Pre-standard Linear Protection Switching in MPLS Transport Profile (MPLS-TP)
RFC 7347

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

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

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

8.3.  Signal Degrade of the Protection Transport Entity

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

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

9.  Protection-Switching State Transition Tables

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

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

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

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

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

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

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

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

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

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

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

   o  EXER: Exercise of the APS protocol.

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

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

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10.  Security Considerations

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

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

11.  Acknowledgements

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

12.  References

12.1.  Normative References

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

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

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

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

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

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

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

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

12.2.  Informative References

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

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

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

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Appendix A.  Operation Examples of the APS Protocol

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Authors' Addresses

   Huub van Helvoort (editor)
   Huawei Technologies

   EMail: huub@van-helvoort.eu

   Jeong-dong Ryoo (editor)
   ETRI

   EMail: ryoo@etri.re.kr

   Haiyan Zhang
   Huawei Technologies

   EMail: zhanghaiyan@huawei.com

   Feng Huang
   Philips

   EMail: feng.huang@philips.com

   Han Li
   China Mobile

   EMail: lihan@chinamobile.com

   Alessandro D'Alessandro
   Telecom Italia

   EMail: alessandro.dalessandro@telecomitalia.it

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