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Versions: (draft-rhd-mpls-tp-psc-sd) 00 01 draft-ietf-mpls-tp-psc-itu

MPLS Working Group                                          J. Ryoo, Ed.
Internet-Draft                                                      ETRI
Updates: 6378 (if approved)                                 E. Gray, Ed.
Intended status: Standards Track                                Ericsson
Expires: May 12, 2014                                    H. van Helvoort
                                                     Huawei Technologies
                                                         A. D'Alessandro
                                                          Telecom Italia
                                                               T. Cheung
                                                                    ETRI
                                                              E. Osborne
                                                     Cisco Systems, Inc.
                                                       November 08, 2013


MPLS Transport Profile (MPLS-TP) Linear Protection in Support of ITU-T's
                              Requirements
                 draft-ryoogray-mpls-tp-psc-itu-01.txt

Abstract

   This document introduces alternate ways to perform certain operations
   defined in RFC6378, "MPLS Transport Profile (MPLS-TP) Linear
   Protection", and also defines additional behaviors.  This set of
   modified and additional behaviors together with the protocol defined
   in RFC6378 meets the ITU-T's protection switching requirements.

   This document introduces capabilities and modes.  A capability is an
   individual behavior.  The capabilities of a node are advertised using
   the method given in this document.  A mode is a particular
   combination of capabilities.  Two modes are defined in this document:
   Protection State Coordination (PSC) mode and Automatic Protection
   Switching (APS) mode.

   This document describes the behavior of the PSC protocol including
   priority logic and state machine when all the capabilities associated
   with the APS mode are enabled.

   This document updates RFC6378 in that the capability advertisement
   method defined here is an addition to that document.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute



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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on May 12, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   4
   3.  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Capability 1: Priority Modification . . . . . . . . . . . . .   5
     4.1.  Motivations for swapping priorities of FS and SF-P  . . .   5
     4.2.  Motivation for raising the priority of Clear SF . . . . .   6
     4.3.  Motivation for introducing Freeze command . . . . . . . .   6
     4.4.  Updates to the PSC RFC  . . . . . . . . . . . . . . . . .   6
   5.  Capability 2: Modification of Non-revertive Operation . . . .   7
   6.  Capability 3: Support of Manual Switch to Working Command . .   7
     6.1.  Motivation for adding Manual Switch to Working  . . . . .   7
     6.2.  Terms modified to support MS-W  . . . . . . . . . . . . .   8
     6.3.  Behavior of MS-P and MS-W . . . . . . . . . . . . . . . .   8
     6.4.  Equal priority resolution for MS  . . . . . . . . . . . .   8
   7.  Capability 4: Support of protection against Signal Degrade  .   9
     7.1.  Motivation for supporting protection against Signal
           Degrade . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.2.  Terms modified to support SD  . . . . . . . . . . . . . .   9
     7.3.  Behavior of protection against SD . . . . . . . . . . . .   9
     7.4.  Equal priority resolution . . . . . . . . . . . . . . . .  11
   8.  Capability 5: Support of Exercise Command . . . . . . . . . .  12



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   9.  Capabilities and Modes  . . . . . . . . . . . . . . . . . . .  13
     9.1.  Capabilities  . . . . . . . . . . . . . . . . . . . . . .  13
       9.1.1.  Sending the Capabilities TLV  . . . . . . . . . . . .  14
       9.1.2.  Receiving the Capabilities TLV  . . . . . . . . . . .  15
       9.1.3.  Handling Capabilities TLV errors  . . . . . . . . . .  15
     9.2.  Modes . . . . . . . . . . . . . . . . . . . . . . . . . .  16
       9.2.1.  PSC Mode  . . . . . . . . . . . . . . . . . . . . . .  16
       9.2.2.  APS Mode  . . . . . . . . . . . . . . . . . . . . . .  16
     9.3.  Backward compatibility  . . . . . . . . . . . . . . . . .  16
   10. PSC Protocol in APS Mode  . . . . . . . . . . . . . . . . . .  17
     10.1.  Request field in PSC protocol message  . . . . . . . . .  17
     10.2.  Priorities of local inputs and remote requests . . . . .  18
   11. State Transition Tables in APS Mode . . . . . . . . . . . . .  20
     11.1.  State transition by local inputs . . . . . . . . . . . .  21
     11.2.  State transition by remote messages  . . . . . . . . . .  23
   12. Security considerations . . . . . . . . . . . . . . . . . . .  25
   13. IANA considerations . . . . . . . . . . . . . . . . . . . . .  25
     13.1.  PSC Request Field  . . . . . . . . . . . . . . . . . . .  25
     13.2.  PSC TLV  . . . . . . . . . . . . . . . . . . . . . . . .  26
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  26
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     15.2.  Informative References . . . . . . . . . . . . . . . . .  26
   Appendix A.  An example of out-of-service scenarios . . . . . . .  27
   Appendix B.  An example of sequence diagram showing
                the problem with the priority level of Clear SF  . .  28
   Appendix C.  Freeze Command . . . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Introduction

   This document introduces alternate ways to perform certain operations
   defined in [RFC6378], "MPLS Transport Profile (MPLS-TP) Linear
   Protection", and also defines additional behaviors.  This set of
   modified and additional behaviors together with the protocol defined
   in [RFC6378] meets the ITU-T's protection switching requirements.

   Alternative behaviors are defined for the following capabilities:

   1.  Priority modification,

   2.  non-revertive behavior modification,

   and the following capabilities have been added to define additional
   behaviors:

   3.  support of Manual Switch to Working (MS-W) command,




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   4.  support of protection against Signal Degrade (SD), and

   5.  support of Exercise command.

   Priority modification includes priority swapping between Signal Fail
   on the Protection path (SF-P) and Forced Switch (FS), and raising the
   priority level of Clear SF.

   Non-revertive behavior is modified to align with the behavior defined
   in [RFC4427] as well as to meet the ITU-T's protection switching
   requirements.

   Support of Manual Switch to Working (MS-W) command to revert traffic
   to the working path in non-revertive operation is covered in this
   document.

   Support of protection switching protocol against Signal Degrade (SD)
   is covered in this document.  The specifics for the method of
   identifying SD is out of the scope of this document similarly to SF
   for [RFC6378].

   Support of Exercise command to test if the Protection State
   Coordination (PSC) communication is operating correctly is also
   covered in this document.  More specifically, the Exercise tests and
   validates the linear protection mechanism and PSC protocol including
   the aliveness of the Local Request logic, the PSC state machine and
   the PSC message generation and reception, and the integrity of the
   protection path, without triggering the actual traffic switching.

   This document introduces capabilities and modes.  A capability is an
   individual behavior, The capabilities of a node are advertised using
   the method given in this document.  A mode is a particular
   combination of capabilities.  Two modes are defined in this document:
   PSC mode and Automatic Protection Switching (APS) mode.

   This document describes the behavior of the PSC protocol including
   priority logic and state machine when all the capabilities associated
   with the APS mode are enabled.

   This document updates [RFC6378] in that the capability advertisement
   method defined here is an addition to that document.  For an existing
   implementation of [RFC6378], it is recommended to be updated with the
   bug-fixes in [I-D.ietf-mpls-psc-updates] and the capability
   adevertisement in this document.

2.  Conventions Used in This Document





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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Acronyms

   This document uses the following acronyms:

   APS     Automatic Protection Switching
   EXER    Exercise
   FS      Forced Switch
   LO      Lockout of protection
   MS      Manual Switch
   MS-P    Manual Switch to Protection
   MS-W    Manual Switch to Working
   MPLS-TP MPLS Transport Profile
   NR      No Request
   OC      Operator Clear
   PSC     Protection State Coordination
   RR      Reverse Request
   SD      Signal Degrade
   SD-P    Signal Degrade on the Protection path
   SD-W    Signal Degrade on the Working path
   SF      Signal Fail
   SFc     Clear Signal Fail
   SF-P    Signal Fail on the Protection path
   SF-W    Signal Fail on the Working path
   WTR     Wait to Restore


4.  Capability 1: Priority Modification

   In this document, the priorities of Forced Switch (FS) and Signal
   Fail on the Protection path (SF-P) are swapped and the priority of
   Clear SF (SFc) is raised.  In addition to the priority modification,
   this document introduces the use of a Freeze command in Appendix C.
   The reasons for these changes are explained in the following sub-
   sections from technical and network operational aspects.

4.1.  Motivations for swapping priorities of FS and SF-P











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   Defining the priority of FS higher than that of Signal Fail on the
   Protection path (SF-P) can result in a situation where the protected
   traffic is taken out-of-service.  Setting the priority of any input
   that is supposed to be signalled to the other end to be higher than
   that of SF-P can result in unpredictable protection switching state,
   when the protection path has failed and consequently the PSC
   communication stopped.  An example of the out-of-service scenarios is
   shown in Appendix A

   According to Section 2.4 of [RFC5654] it MUST be possible to operate
   an MPLS-TP network without using a control plane.  This means that
   external switch commands, e.g., FS, can be transferred to the far end
   only by using the PSC communication channel and should not rely on
   the presence of a control plane.

   As the priority of SF-P has been higher than FS in optical transport
   networks and Ethernet transport networks, for network operators it is
   important that the MPLS-TP protection switching preserves the network
   operation behavior to which network operators have become accustomed.
   Typically, the FS command is issued before network maintenance jobs,
   (e.g., replacing optical cables or other network components).  When
   an operator pulls out a cable on the protection path by mistake, the
   traffic should be protected and the operator expects this behavior
   based on his/her experience on the traditional transport network
   operations.

4.2.  Motivation for raising the priority of Clear SF

   The priority level of SFc defined in [RFC6378] can cause traffic
   disruption when a node that has experienced local signal fails on
   both working and protection paths is recovering from these failures.

   An example of sequence diagram showing the problem with the priority
   level of SFc as defined in [RFC6378] is shown in Appendix B.

4.3.  Motivation for introducing Freeze command

   With the priority swapping between FS and SF-P, the traffic is always
   moved back to the working path when SF-P occurs in Protecting
   Administrative state.  In the case that network operators need an
   option to control their networks so that the traffic can remain on
   the protection path even when the PSC communication channel is
   broken, the Freeze command, which is a local command (i.e., not
   signalled to the other end) can be used.  The use of the Freeze
   command is described in Appendix C.

4.4.  Updates to the PSC RFC




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   The list of local requests in order of priority should be modified as
   follows:

      (from higher to lower)

   o  Clear Signal Fail/Degrade

   o  Signal Fail on the Protection path

   o  Forced Switch

   o  Signal Fail on the Working path

   The change of the PSC control logic including state machine due to
   this priority modification is incorporated in the PSC control logic
   description when all the capabilities are enabled in Section 10 and
   Section 11.

5.  Capability 2: Modification of Non-revertive Operation

   Non-revertive mode of protection switching is defined in [RFC4427].
   In this mode, the traffic does not return to the working path when
   switch-over requests are terminated.

   However, PSC protocol defined in [RFC6378] supports this operation
   only when recovering from a defect condition, but does not operate as
   non-revertive when an operator's switch-over command such as Forced
   Switch or Manual Switch is cleared.  To be aligned with legacy
   transport network behavior and [RFC4427], a node should go into the
   Do-not-Revert (DNR) state not only when a failure condition on a
   working path is cleared but also when an operator command requesting
   switch-over is cleared.

   The change of the PSC control logic including state machine due to
   the modification of non-revertive operation is incorporated into the
   PSC control logic description when all the capabilities are enabled
   in Section 10 and Section 11.

6.  Capability 3: Support of Manual Switch to Working Command

6.1.  Motivation for adding Manual Switch to Working

   Changing the non-revertive operation introduces necessity of a new
   operator command to revert traffic to the working path when in Do-
   not-Revert (DNR) state.  When the traffic is on the protection path
   in DNR state, a Manual Switch to Working (MS-W) command is issued to
   switch the normal traffic back to the working path.  According to
   Section 4.3.3.6 (Do-not-Revert State) in [RFC6378], "to revert back



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   to Normal state, the administrator SHALL issue a Lockout of
   protection (LO) command followed by a Clear command."  However, using
   LO command introduces the potential risk of an unprotected situation
   while the Lockout of protection is in effect.

   Manual Switch-over for recovery LSP/span command, defined in
   [RFC4427] and also defined in [RFC5654], Requirement 83, as one of
   the mandatory external commands, should be used for this purpose, but
   is not included in [RFC6378].  Note that the "Manual Switch-over for
   recovery LSP/span" command is the same as MS-W command.

6.2.  Terms modified to support MS-W

   The term "Manual Switch" and its acronym "MS" used in [RFC6378] are
   replaced respectively by "Manual Switch to Protection" and "MS-P" by
   this document to avoid confusion with "Manual Switch to Working" and
   its acronym "MS-W".

   Also, the term "Protecting administrative state" used in [RFC6378] is
   replaced by "Switching administrative state" by this document to
   include the case where traffic is switched back to the working path
   by administrative Manual Switch to Working command.

6.3.  Behavior of MS-P and MS-W

   The MS-P and MS-W commands SHALL have the same priority.  If one of
   these commands is already issued and accepted, and the other command
   that is issued afterwards SHALL be ignored.  If two LERs are
   requesting opposite operations simultaneously, i.e. one LER is
   sending MS-P while the other LER is sending MS-W, the MS-W SHALL be
   considered to have a higher priority than MS-P, and MS-P SHALL be
   ignored.

   Two commands, MS-P and MS-W are represented by the same Request Field
   value, but differentiated by the FPath value.  When traffic is
   switched to the protection path, 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).  When traffic is
   switched to the working path, the FPath field SHALL indicate that the
   protection path is being blocked (i.e., FPath set to 0), and the Path
   field SHALL indicate that user data traffic is being transported on
   the working path (i.e., Path set to 0).

6.4.  Equal priority resolution for MS

   [RFC6378] defines only one rule for equal priority condition in
   Section 4.3.2 as "The remote message from the far-end LER is assigned



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   a priority just below the similar local input."  In order to support
   the manual switch behavior described in Section 6.3, additional rules
   for equal priority resolution are required.  Since the support of
   protection against signal degrades also requires a similar equal
   priority resolution, the rules are described in Section 7.4.

   The change of the PSC control logic including state machine due to
   the support of MS-W command is incorporated into the PSC control
   logic description when all the capabilities are enabled in Section 10
   and Section 11.

7.  Capability 4: Support of protection against Signal Degrade

7.1.  Motivation for supporting protection against Signal Degrade

   In MPLS-TP survivability framework [RFC6372], fault conditions
   include both Signal Fail (SF) and Signal Degrade (SD) that can be
   used to trigger protection switching.

   [RFC6378], which defines the Protection State Coordination (PSC)
   protocol, does not specify how the SF and SD are declared and
   specifies the protection switching protocol associated with SF only.

   The protection switching protocol associated with SD is covered in
   this document, and the specifics for the method of identifying SD is
   out of the scope of PSC protocol similarly to how to detect SF and
   how MS and FS commands are initiated in a management system and
   signalled to PSC.

7.2.  Terms modified to support SD

   Clear Signal Fail (SFc) includes the clearance of a degraded
   condition in addition to the clearance of a failure condition

   The second paragraph of Section 4.3.3.2 Unavailable State in
   [RFC6378] shows the intention of including Signal Degrade on the
   Protection path (SD-P) in the Unavailable state.  Even though the
   protection path can be partially available under the condition of the
   Signal Degrade on the Protection path, this document follows the same
   state grouping as [RFC6378] for SD on the protection path.

   The bullet item "Protecting failure state" in Section 3.6.  PSC
   Control States in [RFC6378] includes the degraded condition in
   Protection Failure state.  This document follows the same state
   grouping as [RFC6378] for Signal Degrade on the Working path (SD-W).

7.3.  Behavior of protection against SD




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   In order to maintain the network operation behavior to which
   transport network operators have become accustomed, the priorities of
   SD-P and SD-W are defined to be equal as in other transport networks,
   such as OTN and Ethernet.  Once a switch has been completed due to
   Signal Degrade on one path, it will not be overridden by Signal
   Degrade on the other path (first come, first served behavior), to
   avoid protection switching that cannot improve signal quality and
   flapping.

   Signal Degrade (SD) 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.  The FPath
   field SHALL identify the path that is reporting the degrade 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 working path is selected,
   then Path is set to 0; if protection path is selected, then Path is
   set to 1).

   The Wait to Restore (WTR) timer is used when the protected domain is
   configured for revertive behavior and started at the node that
   recovers from a local degraded condition on the working path.

   If the detection of a SD depends on the presence of user data
   packets, such a condition declared on the working path is cleared
   following protection switching to the protection path if a selector
   bridge is used, possibly resulting in flapping.  To avoid flapping,
   the selector bridge should duplicate the user data traffic and feed
   it to both working and protection paths under SD condition.  In
   revertive mode, when WTR timer expires the packet duplication will be
   stopped and the user data traffic will be transported on the working
   path only.  In non-revertive mode, when SD is cleared the packet
   duplication will be stopped and the user data traffic will be
   transported on the protection path only.

   When multiple SDs are detected simultaneously, either as local or
   remote requests on both working and protection paths, the SD on the
   standby path (the path from which the selector does not select the
   user data traffic) is considered as having higher priority than the
   SD on the active path (the path from which the selector selects the
   user data traffic).  Therefore, no unnecessary protection switching
   is performed and the user data traffic continues to be selected from
   the active path.

   In the preceding paragraph, "simultaneously" relates to the
   occurrence of SD on both the active and standby paths at input to the
   Protection State Control Logic in Figure 1 of [RFC6378] at the same
   time, or as long as a SD request has not been acknowledged by the



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   remote end in bidirectional protection switching.  In other words,
   when a local node that has transmitted a SD message receives a SD
   message that indicates a different value of data path (Path) field
   than the value of the Path field in the transmitted SD message, both
   the local and the remote SD requests are considered to occur
   simultaneously.

7.4.  Equal priority resolution

   In order to support the manual switch behavior described in
   Section 6.3 and the protection against Signal Degrade described in
   Section 7.3, the rules to resolve the equal priority requests are
   required.

   For local inputs with same priority, such as MS and SD, first-come,
   first-served rule is applied.  Once a local input is determined as
   the highest priority local input, then a subsequent equal priority
   local input requesting a different action, i.e., the same PSC Request
   Field but different FPath value, to the PSC control logic will not be
   presented to the PSC control logic as the highest local request.
   Furthermore, in the case of MS, the subsequent MS local input
   requesting a different action will be cancelled.

   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 working input assuming that the priority
   modification is in place as in Section 4.4

   However, if the LER is in a remote state due to a remote message, a
   subsequent local input having the same priority but requesting
   different action to the control logic, will be considered as having
   lower priority than the remote message, and will be ignored.  For
   example, if the LER is in remote Unavailable state due to a remote
   SD-P, then subsequent local SD-W input will be ignored.  Likewise, if
   the LER is in remote Switching administrative state due to a remote
   MS-P, then subsequent local MS-W will be ignored and automatically
   cancelled.

   It should be noted that there is a reverse case where one LER
   receives a local input and the other LER receives, simultaneously, an
   input with the same priority but requesting different action.  In
   this case, each of the two LERs receives a subsequent remote message
   having the same priority but requesting different action, while the
   LER is in a local state due to the local input.  In this case, a
   priority must be set for the inputs with the same priority regardless
   of its origin (local input or remote message).  For example, one LER
   receives SD-P as a local input and the other LER receives SP-W as a



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   local input, simultaneously.  Likewise, one LER receives MS-P as a
   local input and the other LER receives MS-W as a local input,
   simultaneously.

   When MS-W and MS-P occur simultaneously at both LERs, MS-W SHALL be
   considered as having higher priority than MS-P at both LERs.

   When SD-W and SD-P occur simultaneously at both LERs, In this case,
   the SD on the standby path (the path from which the selector does not
   select the user data traffic) is considered as having higher priority
   than the SD on the active path (the path from which the selector
   selects the user data traffic) regardless of its origin (local or
   remote message).  Therefore, no unnecessary protection switching is
   performed and the user data traffic continues to be selected from the
   active path.  Giving the higher priority to the SD on the standby
   path SHALL also be applied to the Local Request logic when two SDs
   for different paths happen to be presented to the Local Request logic
   exactly at the same time.

   The change of the PSC control logic including state machine due to
   the support of protection against SD is incorporated into the PSC
   control logic description when all the capabilities are enabled in
   Section 10 and Section 11.

8.  Capability 5: Support of Exercise Command

   Exercise is a command to test if the PSC communication is operating
   correctly.  More specifically, the Exercise is to test and validate
   the linear protection mechanism and PSC protocol including the
   aliveness of the Local Request logic, the PSC state machine and the
   PSC message generation and reception, and the integrity of the
   protection path, without triggering the actual traffic switching.  It
   is used while the working path is either carrying the traffic or not.
   It is lower priority than any "real" switch request.  It is only
   valid in bidirectional switching, since this is the only place where
   one can get a meaningful test by looking for a response.

   This command is documented in R84 of [RFC5654] and it has been
   identified as a requirement from ITU-T.












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   A received EXER message indicates that the remote end point is
   operating under an operator command to validate the protection
   mechanism and PSC protocol including the aliveness of the Local
   Request logic, the PSC state machine and the PSC message generation
   and reception, and the integrity of the protection path, without
   triggering the actual traffic switching.  The valid response to EXER
   message will be an Reverse Request (RR) with the corresponding FPath
   and Path numbers.  The near end will signal a Reverse Request (RR)
   only in response to an EXER command from the far end.

   When Exercise commands are input at both ends, an EXER, instead of
   RR, is transmitted from both ends.

   The following PSC Requests should be added to PSC Request field to
   support Exercise:

      (TBD2) Exercise - indicates that the transmitting end point is
      exercising the protection channel and mechanism.  FPath and Path
      are set to the same value of the NR, RR or DNR request that EXER
      replaces.

      (TBD1) Reverse Request - indicates that the transmitting end point
      is responding to an EXER command from the far end.  FPath and Path
      are set to the same value of the NR, RR or DNR request that EXER
      replaces.

   The priority of Exercise should be inserted between the priorities of
   WTR Expires and No Request.

9.  Capabilities and Modes

9.1.  Capabilities

   A Capability is an individual behavior whose use is signalled in a
   Capabilities TLV, which is placed in Optional TLVs field inside PSC
   messages shown in Figure 2 of [RFC6378].  The format of the
   Capabilities TLV is:

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type = Capabilities          |    Length                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Value = Options                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The value of the Type field is TBD3 pending IANA allocation.



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   The value of the Length field is the length of the Options Value, and
   is in octets.

   The Value of the Capabilities TLV can be any length, as long as it is
   a multiple of 4 octets.  The length of the Value field MUST be the
   minimum required to signal all the required capabilities.  Section 4
   to Section 8 discuss five capabilities that are signalled using the 5
   most significant bits; if a node wishes to signal these five
   capabilities, it MUST send an Options Value of 4 octets.  A node
   would send an Options Value greater than 4 octets only if it had more
   than 32 Capabilities to indicate.  All unused bits MUST be set to
   zero.

   If the bit assigned for an individual capability is set to 1, it
   indicates the sending node's intent to use that capability in the
   protected domain.  If a bit is set to 0, the sending node does not
   intend to use the indicated capability in the protected domain.  Note
   that it is not possible to distinguish between the intent not to use
   a capability and a node's complete non-support (i.e. lack of
   implementation) of a given capability.

   This document defines five specific capabilities that are described
   from Section 4 to Section 8.  Each capability is assigned bit as
   follows:

      0x80000000: priority modification

      0x40000000: modification of non-revertive behavior

      0x20000000: support of Manual Switch to Working (MS-W) command

      0x10000000: support of protection against Signal Degrade (SD)

      0x08000000: support of Exercise command

9.1.1.  Sending the Capabilities TLV

   PSC sends messages in response to external events and in periodic
   retransmission of current status.  It may be expensive to send and to
   parse an Capabilities TLV attached to a packet intended to trigger a
   protection switch or other real- time behavior.  However, if a node
   does not periodically send its Capabilities TLV, the receiving node
   cannot discriminate a deliberate omission of the Capabilities TLV for
   performance reasons from an accidental omission due to an
   implementation issue.  To guard against this, a node MUST include its
   Capabilities TLV in every PSC message that it sends.





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9.1.2.  Receiving the Capabilities TLV

   A node MUST establish a receive timer for the Capabilities TLV.  By
   default this MUST be 3.5 times the periodic retransmission timer of
   five seconds - i.e., 17.5 seconds.  Both the periodic retransmission
   time and the timeout SHOULD be configurable by the operator.  When a
   node receives a Capabilities TLV it resets the timer to 17.5 seconds.
   If the timer expires, the node behaves as in Section 9.1.3.

   [Editor's note: In other packet transport protection technologies,
   Failure of Protocol defect (dFOP) is declared when no protocol
   message is received on the protection path during at least 3.5 times
   the periodic message transmission interval (i.e., at least 17.5
   seconds) and there is no defect on the protection transport entity.
   As the "Capabilities TLV" is included in the PSC message, this error
   of not receiving the Capabilities TLV can be covered by dFOP.  To be
   discussed.]

   When a node receives a Capabilities TLV it MUST compare it to its
   most recent transmitted Capabilities TLV.  If the two are equal, the
   protected domain is said to be running in the mode indicated by that
   set of capabilities (see Section 9.2).  If the sent and received
   Capabilities TLVs are not equal, this indicates a capabilities
   mismatch.  When this happens, the node MUST alert the operator and
   MUST behave as in Section 9.1.3.

9.1.3.  Handling Capabilities TLV errors

   This section covers the two possible errors - a TLV timeout and a TLV
   mismatch - and the error handling procedures in both cases.

9.1.3.1.  Capabilities TLV Timeout

   If the Capabilities TLV receive timer expires, a node is said to have
   timed out.  When this happens, the node MUST alert the operator and
   MUST behave as in Section 9.1.3.3.

9.1.3.2.  Capabilities TLV Mismatch

   If the sent and received Capabilities TLVs are not equal, this
   indicates a capabilities mismatch.  When this happens, the node MUST
   alert the operator and MUST behave as in Section 9.1.3.3.  A node MAY
   retain the received TLV for logging, alert or debug purposes.

9.1.3.3.  Handling Capabilities TLV error conditions

   When a node enters in Capabilities protocol error conditions, the
   following actions MUST be taken:



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   1.  Indicate the error condition (e.g., either mismatch or timeout)
       to the operator by the usual alert mechanisms (e.g., syslog).

   2.  Not make any state transitions based on the contents of any PSC
       Messages

   To expand on point 2 - assume node A is receiving NR(0,0) from its
   PSC peer node Z and is also receiving a mismatched set of
   capabilities (e.g., received 0x4, transmitted 0x5).  If node Z
   detects a local SF-W and wants to initiate a protection switch (that
   is, by sending SF(1,1)), node A MUST NOT react to this input by
   changing its state.  A node MAY increase the severity or urgency of
   its alarms to the operator, but until the operator resolves the
   mismatch in the Capabilities TLV the protected domain will likely
   operate in an inconsistent state.

9.2.  Modes

   A Mode is a given set of Capabilities.  Modes are shorthand;
   referring to a set of capabilities by their individual values or by
   the name of their mode does not change the protocol behavior.  This
   document defines two modes - PSC and APS.

9.2.1.  PSC Mode

   PSC Mode is defined as the lack of any Capabilities - that is, a
   Capabilities set of 0x0.  It is the behavior specified in RFC6378.
   There are two ways to declare PSC Mode.  A node can send a
   Capabilities TLV of 0x0, or it can send no Capabilities TLV at all.
   This is further explored in Section 9.3.

9.2.2.  APS Mode

   APS Mode is defined as the use of all of the five specific
   capabilities, which are described from Section 4 to Section 8 in this
   document.  APS Mode is indicated with a Value of 0xF8000000.

9.3.  Backward compatibility

   As defined in Section 9.2.1, PSC Mode is indicated either with a
   Capabilities TLV of 0x0 or the lack of Capabilities TLV.  This is to
   allow backward compatibility between two nodes - one which can send
   the Capabilities TLV, and one which cannot.

   [RFC6378] does not define how to handle an unrecognized TLV.  There
   may be some implementations that silently discard an unrecognized
   TLV, and some that take more drastic steps like refusing to allow PSC
   to operate.  Thus, a node which has the ability to send and receive



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   the PSC Mode Capabilities TLV MUST be able to both send the PSC Mode
   Capabilities TLV and send no Capabilities TLV at all.  An
   implementation MUST be configurable between these two choices.

   One question that arises from this dual definition of PSC Mode is,
   what happens if a node which was sending a non-null Capabilities TLV
   (e.g., APS Mode) sends PSC packets without any Capabilities TLV?
   This case is handled as follows:

   If a node has never, during the life of a PSC session, received a
   Capabilities TLV from a neighbour, the lack of a Capabilities TLV is
   treated as receipt of a PSC Capabilities TLV.  This allows for
   interop between nodes which support the PSC Mode TLV and nodes which
   do not, and are thus implicitly operating in PSC Mode.

   If a node has received a non-null Capabilities TLV (e.g., APS Mode)
   during the life of a PSC session and then receives a PSC packet with
   no Capabilities TLV, the receiving node MUST treat the lack of
   Capabilities TLV as simply a lack of refresh.  That is, the receipt
   of a PSC packet with no Capabilities TLV simply does not reset the
   receive timer defined in Section 9.1.2.

10.  PSC Protocol in APS Mode

   This section and Section 11 defines the behavior of PSC protocol when
   all of the aforementioned capabilities are enabled, i.e., APS mode.

10.1.  Request field in PSC protocol message

   The values of "Request" field in the PSC protocol message, which is
   shown in Figure 2 of [RFC6378], are defined as follows:

      (14) Lockout of protection

      (12) Forced Switch

      (10) Signal Fail

      (7) Signal Degrade

      (5) Manual Switch

      (4) Wait-to-Restore

      (TBD2) Exercise

      (TBD1) Reverse Request




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      (1) Do-not-Revert

      (0) No Request

10.2.  Priorities of local inputs and remote requests

   Based on the description in Section 3 and Section 4.3.2 in [RFC6378],
   the priorities of multiple outstanding local inputs are evaluated in
   Local Request logic unit, where the highest priority local request is
   determined.  This high-priority local request is passed to the PSC
   Control logic, that will determine the higher priority input (top
   priority global request) between the highest priority local input and
   the last received remote message.  When a remote message comes to the
   PSC Control logic, the top priority global request is determined
   between this remote message and the highest priority local input
   which is present.  The top priority global request is used to
   determine the state transition, which is described in Section 11.

   The priorities for both local and remote requests are defined as
   follows from highest to lowest:

   o  Operator Clear (Local only)

   o  Lockout of protection (Local and Remote)

   o  Clear Signal Fail/Degrade (Local only)

   o  Signal Fail on Protection path (Local and Remote)

   o  Forced Switch (Local and Remote)

   o  Signal Fail on Working path (Local and Remote)

   o  Signal Degrade on either Protection path or Working path (Local
      and Remote)

   o  Manual Switch to either Protection path or Working path (Local and
      Remote)

   o  WTR Expires (Local only)

   o  WTR (Remote only)

   o  Exercise (Local and Remote)

   o  Reverse Request (Remote only)

   o  Do-Not-Revert (Remote only)



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   o  No Request (Remote and Local)

   The remote request from the far-end LER is assigned a priority just
   below the same local request.  However, for the equal priority
   requests, such as Signal Degrade on either Working or protection and
   Manual Switch to either Protection or Working path, the following
   equal priority resolution rules are defined:

   o  If two local inputs having same priority but requesting different
      action come to the Local Request logic, then the input coming
      first SHALL be considered to have a higher priority than the other
      coming later (first-come, first-served).

   o  If the LER receives both a local input and a remote message with
      the same priority and requesting the same action, i.e., the same
      PSC Request Field and the same FPath value, then the local input
      SHALL be considered to have a higher priority than the remote
      message.

   o  If the LER receives both a local input and a remote message with
      the same priority but requesting different actions, i.e., the same
      PSC Request Field but different FPath value, then the first-come,
      first-served rule SHALL be applied.  If the remote message comes
      first, then the state SHALL be a remote state and subsequent local
      input is ignored.  However, if the local input comes first, the
      first-come, first-served rule cannot be applied and must be viewed
      as simultaneous condition.  This is because the subsequent remote
      message will not be an acknowledge of the local input by the far-
      end node.  In this case, the priority SHALL be determined by rules
      for each simultaneous condition.

   o  If the LER receives both MS-P and MS-W requests as both local
      input and remote message and the LER is in a local Switching
      administrative state, then the MS-W request SHALL be considered to
      have a higher priority than the MS-P request.

   o  If the LER receives both SD-P and SD-W requests as both local
      input and remote message and the LER is in a local state, then the
      SD on the standby path (the path from which the selector does not
      select the user data traffic) SHALL be considered as having higher
      priority than the SD on the active path (the path from which the
      selector selects the user data traffic) regardless of its origin
      (local or remote message).  This rule of giving the higher
      priority to the SD on the standby path SHALL also be applied to
      the Local Request logic when two SDs for different paths happen to
      be presented to the Local Request logic exactly at the same time.





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11.  State Transition Tables in APS Mode

   When there is a change in the highest-priority local request or in
   remote PSC messages, the top priority global request is evaluated and
   the state transition tables are looked up in PSC control logic.  The
   following rules are applied to the operation related to the state
   transition table lookup.

   o  If the top priority global request, which determines the state
      transition, is the highest priority local input, the local state
      transition table SHALL be used to decide the next state of the
      LER.  Otherwise, remote messages state transition table SHALL be
      used.

   o  If in remote state, the highest local defect condition (SF-P,
      SF-W, SD-P or SD-W) SHALL always be reflected in the Request Field
      and Fpath.

   o  Operator Clear command, Clear SF/SD (SFc) and WTR Expires are not
      persistent.  Once they appear to the local priority logic and
      complete the operation, they will be disappeared.

   o  For the LER currently in the local state, if the top priority
      global request is changed to OC or SFc causing the next state to
      be Normal, WTR or DNR, then all the local and remote requests
      should be re-evaluated as if the LER is in the state specified in
      the footnotes to the state transition tables, before deciding the
      final state.  This re-evaluation is an internal operation confined
      within the local LER, and PSC messages are generated according to
      the final state.

   o  The WTR timer is started only when the LER which has recovered
      from a local failure/degradation enters the WTR state.  An LER
      which is entering into the WTR state due to a remote WTR message
      does not start the WTR timer.

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

   N        Normal state
   UA:LO:L  Unavailable state due to local LO command
   UA:P:L   Unavailable state due to local SF-P
   UA:DP:L  Unavailable state due to local SD-P
   UA:LO:R  Unavailable state due to remote LO message
   UA:P:R   Unavailable state due to remote SF-P message
   UA:DP:L  Unavailable state due to local SD-P
   PF:W:L   Protecting failure state due to local SF-W
   PF:DW:L  Protecting failure state due to local SD-W
   PF:W:R   Protecting failure state due to remote SF-W message



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   PF:DW:R  Protecting failure state due to remote SD-W message
   SA:F:L   Switching administrative state due to local FS command
   SA:MW:L  Switching administrative state due to local MS-W command
   SA:MP:L  Switching administrative state due to local MS-P command
   SA:F:R   Switching administrative state due to remote FS message
   SA:MW:R  Switching administrative state due to remote MS-W message
   SA:MP:R  Switching administrative state due to remote MS-P message
   E::L     Exercise state due to local EXER command
   E::R     Exercise state due to remote EXER 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 transition tables.

   State    REQ(FP,P)
   -------  ---------
   N        NR(0,0)
   UA:LO:L  LO(0,0)
   UA:P:L   SF(0,0)
   UA:DP:L  SD(0,0)
   UA:LO:R  highest local request(local FPath,0)
   UA:P:R   highest local request(local FPath,0)
   UA:DP:R  highest local request(local FPath,0)
   PF:W:L   SF(1,1)
   PF:DW:L  SD(1,1)
   PF:W:R   highest local request(local FPath,1)
   PF:DW:R  highest local request(local FPath,1)
   SA:F:L   FS(1,1)
   SA:MW:L  MS(0,0)
   SA:MP:L  MS(1,1)
   SA:F:R   highest local request(local FPath,1)
   SA:MW:R  highest local request(local FPath,0)
   SA:MP:R  highest local request(local FPath,1)
   WTR      WTR(0,1)
   DNR      DNR(0,1)
   E::L     EXER(0,x), where x is the existing Path value
                       when Exercise command is issued.
   E::R     RR(0,x), where x is the existing Path value
                     when RR message is generated.


11.1.  State transition by local inputs




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           | OC  | LO      | SFc | SF-P   | FS     | SF-W   |
   --------+-----+---------+-----+--------+--------+--------+
   N       | i   | UA:LO:L | i   | UA:P:L | SA:F:L | PF:W:L |
   UA:LO:L | (1) | i       | i   | i      | i      | i      |
   UA:P:L  | i   | UA:LO:L | (1) | i      | i      | i      |
   UA:DP:L | i   | UA:LO:L | (1) | UA:P:L | SA:F:L | PF:W:L |
   UA:LO:R | i   | UA:LO:L | i   | UA:P:L | i      | PF:W:L |
   UA:P:R  | i   | UA:LO:L | i   | UA:P:L | PF:W:L | PF:W:L |
   UA:DP:R | i   | UA:LO:L | i   | UA:P:L | SA:F:L | PF:W:L |
   PF:W:L  | i   | UA:LO:L | (2) | UA:P:L | SA:F:L | i      |
   PF:DW:L | i   | UA:LO:L | (2) | UA:P:L | SA:F:L | PF:W:L |
   PF:W:R  | i   | UA:LO:L | i   | UA:P:L | SA:F:L | PF:W:L |
   PF:DW:R | i   | UA:LO:L | i   | UA:P:L | SA:F:L | PF:W:L |
   SA:F:L  | (3) | UA:LO:L | i   | UA:P:L | i      | i      |
   SA:MW:L | (1) | UA:LO:L | i   | UA:P:L | SA:F:L | PF:W:L |
   SA:MP:L | (3) | UA:LO:L | i   | UA:P:L | SA:F:L | PF:W:L |
   SA:F:R  | i   | UA:LO:L | i   | UA:P:L | SA:F:L | PF:W:L |
   SA:MW:R | i   | UA:LO:L | i   | UA:P:L | SA:F:L | PF:W:L |
   SA:MP:R | i   | UA:LO:L | i   | UA:P:L | SA:F:L | PF:W:L |
   WTR     | i   | UA:LO:L | i   | UA:P:L | SA:F:L | PF:W:L |
   DNR     | i   | UA:LO:L | i   | UA:P:L | SA:F:L | PF:W:L |
   E::L    | (4) | UA:LO:L | i   | UA:P:L | SA:F:L | PF:W:L |
   E::R    | i   | UA:LO:L | i   | UA:P:L | SA:F:L | PF:W:L |




























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           | SD-P    | SD-W    | MS-W    | MS-P    | WTRExp | EXER
   --------+---------+---------+---------+---------+--------+------
   N       | UA:DP:L | PF:DW:L | SA:MW:L | SA:MP:L | i      | E::L
   UA:LO:L | i       | i       | i       | i       | i      | i
   UA:P:L  | i       | i       | i       | i       | i      | i
   UA:DP:L | i       | i       | i       | i       | i      | i
   UA:LO:R | UA:DP:L | PF:DW:L | i       | i       | i      | i
   UA:P:R  | UA:DP:L | PF:DW:L | i       | i       | i      | i
   UA:DP:R | UA:DP:L | PF:DW:L | i       | i       | i      | i
   PF:W:L  | i       | i       | i       | i       | i      | i
   PF:DW:L | i       | i       | i       | i       | i      | i
   PF:W:R  | UA:DP:L | PF:DW:L | i       | i       | i      | i
   PF:DW:R | UA:DP:L | PF:DW:L | i       | i       | i      | i
   SA:F:L  | i       | i       | i       | i       | i      | i
   SA:MW:L | UA:DP:L | PF:DW:L | i       | i       | i      | i
   SA:MP:L | UA:DP:L | PF:DW:L | i       | i       | i      | i
   SA:F:R  | UA:DP:L | PF:DW:L | i       | i       | i      | i
   SA:MW:R | UA:DP:L | PF:DW:L | SA:MW:L | i       | i      | i
   SA:MP:R | UA:DP:L | PF:DW:L | i       | SA:MP:L | i      | i
   WTR     | UA:DP:L | PF:DW:L | SA:MW:L | SA:MP:L | (6)    | i
   DNR     | UA:DP:L | PF:DW:L | SA:MW:L | SA:MP:L | i      | E::L
   E::L    | UA:DP:L | PF:DW:L | SA:MW:L | SA:MP:L | i      | i
   E::R    | UA:DP:L | PF:DW:L | SA:MW:L | SA:MP:L | i      | E::L


11.2.  State transition by remote messages

           | LO      | SF-P   | FS     | SF-W   | SD-P    | SD-W    |
   --------+---------+--------+--------+--------+---------+---------+
   N       | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
   UA:LO:L | i       | i      | i      | i      | i       | i       |
   UA:P:L  | UA:LO:R | i      | i      | i      | i       | i       |
   UA:DP:L | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | i       | (10)    |
   UA:LO:R | i       | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
   UA:P:R  | UA:LO:R | i      | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
   UA:DP:R | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | i       | PF:DW:R |
   PF:W:L  | UA:LO:R | UA:P:R | SA:F:R | i      | i       | i       |
   PF:DW:L | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | (11)    | i       |
   PF:W:R  | UA:LO:R | UA:P:R | SA:F:R | i      | UA:DP:R | PF:DW:R |
   PF:DW:R | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
   SA:F:L  | UA:LO:R | UA:P:R | i      | i      | i       | i       |
   SA:MW:L | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
   SA:MP:L | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
   SA:F:R  | UA:LO:R | UA:P:R | i      | PF:W:R | UA:DP:R | PF:DW:R |
   SA:MW:R | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
   SA:MP:R | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
   WTR     | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
   DNR     | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |



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   E::L    | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
   E::R    | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |


           | MS-W    | MS-P    | WTR | EXER | RR | DNR | NR
   --------+---------+---------+-----+------+----+-----+----
   N       | SA:MW:R | SA:MP:R | i   | E::R | i  | i   | i
   UA:LO:L | i       | i       | i   | i    | i  | i   | i
   UA:P:L  | i       | i       | i   | i    | i  | i   | i
   UA:DP:L | i       | i       | i   | i    | i  | i   | i
   UA:LO:R | SA:MW:R | SA:MP:R | i   | E::R | i  | i   | N
   UA:P:R  | SA:MW:R | SA:MP:R | i   | E::R | i  | i   | N
   UA:DP:R | SA:MW:R | SA:MP:R | i   | E::R | i  | i   | N
   PF:W:L  | i       | i       | i   | i    | i  | i   | i
   PF:DW:L | i       | i       | i   | i    | i  | i   | i
   PF:W:R  | SA:MW:R | SA:MP:R | (7) | E::R | i  | (8) | (5)
   PF:DW:R | SA:MW:R | SA:MP:R | (7) | E::R | i  | (8) | (5)
   SA:F:L  | i       | i       | i   | i    | i  | i   | i
   SA:MW:L | i       | i       | i   | i    | i  | i   | i
   SA:MP:L | i       | i       | i   | i    | i  | i   | i
   SA:F:R  | SA:MW:R | SA:MP:R | i   | E::R | i  | DNR | N
   SA:MW:R | i       | SA:MP:R | i   | E::R | i  | i   | N
   SA:MP:R | SA:MW:R | i       | i   | E::R | i  | DNR | N
   WTR     | SA:MW:R | SA:MP:R | i   | i    | i  | i   | (9)
   DNR     | SA:MW:R | SA:MP:R | i   | E::R | i  | i   | i
   E::L    | SA:MW:R | SA:MP:R | i   | i    | i  | i   | i
   E::R    | SA:MW:R | SA:MP:R | i   | i    | i  | DNR | N


   NOTES:

   (1)  Re-evaluate to determine final state as if the LER is in the
        Normal state.

   (2)  In the case that both local input and the last received remote
        message are no request after the occurrence of SFc, the LER
        enters into the WTR state when the domain is configured for
        revertive behavior, or the LER enters into the DNR state when
        the domain is configured for non-revertive behavior.  In all the
        other cases, re-evaluate to determine the final state as if the
        LER is in the Normal state.

   (3)  Re-evaluate to determine final state as if the LER is in the
        Normal state when the domain is configured for revertive
        behavior, or as if the LER is in the DNR state when the domain
        is configured for non-revertive behavior,





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   (4)  If Path value is 0, re-evaluate to determine final state as if
        the LER is in the Normal state.  If Path value is 1, re-evaluate
        to determine final state as if the LER is in the DNR state

   (5)  If the received NR message has Path=1, transition to WTR if
        domain configured for revertive behavior, else transition to
        DNR.

   (6)  Remain in WTR, send NR(0,1).

   (7)  Transition to WTR state and continue to send the current
        message.

   (8)  Transition to DNR state and continue to send the current
        message.

   (9)  If the receiving LER's WTR timer is running, maintain current
        state and message.  If the WTR timer is not running, transition
        to N.

   (10) If the active path just before the SD is selected as the highest
        local input was the working path, then ignore.  Otherwise, go to
        PF:DW:R and transmit SD(0,1)

   (11) If the received SD-P message has Path=1, ignore the message.  If
        the received SD-P message has Path=0 and the active path just
        before the SD is selected as the highest local input was the
        working path, then go to UA:DP:R and transmit SD(1,0).  If the
        received SD-P message has Path=0 and the active path just before
        the SD is selected as the highest local input was the protection
        path, then ignore the received SD-P message.

12.  Security considerations

   No specific security issue is raised in addition to those ones
   already documented in [RFC6378]

13.  IANA considerations

13.1.  PSC Request Field

   This document defines two new values in the "MPLS PSC Request
   Registry".

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





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   Value Description              Reference
   ----- --------------------- ---------------
    TBD1 Reverse Request       [this document]
    TBD2 Exercise              [this document]


   [to be removed upon publication: It is requested to assign 2
   (=TBD1)for the Reverse Request value and 3 (=TBD2) for the Exercise
   value to be aligned with the priority levels of those two requests
   defined in this document.]

13.2.  PSC TLV

   This document defines a new value for the Capabilities TLV type in
   the "MPLS PSC TLV Registry".

   Type  TLV Name               Reference
   ----- --------------------- ---------------
   TBD3  Capabilities          [this document]


   [Editor's note: Need to specify a registry for Value (=options)
   inside the Capabilities TLV in a later version of this draft]

14.  Acknowledgements

15.  References

15.1.  Normative References

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

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

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

   [I-D.ietf-mpls-psc-updates]
              Osborne, E., "Updates to PSC", draft-ietf-mpls-psc-
              updates-00 (work in progress), October 2013.

15.2.  Informative References





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

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

Appendix A.  An example of out-of-service scenarios

   The sequence diagram shown is an example of the out-of-service
   scenerios based on the priority level defined in [RFC6378].  The
   first PSC message which differs from the previous PSC message is
   shown.

                    A                  Z
                    |                  |
                (1) |-- NR(0,0) ------>| (1)
                    |<----- NR(0,0) ---|
                    |                  |
                    |                  |
                    | (FS issued at Z) | (2)
                (3) |<------ FS(1,1) --|
                    |-- NR(0,1) ------>|
                    |                  |
                    |                  |
                (4) | (SF on P(A<-Z))  |
                    |                  |
                    |                  |
                    | (Clear FS at Z)  | (5)
                (6) |   X <- NR(0,0) --|
                    |                  |
                    |                  |


   (1) Each end is in Normal state, and transmits NR (0,0) messages.

   (2) When a Forced Switch command is issued at node Z, node Z goes
   into local Protecting Administrative state (PA:F:L) and begins
   transmission of an FS (1,1) messages.

   (3) A remote Forced Switch message causes node A to go into remote
   Protecting Administrative state (PA:F:R), and node A begins
   transmitting NR (0,1) messages.

   (4) When node A detects a unidirectional Signal Fail on the
   Protection path, node A keeps sending NR (0,1) message because SF-P
   is ignored under the state PA:F:R.




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   (5) When a Clear command is issued at node Z, node Z goes into Normal
   state and begins transmission of NR (0,0) messages.

   (6) But node A cannot receive PSC message because of local
   unidirectional Signal Fail on the Protection path.  Because no valid
   PSC message is received, over a period of several successive message
   intervals, the last valid received message remains applicable and the
   node A continue to transmit an NR (0,1) message in the state of
   PA:F:R.

   Now, there exists a mismatch between the bridge/selector positions of
   node A (transmitting an NR (0,1)) and node Z (transmitting an NR
   (0,0)).  It results in out-of-service even when there is neither
   signal fail on working path nor FS.

Appendix B.  An example of sequence diagram showing the problem with the
             priority level of Clear SF

   An example of sequence diagram showing the problem with the priority
   level of Clear SF defined in [RFC6378] is given below.  The following
   sequence diagram is depicted for the case of bidirectional signal
   fails.  However, other cases with unidirectional signal fails can
   result in the same problem.  The first PSC message which differs from
   the previous PSC message is shown.

                    A                  Z
                    |                  |
                (1) |-- NR(0,0) ------>| (1)
                    |<----- NR(0,0) ---|
                    |                  |
                    |                  |
                (2) | (SF on P(A<->Z)) | (2)
                    |-- SF(0,0) ------>|
                    |<------ SF(0,0) --|
                    |                  |
                    |                  |
                (3) | (SF on W(A<->Z)) | (3)
                    |                  |
                    |                  |
                (4) |   (Clear SF-P)   | (4)
                    |                  |
                    |                  |
                (5) |   (Clear SF-W)   | (5)
                    |                  |
                    |                  |


   (1) Each end is in Normal state, and transmits NR (0,0) messages.



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   (2) When signal fail on protection (SF-P) occurs, each node enters
   into [UA:P:L] state and transmits SF (0,0) messages.  Traffic remains
   on working path.

   (3) When signal fail on working (SF-W) occurs, each node remains in
   [UA:P:L] state as SF-W has a lower priority than SF-P.  Traffic is
   still on the working path.  Traffic cannot be delivered as both
   working and protection paths are experiencing signal fails.

   (4) When the signal fail on protection is cleared, local "Clear SF-P"
   request cannot be presented to the PSC control logic, which takes the
   highest priority local request and runs PSC state machine, as the
   priority of "Clear SF-P" is lower than that of SF-W.  Consequently,
   there is no change in state, and the selector and/or bridge keep
   pointing at the working path, which has signal fail condition.

   Now, traffic cannot be delivered while the protection path is
   recovered and available.  It should be noted that the same problem
   will occur in the case that the sequence of SF-P and SF-W events is
   changed.

   If we further continue with this sequence to see what will happen
   after SF-W is cleared,

   (5) When the signal fail on working is cleared, local "Clear SF-W"
   request can be passed to the PSC control logic (state machine) as
   there is no higher priority local request, but this will be ignored
   in the PSC control logic according to the state transition definition
   in [RFC6378].  There will be no change in state or protocol message
   transmitted.

   As the signal fail on working is now cleared and the selector and/or
   bridge are still pointing at the working path, traffic delivery is
   resumed.  However, each node is in [UA:P:L] state and transmitting
   SF(0,0) message, while there exists no outstanding request for
   protection switching.  Moreover, any future legitimate protection
   switching requests, such as SF-W, will be rejected as each node
   thinks the protection path is unavailable.

Appendix C.  Freeze Command

   The "Freeze" command applies only to the near end (local node) of the
   protection group and is not signalled to the far end.  This command
   freezes the state of the protection group.  Until the Freeze is
   cleared, additional near end commands are rejected and condition
   changes and received PSC information are ignored.





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   "Clear Freeze" command clears the local freeze.  When the Freeze
   command is cleared, the state of the protection group is recomputed
   based on the persistent condition of the local triggers.

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

Authors' Addresses

   Jeong-dong Ryoo (editor)
   ETRI
   218 Gajeongno
   Yuseong-gu, Daejeon  305-700
   South Korea

   Phone: +82-42-860-5384
   Email: ryoo@etri.re.kr


   Eric Gray (editor)
   Ericsson

   Email: eric.gray@ericsson.com


   Huub van Helvoort
   Huawei Technologies
   Karspeldreef 4,
   Amsterdam 1101 CJ
   the Netherlands

   Phone: +31 20 4300936
   Email: huub.van.helvoort@huawei.com


   Alessandro D'Alessandro
   Telecom Italia
   via Reiss Romoli, 274
   Torino  10148
   Italy

   Phone: +39 011 2285887
   Email: alessandro.dalessandro@telecomitalia.it







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   Taesik Cheung
   ETRI
   218 Gajeongno
   Yuseong-gu, Daejeon  305-700
   South Korea

   Phone: +82-42-860-5646
   Email: cts@etri.re.kr


   Eric Osborne
   Cisco Systems, Inc.

   Email: eosborne@cisco.com





































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