Internet Engineering Task Force                                 N. Akiya
Internet-Draft                                              C. Pignataro
Updates: 5880 (if approved)                                      D. Ward
Intended status: Standards Track                           Cisco Systems
Expires: February 2, 24, 2015                                     M. Bhatia
                                                          Ionos Networks
                                                           P. K. Santosh
                                                           S. Pallagatti
                                                        Juniper Networks
                                                         August 1, 23, 2014

          Seamless Bidirectional Forwarding Detection (S-BFD)
                    draft-ietf-bfd-seamless-base-02
                    draft-ietf-bfd-seamless-base-03

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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 2, 24, 2015.

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
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   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
   4.  S-BFD Discriminators  . . . . . . . . . . . . . . . . . . . .   5
     4.1.  S-BFD Discriminator Pools Uniqueness  . . . . . . . . . . . . .   5
     4.2.  Discriminator Pools . . . . . .   5
     4.2.  S-BFD Discriminator Uniqueness . . . . . . . . . . . . .   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.  S-BFD Control Packet Demultiplexing . . . . . . . . . . . . . . .   8
     7.2.  Initiator Procedures  . . . . . . . . . . . . . . . . . .   8
       7.2.1.  SBFDInitiator State Machine . . . . . . . . . . . . .   9
       7.2.2.  Details of S-BFD Control Packet Sent by SBFDInitiator . . . .  10
     7.3.  Responder Procedures  . . . . . . . . . . . . . . . . . .  10
       7.3.1.  Responder Demultiplexing  . . . . . . . . . . . . . .  10  11
       7.3.2.  Details of S-BFD Control Packet Sent by SBFDReflector . . . .  11
     7.4.  Diagnostic Values . . . . . . . . . . . . . . . . . . . .  11
     7.5.  The Poll Sequence . . . . . . . . . . . . . . . . . . . .  11
     7.6.  Control Plane Independent (C) . . . . . . . . . . . . . .  11  12
     7.7.  Additional SBFDInitiator Behaviors  . . . . . . . . . . .  12
     7.8.  Additional SBFDReflector Behaviors  . . . . . . . . . . .  12
   8.  Scaling Aspect  . . . . . . . . . . . . . . . . . . . . . . .  13
   9.  Co-existence with Classical BFD Sessions  . . . . . . . . . .  13
   10. S-BFD Echo Function . . . . . . . . . . . . . . . . . . . . .  13
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  14
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   14. Contributing Authors  . . . . . . . . . . . . . . . . . . . .  15
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     15.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Appendix A.  Loop Problem . . . . . . . . . . . . . . . . . . . .  16  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

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].

   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, IP and MPLS
   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 destined to or sourced from for the S-BFD
      mechanism.

   o  S-BFD echo packet - a BFD echo packet for the well-known S-BFD port. 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 (ex: reachability) or specific (ex: 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 packet -->|  |---S-BFD ctrl pkt----->|  SBFDReflector  | |
    | | +-------------+ | |                | | |<--S-BFD ctrl pkt------| +-------------+ | |
    | | | BFD discrim | | |                |    | |S-BFD discrim| | |
    | | |             | |---S-BFD echo pkt---+  | |             | | |
    | | +-------------+ |<-- 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 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, having the knowledge
   of the mapping from a remote entity to an S-BFD discriminator, 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 IS-IS with SystemID xxx (node A) allocates an S-BFD discriminator
   123, and advertises the S-BFD discriminator 123 in an IS-IS TLV.  The
   IS-IS with SystemID yyy (node D) allocates an S-BFD discriminator
   456, and 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.

4.  S-BFD Discriminators

4.1.  S-BFD Discriminator Pools

   This document defines following suggestions for Uniqueness

   One important characteristics of an S-BFD discriminator
   management on SBFDInitiator and SBFDReflector sessions, to minimize
   the collision between required is that it
   MUST be unique within an administrative domain.  If multiple network
   nodes allocated a same S-BFD discriminators discriminator value, then S-BFD control
   packets falsely terminating on a local
   device.

   o  SBFDInitiator is to allocate wrong network node can result in a discriminator from the
   reflector BFD
      discriminator pool. session to generate a response back, due to "your
   discriminator" matching.  This is clearly not desirable.  If the system also supports classical BFD
      that runs on [RFC5880], only IP
   based S-BFD is considered, then it is possible for the reflector BFD discriminator pool SHOULD be
      shared by SBFDInitiator sessions and classical BFD sessions.

   o  SBFDReflector is
   session to allocate a discriminator from the require demultiplexing of incoming S-BFD
      discriminator pool.  The control packets
   with combination of destination IP address and "your discriminator".
   Then S-BFD discriminator pool SHOULD only has to be unique within a
      separate pool 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
   (ex: System-ID, Router-ID) or network targets (ex: 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 (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.

4.2.  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 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 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 application to demultiplex using IP
   destination address.  With PHP, there another
   S-BFD application, that may not be any incoming label
   stack using different values and algorithms
   to aid in demultiplexing either.  Thus, derive S-BFD imposes discriminator values.  If the two applications are
   using S-BFD for a
   requirement that same purpose (ex: network reachability), then the
   colliding S-BFD discriminators MUST discriminator value can be unique within an
   administrative domain. 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
      (ex: 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: The This is a variable introduced by
      [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 state variables defined 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.

7.  S-BFD Procedures

7.1.  S-BFD Control Packet Demultiplexing

   Received BFD control packet MUST first be demultiplexed with
   information from the lower layer (ex: destination UDP port,
   associated channel type).  If the packet is determined to be for an
   SBFDReflector, then the 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.
   If the packet is determined not to be for SBFDReflector, then the
   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
   the located session is a SBFDInitiator, then destination of the
   packet (i.e. destination IP address) SHOULD be validated to be for
   self.

   Details of the initial BFD control packet demultiplexing are
   described in relevant S-BFD data plane documents.

7.2.  Initiator Procedures

   S-BFD control packets transmitted by an SBFDInitiator MUST set "your
   discriminator" field to an S-BFD discriminator corresponding to the
   remote entity.

   Every SBFDInitiator MUST have a locally unique "my discriminator"
   allocated from the BFD discriminator pool.

   Below ASCII art describes high level concept of continuity test using
   S-BFD.  R2 allocates XX as the S-BFD discriminator for its network
   reachability purpose, and advertises XX to neighbors.  ASCII art
   shows R1 and R4 performing 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 BFD session on R2.
   === Links connecting network nodes.
   --- S-BFD control packet traversal.

             Figure 3: S-BFD Continuity Test

7.2.1.  SBFDInitiator State Machine

   An SBFDInitiator may be a persistent session on the initiator with a
   timer for S-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 S-BFD control
   packets "when needed" (stateless SBFDInitiator).  For stateless
   SBFDInitiators, a complete BFD state machine may not be applicable.
   For stateful SBFDInitiators, the states and the state machine
   described in [RFC5880] will not function due to SBFDReflector session
   only sending UP and ADMINDOWN states (i.e.  SBFDReflector 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 of the SBFDInitiator (as received in
   the State field in the S-BFD control 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 the 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 state machine from the DOWN state 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
   in the base BFD specification [RFC5880].

7.2.2.  Details of S-BFD Control Packet Sent by SBFDInitiator

   S-BFD control packets sent by an SBFDInitiator is to have following
   contents:

   o  "my discriminator" assigned by local node.
   o  "your discriminator" corresponding to a remote entity.
   o  "State" MUST be set to a value describing local state.
   o  "Desired Min TX Interval" MUST be set to a value 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 describing locally
      used multiplier value.
   o  Demand (D) bit MUST be set.

7.3.  Responder 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 to perform necessary relevant validations
   described in [RFC5880], [RFC5881], [RFC5883], [RFC5884] and
   [RFC5885].

7.3.1.  Responder Demultiplexing

   When a responder receives an S-BFD control 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 7.3.2, unless prohibited by
   local policies (ex: administrative, security, rate-limiter, etc).

7.3.2.  Details of S-BFD Control Packet Sent by SBFDReflector

   S-BFD control packets sent by an SBFDReflector is to have following
   contents:

   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 7.8.
   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 describing how
      many incoming control packets this reflector BFD session can
      handle.  Further details are described in Section 7.8.
   o  "Required Min Echo RX Interval" SHOULD be set to zero.
   o  "Detection Multiplier" MUST be copied from received "Detection
      Multiplier".
   o  Demand (D) bit MUST be cleared.

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 of 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
   Poll sequence to slow down that rate at which S-BFD control packets
   are generated from an SBFDInitiator.  This is done by the
   SBFDReflector using procedures described in Section 7.8 and setting
   the Poll (P) bit in the reflected S-BFD control packet.  The
   SBFDInitiator is to then send the next S-BFD control packet with the
   Final (F) bit set.  If an SBFDReflector receives an S-BFD control
   packet with Poll (P) bit set, then the SBFDReflector MUST respond
   with an S-BFD control packet with Poll (P) bit cleared 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 have control plane independent (C) bit set.
   If reflector BFD session implementation shares fate with control
   plane, then response S-BFD control packets transmitted MUST NOT have
   control plane independent (C) bit set.

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

   o  If the SBFDReflector wishes to communicate to some or all
      SBFDInitiators that monitored local entity is "temporarily out of
      service", then S-BFD control packets with "state" set to ADMINDOWN
      are sent to those SBFDInitiators.  The SBFDInitiators, upon
      reception of such packets, MUST NOT conclude loss of reachability
      to corresponding remote entity, and MUST 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 local entity that is in
      service, then "state" in response BFD control packets MUST be set
      to UP.

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

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], [RFC5880].  S-BFD echo packets have the
   destination of self, thus S-BFD echo packets are self-generated and self-
   terminated
   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 packets (with BFD control header) packets and S-BFD echo packets (implementation specific) 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 reason reasons
   behind this suggestion:

   o  S-BFD control packets can verify the reachability to intended
      target node, which allows one to conclude 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).

   Implementations MAY set

   The usage of the "Required Min Echo RX Interval" field to
   indicate the rate which SBFDInitiator is sending S-BFD Echo packets
   (in ping) or described
   in Section 7.2.2 and Section 7.3.2.  Because of the rate which stateless nature
   of SBFDReflector wants SBFDInitiators to
   send S-BFD Echo packets (in pong).  However, this is likely more than
   necessary for sessions, a value specified the S-BFD "Required Min Echo function to operate.  Therefore,
   RX Interval" field in both directions is not very meaningful.  Thus
   it is RECOMMENDED that the "Required Min Echo RX Interval" field
   simply be set to zero in both directions.

   Additionally, following

   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 echo packet (ex: 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] 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 sequence number 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 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 accept the S-BFD control packet if it either
      comes with the same sequence number as it had sent or 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  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.

12.  IANA Considerations

   No action is required by IANA for this document.

13.  Acknowledgements

   Authors would like to thank Jeffrey Haas, Greg Mirsky and Marc
   Binderberger 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
   Huawei Technologies
   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, 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.

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-04 (work in
              progress), August 2014.

   [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.

   [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 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 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
   Ionos Networks

   Email: manav@ionosnetworks.com

   Santosh Pallagatti
   Juniper Networks

   Email: santoshpk@juniper.net