Native   |<----------- Pseudowires ----------->|  Native
    Service  |                                     |  Service
     (AC)    |     |<-PSN1-->|     |<-PSN2-->|     |  (AC)
       |     V     V         V     V         V     V   |
       |     +-----+         +-----+         +-----+   |
+----+ |     |T-PE1|=========|S-PE1|=========|T-PE2|   |   +----+
|    |-------|......PW1-Seg1.......|.PW1-Seg2......|-------|    |
| CE1|       |     |=========|     |=========|     |       | CE2|
|    |       +-----+         +-----+         +-----+       |    |
+----+        |.||.|                          |.||.|       +----+
              |.||.|         +-----+          |.||.|
              |.||.|=========|     |========== .||.|
              |.||...PW2-Seg1......|.PW2-Seg2...||.|
              |.| ===========|S-PE2|============ |.|
              |.|            +-----+             |.|
              |.|============+-----+============= .|
              |.....PW3-Seg1.|     | PW3-Seg2......|
               ==============|S-PE3|===============
                             |     |
                             +-----+

Figure 1: Single-Homed CE with Redundant MS-PWs

The procedures in RFCs 6718 and 6870 rely on LDP-based PW status signaling to signal the state of the primary MS-PW that is being protected, and the precedence in which redundant MS-PW(s) should be used to protect the primary MS-PW should it fail. These procedures make use of information carried by the PW Status TLV, which for dynamically signaled PWs is carried by the LDP protocol.

However, statically provisioned PWs (SS-PWs or MS-PWs) do not use the LDP protocol for PW set and signaling, rather they are provisioned by network management systems or other means at each T-PE and S-PE along their path. They also do not use the LDP protocol for status signaling. Rather, they use procedures defined in RFC 6478 [RFC6478] for status signaling via the PW OAM message using the PW Associated Channel Header (ACH). The PW Status TLV carried via this status signaling is itself identical to the PW Status TLV carried via LDP-based status signaling, including the identical PW Status Codes.

Sections 6 and 7 of RFC 6870 describes the management of a primary PW and its secondary PW(s) to provide resiliency to the failure of the primary PW. They use status codes transmitted between endpoint T-PEs using the PW Status TLV transmitted by LDP. For this management to apply to statically provisioned PWs, the PW status signaling defined in RFC 6478 MUST be used for the primary and secondary PWs. In that case, the endpoint T-PEs can then use the PW status signaling provided by RFC 6478 in the place of LDP-based status signaling, but otherwise operate identically as described in RFC 6870. Note that the optional S-PE Bypass Mode defined in Section 5.5 of RFC 6478 cannot be used, as it requires LDP signaling.

3. Operational Considerations

Because LDP is not used between the T-PEs for statically provisioned MS-PWs, the negotiation procedures described in RFC 6870 cannot be used. Thus, operational care must be taken so that the endpoint T-PEs are identically provisioned regarding the use of this document, specifically whether or not MS-PW redundancy is being used, and for each protected MS-PW, the identity of the primary MS-PW and the precedence of the secondary MS-PWs.

4. Security Considerations

The security considerations defined for RFC 6478 apply to this document as well. As the security considerations in RFCs 6718 and 6870 are related to their use of LDP, they are not required for this document.

If the alternative approach in Appendix A is used, then the security considerations defined for RFCs 6378, 7271, and 7324 also apply.

5. IANA Considerations

There are no requests for IANA actions in this document.

Note to the RFC Editor - this section can be removed before publication.

6. Acknowledgements

The authors would like to thank Matthew Bocci, Yaakov Stein, David Sinicrope, Sasha Vainshtein, and Italo Busi for their comments on this document.

Figure 1 and the explanatory paragraph following the figure were taken from RFC 6718. Figure 2 was adapted from RFC 6378.

7. References

7.1. Normative References

7.2. Informative References

Appendix A. Optional Linear Protection Approach

A.1. Introduction

In "MPLS Transport Profile (MPLS-TP) Linear Protection" [RFC6378], as well as in the later updates of this RFC in "MPLS Transport Profile (MPLS-TP) Linear Protection to Match the Operational Expectations of SDH, OTN and Ethernet Transport Network Operators" [RFC7271] and in "Updates to MPLS Transport Profile Linear Protection" [RFC7324], the Protection State Coordination (PSC) protocol was defined for MPLS LSPs only.

This Appendix extends these RFCs to be applicable for PWs (SS-PW and MS-PW) as well. This is useful especially in the case of end-to-end static provisioned MS-PWs running over MPLS-TP where tunnel protection alone cannot be relied upon for end-to-end protection of PWs against S-PE failure. It also enables a uniform operational approach for protection at LSP and PW layers and an easier management integration for networks that already use RFCs 6378, 7271, and 7324.

The protection architectures are those defined in [RFC6378]. For the purposes of this Appendix, we define the protection domain of a point-to-point PW as consisting of two terminating PEs (T-PEs) and the transport paths that connect them (see Figure 2).

              +-----+ //=======================\\ +-----+
              |T-PE1|//     Working Path        \\|T-PE2|
              |    /|                             |\    |
              |  ?< |                             | >?  |
              |    \|\\    Protection Path      //|/    |
              +-----+ \\=======================// +-----+
 
                  |<-------Protection Domain------->|