Internet Engineering Task Force                                 N. Akiya
Internet-Draft                                              C. Pignataro
Updates: 5880 (if approved)                                      D. Ward
Intended status: Standards Track                           Cisco Systems
Expires: December 14, 28, 2014                                     M. Bhatia
                                                          Alcatel-Lucent
                                                          Ionos Networks
                                                           P. K. Santosh
                                                        Juniper Networks
                                                           June 12, 26, 2014

          Seamless Bidirectional Forwarding Detection (S-BFD)
                    draft-ietf-bfd-seamless-base-00
                    draft-ietf-bfd-seamless-base-01

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 14, 28, 2014.

Copyright Notice

   Copyright (c) 2014 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.  Seamless BFD Overview  Terminology . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . .   3
   3.  Seamless BFD Overview . . . . . . . . . . . . . . . . . . . .   4
   4.  BFD Target Identifier Types . . . . . . . . . . . . . . . . .   5
   5.  S-BFD UDP Port  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.
   5.  S-BFD Discriminators  . . . . . . . . . . . . . . . . . . . .   5
   7.
   6.  Reflector BFD Session . . . . . . . . . . . . . . . . . . . .   7
   8.   6
   7.  State Variables . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.
     7.1.  New State Variables . . . . . . . . . . . . . . . . . . .   7
     8.2.
     7.2.  State Variable Initialization and Maintenance . . . . . .   8
   9.  Full Reachability Validations . . . .   7
   8.  S-BFD Procedures  . . . . . . . . . . . .   8
     9.1.  Initiator Behavior . . . . . . . . . .   7
     8.1.  Initiator Procedures  . . . . . . . . .   8
       9.1.1.  Initiator State machine . . . . . . . . .   7
       8.1.1.  SBFDInitiator State Machine . . . . . .   9
     9.2.  Responder Behavior . . . . . . .   8
       8.1.2.  Details of S-BFD Packet Sent by SBFDInitiator . . . .   9
     8.2.  Responder Procedures  . . . . . . . .  10
       9.2.1.  Responder Demultiplexing . . . . . . . . . .   9
       8.2.1.  Responder Demultiplexing  . . . .  10
       9.2.2.  Reflector BFD Session Procedures . . . . . . . . . .  10
     9.3.  Further Packet
       8.2.2.  Details of S-BFD Packet Sent by SBFDReflector . . . . . . . . . . . . . . . . .  12
     9.4.  10
     8.3.  Diagnostic Values . . . . . . . . . . . . . . . . . . . .  12
     9.5.  10
     8.4.  The Poll Sequence . . . . . . . . . . . . . . . . . . . .  13
     9.6.  11
     8.5.  Control Plane Independent (C) . . . . . . . . . . . . . .  13
     9.7.  11
     8.6.  Additional Initiator Behavior . . . SBFDInitiator Behaviors  . . . . . . . . . . .  13
     9.8.  11
     8.7.  Additional Responder Behavior . . SBFDReflector Behaviors  . . . . . . . . . . . .  13
   10. Partial Reachability Validations  . . . . . . . . . . . . . .  14
   11.  12
   9.  Scaling Aspect  . . . . . . . . . . . . . . . . . . . . . . .  14
   12.  12
   10. Co-existence with Traditional BFD . . . . . . . . . . . . . .  15
   13.  12
   11. BFD Echo  . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   14.  12
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  15
   15.  13
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   16.  14
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   17.  14
   15. Contributing Authors  . . . . . . . . . . . . . . . . . . . .  16
   18.  14
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     18.1.  15
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     18.2.  15
     16.2.  Informative References . . . . . . . . . . . . . . . . .  17  15

   Appendix A.  Loop Problem . . . . . . . . . . . . . . . . . . . .  18  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19  17

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 document
   extends BFD to provide solutions to use cases listed in
   [I-D.ietf-bfd-seamless-use-case].  Because defined

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

2.  Seamless BFD Overview

   Each  Terminology

   The reader is expected to be familiar with the BFD, IP and MPLS
   terminologies and protocol instance (e.g.  OSPF/IS-IS) allocates one or more BFD
   discriminators on its network node, ensuring that constructs.  This section describes
   several new terminologies introduced by S-BFD.

   o  S-BFD - Seamless BFD.

   o  S-BFD packet - a BFD discriminators
   allocated are unique within control packet on the well-known S-BFD port.

   o  Entity - a function on a network domain.  Allocated BFD
   discriminators may be advertised by the protocol.  Required result is node that a protocol possess the knowledge of mapping between S-BFD mechanism allows
      remote network
   targets nodes to BFD discriminators.  Each perform connectivity test to.  An entity
      can be abstract (ex: reachability) or specific (ex: IP addresses,
      router-IDs, functions).

   o  SBFDInitiator - an S-BFD session on a network nodes will also create node that performs a
   BFD
      connectivity test to a remote entity by sending S-BFD packets.

   o  SBFDReflector - an S-BFD session instance on a network node that listens
      for incoming BFD control S-BFD packets
   with "your discriminator" having protocol allocated values.  The
   listener BFD session instance, upon receiving a BFD control packet
   targeted to one of local entities and generates
      response S-BFD packets.

   o  Reflector BFD session - synonymous with SBFDReflector.

   o  S-BFD discriminator values, will transmit - a
   response BFD control packet back to the sender.

   Once above setup discriminator allocated for a local
      entity and is complete, any network node, understanding the
   mapping between network targets to BFD discriminators, can quickly
   perform reachability check to these network targets being listened by simply sending an SBFDReflector.

   o  BFD control packets with known discriminator - a BFD discriminator value as "your
   discriminator".

   For example:

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

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

                    Figure 1:
    | +-----------------+ |                | +-----------------+ |
    | |  SBFDInitiator  |--- S-BFD for IS-IS Network

   IS-IS with packet -->|  SBFDReflector  | |
    | | +-------------+ | |                | | +-------------+ | |
    | | | BFD discrim | | |                | | |S-BFD discrim| | |
    | | +-------------+ |<-- S-BFD packet ---| +----------^--+ | |
    | +-----------------+ |                | +------------|----+ |
    |                     |                |              |      |
    |                     |                |          +---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 (ex: OSPF/IS-IS).  Required result is that applications,
   on other network nodes, possess the knowledge of the mapping from
   remote entities to S-BFD discriminators.  The reflector BFD session
   is to, upon receiving an S-BFD packet targeted to one of local S-BFD
   discriminator values, transmit a response S-BFD packet back to the
   initiator.

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

   For example:

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

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

             Figure 2: S-BFD for IS-IS Network

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

   Note that

4.  S-BFD UDP Port

   S-BFD functions on a protocol well-known UDP port: TBD1.

5.  S-BFD Discriminators

   Locally allocated S-BFD discriminator values for entities may create an explicit mapping between a be
   arbitrary allocated or derived from values provided by applications.
   These values may be protocol ID (e.g. IDs (ex: System-ID, Router-ID) to a BFD discriminator.  A
   protocol may also create an explicit mapping between a or
   network target
   (e.g. targets (ex: IP address) to a BFD discriminator.  A protocol may even
   function with implicit mapping address).  To minimize the collision of
   discriminator values between a network target (e.g.  IPv4
   address) to a BFD discriminator, i.e.  IPv4 address and S-BFD, it is used as BFD RECOMMENDED that
   discriminator value.  Decisions pool be separate for BFD and rules on how protocols allocate S-BFD.  Even when
   employing 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 distribute BFD discriminators algorithms to
   derive S-BFD discriminator values.  If the two applications are using
   S-BFD for a same purpose (ex: 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.

3.  Terminology

   The reader

   One important characteristics of an S-BFD discriminator is expected to be familiar with the BFD, IP, MPLS and SR
   terminology and protocol constructs.  This section describes several
   new terminology introduced by Seamless BFD.

   o  BFD Target Identifier: Network entity that is provisioned as it
   MUST be unique within an administrative domain.  If multiple network
   nodes allocated a
      target of Seamless BFD.

   o  BFD Target Identifier Type: Type of same S-BFD discriminator value, then S-BFD packets
   falsely terminating on a wrong network entity that is
      provisioned as node can result in a target of Seamless BFD.

   o  BFD Target Identifier Table: A table containing BFD target
      identifier type, BFD target identifier and corresponding BFD
      discriminator.

   o  Reflector BFD Session: A reflector
   BFD session listening 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 incoming the reflector BFD
      control session to
   require demultiplexing of incoming S-BFD packets destined for with combination of
   destination IP address and "your discriminator".  Then S-BFD
   discriminator only has to be unique within a local BFD target identifier(s).

4.  BFD Target Identifier Types

   This document defines node.  However,
   S-BFD is a generic mechanism where network nodes can
   send BFD control packets to specific network targets to perform
   various tasks.  One task is defined to perform a reachability check (i.e
   requesting immediate response back).  Details run on wide range of this task is further
   defined in sections
   environments: IP, MPLS, etc.  For other transports like MPLS, because
   of the need to follow.  Further tasks (i.e. using use non-routable IP destination address, it is not
   possible for reflector BFD control
   packet session to request specific services from specific network nodes) demultiplex using IP
   destination address.  With PHP, there may not be defined.  Therefore, this document defines any incoming label
   stack to aid in demultiplexing either.  Thus, S-BFD imposes a code point for BFD
   Target Identifier.  Each locally allocated
   requirement that S-BFD discriminator discriminators MUST be associated to unique within an
   administrative domain.

6.  Reflector BFD Target Identifier type, to allow demultiplexing
   to a specific task Session

   Each network node creates one or service. more reflector BFD Target Identifier types:

         Value    BFD Target Identifier Type
        ------    --------------------------
             0    Reserved
             1    Network Target Discriminator

   Procedures defined sessions.  This
   reflector BFD session is a session which transmits S-BFD packets in this document are
   response to be associated received S-BFD packets with BFD
   Target Identifier Type 1 (Network Target Discriminator).

   Note that IP based BFD from [RFC5885] is supported by "your discriminator" having
   S-BFD discriminators allocated for local entities.  Specifically,
   this
   specification, but non-IP based reflector BFD session is outside the scope of this
   document.

   Further identifier types are to be defined as needed basis.

5.  UDP Port have following characteristics:

   o  MUST NOT transmit any S-BFD functions packets based on a well-known UDP port: TBD1.

6.  S-BFD Discriminators

   Protocols (i.e. client of S-BFD) may request local timer expiry.

   o  MUST transmit an arbitrary BFD
   discriminator value, or protocols may request a specific BFD
   discriminator value.  Therefore, it is RECOMMENDED for
   implementations S-BFD packet in response to create a separate discriminator pool for received S-BFD
   sessions to minimize the collision between existing BFD sessions and
      packet having a valid S-BFD sessions.  In such case, incoming BFD control packets MUST be
   demultiplexed first with UDP port to identify the discriminator table
   to look up the session.  Regardless of in the approach, collision can
   happen with following scenarios. "your
      discriminator" field, unless prohibited by local policies (ex:
      administrative, security, rate-limiter, etc).

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

   One reflector BFD session already using a discriminator value that
      collides with specific discriminator value requested may be responsible for handling received
   S-BFD
      session.

      *  Implementation SHOULD allow migrating existing packets targeted to all locally allocated S-BFD discriminators,
   or few reflector BFD sessions to
         free up the discriminator to accommodate specific discriminator
         value requested may each be responsible for subset of
   locally allocated S-BFD session.

   o  S-BFD session already using discriminators.  This policy is a discriminator value, arbitrarily
      allocated, local
   matter, and is outside the scope of this document.

   Note that collides with specific discriminator value
      requested for incoming S-BFD session.  The two packets may be IPv4, IPv6 or MPLS based.
   How such S-BFD sessions are of
      different packets reach an appropriate reflector BFD Target Identifier type.

      *  Protocol requesting arbitrary discriminator value MUST support
         migrating to another discriminator value, session is
   also a local matter, and implementations
         MUST allow migrating existing S-BFD sessions to free up is outside the
         discriminator to accommodate specific discriminator value
         requested for S-BFD session.

   o  S-BFD session already using a discriminator value, arbitrary
      allocated, that collides with specific discriminator value
      requested for S-BFD session.  The two scope of this document.

7.  State Variables

   S-BFD sessions are introduces new state variables, and modifies the usage of same
      BFD Target Identifier type.

      *  No action
   existing ones.

7.1.  New State Variables

   A new state variable is required, as the two can share added to the discriminator.

   One important characteristics base specification in support of
   S-BFD.

   o  bfd.SessionType: The type of this session.  Allowable values are:

      *  SBFDInitiator - an S-BFD discriminator is that it MUST
   be network wide unique.  If multiple network nodes allocated same
   S-BFD discriminator value, then S-BFD control packets falsely
   terminating session on a wrong network node can result in reflector BFD
   session (described in Section 7) to generate that
         performs a response back, due connectivity test to
   "your discriminator" matching.  This is clearly not desirable.  If
   only IP based a target entity by sending
         S-BFD is concerned, then it is possible for packets.

      *  SBFDReflector - an S-BFD
   reflector session to require demultiplexing of on a network node that listens
         for incoming S-BFD control
   packet with combination of destination IP address packets to local entities and "your
   discriminator".  Then generates
         response S-BFD discriminator only has to packets.

   bfd.SessionType variable MUST be unique
   within a local node.  However, initialized to the appropriate type
   when an S-BFD session is a generic mechanism created.

7.2.  State Variable Initialization and Maintenance

   Some state variables defined
   to run on wide range of environments: IP, MPLS, Segment Routing
   ([I-D.previdi-filsfils-isis-segment-routing]), etc.  For other
   transports like MPLS, because in section 6.8.1 of the BFD base
   specification need to use non-routable IP
   destination address, it is not possible for S-BFD reflector be initialized or manipulated differently
   depending on the session
   to demultiplex using IP destination address.  With PHP, there may not type.  Ed-Note: Anything else?.

   o  bfd.DemandMode: This variable MUST be any incoming label stack initialized to aid in demultiplexing either.  Thus,
   S-BFD imposes a requirement that S-BFD discriminators 1 for session
      type SBFDInitiator, and MUST be network
   wide unique.

7.  Reflector BFD Session

   Each network node MUST create one or more reflector BFD sessions.
   This reflector BFD session is a initialized to 0 for session which transmits BFD control type
      SBFDReflector.

8.  S-BFD Procedures

8.1.  Initiator Procedures

   S-BFD packets in response transmitted by an SBFDInitiator MUST set "your
   discriminator" field to received valid locally destined BFD control
   packets.  Specifically, this reflector BFD session is an S-BFD discriminator corresponding to have
   following characteristics:

   o  MUST NOT transmit any BFD control the
   remote entity.

   S-BFD packets based on local timer
      expiry.

   o transmitted by an SBFDInitiator MUST transmit BFD control packet in response NOT set "my
   discriminator" field to a received valid
      locally destined BFD control packet.

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

   One reflector BFD session MAY be responsible an S-BFD discriminator allocated for handling received
   BFD control packets targeted to all a local BFD target identifiers, or
   few reflector BFD sessions MAY each be responsible for subset of
   entity (and is being monitored by a local BFD target identifiers. SBFDReflector).  This policy is to
   prevent incoming response S-BFD packets, from a remote SBFDReflector,
   having "your discriminator" as a S-BFD discriminator of a local matter, and
   entity.  Every SBFDInitiator is
   outside the scope of this document.

   Note that incoming BFD control packets destined to BFD target
   identifier types may have a unique "my discriminator",
   and SHOULD be IPv4, IPv6 or MPLS based.  For those allocated from the BFD
   target identifier types, implementations MAY either allow discriminator pool if the same
   reflector BFD session to handle all incoming BFD control packets in
   address family agnostic fashion, or setup multiple reflector BFD
   sessions to handle incoming BFD control packets with different
   address families.  This policy is again a local matter, and is
   outside
   implementation employs the scope approach of this document.

8.  State Variables

   S-BFD introduces some new state variables, having separate discriminator
   pools for BFD and modifies the usage of
   existing ones.

8.1.  New State Variables

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

   o  bfd.SessionType: The type

   Below ASCII art describes high level concept of this session.  Allowable values are:

      *  SBFDInitiator: Any session on a connectivity test
   using S-BFD.  R2 allocates XX as the S-BFD discriminator for its
   network node that attempts reachability purpose, and advertises XX to
         perform neighbors.  ASCII
   art shows R1 and R4 performing a path monitoring connectivity test to any 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 target identifier session on other R2.
   === Links connecting network nodes.

      *  SBFDReflector: Any
   --- S-BFD packet traversal.

             Figure 3: S-BFD Connectivity Test

8.1.1.  SBFDInitiator State Machine

   An SBFDInitiator may be a persistent session on a network node, which receives
         BFD control packets transmitted by an initiator and responds
         back to the initiator is referred as responder.

   This variable MUST with a
   timer for S-BFD packet transmissions.  An SBFDInitiator may also be initialized to the appropriate type when a
   module, a script or a tool on the
   session is created, according to initiator that transmits one or
   more S-BFD packets "when needed".  For transient SBFDInitiators, the rules
   BFD state machine described in section TBD.

8.2.  State Variable Initialization [RFC5880] may not be applicable.  For
   persistent SBFDInitiators, the states and Maintenance

   Some the state variables defined machine described
   in section 6.8.1 of the BFD base
   specification 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.

9.  Full Reachability Validations

9.1.  Initiator Behavior

   Any network node can attempt to perform a full reachability
   validation to any BFD target identifier on other network nodes, as
   long as destination BFD target identifier is provisioned to use this
   mechanism.  BFD control packets transmitted by the initiator is to
   have "your discriminator" corresponding to destination BFD target
   identifier.

   A node that initiates a BFD control packet MAY create an active BFD
   session to periodically send BFD control packets to a target, or a
   BFD control packet MAY be crafted and sent out on "as needed basis"
   (ex: BFD ping) without any session presence.  In both cases, a BFD
   instance MUST have a unique "my discriminator" value assigned.  If a
   node is to create multiple BFD instances to the same BFD target
   identifier, then each instance MUST have separate "my discriminator"
   values assigned.  A BFD instance MUST NOT use a discriminator
   corresponding to one of local BFD target identifiers as "my
   discriminator".  This is to prevent incoming response BFD control
   packets ("pong" packets) having "your discriminator" as a
   discriminator corresponding to the local BFD target identifier.

   Below ASCII art describes high level concept of full reachability
   validations using this mechanism.  R2 reserves value XX as BFD
   discriminator for its BFD target identifier.  ASCII art shows that R1
   and R4 performing full reachability validation to XX on R2.

    -- md=50/yd=XX (BFD ping) -->
   <-- md=XX/yd=50 (BFD pong) --

                             [*]
    R1 ---------------------- R2 ----------- R3 ----------- R4

                             |  ^
                             |  |
                             |  + - md=60/yd=XX (BFD ping) --
                             + - - -md=XX/yd=60 (BFD pong) -->

   [*] Reflector BFD session on R2.

                    Figure 2: S-BFD path monitoring

   If BFD control packet is to be sent via IP path, then:

   o  Destination IP address MUST be an IP address corresponding to
      target identifier.
   o  Source IP address MUST be a local IP address.
   o  IP TTL MUST be 255 for full reachability validations.  Partial
      reachability validations MAY use smaller TTL value (see
      Section 10).
   o  Well-known UDP destination port(s) for IP based S-BFD.

   If BFD control packet response is determined to explicitly be label
   switched, then:

   o  BFD control packet MUST get imposed with a label stack that is
      expected to reach the target node.
   o  MPLS TTL MUST be 255 for full reachability validations.  Partial
      reachability validations MAY use smaller TTL value (see
      Section 10).
   o  Destination IP address MUST be 127/8 for IPv4 and
      0:0:0:0:0:FFFF:7F00/104 for IPv6.
   o  Source IP address MUST be a local IP address.
   o  IP TTL=1.
   o  Well-known UDP destination port(s) for MPLS based S-BFD

9.1.1.  Initiator State machine

   The following diagram provides an overview of the initiator state
   machine.  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.

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

                    Figure 3: S-BFD Initiator FSM

   Note that the above [RFC5880] will function but are more than necessary.  The
   following diagram provides an optimized state machine is different from the base BFD
   specification[RFC5880].  This is because the Init state is no longer
   applicable for the initiator of the S-BFD session.  Another important
   difference is the transition of the state machine from the Down state
   to the Up state when a packet with State Up is received by the
   initiator. persistent
   SBFDInitiators.  The definitions of the states and the events have the
   same meaning as in the base BFD specification [RFC5880].

9.2.  Responder Behavior

   A network node which receives BFD control packets transmitted by an
   initiator is referred as responder.  Responder, upon reception of BFD
   control packets, is to perform necessary relevant validations
   described in [RFC5880]/[RFC5881]/[RFC5883]/[RFC5884]/[RFC5885].

9.2.1.  Responder Demultiplexing

   When responder receives a BFD control packet, if "your discriminator"
   value is not one of local entries in the BFD target identifier table,
   then this packet MUST NOT be considered for this mechanism.  If "your
   discriminator" value is one of local entries in the BFD target
   identifier table, then the packet is determined to be handled by a
   reflector BFD session responsible for specified BFD targeted
   identifier.  If the packet was determined to be processed further for
   this mechanism, then chosen reflector BFD session is to transmit a
   response BFD control packet using procedures described in
   Section 9.2.2, unless prohibited by local administrative or local
   policy reasons.

9.2.2.  Reflector BFD Session Procedures

   BFD target identifier type MUST be used to determine further
   information notation on how to reach back to each arc represents the initiator.

   In addition, destination IP address state of received BFD control packet
   MUST be examined to determine how to construct response BFD control
   packet to send back to the initiator.

   If destination IP address of
   SBFDInitiator (as received BFD control packet is not 127/8
   for IPv4 in the State field in the S-BFD packet) or 0:0:0:0:0:FFFF:7F00/104 for IPv6, then:

   o  Destination IP address MUST be copied from received source IP
      address.
   o  Source IP address MUST be copied from received destination IP
      address if received destination IP address is a local address.
      Otherwise local IP address MUST be used.
   o  IP TTL MUST be 255.

   Response BFD control packet SHOULD be IP routed back, but MAY
   explicitly be label switched.

   If BFD control packet response is determined to be IP routed, then:

   o  Destination IP address MUST be copied from received source IP
      address.
   o  Source IP address MUST be a local address.
   o  IP TTL MUST be 255.

   If BFD control packet response is determined to explicitly be label
   switched, then:

   o  BFD control packet MUST get label switched back to
   indicates the initiator.
      Determining expiration of the label stack to be imposed on a response BFD
      control packet Detection Timer.

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

             Figure 4: SBFDInitiator FSM

   Note that the above state machine is outside different from the scope of this document.
   o  MPLS TTL MUST be 255.
   o  Destination IP address MUST be 127/8 for IPv4 and
      0:0:0:0:0:FFFF:7F00/104 for IPv6.
   o  Source IP address MUST be a local IP address.
   o  IP TTL MUST be 1.

   Regardless base BFD
   specification[RFC5880].  This is because the Init state is no longer
   applicable for the SBFDInitiator.  Another important difference is
   the transition of the response type, BFD control state machine from the Down state to the Up
   state when a packet being sent with State Up is received by the
   responder MUST perform following procedures:

   o  Copy "my discriminator" from received "your discriminator", initiator.  The
   definitions of the states and
      "your discriminator" from received "my discriminator".
   o  UDP destination port MUST be the events have the same meaning as received UDP destination
      port.

9.3.  Further Packet in
   the base BFD specification [RFC5880].

8.1.2.  Details

   Further details of BFD control S-BFD Packet Sent by SBFDInitiator

   S-BFD packets sent by initiator (ex: active
   BFD session): an SBFDInitiator is to have following contents:

   o  Well-known UDP destination port assigned for S-BFD.
   o  UDP source port as per described in
      [RFC5881]/[RFC5883]/[RFC5884]/[RFC5885]. [RFC5881], [RFC5883],
      [RFC5884] and [RFC5885].
   o  "my discriminator" assigned by local node.
   o  "your discriminator" corresponding to an identifier of target
      node. a remote entity.
   o  "State" MUST be set to a value reflecting describing local state.
   o  "Desired Min TX Interval" MUST be set to a value reflecting describing local
      desired minimum transmit interval.
   o  "Required Min RX Interval" MUST be zero.
   o  "Required Min Echo RX Interval" SHOULD be zero.
   o  "Detection Multiplier" MUST be set to a value reflecting describing locally
      used multiplier value.
   o  "Demand  Demand (D) bit (D)" MUST be set set.

8.2.  Responder Procedures

   A network node which receives S-BFD packets transmitted by an
   initiator is referred as responder.  The responder, upon reception of
   S-BFD packets, is to perform necessary relevant validations described
   in [RFC5880], [RFC5881], [RFC5883], [RFC5884] and [RFC5885].

8.2.1.  Responder Demultiplexing

   A BFD control packet received by a resonder is considered an S-BFD
   packet if the initiator.

   Further details packet is on the well-known S-BFD port.  When a
   responder receives an S-BFD packet, if the value in the "your
   discriminator" field is not one of S-BFD discriminators allocated for
   local entities, then this packet MUST NOT be considered for this
   mechanism.  If the value in the "your discriminator" field is one of
   S-BFD discriminators allocated for local entities, then the packet is
   determined to be handled by a reflector BFD session responsible for
   the S-BFD discriminator.  If the packet was determined to be
   processed further for this mechanism, then chosen reflector BFD
   session is to transmit a response BFD control packet using procedures
   described in Section 8.2.2, unless prohibited by local policies (ex:
   administrative, security, rate-limiter, etc).

8.2.2.  Details of S-BFD Packet Sent by SBFDReflector

   S-BFD packets sent by responder (reflector
   BFD session): an SBFDReflector is to have following contents:

   o  Well-known UDP destination port assigned for S-BFD.
   o  UDP source port as described in
      [RFC5881]/[RFC5883]/[RFC5884]/[RFC5885]. [RFC5881], [RFC5883], [RFC5884]
      and [RFC5885].
   o  "my discriminator" MUST be copied from received "your
      discriminator".
   o  "your discriminator" MUST be copied from received "my
      discriminator".
   o  "State" MUST be UP or ADMINDOWN.  Clarification of reflector BFD
      session state is described in Section 9.8. 8.7.
   o  "Desired Min TX Interval" MUST be copied from received "Desired
      Min TX Interval".
   o  "Required Min RX Interval" MUST be set to a value reflecting describing how
      many incoming control packets this reflector BFD session can
      handle.  Further details are described in Section 8.7.
   o  "Required Min Echo RX Interval" SHOULD be set to zero.
   o  "Detection Multiplier" MUST be copied from received "Detection
      Multiplier".
   o  "Demand  Demand (D) bit (D)" MUST be cleared by the reflector.

9.4. cleared.

8.3.  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 of this specification.

9.5.

8.4.  The Poll Sequence

   Poll sequence MAY be used in both directions.  The Poll sequence MUST
   operate in accordance with [RFC5880].

9.6.  An SBFDReflector MAY use the
   Poll sequence to slow down that rate at which S-BFD packets are
   generated from an SBFDInitiator.  This is done by the SBFDReflector
   using procedures described in Section 8.7 and setting the Poll (P)
   bit in the reflected S-BFD packet.  The SBFDInitiator is to then send
   the next S-BFD packet with the Final (F) bit set.  If an
   SBFDReflector receives an S-BFD packet with Poll (P) bit set, then
   the SBFDReflector MUST respond with an S-BFD packet with Poll (P) bit
   cleared and Final (F) bit set.

8.5.  Control Plane Independent (C)

   Control plane independent (C) bit for BFD instances speaking an SBFDInitiator sending S-BFD
   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 BFD control S-BFD packets transmitted MUST
   have control plane independent (C) bit set.  If reflector BFD session
   implementation shares fate with control plane, then response BFD
   control S-BFD
   packets transmitted MUST NOT have control plane independent (C) bit
   set.

9.7.

8.6.  Additional Initiator Behavior SBFDInitiator Behaviors

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

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

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

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

9.8.

8.7.  Additional Responder Behavior SBFDReflector Behaviors

   o  BFD control  S-BFD packets transmitted by a reflector BFD session the SBFDReflector MUST have "Required
      Min RX Interval" set to a value which reflects expresses how many incoming control
      S-BFD packets this reflector BFD session SBFDReflector can handle.  Responder  The SBFDReflector
      can control how fast initiators SBFInitiators will be sending
      BFD control S-BFD packets
      to self by ensuring "Required Min RX Interval"
      reflects indicates a value
      based on the current load.

   o  If a reflector BFD session the SBFDReflector wishes to communicate to some or all
      initiators
      SBFDInitiators that monitored BFD target identifier local entity is "temporarily out of
      service", then BFD control S-BFD packets with "state" set to ADMINDOWN are
      sent to those initiators.  Initiators, SBFDInitiators.  The SBFDInitiators, upon reception
      of such packets, MUST NOT conclude loss of reachability connectivity to
      corresponding BFD target identifier, remote entity, and MUST back off packet transmission
      interval for the remote entity to corresponding BFD target identifier an interval no faster than 1
      second.  If a reflector BFD session the SBFDReflector is generating a response BFD control S-BFD
      packet for BFD target identifier a local entity that is in service, then "state" in
      response BFD control packets MUST be set to UP.

   o  If a reflector an SBFDReflector receives a an S-BFD packet with D Demand (D) bit
      cleared, the packet MUST be discarded.

10.  Partial Reachability Validations

   Same mechanism as described in "Full Reachability Validations"
   section will be applied with exception of following differences on
   initiator.

   o  When initiator wishes to perform a partial reachability validation
      towards identifier X upto identifier Y, number of hops to
      identifier Y is calculated.

   o  TTL value based on this calculation is used as the IP TTL or MPLS
      TTL on top most label, and "your discriminator" of transmitted BFD
      control packet will carry BFD discriminator corresponding to
      target transit identifier Y.

   o  Imposed label stack or IP destination address will continue to be
      of identifier X.

11.

9.  Scaling Aspect

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

   If traditional BFD technology was used on a network comprised of N
   nodes, and each node monitored M unidirectional paths/LSPs, then
   total number of BFD sessions in such network will be:

   (((N - 1) x M) x 2)

   Assuming that each network node creates one reflector BFD session to
   handle all local BFD target identifiers, then total number of BFD
   sessions in same scenario will be:

   (((N - 1) x M) + N)

12.

10.  Co-existence with Traditional BFD

   This mechanism has no issues being deployed with traditional BFDs
   ([RFC5881]/[RFC5883]/[RFC5884]/[RFC5885])
   ([RFC5881], [RFC5883], [RFC5884] and [RFC5885]) because BFD S-BFD
   discriminators which allow this mechanism to function are explicitly
   reserved and separate UDP port values are used with S-BFD.

13.

11.  BFD Echo

   BFD echo is outside the scope of this document.

14.

12.  Security Considerations

   Same security considerations as [RFC5880], [RFC5881], [RFC5883],
   [RFC5884] and [RFC5885] apply to this document.

   Additionally, implementing the following measures will strengthen
   security aspects of the mechanism described by this document.

   o  Implementations MUST provide filtering capability based on source
      IP addresses or source node segment IDs of received BFD control S-BFD packets: [RFC2827].

   o  Implementations MUST NOT act on received BFD control S-BFD packets containing
      Martian addresses as source IP addresses.

   o  Implementations MUST ensure that response S-BFD packets generated
      to the initiator by the SBFDReflector have a reachable target (ex:
      destination IP addresses or node
      segment IDs are reachable. address).

   o  Initiator  SBFDInitiator MAY pick crypto sequence number based on
      authentication mode configured.

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

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

   o  Reflector  SBFDReflector MUST accept the packet if authentication is
      successful.

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

   o  Initiator  SBFDInitiator MUST accept the S-BFD packet if it either comes with
      the same sequence number as it had sent or its it's within the window
      that it finds acceptable (described in detail in
      [I-D.ietf-bfd-generic-crypto-auth])

   Using the above method,

   o  Reflectors  SBFDReflector continue to remain stateless despite using security.

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

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

15.

13.  IANA Considerations

   BFD Target Identifier types:

         Value    BFD Target Identifier Type
        ------    --------------------------
             0    Reserved
             1    Network Target Discriminator

   New UDP port number, TBD1, will be

   A new value TBD1 is requested for S-BFD.

16. from the "Service Name and Transport
   Protocol Port Number Registry".  The requested registry entry is:

     Service Name (REQUIRED)
       s-bfd
     Transport Protocol(s) (REQUIRED)
       udp
     Assignee (REQUIRED)
       IESG <iesg@ietf.org>
     Contact (REQUIRED)
       BFD Chairs <bfd-chairs@tools.ietf.org>
     Description (REQUIRED)
       Seamless Bidirectional Forwarding Detection (S-BFD)
     Reference (REQUIRED)
       draft-ietf-bfd-seamless-base
     Port Number (OPTIONAL)
       TBD1 (Requesting 7784)

14.  Acknowledgements

   Authors would like to thank Jeffrey Haas for performing thorough
   reviews and providing number of suggestions.  Authors would like to
   thank Girija Raghavendra Rao, Marc Binderberger, Les Ginsberg,
   Srihari Raghavan, Vanitha Neelamegam and Vengada Prasad Govindan from
   Cisco Systems for providing valuable comments.

17.  Authors would also
   like to thank John E.  Drake for providing comments and suggestions.

15.  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
   Huawei Technologies
   Email: aldrin.ietf@gmail.com

18.

16.  References

18.1.

16.1.  Normative References

   [I-D.ietf-bfd-seamless-use-case]
              Aldrin, S., Bhatia, M., Mirsky, G., Kumar, N., and S.
              Matsushima, "Seamless Bidirectional Forwarding Detection
              (BFD) Use Case", draft-ietf-bfd-seamless-use-case-00 (work
              in progress), June 2014.

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

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

   [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.

18.2.

16.2.  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.previdi-filsfils-isis-segment-routing]
              Previdi, S., Filsfils, C., Bashandy, A., Horneffer, M.,
              Decraene, B., Litkowski, S., Milojevic, I., Shakir, R.,
              Ytti, S., Henderickx, W., and J. Tantsura, "Segment
              Routing with IS-IS Routing Protocol", draft-previdi-
              filsfils-isis-segment-routing-02 (work in progress), March
              2013.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

   [RFC5885]  Nadeau, T. and C. Pignataro, "Bidirectional Forwarding
              Detection (BFD) for the Pseudowire Virtual Circuit
              Connectivity Verification (VCCV)", RFC 5885, June 2010.

Appendix A.  Loop Problem

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

      Node A (IP 1.1.1.1) ---------------- 192.0.2.1) ----------------- Node B (IP 2.2.2.2) 192.0.2.2)
                                    |
                                    |
                         Man in the Middle (MiM)

   Assume node A reserved a discriminator 0x01010101 for target
   identifier 1.1.1.1 192.0.2.1 and has a reflector session in listening mode.
   Similarly node B reserved a discriminator 0x02020202 for its target
   identifier 2.2.2.2 192.0.2.2 and also has a reflector session in listening
   mode.

   Suppose MiM sends a spoofed packet with MyDisc = 0x01010101, YourDisc
   = 0x02020202, source IP as 1.1.1.1 192.0.2.1 and dest IP as 2.2.2.2. 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 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
   Cisco Systems

   Email: nobo@cisco.com

   Carlos Pignataro
   Cisco Systems

   Email: cpignata@cisco.com

   Dave Ward
   Cisco Systems

   Email: wardd@cisco.com

   Manav Bhatia
   Alcatel-Lucent
   Ionos Networks

   Email: manav.bhatia@alcatel-lucent.com manav@ionosnetworks.com

   Santosh
   Juniper Networks

   Email: santoshpk@juniper.net