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Versions: 00 01 02 03 04 05 06 draft-ietf-mpls-tp-shared-ring-protection

Network Working Group                                           W. Cheng
Internet-Draft                                                   L. Wang
Intended status: Standards Track                                   H. Li
Expires: August 6, 2015                                     China Mobile
                                                             H. Helvoort
                                                          Hai Gaoming BV
                                                                  K. Liu
                                                                 J. Dong
                                                                   J. He
                                                     Huawei Technologies
                                                                   F. Li
               China Academy of Telecommunication Research, MIIT., China
                                                                 J. Yang
                                               ZTE Corporation P.R.China
                                                                 J. Wang
                      Fiberhome Telecommunication Technologies Co., LTD.
                                                        February 2, 2015


   MPLS-TP Shared-Ring protection (MSRP) mechanism for ring topology
             draft-cheng-mpls-tp-shared-ring-protection-04

Abstract

   This document describes requirements, architecture and solutions for
   MPLS-TP Shared Ring Protection (MSRP) in the ring topology for point-
   to-point (P2P) services.  The mechanism of MSRP is illustrated and
   how it satisfies the requirements in RFC 5654 for optimized ring
   protection is analyzed.

Requirements Language

   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 [RFC2119].

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any



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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 6, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements for MPLS-TP Ring Protection  . . . . . . . . . .   4
     2.1.  Recovery of Multiple Failures . . . . . . . . . . . . . .   4
     2.2.  Smooth Upgrade from Linear Protection to Ring Protection    4
     2.3.  Configuration Complexity  . . . . . . . . . . . . . . . .   4
   3.  Terminology and Notation  . . . . . . . . . . . . . . . . . .   5
   4.  Shared Ring Protection Architecture . . . . . . . . . . . . .   5
     4.1.  Ring Tunnel . . . . . . . . . . . . . . . . . . . . . . .   5
       4.1.1.  Establishment of Ring Tunnel  . . . . . . . . . . . .   6
       4.1.2.  Label Assignment and Distribution . . . . . . . . . .   8
       4.1.3.  Forwarding Operation  . . . . . . . . . . . . . . . .   8
     4.2.  Failure Detection . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Ring Protection . . . . . . . . . . . . . . . . . . . . .  10
       4.3.1.  Wrapping  . . . . . . . . . . . . . . . . . . . . . .  10
       4.3.2.  Short Wrapping  . . . . . . . . . . . . . . . . . . .  12
       4.3.3.  Steering  . . . . . . . . . . . . . . . . . . . . . .  13
     4.4.  Interconnected Ring Protection  . . . . . . . . . . . . .  16
       4.4.1.  Interconnected Ring Topology  . . . . . . . . . . . .  16
       4.4.2.  Interconnected Ring Protection Mechanisms . . . . . .  17
       4.4.3.  Ring Tunnels in Interconnected Rings  . . . . . . . .  18
       4.4.4.  Interconnected Ring Switching Procedure . . . . . . .  20
       4.4.5.  Interconnected Ring Detection Mechanism . . . . . . .  21
   5.  Ring Protection Coordination Protocol . . . . . . . . . . . .  22
     5.1.  RPS Protocol  . . . . . . . . . . . . . . . . . . . . . .  23
       5.1.1.  Transmission and Acceptance of RPS Requests . . . . .  25
       5.1.2.  RPS PDU Format  . . . . . . . . . . . . . . . . . . .  25



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       5.1.3.  Ring Node RPS States  . . . . . . . . . . . . . . . .  26
       5.1.4.  RPS State Transitions . . . . . . . . . . . . . . . .  27
     5.2.  RPS State Machine . . . . . . . . . . . . . . . . . . . .  30
       5.2.1.  Initial States  . . . . . . . . . . . . . . . . . . .  30
       5.2.2.  State transitions When Local Request is Applied . . .  31
       5.2.3.  State Transitions When Remote Request is Applied  . .  34
       5.2.4.  State Transitions When Request Addresses to Another
               Node is Received  . . . . . . . . . . . . . . . . . .  37
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  40
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  40
   8.  Contributing Authors  . . . . . . . . . . . . . . . . . . . .  40
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  40
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  41

1.  Introduction

   As described in 2.5.6.1 of [RFC5654], Ring Protection of MPLS-TP
   requirements , several service providers have expressed much interest
   in operating MPLS-TP in ring topologies and require a high-level
   survivability function in these topologies.  In operational transport
   network deployment, MPLS-TP networks are often constructed with ring
   topologies.  It calls for an efficient and optimized ring protection
   mechanism to achieve simple operation and fast, sub 50 ms, recovery
   performance.

   The requirements for MPLS-TP [RFC5654] state that recovery mechanisms
   which are optimized for ring topologies could be further developed if
   it can provide the following features:

   a.  Minimize the number of OAM entities for protection

   b.  Minimize the number of elements of recovery

   c.  Minimize the required label number

   d.  Minimize the amount of control and management-plane transactions
       during maintenance operation

   e.  Minimize the impact on information exchange during protection if
       a control plane is supported

   This document specifies MPLS-TP Shared-Ring Protection mechanisms
   that can meet all those requirements on ring protection listed in
   [RFC5654].

   The basic concepts and architecture of Shared-Ring protection
   mechanism are specified in this document.  This document focuses on
   the solutions for point-to-point transport paths.  While the basic



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   concepts may also apply to point-to-multipoint transport paths, the
   solution for point-to-multipoint transport paths is under study and
   will be presented in a separate document.

2.  Requirements for MPLS-TP Ring Protection

   The requirements for MPLS-TP ring protection are specified in
   [RFC5654].  This document elaborates on the requirements in detail.

2.1.  Recovery of Multiple Failures

   MPLS-TP is expected to be used in carrier grade metro networks and
   backbone transport networks to provide mobile backhaul, business
   services etc., in which the network survivability is very important.
   According to R106 B in [RFC5654], MPLS-TP recovery mechanisms in a
   ring SHOULD protect against multiple failures.  The following text
   provides some more detailed illustration about "multiple failures".
   In metro and backbone networks, a single risk factor often affects
   multiple links or nodes.  Some examples of risk factors are given as
   follows:

   o  multiple links use fibers in one cable or pipeline

   o  Several nodes share one power supply system

   o  Weather sensitive micro-wave system

   Once one of the above risk factors happens, multiple links or nodes
   failures may occur simultaneously and those failed links or nodes may
   be located on a single ring as well as on interconnected rings.  Ring
   protection against multiple failures should cover both multiple
   failures on a single ring and multiple failures on interconnected
   rings.

2.2.  Smooth Upgrade from Linear Protection to Ring Protection

   It is beneficial for service providers to upgrade the protection
   scheme from linear protection to ring protection in their MPLS-TP
   network without service interruption.  In-service insertion and
   removal of a node on the ring should also be supported.  Therefore,
   the MPLS-TP ring protection mechanism is supposed to be developed and
   optimized for compliance with this smooth upgrading principle.

2.3.  Configuration Complexity

   Ring protection can reduce the dependency of configuration on the
   quantity of services, thus will simplify the network protection
   configuration and operation effort.  This is because the ring



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   protection makes use of the characteristics of ring topology and
   mechanisms on the section layer.  While in the application scenarios
   of deploying linear protection in ring topology MPLS-TP network, the
   configuration of protection has a close relationship with the
   quantities of services carried.  Especially in some large metro
   networks with more than ten thousands of services in the access
   nodes, the LSP linear protection capabilities of the metro core nodes
   needs to be large enough to meet the network planning requirements,
   which also leads to the complexity of network protection
   configurations and operations.

3.  Terminology and Notation

   The following syntax will be used to describe the contents of the
   label stack:

   1.  The label stack will be enclosed in square brackets ("[]").

   2.  Each level in the stack will be separated by the '|' character.
   It should be noted that the label stack may contain additional
   layers.  However, we only present the layers that are related to the
   protection mechanism.

   3.  If the Label is assigned by Node X, the Node Name is enclosed in
   bracket ("()")

4.  Shared Ring Protection Architecture

4.1.  Ring Tunnel

   This document introduces a new logical layer of the ring for shared
   ring protection in MPLS-TP networks.  As shown in Figure 1, the new
   logical layer consists of ring tunnels which provides a server layer
   for the LSPs traverse the ring.  Once a ring tunnel is established,
   the configuration, management and protection of the ring are all
   performed at the ring tunnel level.  One port can carry multiple ring
   tunnels, while one ring tunnel can carry multiple LSPs.














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                                                   +-------------
                                     +-------------|
                       +-------------|             |
         =====PW1======|             |             |
                       |             |   Ring      |  Physical
         =====PW2======|    LSP      |  Tunnel     |  Port
                       |             |             |
         =====PW3======|             |             |
                       +-------------|             |
                                     +-------------|
                                                   +-------------
              Figure 1. The logical layers of the ring

   The label stack used in MPLS-TP Shared Ring Protection mechanism is
   shown as below:

                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   |           Ring tunnel Label         |
                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   |               LSP Label             |
                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   |                PW Label             |
                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   |                Payload              |
                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        Figure 2. Label stack used in MPLS-TP Shared Ring Protection

4.1.1.  Establishment of Ring Tunnel

   The Ring tunnels are established based on the exit node.  The exit
   node is the node where traffic leaves the ring.  LSPs which have the
   same exit node on the ring share the same ring tunnels.  In other
   words, all the LSPs that traverse the ring and exit from the same
   node share the same working ring tunnel and protection ring tunnel.
   For each exit node, four ring tunnels are established:

   o  one clockwise working ring tunnel, which is protected by the
      anticlockwise protection ring tunnel

   o  one anticlockwise protection ring tunnel

   o  one anticlockwise working ring tunnel, which is protected by the
      clockwise protection ring tunnel

   o  one clockwise protection ring tunnel






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   The structure of the protection tunnels are determined by the
   selected protection mechanism.  This will be detailed in subsequent
   sections.

   As shown in Figure 3, LSP 1, LSP 2 and LSP 3 enter the ring from Node
   E, Node A and Node B, respectively, and all leave the ring at Node D.
   To protect these LSPs that traverse the ring, a clockwise working
   ring tunnel (RcW_D) via E->F->A->B->C->D, and its anticlockwise
   protection ring tunnel (RaP_D) via D->C->B->A->F->E->D are
   established, Also, an anti-clockwise working ring tunnel (RaW_D) via
   C->B->A->F->E->D, and its clockwise protection ring tunnel (RcP_D)
   via D->E->F->A->B->C->D are established.  For simplicity Figure 3
   only shows RcW_D and RaP_D.  A similar provisioning should be applied
   for any other node on the ring.  In summary, for each node in
   Figure 3 when acting as exit node, the ring tunnels are created as
   follows:

   o  To Node A: RcW_A, RaW_A, RcP_A, RaP_A

   o  To Node B: RcW_B, RaW_B, RcP_B, RaP_B

   o  To Node C: RcW_C, RaW_C, RcP_C, RaP_C

   o  To Node D: RcW_D, RaW_D, RcP_D, RaP_D

   o  To Node E: RcW_E, RaW_E, RcP_E, RaP_E

   o  To Node F: RcW_F, RaW_F, RcP_F, RaP_F























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                       +---+#############+---+
                       | F |-------------| A | +-- LSP2
                       +---+*************+---+
                       #/*                   *\#
                      #/*                     *\#
                     #/*                       *\#
                   +---+                     +---+
            LSP1-+ | E |                     | B |+-- LSP3
                   +---+                     +---+
                     #\                       */#
                      #\                     */#
                       #\                   */#
                       +---+*************+---+
             LSP1   +--| D |-------------| C |
             LSP2      +---+#############+---+
             LSP3

                      ---- physical links
                      **** RcW_D
                      #### RaP_D

                  Figure 3. Ring tunnels in MSRP

   Through these working and protection ring tunnels, LSPs which enter
   the ring from any node can reach any exit nodes on the ring, and are
   protected from failures on the ring.

4.1.2.  Label Assignment and Distribution

   The ring tunnel labels are downstream-assigned labels as defined in
   [RFC3031].  The ring tunnel labels can be either configured
   statically, provisioned by a controller, or distributed dynamically
   via a control protocol.

4.1.3.  Forwarding Operation

   When an MPLS-TP transport path, such as an LSP, enters the ring, the
   ingress node on the ring pushes the working ring tunnel label
   according to the exit node and sends the traffic to the next hop.
   The transit nodes on the working ring tunnel swap the ring tunnel
   labels and forward the packets to the next hop.  When the packet
   arrives at the exit node, the exit node pops the ring tunnel label
   and forwards the packets based on the inner LSP label and PW label.
   Figure 4 shows the label operation in the MPLS-TP shared ring
   protection mechanism.  Assume that LSP1 enters the ring at Node A and
   exits from Node D, and the following label operations are executed.





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   1.  Ingress node: Packets of LSP1 arrive at Node A with a label stack
       [LSP1] and is supposed to be forwarded in the clockwise direction
       of the ring.  The clockwise working ring tunnel label RcW_D will
       be pushed at Node A, the label stack for the forwarded packet at
       Node A is changed to [RcW_D(B)|LSP1].

   2.  Transit nodes: In this case, Node B and Node C forward the
       packets by swapping the working ring tunnel labels.  For example,
       the label [RcW_D(B)|LSP1] is swapped to [RcW_D(C)|LSP1] at Node
       B.

   3.  Exit node: When the packet arrives at Node D (i.e. the exit node)
       with label stack [RcW_D(D)|LSP1], Node D pops RcW_D(D), and
       subsequently deals with the inner labels of LSP1.

   4.  All the LSPs that exit from the same node share the same set of
       ring tunnel labels.

                      +---+#####[RaP_D(F)]######+---+
                      | F |---------------------| A | +-- LSP1
                      +---+*****[RcW_D(A)]******+---+
                       #/*                        *\#
            [RaP_D(E)]#/*[RcW_D(F)]      [RcW_D(B)]*\#[RaP_D(A)]
                     #/*                            *\#
                   +---+                          +---+
                   | E |                          | B |
                   +---+                          +---+
                     #\                            */#
            [RaP_D(D)]#\                [RxW_D(C)]*/#[RaP_D(B)]
                       #\                        */#
                       +---+*****[RcW_D(D)]****+---+
             LSP1  +-- | D |-------------------| C |
                       +---+#####[RaP_D(C)]####+---+

             -----physical links      ****** RcW_D    ###### RaP_D

                     Figure 4. Label operation of MSRP

4.2.  Failure Detection

   The MPLS-TP section layer OAM is used to monitor the connectivity
   between each two adjacent nodes on the ring using the mechanisms
   defined in [RFC6371].  Protection switching is triggered by the
   failure detected on the ring by the OAM mechanisms.

   Two end ports of a link form a Maintenance Entity Group (MEG), and an
   MEG end point (MEP) function is installed in each ring port.  CC-V
   OAM packets are periodically exchanged between each pair of MEPs to



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   monitor the link health.  Three or more consecutive CC-V packets
   losses will be interpreted as a link failure.

   A node failure is regarded as the failure of two links attached to
   that node.  The two nodes adjacent to the failed node detect the
   failure in the links that are connected to the failed node.

4.3.  Ring Protection

   Taking the topology in Figure 4 as example, the LSP1 enters the ring
   at Node A and leaves the ring at Node D.  In normal state, LSP 1 is
   carried by clockwise working ring tunnel (RcW_D) through the path
   A->B->C->D, the label operation is:

   [LSP1](original data traffic carried by LSP 1) ->
   [RCW_D(B)|LSP1](NodeA) -> [RCW_D(C)|LSP1](NodeB) -> [RCW_D(D)|
   LSP1](NodeC) -> [LSP1](data traffic carried by LSP 1).  Then at node
   D the packet will be forwarded based on label stack of LSP1.

   The following sections describes the protection mechanisms used in
   ring topology.

4.3.1.  Wrapping

   With the wrapping mechanism, the protection ring tunnel is a closed
   ring identified by the exit node.  As shown in Figure 4, the RaP_D is
   the anticlockwise protection ring tunnel for the clockwise working
   ring tunnel RcW_D.  As specified in the following sections, the
   closed ring protection tunnel can protect both the link failure and
   the node failure.

4.3.1.1.  Wrapping for Link Failure

   When a link failure between Node B and Node C occurs, both Node B and
   Node C detect the failure via OAM mechanism.  Node B switches the
   clockwise working ring tunnel (RcW_D) to the anticlockwise protection
   ring tunnel (RaP_D) and Node C switches anticlockwise protection ring
   tunnel(RaP_D) to the clockwise working ring tunnel(RcW_D).  The data
   traffic which enters the ring at Node A and exits at Node D follows
   the path A->B->A->F->E->D->C->D.  The label operation is:

   [LSP1](Original data traffic) -> [RcW_D(B)|LSP1](Node A) ->
   [RaP_D(A)|LSP1](Node B) -> [RaP_D(F)|LSP1](Node A) -> [RaP_D(E)|LSP1]
   (Node F) -> [RaP_D(D)|LSP1] (Node E) -> [RaP_D(C)|LSP1] (Node D) ->
   [RcW_D(D)|LSP1](Node C) -> [LSP1](data traffic exits the ring).






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                         +---+#####[RaP_D(F)]######+---+
                         | F |---------------------| A | +-- LSP1
                         +---+*****[RcW_D(A)]******+---+
                         #/*                        *\#
              [RaP_D(E)]#/*[RcW_D(F)]      [RcW_D(B)]*\#RaP_D(A)
                       #/*                            *\#
                     +---+                          +---+
                     | E |                          | B |
                     +---+                          +---+
                       #\                            *x#
              [RaP_D(D)]#\                [RcW_D(C)]*x#RaP_D(B)
                         #\                        *x#
                         +---+*****[RcW_D(D)]****+---+
               LSP1  +-- | D |-------------------| C |
                         +---+#####[RaP_D(C)]####+---+

                 -----physical links    xxxx Failure Link
                 ****** RcW_D           ###### RaP_D

                      Figure 5.Wrapping for link failure

4.3.1.2.  Wrapping for Node Failure

   When Node B fails, Node A detects the failure between A and B and
   switches the clockwise work ring tunnel (RcW_D) to the anticlockwise
   protection ring tunnel(RaP_D), Node C detects the failure between C
   and B and switches the anticlockwise protection ring tunnel(RaP_D) to
   the clockwise working ring tunnel(RcW_D).  The data traffic which
   enters the ring at Node A and exits at Node D follows the path
   A->F->E->D->C->D.  The label operation is:

   [LSP1](original data traffic carried by LSP 1) ->
   [RaP_D(F)|LSP1](NodeA) -> [RaP_D(E)|LSP1](NodeF) ->
   [RaP_D(D)|LSP1](NodeE) -> [RaP_D(C)|LSP1] (NodeD) -> [RcW_D(D)|LSP1]
   (NodeC) -> [LSP1](data traffic carried by LSP 1).
















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                         +---+#####[RaP_D(F)]######+---+
                         | F |---------------------| A | +-- LSP1
                         +---+*****[RcW_D(A)]******+---+
                         #/*                        *\#
              [RaP_D(E)]#/*[RcW_D(F)]      [RcW_D(B)]*\#RaP_D(A)
                       #/*                            *\#
                     +---+                          xxxxx
                     | E |                          x B x
                     +---+                          xxxxx
                       #\                            */#
              [RaP_D(D)]#\                [RcW_D(C)]*/#RaP_D(B)
                         #\                       */#
                         +---+*****[RcW_D(D)]****+---+
               LSP1  +-- | D |-------------------| C |
                         +---+#####[RaP_D(C)]####+---+

                 -----physical links     xxxxxx  Failure Node
                 *****RcW_D              ######  RaP_D

                    Figure 6. Wrapping for node failure

4.3.2.  Short Wrapping

   With the traditional wrapping protection scheme, Protection switching
   is executed at both nodes detecting the failure, consequently the
   traffic will be wrapped twice.  This mechanism will cause additional
   latency and bandwidth consumption when traffic is switched to the
   protection path.

   With short wrapping protection, data traffic switching is executed
   only at the upstream node detecting the link failure, and exits the
   ring in the protection ring tunnel at the exit node.  This scheme can
   reduce the additional latency and bandwidth consumption when traffic
   is switched to the protection path.

   In the traditional wrapping solution, the protection ring tunnel is a
   closed ring in normal state, while in the short wrapping solution,
   the protection ring tunnel is ended at the exit node, which is
   similar to the working ring tunnel.  Short wrapping is easy to
   implement in shared ring protection because both the working and
   protection ring tunnels are terminated on the exit nodes.  Figure 7
   shows the clockwise working ring tunnel and the anticlockwise
   protection ring tunnel with node D as the exit node.

   As shown in Figure 7, in normal state, LSP 1 is carried by the
   clockwise working ring tunnel (RcW_D) through the path A->B->C->D.
   When a link failure between Node B and Node C occurs, Node B switches
   The working ring tunnel RcW_D to the protection ring tunnel RaP_D in



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   the opposite direction.  The difference occurs in the protection ring
   tunnel at exit node.  In short wrapping protection, Rap_D ends in
   Node D and then traffic will be forwarded based on the LSP labels.
   Thus with short wrapping mechanism, LSP1 will follow the path
   A->B->A->F->E->D when link failure between Node B and Node C happens.
   For node failure, the protection with short wrapping is similar to
   the mechanism with link failure.

                         +---+#####[RaP_D(F)]######+---+
                         | F |---------------------| A | +-- LSP1
                         +---+*****[RcW_D(A)]******+---+
                         #/*                        *\#
              [RaP_D(E)]#/*[RcW_D(F)]      [RcW_D(B)]*\#RaP_D(A)
                       #/*                            *\#
                     +---+                           +---+
                     | E |                           | B |
                     +---+                           +---+
                       #\                            *x#
              [RaP_D(D)]#\                [RcW_D(C)]*x#RaP_D(B)
                         #\                        *x#
                         +---+*****[RcW_D(D)]****+---+
               LSP1  +-- | D |-------------------| C |
                         +---+                   +---+

                 ----- physical links    xxxxx Failure Link
                 ****** RcW_D           ###### RaP_D

                 Figure 7. Short wrapping for link failure

4.3.3.  Steering

   In ring topology, each working ring tunnel is associated with a
   protection ring tunnel in the opposite direction, and every node can
   obtain the ring topology either by configuration or via some topology
   discovery mechanism.  When a failure occurs in the ring, the nodes
   that detect the failure will transmit the failure information in the
   opposite direction of the failure hop by hop on the ring.  When a
   node receives the message that identifies a failure, it can quickly
   determine the location of the fault by using the topology information
   that is maintained by the node, then it can determine whether the
   LSPs entering the ring locally need to switchover or not.  For LSPs
   that needs to switchover, it will switch the LSPs from the working
   ring tunnels to its corresponding protection ring tunnels.








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                                                  +--LSP l
  +-+-+-+-+-+-+-+     +---+ ###[RaP_D(F)]### +---/  +-+-+-+-+-+-+-+
  |F|A|B|C|D|E|F|     | F | ---------------- | A |  |A|B|C|D|E|F|A|
  +-+-+-+-+-+-+-+     +---+ ***[RcW_D(A)]*** +---+  +-+-+-+-+-+-+-+
   |I|I|I|S|I|I|                                     |I|I|S|I|I|I|
   +-+-+-+-+-+-+      #/*                     *\#    +-+-+-+-+-+-+
         [RaP_D(E)]  #/*           [RcW_D(B)]  *\# [RaP_D(A)]
                    #/* [RcW_D(F)]              *\#
 +-+-+-+-+-+-+-+   #/*                           *\#
 |E|F|A|B|C|D|E| +---+                            +---+ +-- LSP 2
 +-+-+-+-+-+-+-+ | E |                            | B |  +-+-+-+-+-+-+-+
  |I|I|I|I|S|I|  +---+                            +---+  |B|C|D|E|F|A|B|
  +-+-+-+-+-+-+     #\*                            */#   +-+-+-+-+-+-+-+
                     #\* [RcW_D(E)]    [RcW_D(C)] */#     |I|S|I|I|I|I|
         [RaP_D(D)]   #\*                        */#      +-+-+-+-+-+-+
                       #\*                      */# [RaP_D(B)]
 +-+-+-+-+-+-+-+       +---+     [RcW_D(D)]    +---+    +-+-+-+-+-+-+-+
 |D|E|F|A|B|C|D|  +--  | D | xxxxxxxxxxxxxxxxx | C |    |C|D|E|F|A|B|C|
 +-+-+-+-+-+-+-+ LSP 1 +---+     [RaP_D(C)]    +---+    +-+-+-+-+-+-+-+
  |I|I|I|I|I|S|  LSP 2                                   |S|I|I|I|I|I|
  +-+-+-+-+-+-+                                          +-+-+-+-+-+-+

        ----- physical links  ***** RcW_D  ##### RaP_D

       Figure 8. Steering operation and protection switching

   As shown in Figure 8, LSP1 enters the ring from Node A while LSP2
   enters the ring from Node B, and both of them have the same
   destination node D.

   In the normal state, LSP 1 is carried by the clockwise working ring
   tunnel (RcW_D) through the path A->B->C->D, the label operation is:
   [LSP1] -> [RcW_D(B)|LSP1](NodeA) -> [RcW_D(C)| LSP1](NodeB) ->
   [RcW_D(D)|LSP1](NodeC) -> [LSP1] (data traffic carried by LSP 1) .

   LSP2 is carried by the clockwise working ring tunnel (RcW_D) throught
   the path B->C->D, the label operation is: [LSP2] ->
   [RcW_D(C)|LSP2](NodeB) -> [RcW_D(D)|LSP2](NodeC) -> [LSP2] (data
   traffic carried by LSP 1) .

   If the link between nodes C and D fails, according to the fault
   detection and distribution mechanisms, Node D will find out that
   there is a failure in the link between C and D, and it will update
   the link state of its ring topology, changing the link between C and
   D from normal to fault.  In the direction that opposite to the
   failure position, Node D will send the state report message to Node
   E, informing Node E of the fault between C and D, and E will update
   the link state of its ring topology accordingly, changing the link



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   between C and D from normal to fault.  In this way, the state report
   message is sent hop by hop in the clockwise direction.  Similar to
   Node D, Node C will send the failure information in the anti-
   clockwise direction.

   When Node A receives the failure report message and updates the link
   state of its ring topology, it is aware that there is a fault on the
   clockwise working ring tunnel to node D (RcW_D), and LSP 1 enters the
   ring locally and is carried by this ring tunnel, thus Node A will
   decide to switch the LSP1 onto the anticlockwise protection ring
   tunnel to node D (RaP_D).  After the switchover, LSP1 will follow the
   path A->F->E->D, the label operation is: [LSP1] -> [RaP_D(F)|
   LSP1](NodeA) -> [RaP_D(E)|LSP1](NodeF) -> [RaP_D(D)|LSP1](NodeE) ->
   [LSP1] (data traffic carried by LSP 1).

   The same also apply to the operation of LSP2.  When Node B updates
   the link state of its ring topology, and finds out that the working
   ring tunnel RcW_D has failed, it will switch the LSP2 to the
   anticlockwise protection tunnel RaP_D.  After the switchover, LSP2
   goes through the path B->A->F->E->D, and the label operation is:
   [LSP2] -> [RaP_D(A)|LSP2](NodeB) -> [RaP_D(F)|LSP2](NodeA) ->
   [RaP_D(E)|LSP2](NodeF) -> [RaP_D(D)|LSP2](NodeE) -> [LSP2](data
   traffic carried by LSP 2).

   Then assume the link between nodes A and B breaks down, as shown in
   Figure 9.  Similar to the above failure case, Node B will detect a
   fault in the link between A and B, and it will update the link state
   of its ring topology, changing the link state between A and B from
   normal to fault.  The state report message is sent hop by hop in the
   clockwise direction, notifying every node that there is a fault
   between node A and B, and every node updates the link state of its
   ring topology.  As a result, Node A will detect a fault in the
   working ring tunnel to node D, and switch LSP1 to the protection ring
   tunnel, while Node B determine that the working ring tunnel for LSP2
   still works fine, and will not perform the switchover.
















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                                                    /-- LSP l
 +-+-+-+-+-+-+-+      +---+ ###[RaP_D(F)]####  +---/  +-+-+-+-+-+-+-+
 |F|A|B|C|D|E|F|      | F | -----------------  | A |  |A|B|C|D|E|F|A|
 +-+-+-+-+-+-+-+      +---+ ***[RcW_D(A)]****  +---+  +-+-+-+-+-+-+-+
  |I|S|I|I|I|I|       #/*                       x      |S|I|I|I|I|I|
  +-+-+-+-+-+-+      #/*                         x     +-+-+-+-+-+-+
        [RaP_D(E)]  #/*[RcW_D(F)]       [RcW_D(B)]x [RaP_D(A)]
                   #/*                             x    +-- LSP 2
 +-+-+-+-+-+-+-+  +---+                             +---++-+-+-+-+-+-+-+
 |E|F|A|B|C|D|E|  | E |                             | B ||B|C|D|E|F|A|B|
 +-+-+-+-+-+-+-+  +---+                             +---++-+-+-+-+-+-+-+
  |I|I|S|I|I|I|     #\*                            */#    |I|I|I|I|I|S|
  +-+-+-+-+-+-+      #\*[RcW_D(E)]    [RcW_D(C)]  */#     +-+-+-+-+-+-+
          [RaP_D(D)]  #\*                        */# [RaP_D(B)]
 +-+-+-+-+-+-+-+       #\*                      */#     +-+-+-+-+-+-+-+
 |D|E|F|A|B|C|D|       +---+ ***[RcW_D(D)]*** +---+     |C|D|E|F|A|B|C|
 +-+-+-+-+-+-+-+  +--  | D | ---------------- | C |     +-+-+-+-+-+-+-+
  |I|I|I|S|I|I|   LSP1 +---+ ###[RaP_D(C)]### +---+      |I|I|I|I|S|I|
  +-+-+-+-+-+-+   LSP2                                   +-+-+-+-+-+-+

           ----- physical links  ***** RcW_D  ##### RaP_D

     Figure 9. Steering operation and protection switching (2)

4.4.  Interconnected Ring Protection

4.4.1.  Interconnected Ring Topology

   Interconnected ring topology is often used in MPLS-TP networks.  This
   document will discuss two typical interconnected ring topologies:

   1.  Single-node interconnected rings

          In single-node interconnected rings, the connection between
          the two rings is through a single node.  Because the
          interconnection node is in fact a single point of failure,
          this topology should be avoided in real transport networks.
          Figure 10 shows the topology of single-node interconnected
          rings.  Node C is the interconnection node between Ring1 and
          Ring2.

   2.  Dual-node interconnected rings

          In dual-node interconnected rings, the connection between the
          two rings is through two nodes.  The two interconnection nodes
          belong to both interconnected rings.  This topology can
          recover from one interconnection node failure.




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   Figure 10 shows the topology of single-node interconnected rings.
   Node C is the interconnection node between Ring1 and Ring2.

          +---+      +---+                        +---+      +---+
          | A |------| B |-----              -----| G |------| H |
          +---+      +---+      \           /     +---+      +---+
            |                    \         /                   |
            |                     \ +---+ /                    |
            |        Ring1          | C |         Ring2        |
            |                     / +---+ \                    |
            |                    /         \                   |
          +---+      +---+      /           \     +---+      +---+
          | F |------| E |-----              -----| J |------| I |
          +---+      +---+                        +---+      +---+

              Figure 10. Single-node interconnected rings

   Figure 11 shows the topology of dual-node interconnected rings.
   Nodes C and Node D are the interconnection nodes between Ring1 and
   Ring2.

             +---+      +---+      +---+      +---+      +---+
             | A |------| B |------| C |------| G |------| H |
             +---+      +---+      +---+      +---+      +---+
               |                    | |                    |
               |                    | |                    |
               |        Ring1       | |       Ring2        |
               |                    | |                    |
               |                    | |                    |
             +---+      +---+      +---+      +---+      +---+
             | F |------| E |------| D |------| J |------| I |
             +---+      +---+      +---+      +---+      +---+

               Figure 11. Dual-node interconnected rings

4.4.2.  Interconnected Ring Protection Mechanisms

   Interconnected rings can be regarded as two independent rings.  Ring
   protection switching protocol operates on each ring independently.
   Failure in one ring only triggers protection switching on the ring
   itself and does not affect the other ring.  Protection switching in a
   single ring is same as the one described in section 4.3.

   The service LSPs that traverse the interconnected rings via the
   interconnection nodes MUST use different ring tunnels in different
   rings.  On the interconnection node, the ring tunnel label used in
   the source ring will be popped, and the ring tunnel label of
   destination ring will be pushed



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   For the protected interconnection node in dual-node interconnected
   ring, the service LSPs in the interconnection nodes should use the
   same LSP label.  So any interconnection node can terminate a source
   ring runnel and push a destination ring tunnel according to the
   service LSP label.

   Two interconnection nodes can be managed as a virtual interconnection
   node group.  Each ring should assign ring tunnels to the virtual
   interconnection node group.  The interconnection nodes in the group
   should terminate the working ring tunnel in each ring.  The
   protection ring tunnel is an open ring to switch with the working
   ring tunnel at the nodes that detect the fault and ends at the egress
   node.

   When the service traffic passes through the interconnection node, the
   direction of the working ring tunnels in each ring for this service
   traffic should be the same.  For example, if the working ring tunnel
   follows the clockwise direction in Ring1, the working ring tunnel for
   the same service traffic in Ring2 also follows the clockwise
   direction when the service leaves Ring1 and enters Ring2.

4.4.3.  Ring Tunnels in Interconnected Rings

   The same ring tunnels as described in section 4.1 are used in each
   ring of the interconnected rings.  Note that ring tunnels to the
   virtual interconnection node group will be established by each ring
   of the interconnected rings, i.e.:

   o  one clockwise working ring tunnel to the virtual interconnection
      node group

   o  one anticlockwise protection ring tunnel to the virtual
      interconnection node group

   o  one anticlockwise working ring tunnel to the virtual
      interconnection node group

   o  one clockwise protection ring tunnel to the virtual
      interconnection node group

   These ring tunnels will terminated at all nodes in the virtual
   interconnection node group.

   For example, all the ring tunnels on Ring1 of Figure 12 are
   established as follows:

   o  To Node A: R1cW_A, R1aW_A, R1cP_A, R1aP_A




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   o  To Node B: R1cW_B, R1aW_B, R1cP_B, R1aP_B

   o  To Node C: R1cW_C, R1aW_C, R1cP_C, R1aP_C

   o  To Node D: R1cW_D, R1aW_D, R1cP_D, R1aP_D

   o  To Node E: R1cW_E, R1aW_E, R1cP_E, R1aP_E

   o  To Node F: R1cW_F, R1aW_F, R1cP_F, R1aP_F

   o  To the virtual interconnection node group (including Node F and
      Node A): R1cW_F&A, R1aW_F&A, R1cP_F&A, R1aP_F&A;

   All the ring tunnels established in Ring2 in Figure 12 are
   provisioned as follows:

   o  To Node A: R2cW_A, R2aW_A, R2cP_A, R2aP_A

   o  To Node F: R2cW_F, R2aW_F, R2cP_F, R2aP_F

   o  To Node G: R2cW_G, R2aW_G, R2cP_G, R2aP_G

   o  To Node H: R2cW_H, R2aW_H, R2cP_H, R2aP_H

   o  To Node I: R2cW_I, R2aW_I, R2cP_I, R2aP_I

   o  To Node J: R2cW_J, R2aW_J, R2cP_J, R2aP_J

   o  To the virtual interconnection node group(including Node F and
      Node A): R2cW_FandA, R2aW_FandA, R2cP_FandA, R2aP_FandA





















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                          +---+cccccccccccc +---+
                          | H |-------------| I |--->LSP1
                          +---+             +---+
                          c/a                   a\
                         c/a                     a\
                        c/a                       a\
                      +---+                     +---+
                      | G |        Ring2        | J |
                      +---+                     +---+
                        c\a                      a/c
                         c\a                    a/c
                          c\a  aaaaaaaaaaaaa   a/c
                          +---+ccccccccccccc+---+
                          | F |-------------| A |
                          +---+ccccccccccccc+---+
                          c/aaaaaaaaaaaaaaaaaaa a\
                         c/                      a\
                        c/                        a\
                      +---+                     +---+
                      | E |        Ring1        | B |
                      +---+                     +---+
                        c\a                      a/c
                         c\a                    a/c
                          c\a                  a/c
                          +---+aaaaaaaaaaaa +---+
                  LSP1--->| D |-------------| C |
                          +---+ccccccccccccc+---+

                           ccccccccccc  R1cW_F&A
                           aaaaaaaaaaa  R1aP_F&A
                           ccccccccccc  R2cW_I
                           aaaaaaaaaaa  R2aP_I
            Figure 12. Ring tunnels for the interconnected rings

4.4.4.  Interconnected Ring Switching Procedure

   As shown in Figure 12, for the service traffic LSP1 which enters
   Ring1 at Node D and exits Ring1 at Node F and continues to enter
   Ring2 at Node F and exits Ring2 at Node I, the protection scheme is
   described below.

   In normal state, LSP1 follows R1cW_F&A in Ring1 and R2CW_I in Ring2.
   The label used for the working ring tunnel R1cW_F&A in Ring1 is
   popped and the label used for the working ring tunnel R2cW_I will be
   pushed based the inner label lookup at the interconnection node F.
   The working path that the service traffic LSP1 follows is:
   LSP1->R1cW_F&A (D->E->F)->R2cW_I(F->G->H->I)->LSP1.




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   In case of link failure, for example, when a failure occurs on the
   link between Node F and Node E, Nodes F and E will detect the failure
   and execute protection switching as described in 2.2.1.1.  The path
   that the service traffic LSP1 follows after switching change to
   LSP1->R1cW_F&A(D->E)->R1aP_F&A(E->D->C->B->A->F)->R1cW_F(F)
   ->R2cW_I(F->G->H->I)->LSP1.

   In case of a non interconnection node failure, for example, when the
   failure occurs at Node E in Ring1, Nodes F and E will detect the
   failure and execute protection switching as described in 2.2.1.2.
   The path that the service traffic LSP1 follows after switching
   becomes: LSP1->R1cW_F&A(D)->R1aP_F&A(D->C->B->A->F)->
   R1cW_F(F)->R2cW_I(F->G->H->I).

   In case of an interconnection node failure, for example, when the
   failure occurs at the interconnection Node F.  Nodes E and A in Ring1
   will detect the failure, and execute protection switching as
   described in 2.2.1.2.  Nodes G and A in Ring2 will also detects the
   failure, and execute protection switching.  The path that the service
   traffic LSP1 follows after switching is:
   LSP1->R1cW_F&A(D->E)->R1aP_F&A(E->D->C->B->A)->R1cW_A(A)
   ->R2aP_I(A->J->I)->LSP1.

4.4.5.  Interconnected Ring Detection Mechanism

   As show in Figure 13, the service traffic LSP1 traverses A->B-C in
   Ring1 and C->G->H->I in Ring2.  Node C and Node D are the
   interconnection nodes.  When both the link between Node C and Node G
   and the link between Node C and Node D fail, the ring tunnel from
   Node C to Node I in Ring 2 becomes unreachable.  However, Node D is
   still available, and LSP1 can still reach Node I.




















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      +---+ *********+---+**********+---+          +---+**********+---+
LSP1->| A |----------| B |----------| C |XXXXXXXXXX| G |----------| H |
      +---+##########+---+##########+---+          +---+##########+---+
        |#                            X                            #|*
        |#                            X                            #|*
        |#           Ring1            X           Ring2            #|*
        |#                            X                            #|*
        |#                            X                            #|*
      +---+##########+---+##########+---+######### +---+##########+---+
      | F |----------| E |----------| D |----------| J |----------| I | ->LSP1
      +---+          +---+          +---+          +---+          +---+

                        ***********  R1cW_C&D
                        ###########  R1aP_C&D
                        ***********  R2cW_I
                        ###########  R2aP_I

                    Figure 13. Interconnected ring

   In order to achieve this, the interconnection nodes need to know the
   ring topology of each ring so that they can judge whether a node is
   reachable.  This judgment is based on the knowledge of each ring
   topology and the fault location as described in section 3.4.  The
   ring topology can be obtained from the NMS or topology discovery
   mechanisms.  The fault location can be obtained by transmitting the
   fault information around the ring.  The nodes that detect the failure
   will transmit the fault information in the opposite direction node by
   node in the ring.  When the interconnection node receives the message
   that informs the failure, it will quickly calculate the location of
   the fault by the topology information that is maintained by itself
   and determines whether the LSPs entering the ring at itself can reach
   the destination.  If the destination node is reachable, the LSP will
   exit the source ring and enter the destination ring.  If the
   destination node is not reachable, the LSP will switch to the
   anticlockwise protection ring tunnel.

   In Figure 13, Node C determines that the ring tunnel to Node I is
   unreachable, the service traffic LSP1 for which the destination node
   on the ring tunnel is Node I should switch to the protection LSP
   (R1aP_C&D) and consequently the service traffic LSP1 traverses the
   interconnected rings at Node D.  Node D will remove the ring tunnel
   label of Ring1 and add the ring tunnel label of Ring2.

5.  Ring Protection Coordination Protocol







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5.1.  RPS Protocol

   The MSRP protection operation MUST be controlled with the help of the
   Ring Protection Switch Protocol (RPS).  The RPS processes in the each
   of the individual ring nodes that form the ring SHOULD communicate
   using the G-ACh channel.

   The RPS protocol MUST carry the ring status information and RPS
   requests, i.e., automatically initiated and externally initiated,
   between the ring nodes.

   Each node on the ring MUST be uniquely identified by assigning it a
   node ID.  The maximum number of nodes on the ring supported by the
   RPS protocol is 127.  The node ID SHOULD be independent of the order
   in which the nodes appear on the ring.  The node ID is used to
   identity the source and destination nodes of each RPS request.

   Each node SHOULD have a ring map containing information about the
   sequence of the nodes around the ring.  The method of configuring the
   nodes with the ring maps is TBD.

   When no protection switches are active on the ring, each node MUST
   dispatch periodically RPS requests to the two adjacent nodes,
   indicating No Request (NR).  When a node determines that a protection
   switching is required, it MUST send the appropriate RPS request in
   both directions.

                   +---+ A->B(NR)    +---+ B->C(NR)    +---+ C->D(NR)
            -------| A |-------------| B |-------------| C |-------
          (NR)F<-A +---+    (NR)A<-B +---+    (NR)B<-C +---+

       Figure 14. RPS communication between the ring nodes in case of
                            no failures in the ring

   A destination node is a node that is adjacent to a node that
   identified a failed span.  When a node that is not the destination
   node receives an RPS request and it has no higher priority local
   request, it MUST transfer in the same direction the RPS request as
   received.  In this way, the switching nodes can maintain direct RPS
   protocol communication in the ring.

                    +---+ C->B(SF)    +---+ B->C(SF)    +---+ C->B(SF)
             -------| A |-------------| B |----- X -----| C |-------
           (SF)C<-B +---+    (SF)C<-B +---+    (SF)B<-C +---+

        Figure 15. RPS communication between the ring nodes in case of
                         failure between nodes B and C




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   Note that in the case of a bidirectional failure such as a cable cut,
   the two adjacent nodes detect the failure and send each other an RPS
   request in opposite directions.

   o  In rings utilizing the wrapping protection.  When the destination
      node receives the RPS request it MUST perform the switch from/to
      the working ring tunnels to/from the protection ring tunnels if it
      has no higher priority active RPS request.

   o  In rings utilizing the steering protection.  When a ring switch is
      required, any node MUST perform the switches if its added/dropped
      traffic is affected by the failure.  Determination of the affected
      traffic SHOULD be performed by examining the RPS requests
      (indicating the nodes adjacent to the failure or failures) and the
      stored ring maps (indicating the relative position of the failure
      and the added traffic destined towards that failure).

   When the failure has cleared and the Wait-to-Restore (WTR) timer has
   expired, the nodes sourcing RPS requests MUST drop their respective
   switches (tail end) and MUST source an RPS request carrying the NR
   code.  The node receiving from both directions such RPS request (head
   end) MUST drop its protection switches.

   A protection switch MUST be initiated by one of the criteria
   specified in Section 3.2.  A failure of the RPS protocol or
   controller MUST NOT trigger a protection switch.

   Ring switches MUST be preempted by higher priority RPS requests.  For
   example, consider a protection switch that is active due to a manual
   switch request on the given span, and another protection switch is
   required due to a failure on another span.  Then an RPS request MUST
   be generated, the former protection switch MUST be dropped, and the
   latter protection switch established.

   MSRP mechanism SHOULD support multiple protection switches in the
   ring, resulting the ring being segmented into two or more separate
   segments.  This may happen when several RPS requests of the same
   priority exist in the ring due to multiple failures or external
   switch commands.

   Proper operation of the MSRP mechanism relies on all nodes having
   knowledge of the state of the ring (nodes and spans) so that nodes do
   not preempt existing RPS request unless they have a higher-priority
   RPS request.  In order to accommodate ring state knowledge, during a
   protection switch the RPS requests MUST be sent in both directions.






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5.1.1.  Transmission and Acceptance of RPS Requests

   A new RPS request MUST be transmitted immediately when a change in
   the transmitted status occurs.

   The first three RPS protocol messages carrying new RPS request SHOULD
   be transmitted as fast as possible.  For fast protection switching
   within 50 ms, the interval of the first three RPS protocol messages
   SHOULD be 3.3 ms.  The successive RPS requests SHOULD be transmitted
   with the interval of 5 seconds.

5.1.2.  RPS PDU Format

   Figure 16 depicts the format of an RPS packet that is sent on the
   G-ACh.  The Channel Type field is set to indicate that the message is
   an RPS message.  The ACH MUST NOT include the ACH TLV Header
   [RFC5586] meaning that no ACH TLVs can be included in the message.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0 0 0 1|0 0 0 0|0 0 0 0 0 0 0 0|    RPS Channel Type (TBD)     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Dest Node ID  | Src Node ID   |   Request     |   Reserved    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                         Figure 16. G-ACh RPS Packet Format

   The following fields MUST be provided:

   o  Destination Node ID: The destination node ID MUST always be set to
      value of the node ID of the adjacent node.  Valid destination node
      ID values are 1-127.

   o  Source node ID: The source node ID MUST always be set to the value
      of the node ID generating the RPS request.  Valid source node ID
      values are 1-127.

   o  RPS request code: A code consisting of four bits as specified
      below:












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          +-------------+-----------------------------+----------+
          |  Bits  4-1  |   Condition, State          | Priority |
          | (MSB - LSB) |  or external Request        |          |
          +-------------------------------------------+----------+
          |   1 1 1 1   |  Lockout of Protection (LP) |  highest |
          |   1 1 0 1   |  Forced Switch (FS)         |          |
          |   1 0 1 1   |  Signal Fail (SF)           |          |
          |   0 1 1 0   |  Manual Switch (MS)         |          |
          |   0 1 0 1   |  Wait-To-Restore (WTR)      |          |
          |   0 0 1 1   |  Exercise (EXER)            |          |
          |   0 0 0 1   |  Reverse Request (RR)       |          |
          |   0 0 0 0   |  No Request (NR)            |  lowest  |
          +-------------+-----------------------------+----------+


5.1.3.  Ring Node RPS States

   Idle state: A node is in the idle state when it has no RPS request
   and is sourcing and receiving NR code to/from both directions.

   Switching state: A node not in the idle or pass-through states is in
   the switching state.

   Pass-through state: A node is in the pass-through state when its
   highest priority RPS request is a request not destined to it or
   sourced by it.  The pass-through is bidirectional.

5.1.3.1.  Idle State

   A node in the idle state MUST source the NR request in both
   directions.

   A node in the idle state MUST terminate RPS requests flow in both
   directions.

   A node in the idle state MUST block the traffic flow on protection
   LSPs/tunnels in both directions.

5.1.3.2.  Switching State

   A node in the switching state MUST source RPS request to adjacent
   node with its highest RPS request code in both directions when it
   detects a failure or receives an external command.

   A node in the switching state MUST terminate RPS requests flow in
   both directions.





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   As soon as it receives an RPS request from the short path, the node
   to which it is addressed MUST acknowledge the RPS request by replying
   with the RR code on the short path, and with the received RPS request
   code on the long path.

   This rule refers to the unidirectional failure detection: the RR
   SHOULD be issued only when the node does not detect the failure
   condition (i.e., the node is a head end), that is, it is not
   applicable when a bidirectional failure is detected, because, in this
   case, both nodes adjacent to the failure will send an RPS request for
   the failure on both paths (short and long).

   The following switches MUST be allowed to coexist:

   o  LP and LP

   o  FS and FS

   o  SF and SF

   o  FS and SF

   When multiple MS RPS requests over different spans exist at the same
   time, no switch SHOULD be executed and existing switches MUST be
   dropped.  The nodes MUST signal, anyway, the MS RPS request code.

   Multiple EXER requests MUST be allowed to coexist in the ring.

   A node in a ring switching state that receives the external command
   LP for the affected span MUST drop its switch and MUST signal NR for
   the locked span if there is no other RPS request on another span.
   Node still SHOULD signal relevant RPS request for another span.

5.1.3.3.  Pass-through State

   When a node is in a pass-through state, it MUST transfer the received
   RPS Request in the same direction.

   When a node is in a pass-through state, it MUST enable the traffic
   flow on protection ring tunnels in both directions.

5.1.4.  RPS State Transitions

   All state transitions are triggered by an incoming RPS request
   change, a WTR expiration, an externally initiated command, or locally
   detected MPLS-TP section failure conditions.





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   RPS requests due to a locally detected failure, an externally
   initiated command, or received RPS request shall pre-empt existing
   RPS requests in the prioritized order given in Section 3.1.2, unless
   the requests are allowed to coexist.

5.1.4.1.  Transitions Between Idle and Pass-through States

   The transition from the idle state to pass-through state MUST be
   triggered by a valid RPS request change, in any direction, from the
   NR code to any other code, as long as the new request is not destined
   to the node itself.  Both directions move then into a pass-through
   state, so that, traffic entering the node through the protection Ring
   tunnels are transferred transparently through the node.

   A node MUST revert from pass-through state to the idle state when it
   detects NR codes incoming from both directions.  Both directions
   revert simultaneously from the pass-through state to the idle state.

5.1.4.2.  Transitions Between Idle and Switching States

   Transition of a node from the idle state to the switching state MUST
   be triggered by one of the following conditions:

   o  A valid RPS request change from the NR code to any code received
      on either the long or the short path and destined to this node

   o  An externally initiated command for this node

   o  The detection of an MPLS-TP section layer failure at this node

   Actions taken at a node in the idle state upon transition to
   switching state are:

   o  For all protection switch requests, except EXER and LP, the node
      MUST execute the switch

   o  For EXER, and LP, the node MUST signal appropriate request but not
      execute the switch

   A node MUST revert from the switching state to the idle state when it
   detects NR codes received from both directions.

   o  At the tail end: When a WTR time expires or an externally
      initiated command is cleared at a node, the node MUST drop its
      switch, transit to the Idle State and signal the NR code in both
      directions.





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   o  At the head end: Upon reception of the NR code, from both
      directions, the head-end node MUST drop its switch, transition to
      Idle State and signal the NR code in both directions.

5.1.4.3.  Transitions Between Switching States

   When a node that is currently executing any protection switch
   receives a higher priority RPS request (due to a locally detected
   failure, an externally initiated command, or a ring protection switch
   request destined to it) for the same span, it MUST update the
   priority of the switch it is executing to the priority of the
   received RPS request.

   When a failure condition clears at a node, the node MUST enter WTR
   condition and remain in it for the appropriate time-out interval,
   unless:

   o  A different RPS request with a higher priority than WTR is
      received

   o  Another failure is detected

   o  An externally initiated command becomes active

   The node MUST send out a WTR code on both the long and short paths.

   When a node that is executing a switch in response to incoming SF RPS
   request (not due to a locally detected failure) receives a WTR code
   (unidirectional failure case), it MUST send out RR code on the short
   path and the WTR on the long path.

5.1.4.4.  Transitions Between Switching and Pass-through States

   When a node that is currently executing a switch receives an RPS
   request for a non-adjacent span of higher priority than the switch it
   is executing, it MUST drop its switch immediately and enter the pass-
   through state.

   The transition of a node from pass-through to switching state MUST be
   triggered by:

   o  An equal priority, a higher priority, or an allowed coexisting
      externally initiated command

   o  The detection of an equal priority, a higher priority, or an
      allowed coexisting automatic initiated command





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   o  The receipt of an equal, a higher priority, or an allowed
      coexisting RPS request destined to this node

5.2.  RPS State Machine

5.2.1.  Initial States

            +-----------------------------------+----------------+
            |        State                      |  Signaled RPS  |
            +-----------------------------------+----------------+
            |  A  |  Idle                       |  NR            |
            |     |  Working: no switch         |                |
            |     |  Protection: no switch      |                |
            +-----+-----------------------------+----------------+
            |  B  |  Pass-trough                |  N/A           |
            |     |   Working: no switch        |                |
            |     |   Protection: pass through  |                |
            +-----+-----------------------------+----------------+
            |  C  |  Switching - LP             |  LP            |
            |     |  Working: no switch         |                |
            |     |  Protection: no switch      |                |
            +-----+-----------------------------+----------------+
            |  D  |  Idle - LW                  |  NR            |
            |     |  Working: no switch         |                |
            |     |  Protection: no switch      |                |
            +-----+-----------------------------+----------------+
            |  E  |  Switching - FS             |  FS            |
            |     |  Working: switched          |                |
            |     |  Protection: switched       |                |
            +-----+-----------------------------+----------------+
            |  F  |  Switching - SF             |  SF            |
            |     |  Working: switched          |                |
            |     |  Protection: switched       |                |
            +-----+-----------------------------+----------------+
            |  G  |  Switching - MS             |  MS            |
            |     |  Working: switched          |                |
            |     |  Protection: switched       |                |
            +-----+-----------------------------+----------------+
            |  H  |  Switching - WTR            |  WTR           |
            |     |  Working: switched          |                |
            |     |  Protection: switched       |                |
            +-----+-----------------------------+----------------+
            |  I  |  Switching - EXER           |  EXER          |
            |     |  Working: no switch         |                |
            |     |  Protection: no switch      |                |
            +-----+-----------------------------+----------------+





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5.2.2.  State transitions When Local Request is Applied

   In the state description below 'O' means that new local request will
   be rejected because of exiting request.

   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   A (Idle)             LP                C (Switching - LP)
                        LW                D (Idle - LW)
                        FS                E (Switching - FS)
                        SF                F (Switching - SF)
                        Recover from SF   N/A
                        MS                G (Switching - MS)
                        Clear             N/A
                        WTR expires       N/A
                        EXER              I (Switching - EXER)
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   B (Pass-trough)      LP                C (Switching - LP)
                        LW                B (Pass-trough)
                        FS                O - if current state is due to
                                              LP sent by another node
                                          E (Switching - FS) - otherwise
                        SF                O - if current state is due to
                                              LP sent by another node
                                          F (Switching - SF) - otherwise
                        Recover from SF   N/A
                        MS                O - if current state is due to
                                              LP, SF or FS sent by
                                              another node
                                          G (Switching - MS) - otherwise
                        Clear             N/A
                        WTR expires       N/A
                        EXER              O
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   C (Switching - LP)   LP                N/A
                        LW                O
                        FS                O
                        SF                O
                        Recover from SF   N/A
                        MS                O
                        Clear             A (Idle) - if there is no
                                             failure in the ring
                                          F (Switching - SF) - if there



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                                             is a failure at this node
                                          B (Pass-trough) - if there is
                                             a failure at another node
                        WTR expires       N/A
                        EXER              O
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   D (Idle - LW)        LP                C (Switching - LP)
                        LW                N/A - if on the same span
                                          D (Idle - LW) - if on another
                                             span
                        FS                O - if on the same span
                                          E (Switching - FS) - if on
                                             another span
                        SF                O - if on the addressed span
                                          F (Switching - SF) - if on
                                             another span
                        Recover from SF   N/A
                        MS                O - if on the same span
                                          G (Switching - MS) - if on
                                             another span
                        Clear             A (Idle) - if there is no
                                             failure on addressed span
                                          F (Switching - SF) - if there
                                             is a failure on this span
                        WTR expires       N/A
                        EXER              O
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   E (Switching - FS)   LP                C (Switching - LP)
                        LW                O - if on another span
                                          D (Idle - LW) - if on the same
                                             span
                        FS                N/A - if on the same span
                                          E (Switching - FS) - if on
                                             another span
                        SF                O - if on the addressed span
                                          E (Switching - FS) - if on
                                             another span
                        Recover from SF   N/A
                        MS                O
                        Clear             A (Idle) - if there is no
                                             failure in the ring
                                          F (Switching - SF) - if there
                                             is a failure at this node
                                          B (Pass-trough) - if there is



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                                             a failure at another node
                        WTR expires       N/A
                        EXER              O
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   F (Switching - SF)   LP                C (Switching - LP)
                        LW                O - if on another span

                                          D (Idle - LW) - if on the same
                                             span
                        FS                E (Switching - FS)
                        SF                N/A - if on the same span
                                          F (Switching - SF) - if on
                                             another span
                        Recover from SF   H (Switching - WTR)
                        MS                O
                        Clear             N/A
                        WTR expires       N/A
                        EXER              O
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   G (Switching - MS)   LP                C (Switching - LP)
                        LW                O - if on another span
                                          D (Idle - LW) - if on the same
                                             span
                        FS                E (Switching - FS)
                        SF                F (Switching - SF)
                        Recover from SF   N/A
                        MS                N/A - if on the same span
                                          G (Switching - MS) - if on
                                             another span release the
                                             switches but signal MS
                        Clear             A
                        WTR expires       N/A
                        EXER              O
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   H (Switching - WTR)  LP                C (Switching - LP)
                        LW                D (Idle - W)
                        FS                E (Switching - FS)
                        SF                F (Switching - SF)
                        Recover from SF   N/A
                        MS                G (Switching - MS)
                        Clear             A
                        WTR expires       A



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                        EXER              O
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   I (Switching - EXER) LP                C (Switching - LP)
                        LW                D (idle - W)
                        FS                E (Switching - FS)
                        SF                F (Switching - SF)
                        Recover from SF   N/A
                        MS                G (Switching - MS)
                        Clear             A
                        WTR expires       N/A
                        EXER              N/A - if on the same span
                                          I (Switching - EXER)
   =====================================================================


5.2.3.  State Transitions When Remote Request is Applied

   The priority of a remote request does not depend on the side from
   which the request is received.

   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   A (Idle)             LP                C (Switching - LP)
                        FS                E (Switching - FS)
                        SF                F (Switching - SF)
                        MS                G (Switching - MS)
                        WTR               N/A
                        EXER              I (Switching - EXER)
                        RR                N/A
                        NR                A (Idle)
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   B (Pass-trough)      LP                C (Switching - LP)
                        FS                N/A - cannot happen when there
                                               is LP request in the ring
                                          E (Switching - FS) - otherwise
                        SF                N/A - cannot happen when there
                                               is LP request in the ring
                                          F (Switching - SF) - otherwise
                        MS                N/A - cannot happen when there
                                                is LP, FS or SF request
                                                in the ring
                                          G (Switching - MS) - otherwise
                        WTR               N/A - cannot happen when there



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                                                is LP, FS, SF or MS
                                                request in the ring
                        EXER              N/A - cannot happen when there
                                                is LP, FS, SF, MS or WTR
                                                request in the ring
                                          I (Switching - EXER) -
                                                otherwise
                        RR                N/A
                        NR                A (Idle) - if received from
                                                     both sides
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   C (Switching - LP)   LP                C (Switching - LP)
                        FS                N/A - cannot happen when there
                                               is LP request in the ring
                        SF                N/A - cannot happen when there
                                               is LP request in the ring
                        MS                N/A - cannot happen when there
                                               is LP request in the ring
                        WTR               N/A
                        EXER              N/A - cannot happen when there
                                               is LP request in the ring
                        RR                C (Switching - LP)
                        NR                N/A
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   D (Idle - LW)        LP                C (Switching - LP)
                        FS                E (Switching - FS)
                        SF                F (Switching - SF)
                        MS                G (Switching - MS)
                        WTR               N/A
                        EXER              I (Switching - EXER)
                        RR                N/A
                        NR                D (Idle - LW)
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   E (Switching - FS)   LP                C (Switching - LP)
                        FS                E (Switching - FS)
                        SF                E (Switching - FS)
                        MS                N/A - cannot happen when there
                                               is FS request in the ring
                        WTR               N/A
                        EXER              N/A - cannot happen when there
                                               is FS request in the ring
                        RR                E (Switching - FS)



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                        NR                N/A
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   F (Switching - SF)   LP                C (Switching - LP)
                        FS                F (Switching - SF)
                        SF                F (Switching - SF)
                        MS                N/A - cannot happen when there
                                               is SF request in the ring
                        WTR               N/A
                        EXER              N/A - cannot happen when there
                                               is SF request in the ring
                        RR                F (Switching - SF)
                        NR                N/A
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   G (Switching - MS)   LP                C (Switching - LP)
                        FS                E (Switching - FS)
                        SF                F (Switching - SF)
                        MS                G (Switching - MS) - release
                                             the switches but signal MS
                        WTR               N/A
                        EXER              N/A - cannot happen when there
                                               is MS request in the ring
                        RR                G (Switching - MS)
                        NR                N/A
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   H (Switching - WTR)  LP                C (Switching - LP)
                        FS                E (Switching - FS)
                        SF                F (Switching - SF)
                        MS                G (Switching - MS)
                        WTR               H (Switching - WTR)
                        EXER              N/A - cannot happen when there
                                              is WTR request in the ring
                        RR                H (Switching - WTR)
                        NR                N/A
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   I (Switching - EXER) LP                C (Switching - LP)
                        FS                E (Switching - FS)
                        SF                F (Switching - SF)
                        MS                G (Switching - MS)
                        WTR               N/A
                        EXER              I (Switching - EXER)



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                        RR                I (Switching - EXER)
                        NR                N/A
   =====================================================================


5.2.4.  State Transitions When Request Addresses to Another Node is
        Received

   The priority of a remote request does not depend on the side from
   which the request is received.

   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   A (Idle)             LP                B (Pass-trough)
                        FS                B (Pass-trough)
                        SF                B (Pass-trough)
                        MS                B (Pass-trough)
                        WTR               B (Pass-trough)
                        EXER              B (Pass-trough)
                        RR                N/A
                        NR                N/A
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   B (Pass-trough)      LP                B (Pass-trough)
                        FS                N/A - cannot happen when there
                                               is LP request in the ring
                                          B (Pass-trough) - otherwise
                        SF                N/A - cannot happen when there
                                               is LP request in the ring
                                          B (Pass-trough) - otherwise
                        MS                N/A - cannot happen when there
                                                is LP, FS or SF request
                                                in the ring
                                          B (Pass-trough) - otherwise
                        WTR               N/A - cannot happen when there
                                                is LP, FS, SF or MS
                                                request in the ring
                                          B (Pass-trough) - otherwise
                        EXER              N/A - cannot happen when there
                                                is LP, FS, SF, MS or WTR
                                                request in the ring
                                          B (Pass-trough) - otherwise
                        RR                N/A
                        NR                B (Pass-trough)
   =====================================================================
   Initial state        New request       New state



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   -------------        -----------       ---------
   C (Switching - LP)   LP                C (Switching - LP)
                        FS                N/A - cannot happen when there
                                               is LP request in the ring
                        SF                N/A - cannot happen when there
                                               is LP request in the ring
                        MS                N/A - cannot happen when there
                                               is LP request in the ring
                        WTR               N/A - cannot happen when there
                                               is LP in the ring
                        EXER              N/A - cannot happen when there
                                               is LP request in the ring
                        RR                N/A
                        NR                N/A
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   D (Idle - LW)        LP                B (Pass-trough)
                        FS                B (Pass-trough)
                        SF                B (Pass-trough)
                        MS                B (Pass-trough)
                        WTR               B (Pass-trough)
                        EXER              B (Pass-trough)
                        RR                N/A
                        NR                N/A
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   E (Switching - FS)   LP                B (Pass-trough)
                        FS                E (Switching - FS)
                        SF                E (Switching - FS)
                        MS                N/A - cannot happen when there
                                               is FS request in the ring
                        WTR               N/A - cannot happen when there
                                               is FS request in the ring
                        EXER              N/A - cannot happen when there
                                               is FS request in the ring
                        RR                N/A
                        NR                N/A
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   F (Switching - SF)   LP                B (Pass-trough)
                        FS                F (Switching - SF)
                        SF                F (Switching - SF)
                        MS                N/A - cannot happen when there
                                               is SF request in the ring
                        WTR               N/A - cannot happen when there



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                                               is SF request in the ring
                        EXER              N/A - cannot happen when there
                                               is SF request in the ring
                        RR                N/A
                        NR                N/A
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   G (Switching - MS)   LP                B (Pass-trough)
                        FS                B (Pass-trough)
                        SF                B (Pass-trough)
                        MS                G (Switching - MS) - release
                                             the switches but signal MS
                        WTR               N/A - cannot happen when there
                                               is MS request in the ring
                        EXER              N/A - cannot happen when there
                                               is MS request in the ring
                        RR                N/A
                        NR                N/A
   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   H (Switching - WTR)  LP                B (Pass-trough)
                        FS                B (Pass-trough)
                        SF                B (Pass-trough)
                        MS                B (Pass-trough)
                        WTR               N/A
                        EXER              N/A - cannot happen when there
                                              is WTR request in the ring
                        RR                N/A
                        NR                N/A
   =====================================================================
   Initial state        New request       New state
   I (Switching - EXER) LP                B (Pass-trough)
                        FS                B (Pass-trough)
                        SF                B (Pass-trough)
                        MS                B (Pass-trough)
                        WTR               N/A
                        EXER              I (Switching - EXER)
                        RR                N/A
                        NR                N/A
   =====================================================================









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6.  IANA Considerations

   The Channel Types for the Generic Associated Channel are allocated
   from the IANA PW Associated Channel Type registry defined in
   [RFC4446] and updated by [RFC5586].

   IANA is requested to allocate a further Channel Type as follows:

   o  TBA   Ring Protection Switching (RPS)

   Note to RFC Editor: this section may be removed on publication as an
   RFC.

7.  Security Considerations

   This document does not by itself raise any particular security
   considerations.

8.  Contributing Authors

   Wen Ye, Minxue Wang, Sheng Liu (China Mobile)

9.  Normative References

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

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031, January 2001.

   [RFC4446]  Martini, L., "IANA Allocations for Pseudowire Edge to Edge
              Emulation (PWE3)", BCP 116, RFC 4446, April 2006.

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

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

   [RFC6371]  Busi, I. and D. Allan, "Operations, Administration, and
              Maintenance Framework for MPLS-Based Transport Networks",
              RFC 6371, September 2011.








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Authors' Addresses

   Weiqiang Cheng
   China Mobile

   Email: chengweiqiang@chinamobile.com


   Lei Wang
   China Mobile

   Email: wangleiyj@chinamobile.com


   Han Li
   China Mobile

   Email: lihan@chinamobile.com


   Huub van Helvoort
   Hai Gaoming BV

   Email: huubatwork@gmail.com


   Kai Liu
   Huawei Technologies

   Email: alex.liukai@huawei.com


   Jie Dong
   Huawei Technologies

   Email: jie.dong@huawei.com


   Jia He
   Huawei Technologies

   Email: hejia@huawei.com


   Fang Li
   China Academy of Telecommunication Research, MIIT., China

   Email: lifang@ritt.cn



Cheng, et al.            Expires August 6, 2015                [Page 41]


Internet-Draft                    MSRP                     February 2015


   Jian Yang
   ZTE Corporation P.R.China

   Email: yang.jian90@zte.com.cn


   Junfang Wang
   Fiberhome Telecommunication Technologies Co., LTD.

   Email: wjf@fiberhome.com.cn









































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