RFC8227: MPLS-TP Shared-Ring Protection (MSRP) Mechanism for Ring Topology

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Internet Engineering Task Force (IETF)                          W. Cheng
Request for Comments: 8227                                       L. Wang
Category: Standards Track                                          H. Li
ISSN: 2070-1721                                             China Mobile
                                                         H. van Helvoort
                                                          Hai Gaoming BV
                                                                 J. Dong
                                                     Huawei Technologies
                                                             August 2017


   MPLS-TP Shared-Ring Protection (MSRP) Mechanism for Ring Topology

Abstract

   This document describes requirements, architecture, and solutions for
   MPLS-TP Shared-Ring Protection (MSRP) in a ring topology for point-
   to-point (P2P) services.  The MSRP mechanism is described to meet the
   ring protection requirements as described in RFC 5654.  This document
   defines the Ring Protection Switching (RPS) protocol that is used to
   coordinate the protection behavior of the nodes on an MPLS ring.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8227.
















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Copyright Notice

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





































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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Terminology and Notation  . . . . . . . . . . . . . . . . . .   4
   3.  MPLS-TP Ring Protection Criteria and Requirements . . . . . .   5
   4.  Shared-Ring Protection Architecture . . . . . . . . . . . . .   6
     4.1.  Ring Tunnel . . . . . . . . . . . . . . . . . . . . . . .   6
       4.1.1.  Establishment of the Ring Tunnel  . . . . . . . . . .   8
       4.1.2.  Label Assignment and Distribution . . . . . . . . . .   9
       4.1.3.  Forwarding Operation  . . . . . . . . . . . . . . . .   9
     4.2.  Failure Detection . . . . . . . . . . . . . . . . . . . .  10
     4.3.  Ring Protection . . . . . . . . . . . . . . . . . . . . .  11
       4.3.1.  Wrapping  . . . . . . . . . . . . . . . . . . . . . .  12
       4.3.2.  Short-Wrapping  . . . . . . . . . . . . . . . . . . .  14
       4.3.3.  Steering  . . . . . . . . . . . . . . . . . . . . . .  17
     4.4.  Interconnected Ring Protection  . . . . . . . . . . . . .  21
       4.4.1.  Interconnected Ring Topology  . . . . . . . . . . . .  21
       4.4.2.  Interconnected Ring Protection Mechanisms . . . . . .  22
       4.4.3.  Ring Tunnels in Interconnected Rings  . . . . . . . .  23
       4.4.4.  Interconnected Ring-Switching Procedure . . . . . . .  25
       4.4.5.  Interconnected Ring Detection Mechanism . . . . . . .  26
   5.  Ring Protection Coordination Protocol . . . . . . . . . . . .  27
     5.1.  RPS and PSC Comparison on Ring Topology . . . . . . . . .  27
     5.2.  RPS Protocol  . . . . . . . . . . . . . . . . . . . . . .  28
       5.2.1.  Transmission and Acceptance of RPS Requests . . . . .  30
       5.2.2.  RPS Protocol Data Unit (PDU) Format . . . . . . . . .  31
       5.2.3.  Ring Node RPS States  . . . . . . . . . . . . . . . .  32
       5.2.4.  RPS State Transitions . . . . . . . . . . . . . . . .  34
     5.3.  RPS State Machine . . . . . . . . . . . . . . . . . . . .  36
       5.3.1.  Switch Initiation Criteria  . . . . . . . . . . . . .  36
       5.3.2.  Initial States  . . . . . . . . . . . . . . . . . . .  39
       5.3.3.  State Transitions When Local Request Is Applied . . .  40
       5.3.4.  State Transitions When Remote Request is Applied  . .  44
       5.3.5.  State Transitions When Request Addresses to Another
               Node is Received  . . . . . . . . . . . . . . . . . .  47
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  51
     6.1.  G-ACh Channel Type  . . . . . . . . . . . . . . . . . . .  51
     6.2.  RPS Request Codes . . . . . . . . . . . . . . . . . . . .  51
   7.  Operational Considerations  . . . . . . . . . . . . . . . . .  52
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  52
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  53
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  53
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  54
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  55
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  55
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  56




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1.  Introduction

   As described in Section 2.5.6.1 of [RFC5654], several service
   providers have expressed much interest in operating an MPLS Transport
   Profile (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 using ring
   topologies.  This calls for an efficient and optimized ring
   protection mechanism to achieve simple operation and fast, sub 50 ms,
   recovery performance.

   This document specifies an MPLS-TP Shared-Ring Protection mechanism
   that meets the criteria for ring protection and the ring protection
   requirements described in Section 2.5.6.1 of [RFC5654].

   The basic concept and architecture of the MPLS-TP Shared-Ring
   Protection mechanism are specified in this document.  This document
   describes the solutions for point-to-point transport paths.  While
   the basic concept may also apply to point-to-multipoint transport
   paths, the solution for point-to-multipoint transport paths is out of
   the scope of this document.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Terminology and Notation

   Terminology:

   Ring node:  All nodes in the ring topology are ring nodes, and they
      MUST actively participate in the ring protection.

   Ring tunnel:  A ring tunnel provides a server layer for the Label
      Switched Paths (LSPs) traversing the ring.  The notation used for
      a ring tunnel is: R<d><p><X> where <d> = c (clockwise) or a
      (anticlockwise), <p> = W (working) or P (protecting), and <X> =
      the node name.









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   Ring map:  A ring map is present in each ring node.  The ring map
      contains the ring topology information, i.e., the nodes in the
      ring, the adjacency of the ring nodes, and the status of the links
      between ring nodes (Intact or Severed).  The ring map is used by
      every ring node to determine the switchover behavior of the ring
      tunnels.

   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
       parentheses ("()").

3.  MPLS-TP Ring Protection Criteria and Requirements

   The generic requirements for MPLS-TP protection are specified in
   [RFC5654].  The requirements specific for ring protection are
   specified in Section 2.5.6.1 of [RFC5654].  This section describes
   how the criteria for ring protection are met:

   a.  The number of Operations, Administration, and Maintenance (OAM)
       entities needed to trigger protection

       Each ring node requires only one instance of the RPS protocol per
       ring.  The OAM of the links connected to the adjacent ring nodes
       has to be forwarded to only this instance in order to trigger
       protection.  For detailed information, see Section 5.2.

   b.  The number of elements of recovery in the ring

       Each ring node requires only one instance of the RPS protocol and
       is independent of the number of LSPs that are protected.  For
       detailed information, see Section 5.2.









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   c.  The required number of labels required for the protection paths

       The RPS protocol uses ring tunnels, and each tunnel has a set of
       labels.  The number of ring tunnel labels is related to the
       number of ring nodes and is independent of the number of
       protected LSPs.  For detailed information, see Section 4.1.2.

   d.  The amount of control and management-plane transactions

       Each ring node requires only one instance of the RPS protocol per
       ring.  This means that only one maintenance operation is required
       per ring node.  For detailed information, see Section 5.2.

   e.  Minimize the signaling and routing information exchange during
       protection

       Information exchange during a protection switch is using the
       in-band RPS and OAM messages.  No control-plane interactions are
       required.  For detailed information, see Section 5.2.

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 that provide a server layer
   for the LSPs traversing the ring.  Once a ring tunnel is established,
   the forwarding and protection switching of the ring are all performed
   at the ring tunnel level.  A port can carry multiple ring tunnels,
   and a ring tunnel can carry multiple LSPs.




















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                                              +-------------
                                +-------------|
                  +-------------|             |
    ===Service1===|             |             |
    ===Service2===|    LSP1     |             |
                  +-------------|             |
                                |Ring-Tunnel1 |
                  +-------------|             |
    ===Service3===|             |             |
    ===Service4===|    LSP2     |             |
                  +-------------|             |
                                +-------------|  Physical
                                +-------------|
                  +-------------|             |    Port
    ===Service5===|             |             |
    ===Service6===|    LSP3     |             |
                  +-------------|             |
                                |Ring-Tunnel2 |
                  +-------------|             |
    ===Service7===|             |             |
    ===Service8===|    LSP4     |             |
                  +-------------|             |
                                +-------------|
                                              +-------------

                 Figure 1: The Logical Layers of the Ring

   The label stack used in the MPLS-TP Shared-Ring Protection mechanism
   is [Ring Tunnel Label|LSP Label|Service Label](Payload) as
   illustrated in Figure 2.

                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   |           Ring Tunnel Label         |
                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   |               LSP Label             |
                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   |             Service Label           |
                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   |                Payload              |
                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 2: Label Stack Used in MPLS-TP Shared-Ring Protection









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4.1.1.  Establishment of the Ring Tunnel

   The Ring tunnels are established based on the egress nodes.  The
   egress node is the node where traffic leaves the ring.  LSPs that
   have the same egress node on the ring and travel along the ring in
   the same direction (clockwise or anticlockwise) share the same ring
   tunnels.  In other words, all the LSPs that traverse the ring in the
   same direction and exit from the same node share the same working
   ring tunnel and protection ring tunnel.  For each egress 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

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

   As shown in Figure 3, LSP1, LSP2, and LSP3 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 anticlockwise 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 an egress 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 that enter
   the ring from any node can reach any egress 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 on each hop of the ring tunnel can
   be either configured statically, provisioned by a controller, or
   distributed dynamically via a control protocol.  For an LSP that
   traverses the ring tunnel, the ingress ring node and the egress ring
   node are considered adjacent at the LSP layer, and LSP label needs to
   be allocated at these two ring nodes.  The control plane for label
   distribution is outside the scope of this document.

4.1.3.  Forwarding Operation

   When an MPLS-TP transport path, i.e., an LSP, enters the ring, the
   ingress node on the ring pushes the working ring tunnel label that is
   used to reach the specific egress 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 egress node, the egress node pops the ring
   tunnel label and forwards the packets based on the inner LSP label




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   and service 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.

   1.  Ingress node: Packets of LSP1 arrive at Node A with a label stack
       [LSP1] and are supposed to be forwarded in the clockwise
       direction of the ring.  The label of the clockwise working ring
       tunnel 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, Nodes B and 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.  Egress node: When the packet arrives at Node D (i.e., the egress
       node) with label stack [RcW_D(D)|LSP1], Node D pops RcW_D(D) and
       subsequently deals with the inner labels of LSP1.

                      +---+#####[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.




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   Two ports of a link form a Maintenance Entity Group (MEG), and a MEG
   End Point (MEP) function is installed in each ring port.  Continuity
   Check (CC) OAM packets are periodically exchanged between each pair
   of MEPs to monitor the link health.  Three consecutive lost CC
   packets MUST 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

   This section specifies the ring protection mechanisms in detail.  In
   general, the description uses the clockwise working ring tunnel and
   the corresponding anticlockwise protection ring tunnel as an example,
   but the mechanism is applicable in the same way to the anticlockwise
   working and clockwise protection ring tunnels.

   In a ring network, each working ring tunnel is associated with a
   protection ring tunnel in the opposite direction, and every node MUST
   obtain the ring topology either by configuration or via a topology
   discovery mechanism.  The ring topology and the connectivity (Intact
   or Severed) between two adjacent ring nodes form the ring map.  Each
   ring node maintains the ring map and uses it to perform ring
   protection switching.

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

   [LSP1](Payload) -> [RCW_D(B)|LSP1](NodeA) -> [RCW_D(C)|LSP1](NodeB)
   -> [RCW_D(D)| LSP1](NodeC) -> [LSP1](Payload).

   Then at Node D, the packet will be forwarded based on the label stack
   of LSP1.

   Three typical ring protection mechanisms are described in this
   section: wrapping, short-wrapping, and steering.  All nodes on the
   same ring MUST use the same protection mechanism.  If the RPS
   protocol in any node detects an RPS message with a protection-
   switching mode that was not provisioned in that node, a failure of
   protocol will be reported, and the protection mechanism will not be
   activated.

   Wrapping ring protection: the node that detects a failure or accepts
   a switch request switches the traffic impacted by the failure or the
   switch request to the opposite direction (away from the failure).  In



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   this way, the impacted traffic is switched to the protection ring
   tunnel by the switching node upstream of the failure, then it travels
   around the ring to the switching node downstream of the failure
   through the protection ring tunnel, where it is switched back onto
   the working ring tunnel to reach the egress node.

   Short-wrapping ring protection provides some optimization to wrapping
   protection, in which the impacted traffic is only switched once to
   the protection ring tunnel by the switching node upstream to the
   failure.  At the egress node, the traffic leaves the ring from the
   protection ring tunnel.  This can reduce the traffic detour of
   wrapping protection.

   Steering ring protection implies that the node that detects a failure
   sends a request along the ring to the other node adjacent to the
   failure, and all nodes in the ring process this information.  For the
   impacted traffic, the ingress node (which adds traffic to the ring)
   performs switching of the traffic from working to the protection ring
   tunnel, and the egress node will drop the traffic received from the
   protection ring tunnel.

   The following sections describe these protection mechanisms in
   detail.

4.3.1.  Wrapping

   With the wrapping mechanism, the protection ring tunnel is a closed
   ring identified by the egress 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 link failures and node
   failures.  Wrapping can be applicable for the protection of
   Point-to-Multipoint (P2MP) LSPs on the ring; the details of which are
   outside the scope of this document.

4.3.1.1.  Wrapping for Link Failure

   When a link failure between Nodes B and C occurs, if it is a
   bidirectional failure, both Nodes B and C can detect the failure via
   the OAM mechanism; if it is a unidirectional failure, one of the two
   nodes would detect the failure via the OAM mechanism.  In both cases,
   the node at the other side of the detected failure will be determined
   by the ring map and informed using the RPS protocol, which is
   specified in Section 5.  Then Node B switches the clockwise working
   ring tunnel (RcW_D) to the anticlockwise protection ring tunnel
   (RaP_D), and Node C switches the anticlockwise protection ring tunnel
   (RaP_D) back to the clockwise working ring tunnel (RcW_D).  The




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   payload that enters the ring at Node A and leaves the ring at Node D
   follows the path A->B->A->F->E->D->C->D.  The label operation is:

   [LSP1](Payload) -> [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](Payload).

                      +---+#####[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    xxxxx Failure Links
                 ***** RcW_D             ##### RaP_D

                    Figure 5: Wrapping for Link Failure

4.3.1.2.  Wrapping for Node Failure

   As shown in Figure 6, when Node B fails, Node A detects the failure
   between A and B and switches the clockwise working 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 node at the other side of the failed node will be
   determined by the ring map and informed using the RPS protocol
   specified in Section 5.

   The payload that 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](Payload)-> [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](Payload).





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   In one special case where Node D fails, all the ring tunnels with
   Node D as the egress will become unusable.  The ingress node will
   update its ring map according to received RPS messages and determine
   that the egress node is not reachable; thus, it will not send traffic
   to either the working or the protection tunnel.  However, before the
   failure location information is propagated to all the ring nodes, the
   wrapping protection mechanism may cause a temporary traffic loop:
   Node C detects the failure and switches the traffic from the
   clockwise working ring tunnel (RcW_D) to the anticlockwise protection
   ring tunnel (RaP_D); Node E also detects the failure and switches the
   traffic from the anticlockwise protection ring tunnel (RaP_D) back to
   the clockwise working ring tunnel (RcW_D).  A possible mechanism to
   mitigate the temporary loop problem is: the TTL of the ring tunnel
   label is set to 2*N by the ingress ring node of the traffic, where N
   is the number of nodes on the ring.

                         +---+#####[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    xxxxx Failure Nodes
                    ***** RcW_D             ##### RaP_D

                    Figure 6: Wrapping for Node Failure

4.3.2.  Short-Wrapping

   With the wrapping protection scheme, protection switching is executed
   at both nodes adjacent to 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.







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   With short-wrapping protection, protection switching is executed only
   at the node upstream to the failure, and the packet leaves the ring
   in the protection ring tunnel at the egress node.  This scheme can
   reduce the additional latency and bandwidth consumption when traffic
   is switched to the protection path.  However, the two directions of a
   protected bidirectional LSP are no longer co-routed under the
   protection-switching conditions.

   In the traditional wrapping solution, the protection ring tunnel is
   configured as a closed ring, while in the short-wrapping solution,
   the protection ring tunnel is configured as ended at the egress 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 egress nodes.  Figure 7
   shows the clockwise working ring tunnel and the anticlockwise
   protection ring tunnel with Node D as the egress node.

4.3.2.1.  Short-Wrapping for Link Failure

   As shown in Figure 7, in normal state, LSP1 is carried by the
   clockwise working ring tunnel (RcW_D) through the path A->B->C->D.
   When a link failure between Nodes B and C occurs, Node B switches the
   working ring tunnel RcW_D to the protection ring tunnel RaP_D in the
   opposite direction.  The difference with wrapping occurs in the
   protection ring tunnel at the egress node.  In short-wrapping
   protection, Rap_D ends in Node D, and then traffic will be forwarded
   based on the LSP labels.  Thus, with the short-wrapping mechanism,
   LSP1 will follow the path A->B->A->F->E->D when a link failure
   between Node B and Node C happens.  The protection switch at Node D
   is based on the information from its ring map and the information
   received via the RPS protocol.




















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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 |
                         +---+                   +---+

                   ----- Physical Links    xxxxx Failure Links
                   ***** RcW_D             ##### RaP_D

                 Figure 7: Short-Wrapping for Link Failure

4.3.2.2.  Short-Wrapping for Node Failure

   For the node failure that happens on a non-egress node, the short-
   wrapping protection switching is similar to the link failure case as
   described in the previous section.  This section specifies the
   scenario of an egress node failure.

   As shown in Figure 8, LSP1 enters the ring on Node A and leaves the
   ring on Node D.  In normal state, LSP1 is carried by the clockwise
   working ring tunnel (RcW_D) through the path A->B->C->D.  When Node D
   fails, the traffic of LSP1 cannot be protected by any ring tunnels
   that use Node D as the egress node.  The ingress node will update its
   ring map according to received RPS messages and determine that the
   egress node is not reachable; thus, it will not send traffic to
   either the working or the protection tunnel.  However, before the
   failure location information is propagated to all the ring nodes
   using the RPS protocol, Node C switches all the traffic on the
   working ring tunnel RcW_D to the protection ring tunnel RaP_D in the
   opposite direction based on the information in the ring map.  When
   the traffic arrives at Node E, which also detects the failure of Node
   D, the protection ring tunnel RaP_D cannot be used to forward traffic
   to Node D.  With the short-wrapping mechanism, protection switching
   can only be performed once from the working ring tunnel to the
   protection ring tunnel; thus, Node E MUST NOT switch the traffic that
   is already carried on the protection ring tunnel back to the working





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   ring tunnel in the opposite direction.  Instead, Node E will discard
   the traffic received on RaP_D locally.  This can avoid the temporary
   traffic loop when the failure happens on the egress node of the ring
   tunnel.  This also illustrates one of the benefits of having separate
   working and protection ring tunnels in each ring direction.

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

                   ----- Physical Links    xxxxx Failure Nodes
                   ***** RcW_D             ##### RaP_D

             Figure 8: Short-Wrapping for Egress Node Failure

4.3.3.  Steering

   With the steering protection mechanism, the ingress node (which adds
   traffic to the ring) performs switching from the working to the
   protection ring tunnel, and at the egress node, the traffic leaves
   the ring from the protection ring tunnel.

   When a failure occurs in the ring, the node that detects the failure
   with an OAM mechanism sends the failure information in the opposite
   direction of the failure hop by hop along the ring using an RPS
   request message and the ring-map information.  When a ring node
   receives the RPS message that identifies a failure, it can determine
   the location of the fault by using the topology information of the
   ring map and updating the ring map accordingly; then, it can
   determine whether the LSPs entering the ring locally need to switch
   over or not.  For LSPs that need to switch over, it will switch the
   LSPs from the working ring tunnels to their corresponding protection
   ring tunnels.






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4.3.3.1.  Steering for Link Failure

   Ring Map of F                                  +--LSP1
  +-+-+-+-+-+-+-+     +---+ ###[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| +---+                            +---+ +-- LSP2
 +-+-+-+-+-+-+-+ | 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|
 +-+-+-+-+-+-+-+ LSP1  +---+     [RaP_D(C)]    +---+    +-+-+-+-+-+-+-+
  |I|I|I|I|I|S|  LSP2                                    |S|I|I|I|I|I|
  +-+-+-+-+-+-+                                          +-+-+-+-+-+-+

                            ----- Physical Links
                            ***** RcW_D
                            ##### RaP_D
                               I: Intact
                               S: Severed

           Figure 9: Steering Operation and Protection Switching
                            When Link C-D Fails

   As shown in Figure 9, LSP1 enters the ring from Node A while LSP2
   enters the ring from Node B, and both of them have the same
   destination, which is Node D.

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

   LSP2 is carried by the clockwise working ring tunnel (RcW_D) through
   the path B->C->D, and the label operation is: [LSP2](Payload) ->
   [RcW_D(C)|LSP2](NodeB) -> [RcW_D(D)|LSP2](NodeC) -> [LSP2](Payload).






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   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 is 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
   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 anticlockwise
   direction.

   When Node A receives the failure report message and updates the link
   state of its ring map, it is aware that there is a fault on the
   clockwise working ring tunnel to Node D (RcW_D), and LSP1 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, and the label operation is: [LSP1](Payload) ->
   [RaP_D(F)| LSP1](NodeA) -> [RaP_D(E)|LSP1](NodeF) ->
   [RaP_D(D)|LSP1](NodeE) -> [LSP1](Payload).

   The same procedure also applies 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](Payload) -> [RaP_D(A)|LSP2](NodeB) -> [RaP_D(F)|LSP2](NodeA)
   -> [RaP_D(E)|LSP2](NodeF) -> [RaP_D(D)|LSP2](NodeE) ->
   [LSP2](Payload).

   Assume the link between Nodes A and B breaks down, as shown in
   Figure 10.  Similar to the above failure case, Node B will detect a
   fault in the link between A and B, and it will update its ring map,
   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 Nodes 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 determines
   that the working ring tunnel for LSP2 still works fine, and it will
   not perform the switchover.








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                                                   /+-- LSP1
+-+-+-+-+-+-+-+      +---+ ###[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     /+-- LSP2
+-+-+-+-+-+-+-+  +---+                             +---/ +-+-+-+-+-+-+-+
|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 10: Steering Operation and Protection Switching
                            When Link A-B Fails

4.3.3.2.  Steering for Node Failure

   For a node failure that happens on a non-egress node, steering
   protection switching is similar to the link failure case as described
   in the previous section.

   If the failure occurs at the egress node of the LSP, the ingress node
   will update its ring map according to the received RPS messages; it
   will also determine that the egress node is not reachable after the
   failure, thus it will not send traffic to either the working or the
   protection tunnel, and a traffic loop can be avoided.













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4.4.  Interconnected Ring Protection

4.4.1.  Interconnected Ring Topology

   Interconnected ring topology is widely used in MPLS-TP networks.  For
   a given ring, the interconnection node acts as the egress node for
   that ring, meaning that all LSPs using the interconnection node as an
   egress from one specific ring to another will use the same group of
   ring tunnels within the ring.  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 11 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 11: Single-Node Interconnected Rings

   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.

          Figure 12 shows the topology of dual-node interconnected
          rings.  Nodes C and D are the interconnection nodes between
          Ring1 and Ring2.





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

                 Figure 12: Dual-Node Interconnected Rings

4.4.2.  Interconnected Ring Protection Mechanisms

   Interconnected rings can be treated as two independent rings.  The
   RPS protocol operates on each ring independently.  A failure that
   happens in one ring only triggers protection switching in the ring
   itself and does not affect the other ring, unless the failure is on
   the interconnection node.  In this way, protection switching on each
   ring is the same as the mechanisms described in Section 4.3.

   The service LSPs that traverse the interconnected rings use the ring
   tunnels in each ring; within a given ring, the tunnel is selected
   using normal ring-selection procedures.  The traversing LSPs are
   stitched on the interconnection node.  On the interconnection node,
   the ring tunnel label of the source ring is popped, then LSP label is
   swapped; after that, the ring tunnel label of the destination ring is
   pushed.

   In the dual-node interconnected ring scenario, the two
   interconnection nodes can be managed as a virtual node group.  In
   addition to the ring tunnels to each physical ring node, each ring
   SHOULD assign the working and protection ring tunnels to the virtual
   interconnection node group.  In addition, on both nodes in the
   virtual interconnection node group, the same LSP label is assigned
   for each traversed LSP.  This way, any interconnection node in the
   virtual node group can terminate the working or protection ring
   tunnels targeted to the virtual node group and stitch the service LSP
   from the source ring tunnel to the destination ring tunnel.

   When the service LSP passes through the interconnected rings, the
   direction of the working ring tunnels used on both rings SHOULD be
   the same.  In dual-node interconnected rings, this ensures that in
   normal state the traffic passes only one of the two interconnection
   nodes and does not pass the link between the two interconnection




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   nodes.  The traffic will then only be switched to the protection path
   if the interconnection node that is in working path fails.  For
   example, if the service LSP uses the clockwise working ring tunnel on
   Ring1, when the service LSP leaves Ring1 and enters Ring2, the
   working ring tunnel used on Ring2 should also follow the clockwise
   direction.

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.  In addition, ring tunnels to the
   virtual interconnection node group are established on each ring of
   the interconnected rings, that is:

   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

   The ring tunnels to the virtual interconnection node group are shared
   by all LSPs that need to be forwarded to other rings.  These ring
   tunnels can terminate at any node in the virtual interconnection node
   group.

   For example, all the ring tunnels on Ring1 in Figure 13 are
   provisioned as follows:

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

   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 Nodes F and
      A): R1cW_F&A, R1aW_F&A, R1cP_F&A, R1aP_F&A



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   All the ring tunnels on Ring2 in Figure 13 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 Nodes F and
      A): R2cW_F&A, R2aW_F&A, R2cP_F&A, R2aP_F&A


































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                          +---+ccccccccccccc+---+
                          | 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
                          +---+aaaaaaaaaaaaa+---+
                  LSP1--->| D |-------------| C |
                          +---+ccccccccccccc+---+

                          Ring1:
                           ccccccccccc  R1cW_F&A
                           aaaaaaaaaaa  R1aP_F&A

                          Ring2:
                           ccccccccccc  R2cW_I
                           aaaaaaaaaaa  R2aP_I

           Figure 13: Ring Tunnels for the Interconnected Rings

4.4.4.  Interconnected Ring-Switching Procedure

   As shown in Figure 13, for the service LSP1 that enters Ring1 at Node
   D and leaves Ring1 at Node F and continues to enter Ring2 at Node F
   and leaves Ring2 at Node I, the short-wrapping protection scheme is
   described as below.







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

   In case of link failure, for example, when a failure occurs on the
   link between Nodes F and E, Node E will detect the failure and
   execute protection switching as described in Section 4.3.2.  The path
   that the service LSP1 follows after switching change to: LSP1->R1cW_F
   &A(D->E)->R1aP_F&A(E->D->C->B->A)->R2cW_I(A->F->G->H->I)->LSP1.

   In case of a non-interconnection node failure, for example, when the
   failure occurs at Node E in Ring1, Node D will detect the failure and
   execute protection switching as described in Section 4.3.2.  The path
   that the service LSP1 follows after switching becomes:
   LSP1->R1aP_F&A(D->C->B->A)->R2cW_I(A->F->G->H->I)->LSP1.

   In case of an interconnection node failure, for example, when the
   failure occurs at the interconnection Node F, Node E in Ring1 will
   detect the failure and execute protection switching as described in
   Section 4.3.2.  Node A in Ring2 will also detect the failure and
   execute protection switching as described in Section 4.3.2.  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)->R2aP_I(A->J->I)->LSP1.

4.4.5.  Interconnected Ring Detection Mechanism

   As shown in Figure 13, in normal state, the service traffic LSP1
   traverses D->E->F in Ring1 and F->G->H->I in Ring2.  Nodes A and F
   are the interconnection nodes.  When both links between Nodes F and G
   and between Nodes F and A fail, the ring tunnel from Node F to Node I
   in Ring2 becomes unreachable.  However, the other interconnection
   Node A is still available, and LSP1 can still reach Node I via Node
   A.

   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 the ring map
   and the fault location.  The ring map can be obtained from the
   Network Management System (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 hop by hop
   using the RPS protocol message.  When the interconnection node
   receives the message that informs the failure, it will calculate the



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   location of the fault according to 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 leave 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 F determines that the ring tunnel to Node I is
   unreachable; the service LSP1 for which the destination node on Ring2
   is Node I MUST switch to the protection ring tunnel (R1aP_F&A), and
   consequently, the service traffic LSP1 traverses the interconnected
   rings at Node A.  Node A will pop the ring tunnel label of Ring1 and
   push the ring tunnel label of Ring2 and send the traffic to Node I
   via the ring tunnel (R2aW_I).

5.  Ring Protection Coordination Protocol

5.1.  RPS and PSC Comparison on Ring Topology

   This section provides comparison between RPS and Protection State
   Coordination (PSC) [RFC6378] [RFC6974] on ring topologies.  This can
   be helpful to explain the reason of defining a new protocol for ring
   protection switching.

   The PSC protocol [RFC6378] is designed for point-to-point LSPs, on
   which the protection switching can only be performed on one or both
   of the endpoints of the LSP.  The RPS protocol is designed for ring
   tunnels, which consist of multiple ring nodes, and the failure could
   happen on any segment of the ring; thus, RPS is capable of
   identifying and handling the different failures on the ring and
   coordinating the protection-switching behavior of all the nodes on
   the ring.  As will be specified in the following sections, this is
   achieved with the introduction of the "pass-through" state for the
   ring nodes, and the location of the protection request is identified
   via the node IDs in the RPS request message.

   Taking a ring topology with N nodes as an example:

   With the mechanism specified in [RFC6974], on every ring node, a
   linear protection configuration has to be provisioned with every
   other node in the ring, i.e., with (N-1) other nodes.  This means
   that on every ring node there will be (N-1) instances of the PSC
   protocol.  And in order to detect faults and to transport the PSC
   message, each instance shall have a MEP on the working path and a MEP
   on the protection path, respectively.  This means that every node on
   the ring needs to be configured with (N-1) * 2 MEPs.





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   With the mechanism defined in this document, on every ring node there
   will only be a single instance of the RPS protocol.  In order to
   detect faults and to transport the RPS message, each node only needs
   to have a MEP on the section to its adjacent nodes, respectively.  In
   this way, every ring node only needs to be configured with 2 MEPs.

   As shown in the above example, RPS is designed for ring topologies
   and can achieve ring protection efficiently with minimum protection
   instances and OAM entities, which meets the requirements on topology-
   specific recovery mechanisms as specified in [RFC5654].

5.2.  RPS Protocol

   The RPS protocol defined in this section is used to coordinate the
   protection-switching action of all the ring nodes in the same ring.

   The protection operation of the ring tunnels is controlled with the
   help of the RPS protocol.  The RPS processes in each of the
   individual ring nodes that form the ring MUST communicate using the
   Generic Associated Channel (G-ACh).  The RPS protocol is applicable
   to all the three ring protection modes.  This section takes the
   short-wrapping mechanism described in Section 4.3.2 as an example.

   The RPS protocol is used to distribute the ring status information
   and RPS requests to all the ring nodes.  Changes in the ring status
   information and RPS requests can be initiated automatically based on
   link status or caused by external commands.

   Each node on the ring is uniquely identified by assigning it a node
   ID.  The node ID MUST be unique on each ring.  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 identify the source and destination
   nodes of each RPS request.

   Every node obtains the ring topology either by configuration or via
   some topology discovery mechanism.  The ring map consists of the ring
   topology information, and connectivity status (Intact or Severed)
   between the adjacent ring nodes, which is determined via the OAM
   message exchanged between the adjacent nodes.  The ring map is used
   by every ring node to determine the switchover behavior of the ring
   tunnels.









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   As shown in Figure 14, when no protection switching is active on the
   ring, each node MUST send RPS requests with No Request (NR) to its
   two adjacent nodes periodically.  The transmission interval of RPS
   requests is specified in Section 5.2.1.

                   +---+ 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 Failure in the Ring

   As shown in Figure 15, when a node detects a failure and determines
   that protection switching is required, it MUST send the appropriate
   RPS request in both directions to the destination node.  The
   destination node is the other node that is adjacent to the identified
   failure.  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 RPS protocol communication in
   the ring.  The RPS request MUST be terminated by the destination node
   of the message.  If an RPS request with the node itself set as the
   source node is received, this message MUST be dropped and not be
   forwarded to the next node.

                    +---+ 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

   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, each node detects the
      failure or receives the RPS request as the destination node 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 short-wrapping protection, each node
      detects the failure or receives the RPS request as the destination
      node MUST perform the switch only from the working ring tunnels to
      the protection ring tunnels.





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   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 MUST be performed by examining the RPS requests
      (indicating the nodes adjacent to the failure or failures) and the
      stored ring map (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 that generate the RPS requests MUST drop their
   respective switches and MUST generate an RPS request carrying the NR
   code.  The node receiving such an RPS request from both directions
   MUST drop its protection switches.

   A protection switch MUST be initiated by one of the criteria
   specified in Section 5.3.  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 link, and another protection switch is
   required due to a failure on another link.  Then an RPS request MUST
   be generated, the former protection switch MUST be dropped, and the
   latter protection switch established.

   The MPLS-TP Shared-Ring Protection mechanism supports multiple
   protection switches in the ring, resulting in 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 using
   their ring map to determine the state of the ring (nodes and links).
   In order to accommodate ring state knowledge, the RPS requests MUST
   be sent in both directions during a protection switch.

5.2.1.  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 a new RPS request MUST
   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.  A ring node that is not the
   destination of the received RPS message MUST forward it to the next
   node along the ring immediately.



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5.2.2.  RPS Protocol Data Unit (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.

       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|Version|   Reserved    |    RPS Channel Type (0x002A)  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Dest Node ID  | Src Node ID   |   Request     | M | 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
      the value of the node ID of the adjacent node.  The node ID MUST
      be unique on each ring.  Valid destination node ID values are
      1-127.

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

   o  Protection-Switching Mode (M): This 2-bit field indicates the
      protection-switching mode used by the sending node of the RPS
      message.  This can be used to check that the ring nodes on the
      same ring use the same protection-switching mechanism.  The
      defined values of the M field are listed as below:

             +------------------+-----------------------------+
             | Bits (MSB - LSB) |  Protection-Switching Mode  |
             +------------------+-----------------------------+
             |       0 0        |         Reserved            |
             |       0 1        |         Wrapping            |
             |       1 0        |       Short-Wrapping        |
             |       1 1        |         Steering            |
             +------------------+-----------------------------+

             Note:
             MSB = most significant bit
             LSB = least significant bit






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   o  RPS Request Code: A code consisting of 8 bits as specified below:

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

5.2.3.  Ring Node RPS States

   Idle state: A node is in the idle state when it has no RPS request
   and is sending and receiving an 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
   generated by it.  The pass-through is bidirectional.

5.2.3.1.  Idle State

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

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

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

5.2.3.2.  Switching State

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







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   In a bidirectional failure condition, both of the nodes adjacent to
   the failure detect the failure and send the RPS request in both
   directions with the destination set to each other; while each node
   can only receive the RPS request via the long path, the message sent
   via the short path will get lost due to the bidirectional failure.
   Here, the short path refers to the shorter path on the ring between
   the source and destination node of the RPS request, and the long path
   refers to the longer path on the ring between the source and
   destination node of the RPS request.  Upon receipt of the RPS request
   on the long path, the destination node of the RPS request MUST send
   an RPS request with its highest request code periodically along the
   long path to the other node adjacent to the failure.

   In a unidirectional failure condition, the node that detects the
   failure MUST send the RPS request in both directions with the
   destination node set to the other node adjacent to the failure.  The
   destination node of the RPS request cannot detect the failure itself
   but will receive an RPS request from both the short path and the long
   path.  The destination node MUST acknowledge the received RPS
   requests by replying with an RPS request with the RR code on the
   short path and an RPS request with the received RPS request code on
   the long path.  Accordingly, when the node that detects the failure
   receives the RPS request with RR code on the short path, then the RPS
   request received from the same node along the long path SHOULD be
   ignored.

   A node in the switching state MUST terminate the received RPS
   requests in both directions and not forward it further along the
   ring.

   The following switches as defined in Section 5.3.1 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 exist at the same time addressing
   different links and there is no higher-priority request on the ring,
   no switch SHOULD be executed and existing switches MUST be dropped.
   The nodes MUST still signal an RPS request with the MS code.

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




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

5.2.3.3.  Pass-Through State

   When a node is in a pass-through state, it MUST transfer the received
   RPS request unchanged 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.

5.2.4.  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.

   RPS requests due to a locally detected failure, an externally
   initiated command, or a received RPS request shall preempt existing
   RPS requests in the prioritized order given in Section 5.2.2, unless
   the requests are allowed to coexist.

5.2.4.1.  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 an
   RPS request with an NR code is received in both directions.  Then
   both directions revert simultaneously from the pass-through state to
   the idle state.

5.2.4.2.  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 is destined to this node

   o  An externally initiated command for this node




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   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 the
   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 the appropriate request but
      not execute the switch

   In one of the following conditions, transition from the switching
   state to the idle state MUST be triggered:

   o  On the node that triggers the protection switching, when the WTR
      time expires or an externally initiated command is cleared, the
      node MUST transit from switching state to Idle State and signal
      the NR code using RPS message in both directions.

   o  On the node that enters the switching state due to the received
      RPS request: 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.

5.2.4.3.  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 link, 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.






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

5.2.4.4.  Transitions between Switching and Pass-Through States

   When a node that is currently executing a switch receives an RPS
   request for a non-adjacent link 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

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

5.3.  RPS State Machine

5.3.1.  Switch Initiation Criteria

5.3.1.1.  Administrative Commands

   Administrative commands can be initiated by the network operator
   through the Network Management System (NMS).  The operator command
   may be transmitted to the appropriate node via the MPLS-TP RPS
   message.

   The following commands can be transferred by the RPS message:

   o  Lockout of Protection (LP): This command prevents any protection
      activity and prevents using ring switches anywhere in the ring.
      If any ring switches exist in the ring, this command causes the
      switches to drop.










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   o  Forced Switch (FS) to protection: This command performs the ring
      switch of normal traffic from the working entity to the protection
      entity for the link between the node at which the command is
      initiated and the adjacent node to which the command is directed.
      This switch occurs regardless of the state of the MPLS-TP section
      for the requested link, unless a higher-priority switch request
      exists.

   o  Manual Switch (MS) to protection: This command performs the ring
      switch of the normal traffic from the working entity to the
      protection entity for the link between the node at which the
      command is initiated and the adjacent node to which the command is
      directed.  This occurs if the MPLS-TP section for the requested
      link is not satisfying an equal or higher priority switch request.

   o  Exercise (EXER): This command exercises ring protection switching
      on the addressed link without completing the actual switch.  The
      command is issued and the responses (RRs) are checked, but no
      normal traffic is affected.

   The following commands are not transferred by the RPS message:

   o  Clear: This command clears the administrative command and WTR
      timer at the node to which the command was addressed.  The
      node-to-node signaling after the removal of the externally
      initiated commands is performed using the NR code.

   o  Lockout of Working (LW): This command prevents the normal traffic
      transported over the addressed link from being switched to the
      protection entity by disabling the node's capability of requesting
      a switch for this link in case of failure.  If any normal traffic
      is already switched on the protection entity, the switch is
      dropped.  If no other switch requests are active on the ring, the
      NR code is transmitted.  This command has no impact on any other
      link.  If the node receives the switch request from the adjacent
      node from any side, it will perform the requested switch.  If the
      node receives the switch request addressed to the other node, it
      will enter the pass-through state.

5.3.1.2.  Automatically Initiated Commands

   Automatically initiated commands can be initiated based on MPLS-TP
   section-layer OAM indication and the received switch requests.

   The node can initiate the following switch requests automatically:

   o  Signal Fail (SF): This command is issued when the MPLS-TP section-
      layer OAM detects a signal failure condition.



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   o  Wait-to-Restore (WTR): This command is issued when the MPLS-TP
      section detects that the SF condition has cleared.  It is used to
      maintain the state during the WTR period unless it is preempted by
      a higher-priority switch request.  The WTR time may be configured
      by the operator in 1 minute steps between 0 and 12 minutes; the
      default value is 5 minutes.

   o  Reverse Request (RR): This command is transmitted to the source
      node of the received RPS message over the short path as an
      acknowledgment for receiving the switch request.









































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5.3.2.  Initial States

   This section describes the possible states of a ring node, the
   corresponding action of the working and protection ring tunnels on
   the node, and the RPS request that should be generated in that state.

            +-----------------------------------+----------------+
            |        State                      |  Signaled RPS  |
            +-----------------------------------+----------------+
            |  A  |  Idle                       |  NR            |
            |     |  Working: no switch         |                |
            |     |  Protection: no switch      |                |
            +-----+-----------------------------+----------------+
            |  B  |  Pass-through               |  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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RFC 8227       MSRP Protection Mechanism for Ring Topology   August 2017


5.3.3.  State Transitions When Local Request Is Applied

   In the state description below, 'O' means that a new local request
   will be rejected because of an existing 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-through)     LP                C (Switching - LP)
                        LW                B (Pass-through)
                        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















Cheng, et al.                Standards Track                   [Page 40]

RFC 8227       MSRP Protection Mechanism for Ring Topology   August 2017


   =====================================================================
   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
                                             is a failure at this node
                                          B (Pass-through) - 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 link
                                          D (Idle - LW) - if on another
                                             link
                        FS                O - if on the same link
                                          E (Switching - FS) - if on
                                             another link
                        SF                O - if on the addressed link
                                          F (Switching - SF) - if on
                                             another link
                        Recover from SF   N/A
                        MS                O - if on the same link
                                          G (Switching - MS) - if on
                                             another link
                        Clear             A (Idle) - if there is no
                                             failure on addressed link
                                          F (Switching - SF) - if there
                                             is a failure on this link
                        WTR expires       N/A
                        EXER              O











Cheng, et al.                Standards Track                   [Page 41]

RFC 8227       MSRP Protection Mechanism for Ring Topology   August 2017


   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   E (Switching - FS)   LP                C (Switching - LP)
                        LW                O - if on another link
                                          D (Idle - LW) - if on the same
                                             link
                        FS                N/A - if on the same link
                                          E (Switching - FS) - if on
                                             another link
                        SF                O - if on the addressed link
                                          E (Switching - FS) - if on
                                             another link
                        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-through) - if there is
                                             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 link
                                          D (Idle - LW) - if on the same
                                             link
                        FS                E (Switching - FS)
                        SF                N/A - if on the same link
                                          F (Switching - SF) - if on
                                             another link
                        Recover from SF   H (Switching - WTR)
                        MS                O
                        Clear             N/A
                        WTR expires       N/A
                        EXER              O












Cheng, et al.                Standards Track                   [Page 42]

RFC 8227       MSRP Protection Mechanism for Ring Topology   August 2017


   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   G (Switching - MS)   LP                C (Switching - LP)
                        LW                O - if on another link
                                          D (Idle - LW) - if on the same
                                             link
                        FS                E (Switching - FS)
                        SF                F (Switching - SF)
                        Recover from SF   N/A
                        MS                N/A - if on the same link
                                          G (Switching - MS) - if on
                                             another link, 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
                        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 link
                                          I (Switching - EXER)
   =====================================================================








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RFC 8227       MSRP Protection Mechanism for Ring Topology   August 2017


5.3.4.  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-through)     LP                C (Switching - LP)
                        FS                N/A - cannot happen when there
                                                is an LP request in the
                                                ring
                                          E (Switching - FS) - otherwise
                        SF                N/A - cannot happen when there
                                                is an LP request in the
                                                ring
                                          F (Switching - SF) - otherwise
                        MS                N/A - cannot happen when there
                                                is an LP, FS, or SF
                                                request in the ring
                                          G (Switching - MS) - otherwise
                        WTR               N/A - cannot happen when there
                                                is an LP, FS, SF, or MS
                                                request in the ring
                        EXER              N/A - cannot happen when there
                                                is an LP, FS, SF, MS, or
                                                a WTR request in the
                                                ring
                                          I (Switching - EXER) -
                                                otherwise
                        RR                N/A
                        NR                A (Idle) - if received from
                                                     both sides







Cheng, et al.                Standards Track                   [Page 44]

RFC 8227       MSRP Protection Mechanism for Ring Topology   August 2017


   =====================================================================
   Initial state        New request       New state
   -------------        -----------       ---------
   C (Switching - LP)   LP                C (Switching - LP)

                        FS                N/A - cannot happen when there
                                                is an LP request in the
                                                ring
                        SF                N/A - cannot happen when there
                                                is an LP request in the
                                                ring
                        MS                N/A - cannot happen when there
                                                is an LP request in the
                                                ring
                        WTR               N/A
                        EXER              N/A - cannot happen when there
                                                is an 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 an FS request in the
                                                ring
                        WTR               N/A
                        EXER              N/A - cannot happen when there
                                                is an FS request in the
                                                ring
                        RR                E (Switching - FS)
                        NR                N/A





Cheng, et al.                Standards Track                   [Page 45]

RFC 8227       MSRP Protection Mechanism for Ring Topology   August 2017


   =====================================================================
   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 an SF request in the
                                                ring
                        WTR               N/A
                        EXER              N/A - cannot happen when there
                                                is an 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 an 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 a WTR request in the
                                                ring
                        RR                H (Switching - WTR)
                        NR                N/A









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RFC 8227       MSRP Protection Mechanism for Ring Topology   August 2017


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


5.3.5.  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-through)
                        FS                B (Pass-through)
                        SF                B (Pass-through)
                        MS                B (Pass-through)
                        WTR               B (Pass-through)
                        EXER              B (Pass-through)
                        RR                N/A
                        NR                N/A




















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RFC 8227       MSRP Protection Mechanism for Ring Topology   August 2017


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



Cheng, et al.                Standards Track                   [Page 48]

RFC 8227       MSRP Protection Mechanism for Ring Topology   August 2017


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






Cheng, et al.                Standards Track                   [Page 49]

RFC 8227       MSRP Protection Mechanism for Ring Topology   August 2017


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










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RFC 8227       MSRP Protection Mechanism for Ring Topology   August 2017


6.  IANA Considerations

   IANA has assigned the values listed in the sections below.

6.1.  G-ACh Channel Type

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

   IANA has allocated the following new G-ACh Channel Type in the "MPLS
   Generalized Associated Channel (G-ACh) Types (including Pseudowire
   Associated Channel Types)" registry:

      Value |          Description            | Reference
     -------+---------------------------------+--------------
     0x002A | Ring Protection Switching (RPS) | this document
            | Protocol                        |
     -------+---------------------------------+--------------

6.2.  RPS Request Codes

   IANA has created the subregistry "MPLS RPS Request Code Registry"
   under the "Generic Associated Channel (G-ACh) Parameters" registry.
   All code points within this registry shall be allocated according to
   the "Specification Required" procedure as specified in [RFC8126].

   The RPS request field is 8 bits; the allocated values are as follows:

      Value    Description                  Reference
      -------  ---------------------------  -------------
         0     No Request (NR)              this document
         1     Reverse Request (RR)         this document
         2     Unassigned
         3     Exercise (EXER)              this document
         4     Unassigned
         5     Wait-to-Restore (WTR)        this document
         6     Manual Switch (MS)           this document
        7-10   Unassigned
        11     Signal Fail (SF)             this document
        12     Unassigned
        13     Forced Switch (FS)           this document
        14     Unassigned
        15     Lockout of Protection (LP)   this document
      16-254   Unassigned
        255    Reserved






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RFC 8227       MSRP Protection Mechanism for Ring Topology   August 2017


7.  Operational Considerations

   This document describes three protection modes of the RPS protocol.
   Operators could choose the appropriate protection mode according to
   their network and service requirement.

   Wrapping mode provides a ring protection mechanism in which the
   protected traffic will reach every node of the ring and is applicable
   to protect both the point-to-point LSPs and LSPs that need to be
   dropped in several ring nodes, i.e., the point-to-multipoint
   applications.  When protection is inactive, the protected traffic is
   switched (wrapped) to/from the protection ring tunnel at both sides
   of the defective link/node.  Due to the wrapping, the additional
   propagation delay and bandwidth consumption of the protection tunnel
   are considerable.  For bidirectional LSPs, the protected traffic in
   both directions is co-routed.

   Short-wrapping mode provides a ring protection mechanism that can be
   used to protect only point-to-point LSPs.  When protection is
   inactive, the protected traffic is wrapped to the protection ring
   tunnel at the defective link/node and leaves the ring when the
   protection ring tunnel reaches the egress node.  Compared with the
   wrapping mode, short-wrapping can reduce the propagation latency and
   bandwidth consumption of the protection tunnel.  However, the two
   directions of a protected bidirectional LSP are not totally co-
   routed.

   Steering mode provides a ring protection mechanism that can be used
   to protect only point-to-point LSPs.  When protection is inactive,
   the protected traffic is switched to the protection ring tunnel at
   the ingress node and leaves the ring when the protection ring tunnel
   reaches the egress node.  The steering mode has the least propagation
   delay and bandwidth consumption of the three modes, and the two
   directions of a protected bidirectional LSP can be kept co-routed.

   Note that only one protection mode can be provisioned in the whole
   ring for all protected traffic.

8.  Security Considerations

   MPLS-TP is a subset of MPLS, thus it builds upon many of the aspects
   of the security model of MPLS.  Please refer to [RFC5920] for generic
   MPLS security issues and methods for securing traffic privacy and
   integrity.

   The RPS message defined in this document is used for protection
   coordination on the ring; if it is injected or modified by an
   attacker, the ring nodes might not agree on the protection action,



Cheng, et al.                Standards Track                   [Page 52]

RFC 8227       MSRP Protection Mechanism for Ring Topology   August 2017


   and the improper protection-switching action may cause a temporary
   break to services traversing the ring.  It is important that the RPS
   message is used within a trusted MPLS-TP network domain as described
   in [RFC6941].

   The RPS message is carried in the G-ACh [RFC5586], so it is dependent
   on the security of the G-ACh itself.  The G-ACh is a generalization
   of the Associated Channel defined in [RFC4385].  Thus, this document
   relies on the security mechanisms provided for the Associated Channel
   as described in those two documents.

   As described in the security considerations of [RFC6378], the G-ACh
   is essentially connection oriented, so injection or modification of
   control messages requires the subversion of a transit node.  Such
   subversion is generally considered hard in connection-oriented MPLS
   networks and impossible to protect against at the protocol level.
   Management-level techniques are more appropriate.  The procedures and
   protocol extensions defined in this document do not affect the
   security model of MPLS-TP linear protection as defined in [RFC6378].

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <https://www.rfc-editor.org/info/rfc3031>.

   [RFC4385]  Bryant, S., Swallow, G., Martini, L., and D. McPherson,
              "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
              Use over an MPLS PSN", RFC 4385, DOI 10.17487/RFC4385,
              February 2006, <https://www.rfc-editor.org/info/rfc4385>.

   [RFC4446]  Martini, L., "IANA Allocations for Pseudowire Edge to Edge
              Emulation (PWE3)", BCP 116, RFC 4446,
              DOI 10.17487/RFC4446, April 2006,
              <https://www.rfc-editor.org/info/rfc4446>.

   [RFC5586]  Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
              "MPLS Generic Associated Channel", RFC 5586,
              DOI 10.17487/RFC5586, June 2009,
              <https://www.rfc-editor.org/info/rfc5586>.



Cheng, et al.                Standards Track                   [Page 53]

RFC 8227       MSRP Protection Mechanism for Ring Topology   August 2017


   [RFC5654]  Niven-Jenkins, B., Ed., Brungard, D., Ed., Betts, M., Ed.,
              Sprecher, N., and S. Ueno, "Requirements of an MPLS
              Transport Profile", RFC 5654, DOI 10.17487/RFC5654,
              September 2009, <https://www.rfc-editor.org/info/rfc5654>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

   [RFC5920]  Fang, L., Ed., "Security Framework for MPLS and GMPLS
              Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,
              <https://www.rfc-editor.org/info/rfc5920>.

   [RFC6371]  Busi, I., Ed. and D. Allan, Ed., "Operations,
              Administration, and Maintenance Framework for MPLS-Based
              Transport Networks", RFC 6371, DOI 10.17487/RFC6371,
              September 2011, <https://www.rfc-editor.org/info/rfc6371>.

   [RFC6378]  Weingarten, Y., Ed., Bryant, S., Osborne, E., Sprecher,
              N., and A. Fulignoli, Ed., "MPLS Transport Profile (MPLS-
              TP) Linear Protection", RFC 6378, DOI 10.17487/RFC6378,
              October 2011, <https://www.rfc-editor.org/info/rfc6378>.

   [RFC6941]  Fang, L., Ed., Niven-Jenkins, B., Ed., Mansfield, S., Ed.,
              and R. Graveman, Ed., "MPLS Transport Profile (MPLS-TP)
              Security Framework", RFC 6941, DOI 10.17487/RFC6941, April
              2013, <https://www.rfc-editor.org/info/rfc6941>.

   [RFC6974]  Weingarten, Y., Bryant, S., Ceccarelli, D., Caviglia, D.,
              Fondelli, F., Corsi, M., Wu, B., and X. Dai,
              "Applicability of MPLS Transport Profile for Ring
              Topologies", RFC 6974, DOI 10.17487/RFC6974, July 2013,
              <https://www.rfc-editor.org/info/rfc6974>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.











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RFC 8227       MSRP Protection Mechanism for Ring Topology   August 2017


Acknowledgements

   The authors would like to thank Gregory Mirsky, Yimin Shen, Eric
   Osborne, Spencer Jackson, and Eric Gray for their valuable comments
   and suggestions.

Contributors

   The following people contributed significantly to the content of this
   document and should be considered co-authors:

   Kai Liu
   Huawei Technologies
   Email: alex.liukai@huawei.com

   Jia He
   Huawei Technologies
   Email: hejia@huawei.com

   Fang Li
   China Academy of Telecommunication Research MIIT
   China
   Email: lifang@catr.cn

   Jian Yang
   ZTE Corporation
   China
   Email: yang.jian90@zte.com.cn

   Junfang Wang
   Fiberhome Telecommunication Technologies Co., LTD.
   Email: wjf@fiberhome.com.cn

   Wen Ye
   China Mobile
   Email: yewen@chinamobile.com

   Minxue Wang
   China Mobile
   Email: wangminxue@chinamobile.com

   Sheng Liu
   China Mobile
   Email: liusheng@chinamobile.com

   Guanghui Sun
   Huawei Technologies
   Email: sunguanghui@huawei.com



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RFC 8227       MSRP Protection Mechanism for Ring Topology   August 2017


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


   Jie Dong
   Huawei Technologies

   Email: jie.dong@huawei.com





















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