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Versions: (draft-weingarten-mpls-tp-linear-protection) 00 01 02 03 04 05 06 07 08 09 RFC 6378

Network Working Group                                          S. Bryant
Internet-Draft                                                E. Osborne
Intended status: Standards Track                                   Cisco
Expires: December 4, 2011                                    N. Sprecher
                                                  Nokia Siemens Networks
                                                       A. Fulignoli, Ed.
                                                                Ericsson
                                                      Y. Weingarten, Ed.
                                                  Nokia Siemens Networks
                                                            June 2, 2011


                       MPLS-TP Linear Protection
              draft-ietf-mpls-tp-linear-protection-07.txt

Abstract

   The Transport Profile for Multiprotocol Label Switching (MPLS-TP) is
   being specified jointly by IETF and ITU-T.  This document addresses
   the functionality described in the MPLS-TP Survivability Framework
   document [SurvivFwk] and defines a protocol that may be used to
   fulfill the function of the Protection State Coordination for linear
   protection, as described in that document.

   This document is a product of a joint Internet Engineering Task Force
   (IETF) / International Telecommunications Union Telecommunications
   Standardization Sector (ITU-T) effort to include an MPLS Transport
   Profile within the IETF MPLS and PWE3 architectures to support the
   capabilities and functionalities of a packet transport network as
   defined by the ITU-T.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 4, 2011.




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Copyright Notice

   Copyright (c) 2011 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
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
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   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Protection architectures . . . . . . . . . . . . . . . . .  4
     1.2.  Scope of the document  . . . . . . . . . . . . . . . . . .  5
     1.3.  Contributing authors . . . . . . . . . . . . . . . . . . .  6
   2.  Conventions used in this document  . . . . . . . . . . . . . .  6
     2.1.  Acronyms . . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.2.  Definitions and Terminology  . . . . . . . . . . . . . . .  7
   3.  Protection switching control logic . . . . . . . . . . . . . .  7
     3.1.  Local Request Logic  . . . . . . . . . . . . . . . . . . .  8
     3.2.  Remote Requests  . . . . . . . . . . . . . . . . . . . . . 10
     3.3.  PSC Control Logic  . . . . . . . . . . . . . . . . . . . . 11
     3.4.  PSC Message Generator  . . . . . . . . . . . . . . . . . . 12
     3.5.  Wait-to-Restore (WTR) timer  . . . . . . . . . . . . . . . 12
     3.6.  PSC Control States . . . . . . . . . . . . . . . . . . . . 12
       3.6.1.  Local and Remote state . . . . . . . . . . . . . . . . 14
   4.  Protection state coordination (PSC) protocol . . . . . . . . . 14
     4.1.  Transmission and acceptance of PSC control packets . . . . 15
     4.2.  Protocol format  . . . . . . . . . . . . . . . . . . . . . 15
       4.2.1.  PSC Ver field  . . . . . . . . . . . . . . . . . . . . 16
       4.2.2.  PSC Request field  . . . . . . . . . . . . . . . . . . 16
       4.2.3.  Protection Type (PT) . . . . . . . . . . . . . . . . . 17
       4.2.4.  Revertive (R) field  . . . . . . . . . . . . . . . . . 18
       4.2.5.  Fault path (FPath) field . . . . . . . . . . . . . . . 18
       4.2.6.  Data path (Path) field . . . . . . . . . . . . . . . . 18
       4.2.7.  Additional TLV information . . . . . . . . . . . . . . 19
     4.3.  Principles of Operation  . . . . . . . . . . . . . . . . . 19
       4.3.1.  Basic operation  . . . . . . . . . . . . . . . . . . . 19
       4.3.2.  Priority of inputs . . . . . . . . . . . . . . . . . . 20
       4.3.3.  Operation of PSC States  . . . . . . . . . . . . . . . 21
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 31
     5.1.  Pseudowire Associated Channel Type . . . . . . . . . . . . 32
     5.2.  PSC Request Field  . . . . . . . . . . . . . . . . . . . . 32
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 33
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 33
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 33
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 33
   Appendix A.  PSC state machine tables  . . . . . . . . . . . . . . 35
   Appendix B.  Exercising the protection domain  . . . . . . . . . . 39
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 40









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1.  Introduction

   The MPLS Transport Profile (MPLS-TP) [TPFwk] is a framework for the
   construction and operation of packet-switched transport networks
   based on the architectures for MPLS ([RFC3031] and [RFC3032]) and for
   Pseudowires (PWs) ([RFC3985] and [RFC5659]) and the requirements of
   [RFC5654].

   Network survivability is the ability of a network to recover traffic
   delivery following failure, or degradation of network resources.  The
   MPLS-TP Survivability Framework [SurvivFwk] is a framework for
   survivability in MPLS-TP networks, and describes recovery elements,
   types, methods, and topological considerations, focusing on
   mechanisms for recovering MPLS-TP Label Switched Paths (LSPs).

   Linear protection in mesh networks - networks with arbitrary
   interconnectivity between nodes - is described in Section 4.7 of
   [SurvivFwk].  Linear protection provides rapid and simple protection
   switching.  In a mesh network, linear protection provides a very
   suitable protection mechanism because it can operate between any pair
   of points within the network.  It can protect against a defect in an
   intermediate node, a span, a transport path segment, or an end-to-end
   transport path.

1.1.  Protection architectures

   Protection switching is a fully allocated survivability mechanism.
   It is fully allocated in the sense that the route and bandwidth of
   the protection path is reserved for a selected working path or set of
   working paths.  It provides a fast and simple survivability
   mechanism, that allows the network operator to easily grasp the
   active state of the network, compared to other survivability
   mechanisms.

   As specified in the Survivability Framework document [SurvivFwk],
   protection switching is applied to a protection domain.  For the
   purposes of this document, we define the protection domain of a P2P
   LSP as consisting of two Label Edge Routers (LER) and the transport
   paths that connect them (see Figure 3 below).  For a P2MP LSP the
   protection domain includes the root (or source) LER, the destination
   (or sink) LERs, and the transport paths that connect them.

   In 1+1 unidirectional architecture as presented in [SurvivFwk], a
   protection transport path is dedicated to the working transport path.
   Normal traffic is bridged (as defined in [RFC4427])and fed to both
   the working and the protection paths by a permanent bridge at the
   source of the protection domain.  The sink of the protection domain
   uses a selector to select either the working or protection paths to



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   receive the traffic from, based on a predetermined criteria, e.g.
   server defect indication.  When used for bidirectional switching the
   1+1 protection architecture must also support a Protection State
   Coordination (PSC) protocol.  This protocol is used to help
   coordinate between both ends of the protection domain in selecting
   the proper traffic flow.

   In the 1:1 architecture, a protection transport path is dedicated to
   the working transport path of a single service and the traffic is
   only transmitted either on the working or the protection path, by
   using a selector at the source of the protection domain.  A selector
   at the sink of the protection domain then selects the path that
   carries the normal traffic.  Since the source and sink need to be
   coordinated to ensure that the selector at both ends select the same
   path, this architecture must support a PSC protocol.

   The 1:n protection architecture extends the 1:1 architecture above by
   sharing the protection path among n services.  Again, the protection
   path is fully allocated and disjoint from any of the n working
   transport paths that it is being used to protect.  The normal data
   traffic for each service is transmitted either on the normal working
   path for that service or, in cases that trigger protection switching
   (as defined in [SurvivFwk]), may be sent on the protection path.  The
   switching action is similar to the 1:1 case where a selector is used
   at the source.  It should be noted that in cases where multiple
   working path services have triggered protection switching that some
   services, dependent upon their Service Level Agreement (SLA), may not
   be transmitted as a result of limited resources on the protection
   path.  In this architecture there may be a need for coordination of
   the protection switching, and also for resource allocation
   negotiation.  The procedures for this are for further study and may
   be addressed in future documents.

1.2.  Scope of the document

   As was pointed out in the Survivability Framework [SurvivFwk] and
   highlighted above, there is a need for coordination between the end
   points of the protection domain when employing bidirectional
   protection schemes.  This is especially true when there is a need to
   maintain traffic over a co-routed bidirectional LSP.

   The scope of this draft is to present a protocol for the Protection
   State Coordination of Linear Protection.  The protocol addresses the
   protection of LSPs in an MPLS-TP network as required by [RFC5654] (in
   particular requirements 63-65 and 74-79) and described in
   [SurvivFwk].  The basic protocol is designed for use in conjunction
   with the 1:1 protection architecture bidirectional protection and for
   1+1 protection of a bidirectional path (for both unidirectional and



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   bidirectional protection switching).  Applicability of the protocol
   for 1:1 unidirectional protection and for 1:n protection schemes may
   be documented in a future document.  The applicability of this
   protocol to additional MPLS-TP constructs and topologies may be
   documented in future documents.

   While the unidirectional 1+1 protection architecture does not require
   the use of a coordination protocol, the protocol may be used by the
   ingress node of the path to notify the far-side end point that a
   switching condition has occurred and verify the consistency of the
   end point configuration.  This use may be especially useful for
   point-to-multipoint transport paths, that are unidirectional by
   definition of [RFC5654].

1.3.  Contributing authors

   Hao Long (Huawei), Dan Frost (Cisco), Davide Chiara (Ericsson),
   Francesco Fondelli (Ericsson),


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].


























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2.1.  Acronyms

   This draft uses the following acronyms:

   DNR     Do not revert
   FS      Forced Switch
   G-ACh   Generic Associated Channel
   LER     Label Edge Router
   LO      Lockout of protection
   MPLS-TP Transport Profile for MPLS
   MS      Manual Switch
   NR      No Request
   P2P     Point-to-point
   P2MP    Point-to-multipoint
   PSC     Protection State Coordination Protocol
   SD      Signal Degrade
   SF      Signal Fail
   SLA     Service Level Agreement
   WTR     Wait-to-Restore

2.2.  Definitions and Terminology

   The terminology used in this document is based on the terminology
   defined in [RFC4427] and further adapted for MPLS-TP in [SurvivFwk].
   In addition, we use the term LER to refer to a MPLS-TP Network
   Element, whether it is a LSR, LER, T-PE, or S-PE.


3.  Protection switching control logic

   Protection switching processes the local triggers described in
   requirements 74-79 of [RFC5654] together with inputs received from
   the far-end LER.  Based on these inputs the LER will take certain
   protection switching actions, e.g. switching the selector to transmit
   on the working or protection path for 1:1 protection or switching the
   selector to receive the traffic for either 1:1 or 1+1 protection, and
   transmit different protocol messages.

   The following figure shows the logical decomposition of the
   Protection Switching Control Logic into different logical processing
   units.  These processing units are presented in subsequent
   subsections of this document.  This logical decomposition is only
   intended for descriptive purposes, any implementation that produces
   the external behavior described in section 4 is acceptable.







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                  Server Indication     Control Plane Indication
                  -----------------+  +-------------
                Operator Command   |  |   OAM Indication
                ----------------+  |  |  +---------------
                                |  |  |  |
                                V  V  V  V
                             +---------------+         +-------+
                             | Local Request |<--------|  WTR  |
                             |    logic      |WTR Exps | Timer |
                             +---------------+         +-------+
                                    |                      ^
                       Highest local|request               |
                                    V                      | Start/Stop
                            +-----------------+            |
                Remote PSC  |  PSC  Control   |------------+
               ------------>|      logic      |
                  Request   +-----------------+
                                    |
                                    |  Action         +------------+
                                    +---------------->|  Message   |
                                                      | Generator  |
                                                      +------------+
                                                            |
                                                 Output PSC | Message
                                                            V


               Figure 1: Protection switching control logic

   Figure 1 describes the logical architecture of the protection
   switching control.  The Local Request logic unit accepts the triggers
   from the OAM, external operator commands, from the local control
   plane (when present), and the Wait-to-Restore timer.  By considering
   all of these local request sources it determines the highest priority
   local request.  This high-priority request is passed to the PSC
   Control logic, that will cross-check this local request with the
   information received from the far-end LER.  The PSC Control logic
   uses this input to determine what actions need to be taken, e.g.
   local actions at the LER, or what message should be sent to the far-
   end LER, and the current status of the protection domain.

3.1.  Local Request Logic

   The Local Request logic processes input triggers from five sources:

   o  Operator command - the network operator may issue local
      administrative commands on the LER that trigger protection
      switching.  The supported commands are Forced Switch, Manual



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      Switch, Clear, Lockout of Protection, (see definitions in
      [RFC4427]).  An implementation MAY provide additional commands for
      operator use; providing that these commands do not introduce
      incompatable behavior between two arbitrary implementations, they
      are outside the scope of this document.  For example, an
      implementation could provide a command to manually trigger a "WTR
      expires" trigger (see below) input without waiting for the
      duration of the WTR timer; as this merely hastens the transition
      from one state to another and has no impact on the state machine
      itself, it would be perfectly valid.

   o  Server layer alarm indication - the underlying server layer of the
      network detects failure conditions at the underlying layer and may
      issue an indication to the MPLS-TP layer.  The server layer may
      employ its own protection switching mechanism, and therefore this
      input MAY be controlled by a holdoff-timer that SHOULD be
      configurable by the network operator.  The holdoff-timer is
      described in greater detail in [SurvivFwk].

   o  Control plane - if there is a control plane active in the network
      (either signaling or routing), it MAY trigger protection switching
      based on conditions detected by the control plane.  If the control
      plane is based on GMPLS [RFC3945] then the recovery process SHALL
      comply with the process described in [RFC4872].

   o  OAM indication - OAM fault management or performance measurement
      tools may detect a failure or degrade condition on either the
      working or protection transport path and this SHOULD input an
      indication to the Local Request Logic.

   o  WTR expires - The Wait-to-Restore timer is used in conjunction
      with recovery from failure conditions on the working path in
      revertive mode.  The timer SHALL signal the PSC control process
      when it expires and the end point SHOULD revert to the normal
      transmission of the user data traffic.

   The Local request logic SHALL process these different input sources
   and, based on the priorities between them (see section 4.3.2), shall
   produce a current local request.  If more than one local input source
   generates an indicator, then the Local request logic shall select the
   higher priority indicator and block any lower priority indicator.  As
   a result, there is a single current local request that is passed to
   the PSC Control logic.  The different local requests that may be
   output from the Local Request Logic are:

   o  Clear - if the operator cancels an active local administrative
      command, i.e.  LO/FS/MS.




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   o  Lockout of Protection (LO) - if the operator requested to prevent
      switching data traffic to the protection path, for any purpose.

   o  Signal Fail (SF) - if any of the Server Layer, Control plane, or
      OAM indications signaled a failure condition on either the
      protection path or one of the working paths.

   o  Signal Degrade (SD) - if any of the Server Layer, Control plane,
      or OAM indications signaled a degraded transmission condition on
      either the protection path or one of the working paths.  The
      determination and actions for SD are for further study and may
      appear in a separate document.  All references to SD input are
      place-holders for this extension.

   o  Clear Signal Fail - if all of the Server Layer, Control plane, or
      OAM indications are no longer indicating a failure condition on a
      path that was previously indicating a failure condition.

   o  Forced Switch (FS) - if the operator requested that traffic be
      switched from one of the working paths to the protection path.

   o  Manual Switch (MS) - if the operator requested that traffic be
      switched from the working path to the protection path.  This is
      only relevant if there is no currently active fault condition or
      Operator command.

   o  WTR Expires - generated by the WTR timer completing its period.

   If none of the input sources have generated any input then the Local
   request logic should generate a No Request (NR) request as the
   current local request .

3.2.  Remote Requests

   In addition to the local requests, generated as a result of the local
   triggers, indicated in the previous subsection, the PSC Control Logic
   SHALL accept PSC messages from the far-end LER of the transport path.
   These remote messages indicate the status of the transport path from
   the viewpoint of the far-end LER, and may indicate if the local MEP
   SHOULD initiate a protection switch operation.

   The following remote requests may be received by the PSC process:

   o  Remote LO - indicates that the remote end point is in Unavailable
      state due to a Lockout of Protection operator command.

   o  Remote SF - indicates that the remote end point has detected a
      Signal Fail condition on one of the transport paths in the



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      protection domain.  This remote message SHALL include an
      indication of which transport path is affected by the SF
      condition.  In addition, it should be noted that the SF condition
      may be either a unidirectional or a bidirectional failure, even if
      the transport path is bidirectional.

   o  Remote SD - indicates that the remote end point has detected a
      Signal Degrade condition on one of the transport paths in the
      protection domain.  This remote message SHALL include an
      indication of which transport path is affected by the SD
      condition.  In addition, it should be noted that the SD condition
      may be either a unidirectional or a bidirectional failure, even if
      the transport path is bidirectional.

   o  Remote FS - indicates that the remote end point is operating under
      an operator command to switch the traffic to the protection path.

   o  Remote MS - indicates that the remote end point is operating under
      an operator command to switch the traffic to the path that was not
      being used previously.

   o  Remote WTR - indicates that the remote end point has determined
      that the failure condition has recovered and has started its WTR
      timer in preparation for reverting to the Normal state.

   o  Remote DNR - indicates that the remote end point has determined
      that the failure condition has recovered and will continue
      transporting traffic on the protection path due to operator
      configuration that prevents automatic reversion to the Normal
      state.

   o  Remote NR - indicates that the remote end point has no abnormal
      condition to report.

3.3.  PSC Control Logic

   The PSC Control Logic SHALL accept as input -

   a.  the current local request output from the Local Request Logic
       (see section 3.1),

   b.  the remote request message from the remote end point of the
       transport path (see section 3.2), and

   c.  the current state of the PSC Control Logic (maintained internally
       by the PSC Control Logic).

   Based on the priorities between the different inputs, the PSC Control



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   Logic SHALL determine the new state of the PSC Control Logic and what
   actions need to be taken.

   The new state information SHALL be retained by the PSC Control Logic,
   while the requested action should be sent to the PSC Message
   Generator (see subsection 3.4) to generate and transmit the proper
   PSC message to be transmitted to the remote end point of the
   protection domain.

3.4.  PSC Message Generator

   Based on the action output from the PSC Control Logic this unit
   formats the PSC protocol message that is transmitted to the remote
   end point of the protection domain.  This message may either be the
   same as the previously transmitted message or change when the PSC
   control state (see section 3.6) has changed.  The messages should be
   transmitted as described in section 4.1 of this document.

3.5.  Wait-to-Restore (WTR) timer

   The WTR timer is used to delay reversion to Normal state when
   recovering from a failure condition on the working path and the
   protection domain is configured for revertive behavior.  The length
   of the timer may be provisioned by the operator.  The WTR may be in
   one of two states - either Running or Stopped.  The control of the
   WTR timer is managed by the PSC Control Logic, by use of internal
   signals to start and stop, i.e. reset, the WTR timer.

   If the WTR timer expires prior to being stopped it shall generate a
   WTR Expires local signal that shall be processed by the Local Request
   Logic.  If the WTR timer is running, sending a Stop command shall
   reset the timer, and put the WTR timer into Stopped state, but shall
   not generate a WTR Expires local signal.  If the WTR timer is
   stopped, a Stop command shall be ignored.

3.6.  PSC Control States

   The PSC Control Logic SHOULD maintain information on the current
   state of the protection domain.  Information on the state of the
   domain is maintained by each LER within the protection domain.  The
   state information SHALL include information of the current state of
   the protection domain, an indication of the cause for the current
   state (e.g. unavailable due to local LO command, protecting due to
   remote FS), and, for each LER, SHOULD include an indication if the
   state is related to a remote or local condition.  If there are both a
   local indicator and remote indicator for the state then the state
   shall be considered a local state.  For example, if the protection
   domain enters into a Protecting failure state and the LER identifies



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   this due to a remote SF input, and then a local SF indication is
   received then even though this was initially a remote Protecting
   failure state, by receiving the local SF input the LER is considered
   to be in local Protecting failure state.

   It should be noted that when referring to the "transport" of the data
   traffic, in the following descriptions and later in the document that
   the data will be transmitted on both the working and the protection
   paths when using 1+1 protection, and on either the working or the
   protection path exclusively when using 1:1 protection.  When using
   1+1 protection, the receiving LER should select the proper
   transmission, according to the state of the protection domain.

   The protection domain states that are supported by the PSC Control
   Logic are:

   o  Normal state - Both the protection and working paths are fully
      allocated and active, data traffic is being transported over (or
      selected from) the working path, and no trigger events are
      reported within the domain.

   o  Unavailable state - The protection path is unavailable - either as
      a result of an operator Lockout command or a failure condition
      detected on the protection path.

   o  Protecting failure state - The working path has reported a
      failure/degrade condition and the user traffic is being
      transported (or selected) on the protection path.

   o  Protecting administrative state - The operator has issued a
      command switching the user traffic to the protection path.

   o  Wait-to-restore state - The protection domain is recovering from a
      SF/SD condition on the working path that is being controlled by
      the Wait-to-Restore (WTR) timer.

   o  Do-not-revert state - The protection domain is recovering from a
      Protecting state, but the operator has configured the protection
      domain to not automatically revert to the Normal state upon
      recovery.  The protection domain SHALL remain in this state until
      the operator issues a command to revert to the Normal state or
      there is a new trigger to switch to a different state.

   See section 4.3.3 for details on what actions are taken by the PSC
   Process Logic for each state and the relevant input.






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3.6.1.  Local and Remote state

   An end-point may be in a given state as a result of either a local
   input indicator, e.g.  OAM, WTR timer, or as a result of receiving a
   PSC message from the far-end LER.  If the state is entered as a
   result of a local input indicator, then the state SHOULD be
   considered a local state.  If the state is entered as a result of a
   PSC message, in the absence of a local input, then the state SHOULD
   be considered a remote state.  This differentiation affects how the
   LER should react to different inputs, as described in section 4.3.3.
   The PSC Control logic should maintain, together with the current
   protection domain state, an indication of whether this is a local or
   remote state, for this LER.

   In any instance where the LER has both a local and remote indicators
   that cause the protection domain to enter a particular state, then
   the state SHOULD be considered a local state, regardless of the order
   in which the indicators were processed.  If, however, the LER has
   local and remote indicators that would cause the protection domain to
   enter different states, e.g. a Local SF on working and a Remote
   Lockout message, then the input with the higher priority (see section
   4.3.2) will be the deciding factor and the source of that indicator
   will determine whether it is local or remote.  In the given example
   the result would be a Remote Unavailable state transmitting SF(1,0)
   messages.


4.  Protection state coordination (PSC) protocol

   Bidirectional protection switching, as well as unidirectional 1:1
   protection, requires coordination between the two end points in
   determining which of the two possible paths, the working or
   protection path, is transmitting the data traffic in any given
   situation.  When protection switching is triggered as described in
   section 3, the end points must inform each other of the switch-over
   from one path to the other in a coordinated fashion.

   There are different possibilities for the type of coordinating
   protocol.  One possibility is a two-phased coordination in which the
   LER that is initiating the protection switching sends a protocol
   message indicating the switch but the actual switch-over is performed
   only after receiving an 'Ack' from the far-end LER.  The other
   possibility is a single-phased coordination, in which the initiating
   LER performs the protection switchover to the alternate path and
   informs the far-end LER of the switch, and the far-end LER MUST
   complete the switchover.

   This protocol is a single-phased protocol, as described above.  In



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   the following subsections we describe the protocol messages that
   SHALL be used between the two end points of the protection domain.

4.1.  Transmission and acceptance of PSC control packets

   The PSC control packets SHALL be transmitted over the protection path
   only.  This allows the transmission of the messages without affecting
   the normal data traffic in the most prevalent case, i.e. the Normal
   state.  In addition, limiting the transmission to a single path
   avoids possible conflicts and race conditions that could develop if
   the PSC messages were sent on both paths.

   When the protection domain state is changed due to a local input,
   three PSC messages SHOULD be transmitted as quickly as possible, to
   allow for rapid protection switching.  This set of three rapid
   messages allows for fast protection switching even if one or two of
   these packets are lost or corrupted.  When the protection domain
   state changes due to a remote message the LER MAY send the three
   rapid messages, but is not required to.  However, when the LER
   tranfers from WTR state to Normal state as a result of a remote NR
   message, the three rapid messages SHOULD be transmitted.

   The frequency of the three rapid messages and the separate frequency
   of the continual transmission SHOULD be configurable by the operator.
   For protection switching within 50ms, it is RECOMMENDED that the
   default interval of the first three PSC messages SHOULD be no larger
   than 3.3ms.  The subsequent messages SHOULD be continuously
   transmitted with an interval of 5 seconds.

   If no valid PSC message is received, the last valid received message
   remains applicable.

4.2.  Protocol format

   The protocol messages SHALL be sent over the G-ACh as described in
   [RFC5586].  There is a single channel type for the set of PSC
   messages.  The actual message function SHALL be identified by the
   Request field of the ACH payload as described below.

   The channel type for the PSC messages SHALL be PSC-CT=0xHH (to be
   assigned by IANA)

   The following figure shows the format for the complete PSC message:








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        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|Version|  Reserved     |          PSC-CT               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Ver|Request|PT |R|  Reserved   |     FPath     |     Path      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         TLV Length            |          Reserved             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                         Optional TLVs                         ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 2: Format of PSC packet with a G-ACh header

   Where:

   o  The following subsections describe the fields of the PSC payload.

4.2.1.  PSC Ver field

   The Ver field identifies the version of the protocol.  For this
   version of the document the value SHALL be 1.

4.2.2.  PSC Request field

   The PSC protocol SHALL support transmission of the following requests
   between the two end points of the protection domain:

   o  (1110) Lockout of protection - indicates that the end point has
      disabled the protection path as a result of an administrative
      command.  Both the FPath and Path fields SHALL be set to 0.

   o  (1100) Forced switch - indicates that the transmitting end point
      has switched traffic to the protection path as a result of an
      administrative command.  The Fpath field SHALL indicate that the
      working path is being blocked (i.e.  Fpath set to 1), and the Path
      field SHALL indicate that user data traffic is being transported
      on the protection path (i.e.  Path set to 1).

   o  (1010) Signal Fail - indicates that the transmitting end point has
      identified a signal fail condition on either the working or
      protection path.  The Fpath field SHALL identify the path that is
      reporting the failure condition (i.e. if protection path then
      Fpath is set to 0 and if working path then Fpath is set to 1), and
      the Path field SHALL indicate where the data traffic is being
      transported (i.e. if protection path is blocked then Path is set
      to 0 and if working path is blocked then Path is set to 1).




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   o  (0111) Signal Degrade - indicates that that the transmitting end
      point has identified a degradation of the signal, or integrity of
      the packet transmission on either the working or protection path.
      This request is presented here only as a place-holder.  The
      specifics for the method of identifying this degradation is out-
      of-scope for this document.  The details of the actions to be
      taken for this situation is left for future specification.

   o  (0101) Manual switch - indicates that the transmitting end point
      has switched traffic to the protection path as a result of an
      administrative Manual Switch command.  The Fpath field SHALL
      indicate that the working path is being blocked (i.e.  Fpath set
      to 1), and the Path field SHALL indicate that user data traffic is
      being transported on the protection path (i.e.  Path set to 1).

   o  (0100) Wait to restore - indicates that the transmitting end point
      is recovering from a failure condition of the working path and has
      started the Wait-to-Restore timer.  Fpath SHALL be set to 0 and
      ignored upon receipt.  Path SHALL indicate the working path that
      is currently being protected (i.e.  Path set to 1).

   o  (0001) Do not revert - indicates that the transmitting end point
      is recovering from a failure/blocked condition, but due to the
      local settings is requesting that the protection domain continues
      to transmit data over the protection path, rather than revert to
      the Normal state.  Fpath SHALL be set to 0 and ignored upon
      receipt.  Path SHALL indicate the working path that is currently
      being protected (i.e.  Path set to 1).

   o  (0000) No request - indicates that the transmitting end point has
      nothing to report, Fpath and Path fields SHALL be set to according
      to the state of the end point, see section 4.3.3 for detailed
      scenarios.

4.2.3.  Protection Type (PT)

   The PT field indicates the currently configured protection
   architecture type, this SHOULD be validated to be consistent for both
   ends of the protection domain.  If an inconsistency is detected then
   an alarm SHALL be sent to the management system.  The following are
   the possible values:

   o  11: bidirectional switching using a permanent bridge

   o  10: bidirectional switching using a selector bridge

   o  01: unidirectional switching using a permanent bridge




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   o  00: for future extensions

   As described in the introduction (section 1.1) a 1+1 protection
   architecture is characterized by the use of a permanent bridge at the
   source node, whereas the 1:1 and 1:n protection architectures are
   characterized by the use of a selector bridge at the source node.

4.2.4.  Revertive (R) field

   This field indicates that the transmitting end point is configured to
   work in revertive mode.  If there is an inconsistency between the two
   end points, i.e. one end point is configured for revertive action and
   the second end point is in non-revertive mode, then the management
   system SHOULD be notified.  Possible values are:

   o  0 - non-revertive mode

   o  1 - revertive mode

4.2.5.  Fault path (FPath) field

   The Fpath field indicates which path (i.e. working or protection) is
   identified to be in a fault condition or affected by an
   administrative command.  The following are the possible values:

   o  0: indicates that the anomaly condition is on the protection path

   o  1: indicates that the anomaly condition is on the working path

   o  2-255: for future extensions

4.2.6.  Data path (Path) field

   The Path field indicates which data is being transmitted on the
   protection path.  Under normal conditions, the protection path
   (especially in 1:1 or 1:n architecture) does not need to carry any
   user data traffic.  If there is a failure/degrade condition on one of
   the working paths, then that working path's data traffic will be
   transmitted over the protection path.  The following are the possible
   values:

   o  0: indicates that the protection path is not transporting user
      data traffic (in 1:n architecture) or transporting redundant user
      data traffic (in 1+1 architecture).

   o  1: indicates that the protection path is transmitting user traffic
      replacing the use of the working path.




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   o  2-255: for future extensions

4.2.7.  Additional TLV information

   It may be necessary for future applications of the protocol to
   include additional information for the proper processing of the
   requests.  For this purpose, we provide for optional additional
   information to be included in the PSC payload.  This information MUST
   include a header that indicates the total length (in bytes) of the
   additional information.

   This information includes the following fields:

   o  TLV Length -- indicates the number of bytes included in the
      optional TLV information.  For the basic PSC protocol operation
      described in this document this value SHOULD be 0.

   o  Reserved -- this field SHALL be 0.

   o  Optional TLVs -- this includes any additional information
      formatted as TLV units.  There are no TLV units defined for the
      basic PSC operation.

4.3.  Principles of Operation

   In all of the following subsections, assume a protection domain
   between LER-A and LER-Z, using paths W (working) and P (protection)
   as shown in figure 3.

                 +-----+ //=======================\\ +-----+
                 |LER-A|//     Working Path        \\|LER-Z|
                 |    /|                             |\    |
                 |  ?< |                             | >?  |
                 |    \|\\    Protection Path      //|/    |
                 +-----+ \\=======================// +-----+

                     |--------Protection Domain--------|


                        Figure 3: Protection domain

4.3.1.  Basic operation

   The purpose of the PSC protocol is to allow an end point of the
   protection domain to notify its peer of the status of the domain that
   is known at the end point and coordinate the transmission of the data
   traffic.  The current state of the end point is expressed in the
   values of the Request field [reflecting the local requests at that



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   end point] and the Fpath field [reflecting knowledge of a blocked
   path].  The coordination between the end points is expressed by the
   value of the Path field [indicating where the user data traffic is
   being transmitted].  The value of the Path field SHOULD be identical
   for both end points at any particular time.  The values of the
   Request and Fpath fields may not be identical between the two end
   points.  In particular it should be noted that a remote message MAY
   not cause the end point to change the Request field that is being
   transmitted while it does affect the Path field (see details in the
   following subsections).

   The protocol is a single-phased protocol.  Single-phased implies that
   each end point notifies its peer of a change in the operation
   (switching to or from the protection path) and makes the switch
   without waiting for acknowledgement.

   The following subsections will identify the messages that SHALL be
   transmitted by the end point in different scenarios.  The messages
   are described as REQ(FP, P) - where REQ is the value of the Request
   field, FP is the value of the Fpath field, and P is the value of the
   Path field.  All examples assume a protection domain between LER-A
   and LER-Z with a single working path and single protection path (as
   shown in figure 3).  Again it should be noted that when using 1:1
   protection the data traffic will be transmitted exclusively on either
   the protection or working path, while when using 1+1 protection the
   traffic will be transmitted on both paths and the receiving LER
   should select the appropriate signal based on the state.  The text
   will refer to this transmission/selection as "transport" of the data
   traffic.

4.3.2.  Priority of inputs

   As noted above (in section 3.1) the PSC Control Process accepts input
   from five local input sources.  There is a definition of priority
   between the different inputs that may be triggered locally.  The list
   of local requests in order of priority are (from highest to lowest
   priority):

   1.   Clear (Operator command)

   2.   Lockout of protection (Operator command)

   3.   Signal Fail on protection (OAM/Control Plane/Server Indication)

   4.   Forced switch (Operator command)

   5.   Signal Fail on working (OAM/Control Plane/Server Indication)




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   6.   Signal Degrade on working (OAM/Control Plane/Server Indication)

   7.   Clear Signal Fail/Degrade (OAM/Control Plane/Server Indication)

   8.   Manual switch (Operator command)

   9.   WTR expires (WTR Timer)

   10.  No request (default)

   As was noted above, the Local request logic SHALL always select the
   local input indicator with the highest priority as the current local
   request, i.e. only the highest priority local input will be used to
   affect the control logic.  All local inputs with lower priority than
   this current local request will be ignored.

   The remote message from the far-end LER is assigned a priority just
   below the similar local input.  For example, a remote Signal Fail on
   protection would have a priority just below a local Signal Fail on
   protection but above a local Forced Switch input.  As mentioned in
   section 3.6.1, the state transition is determined by the higher
   priority input between the highest priority local input and the
   remote message.  This also determines the classification of the state
   as local or remote.  The following subsections detail the transition
   based on the current state and the higher priority of these two
   inputs.

4.3.3.  Operation of PSC States

   The following sub-sections present the operation of the different
   states defined in section 3.6.  For each state we define the
   reaction, i.e. the new state and the message to transmit, to each
   possible input - either the highest priority local input or the PSC
   message from the remote LER.  It should be noted that the new state
   of the protection domain is described from the point of view of the
   LER that is reporting the state, therefore, the language of "the LER
   goes into a state" is referring to the LER reporting that the
   protection domain is now in this new state.  If the definition states
   to "ignore" the message, the intention is that the protection domain
   should remain in its current state and the LER should continue
   transmitting (as presented in section 4.1) the current PSC message.

   When a LER is in a remote state, i.e. state transition in reaction to
   a PSC message recieved from the far-end LER, and receives a new PSC
   message from the far-end LER that indicates a contradictory state,
   e.g. in remote Unavailable state receiving a remote FS(1,1) message,
   then the PSC Control Logic should reevaluate all inputs (both the
   local input and the remote message) as if the LER is in the Normal



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   state.

4.3.3.1.  Normal State

   When the protection domain has no special condition in effect, the
   ingress LER SHALL forward the user data along the working path, and,
   in the case of 1+1 protection, the Permanent Bridge will bridge the
   data to the protection path as well.  The receiving LER SHALL read
   the data from the working path.

   When the protection domain is in Normal State the end-point SHALL
   transmit a NR(0,0) message, indicating - Nothing to report and data
   traffic is being transported on the working path.

   When the protection domain is in Normal State the following
   transitions are relevant in reaction to a local input (new state
   SHOULD be marked as local) to the LER:

   o  A local Lockout of protection input SHALL cause the LER to go into
      local Unavailable State and begin transmission of a LO(0,0)
      message.

   o  A local Forced switch input SHALL cause the LER to go into local
      Protecting administrative state and begin transmission of a
      FS(1,1) message.

   o  A local Signal Fail indication on the protection path SHALL cause
      the LER to go into local Unavailable state and begin transmission
      of a SF(0,0) message.

   o  A local Signal Fail indication on the working path SHALL cause the
      LER to go into local Protecting failure state and begin
      transmission of a SF(1,1) message.

   o  A local Manual switch input SHALL cause the LER to go into local
      Protecting administrative state and begin transmission of a
      MS(1,1) message.

   o  All other local inputs SHALL be ignored.

   In Normal state, remote messages would cause the following reaction
   from the LER (new state SHOULD be marked as remote):

   o  A remote Lockout of protection message SHALL cause the LER to go
      into remote Unavailable state, while continuing to transmit the
      NR(0,0) message.





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   o  A remote Forced switch message SHALL cause the LER to go into
      remote Protecting administrative state, and begin transmitting a
      NR(0,1) message.

   o  A remote Signal Fail message that indicates that the failure is on
      the protection path SHALL cause the LER (LER-A) to go into remote
      Unavailable state, while continuing to transmit the NR(0,0)
      message.

   o  A remote Signal Fail message that indicates that the failure is on
      the working path SHALL cause the LER to go into remote Protecting
      failure state, and transmit a NR(0,1) message.

   o  A remote Manual switch message SHALL cause the LER to go into
      remote Protecting administrative state, and transmit a NR(0,1)
      message.

   o  All other remote messages SHALL be ignored.

4.3.3.2.  Unavailable State

   When the protection path is unavailable - either as a result of a
   Lockout operator command, or as a result of a SF detected on the
   protection path - then the protection domain is in the unavailable
   state.  In this state, the data traffic is transported on the working
   path and is not protected.  When the domain is in unavailable state
   the PSC messages may not get through and therefore the protection is
   more dependent on the local inputs rather than the remote messages
   (that may not be received).

   The protection domain will exit the unavailable state and revert to
   the normal state when, either the operator clears the Lockout command
   or the protection path recovers from the signal fail or degraded
   situation.  Both ends will continue to send the PSC messages over the
   protection path, as a result of this recovery.

   When the LER (assume LER-A) is in Unavailable State the following
   transitions are relevant in reaction to a local input (new state
   SHOULD be marked as local):

   o  A local Clear input SHOULD be ignored if the LER is in remote
      Unavailable state.  If in local Unavailable state due to a Lockout
      command, then the input SHALL cause the LER to go to Normal state
      and begin transmitting a NR(0,0) message.

   o  A local Lockout of protection input SHALL cause the LER to remain
      in local Unavailable State and transmit a LO(0,0) message to the
      far-end LER (LER-Z).



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   o  A local Clear SF of the protection path in local Unavailable state
      that is due to a SF on the protection path SHALL cause the LER to
      go to Normal state and begin transmitting a NR(0,0) message.  If
      the LER is in remote Unavailable state but has an active local SF
      condition, then the local Clear SF SHALL clear the SF local
      condition and the LER SHALL remain in remote Unavailable state and
      begin transmitting NR(0,0) messages.  In all other cases the local
      Clear SF SHOULD be ignored.

   o  A local Forced switch SHALL be ignored by the PSC Control Logic.

   o  A local Signal Fail on the protection path input when in local
      Unavailable state [by implication this is due to a local SF on
      protection] SHALL cause the LER to remain in local Unavailable
      state and transmit a SF(0,0) message.

   o  A local Signal Fail on the working path input when in remote
      Unavailable state SHALL cause the LER to remain in remote
      Unavailable state and transmit a SF(1,0) message.

   o  All other local inputs SHALL be ignored.

   If remote messages are being received over the protection path then
   they would have the following affect:

   o  A remote Lockout of protection message SHALL cause the LER to
      remain in Unavailable state, (note that if the LER was previously
      in local Unavailable state due to a Signal Fail on the protection
      path, then it will now be in remote Unavailable state) and
      continue transmission of the current message (either NR(0,0) or
      LO(0,0) or SF(0,0))

   o  A remote Signal Fail message that indicates that the failure is on
      the protection path SHALL cause the LER to remain in Unavailable
      state and continue transmission of the current message (either
      NR(0,0) or SF(0,0) or LO(0,0)).

   o  A remote No Request, when the LER is in remote Unavailable state
      and there is no active local Signal Fail SHALL cause the LER to go
      into Normal state and continue transmission of the current
      message.  If there is a local Signal Fail on the protection path,
      the LER SHALL remain in local Unavailable state and transmit a
      SF(0,0) message.  If there is a local Signal Fail on the working
      path, the LER SHALL go into local Protecting Failure state and
      transmit a SF(1,1) message.  When in local Unavailable state, the
      remote message SHALL be ignored.





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   o  All other remote messages SHALL be ignored.

4.3.3.3.  Protecting administrative state

   In the protecting state the user data traffic is being transported on
   the protection path, while the working path is blocked due to an
   operator command, i.e.  Forced Switch or Manual Switch.  The
   difference between a local FS and local MS affects what local
   indicators may be received - the Local request logic will block any
   local SF when under the influence of a local FS, whereas the SF would
   override a local MS.  In general, a MS will be canceled in case of
   either a local or remote SF or LO condition.

   The following describe the reaction to local input:

   o  A local Clear SHOULD be ignored if in remote Protecting
      administrative state.  If in local Protecting administrative state
      then this input SHALL cause the LER to go into Normal state and
      begin transmitting a NR(0,0) message.

   o  A local Lockout of protection input SHALL cause the LER to go into
      local Unavailable state and begin transmission of a LO(0,0)
      message.

   o  A local Forced switch input SHALL cause the LER to remain in local
      Protecting administrative state and transmit a FS(1,1) message.

   o  A local Signal Fail indication on the protection path SHALL cause
      the LER to go into local Unavailable state (i.e. overriding the MS
      or FS related Protection administrative state) and begin
      transmission of a SF(0,0) message.

   o  A local Signal Fail indication on the working path SHALL cause the
      LER to go into local Protecting failure state and begin
      transmitting a SF(1,1) message, if the current state is due to a
      (local or remote) Manual switch operator command.  If the LER is
      in remote Protecting administrative state due to a remote Forced
      Switch command, then this local indication SHALL cause the LER to
      remain in remote Protecting administrative state and transmit a
      SF(1,1) message.  If the LER is in local Protecting administrative
      state due to a local Forced Switch command then this indication
      SHALL be ignored (i.e. the indication should have been blocked by
      the Local request logic).

   o  A local Clear SF when in remote Protecting administrative state
      SHOULD clear any local SF condition that may exist.  The LER SHALL
      stop transmitting the SF(x,1) message and begin transmitting an
      NR(0,1) message.



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   o  A local Manual switch input SHALL be ignored if in remote
      Protecting administrative state is due to a remote Forced switch
      command.  If the current state is due to a (local or remote)
      Manual switch operator command, it SHALL cause the LER to remain
      in local Protecting administrative state and transmit a MS(1,1)
      message.

   o  All other local inputs SHALL be ignored.

   While in Protecting administrative state the LER may receive and
   react as follows to remote PSC messages:

   o  A remote Lockout of protection message SHALL cause the LER to go
      into remote Unavailable state and begin transmitting a NR(0,0)
      message.  It should be noted that this automatically cancels the
      current Forced switch or Manual switch command and data traffic is
      reverted to the working path.

   o  A remote Forced switch message SHOULD be ignored by the PSC
      Process Logic if there is an active local Forced switch operator
      command.  If the Protecting administrative state is due to a
      remote Forced switch message then the LER SHALL remain in remote
      Protecting administrative state and continue transmitting the last
      message.  If the Protecting administrative state is due to either
      a local or remote Manual switch then the LER SHALL remain in
      remote Protecting administrative state (updating the state
      information with the proper relevant information) and begin
      transmitting a NR(0,1) message.

   o  A remote Signal Fail message indicating a failure on the
      protection path SHALL cause the LER to go into remote Unavailable
      state and begin transmitting a NR(0,0) message.  It should be
      noted that this automatically cancels the current Forced switch or
      Manual switch command and data traffic is reverted to the working
      path.

   o  A remote Signal Fail message indicating a failure on the working
      path SHALL be ignored if there is an active local Forced switch
      command.  If the Protecting state is due to a local or remote
      Manual switch then the LER SHALL go to remote Protecting failure
      state and begin transmitting a NR(0,1) message.

   o  A remote Manual switch message SHALL be ignored by the PSC Control
      Logic if in Protecting administrative state due to a local or
      remote Forced switch.  If in Protecting administrative state due
      to a remote Manual switch then the LER SHALL remain in remote
      Protecting administrative state and continue transmitting the
      current message.  If in local Protecting administrative state due



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      to an active Manual switch then the LER SHALL remain in local
      Protecting administrative state and continue transmission of the
      MS(1,1) message.

   o  A remote DNR(0,1) message SHALL be ignored if in local Protecting
      administrative state.  If in remote Protecting administrative
      state then the LER SHALL go to Do-not-revert state and continue
      transmitting the current message.

   o  A remote NR(0,0) message SHALL be ignored if in local Protecting
      administrative state.  If in remote Protecting administrative
      state and there is no active local Signal Fail indication then the
      LER SHALL go to Normal state and begin transmitting a NR(0,0)
      message.  If there is a local Signal Fail on the working path, the
      LER SHALL go to local Protecting failure state and begin
      transmitting a SF(1,1) message.

   o  All other remote messages SHOULD be ignored.

4.3.3.4.  Protecting failure state

   When the protection mechanism has been triggered and the protection
   domain has performed a protection switch, the domain is in the
   protecting failure state.  In this state the normal data traffic is
   transported on the protection path.  When an LER is in this state it
   implies that there was either a local SF condition or received a
   remote SF PSC message.  The SF condition or message indicated that
   the failure is on the working path.

   This state may be overridden by the Unavailable state triggers, i.e.
   Lockout of Protection or SF on the protection path, or by issuing a
   FS operator command.  This state will be cleared when the SF
   condition is cleared.  In order to prevent flapping due to an
   intermittent fault, the LER SHOULD employ a Wait-to-restore timer to
   delay return to Normal state until the network has stabilized (see
   section 3.5)

   The following describe the reaction to local input:

   o  A local Clear SF SHALL be ignored if in remote Protecting failure
      state.  If in local Protecting failure state and the LER is
      configured for revertive behavior then this input SHALL cause the
      LER to go into Wait-to-restore state, start the WTR timer, and
      begin transmitting a WTR(0,1) message.  If in local Protecting
      failure state and the LER is configured for non-revertive behavior
      then this input SHALL cause the LER to go into Do-not-revert state
      and begin transmitting a DNR(0,1) message.




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   o  A local Lockout of protection input SHALL cause the LER to go into
      Unavailable state and begin transmission of a LO(0,0) message.

   o  A local Forced switch input SHALL cause the LER to go into
      Protecting administrative state and begin transmission of a
      FS(1,1) message.

   o  A local Signal Fail indication on the protection path SHALL cause
      the LER to go into Unavailable state and begin transmission of a
      SF(0,0) message.

   o  A local Signal Fail indication on the working path SHALL cause the
      LER to remain in local Protecting failure state and transmit a
      SF(1,1) message.

   o  All other local inputs SHOULD be ignored.

   While in Protecting failure state the LER may receive and react as
   follows to remote PSC messages:

   o  A remote Lockout of protection message SHALL cause the LER to go
      into remote Unavailable state and if in local Protecting failure
      state then the LER SHALL transmit a SF(1,0) message, otherwise it
      SHALL transmit a NR(0,0) message.  It should be noted that this
      may cause loss of user data since the working path is still in a
      failure condition.

   o  A remote Forced switch message SHALL cause the LER go into remote
      Protecting administrative state and if in local Protecting failure
      state the LER SHALL transmit the SF(1,1) message, otherwise it
      SHALL transmit NR(0,1).

   o  A remote Signal Fail message indicating a failure on the
      protection path SHALL cause the LER to go into remote Unavailable
      state and if in local Protecting failure state then the LER SHALL
      transmit a SF(1,0) message, otherwise it SHALL transmitting
      NR(0,0) message.  It should be noted that this may cause loss of
      user data since the working path is still in a failure condition.

   o  If in remote Protecting failure state, a remote Wait-to-Restore
      message SHALL cause the LER to go into remote Wait-to-Restore
      state and continue transmission of the current message.

   o  If in remote Protecting failure state, a remote Do-not-revert
      message SHALL cause the LER to go into remote Do-not-revert state
      and continue transmission of the current message.





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   o  If in remote Protecting failure state, a remote NR(0,0) SHALL
      cause the LER to go to Normal state and transmit an NR(0,0)
      message.

   o  All other remote messages SHOULD be ignored.

4.3.3.5.  Wait-to-restore state

   The Wait-to-Restore state is used by the PSC protocol to delay
   reverting to the normal state, when recovering from a failure
   condition on the working path, for the period of the WTR timer to
   allow the recovering failure to stabilize.  While in the Wait-to-
   Restore state the data traffic SHALL continue to be transported on
   the protection path.  The natural transition from the Wait-to-Restore
   state to Normal state will occur when the WTR timer expires.

   When in Wait-to-Restore state the following describe the reaction to
   local inputs:

   o  A local Lockout of protection command SHALL cause the LER to Stop
      the WTR timer, go into local Unavailable state, and begin
      transmitting a LO(0,0) message.

   o  A local Forced switch command SHALL cause the LER to Stop the WTR
      timer, go into local Protecting administrative state, and begin
      transmission of a FS(1,1) message.

   o  A local Signal Fail indication on the protection path SHALL cause
      the LER to Stop the WTR timer, go into local Unavailable state,
      and begin transmission of a SF(0,0) message.

   o  A local Signal Fail indication on the working path SHALL cause the
      LER to Stop the WTR timer, go into local Protecting failure state,
      and begin transmission of a SF(1,1) message.

   o  A local Manual switch input SHALL cause the LER to Stop the WTR
      timer, go into local Protecting administrative state and begin
      transmission of a MS(1,1) message.

   o  A local WTR expires input SHALL cause the LER to remain in Wait-
      to-Restore state and begin transmitting a NR(0,1) message.

   o  All other local inputs SHOULD be ignored.

   When in Wait-to-Restore state the following describe the reaction to
   remote messages:





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   o  A remote Lockout of protection message SHALL cause the LER to Stop
      the WTR timer, go into remote Unavailable state, and begin
      transmitting a NR(0,0) message.

   o  A remote Forced switch message SHALL cause the LER to Stop the WTR
      timer, go into remote Protecting administrative state, and begin
      transmission of a NR(0,1) message.

   o  A remote Signal Fail message for the protection path SHALL cause
      the LER to Stop the WTR timer, go into remote Unavailable state,
      and begin transmission of a NR(0,0) message.

   o  A remote Signal Fail message for the working path SHALL cause the
      LER to Stop the WTR timer, go into remote Protecting failure
      state, and begin transmission of a NR(0,1) message.

   o  A remote Manual switch message SHALL cause the LER to Stop the WTR
      timer, go into remote Protecting administrative state and begin
      transmission of a NR(0,1) message.

   o  If the WTR timer is running then a remote NR message SHALL be
      ignored.  If the WTR timer is stopped then a remote NR message
      SHALL cause the LER to go into Normal state and begin transmitting
      a NR(0,0) message.

   o  All other remote messages SHOULD be ignored.

4.3.3.6.  Do-not-revert state

   Do-not-revert state is a continuation of the Protecting failure
   state.  When the protection domain is configured for non-revertive
   behavior.  While in Do-not-revert state, data traffic continues to be
   transported on the protection path until the administrator sends a
   command to revert to the Normal state.  It should be noted that there
   is a fundamental difference between this state and Normal - whereas
   Forced Switch in Normal state actually causes a switch in the
   transport path used, in Do-not-revert state the Forced switch just
   switches the state (to Protecting administrative state) but the
   traffic would continue to be transported on the protection path!  To
   revert back to Normal state the administrator SHALL issue a Lockout
   of protection command followed by a Clear command.

   When in Do-not-revert state the following describe the reaction to
   local input:

   o  A local Lockout of protection command SHALL cause the LER to go
      into local Unavailable state and begin transmitting a LO(0,0)
      message.



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   o  A local Forced switch command SHALL cause the LER to go into local
      Protecting administrative state and begin transmission of a
      FS(1,1) message.

   o  A local Signal Fail indication on the protection path SHALL cause
      the LER to go into local Unavailable state and begin transmission
      of a SF(0,0) message.

   o  A local Signal Fail indication on the working path SHALL cause the
      LER to go into local Protecting failure state and begin
      transmission of a SF(1,1) message.

   o  A local Manual switch input SHALL cause the LER to go into local
      Protecting administrative state and begin transmission of a
      MS(1,1) message.

   o  All other local inputs SHOULD be ignored.

   When in Do-not-revert state the following describe the reaction to
   remote messages:

   o  A remote Lockout of protection message SHALL cause the LER to go
      into remote Unavailable state and begin transmitting a NR(0,0)
      message.

   o  A remote Forced switch message SHALL cause the LER to go into
      remote Protecting administrative state and begin transmission of a
      NR(0,1) message.

   o  A remote Signal Fail message for the protection path SHALL cause
      the LER to go into remote Unavailable state and begin transmission
      of a NR(0,0) message.

   o  A remote Signal Fail message for the working path SHALL cause the
      LER to go into remote Protecting failure state, and begin
      transmission of a NR(0,1) message.

   o  A remote Manual switch message SHALL cause the LER to go into
      remote Protecting administrative state and begin transmission of a
      NR(0,1) message.

   o  All other remote messages SHOULD be ignored.


5.  IANA Considerations






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5.1.  Pseudowire Associated Channel Type

   In the "Pseudowire Name Spaces (PWE3) IANA" maintains the "
   Pseudowire Associated Channel Types Registry".

   IANA is requested to assign a new code point from this registry.  The
   code point shall be assigned form the code point space that requires
   "IETF Review" as follows:

   Registry:

   Value       Description       TLV Follows    Reference
   ----- ----------------------- ----------- ---------------
    0xHH     Protection State         no     [this document]
         Coordination Protocol -
          Channel Type (PSC-CT)

5.2.  PSC Request Field

   The IANA is instructed to create and maintain a new registry within
   the "Multiprotocol Label Switching Architecture (MPLS)" called "MPLS
   PSC Request Registry".  All code points within this registry shall be
   allocated according to the "Standards Action" procedures as specified
   in [RFC5226].

   The PSC Request Field is 4 bits and the values shall be allocated as
   follows:

       Value     Description              Reference
   ------------- --------------------- ---------------
       b0000     No Request            [this document]
       b0001     Do not revert         [this document]
   b0010 - b0011 Unassigned
       b0100     Wait to restore       [this document]
       b0101     Manual switch         [this document]
       b0110     Unassigned
       b0111     Signal Degrade        [this document]
   b1000 - b1001 Unassigned
       b1010     Signal Fail           [this document]
       b1011     Unassigned
       b1100     Forced switch         [this document]
       b1101     Unassigned
       b1110     Lockout of protection [this document]
       b1111     Unassigned







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6.  Security Considerations

   The generic security considerations for the data-plane of MPLS-TP are
   described in the security framework document [SecureFwk] together
   with the required mechanisms needed to address them.  The security
   considerations for the generic associated control channel are
   described in [RFC5586].  The security considerations for protection
   and recovery aspects of MPLS-TP are addressed in [SurvivFwk].

   The protocol described in this document is based on the use of the
   Generic Associated Channel as defined in [RFC5586].  Any new security
   risk introduced may be in the treatment of corrupted protocol units.
   The main concern is around the Request, FPath and Path fields as a
   change to these fields would change the behavior of the peer
   endpoint.  Although there is no way to avoid a change in network
   behavior upon receipt of a message indicating a change in protection
   status, the transition between states will converge on a known and
   stable behavior in the face of messages which do not match reality.


7.  Acknowledgements

   The authors would like to thank all members of the teams (the Joint
   Working Team, the MPLS Interoperability Design Team in IETF and the
   T-MPLS Ad Hoc Group in ITU-T) involved in the definition and
   specification of MPLS Transport Profile.


8.  References

8.1.  Normative References

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

   [RFC5654]  Niven-Jenkins, B., Brungard, D., Betts, M., Sprecher, N.,
              and S. Ueno, "Requirements of an MPLS Transport Profile",
              RFC 5654, September 2009.

8.2.  Informative References

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031, Jan 2001.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, Jan 2001.




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   [RFC5659]  Bocci, M. and S. Bryant, "An Architecture for Multi-
              Segment Pseudowire Emulation  Edge-to-Edge", RFC 5659,
              October 2009.

   [RFC3985]  Bryant, S. and P. Pate, "Pseudowire Emulation Edge-to-Edge
              (PWE3) Architecture", RFC 3985, March 2005.

   [RFC5085]  Nadeau, T. and C. Pignataro, "Pseudowire Virtual Circuit
              Connectivity Verification (VCCV): A Control Channel for
              Pseudowires", RFC 5085, December 2007.

   [TPFwk]    Bocci, M., Bryant, S., and L. Levrau, "A Framework for
              MPLS in Transport Networks",
              ID draft-ietf-mpls-tp-framework-06.txt, July 2009.

   [RFC5586]  Vigoureux,, M., Bocci, M., Swallow, G., Aggarwal, R., and
              D. Ward, "MPLS Generic Associated Channel", RFC 5586,
              May 2009.

   [RFC4427]  Mannie, E. and D. Papadimitriou, "Recovery Terminology for
              Generalized Multi-Protocol Label Switching", RFC 4427,
              Mar 2006.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [SurvivFwk]
              Sprecher, N., Farrel, A., and H. Shah, "Multi-protocol
              Label Switching Transport Profile Survivability
              Framework", ID draft-ietf-mpls-tp-survive-fwk-02.txt,
              Feb 2009.

   [SecureFwk]
              Fang, L., Niven-Jenkins, B., Mansfield, S., Zhang, R.,
              Bitar, N., Daikoku, M., and L. Wang, "MPLS-TP Security
              Framework",
              ID draft-ietf-mpls-tp-security-framework-00.txt, Feb 2011.

   [RFC4872]  Lang, J., Papadimitriou, D., and Y. Rekhter, "RSVP-TE
              Extensions in Support of End-to-End Generalized Multi-
              Protocol Label Switching (GMPLS) Recovery", RFC 4872,
              May 2007.

   [RFC3945]  Mannie, E., "Generalized Multi-Protocol Label Switching
              (GMPLS) Architecture", RFC 3945, Oct 2004.





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Appendix A.  PSC state machine tables

   The PSC state machine is described in section 4.3.3.  This appendix
   provides the same information but in tabular format.  In the event of
   a mismatch between these tables and the text in section 4.3.3, the
   text is authoritative.  Note that this appendix is intended to be a
   functional description, not an implementation specification.

   For the sake of clarity of the table the six states listed in the
   text are split into thirteen states.  The logic of the split is to
   differentiate between the different cases given in the conditional
   statements in the descriptions of each state in the text.  In
   addition, the remote and local states were split for the Unavailable,
   Protecting failure, and Protecting administrative states.

   There is only one table for the PSC state machine, but it is broken
   into two parts for space reasons.  The first part lists the thirteen
   possible states, the eight possible local inputs (that is, inputs
   which are generated by the node in question) and the action taken
   when a given input is received when the node is in a particular
   state.  The second part of the table lists the thirteen possible
   states and the eight remote inputs (inputs which come from a node
   other than the one executing the state machine).

   There are thirteen rows in the table, headers notwithstanding.  These
   rows are the thirteen possible extended states in the state machine.

   The text in the first column is the current state.  Those states
   which have both source and cause are formatted as State:Cause:Source.
   For example, the string UA:LO:L indicates that the current state is
   'Unavailable', that the cause of the current state is a Lockoutof
   protection that was a Local input.  In contrast, the state N simply
   is Normal; there is no need to track the cause for entry into Normal
   state.

















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   The thirteen extended states, as they appear in the table, are:

   N       Normal state
   UA:LO:L Unavailable state due to local Lockout
   UA:P:L  Unavailable state due to local SF on protection path
   UA:LO:R Unavailable state due to remote Lockout message
   UA:P:R  Unavailable state due to remote SF message on protection path
   PF:W:L  Protecting failure state due to local SF on working path
   PF:W:R  Protecting failure state due to remote SF message on working
           path
   PA:F:L  Protecting administrative state due to local FS operator
           command
   PA:M:L  Protecting administrative state due to local MS operator
           command
   PA:F:R  Protecting administrative state due to remote FS message
   PA:M:R  Protecting administrative state due to remote MS message
   WTR     Wait-to-restore state
   DNR     Do-not-revert state

   Each state corresponds to the transmission of a particular set of
   Request, FPath and Path bits.  The table below lists the message that
   is generally sent in each particular state.  If the message to be
   sent in a particular state deviates from the table below, it is noted
   in the footnotes to the state-machine table.

   State   REQ(FP,P)
   ------- ---------
   N       NR(0,0)
   UA:LO:L LO(0,0)
   UA:P:L  SF(0,0)
   UA:LO:R NR(0,0)
   UA:P:R  NR(0,0)
   PF:W:L  SF(1,1)
   PF:W:R  NR(0,1)
   PA:F:L  FS(1,1)
   PA:M:L  MS(1,1)
   PA:F:R  NR(0,1)
   PA:M:R  NR(0,1)
   WTR     WTR(0,1)
   DNR     DNR(0,1)











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   The top row in each table is the list of possible inputs.  The local
   inputs are:

   NR     No Request
   OC     Operator Clear
   LO     Lockout of protection
   SF-P   Signal Fail on protection path
   SF-W   Signal Fail on working path
   FS     Forced Switch
   SFc    Clear Signal Fail
   MS     Manual Switch
   WTRExp WTR Expired

   and the remote inputs are:

   LO   remote LO message
   SF-P remote SF message indicating protection path
   SF-W remote SF message indicating working path
   FS   remote FS message
   MS   remote MS message
   WTR  remote WTR message
   DNR  remote DNR message
   NR   remote NR message

   Section 4.3.3 refers to some states as 'remote' and some as 'local'.
   By definition, all states listed in the table of local sources are
   local states, and all states listed in the table of remote sources
   are remote states.  For example, section 4.3.3.1 says "A local
   Lockout of protection input SHALL cause the LER to go into local
   Unavailable State".  As the trigger for this state change is a local
   one, 'local Unavailable State' is by definition displayed in the
   table of local sources.  Similarly, "A remote Lockout of protection
   message SHALL cause the LER to go into remote Unavailable state"
   means that the state represented in the Unavailable rows in the table
   of remote sources is by definition a remote Unavailable state.

   Each cell in the table below contains either a state, a footnote, or
   the letter 'i'. 'i' stands for Ignore, and is an indication to
   continue with the current behavior.  See section 4.3.3.  The
   footnotes are listed below the table.











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   Part 1: Local input state machine

               | OC  | LO    | SF-P | FS   | SF-W | SFc  | MS   | WTRExp
       --------+-----+-------+------+------+------+------+------+-------
       N       | i   |UA:LO:L|UA:P:L|PA:F:L|PF:W:L| i    |PA:M:L| i
       UA:LO:L | N   | i     | i    | i    | i    | i    | i    | i
       UA:P:L  | i   |UA:LO:L| i    | i    | i    | [5]  | i    | i
       UA:LO:R | i   |UA:LO:L| [1]  | i    | [2]  | [6]  | i    | i
       UA:P:R  | i   |UA:LO:L|UA:P:L| i    | [3]  | [6]  | i    | i
       PF:W:L  | i   |UA:LO:L|UA:P:L|PA:F:L| i    | [7]  | i    | i
       PF:W:R  | i   |UA:LO:L|UA:P:L|PA:F:L|PF:W:L| i    | i    | i
       PA:F:L  | N   |UA:LO:L|UA:P:L| i    | i    | i    | i    | i
       PA:M:L  | N   |UA:LO:L|UA:P:L|PA:F:L|PF:W:L| i    | i    | i
       PA:F:R  | i   |UA:LO:L|UA:P:L|PA:F:L| [4]  | [8]  | i    | i
       PA:M:R  | i   |UA:LO:L|UA:P:L|PA:F:L|PF:W:L| i    |PA:M:L| i
       WTR     | i   |UA:LO:L|UA:P:L|PA:F:L|PF:W:L| i    |PA:M:L| [9]
       DNR     | i   |UA:LO:L|UA:P:L|PA:F:L|PF:W:L| i    |PA:M:L| i

   Part 2: Remote messages state machine

               | LO    | SF-P | FS   | SF-W | MS   | WTR  | DNR  | NR
       --------+-------+------+------+------+------+------+------+------
       N       |UA:LO:R|UA:P:R|PA:F:R|PF:W:R|PA:M:R| i    | i    | i
       UA:LO:L | i     | i    | i    | i    | i    | i    | i    | i
       UA:P:L  | [10]  | i    | i    | i    | i    | i    | i    | i
       UA:LO:R | i     | i    | i    | i    | i    | i    | i    | [16]
       UA:P:R  |UA:LO:R| i    | i    | i    | i    | i    | i    | [16]
       PF:W:L  | [11]  | [12] |PA:F:R| i    | i    | i    | i    | i
       PF:W:R  |UA:LO:R|UA:P:R|PA:F:R| i    | i    | [14] | [15] | N
       PA:F:L  |UA:LO:R|UA:P:R| i    | i    | i    | i    | i    | i
       PA:M:L  |UA:LO:R|UA:P:R|PA:F:R| [13] | i    | i    | i    | i
       PA:F:R  |UA:LO:R|UA:P:R| i    | i    | i    | i    | i    | [17]
       PA:M:R  |UA:LO:R|UA:P:R|PA:F:R| [13] | i    | i    | i    | N
       WTR     |UA:LO:R|UA:P:R|PA:F:R|PF:W:R|PA:M:R| i    | i    | [18]
       DNR     |UA:LO:R|UA:P:R|PA:F:R|PF:W:R|PA:M:R| i    | i    | i

   The following are the footnotes for the table:

   [1] Remain in the current state (UA:LO:R) and transmit SF(0,0)

   [2] Remain in the current state (UA:LO:R) and transmit SF(1,0)

   [3] Remain in the current state (UA:P:R) and transmit SF(1,0)

   [4] Remain in the current state (PA:F:R) and transmit SF(1,1)

   [5] If the SF being cleared is SF-P, Transition to N. If it's SF-W,
   ignore the clear.



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   [6] Remain in current state (UA:x:R), if the SFc corresponds to a
   previous SF then begin transmitting NR(0,0).

   [7] If domain configured for revertive behavior transition to WTR,
   else transition to DNR

   [8] Remain in PA:F:R and transmit NR(0,1)

   [9] Remain in WTR, send NR(0,1)

   [10] Transition to UA:LO:R continue sending SF(0,0)

   [11] Transition to UA:LO:R and send SF(1,0)

   [12] Transition to UA and send SF(1,0)

   [13] Transition to PF:W:R and send NR(0,1)

   [14] Transition to WTR state and continue to send the current
   message.

   [15] Transition to DNR state and continue to send the current
   message.

   [16] If the local input is SF-P then transition to UA:P:L. If the
   local input is SF-W then transition to PF:W:L. Else - transition to N
   state and continue to send the current message.

   [17] If the local input is SF-W then transition to PF:W:L. Else -
   transition to N state and continue to send the current message.

   [18] If the receiving LER's WTR timer is running, maintain current
   state and message.  If the WTR timer is stopped, transition to N.


Appendix B.  Exercising the protection domain

   There is a requirement in [RFC5654] (number 84) that discusses a
   requirement to verify that the protection path is viable.  While the
   PSC protocol does not define a specific operation for this
   functionality, it is possible to perform this operation by combining
   operations of the PSC and other OAM functionalities.  One such
   possible combination would be to issue a Lockout of Protection
   operation and then use the OAM function for diagnostic testing of the
   protection path.  Similarly, to test the paths when the working path
   is not active would involve performing a Forced Switch to protection
   and then perform the diagnostic function on either the working or
   protection path.



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Internet-Draft                 MPLS-TP LP                      June 2011


Authors' Addresses

   Stewart Bryant
   Cisco
   United Kingdom

   Email: stbryant@cisco.com


   Eric Osborne
   Cisco
   United States

   Email: eosborne@cisco.com


   Nurit Sprecher
   Nokia Siemens Networks
   3 Hanagar St. Neve Ne'eman B
   Hod Hasharon,   45241
   Israel

   Email: nurit.sprecher@nsn.com


   Annamaria Fulignoli (editor)
   Ericsson
   Italy

   Email: annamaria.fulignoli@ericsson.com


   Yaacov Weingarten (editor)
   Nokia Siemens Networks
   3 Hanagar St. Neve Ne'eman B
   Hod Hasharon,   45241
   Israel

   Email: yaacov.weingarten@nsn.com












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