words, for very first S-BFD
      packet transmitted by the SBFDInitiator, an implementation MUST
      NOT expect response S-BFD packet to be received for time
      equivalent to sum of latencies: initiator to responder and
      responder back to initiator.

   o  If the SBFDInitiator receives an S-BFD control packet with Demand
      (D) bit set, the packet MUST be discarded.

7.8.  Additional SBFDReflector Behaviors

   o  S-BFD control packets transmitted by the SBFDReflector MUST have
      "Required Min RX Interval" set to a

7.4.  Diagnostic Values

   Diagnostic value which expresses how many
      incoming S-BFD control packets this SBFDReflector can handle.  The
      SBFDReflector can control how fast SBFInitiators will in both directions MAY be sending
      S-BFD control packets set to self by ensuring "Required Min RX
      Interval" indicates a value based on the current load.

   o  If the SBFDReflector wishes certain value, to
   attempt to communicate further information to some or all
      SBFDInitiators that monitored local entity is "temporarily out both ends.
   Implementation MAY use already existing diagnostic values defined in
   Section 4.1 of
      service", then S-BFD control packets with "state" set to ADMINDOWN
      are sent to those SBFDInitiators.  The SBFDInitiators, upon
      reception [RFC5880].  However, details of such packets, MUST NOT conclude loss are outside the
   scope of reachability
      to corresponding remote entity, and MUST back off packet
      transmission interval for this specification.

7.5.  The Poll Sequence

   Poll sequence MAY be used in both directions.  The Poll sequence MUST
   operate in accordance with [RFC5880].  An SBFDReflector MAY use the remote entity
   Poll sequence to slow down that rate at which S-BFD control packets
   are generated from an interval no
      faster than 1 second.  If SBFDInitiator.  This is done by the
   SBFDReflector is generating a
      response using procedures described in Section 7.2.3 and setting
   the Poll (P) bit in the reflected S-BFD control packet for a local entity that packet.  The
   SBFDInitiator is in
      service, to then "state" in response BFD send the next S-BFD control packets MUST be set
      to UP.

   o packet with the
   Final (F) bit set.  If an SBFDReflector receives an S-BFD control
   packet with Demand
      (D) Poll (P) bit cleared, set, then the packet SBFDReflector MUST be discarded. respond
   with an S-BFD control packet with Poll (P) bit cleared and Final (F)
   bit set.

8.  Scaling Aspect

   This mechanism brings forth one noticeable difference in terms of
   scaling aspect: number of SBFDReflector.  This specification
   eliminates the need for egress nodes to have fully active BFD
   sessions when only one side desires to perform continuity tests.
   With introduction of reflector BFD concept, egress no longer is
   required to create any active BFD session per path/LSP/function
   basis.  Due to this, total number of BFD sessions in a network is
   reduced.

9.  Co-existence with Classical BFD Sessions

   Initial packet demultiplexing requirement is described in
   Section 7.1.  Because of this, S-BFD mechanism can co-exist with
   classical BFD sessions.

10.  S-BFD Echo Function

   The concept of the S-BFD Echo function is similar to the BFD Echo
   function described in [RFC5880].  S-BFD echo packets have the
   destination of self, thus S-BFD echo packets are self-generated and
   self-terminated after traversing a link/path.  S-BFD echo packets are
   expected to u-turn on the target node in the data plane and MUST NOT
   be processed by any reflector BFD sessions on the target node.

   When using the S-BFD Echo function, it is RECOMMENDED that:

   o  Both S-BFD control packets and S-BFD echo packets be sent.

   o  Both S-BFD control packets and S-BFD echo packets have the same
      semantics in the forward direction to reach the target node.

   In other words, it is not preferable to send just S-BFD echo packets
   without also sending S-BFD control packets.  There are two reasons
   behind this suggestion:

   o  S-BFD control packets can verify the reachability to intended
      target node, which allows one to have confidence that S-BFD echo
      packets are u-turning on the expected target node.

   o  S-BFD control packets can detect when the target node is going out
      of service (i.e., via receiving back ADMINDOWN state).

   The usage of the "Required Min Echo RX Interval" field is described
   in Section 7.2.2 7.3.2 and Section 7.3.2. 7.2.2.  Because of the stateless nature
   of SBFDReflector sessions, a value specified the "Required Min Echo
   RX Interval" field in both directions is not very meaningful. meaningful at SBFDReflector.  Thus it
   is RECOMMENDED that the "Required Min Echo RX Interval" field simply
   be set to zero in both directions. from SBFDInitiator.  SBFDReflector MAY set to
   appropriate value to control the rate at which it wants to receives
   SBFD echo packets.

   Following aspects of S-BFD Echo functions are left as implementation
   details, and are outside the scope of this document:

   o  Format of the S-BFD echo packet (e.g., data beyond UDP header).

   o  Procedures on when and how to use the S-BFD Echo function.

11.  Security Considerations

   Same security considerations as [RFC5880], [RFC5881], [RFC5883],
   [RFC5884] and [RFC5885] [RFC5880] apply to this document.
   Additionally, implementing the following measures will strengthen
   security aspects of the mechanism described by this document:

   o  SBFDInitiator MAY pick crypto a sequence number to be set in "sequence
      Number" in authentication section based on authentication mode
      configured.

   o  SBFDReflector MUST NOT look at the crypto sequence number before
      accepting the packet.

   o  SBFDReflector MAY look at the Auth Key ID
      [I-D.ietf-bfd-generic-crypto-auth] in the incoming packet
      and verify the authentication data.

   o  SBFDReflector MUST accept the packet if authentication is
      successful.

   o  SBFDReflector MUST compute the Authentication data and MUST use
      the same sequence number that it received in the S-BFD control
      packet that it is responding to.

   o  SBFDInitiator MUST SHOULD accept the S-BFD control packet if it either
      comes with the same sequence
      number as it had sent or it's within permissible window.  One potential approach is the window that it finds acceptable (described in detail
      procedure explained in
      [I-D.ietf-bfd-generic-crypto-auth]) [I-D.ietf-bfd-generic-crypto-auth].

   Using the above method,

   o  SBFDReflector continue to remain stateless despite using security.

   o  SBFDReflector are not susceptible to replay attacks as they always
      respond to S-BFD control packets irrespective of the sequence
      number carried.

   o  An attacker cannot impersonate the responder since the
      SBFDInitiator will only accept S-BFD control packets that come
      with the sequence number that it had originally used when sending
      the S-BFD control packet.

   Considerations about loop problems are covered in Appendix A.

12.  IANA Considerations

   No action is required by IANA for this document.

13.  Acknowledgements

   Authors would like to thank Jeffrey Haas, Greg Mirsky and Mirsky, Marc
   Binderberger
   Binderberger, and Alvaro Retana for performing thorough reviews and
   providing number of suggestions.  Authors would like to thank Girija
   Raghavendra Rao, Les Ginsberg, Srihari Raghavan, Vanitha Neelamegam
   and Vengada Prasad Govindan from Cisco Systems for providing valuable
   comments.  Authors would also like to thank John E.  Drake and Pablo
   Frank for providing comments and suggestions.

14.  Contributing Authors

   Tarek Saad
   Cisco Systems
   Email: tsaad@cisco.com

   Siva Sivabalan
   Cisco Systems
   Email: msiva@cisco.com

   Nagendra Kumar
   Cisco Systems
   Email: naikumar@cisco.com

   Mallik Mudigonda
   Cisco Systems
   Email: mmudigon@cisco.com

   Sam Aldrin
   Google
   Email: aldrin.ietf@gmail.com

15.  References

15.1.  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. 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010.

   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, June
              2010.

   [RFC5883]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for Multihop Paths", RFC 5883, June 2010.

   [RFC5884]  Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
              "Bidirectional Forwarding Detection (BFD) for MPLS Label
              Switched Paths (LSPs)", RFC 5884, June 2010. 2010,
              <http://www.rfc-editor.org/info/rfc5880>.

15.2.  Informative References

   [I-D.ietf-bfd-generic-crypto-auth]
              Bhatia, M., Manral, V., Zhang, D., and M. Jethanandani,
              "BFD Generic Cryptographic Authentication", draft-ietf-
              bfd-generic-crypto-auth-06 (work in progress), April 2014.

   [I-D.ietf-bfd-multipoint]
              Katz, D., Ward, D., and J. Networks, "BFD for Multipoint
              Networks", draft-ietf-bfd-multipoint-06 (work in
              progress), May 2015.

   [I-D.ietf-bfd-seamless-use-case]
              Bhatia, M., Matsushima, S., Mirsky, G., and N. Kumar,
              "Seamless Bidirectional Forwarding Detection (BFD) Use
              Case", draft-ietf-bfd-seamless-use-case-02 draft-ietf-bfd-multipoint-07 (work in
              progress), April August 2015.

   [RFC5885]  Nadeau, T.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <http://www.rfc-editor.org/info/rfc791>.

   [RFC2460]  Deering, S. and C. Pignataro, "Bidirectional Forwarding
              Detection (BFD) for the Pseudowire Virtual Circuit
              Connectivity Verification (VCCV)", R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 5885, June 2010. 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <http://www.rfc-editor.org/info/rfc3031>.

Appendix A.  Loop Problem

   Consider a scenario where we have two nodes and both are S-BFD
   capable.

      Node A (IP 192.0.2.1) ----------------- Node B (IP 192.0.2.2)
                                    |
                                    |
                         Man in the Middle (MiM)

   Assume node A reserved a discriminator 0x01010101 for target
   identifier 192.0.2.1 and has a reflector session in listening mode.
   Similarly node B reserved a discriminator 0x02020202 for its target
   identifier 192.0.2.2 and also has a reflector session in listening
   mode.

   Suppose MiM sends a spoofed packet with MyDisc = 0x01010101, YourDisc
   = 0x02020202, source IP as 192.0.2.1 and dest IP as 192.0.2.2.  When
   this packet reaches Node B, the reflector session on Node B will swap
   the discriminators and IP addresses of the received packet and
   reflect it back, since YourDisc of the received packet matched with
   reserved discriminator of Node B.  The reflected packet that reached
   Node A will have MyDdisc=0x02020202 and YourDisc=0x01010101.  Since
   YourDisc of the received packet matched the reserved discriminator of
   Node A, Node A will swap the discriminators and reflects the packet
   back to Node B.  Since reflectors MUST must set the TTL of the reflected
   packets to 255, the above scenario will result in an infinite loop
   with just one malicious packet injected from MiM.

   FYI: Packet fields do not carry any direction information, i.e., if
   this is Ping packet or reply packet.

   Solutions

   The current proposals to avoid the loop problem are:

   o  Overload "D" bit (Demand mode bit): Initiator always sets the 'D'
      bit and reflector clears it.  This way we can identify if a
      received packet was a reflected packet and avoid reflecting it
      back.  However this changes the interpretation of 'D' bit.

   o  Use of State field in the BFD control packets: Initiator will
      always send packets with State set to DOWN and reflector will send
      back packets with state field set to UP.  Reflectors will never
      reflect any received packets with state as UP.  However the only
      issue is the use of state field differently i.e., state in the
      S-BFD control packet from initiator does not reflect the local
      state which is anyway not significant at reflector.

   o  Use of local discriminator as My Disc at reflector: Reflector will
      always fill in My Discriminator with a locally allocated
      discriminator value (not reserved discriminators) and will not
      copy it from the received packet.

Authors' Addresses

   Nobo Akiya
   Big Switch Networks

   Email: nobo.akiya.dev@gmail.com

   Carlos Pignataro
   Cisco Systems

   Email: cpignata@cisco.com

   Dave Ward
   Cisco Systems

   Email: wardd@cisco.com
   Manav Bhatia
   Ionos Networks

   Email: manav@ionosnetworks.com

   Santosh Pallagatti
   Juniper Networks

   Email: santoshpk@juniper.net santosh.pallagatti@gmail.com