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Versions: (draft-akiya-bfd-seamless-base) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 7880

Internet Engineering Task Force                             C. Pignataro
Internet-Draft                                                   D. Ward
Updates: 5880 (if approved)                                        Cisco
Intended status: Standards Track                                N. Akiya
Expires: November 7, 2016                            Big Switch Networks
                                                               M. Bhatia
                                                          Ionos Networks
                                                           S. Pallagatti
                                                             May 6, 2016


          Seamless Bidirectional Forwarding Detection (S-BFD)
                    draft-ietf-bfd-seamless-base-11

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 November 7, 2016.





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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Seamless BFD Overview . . . . . . . . . . . . . . . . . . . .   5
   4.  S-BFD Discriminators  . . . . . . . . . . . . . . . . . . . .   6
     4.1.  S-BFD Discriminator Uniqueness  . . . . . . . . . . . . .   6
     4.2.  Discriminator Pools . . . . . . . . . . . . . . . . . . .   7
   5.  Reflector BFD Session . . . . . . . . . . . . . . . . . . . .   7
   6.  State Variables . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  New State Variables . . . . . . . . . . . . . . . . . . .   8
     6.2.  State Variable Initialization and Maintenance . . . . . .   9
   7.  S-BFD Procedures  . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Demultiplexing of S-BFD Control Packet  . . . . . . . . .   9
     7.2.  Responder Procedures  . . . . . . . . . . . . . . . . . .  10
       7.2.1.  Responder Demultiplexing  . . . . . . . . . . . . . .  10
       7.2.2.  Transmission of S-BFD Control Packet by SBFDReflector  10
       7.2.3.  Additional SBFDReflector Behaviors  . . . . . . . . .  11
     7.3.  Initiator Procedures  . . . . . . . . . . . . . . . . . .  12
       7.3.1.  SBFDInitiator State Machine . . . . . . . . . . . . .  12
       7.3.2.  Transmission of S-BFD Control Packet by SBFDInitiator  13
       7.3.3.  Additional SBFDInitiator Behaviors  . . . . . . . . .  14
     7.4.  Diagnostic Values . . . . . . . . . . . . . . . . . . . .  14
     7.5.  The Poll Sequence . . . . . . . . . . . . . . . . . . . .  15
   8.  Operational Considerations  . . . . . . . . . . . . . . . . .  15
     8.1.  Scaling Aspect  . . . . . . . . . . . . . . . . . . . . .  15
     8.2.  Congestion Considerations . . . . . . . . . . . . . . . .  16
   9.  Co-existence with Classical BFD Sessions  . . . . . . . . . .  16
   10. S-BFD Echo Function . . . . . . . . . . . . . . . . . . . . .  16
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  17
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18
   14. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  19



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   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     15.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Appendix A.  Loop Problem and Solution  . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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 that 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.  Additional use
   cases are listed in [I-D.ietf-bfd-seamless-use-case].

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

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

2.  Terminology

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

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

   o  S-BFD - Seamless BFD.




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

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

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

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

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

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

   o  Reflector BFD session - synonymous with SBFDReflector.

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

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

   o  Initiator - a network node hosting an SBFDInitiator.

   o  Responder - a network node hosting an SBFDReflector.

   Figure 1 describes the relationship between S-BFD terminologies.



















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

                 Figure 1: S-BFD Terminology Relationship

3.  Seamless BFD Overview

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

   Once the above setup is complete, any network node, having the
   knowledge of the S-BFD discriminator allocated by a remote node to
   remote entity/entities, 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.

   This is exemplified in Figure 2.














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      <------- 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

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

   When a node allocates multiple S-BFD discriminators, how remote nodes
   determine which of the discriminators is associated with a specific
   entity is currently unspecified.  The use of multiple S-BFD
   discriminators by a single network node is therefore discouraged
   until a means of learning the mapping is defined.

4.  S-BFD Discriminators

4.1.  S-BFD Discriminator Uniqueness

   One important characteristic of an S-BFD discriminator is that it
   MUST be unique within an administrative domain.  If multiple network
   nodes allocated the same S-BFD discriminator value, then S-BFD
   control packets falsely terminating on a wrong network node can
   result in a reflector BFD session to generate a response back, due to
   "your discriminator" matching.  This is clearly not desirable.








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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.

   The remainder of this subsection describes the reasons for the
   suggestions above.

   Locally allocated S-BFD discriminator values for entities, listened
   by SBFDReflector sessions, may be arbitrary allocated or derived from
   values provided by applications.  These values may be protocol IDs
   (e.g., System-ID, Router-ID) or network targets (e.g., IP address).
   To avoid derived S-BFD discriminator values already being assigned to
   other BFD sessions (i.e., SBFDInitiator sessions and classical BFD
   sessions), it is RECOMMENDED that the 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 the same purpose (e.g., network reachability), then
   the colliding S-BFD discriminator value can be shared.  If the two
   applications are using S-BFD for a different purpose, then the
   collision must be addressed.  The use of multiple S-BFD
   discriminators by a single network node, however, is discouraged (see
   Section 3).

5.  Reflector BFD Session

   Each network node creates one or more reflector BFD sessions.  This
   reflector BFD session is a session that 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 has the following
   characteristics:




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

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

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

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

   Note that incoming S-BFD control packets may be IPv4, IPv6 or MPLS
   based [I-D.ietf-bfd-seamless-ip], and other options are possible and
   can be defined in future documents.  How such S-BFD control packets
   reach an appropriate reflector BFD session is also a local matter,
   and is outside the scope of this document.

6.  State Variables

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

6.1.  New State Variables

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

   o  bfd.SessionType: This is a new state variable that 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.







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6.2.  State Variable Initialization and Maintenance

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

   o  bfd.DemandMode: This variable MUST be initialized to 1 for session
      type SBFDInitiator, and MUST be initialized to 0 for session type
      SBFDReflector.  This is done to prevent loops (see Appendix A).

7.  S-BFD Procedures

7.1.  Demultiplexing of S-BFD Control Packet

   S-BFD packet MUST be demultiplexed with lower layer information
   (e.g., dedicated destination UDP port [I-D.ietf-bfd-seamless-ip],
   associated channel type [I-D.ietf-pals-seamless-vccv]).  The
   following procedure SHOULD be executed on both initiator and
   reflector.

      If S-BFD packet

         If S-BFD packet is for SBFDReflector

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

         Else

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

            If session is SBFDInitiator

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

            Else

               Packet MUST be discarded

      Else

         Procedure described in [RFC5880] MUST be applied.



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

7.2.  Responder Procedures

   A network node that 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].

7.2.1.  Responder Demultiplexing

   S-BFD packet MUST be demultiplexed with lower layer information.  The
   following procedure SHOULD be executed by the responder:

      If "your discriminator" not one of the entry allocated for local
      entities

         Packet MUST be discarded.

      Else

         Packet is determined to be handled by a reflector BFD session
         responsible for that S-BFD discriminator.

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

            Chosen reflector BFD session SHOULD transmit a response BFD
            control packet using procedures described in Section 7.2.2.

7.2.2.  Transmission of S-BFD Control Packet by SBFDReflector

   Contents of S-BFD control packets sent by an SBFDReflector MUST be
   set as per Section 6.8.7 of [RFC5880].  There are a few fields that
   needs to be set differently from [RFC5880] as follows:

      State (Sta)

         Set to bfd.SessionState (either UP or ADMINDOWN only).
         Clarification of reflector BFD session state is described in
         Section 7.2.3.

      Demand (D)

         Set to 0, to identify the S-BFD packet is sent by the
         SBFDReflector.




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      Detect Mult

         Value to be copied from "Detection Multiplier" filed of
         received BFD packet.

      My Discriminator

         Value be copied from "your discriminator" filed of received BFD
         packet.

      Your Discriminator

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

      Desired Min TX Interval

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

      Required Min RX Interval

         Set to a bfd.RequiredMinRxInterval, value describing minimum
         interval, in microseconds between received SBFD Control
         packets.  Further details are described in Section 7.2.3.

      Required Min Echo RX Interval

         If device supports looping back S-BFD echo packets

            Set to the minimum required Echo packet receive interval for
            this session.

         Else

            Set to 0.

7.2.3.  Additional SBFDReflector Behaviors

   o  S-BFD control packets transmitted by the SBFDReflector MUST have
      "Required Min RX Interval" set to a value that expresses, in
      microseconds, the minimum interval between 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.





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   o  When the SBFDReflector receives an S-BFD control packet from an
      SBFDInitiator, then the SBFDReflector needs to determine what
      "state" to send in the response S-BFD control packet.  If the
      monitored local entity is in service, then the "state" MUST be set
      to UP.  If the monitored local entity is "temporarily out of
      service", then the "state" SHOULD be set to ADMINDOWN.

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

7.3.  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.

   Figure 3 describes the 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

7.3.1.  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



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   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.  See Figure 4.

                       +--+
          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].

7.3.2.  Transmission of S-BFD Control Packet by SBFDInitiator

   Contents of S-BFD control packets sent by an SBFDInitiator MUST be
   set as per Section 6.8.7 of [RFC5880].  There are few fields which
   needs to be set differently from [RFC5880] as follows:

      Demand (D)

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

      Your Discriminator

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




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      Required Min RX Interval

         Set to 0.

      Required Min Echo RX Interval

         Set to 0.

7.3.3.  Additional SBFDInitiator Behaviors

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

   o  When an SBFDInitiator receives a response S-BFD control packet, if
      the state specified is ADMINDOWN, the SBFDInitiator MUST NOT
      conclude loss of reachability to the corresponding remote entity,
      and MUST back off packet transmission interval for the remote
      entity to an interval no faster than 1 second.

   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; the action MAY include
      logging an error.

   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 (see Appendix A).

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.
   Implementation MAY use already existing diagnostic values defined in
   Section 4.1 of [RFC5880].  However, details of such are outside the
   scope of this specification.





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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 7.2.3 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.

8.  Operational Considerations

   S-BFD provides a smooth and continuous (i.e., seamless) operational
   experience as an Operations, Administration, and Maintenance (OAM)
   mechanism for connectivity check and connection verification.  This
   is achieved by providing a simplified mechanism with large portions
   of negotiation aspects eliminated, resulting in a faster and simpler
   provisioning.

   Because of this simplified mechanism, due to a misconfiguration, an
   SBFDInitiator could send S-BFD control packets to a target that does
   not exist or that is outside the S-BFD administrative domain.  As
   explained in Section 7.3.1, an SBFDInitiator can be a "persistent"
   initiator or a "when needed" one.  When an S-BFD "persistent"
   SBFDInitiator is used, it SHOULD be controlled that S-BFD control
   packet do not propagate for an extended period of time outside of the
   administrative domain that uses it.  Further, operational measures
   SHOULD be taken to identify if S-BFD packets are not responded to for
   an extended period of time, and remediate the situation.  These
   potential concerns are largely mitigated by dynamic advertisement
   mechanisms for S-BFD, and with automation checks before applying
   configurations.

8.1.  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.




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8.2.  Congestion Considerations

   S-BFD performs failure detection by consuming resources, including
   bandwidth and CPU processing.  It is therefore imperative that
   operators correctly provision the rates at which S-BFD is transmitted
   to avoid congestion.  When BFD is used across multiple hops, a
   congestion control mechanism MUST be implemented, and when congestion
   is detected, the BFD implementation MUST reduce the amount of traffic
   it generates.  The exact mechanism used to detect congestion is
   outside the scope of this specification, but may include detection of
   lost BFD control packets or other means.  The SBFDReflector can limit
   the rate at which an SBFInitiators will be sending S-BFD control
   packets utilizing the "Required Min RX Interval", at the expense of
   increasing the detection time.

9.  Co-existence with Classical BFD Sessions

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

10.  S-BFD Echo Function

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

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

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

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

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

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

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




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   S-BFD Echo packets can be spoofed, and can u-turn in a transit node
   before reaching the expected target node.  When the S-BFD Echo
   function is used, it is RECOMMENDED in this specification that both
   S-BFD control packets and S-BFD echo packets be sent.  While the
   additional use of S-BFD control packets alleviates these two
   concerns, some form of authentication MAY still be included.

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

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

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

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

11.  Security Considerations

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

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

   o  SBFDReflector MUST NOT use the crypto sequence number to make a
      decision about accepting the packet.  This is because the
      SBFDReflector does not maintain S-BFD peer state, and because the
      SBFDReflector can receive S-BFD packets from multiple
      SBFDInitiators.  Consequently, BFD authentication can be used but
      not the sequence number.

   o  SBFDReflector MAY use the Auth Key ID in the incoming packet to
      verify the authentication data.

   o  SBFDReflector MUST accept the packet if authentication is
      successful.






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   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 SHOULD accept S-BFD control packet with sequence
      number within permissible window.  One potential approach is the
      procedure explained 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.

   Additionally, the use of strong forms of authentication is strongly
   encouraged for S-BFD.  The use of Simple Password authentication
   potentially puts other services at risk, if S-BFD packets can be
   intercepted and if those password values are reused for other
   services.

   Considerations about loop problems are covered in Appendix A.

12.  IANA Considerations

   No action is required by IANA for this document.

13.  Acknowledgements

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








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14.  Contributors

   The following are key contributors to this document:

      Tarek Saad, Cisco Systems, Inc.
      Siva Sivabalan, Cisco Systems, Inc.
      Nagendra Kumar, Cisco Systems, Inc.
      Mallik Mudigonda, Cisco Systems, Inc.
      Sam Aldrin, Google

15.  References

15.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
              <http://www.rfc-editor.org/info/rfc5880>.

15.2.  Informative References

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

   [I-D.ietf-bfd-seamless-ip]
              Akiya, N., Pignataro, C., and D. Ward, "Seamless
              Bidirectional Forwarding Detection (S-BFD) for IPv4, IPv6
              and MPLS", draft-ietf-bfd-seamless-ip-04 (work in
              progress), April 2016.

   [I-D.ietf-bfd-seamless-use-case]
              Aldrin, S., Pignataro, C., Mirsky, G., and N. Kumar,
              "Seamless Bidirectional Forwarding Detection (S-BFD) Use
              Cases", draft-ietf-bfd-seamless-use-case-06 (work in
              progress), April 2016.

   [I-D.ietf-pals-seamless-vccv]
              Govindan, V. and C. Pignataro, "Seamless BFD for VCCV",
              draft-ietf-pals-seamless-vccv-03 (work in progress), April
              2016.





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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

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

Appendix A.  Loop Problem and Solution

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

    Node A (IP 2001:db8::1) ----------------- Node B (IP 2001:db8::2)
                                    |
                                    |
                         Man in the Middle (MiM)

   Assume node A reserved a discriminator 0x01010101 for target
   identifier 2001:db8::1 and has a reflector session in listening mode.
   Similarly node B reserved a discriminator 0x02020202 for its target
   identifier 2001:db8::2 and also has a reflector session in listening
   mode.

   Suppose MiM sends a spoofed packet with MyDisc = 0x01010101, YourDisc
   = 0x02020202, source IP as 2001:db8::1 and dest IP as 2001:db8::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.

   The solution to avoid the loop problem uses the "D" bit (Demand mode
   bit).  The Initiator always sets the 'D' bit and the reflector always
   clears it.  This way we can identify if a received packet was a
   reflected packet and avoid reflecting it back.





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Authors' Addresses

   Carlos Pignataro
   Cisco Systems, Inc.

   Email: cpignata@cisco.com


   Dave Ward
   Cisco Systems, Inc.

   Email: wardd@cisco.com


   Nobo Akiya
   Big Switch Networks

   Email: nobo.akiya.dev@gmail.com


   Manav Bhatia
   Ionos Networks

   Email: manav@ionosnetworks.com


   Santosh Pallagatti

   Email: santosh.pallagatti@gmail.com






















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