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RFC 7347

Informational
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Pre-standard Linear Protection Switching in MPLS Transport Profile (MPLS-TP)

 


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Independent Submission                              H. van Helvoort, Ed.
Request for Comments: 7347                           Huawei Technologies
Category: Informational                                     J. Ryoo, Ed.
ISSN: 2070-1721                                                     ETRI
                                                                H. Zhang
                                                     Huawei Technologies
                                                                F. Huang
                                                                 Philips
                                                                   H. Li
                                                            China Mobile
                                                         A. D'Alessandro
                                                          Telecom Italia
                                                          September 2014


              Pre-standard Linear Protection Switching in
                    MPLS Transport Profile (MPLS-TP)

Abstract

   The IETF Standards Track solution for MPLS Transport Profile
   (MPLS-TP) Linear Protection is provided in RFCs 6378, 7271, and 7324.

   This document describes the pre-standard implementation of MPLS-TP
   Linear Protection that has been deployed by several network operators
   using equipment from multiple vendors.  At the time of publication,
   these pre-standard implementations were still in operation carrying
   live traffic.

   The specified mechanism supports 1+1 unidirectional/bidirectional
   protection switching and 1:1 bidirectional protection switching.  It
   is purely supported by the MPLS-TP data plane and can work without
   any control plane.

Page 2 
Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7347.

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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   5
   3.  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Linear Protection-Switching Overview  . . . . . . . . . . . .   6
     4.1.  Protection Architecture Types . . . . . . . . . . . . . .   6
       4.1.1.  1+1 Architecture  . . . . . . . . . . . . . . . . . .   6
       4.1.2.  1:1 Architecture  . . . . . . . . . . . . . . . . . .   6
       4.1.3.  1:n Architecture  . . . . . . . . . . . . . . . . . .   7
     4.2.  Protection Switching Type . . . . . . . . . . . . . . . .   7
     4.3.  Protection Operation Type . . . . . . . . . . . . . . . .   7
   5.  Protection-Switching Trigger Conditions . . . . . . . . . . .   8
     5.1.  Fault Conditions  . . . . . . . . . . . . . . . . . . . .   8
     5.2.  External Commands . . . . . . . . . . . . . . . . . . . .   8
       5.2.1.  End-to-End Commands . . . . . . . . . . . . . . . . .   8
       5.2.2.  Local Commands  . . . . . . . . . . . . . . . . . . .   9
   6.  Protection-Switching Schemes  . . . . . . . . . . . . . . . .  10
     6.1.  1+1 Unidirectional Protection Switching . . . . . . . . .  10
     6.2.  1+1 Bidirectional Protection Switching  . . . . . . . . .  11
     6.3.  1:1 Bidirectional Protection Switching  . . . . . . . . .  12
   7.  APS Protocol  . . . . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  APS PDU Format  . . . . . . . . . . . . . . . . . . . . .  13
     7.2.  APS Transmission  . . . . . . . . . . . . . . . . . . . .  16
     7.3.  Hold-Off Timer  . . . . . . . . . . . . . . . . . . . . .  17
     7.4.  WTR Timer . . . . . . . . . . . . . . . . . . . . . . . .  17
     7.5.   Command Acceptance and Retention . . . . . . . . . . . .  18
     7.6.  Exercise Operation  . . . . . . . . . . . . . . . . . . .  18
   8.  Protection-Switching Logic  . . . . . . . . . . . . . . . . .  19
     8.1.  Principle of Operation  . . . . . . . . . . . . . . . . .  19
     8.2.  Equal Priority Requests . . . . . . . . . . . . . . . . .  21
     8.3.  Signal Degrade of the Protection Transport Entity . . . .  22
   9.  Protection-Switching State Transition Tables  . . . . . . . .  22
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  24
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  24
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     12.2.  Informative References . . . . . . . . . . . . . . . . .  25
   Appendix A.  Operation Examples of the APS Protocol . . . . . . .  26

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1.  Introduction

   The IETF Standards Track solution for MPLS Transport Profile
   (MPLS-TP) Linear Protection is provided in [RFC6378], [RFC7271], and
   [RFC7324].

   This document describes the pre-standard implementation of MPLS-TP
   Linear Protection that has been deployed by several network operators
   using equipment from multiple vendors.  At the time of publication,
   these pre-standard implementations were still in operation carrying
   live traffic.

   This implementation was considered in the MPLS WG; however, a
   different path was chosen.

   This document may be useful in the future if a vendor or operator is
   trying to interwork with a different vendor or operator who has
   deployed the pre-standard implementation, and it provides a permanent
   record of the pre-standard implementation.  It is also worth noting
   that the experience gained during deployment of the implementations
   of this document was used to refine [RFC7271].

   MPLS-TP is defined as the transport profile of MPLS technology to
   allow its deployment in transport networks.  A typical feature of a
   transport network is that it can provide fast protection switching
   for end-to-end transport paths and transport path segments.  The
   protection-switching time is generally required to be less than 50 ms
   to meet the strict requirements of services such as voice, private
   line, etc.

   The goal of a linear protection-switching mechanism is to satisfy the
   requirement of fast protection switching for an MPLS-TP network.
   Linear protection switching means that, for one or more working
   transport entities (working paths), there is one protection transport
   entity (protection path), which is disjoint from any of the working
   transport entities, ready to take over the service transmission when
   a working transport entity has failed.

   This document specifies a 1+1 unidirectional protection-switching
   mechanism for a unidirectional transport entity (either point to
   point or point to multipoint) as well as a bidirectional point-to-
   point transport entity and a 1+1/1:1 bidirectional protection-
   switching mechanism for a point-to-point bidirectional transport
   entity.  Since bidirectional protection switching needs the
   coordination of the two endpoints of the transport entity, this
   document also specifies the Automatic Protection Switching (APS)
   protocol, which is used for this purpose.

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   The linear protection mechanism described in this document is
   applicable to both Label Switched Paths (LSPs) and Pseudowires (PWs).

   The APS protocol specified in this document is based on the same
   principles and behavior of the APS protocol designed for Synchronous
   Optical Network (SONET) [T1.105.01] / Synchronous Digital Hierarchy
   (SDH) [G.841], Optical Transport Network (OTN) [G.873.1], and
   Ethernet [G.8031] and provides commonality with the established
   operation models utilized in transport network technologies (e.g.,
   SDH/SONET, OTN, and Ethernet).

2.  Conventions Used in This Document

   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 [RFC2119].

3.  Acronyms

   This document uses the following acronyms:

   APS     Automatic Protection Switching
   DNR     Do not Revert
   EXER    Exercise
   G-ACh   Generic Associated Channel
   FS      Forced Switch
   LO      Lockout of Protection
   LSP     Label Switched Path
   MPLS-TP MPLS Transport Profile
   MS      Manual Switch
   MS-P    Manual Switch to Protection transport entity
   MS-W    Manual Switch to Working transport entity
   NR      No Request
   OAM     Operations, Administration, and Maintenance
   OTN     Optical Transport Network
   PDU     Protocol Data Unit
   PW      Pseudowire
   RR      Reverse Request
   SD      Signal Degrade
   SD-P    Signal Degrade on Protection transport entity
   SD-W    Signal Degrade on Working transport entity
   SDH     Synchronous Digital Hierarchy
   SF      Signal Fail
   SF-P    Signal Fail on Protection transport entity
   SF-W    Signal Fail on Working transport entity
   SONET   Synchronous Optical Network
   WTR     Wait to Restore

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4.  Linear Protection-Switching Overview

   To guarantee the protection-switching time for a working transport
   entity, its protection transport entity is always preconfigured
   before the failure occurs.  Normally, traffic will be transmitted and
   received on the working transport entity.  Switching to the
   protection transport entity is usually triggered by link or node
   failure, external commands, etc.  Note that external commands are
   often used in transport networks by operators, and they are very
   useful in cases of service adjustment, path maintenance, etc.

4.1.  Protection Architecture Types

4.1.1.  1+1 Architecture

   In the 1+1 architecture, the protection transport entity is
   associated with a working transport entity.  The normal traffic is
   permanently bridged onto both the working transport entity and the
   protection transport entity at the source endpoint of the protected
   domain.  The normal traffic on working and protection transport
   entities is transmitted simultaneously to the destination sink
   endpoint of the protected domain, where a selection between the
   working and protection transport entity is made based on
   predetermined criteria, such as signal fail and signal degrade
   indications.

4.1.2.  1:1 Architecture

   In the 1:1 architecture, the protection transport entity is
   associated with a working transport entity.  When the working
   transport entity is determined to be impaired, the normal traffic
   MUST be transferred from the working to the protection transport
   entity at both the source and sink endpoints of the protected domain.
   The selection between the working and protection transport entities
   is made based on predetermined criteria, such as signal fail and
   signal degrade indications from the working or protection transport
   entity.

   The bridge at the source endpoint can be realized in two ways: it is
   either a selector bridge or a broadcast bridge.  With a selector
   bridge, the normal traffic is connected either to the working
   transport entity or the protection transport entity.  With a
   broadcast bridge, the normal traffic is permanently connected to the
   working transport entity, and in case a protection switch is active,
   it is also connected to the protection transport entity.  The
   broadcast bridge is recommended to be used in revertive mode only.

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4.1.3.  1:n Architecture

   Details for the 1:n protection-switching architecture are out of
   scope of this document and will be provided in a different document
   in the future.

   It is worth noting that the APS protocol defined here is capable of
   supporting 1:n operations.

4.2.  Protection Switching Type

   The linear protection-switching types can be a unidirectional
   switching type or a bidirectional switching type.

   o  Unidirectional switching type: Only the affected direction of the
      working transport entity is switched to the protection transport
      entity; the selectors at each endpoint operate independently.
      This switching type is recommended to be used for 1+1 protection
      in this document.

   o  Bidirectional switching type: Both directions of the working
      transport entity, including the affected direction and the
      unaffected direction, are switched to the protection transport
      entity.  For bidirectional switching, the APS protocol is required
      to coordinate the two endpoints so that both have the same bridge
      and selector settings, even for a unidirectional failure.  This
      type is applicable for 1+1 and 1:1 protection.

4.3.  Protection Operation Type

   The linear protection operation types can be a non-revertive
   operation type or a revertive operation type.

   o  Non-revertive operation: The normal traffic will not be switched
      back to the working transport entity even after a protection
      switching cause has cleared.  This is generally accomplished by
      replacing the previous switch request with a "Do not Revert (DNR)"
      request, which has a low priority.

   o  Revertive operation: The normal traffic is restored to the working
      transport entity after the condition(s) causing the protection
      switching has cleared.  In the case of clearing a command (e.g.,
      Forced Switch), this happens immediately.  In the case of clearing
      a defect, this generally happens after the expiry of a "Wait to
      Restore (WTR)" timer, which is used to avoid chattering of
      selectors in the case of intermittent defects.

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5.  Protection-Switching Trigger Conditions

5.1.  Fault Conditions

   Fault conditions mean the requests generated by the local Operations,
   Administration, and Maintenance (OAM) function.

   o  Signal Fail (SF): If an endpoint detects a failure by an OAM
      function or other mechanism, it will submit a local signal failure
      (local SF) to the APS module to request a protection switch.  The
      local SF could be on the working transport entity (Signal Fail on
      Working transport entity (SF-W)) or the protection transport
      entity (Signal Fail on Protection transport entity (SF-P)).

   o  Signal Degrade (SD): If an endpoint detects signal degradation by
      an OAM function or other mechanism, it will submit a local signal
      degrade (local SD) to the APS module to request a protection
      switching.  The local SD could be on the working transport entity
      (Signal Degrade on Working transport entity (SD-W)) or the
      protection transport entity (Signal Degrade on Protection
      transport entity (SD-P)).

5.2.  External Commands

   The external command issues an appropriate external request to the
   protection process.

5.2.1.  End-to-End Commands

   These commands are applied to both local and remote nodes.  When the
   APS protocol is present, these commands, except the Clear command,
   are signaled to the far end of the connection.  In bidirectional
   switching, these commands affect the bridge and selector at both
   ends.

   o  Lockout of Protection (LO): This command is used to provide the
      operator a tool for temporarily disabling access to the protection
      transport entity.

   o  Manual Switch (MS): This command is used to provide the operator a
      tool for temporarily switching normal traffic to the working
      transport entity (Manual Switch to Working transport entity (MS-
      W)) or to the protection transport entity (Manual Switch to
      Protection transport entity (MS-P)), unless a higher priority
      switch request (i.e., LO, FS, or SF) is in effect.

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   o  Forced Switch (FS): This command is used to provide the operator a
      tool for temporarily switching normal traffic from the working
      transport entity to the protection transport entity, unless a
      higher priority switch request (i.e., LO or SF-P) is in effect.

   o  Exercise (EXER): Exercise is a command to test if the APS
      communication is operating correctly.  The EXER command SHALL NOT
      affect the state of the protection selector and bridge.

   o  Clear: This command between management and the local protection
      process is not a request sent by APS to other endpoints.  It is
      used to clear the active near-end external command or WTR state.

5.2.2.  Local Commands

   These commands apply only to the near end (local node) of the
   protection group.  Even when an APS protocol is supported, they are
   not signaled to the far end.

   o  Freeze: This command freezes the state of the protection group.
      Until the freeze is cleared, additional near-end commands are
      rejected, and condition changes and received APS information are
      ignored.  When the Freeze command is cleared, the state of the
      protection group is recomputed based on the condition and received
      APS information.

      Because the freeze is local, if the freeze is issued at one end
      only, a failure of protocol can occur as the other end is open to
      accept any operator command or fault condition.

   o  Clear Freeze: This command clears the local freeze.

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6.  Protection-Switching Schemes

6.1.  1+1 Unidirectional Protection Switching

     +-----------+                                       +-----------+
     |           |---------------------------------------|           |
     |          -+---------------------------------------+-          |
     |         / |---------------------------------------| \         |
     |        /  |       Working transport entity        |  \        |
   --+------->   |                                       |   --------+->
     |        \  |                                       |           |
     |         \ |---------------------------------------|           |
     |          -+---------------------------------------|           |
     |  source   |---------------------------------------|    sink   |
     +-----------+       Protection transport entity     +-----------+
                            (normal condition)

     +-----------+                                       +-----------+
     |           |---------------------------------------|           |
     |          -+------------------XX-------------------+           |
     |         / |---------------------------------------|           |
     |        /  |   Working transport entity (failure)  |           |
   --|------->   |                                       |   --------+->
     |        \  |                                       |  /        |
     |         \ |---------------------------------------| /         |
     |          -+---------------------------------------+-          |
     |  source   |---------------------------------------|    sink   |
     +-----------+     Protection transport entity       +-----------+
                           (failure condition)

         Figure 1: 1+1 Unidirectional Linear Protection Switching

   1+1 unidirectional protection switching is the simplest protection
   switching mechanism.  The normal traffic is permanently bridged on
   both the working and protection transport entities at the source
   endpoint of the protected domain.  In the normal condition, the sink
   endpoint receives traffic from the working transport entity.  If the
   sink endpoint detects a failure on the working transport entity, it
   will switch to receive traffic from the protection transport entity.
   1+1 unidirectional protection switching is recommended to be used for
   unidirectional transport.

   Note that 1+1 unidirectional protection switching does not use the
   APS coordination protocol since it only performs protection switching
   based on the local request.

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6.2.  1+1 Bidirectional Protection Switching

     +-----------+                                       +-----------+
     |           |---------------------------------------|           |
     |          -+<--------------------------------------+-          |
     |         / +-------------------------------------->+ \         |
     | sink   / /|---------------------------------------|\ \   sink |
   <-+-------/ / |        Working transport entity       | --\-------+->
   --+-------->  |                                       |    <------+--
     | source  \ |                                       |   / source|
     |          \|---------------------------------------|  /        |
     |           +-------------------------------------->| /         |
     |           |<--------------------------------------+-          |
     | APS <...................................................> APS |
     |           |---------------------------------------+           |
     +-----------+      Protection transport entity      +-----------+
                            (normal condition)

     +-----------+                                       +-----------+
     |           |---------------------------------------|           |
     |           +<----------------XX--------------------+-          |
     |           +-------------------------------------->+ \         |
     |          /|---------------------------------------|  \        |
     | source  / |   Working transport entity (failure)  |   \ source|
   --+-------->  |                                       |    \<-----+--
   <-+-------  \ |                                       |  --/------+->
     | sink  \  \|---------------------------------------| / /  sink |
     |        \  +-------------------------------------->+- /        |
     |         --+<--------------------------------------+-/         |
     | APS <...................................................> APS |
     |           |---------------------------------------+           |
     +-----------+      Protection transport entity      +-----------+
                             (failure condition)

          Figure 2: 1+1 Bidirectional Linear Protection Switching

   In 1+1 bidirectional protection switching, for each direction, the
   normal traffic is permanently bridged on both the working and
   protection transport entities at the source endpoint of the protected
   domain.  In the normal condition, for each direction, the sink
   endpoint receives traffic from the working transport entity.

   If the sink endpoint detects a failure on the working transport
   entity, it will switch to receive traffic from the protection
   transport entity.  It will also send an APS message to inform the
   sink endpoint on the other direction to switch to receive traffic
   from the protection transport entity.

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   The APS mechanism is necessary to coordinate the two endpoints of the
   transport entity and to implement 1+1 bidirectional protection
   switching even for a unidirectional failure.

6.3.  1:1 Bidirectional Protection Switching

     +-----------+                                       +-----------+
     |           |---------------------------------------|           |
     |          -+<--------------------------------------+-          |
     |         / +-------------------------------------->+ \         |
     | sink   / /|---------------------------------------|\ \  source|
   <-+-------/ / |        Working transport entity       | \ <-------+--
   --+-------->  |                                       |  ---------+->
     | source    |                                       |      sink |
     |           |---------------------------------------|           |
     |           |                                       |           |
     |           |                                       |           |
     | APS <...................................................> APS |
     |           |---------------------------------------|           |
     +-----------+      Protection transport entity      +-----------+
                           (normal condition)

     +-----------+                                       +-----------+
     |           |---------------------------------------|           |
     |           |                 \/                    |           |
     |           |                 /\                    |           |
     |           |---------------------------------------|           |
     | source    |   Working transport entity (failure)  |      sink |
   --+------->   |                                       |   --------+->
   <-+------- \  |                                       |  / <------+--
     | sink  \ \ |---------------------------------------| / / source|
     |        \ -+-------------------------------------->+- /        |
     |         --+<--------------------------------------+--         |
     | APS <...................................................> APS |
     |           |---------------------------------------+           |
     +-----------+      Protection transport entity      +-----------+
                           (failure condition)

          Figure 3: 1:1 Bidirectional Linear Protection Switching

   In 1:1 bidirectional protection switching, for each direction, the
   source endpoint sends traffic on either the working transport entity
   or the protection transport entity.  The sink endpoint receives the
   traffic from the same transport entity on which the source endpoint
   sends the traffic.

   In the normal condition, for each direction, the source and sink
   endpoints send and receive traffic from the working transport entity.

Top      ToC       Page 13 
   If the sink endpoint detects a failure on the working transport
   entity, it will switch to send and receive traffic from the
   protection transport entity.  It will also send an APS message to
   inform the sink endpoint on another direction to switch to send and
   receive traffic from the protection transport entity.

   The APS mechanism is necessary to coordinate the two endpoints of the
   transport entity and implement 1:1 bidirectional protection switching
   even for a unidirectional failure.

7.  APS Protocol

   This APS protocol is based upon the APS protocol defined in
   Section 11 of [G.8031].  See that reference for further definition of
   the Protocol Data Unit (PDU) fields and protocol details beyond the
   description in this document.

7.1.  APS PDU Format

   APS packets MUST be sent over a Generic Associated Channel (G-ACh) as
   defined in [RFC5586].

   The format of APS PDU is specified in Figure 4 below.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 0 0 1|0 0 0 0|0 0 0 0 0 0 0 0|     Channel Type (=0x7FFA)    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | MEL | Version |    OpCode     |     Flags     |   TLV Offset  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  APS Specific Information                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    End TLV    |
    +-+-+-+-+-+-+-+-+

                         Figure 4: APS PDU Format

   The following values MUST be used for APS PDU:

   o  Channel Type: The Channel Type MUST be configurable by the
      implementation.  During deployment, the local system administrator
      provisioned the value 0x7FFA.  This is a code point value in the
      range of experimental Channel Types as described in RFC 5586,
      Section 10.

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   o  Maintenance Entity group Level (MEL): The MEL value to set and
      check MUST be configurable.  The DEFAULT value MUST be "111".
      With co-routed bidirectional transport paths, the configured MEL
      MUST be the same in both directions.

   o  Version: 0x00

   o  OpCode: 0x27 (=0d39)

   o  Flags: 0x00

   o  TLV Offset: 4

   o  End TLV: 0x00

   The format of the APS-specific information is defined in Figure 5.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Request|Pr.Type|   Requested   |   Bridged     | |             |
    |   /   |-+-+-+-|               |               |T|  Reserved(0)|
    | State |A|B|D|R|    Signal     |    Signal     | |             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 5: APS-Specific Information Format

   All bits defined as "Reserved" MUST be transmitted as 0 and ignored
   on reception.

   o  Request/State:

      The four bits indicate the protection-switching request type.  See
      Figure 6 for the code of each request/state type.

Top      ToC       Page 15 
      In case that there are multiple protection-switching requests,
      only the protection-switching request with the highest priority
      MUST be processed.

          +------------------------------------+---------------+
          |            Request/State           | Code/Priority |
          +------------------------------------+---------------+
          |Lockout of Protection (LO)          | 1111 (highest)|
          +------------------------------------+---------------+
          |Signal Fail on Protection (SF-P)    | 1110          |
          +------------------------------------+---------------+
          |Forced Switch (FS)                  | 1101          |
          +------------------------------------+---------------+
          |Signal Fail on Working (SF-W)       | 1011          |
          +------------------------------------+---------------+
          |Signal Degrade (SD)                 | 1001          |
          +------------------------------------+---------------+
          |Manual Switch (MS)                  | 0111          |
          +------------------------------------+---------------+
          |Wait to Restore (WTR)               | 0101          |
          +------------------------------------+---------------+
          |Exercise (EXER)                     | 0100          |
          +------------------------------------+---------------+
          |Reverse Request (RR)                | 0010          |
          +------------------------------------+---------------+
          |Do Not Revert (DNR)                 | 0001          |
          +------------------------------------+---------------+
          |No Request (NR)                     | 0000 (lowest) |
          +------------------------------------+---------------+

           Figure 6: Protection-Switching Request Code/Priority

   o  Protection Type (Pr.Type):

      The four bits are used to specify the protection type.

      A: reserved (set by default to 1)
      B: 0 - 1+1 (permanent bridge)
      1 - 1:1 (no permanent bridge)
      D: 0 - Unidirectional switching
      1 - Bidirectional switching
      R: 0 - Non-revertive operation
      1 - Revertive operation

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   o  Requested Signal:

      This byte is used to indicate the traffic that the near-end
      requests to be carried over the protection entity.

      value = 0: Null traffic
      value = 1: Normal traffic 1
      value = 2~255: Reserved

   o  Bridged Signal:

      This byte is used to indicate the traffic that is bridged onto the
      protection entity.

      value = 0: Null traffic
      value = 1: Normal traffic 1
      value = 2~255: Reserved

   o  Bridge Type (T):

      This bit is used to further specify the type of non-permanent
      bridge for 1:1 protection switching.

      value = 0: Selector bridge
      value = 1: Broadcast bridge

   o  Reserved:

      This field MUST be set to zero.

7.2.  APS Transmission

   The APS message MUST be transported on the protection transport
   entity by encapsulation with the protection transport entity label
   (the label of the LSP used to transport protection traffic).  If an
   endpoint receives APS-specific information from the working transport
   entity, it MUST ignore this information and MUST report the failure
   of protocol defect (see Section 8.1) to the operator.

   A new APS packet MUST be transmitted immediately when a change in the
   transmitted status occurs.  The first three APS packets MUST be
   transmitted as fast as possible only if the APS information to be
   transmitted has been changed so that fast protection switching is
   possible, even if one or two APS packets are lost or corrupted.  The
   interval of the first three APS packets SHOULD be 3.3 ms.  APS
   packets after the first three MUST be transmitted with the interval
   of 5 seconds.

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   If no valid APS-specific information is received, the last valid
   received information remains applicable.

7.3.  Hold-Off Timer

   In order to coordinate timing of protection switches at multiple
   layers, a hold-off timer MAY be required.  The purpose is to allow a
   server-layer protection switch to have a chance to fix the problem
   before switching at a client layer.

   Each selector SHOULD have a provisioned hold-off timer.  The
   suggested range of the hold-off timer is 0 to 10 seconds in steps of
   100 ms (accuracy of +/-5 ms).

   When a new defect or more severe defect occurs (new SF or SD) on the
   active transport entity (the transport entity that currently carries
   and selects traffic), this event will not be reported immediately to
   protection switching if the provisioned hold-off timer value is non-
   zero.  Instead, the hold-off timer SHALL be started.  When the hold-
   off timer expires, it SHALL be checked whether a defect still exists
   on the transport entity that started the timer.  If it does, that
   defect SHALL be reported to protection switching.  The defect need
   not be the same one that started the timer.

   This hold-off timer mechanism SHALL be applied for both working and
   protection transport entities.

7.4.  WTR Timer

   In revertive mode of operation, to prevent frequent operation of the
   protection switch due to an intermittent defect, a failed working
   transport entity MUST become fault free.  After the failed working
   transport entity meets this criterion, a fixed period of time SHALL
   elapse before a normal traffic signal uses it again.  This period,
   called a WTR period, MAY be configured by the operator in 1 minute
   steps between 5 and 12 minutes; the default value is 5 minutes.  An
   SF or SD condition will override the WTR.  To activate the WTR timer
   appropriately, even when both ends concurrently detect clearance of
   SF-W and SD-W, when the local state transits from SF-W or SD-W to No
   Request (NR) with the requested signal number 1, the previous local
   state, SF-W or SD-W, MUST be memorized.  If both the local state and
   far-end state are NR with the requested signal number 1, the local
   state transits to WTR only when the previous local state is SF-W or
   SD-W.  Otherwise, the local state transits to NR with the requested
   signal number 0.

Top      ToC       Page 18 
   In revertive mode of operation, when the protection is no longer
   requested, i.e., the failed working transport entity is no longer in
   SF or SD condition (and assuming no other requesting transport
   entities), a local WTR state will be activated.  Since this state
   becomes the highest in priority, it is indicated on the APS signal
   and maintains the normal traffic signal from the previously failed
   working transport entity on the protection transport entity.  This
   state SHALL normally time out and become an NR state.  The WTR timer
   deactivates earlier when any request of higher priority request
   preempts this state.

7.5.  Command Acceptance and Retention

   The commands Clear, LO, FS, MS, and EXER are accepted or rejected in
   the context of previous commands, the condition of the working and
   protection entities in the protection group, and (in bidirectional
   switching only) the APS information received.

   The Clear command MUST be only valid if a near-end LO, FS, MS, or
   EXER command is in effect or if a WTR state is present at the near
   end and rejected otherwise.  This command will remove the near-end
   command or WTR state, allowing the next lower-priority condition or
   (in bidirectional switching) APS request to be asserted.

   Other commands MUST be rejected unless they are higher priority than
   the previously existing command, condition, or (in bidirectional
   switching) APS request.  If a new command is accepted, any previous,
   lower-priority command that is overridden MUST be forgotten.  If a
   higher priority command overrides a lower-priority condition or (in
   bidirectional switching) APS request, that other request will be
   reasserted if it still exists at the time the command is cleared.  If
   a command is overridden by a condition or (in bidirectional
   switching) APS request, that command MUST be forgotten.

7.6.  Exercise Operation

   Exercise is a command to test if the APS communication is operating
   correctly.  It is lower priority than any "real" switch request.  It
   is only valid in bidirectional switching, since this is the only
   place where you can get a meaningful test by looking for a response.

   The Exercise command SHALL issue the command with the same requested
   and bridged signal numbers of the NR, Reverse Request (RR), or DNR
   request that it replaces.  The valid response will be an RR with the
   corresponding requested and bridged signal numbers.  When Exercise
   commands are input at both ends, an EXER, instead of RR, MUST be
   transmitted from both ends.  The standard response to DNR MUST be DNR
   rather than NR.  When the exercise command is cleared, it MUST be

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   replaced with NR or RR if the requested signal number is 0 and DNR or
   RR if the requested signal number is 1.

8.  Protection-Switching Logic

8.1.  Principle of Operation

                +-------------+ Persistent +----------+
    SF,SD       | Hold-off    | fault      | Local    |
    ----------->| timer logic |----------->| request  |
                +-------------+            | logic    |
    Other local requests ----------------->|          |
    (LO, FS, MS, EXER, Clear)              +----------+
                                               |
                                               | Highest
                                               | local request
                                               |
    Remote APS                                 V
    message       +-------+ Remote APS    +----------------+
    ------------->|  APS  | request/state |  APS process   |
    (received     | check |-------------->|  logic         |
    from far end) +-------+               +----------------+
                    |   ^                   |            |
                    |   |                   | Signaled   |
                    |   |                   | APS        |
                    |   | Txed              |            |
                    |   | "Requested        V            |
                    |   | Signal"         +-----------+  |
                    |   +-----------------| APS mess. |  |
                    |                     | generator |  |
                    |                     +-----------+  |
                    |                       |            |
                    V                       |            |
                Failure of                  V            |
                protocol                  APS message    |
                detection                                V
                                                    Set local
                                                    bridge/selector

                   Figure 7: Protection-Switching Logic

   Figure 7 describes the protection-switching logic.

   One or more local protection-switching requests may be active.  The
   "local request logic" determines which of these requests is highest
   using the order of priority given in Figure 6.  This highest local
   request information SHALL be passed on to the "APS process logic".
   Note that an accepted Clear command, clearance of SF or SD, or

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   expiration of the WTR timer SHALL NOT be processed by the local
   request logic but SHALL be considered as the highest local request
   and submitted to the APS process logic for processing.

   The remote APS message is received from the far end and is subjected
   to the validity check and mismatch detection in "APS check".  Failure
   of protocol situations are as follows:

   o  The "B" field mismatch due to incompatible provisioning;

   o  The reception of the APS message from the working entity due to
      working/protection configuration mismatch;

   o  No match in sent "Requested Signal" and received "Requested
      Signal" for more than 50 ms;

   o  No APS message is received on the protection transport entity
      during at least 3.5 times the long APS interval (e.g., at least
      17.5 seconds), and there is no defect on the protection transport
      entity.

   Provided the "B" field matches:

   o  If the "D" bit mismatches, the bidirectional side will fall back
      to unidirectional switching.

   o  If the "R" bit mismatches, one side will clear switches to WTR and
      the other will clear to DNR.  The two sides will interwork and the
      traffic is protected.

   o  If the "T" bit mismatches, the side using a broadcast bridge will
      fall back to using a selector bridge.

   The APS message with invalid information MUST be ignored, and the
   last valid received information remains applicable.

   The linear protection-switching algorithm SHALL commence immediately
   every time one of the input signals changes, i.e., when the status of
   any local request changes, or when different APS-specific information
   is received from the far end.  The consequent actions of the
   algorithm are also initiated immediately, i.e., change the local
   bridge/selector position (if necessary), transmit new APS-specific
   information (if necessary), or detect the failure of protocol defect
   if the protection switching is not completed within 50 ms.

Top      ToC       Page 21 
   The state transition is calculated in the "APS process logic" based
   on the highest local request, the request of the last received
   "Request/State" information, and state transition tables defined in
   Section 9, as follows:

   o  If the highest local request is Clear, clearance of SF or SD, or
      expiration of WTR, a state transition is calculated first based on
      the highest local request and state machine table for local
      requests to obtain an intermediate state.  This intermediate state
      is the final state in case of clearance of SF-P; otherwise,
      starting at this intermediate state, the last received far-end
      request and the state machine table for far-end requests are used
      to calculate the final state.

   o  If the highest local request is neither Clear nor clearance of SF
      or of SD nor expiration of WTR, the APS process logic compares the
      highest local request with the request of the last received
      "Request/State" information based on Figure 6.

      1.  If the highest local request has higher or equal priority, it
          is used with the state transition table for local requests
          defined in Section 9 to determine the final state; otherwise,

      2.  The request of the last received "Request/State" information
          is used with the state transition table for far-end requests
          defined in Section 9 to determine the final state.

   The "APS message generator" generates APS-specific information with
   the signaled APS information for the final state from the state
   transition calculation (with coding as described in Figure 5).

8.2.  Equal Priority Requests

   In general, once a switch has been completed due to a request, it
   will not be overridden by another request of the same priority
   (first-come, first-served policy).  Equal priority requests from both
   sides of a bidirectional protection group are both considered valid,
   as follows:

   o  If the local state is NR, with the requested signal number 1, and
      the far-end state is NR, with the requested signal number 0, the
      local state transits to NR with the requested signal number 0.
      This applies to the case when the remote request for switching to
      the protection transport entity has been cleared.

Top      ToC       Page 22 
   o  If both the local and far-end states are NR, with the requested
      signal number 1, the local state transits to the appropriate new
      state (DNR state for non-revertive mode and WTR state for
      revertive mode).  This applies to the case when the old request
      has been cleared at both ends.

   o  If both the local and far-end states are RR, with the same
      requested signal number, both ends transit to the appropriate new
      state according to the requested signal number.  This applies to
      the case of concurrent deactivation of EXER from both ends.

   o  In other cases, no state transition occurs, even if equal priority
      requests are activated from both ends.  Note that if MSs are
      issued simultaneously to both working and protection transport
      entities, either as local or far-end requests, the MS to the
      working transport entity is considered as having higher priority
      than the MS to the protection transport entity.

8.3.  Signal Degrade of the Protection Transport Entity

   Signal degrade on the protection transport entity has the same
   priority as signal degrade on the working transport entity.  As a
   result, if an SD condition affects both transport entities, the first
   SD detected MUST NOT be overridden by the second SD detected.  If the
   SD is detected simultaneously, either as local or far-end requests on
   both working and protection transport entities, then the SD on the
   standby transport entity MUST be considered as having higher priority
   than the SD on the active transport entity, and the normal traffic
   signal continues to be selected from the active transport entity
   (i.e., no unnecessary protection switching is performed).

   In the preceding sentence, "simultaneously" relates to the occurrence
   of SD on both the active and standby transport entities at input to
   the protection-switching process at the same time, or as long as an
   SD request has not been acknowledged by the remote end in
   bidirectional protection switching.

9.  Protection-Switching State Transition Tables

   In this section, state transition tables for the following protection
   switching configurations are described.

   o  1:1 bidirectional (revertive mode, non-revertive mode);

   o  1+1 bidirectional (revertive mode, non-revertive mode);

   o  1+1 unidirectional (revertive mode, non-revertive mode).

Top      ToC       Page 23 
   Note that any other global or local request that is not described in
   state transition tables does not trigger any state transition.

   The states specified in the state transition tables can be described
   as follows:

   o  NR: NR is the state entered by the local priority under all
      conditions where no local protection-switching requests (including
      WTR and DNR) are active.  NR can also indicate that the highest
      local request is overridden by the far-end request, whose priority
      is higher than the highest local request.  Normal traffic signal
      is selected from the corresponding transport entity.

   o  LO, SF-P, SD-P: The access by the normal traffic to the protection
      transport entity is NOT allowed in this state.  The normal traffic
      is carried by the working transport entity, regardless of the
      fault/degrade condition possibly present (due to the highest
      priority of the switching triggers leading to this state).

   o  FS, SF-W, SD-W, MS-W, MS-P: A switching trigger NOT resulting in
      the protection transport entity unavailability is present.  The
      normal traffic is selected either from the corresponding working
      transport entity or from the protection transport entity,
      according to the behavior of the specific switching trigger.

   o  WTR: In revertive operation, after the clearing of an SF-W or SD-
      W, this maintains normal traffic as selected from the protection
      transport entity until the WTR timer expires or another request
      with higher priority, including the Clear command, is received.
      This is used to prevent frequent operation of the selector in the
      case of intermittent failures.

   o  DNR: In non-revertive operation, this is used to maintain a normal
      traffic to be selected from the protection transport entity.

   o  EXER: Exercise of the APS protocol.

   o  RR: The near end will enter and signal Reverse Request only in
      response to an EXER from the far end.

   [State transition tables are shown at the end of the PDF form of this
   document.]

Top      ToC       Page 24 
10.  Security Considerations

   MPLS-TP is a subset of MPLS and so builds upon many of the aspects of
   the security model of MPLS.  MPLS networks make the assumption that
   it is very hard to inject traffic into a network and equally hard to
   cause traffic to be directed outside the network.  The control-plane
   protocols utilize hop-by-hop security and assume a "chain-of-trust"
   model such that end-to-end control-plane security is not used.  For
   more information on the generic aspects of MPLS security, see
   [RFC5920].

   This document describes a protocol carried in the G-ACh [RFC5586] and
   so is dependent on the security of the G-ACh, itself.  The G-ACh is a
   generalization of the associated channel defined in [RFC4385].  Thus,
   this document relies heavily on the security mechanisms provided for
   the associated channel and described in those two documents.

11.  Acknowledgements

   The authors would like to thank Hao Long, Vincenzo Sestito, Italo
   Busi, Igor Umansky, and Andy Malis for their input to and review of
   the current document.

12.  References

12.1.  Normative References

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

   [RFC4385]  Bryant, S., Swallow, G., Martini, L., and D. McPherson,
              "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
              Use over an MPLS PSN", RFC 4385, February 2006.

   [RFC5586]  Bocci, M., Vigoureux, M., and S. Bryant, "MPLS Generic
              Associated Channel", RFC 5586, June 2009.

   [RFC5920]  Fang, L., "Security Framework for MPLS and GMPLS
              Networks", RFC 5920, July 2010.

   [G.841]    International Telecommunications Union, "Types and
              characteristics of SDH network protection architectures",
              ITU-T Recommendation G.841, October 1998.

   [G.873.1]  International Telecommunications Union, "Optical Transport
              Network (OTN): Linear protection", ITU-T Recommendation
              G.873.1, May 2014.

Top      ToC       Page 25 
   [G.8031]   International Telecommunications Union, "Ethernet linear
              protection switching", ITU-T Recommendation G.8031/Y.1342,
              June 2011.

   [T1.105.01]
              American National Standards Institute, "Synchronous
              Optical Network (SONET) - Automatic Protection Switching",
              ANSI 0900105.01:2000 (R2010), March 2000.

12.2.  Informative References

   [RFC6378]  Weingarten, Y., Bryant, S., Osborne, E., Sprecher, N., and
              A. Fulignoli, "MPLS Transport Profile (MPLS-TP) Linear
              Protection", RFC 6378, October 2011.

   [RFC7271]  Ryoo, J., Gray, E., van Helvoort, H., D'Alessandro, A.,
              Cheung, T., and E. Osborne, "MPLS Transport Profile (MPLS-
              TP) Linear Protection to Match the Operational
              Expectations of Synchronous Digital Hierarchy, Optical
              Transport Network, and Ethernet Transport Network
              Operators", RFC 7271, June 2014.

   [RFC7324]  Osborne, E., "Updates to MPLS Transport Profile Linear
              Protection", RFC 7324, July 2014.

Top      ToC       Page 26 
Appendix A.  Operation Examples of the APS Protocol

   The sequence diagrams shown in this section are only a few examples
   of the APS operations.  The first APS message, which differs from the
   previous APS message, is shown.  The operation of hold-off timer is
   omitted.  The fields whose values are changed during APS packet
   exchange are shown in the APS packet exchange.  They are Request/
   State, requested traffic, and bridged traffic.  For an example,
   SF(0,1) represents an APS packet with the following field values:
   Request/State = SF, Requested Signal = 0, and Bridged Signal = 1.
   The values of the other fields remain unchanged from the initial
   configuration.  The signal numbers 0 and 1 refer to null signal and
   normal traffic signal, respectively.  W(A->Z) and P(A->Z) indicate
   the working and protection paths in the direction of A to Z,
   respectively.

   Example 1. 1:1 bidirectional protection switching (revertive mode) -
   Unidirectional SF case

                       A                  Z
                       |                  |
                   (1) |---- NR(0,0)----->|
                       |<----- NR(0,0)----|
                       |                  |
                       |                  |
                   (2) | (SF on W(Z->A))  |
                       |---- SF(1,1)----->| (3)
                       |<----- NR(1,1)----|
                   (4) |                  |
                       |                  |
                   (5) | (Recovery)       |
                       |---- WTR(1,1)---->|
                      /|                  |
             WTR timer |                  |
                      \|                  |
                   (6) |---- NR(0,0)----->| (7)
                   (8) |<----- NR(0,0)----|
                       |                  |

   (1)  The protected domain is operating without any defect, and the
        working entity is used for delivering the normal traffic.

   (2)  Signal Fail occurs on the working entity in the Z to A
        direction.  Selector and bridge of node A select protection
        entity.  Node A generates an SF(1,1) message.

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   (3)  Upon receiving SF(1,1), node Z sets selector and bridge to
        protection entity.  As there is no local request in node Z, node
        Z generates an NR(1,1) message.

   (4)  Node A confirms that the far end is also selecting protection
        entity.

   (5)  Node A detects clearing of the SF condition, starts the WTR
        timer, and sends a WTR(1,1) message.

   (6)  At expiration of the WTR timer, node A sets selector and bridge
        to working entity and sends an NR(0,0) message.

   (7)  Node Z is notified that the far-end request has been cleared and
        sets selector and bridge to working entity.

   (8)  It is confirmed that the far end is also selecting working
        entity.

   Example 2. 1:1 bidirectional protection switching (revertive mode) -
   Bidirectional SF case

                       A                  Z
                       |                  |
                   (1) |---- NR(0,0)----->| (1)
                       |<----- NR(0,0)----|
                       |                  |
                       |                  |
                   (2) | (SF on W(Z<->A)) | (2)
                       |<---- SF(1,1)---->|
                   (3) |                  | (3)
                       |                  |
                   (4) |    (Recovery)    | (4)
                       |<---- NR(1,1)---->|
                   (5) |<--- WTR(1,1)---->| (5)
                      /|                  |\
             WTR timer |                  | WTR timer
                      \|                  |/
                   (6) |<---- NR(1,1)---->| (6)
                   (7) |<----- NR(0,0)--->| (7)
                   (8) |                  | (8)

   (1)  The protected domain is operating without any defect, and the
        working entity is used for delivering the normal traffic.

   (2)  Nodes A and Z detect local SF conditions on the working entity,
        set selector and bridge to protection entity, and generate
        SF(1,1) messages.

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   (3)  Upon receiving SF(1,1), each node confirms that the far end is
        also selecting protection entity.

   (4)  Each node detects clearing of the SF condition and sends an
        NR(1,1) message as the last received APS message was SF.

   (5)  Upon receiving NR(1,1), each node starts the WTR timer and sends
        WTR(1,1).

   (6)  At expiration of the WTR timer, each node sends NR(1,1) as the
        last received APS message was WTR.

   (7)  Upon receiving NR(1,1), each node sets selector and bridge to
        working entity and sends an NR(0,0) message.

   (8)  It is confirmed that the far end is also selecting working
        entity.

   Example 3. 1:1 bidirectional protection switching (revertive mode) -
   Bidirectional SF case - Inconsistent WTR timers

                       A                  Z
                       |                  |
                   (1) |---- NR(0,0)----->| (1)
                       |<----- NR(0,0)----|
                       |                  |
                       |                  |
                   (2) | (SF on W(Z<->A)) | (2)
                       |<---- SF(1,1)---->|
                   (3) |                  | (3)
                       |                  |
                   (4) |    (Recovery)    | (4)
                       |<---- NR(1,1)---->|
                   (5) |<--- WTR(1,1)---->| (5)
                      /|                  |\
             WTR timer |                  | |
                      \|                  | WTR timer
                   (6) |----- NR(1,1)---->| | (7)
                       |                  |/
                   (9) |<----- NR(0,0)----| (8)
                       |---- NR(0,0)----->| (10)

   (1)   The protected domain is operating without any defect, and the
         working entity is used for delivering the normal traffic.

   (2)   Nodes A and Z detect local SF conditions on the working entity,
         set selector and bridge to protection entity, and generate
         SF(1,1) messages.

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   (3)   Upon receiving SF(1,1), each node confirms that the far end is
         also selecting protection entity.

   (4)   Each node detects clearing of the SF condition and sends an
         NR(1,1) message as the last received APS message was SF.

   (5)   Upon receiving NR(1,1), each node starts the WTR timer and
         sends WTR(1,1).

   (6)   At expiration of the WTR timer in node A, node A sends an
         NR(1,1) message as the last received APS message was WTR.

   (7)   At node Z, the received NR(1,1) is ignored as the local WTR has
         a higher priority.

   (8)   At expiration of the WTR timer in node Z, node Z sets selector
         and bridge to working entity and sends an NR(0,0) message.

   (9)   Upon receiving NR(0,0), node A sets selector and bridge to
         working entity and sends an NR(0,0) message.

   (10)  It is confirmed that the far end is also selecting working
         entity.

   Example 4. 1:1 bidirectional protection switching (non-revertive
   mode) - Unidirectional SF on working followed by unidirectional SF on
   protection

Top      ToC       Page 30 
                       A                  Z
                       |                  |
                   (1) |---- NR(0,0)----->| (1)
                       |<----- NR(0,0)----|
                       |                  |
                       |                  |
                   (2) | (SF on W(Z->A))  |
                       |----- SF(1,1)---->| (3)
                   (4) |<----- NR(1,1)----|
                       |                  |
                       |                  |
                   (5) |    (Recovery)    |
                       |----- DNR(1,1)--->| (6)
                       |<--- DNR(1,1)---->|
                       |                  |
                       |                  |
                       | (SF on P(A->Z))  | (7)
                   (8) |<--- SF-P(0,0)----|
                       |---- NR(0,0)----->|
                       |                  |
                       |                  |
                       |     (Recovery)   | (9)
                       |<----- NR(0,0)----|
                       |                  |

   (1)  The protected domain is operating without any defect, and the
        working entity is used for delivering the normal traffic.

   (2)  Signal Fail occurs on the working entity in the Z to A
        direction.  Selector and bridge of node A select the protection
        entity.  Node A generates an SF(1,1) message.

   (3)  Upon receiving SF(1,1), node Z sets selector and bridge to
        protection entity.  As there is no local request in node Z, node
        Z generates an NR(1,1) message.

   (4)  Node A confirms that the far end is also selecting protection
        entity.

   (5)  Node A detects clearing of the SF condition and sends a DNR(1,1)
        message.

   (6)  Upon receiving DNR(1,1), node Z also generates a DNR(1,1)
        message.

   (7)  Signal Fail occurs on the protection entity in the A to Z
        direction.  Selector and bridge of node Z select the working
        entity.  Node Z generates an SF-P(0,0) message.

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   (8)  Upon receiving SF-P(0,0), node A sets selector and bridge to
        working entity and generates an NR(0,0) message.

   (9)  Node Z detects clearing of the SF condition and sends an NR(0,0)
        message.

   Exmaple 5. 1:1 bidirectional protection switching (non-revertive
   mode) - Bidirectional SF on working followed by bidirectional SF on
   protection

                       A                  Z
                       |                  |
                   (1) |---- NR(0,0)----->| (1)
                       |<----- NR(0,0)----|
                       |                  |
                       |                  |
                   (2) | (SF on W(A<->Z)) | (2)
                   (3) |<---- SF(1,1)---->| (3)
                       |                  |
                       |                  |
                   (4) |    (Recovery)    | (4)
                   (5) |<---- NR(1,1)---->| (5)
                       |<--- DNR(1,1)---->|
                       |                  |
                       |                  |
                   (6) | (SF on P(A<->Z)) | (6)
                   (7) |<--- SF-P(0,0)--->| (7)
                       |                  |
                       |                  |
                   (8) |     (Recovery)   | (8)
                       |<---- NR(0,0)---->|
                       |                  |

   (1)  The protected domain is operating without any defect, and the
        working entity is used for delivering the normal traffic.

   (2)  Nodes A and Z detect local SF conditions on the working entity,
        set selector and bridge to protection entity, and generate
        SF(1,1) messages.

   (3)  Upon receiving SF(1,1), each node confirms that the far end is
        also selecting protection entity.

   (4)  Each node detects clearing of the SF condition and sends an
        NR(1,1) message as the last received APS message was SF.

   (5)  Upon receiving NR(1,1), each node sends DNR(1,1).

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   (6)  Signal Fail occurs on the protection entity in both directions.
        Selector and bridge of each node selects the working entity.
        Each node generates an SF-P(0,0) message.

   (7)  Upon receiving SF-P(0,0), each node confirms that the far end is
        also selecting working entity.

   (8)  Each node detects clearing of the SF condition and sends an
        NR(0,0) message.

Authors' Addresses

   Huub van Helvoort (editor)
   Huawei Technologies

   EMail: huub@van-helvoort.eu


   Jeong-dong Ryoo (editor)
   ETRI

   EMail: ryoo@etri.re.kr


   Haiyan Zhang
   Huawei Technologies

   EMail: zhanghaiyan@huawei.com


   Feng Huang
   Philips

   EMail: feng.huang@philips.com


   Han Li
   China Mobile

   EMail: lihan@chinamobile.com


   Alessandro D'Alessandro
   Telecom Italia

   EMail: alessandro.dalessandro@telecomitalia.it