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Versions: 00 01 02 draft-ietf-pwe3-redundancy-bit

Network Working Group                                    Praveen Muley
Internet Draft                                       Mustapha Aissaoui
Expires: May 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.


                                                      November 19, 2007

               Preferential Forwarding Status bit definition
               draft-muley-dutta-pwe3-redundancy-bit-02.txt


Status of this Memo



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   This Internet-Draft will expire on May 19, 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..................................................4
   3. Terminology.................................................5
   4. Modes of Operation..........................................5
      4.1. Independent Mode:.......................................5
      4.2. Master/Slave Mode:......................................6
   5. Signaling procedures of PW State Transition..................7
      5.1. PW Standby notification procedures in Independent mode...8
      5.2. PW Standby notification procedures in Master/Slave mode..8
         5.2.1. PW State Machine...................................9
      5.3. Coordination of PW Path Switchover.....................11
         5.3.1. Procedures at the requesting endpoint.............12
         5.3.2. Procedures at the receiving endpoint..............13
   6. Applicability and Backward Compatibility....................14
   7. Security Considerations.....................................14
   8. IANA Considerations........................................14
      8.1. Status Code for PW Preferential Forwarding Status.......14
      8.2. Status Code for PW Request Switchover Status...........15
   9. Acknowledgments............................................15


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   10. References................................................15
   Author's Addresses............................................15
   Full Copyright Statement.......................................16
   Intellectual Property Statement................................17
   Acknowledgment................................................17

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 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 [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 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.



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


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   +-----+    |     | 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, PW redundancy is important to provide
   resilience in the event of failure of S-PE node since PSN protection
   mechanisms cannot.

   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

   The PWE3 control protocol [2] defines the following status codes 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

   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.


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   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..

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
             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 traffic.

   Standby PW: An UP PW that is not used for forwarding user traffic.

   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.

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


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   operationally UP at both endpoints and that shows Active states at
   both the local and remote endpoint.

   In steady state, a PE will always find an Active PW. However, it is
   possible that due to a misconfiguration, such a PW is not found. The
   behavior of a PE in this case is outside the scope of this document.

   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
   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.

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 forward user


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   traffic to any other of the PW's in the redundant 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.

   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 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.

   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.

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.



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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
   application. They advertise the corresponding Active/Standby
   forwarding state for each PW. 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


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   status message contains either the PW FEC TLV with only the Group ID
   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





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                    Receive PW preferential forwarding    ACTIVE

                    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 LDP status 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 set in the requested PW, 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 set to the newly
               active PW 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:

            i. Abort the timer.


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           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 set to the newly
               active PW 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
      one, and the requested PW is operationally Down, the receiving
      endpoint must perform the following:

            i. Ignore the request and do nothing.


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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

              active".



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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 and Philippe Niger 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-05.txt, July 2007.

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

   [5]  Praveen, Pranjal et al., "draft-muley-pwe3-redundancy-01.txt",
         March 2007.



Author's Addresses

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






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   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

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   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
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   retain all their rights.



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