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rfc:rfc8227

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.

Cheng, et al. Standards Track [Page 1] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

Copyright Notice

 Copyright (c) 2017 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Cheng, et al. Standards Track [Page 2] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

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

Cheng, et al. Standards Track [Page 3] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

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.

Cheng, et al. Standards Track [Page 4] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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.

Cheng, et al. Standards Track [Page 5] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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.

Cheng, et al. Standards Track [Page 6] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

                                            +-------------
                              +-------------|
                +-------------|             |
  ===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

Cheng, et al. Standards Track [Page 7] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

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

Cheng, et al. Standards Track [Page 8] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

                     +---+#############+---+
                     | F |-------------| A | +-- LSP2
                     +---+*************+---+
                     #/*                   *\#
                    #/*                     *\#
                   #/*                       *\#
                 +---+                     +---+
        LSP1 --+ | E |                     | B |+-- LSP3
                 +---+                     +---+
                   #\                       */#
                    #\                     */#
                     #\                   */#
                     +---+*************+---+
             LSP1 +--| D |-------------| C |
             LSP2    +---+#############+---+
             LSP3
  1. —- 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

Cheng, et al. Standards Track [Page 9] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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)]####+---+
  1. —- 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.

Cheng, et al. Standards Track [Page 10] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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

Cheng, et al. Standards Track [Page 11] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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

Cheng, et al. Standards Track [Page 12] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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)]####+---+
  1. —- 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).

Cheng, et al. Standards Track [Page 13] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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)]####+---+
  1. —- 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.

Cheng, et al. Standards Track [Page 14] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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.

Cheng, et al. Standards Track [Page 15] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

                       +---+#####[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 |
                       +---+                   +---+
  1. —- 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

Cheng, et al. Standards Track [Page 16] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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                   +---+
  1. —- 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.

Cheng, et al. Standards Track [Page 17] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

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|
+-+-+-+-+-+-+                                          +-+-+-+-+-+-+
  1. —- 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).

Cheng, et al. Standards Track [Page 18] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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.

Cheng, et al. Standards Track [Page 19] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

                                                 /+-- LSP1

+-+-+-+-+-+-+-+ +—+ ###[RaP_D(F)]#### +—/ +-+-+-+-+-+-+-+

FABCDEF F —————– A ABCDEFA

+-+-+-+-+-+-+-+ +—+ *[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.

Cheng, et al. Standards Track [Page 20] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

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.

Cheng, et al. Standards Track [Page 21] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

           +---+      +---+      +---+      +---+      +---+
           | 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

Cheng, et al. Standards Track [Page 22] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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

Cheng, et al. Standards Track [Page 23] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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

Cheng, et al. Standards Track [Page 24] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

                        +---+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.

Cheng, et al. Standards Track [Page 25] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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

Cheng, et al. Standards Track [Page 26] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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.

Cheng, et al. Standards Track [Page 27] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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.

Cheng, et al. Standards Track [Page 28] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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.

Cheng, et al. Standards Track [Page 29] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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.

Cheng, et al. Standards Track [Page 30] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

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

Cheng, et al. Standards Track [Page 31] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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.

Cheng, et al. Standards Track [Page 32] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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.

Cheng, et al. Standards Track [Page 33] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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

Cheng, et al. Standards Track [Page 34] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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.

Cheng, et al. Standards Track [Page 35] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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.

Cheng, et al. Standards Track [Page 36] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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.

Cheng, et al. Standards Track [Page 37] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

 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.

Cheng, et al. Standards Track [Page 38] RFC 8227 MSRP Protection Mechanism for Ring Topology August 2017

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      |                |
          +-----+-----------------------------+----------------+

Cheng, et al. Standards Track [Page 39] 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)
 =====================================================================

Cheng, et al. Standards Track [Page 43] 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

Cheng, et al. Standards Track [Page 46] 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

Cheng, et al. Standards Track [Page 47] 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
 =====================================================================

Cheng, et al. Standards Track [Page 50] 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

Cheng, et al. Standards Track [Page 51] 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>.

Cheng, et al. Standards Track [Page 54] 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

Cheng, et al. Standards Track [Page 55] 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

Cheng, et al. Standards Track [Page 56]

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