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RFC 6378 - MPLS Transport Profile (MPLS-TP) Linear Protection


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Internet Engineering Task Force (IETF)                Y. Weingarten, Ed.
Request for Comments: 6378                        Nokia Siemens Networks
Category: Standards Track                                      S. Bryant
ISSN: 2070-1721                                               E. Osborne
                                                                   Cisco
                                                             N. Sprecher
                                                  Nokia Siemens Networks
                                                       A. Fulignoli, Ed.
                                                                Ericsson
                                                            October 2011

           MPLS Transport Profile (MPLS-TP) Linear Protection

Abstract

   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 Pseudowire Emulation Edge-to-Edge
   (PWE3) architectures to support the capabilities and functionalities
   of a packet transport network as defined by the ITU-T.

   This document addresses the functionality described in the MPLS-TP
   Survivability Framework document (RFC 6372) 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.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in 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/rfc6378.

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
   Provisions Relating to IETF Documents
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   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
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   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
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   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1. Introduction ....................................................4
      1.1. Protection Architectures ...................................4
      1.2. Scope of the Document ......................................5
   2. Conventions Used in This Document ...............................6
      2.1. Acronyms ...................................................6
      2.2. Definitions and Terminology ................................7
   3. Protection State Control Logic ..................................7
      3.1. Local Request Logic ........................................9
      3.2. Remote Requests ...........................................11
      3.3. PSC Control Logic .........................................12
      3.4. PSC Message Generator .....................................12
      3.5. Wait-to-Restore (WTR) Timer ...............................12
      3.6. PSC Control States ........................................13
           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 ...........................................16
           4.2.1. PSC Ver Field ......................................16
           4.2.2. PSC Request Field ..................................17
           4.2.3. Protection Type (PT) Field .........................18
           4.2.4. Revertive (R) Field ................................18
           4.2.5. Fault Path (FPath) Field ...........................19
           4.2.6. Data Path (Path) Field .............................19
           4.2.7. Additional TLV Information .........................19
      4.3. Principles of Operation ...................................20
           4.3.1. Basic Operation ....................................20
           4.3.2. Priority of Inputs .................................21
           4.3.3. Operation of PSC States ............................22
   5. IANA Considerations ............................................33
      5.1. Pseudowire Associated Channel Type ........................33
      5.2. PSC Request Field .........................................33
      5.3. Additional TLVs ...........................................34
   6. Security Considerations ........................................34
   7. Acknowledgements ...............................................35
   8. Contributing Authors ...........................................36
   9. References .....................................................37
      9.1. Normative References ......................................37
      9.2. Informative References ....................................37
   Appendix A. PSC State Machine Tables ..............................39
   Appendix B. Exercising the Protection Domain ......................44

1.  Introduction

   The MPLS Transport Profile (MPLS-TP) [RFC5921] 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 [RFC6372] 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
   [RFC6372].  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 resources of
   the protection path are 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 and that can operate between any pair of points
   within the network.

   As described in the Survivability Framework document [RFC6372],
   protection switching is applied to a protection domain.  For the
   purposes of this document, we define the protection domain of a
   point-to-point LSP as consisting of two Label Edge Routers (LERs) and
   the transport paths that connect them (see Figure 3).  For a point-
   to-multipoint 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 [RFC6372], 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 choose either the working or protection path from
   which to receive the traffic, based on 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 on either 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 listed in [RFC6372]), 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.  In cases where multiple working path services have
   triggered protection switching, it should be noted 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 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 [RFC6372] 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
   verify that the traffic continues to be transported on a
   bidirectional LSP that is co-routed.

   The scope of this document 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 [RFC6372].  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 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 and is out of scope
   for this 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].  The use of this protocol for point-to-
   multipoint paths is out of scope for this document and may be
   addressed in a future applicability document.

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].

2.1.  Acronyms

   This document uses the following acronyms:

   CT      Channel Type
   DNR     Do-not-Revert
   FS      Forced Switch
   G-ACh   Generic Associated Channel
   LER     Label Edge Router
   LO      Lockout of protection
   LSR     Label Switching Router
   MEG     Managed Entity Group
   MEP     MEG End Point
   MPLS-TP Transport Profile for MPLS
   MS      Manual Switch
   NR      No Request
   OAM     Operations, Administration, and Maintenance
   PSC     Protection State Coordination Protocol
   S-PE    Switching Provider Edge
   SD      Signal Degrade
   SF      Signal Fail
   SFc     Clear Signal Fail
   SLA     Service Level Agreement

   T-PE    Terminating Provider Edge
   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 [RFC6372].
   In addition, we use the term "LER" to refer to an MPLS-TP Network
   Element, whether it is an LSR, LER, T-PE, or S-PE.

3.  Protection State 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 State 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.

                  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 State Control Logic

   Figure 1 describes the logical architecture of the protection
   switching control.  The Local Request logic unit accepts the triggers
   from the OAM, server layer, external operator commands, 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 commands Forced Switch, Manual Switch, Clear,
      Lockout of protection (defined in [RFC4427] as Forced switch-over,
      Manual switch-over, Clear, and Lockout of recovery LSP/span,
      respectively) MUST be supported.  An implementation MAY provide
      additional commands for operator use; providing that these
      commands do not introduce incompatible behavior between two
      arbitrary implementations, they are outside the scope of this
      document.  For example, an implementation could provide a command
      to manually set off 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; therefore, this
      input MAY be controlled by a hold-off timer that SHOULD be
      configurable by the network operator.  The hold-off timer is
      described in greater detail in [RFC6372].

   o  Control-Plane Indication - 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] and [RFC4873].

   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 MUST 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 SHALL revert to the normal
      transmission of the user data traffic.

   The input from these sources SHOULD be retained persistently for the
   duration of the condition that initiated the trigger.  The Local
   Request logic processes these different input sources and, based on

   the priorities between them (see Section 4.3.2), produces a current
   local request.  If more than one local input source generates a
   trigger, then the Local Request logic selects the higher priority
   indicator and ignores 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 as follows:

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

   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
      placeholders for this extension.

   o  Clear Signal Fail (SFc) - 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 (WTRExp) - generated by the WTR timer completing its
      period.

   If none of the input sources have generated any triggers, then the
   Local Request logic should generate a No Request (NR) 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.
   Remote messages indicate the status of the transport path from the
   viewpoint of the far-end LER.  These messages may drive state changes
   on the local MEP, as defined later in this document.  When using 1+1
   unidirectional protection, an LER that receives a remote request
   SHALL NOT perform any protection switching action, i.e., will
   continue to select traffic from the working path and transport
   traffic on both paths.

   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
      protection domain.  This remote message includes 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 includes 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 from the working path to
      the protection path.

   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 accepts the following 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
   logic determines the new state of the PSC Control logic and what
   actions need to be taken.

   The new state information is retained by the PSC Control logic, while
   the requested action should be sent to the PSC Message Generator (see
   Section 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 are
   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: 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 is 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 would 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.

   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 as follows:

   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
      an 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 has recovered from a
      Protecting state, but the operator has configured the protection
      domain not to 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.

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 is 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 is considered a
   remote state.  This differentiation affects how the LER reacts 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 indicator
   that cause the protection domain to enter a particular state, then
   the state is 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 of
   protection 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
   PSC messages that indicate an SF condition on the working path and
   that the protection path is not being used to transport protected
   traffic (as described in the next section).

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 switchover
   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 switchover 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 will
   complete the switchover.

   This protocol is a single-phased protocol, as described above.  In
   the following subsections, we describe the protocol messages that are
   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 SHALL 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 SHOULD send the three
   rapid messages.  However, when the LER transfers from WTR state to
   Normal state as a result of a remote NR message, the three rapid
   messages SHALL be transmitted.  After the transmission of the three
   rapid messages, the LER MUST retransmit the most recently transmitted
   PSC message on a continual basis.

   Both the default frequency of the three rapid messages as well as the
   default frequency of the continual message transmission SHALL be
   configurable by the operator.  The actual frequencies used MAY be
   configurable, at the time of establishment, for each individual
   protected LSP.  For management purposes, the operator SHOULD be able
   to retrieve the current default frequency values as well as the
   actual values for any specific LSP.  For protection switching within
   50 ms, it is RECOMMENDED that the default interval of the first three
   rapid PSC messages SHOULD be no longer than 3.3 ms.  Using this

   frequency would allow the far-end to be guaranteed of receiving the
   trigger indication within 10 ms and completion of the switching
   operation within 50 ms.  Subsequent messages SHOULD be continuously
   transmitted with a default interval of 5 seconds.  The purpose of the
   continual messages is to verify that the PSC session is still alive.

   If no valid PSC message is received, over a period of several
   continual messages intervals, 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=0x0024.

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

        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|  Reserved1  |     FPath     |     Path      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         TLV Length            |          Reserved2            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                         Optional TLVs                         ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 2: Format of PSC Packet with a G-ACh Header

   Where:

   o  Both Reserved1 and Reserved2 fields MUST be set to 0 and ignored
      upon receipt.

   o  The following subsections describe the remaining 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  (14) 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  (12) 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  (10) 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; 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; if working path is blocked, then Path is set to 1).

   o  (7) Signal Degrade - indicates 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 placeholder.  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 are left for future specification.

   o  (5) 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  (4) 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  (1) Do-not-Revert - indicates that the transmitting end point has
      recovered from a failure/blocked condition, but due to the local
      settings, is requesting that the protection domain continues to
      transport the data as if it is in a protecting state, 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  (0) No Request - indicates that the transmitting end point has
      nothing to report, FPath and Path fields SHALL be set according to
      the transmission state of the end point, see Section 4.3.3 for
      detailed scenarios.

   All other values are for future extensions (to be administered by
   IANA) and SHALL be ignored upon receipt.

4.2.3.  Protection Type (PT) Field

   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  3: bidirectional switching using a permanent bridge

   o  2: bidirectional switching using a selector bridge

   o  1: unidirectional switching using a permanent bridge

   o  0: 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.  The following are the possible
   values:

   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, when a fault or command is indicated by the
   Request field to be in effect.  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 and SHALL be ignored by this version
      of the protocol.

4.2.6.  Data Path (Path) Field

   The Path field indicates which data is being transported 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
   transported 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.

   o  2-255: for future extensions and SHALL be ignored by this version
      of the protocol.

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 MUST 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
   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).  Except during a protection switch, 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.  As a side effect of using a
   single-phased protocol, there will be a short period during state
   transitions of one-sided triggers (e.g., operator commands or
   unidirectional SF) when one LER may be transporting/selecting the
   data from one transport path while the other end point is
   transporting/selecting from the other transport path.  This should
   become coordinated once the remote message is received and the far-
   end LER performs the protection switching operation.

   The following subsections will identify the messages that will 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; whereas 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.  For 1+1 unidirectional protection, the state of the
   selector will only be switched in reaction to a local message.  When
   receiving a remote message, a LER that is configured for 1+1
   unidirectional protection, will transfer to the new remote state;
   however, it will continue to select data according to the latest
   known local state.  When the LER transitions into the Normal state,
   the PSC Control Process SHALL check the persistent state of the local
   triggers to decide if it should further transition into a new state.

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.   Forced Switch (operator command)

   4.   Signal Fail on protection (OAM / control-plane / server
        indication)

   5.   Signal Fail on working (OAM / control-plane / server indication)

   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 Forced Switch
   would have a priority just below a local Forced Switch but above a
   local Signal Fail on protection 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 subsections 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
   SHALL remain in its current state and the LER SHALL 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 received 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 SHALL reevaluate all inputs (both
   the local input and the remote message) as if the LER is in the
   Normal 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 LER transitions into the Normal state, the PSC Control
   Process SHALL check the persistent state of the local triggers to
   decide if it should further transition into a new state.  If the
   result of this check is a transition into a new state, the LER SHALL
   transmit the corresponding message described in this section and
   SHALL use the data path corresponding to the new state.  When the
   protection domain remains in Normal state, the end point SHALL
   transmit an 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 to the LER:

   o  A local Lockout of protection input SHALL cause the LER to go into
      local Unavailable state and begin transmission of an 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 an
      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 an 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 an 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 an
      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:

   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.

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

   o  A remote Manual Switch message SHALL cause the LER to go into
      remote Protecting administrative state, and transmit an 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 SHALL be transported on the
   working path and is not protected.  When the domain is in Unavailable
   state, the PSC messages may not get through: therefore, the
   protection is more dependent on the local inputs 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:

   o  A local Clear input SHALL 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.

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

   o  A local Clear SF of the protection path in local Unavailable state
      that is due to an SF on the protection path SHALL cause the LER to
      go to Normal state.  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 SHALL be ignored.

   o  A local Forced Switch SHALL be ignored by the PSC Control logic
      when in Unavailable state as a result of a (local or remote)
      Lockout of protection.  If in Unavailable state due to an SF on
      protection, then the FS SHALL cause the LER to go into local
      Protecting administrative state and begin transmitting an FS(1,1)
      message.  It should be noted that due to the unavailability of the
      protection path (i.e., due to the SF condition) that this FS may
      not be received by the far-end until the SF condition is cleared.

   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 an 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 an 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 effect:

   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 Forced Switch message SHALL be ignored by the PSC Control
      logic when in Unavailable state as a result of a (local or remote)
      Lockout of protection.  If in Unavailable state due to a local or
      remote SF on protection, then the FS SHALL cause the LER to go
      into remote Protecting administrative state; if in Unavailable
      state due to local SF, begin transmitting an SF(0,1) message.

   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 an
      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 an SF(1,1) message.  When in local Unavailable state, the
      remote message SHALL be ignored.

   o  All other remote messages SHALL be ignored.

4.3.3.3.  Protecting Administrative State

   In the Protecting administrative state, the user data traffic SHALL
   be 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, an 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 SHALL 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.

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

   o  A local Forced Switch input SHALL cause the LER to remain in local
      Protecting administrative state and transmit an 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 an SF(0,0) message, if the current state is due to a (local or
      remote) Manual Switch operator command.  If the LER is in (local
      or remote) Protecting administrative state due to an FS situation,
      then the SF on protection SHALL be ignored.

   o  A local Signal Fail indication on the working path SHALL cause the
      LER to go into local Protecting failure state and begin
      transmitting an 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 an
      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 SHALL clear any local SF condition that may
      exist.  If in remote Protecting administrative state, the LER
      SHALL stop transmitting the SF(x,1) message and begin transmitting
      an NR(0,1) message.

   o  A local Manual Switch input SHALL be ignored if in remote
      Protecting administrative state 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 an 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 an 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 SHALL 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 an
      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 an NR(0,0) message, if the Protecting
      administrative state is due to a Manual Switch command.  It should
      be noted that this automatically cancels the current 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 an 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
      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 an 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 an SF(1,1) message.

   o  All other remote messages SHALL 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

   SHALL be transported on the protection path.  When an LER is in this
   state, it implies that there either was a local SF condition or it
   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 an
   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.

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

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

   o  All other local inputs SHALL 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 an SF(1,0) message; otherwise,

      it SHALL transmit an 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 an SF(1,0) message; otherwise, it SHALL transmit an
      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.

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

   o  All other remote messages SHALL be ignored.

4.3.3.5.  Wait-to-Restore State

   When recovering from a failure condition on the working path, the
   Wait-to-Restore state is used by the PSC protocol to delay reverting
   to the Normal state, 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 send the Stop command
      to the WTR timer, go into local Unavailable state, and begin
      transmitting an LO(0,0) message.

   o  A local Forced Switch command SHALL send the Stop command to the
      WTR timer, go into local Protecting administrative state, and
      begin transmission of an FS(1,1) message.

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

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

   o  A local Manual Switch input SHALL send the Stop command to the WTR
      timer, go into local Protecting administrative state, and begin
      transmission of an MS(1,1) message.

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

   o  All other local inputs SHALL be ignored.

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

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

   o  A remote Forced Switch message SHALL send the Stop command to the
      WTR timer, go into remote Protecting administrative state, and
      begin transmission of an NR(0,1) message.

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

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

   o  A remote Manual Switch message SHALL send the Stop command to the
      WTR timer, go into remote Protecting administrative state, and
      begin transmission of an 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.

   o  All other remote messages SHALL 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 SHALL continue to be
   transported on the protection path until the administrator sends a
   command to revert to 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 an LO(0,0)
      message.

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

   o  All other local inputs SHALL 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 an 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
      an 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 an 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 an 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
      an NR(0,1) message.

   o  All other remote messages SHALL be ignored.

5.  IANA Considerations

5.1.  Pseudowire Associated Channel Type

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

   IANA has assigned a new code point from this registry.  The code
   point has been assigned from the code point space that requires "IETF
   Review" as follows:

   Registry:

    Value       Description       TLV Follows    Reference
   ------ ----------------------- ----------- ---------------
   0x0024     Protection State         no     [this document]
          Coordination Protocol -
           Channel Type (PSC-CT)

5.2.  PSC Request Field

   IANA has created and maintains a new sub-registry within the
   "Multiprotocol Label Switching (MPLS) Operations, Administration, and
   Management (OAM) Parameters" registry called the "MPLS PSC Request
   Registry".  All code points within this registry shall be allocated
   according to the "Standards Action" procedure as specified in
   [RFC5226].

   The PSC Request Field is 4 bits, and the values have been allocated
   as follows:

   Value Description              Reference
   ----- --------------------- ---------------
     0   No Request            [this document]
     1   Do-not-Revert         [this document]
   2 - 3 Unassigned
     4   Wait-to-Restore       [this document]
     5   Manual Switch         [this document]
     6   Unassigned
     7   Signal Degrade        [this document]
   8 - 9 Unassigned
     10  Signal Fail           [this document]
     11  Unassigned
     12  Forced Switch         [this document]
     13  Unassigned
     14  Lockout of protection [this document]
     15  Unassigned

5.3.  Additional TLVs

   The IANA has created and maintains a new sub-registry within the
   "Multiprotocol Label Switching (MPLS) Operations, Administration, and
   Management (OAM) Parameters" registry called the "MPLS PSC TLV
   Registry".  All code points within this registry shall be allocated
   according to the "IETF Review" procedure as specified in [RFC5226].

6.  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.

   A specific concern for the G-ACh is that is can be used to provide a
   covert channel.  This problem is wider than the scope of this
   document and does not need to be addressed here, but it should be
   noted that the channel provides end-to-end connectivity and SHOULD

   NOT be policed by transit nodes.  Thus, there is no simple way of
   preventing any traffic being carried between in the G-ACh consenting
   nodes.

   A good discussion of the data-plane security of an associated channel
   may be found in [RFC5085].  That document also describes some
   mitigation techniques.

   It should be noted that the G-ACh is essentially connection oriented
   so injection or modification of control messages specified in this
   document require the subversion of a transit node.  Such subversion
   is generally considered hard in MPLS networks and impossible to
   protect against at the protocol level.  Management level techniques
   are more appropriate.

   However, a new concern for this document is the accidental corruption
   of messages (through faulty implementations or random corruption).
   The main concern is around the Request, FPath, and Path fields as a
   change to these fields would change the behavior of the peer end
   point.  Although this document does not define a 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 that
   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 the IETF, and
   the T-MPLS Ad Hoc Group in ITU-T) involved in the definition and
   specification of the MPLS Transport Profile.

8.  Contributing Authors

   Hao Long
   Huawei Technologies Co., Ltd.
   F3 Building, Huawei Industrial Park
   Bantian, Shenzhen, China

   EMail: longhao@huawei.com

   Davide Chiara
   Ericsson
   Via Calda 5, 16152 Genova Italy

   EMail: davide.chiara@ericsson.com

   Dan Frost
   Cisco Systems

   EMail: danfrost@cisco.com

   Francesco Fondelli
   Ericsson
   via Moruzzi 1
   56100, Pisa
   Italy

   EMail: francesco.fondelli@ericsson.com

9.  References

9.1.  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.

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

9.2.  Informative References

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

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

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

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

   [RFC4427]  Mannie, E. and D. Papadimitriou, "Recovery (Protection and
              Restoration) Terminology for Generalized Multi-Protocol
              Label Switching (GMPLS)", RFC 4427, March 2006.

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

   [RFC4873]  Berger, L., Bryskin, I., Papadimitriou, D., and A. Farrel,
              "GMPLS Segment Recovery", RFC 4873, May 2007.

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

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

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

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

   [RFC5921]  Bocci, M., Bryant, S., Frost, D., Levrau, L., and L.
              Berger, "A Framework for MPLS in Transport Networks",
              RFC 5921, July 2010.

   [RFC6372]  Sprecher, N., Ed. and A. Farrel, Ed., "MPLS Transport
              Profile (MPLS-TP) Survivability Framework", RFC 6372,
              September 2011.

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 13 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 13
   possible states, the eight possible local inputs (that is, inputs
   that 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 13 possible states and
   the eight remote inputs (inputs that come from a node other than the
   one executing the state machine).

   There are 13 rows in the table, headers notwithstanding.  These rows
   are the 13 possible extended states in the state machine.

   The text in the first column is the current state.  Those states that
   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 Lockout of
   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.

   The 13 extended states, as they appear in the table, are as follows:

   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 of protection 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)

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

   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 as follows:

   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, Section 4.3.3.1 also states that

   "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.

   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    |PA:F:L| 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|PA:F:L| [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| i    | 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| i    |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    | [19] | i    | i    | i    | i    | i
       UA:LO:R | i     | i    | i    | i    | i    | i    | i    | [16]
       UA:P:R  |UA:LO:R| i    |PA:F:R| 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| i    | 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| i    | i    | i    | i    | i    | DNR  | [17]
       PA:M:R  |UA:LO:R|UA:P:R|PA:F:R| [13] | i    | i    | DNR  | 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.

   [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.

   [19]  Transition to PA:F:R and send SF (0,1).

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.

Authors' Addresses

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

   EMail: yaacov.weingarten@nsn.com

   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
   Via Moruzzi
   Pisa  56100
   Italy

   EMail: annamaria.fulignoli@ericsson.com

 

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