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Versions: (draft-muley-dutta-pwe3-redundancy-bit) 00 01 02 03 04 05 06 07 08 09 RFC 6870

Network Working Group                                     Praveen Muley
Internet Draft                                        Mustapha Aissaoui
Expires: August 2008                                      Matthew Bocci
                                                     Pranjal Kumar Dutta
                                                           Marc Lasserre
                                                          Alcatel-Lucent

                                                         Jonathan Newton
                                                        Cable & Wireless

                                                             Olen Stokes
                                                        Extreme Networks

                                                       Hamid Ould-Brahim
                                                                  Nortel

                                                            Luca Martini
                                                      Cisco Systems Inc.

                                                             Giles Heron
                                                           Thomas Nadeau
                                                                      BT


                                                       February 25, 2008

               Preferential Forwarding Status bit definition
                   draft-ietf-pwe3-redundancy-bit-00.txt


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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on August 25, 2008.

Abstract

   This document describes a mechanism for standby status signaling of
   redundant PWs between their termination points. A set of redundant
   PWs is configured between PE nodes in SS-PW applications, or between
   T-PE nodes in MS-PW applications.

   In order for the PE/T-PE nodes to indicate the preferred PW path to
   forward to one another, a new status bit is needed to indicate a
   preferential forwarding status of active or standby for each PW in
   the redundancy set.

   In addition, a second status bit is defined to allow peer PE/T-PE
   nodes to coordinate a switchover operation of the PW/MS-PW path.



Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC-2119 [1].

Table of Contents


   1. Introduction...................................................3
   2. Motivation and Scope...........................................4
   3. Terminology....................................................6
   4. Modes of Operation.............................................7
      4.1. Independent Mode:.........................................8
      4.2. Master/Slave Mode:........................................9
   5. Signaling procedures of PW State Transition...................10
      5.1. PW Standby notification procedures in Independent mode...10
      5.2. PW Standby notification procedures in Master/Slave mode..11
         5.2.1. PW State Machine....................................12
      5.3. Coordination of PW Path Switchover.......................14
         5.3.1. Procedures at the requesting endpoint...............15
         5.3.2. Procedures at the receiving endpoint................16
   6. Applicability and Backward Compatibility......................17
   7. Security Considerations.......................................17
   8. IANA Considerations...........................................17


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      8.1. Status Code for PW Preferential Forwarding Status........17
      8.2. Status Code for PW Request Switchover Status.............18
   9. Acknowledgments...............................................18
   10. References...................................................18
   11. Appendix A - Applications of PW Redundancy Procedures........18
      11.1. One Multi-homed CE with single SS-PW redundancy.........19
      11.2. Multiple Multi-homed CEs with single SS-PW redundancy...20
      11.3. Multi-homed CE with MS-PW redundancy....................22
      11.4. Single Homed CE with MS-PW redundancy...................23
      11.5. PW redundancy between H-VPLS MTU-s and PE-rs............24
   Author's Addresses...............................................26
   Full Copyright Statement.........................................27
   Intellectual Property Statement..................................27
   Acknowledgment...................................................28

1. Introduction

   In single-segment PW (SS-PW) services such as VPWS and VPLS,
   protection for the PW is provided by the PSN layer. This may be an
   RSVP-TE LSP with a FRR backup and/or an end-to-end backup LSP. There
   are however applications where PSN protection is insufficient to
   fully protect the PWE3 service and pseudowire redundancy is required.
   These scenarios are described in the PW redundancy and framework
   document [5].

   In a VPWS service, this is used to provide access AC redundancy to a
   CE device which is dual-homed to target PE nodes. In a HVPLS service,
   this is used to provide access PW redundancy to the MTU device which
   is dual-homed to two PE-r devices. PSN protection mechanisms cannot
   protect against failure of the target PE node or the failure of the
   remote AC. These scenarios rely on a set of two or more pseudowires
   to protect a given PWE3 service. Only one of these pseudowires is
   used by the PEs to forward user traffic on at any given time. This is
   the active PW. The other PWs in the set are considered standby and
   are not used for forwarding unless they become active. In essence,
   this provides a 1:1 or N:1 PW protection with the possibility of
   multi-homing.

   In order to support access AC or access PW redundancy, at least one
   of the PEs on which a PW terminates must be different from that on
   which the primary PW terminates, as described in [5]. Figure 1
   illustrates an application of Active and Standby PWs.







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        |<-------------- Emulated Service ---------------->|
         |                                                  |
         |          |<------- Pseudo Wire ------>|          |
         |          |                            |          |
         |          |    |<-- PSN Tunnels-->|    |          |
         |          V    V                  V    V          |
         V    AC    +----+                  +----+     AC   V
   +-----+    |     | PE1|==================|    |     |    +-----+
   |     |----------|....|...PW1.(active)...|....|----------|     |
   |     |          |    |==================|    |          | CE2 |
   | CE1 |          +----+                  |PE2 |          |     |
   |     |          +----+                  |    |          +-----+
   |     |          |    |==================|    |
   |     |----------|....|...PW2.(standby)..|    |
   +-----+    |     | PE3|==================|    |
              AC    +----+                  +----+


                  Figure 1: Reference Model for PW Redundancy

   In multi-segment PW (MS-PW) applications, multiple MS-PWs are
   configured between a pair of T-PE nodes. The paths of these MS-PWs
   are diverse and are switched at different S-PE nodes. Only one of
   these MS-PWs is active at any given time. The others are put in
   standby. In these applications, 1:1 or N:1 PW redundancy is important
   to provide resilience in the event of failure of S-PE node since PSN
   protection mechanisms cannot, and the desire is that MS-PW based
   services have resiliency similar to that of SS-PW  based services.

   This document specifies a new PW status bit to indicate the
   preferential forwarding status of the PW for the purpose of notifying
   the remote PE of the active/standby state of each PW in the
   redundancy set. This status bit is different from the operational
   status bits already defined in the PWE3 control protocol [2]. In
   addition, a second status bit is defined to allow peer PE/T-PE nodes
   to coordinate a switchover operation of the PW/MS-PW path.

2. Motivation and Scope

   The PWE3 control protocol [2] defines the following status codes in
   PW the status TLV to indicate the operational state for an AC and a
   PW:

   0x00000000 - Pseudowire forwarding (clear all failures)

   0x00000001 - Pseudowire Not Forwarding

   0x00000002 - Local Attachment Circuit (ingress) Receive Fault


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   0x00000004 - Local Attachment Circuit (egress) Transmit Fault

   0x00000008 - Local PSN-facing PW (ingress) Receive Fault

   0x00000010 - Local PSN-facing PW (egress) Transmit Fault

   The scenarios defined in [5] allow the provisioning of a primary PW
   and one or many secondary PWs in the same VPWS or VPLS service.

   A PE node makes a selection of which PW to activate at any given time
   for the purpose of forwarding user packets. This selection takes into
   account the local operational state of the PW as well as the remote
   operational state of the PW as indicated in the status bits of the PW
   it received from the peer PE node.

   In the absence of faults, all PWs are operationally UP both locally
   and remotely and a PE node needs to select a single PW to forward
   user packets to. This is referred to as the active PW. All other PWs
   will be in standby and must not be used to forward user packets.

   In order for both ends of the service to select the same PW for
   forwarding user packets, it is proposed that a PE node communicates a
   new status bit to indicate the forwarding state of a PW to its peer
   PE node.

   In addition, a second status bit is defined to allow peer PE/T-PE
   nodes to coordinate a switchover operation of the PW/MS-PW path if
   required by the application.

   Together, the mechanisms described in this document achieve the
   following PW protection capabilities:

      a. A mandatory 1:1 PW protection with a single active PW and one
         standby PW. An active PW can forward data traffic and control
         plane traffic, such as OAM packets. A standby PW does not
         carry data traffic.

      b. An optional N:1 PW protection scheme with a single active PW
         and N standby PWs.

      c. An independent mode of operation in which each PW endpoint
         node uses its own local rule to select which PW it intends to
         activate at any given time and advertises it to the remote
         endpoints. Only a PW which is operationally UP and which
         indicated Active status bit locally and remotely is in the
         Active state and can be used to forward data packets. This is
         described in Section 4.1. .


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      d. An optional mechanism to allow PW endpoints to coordinate the
         switchover to a given PW path by using an explicit
         request/acknowledgment switchover procedure. This mechanism is
         complementary to the Independent mode of operation and is
         described in Section 5.3. . This mechanism can be invoked
         manually by the user, effectively providing a manual
         switchover capability. It can also be invoked automatically to
         resolve a situation where the PW endpoints could not match the
         two directions of the PW.

      e. A Master/Slave mode in which one PW endpoint, the Master
         endpoint, selects and dictates to the other endpoint(s), the
         Slave endpoint(s), which PW to activate. This is described in
         Section 4.2. .

      f. Multi-homing of a CE device to two or more PE/T-PE nodes.

      g. Multi-homing of a PE/T-PE node to two or more PE/T-PE nodes.

      h. Optionally, implementations can add support for precedence
         parameter to govern the selection of a PW when more than one
         PW qualify for the active state, as defined in sections 4.1.
         and 4.2. The PW with the lowest precedence value has the
         highest priority. This is a local configuration parameter to
         the PW endpoint.

      i. Optionally, implementations can designate by configuration one
         PW in the 1:1 or N:1 set as a primary PW and the remaining as
         secondary PWs. If more than one PW qualify for the active
         state, as defined in sections 4.1. and 4.2. , a PE/T-PE node
         selects the primary PW in preference to a secondary PW. In
         other words, the primary PW has implicitly the lowest
         precedence value. Furthermore, implementations can provide the
         option to revert to the primary PW immediately after it comes
         back up or after the expiration of a delay. The PE/T-PE node
         can use the PW precedence parameter to select a secondary PW
         among many that qualify for active state.

   Appendix A shows how the mechanisms described in this document are
   used to achieve the desired protection behavior for the scenarios
   described in the PW redundancy and framework document [5].

3. Terminology

   UP PW:   A PW which has been configured (label mapping exchanged
            between PEs) and is not in any of the PW defect states



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            specified in [2]. Such a PW is available for forwarding
            traffic.

   DOWN PW: A PW that has either not been fully configured or has been
            and is in any of the PW defect states specified in [2].
            Such a PW is not available for forwarding traffic.

   Active PW:  An UP PW used for forwarding user and OAM traffic.

   Standby PW: An UP PW that is not used for forwarding user traffic,
           but may forward OAM traffic.

   Primary PW: the PW which a PW endpoint activates in preference to any
           other PW when more than one PW qualify for active state.
           When the primary PW comes back up after a failure and
           qualifies for active state, the PW endpoint always reverts
           to it. The designation of Primary is performed by local
           configuration at the PW endpoint.

   Secondary PW: when it qualifies for active state, a Secondary PW is
           only selected if no Primary PW is configured or if the
           configured primary PW does not qualify for active state
           (e.g., is operationally Down). By default, a PW in a
           redundancy PW set is considered secondary. There is no
           Revertive mechanism among secondary PWs.

   PW Precedence: this is a configuration parameter that dictates the
           order in which a PW endpoint activates a PW among multiple
           PWs which qualify for active state. The lowest the
           precedence value, the highest is the priority of selecting
           the PW.

   PW Endpoint: A PE where a PW terminates on an NSP e.g. A SS-PW PE, an
           MS-PW T-PE, or a VPLS MTU or PE-r.

4. Modes of Operation

   There are two modes of operation for the use of the PW preferential
   forwarding status bits:

   o  Independent mode

   o  Master/Slave mode.






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4.1. Independent Mode:

   PW endpoint nodes independently select which PW they intend to make
   active and which PWs they intend to make standby. They advertise the
   corresponding Active/Standby forwarding state for each PW. Each PW
   endpoint compares local and remote status and uses the PW that is
   operationally UP at both endpoints and that shows Active states at
   both the local and remote endpoint.

   If more than one PW qualify for the Active state, and the PW endpoint
   implements the precedence parameter, it must use the PW with the
   lowest precedence value. The precedence parameter is optional.

   If more than one PW qualify for the Active state, and the PW endpoint
   implements the primary/secondary procedures, it must use the primary
   PW in preference to all secondary PWs. If a primary PW is not
   available, it must use the secondary PW with the lowest precedence
   value. If the primary PW becomes available, a PW endpoint may revert
   to it immediately or after the expiration of a configurable delay.
   These primary/secondary procedures are optional.

   In steady state with consistent configuration, a PE will always find
   an Active PW. However, it is possible that due to a misconfiguration,
   such a PW is not found. In the event that an active PW is not found,
   a management indication should be generated. If a management
   indication for failure to find an active PW was generated and an
   active PW is subsequently found, a management indication should be
   generated, effectively clearing the previous failure indication.
   Additionally, a PE may use the optional request switchover procedures
   described in Section 5.3.  to have both PE nodes switch to a common
   PW path.

   There may also be transient conditions where endpoints do not share a
   common view of the active/standby state of the PWs. This could be
   caused by propagation delay of the T-LDP status messages between
   endpoints. In this case, the behavior of the receiving endpoint is
   outside the scope of this document.

   Thus, in this mode of operation, the following definition of Active
   and Standby PW states apply:

   o  Active State

   A PW is considered to be in Active state when the PW labels are
   exchanged between its two endpoints in control plane, and the status
   bits exchanged between the endpoints indicate the PW is UP and Active



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   at both endpoints. In this state user traffic can flow over the PW in
   both directions.

   o  Standby State

   A PW is considered to be in Standby state when the PW labels are
   exchanged between its two endpoints in the control plane, but the
   status bits exchanged indicate the PW is in Standby state at one or
   both endpoints. In this state the endpoints MUST NOT forward data
   traffic over the PW but MAY allow PW OAM packets, e.g., VCCV, to be
   sent and received in order to test the liveliness of standby PWs. If
   the PW is a spoke in H-VPLS, any MAC addresses learned via the PW
   should be flushed when it transitions to Standby state.

4.2. Master/Slave Mode:

   One endpoint node of the redundant set of PWs is designated the
   Master and is responsible for selecting which PW both endpoints must
   use to forward user traffic.

   The Master indicates the forwarding state in the Active/Standby
   status bit. The other endpoint node, the Slave, MUST follow the
   decision of the Master node based on the received status bits.

   One endpoint of the PW, the Master, actively selects which PW to
   activate and uses it for forwarding user traffic. This status is
   indicated to the Slave node by setting the preferential forwarding
   status bit in the status bit TLV to Active. It does not forward user
   traffic to any other of the PW's in the redundancy set to the slave
   node and indicates this by setting the preferential forwarding status
   bit in the status bit TLV to Standby for those PWs. The master node
   MUST ignore any Active/Standby status bits received from the Slave
   nodes.

   If more than one PW qualify for the Active state, and the PW endpoint
   implements the precedence parameter, it must use the PW with the
   lowest precedence value. The precedence parameter is optional.

   If more than one PW qualify for the Active state, and the PW endpoint
   implements the primary/secondary procedures, it must use the primary
   PW in preference to all secondary PWs. If a primary PW is not
   available, it must use the secondary PW with the lowest precedence
   value. If the primary PW becomes available, a PW endpoint may revert
   to it immediately or after the expiration of a configurable delay.
   These primary/secondary procedures are optional. The Slave
   endpoint(s) are required to act on the status bits received from the
   Master. When the received status bit transitions from Active to


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   Standby, a Slave node MUST stop forwarding over the previously active
   PW. When the received status bit transitions from Standby to Active
   for a given PW, the Slave node MUST start forwarding user traffic
   over this PW.

   There is a single PE/T-PE Master PW endpoint node and one or many
   PE/T-PE PW endpoint Slave nodes. The assignment of Master/Slave roles
   to the PW endpoints is performed by local configuration. Note that
   the above behavior assumes correct configuration of the Master and
   Slave endpoints. This document does not define a mechanism to detect
   errors in the configuration.

   In this mode of operation, the following definition of Active and
   Standby PW states apply:

   o  Active State

   A PW is considered to be in Active state when the PW labels are
   exchanged between its two endpoints in control plane, and the status
   bits exchanged between the endpoints indicate the PW is UP at both
   endpoints, and the forwarding status sent by the Master endpoint
   indicates Active state. In this state user traffic can flow over the
   PW in both directions.

   o  Standby State

   A PW is considered to be in Standby state when the PW labels are
   exchanged between its two endpoints in the control plane, but the
   status bits sent by the Master endpoint indicate the PW is in Standby
   state. In this state the endpoints MUST NOT forward data traffic over
   the PW but MAY allow PW OAM packets, e.g., VCCV, to be sent and
   received. If the PW is a spoke in H-VPLS, any MAC addresses learned
   via the PW should be flushed when it transitions to Standby state.

5. Signaling procedures of PW State Transition

   This section describes the extensions proposed and the processing
   rules for the extensions. It defines a new "PW preferential
   forwarding" bit in Status Code that is to be used with PW Status TLV
   proposed in RFC 4447 [2]. The PW preferential forwarding bit when set
   is used to signal Standby state of PW by one terminating point to the
   other end.

5.1. PW Standby notification procedures in Independent mode

   PW endpoint nodes independently select which PW they intend to use
   for forwarding, and which PWs they do not, based on the specific


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   application. They advertise the corresponding Active/Standby
   forwarding state for each PW. An active forwarding state is indicated
   by clearing the Active/Standby status bit in the PW status TLV. A
   standby forwarding state is indicated by setting the Active/Standby
   status bit in the PW status TLV. This advertisement occurs in both
   the initial label mapping message and in a subsequent notification
   message when the forwarding state transitions as a result of a state
   change in the specific application.

   Each endpoint compares the updated local and remote status and
   effectively activates the PW which is operationally UP at both
   endpoints and which shows both local Active and remote Active states.

   When a PW is in active state, the endpoints can forward both user
   packets and OAM packets.

   When a PW is in standby state, the endpoints MUST NOT forward user
   packets over the PW but MAY forward PW OAM packets.

   For MS-PWs, S-PEs MUST relay the PW status notification containing
   both the operational and preferential forwarding status bits between
   ingress and egress PWs.

5.2. PW Standby notification procedures in Master/Slave mode

   Whenever the Master PW endpoint "actively" selects or deselects a PW
   for forwarding user traffic at its end, it explicitly notifies the
   event to the remote Slave endpoint.  The slave endpoint carries out
   the corresponding action on receiving the PW state change
   notification.

   If the PW preferential forwarding bit in PW Status TLV received by
   the slave is set, it indicates that the PW at the Master end is not
   used for forwarding and is thus kept in the Standby state, the PW
   MUST also not be used for forwarding at Slave endpoint. Clearance of
   the PW Preferential Forwarding bit in PW Status TLV indicates that
   the PW at the Master endpoint is used for forwarding and is in Active
   state, and the receiving Slave endpoint MUST activate the PW if it
   was previously not used for forwarding.

   This mechanism is RECOMMENDED to be used with PWs signaled in groups
   with common group-id in PWid FEC Element or Grouping TLV in
   Generalized PWid FEC Element defined in [2]. When PWs are provisioned
   with such grouping a termination point sends a single "wildcard"
   Notification message using a PW FEC TLV with only the group ID set,
   to denote this change in status for all affected PW connections. This
   status message contains either the PW FEC TLV with only the Group ID


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   set, or else it contains the PW Generalized FEC TLV with only the PW
   Grouping ID TLV. As mentioned in [2], the Group ID field of the PWid
   FEC Element, or the PW Grouping TLV used with the Generalized ID FEC
   Element, can be used to send status notification for all arbitrary
   set of PWs. For example, Group-ID in PWiD may be used to send
   wildcard status notification message for a group of PWs when PWid FEC
   element is used for PW state signaling. When Generalized PWiD FEC
   Element defined is used in PW state signaling, PW Grouping TLV may be
   used for wildcard status notification for a group of PWs.

   For MS-PWs, S-PEs MUST relay the PW status notification containing
   both the operational and preferential forwarding status bits between
   ingress and egress PW segments.

5.2.1. PW State Machine

   It is convenient to describe the PW state change behavior in terms of
   a state machine. The PW state machine is explained in detail in the
   two defined states and the behavior is presented as a state
   transition table. The same state machine is seamlessly applicable to
   PW Groups.



                     PW State Transition State Table



      STATE         EVENT                                   NEW STATE



      ACTIVE        PW put in Standby (master)              STANDBY

                    Action: Transmit PW preferential forwarding bit set



                    Receive PW preferential forwarding      STANDBY

                    bit set

                    Action: Stop forwarding over PW



                    Receive PW preferential forwarding      ACTIVE


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                    bit set but bit not supported

                    Action: None



                    Receive PW preferential forwarding      ACTIVE

                    bit clear

                    Action: None.





      STANDBY       PW activated (master)                   ACTIVE

                    Action: Transmit PW preferential forwarding bit
                    clear



                    Receive PW preferential forwarding      ACTIVE

                    bit clear

                    Action: Activate PW



                    Receive PW preferential forwarding      STANDBY

                    bit clear but bit not supported

                    Action: None



                    Receive PW preferential forwarding      STANDBY

                    bit set

                    Action: No action





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5.3. Coordination of PW Path Switchover

   There are PW redundancy applications which require that PE/T-PE nodes
   coordinate the switchover to a PW/MS-PW path such that both endpoints
   will be forwarding over the same path at any given time. One such
   application of redundant MS-PW paths is identified in [5]. Multiple
   MS-PWs are configured between a pair of T-PE nodes. The paths of
   these MS-PWs are diverse and are switched at different S-PE nodes.
   Only one of these MS-PWs is active at any given time. The others are
   put in standby. The endpoints follow the Independent Mode procedures
   to activate the PW which is UP and advertised Active 'preferential
   forwarding' status bit by both endpoints.

   The trigger for sending a request to switchover of the path of the
   MS-PW by one endpoint can be due to an operational event, example a
   failure, which caused the endpoints to not be able to match the
   Active 'preferential forwarding' status bit. The other trigger is the
   execution of an administrative maintenance operation by the network
   operator in order to move the traffic away from the node/links to be
   serviced.

   Unlike the case of a Master/Slave mode of operation, the endpoint
   requesting the switchover requires explicit acknowledgement from the
   peer endpoint that the request is honored before it switches the path
   of the PW. Furthermore, any of the endpoints can make the request to
   switchover.

   A new status bit is proposed to have a PE/T-PE node request the
   switchover to its peer. This bit will be referred to as 'request PW
   switchover' status bit. The 'preferential forwarding' status bit
   continues to be used by each endpoint to indicate its current local
   settings of the active/standby state of each PW in the redundancy
   set. In other words, like in the Independent mode, it indicates to
   the far-end which of the PWs is being used to forward packets and
   which is being put in standby. It can thus be used as a way for the
   far-end to acknowledge the requested switchover operation.

   The following procedures must be followed by both endpoints of a
   PW/MS-PW to coordinate the switchover of the PW/MS-PW path. These
   procedures are enabled only when the user configured the use of the
   'request switchover' status bit at both endpoints.

   S-PEs nodes MUST relay the PW status notification containing the
   operational status bits, as well as the 'preferential forwarding' and
   'request switchover' status bits between ingress and egress PW
   segments.



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5.3.1. Procedures at the requesting endpoint

   a. The requesting endpoint sends a Status TLV in the LDP
      notification message with the 'request switchover' bit set on the
      PW it desires to switch to.

   b. The endpoint does not activate forwarding on that PW/MS-PW at
      this point in time. It may however enable receiving on that
      PW/MS-PW. Thus the 'preferential forwarding' status bit still
      reflects the currently used PW path.

   c. The requesting endpoint starts a timer while waiting the remote
      endpoint to acknowledge the request.

   d. If while waiting for the acknowledgment, the requesting endpoint
      receives a request from its peer to switchover to the same or a
      different PW path, it must perform the following:

            i. If its system IP address is higher than that of the peer,
               this endpoint ignores the request and continues to wait
               for the acknowledgement from its peer.

           ii. If its system IP address is lower than that of its peer,
               it aborts the timer and immediately starts the
               procedures of the receiving endpoint in Section 5.3.2.

   e.    If while waiting for the acknowledgment, the requesting
      endpoint receives a status notification message from its peer
      with the 'preferential forwarding' status bit cleared in the
      requested PW and set in all other PWs, it must treat this as an
      explicit acknowledgment of the  request and must perform the
      following:

            i. Abort the timer.

           ii. Activate the PW path.

          iii. Send an update status notification message with the
               'preferential forwarding' status bit clear on the newly
               active PW and set in all other PWs in the redudancy set,
               and the 'request switchover' bit reset in all PWs in the
               redundancy set..

   f. If while waiting for the acknowledgment, the requesting endpoint
      detects that the requested PW went into operational Down state
      locally, and could use an alternate PW which is operationally UP,
      it must perform the following:


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            i. Abort the timer.

           ii. Issue a new request to switchover to the alternate PW.

          iii. Re-start the timer.

   g. If while waiting for the acknowledgment, the requesting endpoint
      detects that the requested PW went into operational Down state
      locally, and could not use an alternate PW which is operationally
      UP, it must perform the following:

            i. Abort the timer.

           ii. Send an update status notification message with the
               'preferential forwarding' status bit unchanged and the
               'request switchover' bit reset in all PWs in the
               redundancy set.

   h. If while waiting for the acknowledgment, the timer expired, the
      requesting endpoint assumes the request is rejected and will
      either issue a new request or do nothing.

   i. If the requesting node receives the acknowledgment after the
      request expired, it will treat it as if the remote endpoint
      unilaterally switched the path of the PW without issuing a
      request. In that case, it may issue a new request and follow the
      requesting endpoint procedures to synchronize transmit and
      receive paths of the PW.

5.3.2. Procedures at the receiving endpoint

   a. Upon receiving a status notification message with the 'request
      switchover' bit set on a PW different from the currently active
      one, and the requested PW is operationally UP, the receiving
      endpoint must perform the following:

           i. Activate the PW.

          ii. Send an update status notification message with the
               'preferential forwarding' status bit clear on the newly
               active PW and set in all other PWs in the redudancy set,
               and the 'request switchover' bit reset in all PWs in the
               redundancy set.

   b. Upon receiving a status notification message with the 'request
      switchover' bit set on a PW different from the currently active



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      one, and the requested PW is operationally Down, the receiving
      endpoint must perform the following:

           i. Ignore the request and do nothing.

6. Applicability and Backward Compatibility

   The mechanism defined in this document is OPTIONAL and is applicable
   to PWE3 applications where standby state signaling of PW or PW group
   is required.

   A PE implementation that uses the mechanisms described in this
   document MUST negotiate the use of PW status TLV between its T-LDP
   peers as per RFC 4447 [2]. If PW Status TLV is found to be not
   supported by either of its endpoint after status negotiation
   procedures, then the mechanisms specified in this document cannot be
   used.

   A PE implementation compliant to RFC 4447 [2], and which does not
   support the generation or processing of the 'preferential forwarding'
   status bit or of  the 'request switchover'status bit, will not set
   these bits in the status bits transmitted to a peer PE and will not
   examine them in the received status bits from a peer PE. The
   mechanisms specified in this document cannot be used.

7. Security Considerations

   This document uses the LDP extensions that are needed for protecting
   pseudo-wires. It will have the same security properties as in the
   PWE3 control protocol [2].

8. IANA Considerations


   We have defined the following codes for the pseudo-wire redundancy
   application.


8.1. Status Code for PW Preferential Forwarding Status


   0x00000020 When the bit is set, it indicates "PW forwarding

              standby".

              When the bit is cleared, it indicates "PW forwarding



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              active".

8.2. Status Code for PW Request Switchover Status


   0x00000040  When the bit is set, it represents "Request switchover to
               this PW".

               When the bit is cleared, it represents no specific
               action.

9. Acknowledgments

   The authors would like to thank Vach Kompella, Kendall Harvey,
   Tiberiu Grigoriu, John Rigby, Prashanth Ishwar, Neil Hart, Kajal
   Saha, Florin Balus, Philippe Niger, Dave McDysan, and Roman
   Krzanowski for their valuable comments and suggestions.

10. References

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

   [2]   Martini, L., et al., "Pseudowire Setup and Maintenance using
         LDP", RFC 4447, April 2006.

   [3]   Martini, L., et al., "Segmented Pseudo Wire", draft-ietf-pwe3-
         segmented-pw-06.txt, November 2007.

   [4]   Bryant, S., et al., " Pseudo Wire Emulation Edge-to-Edge (PWE3)
         Architecture", RFC 3985, March 2005

   [5]   Muley, P., et al., "Pseudowire (PW) Redundancy", draft-ietf-
         pwe3-redundancy-00.txt", February 2008.

   [6]   Kompella,V., Lasserrre, M. , et al., "Virtual Private LAN
         Service (VPLS) Using LDP Signalling", RFC 4762, January 2007.



11. Appendix A - Applications of PW Redundancy Procedures

   This section shows how the mechanisms described in this document are
   used to achieve the desired protection behavior for the scenarios
   described in the PW redundancy requirements and framework document
   [5].



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11.1. One Multi-homed CE with single SS-PW redundancy

   The following figure illustrates an application of single segment
   pseudo-wire redundancy.

         |<-------------- Emulated Service ---------------->|
         |                                                  |
         |          |<------- Pseudo Wire ------>|          |
         |          |                            |          |
         |          |    |<-- PSN Tunnels-->|    |          |
         |          V    V                  V    V          |
         V    AC    +----+                  +----+     AC   V
   +-----+    |     | PE1|==================|    |     |    +-----+
   |     |----------|....|...PW1.(active)...|....|----------|     |
   |     |          |    |==================|    |          | CE2 |
   | CE1 |          +----+                  |PE2 |          |     |
   |     |          +----+                  |    |          +-----+
   |     |          |    |==================|    |
   |     |----------|....|...PW2.(standby)..|    |
   +-----+    |     | PE3|==================|    |
              AC    +----+                  +----+

           Figure 2 Multi-homed CE with single SS-PW redundancy

   The application in Figure 2 makes use of the Independent mode of
   operation.

   CE1 is dual homed to PE1 and to PE3 by attachment circuits. The
   method for dual-homing of CE1 to PE1 and to PE3 nodes, and the used
   protocols such as Multi-chassis Link Aggregation Group (MC-LAG), is
   outside the scope of this document.

   In normal operation, PE1 and PE3 will advertise "Active" and
   "Standby" 'preferential forwarding' status bit respectively to PE2.
   This status reflects the forwarding state of the two AC's to CE1 as
   determined by the AC dual-homing protocol used. PE2 advertises
   'preferential forwarding' status bit of "Active" on both PW1 and PW2
   since the AC to CE2 is single homed. As both the local and remote
   operational and preferential forwarding status for PW1 are UP and
   Active, traffic is forwarded over PW1 in both directions.

   On failure of the AC between CE1 and PE1, the forwarding state of the
   AC on PE3 transitions to Active. For example the MC-LAG control
   protocol changes the link state on PE3 to active. PE3 then announces
   the newly changed 'preferential forwarding' status bit of "active" to
   PE2. PE1 will advertise a PW status notification message indicating
   that the AC between CE1 and PE1 is operationally down. PE2 matches


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   the local and remote preferential forwarding status of "active" and
   operational status "Up" and select PW2 as the new active pseudo-wire
   to send traffic to.

   On failure of PE1 node, PE3 will detect it and will transition the
   forwarding state of its AC to Active. The method by which PE3 detects
   that PE1 is down is outside the scope of this document. PE3 then
   announces the newly changed 'preferential forwarding' status bit of
   "active" to PE2. PE3 and PE2 match the local and remote preferential
   forwarding status of "active" and operational status "Up" and select
   PW2 as the new active pseudo-wire to send traffic to. Note that PE2
   may have detected that the PW to PE1 went down via T-LDP Hello
   timeout or via other means. However, it will not be able to forward
   user traffic until it received the updated status bit from PE3.

   Note in this example, the receipt of the operational status of the AC
   on the CE1-PE1 link is normally sufficient to have PE2 switch the
   path to PW2. However, the operator may want to trigger the switchover
   of the path of the PW for administrative reasons, i.e., maintenance,
   and thus the use of the 'preferential forwarding' status bit is
   required to notify PE2 to trigger the switchover.

   Note that the primary/secondary procedures do not apply in this case
   as the PW Active/Standby status is driven by the AC forwarding state
   as determined by the AC dual-homing protocol used.

11.2. Multiple Multi-homed CEs with single SS-PW redundancy

             |<-------------- Emulated Service ---------------->|
             |                                                  |
             |          |<------- Pseudo Wire ------>|          |
             |          |                            |          |
             |          |    |<-- PSN Tunnels-->|    |          |
             |          V    V                  V    V          |
             V    AC    +----+                  +----+     AC   V
       +-----+    |     |....|.......PW1........|....|     |    +-----+
       |     |----------| PE1|......   .........| PE3|----------|     |
       | CE1 |          +----+      \ /  PW3    +----+          | CE2 |
       |     |          +----+       X          +----+          |     |
       |     |          |    |....../ \..PW4....|    |          |     |
       |     |----------| PE2|                  | PE4|--------- |     |
       +-----+    |     |....|.....PW2..........|....|     |    +-----+
                  AC    +----+                  +----+    AC


      Figure 3 Multiple Multi-homed CEs with single SS-PW redundancy



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   The application in Figure 3 makes use of the Independent mode of
   operation.

   CE1 is dual-homed to PE1 and PE2. CE2 is dual-homed PE3 and PE4. The
   method for dual-homing and the used protocols such as Multi-chassis
   Link Aggregation Group (MC-LAG) are outside the scope of this
   document.  Note that the PSN tunnels are not shown in this figure for
   clarity. However, it can be assumed that each of the PWs shown is
   encapsulated in a separate PSN tunnel.

   PE1 advertises the preferential status "active" and operational
   status "UP" for pseudo-wires PW1 and PW4 connected to PE3 and PE4.
   This status reflects the forwarding state of the AC attached to PE1.
   PE2 advertises preferential status "standby" where as operational
   status "UP" for pseudo-wires PW2 and PW3 to PE3 and PE4. PE3
   advertises preferential status "standby" where as operational status
   "UP" for pseudo-wires PW1 and PW3 to PE1 and PE2. PE4 advertise the
   preferential status "active" and operational status "UP" for pseudo-
   wires PW2 and PW4 to PE2 and PE1 respectively. The method of
   deriving Active/Standby status of the AC is outside the scope of
   this document. In case of MC-LAG it is derived by the Link
   Aggregation Control protocol (LACP) negotiation. Thus by matching
   the local and remote preferential forwarding status of "active" and
   operational status "Up" of pseudo-wire, the PE nodes determine which
   PW should be in the Active state. In this case it is PW4 that will
   be selected.


   On failure of the AC between CE1 and PE1, the forwarding state of
   the AC on PE2 is changed to Active. For example the MC-LAG control
   protocol changes the link state on PE2 to active. PE2 then announces
   the newly changed 'preferential forwarding' status bit of "active"
   to PE3 and PE4. PE1 will advertise a PW status notification message
   indicating that the AC between CE1 and PE1 is operationally down.
   PE2 and PE4 match the local and remote preferential forwarding
   status of "active" and operational status "Up" and select PW2 as the
   new active pseudo-wire to send traffic to.


   On failure of PE1 node, PE2 will detect it and will transition the
   forwarding state of its AC to Active. The method by which PE2
   detects that PE1 is down is outside the scope of this document. PE2
   then announces the newly changed 'preferential forwarding' status
   bit of "active" to PE3 and PE4. PE2 and PE4 match the local and
   remote preferential forwarding status of "active" and operational
   status "Up" and select PW2 as the new active pseudo-wire to send
   traffic to. Note that PE3 and PE4 may have detected that the PW to


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   PE1 went down via T-LDP Hello timeout or via other means. However,
   they will not be able to forward user traffic until they received
   the updated status bit from PE2.


   Because each dual-homing algorithm running on the two node sets,
   i.e., {CE1, PE1, PE2} and {CE2, PE3, PE4}, selects the active AC
   independently, there is a need to signal the active status of the AC
   such that the PE nodes can select a common active PW path for end-to-
   end forwarding between CE1 and CE2 as per the procedures in the
   independent mode.

   Note that the primary/secondary procedures do not apply in this use
   case as the Active/Standby status is driven by the AC forwarding
   state as determined by the AC dual-homing protocol used.

11.3. Multi-homed CE with MS-PW redundancy

   The following figure illustrates an application of multi-segment
   pseudo-wire redundancy.

           Native   |<-----------Pseudo Wire----------->|  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|=========|T-PE4|   |   +----+
                                    +-----+         +-----+   AC



             Figure 4 Multi-homed CE with MS-PW redundancy

   The application in Figure 4 makes use of the Independent mode of
   operation.

   CE2 is dual-homed to T-PE2 and T-PE4. PW1 and PW2 are used to extend
   the resilient connectivity all the way to T-PE1. PW1 has two segments
   and is active pseudo-wire while PW2 has two segments and is a standby



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   pseudo-wire. This application requires support for MS-PW with
   segments of the same type as described in [3].

   The operation in this case is the same as in the case of SS-PW as
   described in Section 11.1. . The only difference is that the S-PE
   nodes need to relay the PW status notification containing both the
   operational and forwarding status to the T-PE nodes.

11.4. Single Homed CE with MS-PW redundancy

   The following is an application of the independent mode of operation
   along with the optional request switchover procedures in order to
   provide N:1 PW protection. A revertive behavior to a primary PW is
   shown as an example of configuring and using the primary/secondary
   procedures.

           Native   |<------------Pseudo Wire------------>|  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 5 Single homed CE with multi-segment pseudo-wire redundancy

   CE1 is connected to PE1 in provider Edge 1 and CE2 to PE2 in provider
   edge 2 respectively. There are three segmented PWs. A primary PW,
   PW1, is switched at S-PE1. A primary PW has the highest precedence. A
   secondary PW, PW2, which is switched at S-PE2 and has a precedence of
   1. Finally, another secondary PW, PW3, is switched at S-PE3 and has a
   precedence of 2. The precedence is locally configured at the
   endpoints of the PW, i.e., T-PE1 and T-PE2.



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   T-PE1 and T-PE2 will select the PW they intend to activate based on
   their local and remote operational state as well as the local
   primary/precedence configuration. In this case, they will both
   advertise preferential forwarding' status bit of "active" on PW1 and
   of "standby" on PW2 and PW3. Assuming all PWs are operationally UP,
   T-PE1 and T-PE2 will use PW1 to forward user packets.

   If PW1 fails, then the T-PE detecting the failure will send a status
   notification to the remote T-PE with a "PW Down" bit set as well as
   the 'preferential forwarding' status bit set on PW1. It will also
   clear 'preferential forwarding' status bit on PW2 as it is the next
   it has the next lowest precedence value. T-PE2 will also perform the
   same steps as soon as it is informed of the failure of PW1. Both T-PE
   nodes will perform a match on the 'preferential forwarding' status
   of "active" and operational status of "Up" and will use PW2 to
   forward user packets.

   However this does not guarantee that paths of the PW are synchronized
   at all times. This may be due to a mismatch of the configuration of
   the PW priority in each T-PE. This may also be due to a failure which
   caused the endpoints to not be able to match the Active 'preferential
   forwarding' status bit and operational status bits. In this case, T-
   PE1 and/or T-PE2 can invoke the optional request
   switchover/acknowledgement procedures to synchronize the PW paths in
   both directions.

   The trigger for sending a request to switchover can also be the
   execution of an administrative maintenance operation by the network
   operator in order to move the traffic away from the T-PE/S-PE nodes
   /links to be serviced.

   In case the request switchover is sent by both endpoints
   simultaneously, both T-PEs send status notification with the newly
   selected PW with 'request switchover' bit set, waiting for response
   from the other endpoint. In such situation, the T-PE with greater
   system address request is given precedence. This helps in
   synchronizing paths in event of mismatch of priority configuration as
   well.

11.5. PW redundancy between H-VPLS MTU-s and PE-rs

   Following figure illustrates the application of use of PW redundancy
   in H-VPLS for the purpose of dual-homing an MTU-s node to PE nodes
   using PW spokes. This application makes use of the Master/Slave mode
   of operation.




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                         |<-PSN1-->|     |<-PSN2-->|
                         V         V     V         V
                   +-----+         +--------+
                   |MTU-s|=========|PE1-rs  |========
                   |..Active PW group....   | H-VPLS-core
                   |     |=========|        |=========
                   +-----+         +--------+
                      |.|
                      |.|           +--------+
                      |.|===========|        |==========
                      |...Standby PW group   |.H-VPLS-core
                       =============|  PE2-rs|==========
                                    +--------+

               Figure 6 Multi-homed MTU-s in H-VPLS core

   MTU-s is dual homed to PE1-rs and PE2-rs. The primary spoke PWs from
   MTU-s are connected to PE1-rs while the secondary PWs are connected
   to PE2. PE1-rs and PE2-rs are connected to H-VPLS core on the other
   side of network. MTU-s communicates to PE1-rs and PE2-rs the
   forwarding status of its member PWs for a set of VSIs having common
   status Active/Standby. It may be signaled using PW grouping with
   common group-id in PWid FEC Element or Grouping TLV in Generalized
   PWid FEC Element as defined in [2] to scale better.  MTU-s derives
   the status of the PWs based on local policy configuration. In this
   example, the primary/secondary procedures are used but this can be
   based on any other policy.

   Whenever MTU-s performs a switchover, it sends a wildcard
   Notification Message to PE2-rs for the Standby PW group containing PW
   Status TLV with PW Standby bit cleared. On receiving the notification
   PE-2rs unblocks all member PWs identified by the PW group and state
   of PW group changes from Standby to Active. All procedures described
   in Section 5.2. are applicable.

   The use of the 'preferential forwarding' status bit in Master/Slave
   mode is similar to Topology Change Notification in RSTP controlled
   IEEE Ethernet Bridges but is restricted over a single hop. When these
   procedures are implemented, PE-rs devices are aware of switchovers at
   MTU-s and could generate MAC Withdraw Messages to trigger MAC
   flushing within the H-VPLS full mesh. By default, MTU-s devices
   should still trigger MAC Withdraw messages as currently defined in
   [6] to prevent two copies of MAC withdraws to be sent, one by MTU-s
   and another one by PE-rs nodes. Mechanisms to disable MAC Withdraw
   trigger in certain devices is out of the scope of this document



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Author's Addresses

   Praveen Muley
   Alcatel-lucent
   701 E. Middlefiled Road
   Mountain View, CA, USA
   Email: Praveen.muley@alcatel-lucent.com

   Mustapha Aissaoui
   Alcatel-lucent
   600 March Rd
   Kanata, ON, Canada K2K 2E6
   Email: mustapha.aissaoui@alcatel-lucent.com

   Matthew Bocci
   Alcatel-Lucent
   Voyager Place, Shoppenhangers Rd
   Maidenhead, Berks, UK SL6 2PJ
   Email: matthew.bocci@alcatel-lucent.co.uk

   Pranjal Kumar Dutta
   Alcatel-Lucent
   Email: pdutta@alcatel-lucent.com

   Marc Lasserre
   Alcatel-Lucent
   Email: mlasserre@alcatel-lucent.com

   Jonathan Newton
   Cable & Wireless
   Email: Jonathan.Newton@cw.com

   Olen Stokes
   Extreme Networks
   Email: ostokes@extremenetworks.com

   Hamid Ould-Brahim
   Nortel
   Email: hbrahim@nortel.com

   Luca Martini
   Cisco Systems, Inc.
   9155 East Nichols Avenue, Suite 400
   Englewood, CO, 80112
   Email: lmartini@cisco.com

   Thomas Nadeau


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   BT
   tnadeau@lucidvision.com

   Giles Heron
   BT
   giles.heron@gmail.com


Full Copyright Statement

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   rights that may cover technology that may be required to implement
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