Dual-Homing Coordination for MPLS Transport Profile (MPLS-TP) Pseudowires Protection
draft-ietf-pals-mpls-tp-dual-homing-coordination-06

Network Working Group                                           W. Cheng
Internet-Draft                                                   L. Wang
Intended status: Standards Track                                   H. Li
Expires: October 28, 2017                                   China Mobile
                                                                 J. Dong
                                                     Huawei Technologies
                                                         A. D'Alessandro
                                                          Telecom Italia
                                                          April 26, 2017

     Dual-Homing Coordination for MPLS Transport Profile (MPLS-TP)
                         Pseudowires Protection
          draft-ietf-pals-mpls-tp-dual-homing-coordination-06

Abstract

   In some scenarios, MPLS Transport Profile (MPLS-TP) Pseudowires (PWs)
   (RFC 5921) may be statically configured, when a dynamic control plane
   is not available.  A fast protection mechanism for MPLS-TP PWs is
   needed to protect against the failure of an Attachment Circuit (AC),
   the failure of a Provider Edge (PE), or a failure in the Packet
   Switched Network (PSN).  The framework and typical scenarios of dual-
   homing PW local protection are described in [draft-ietf-pals-mpls-tp-
   dual-homing-protection].  This document proposes a dual-homing
   coordination mechanism for MPLS-TP PWs, which is used for state
   exchange and switchover coordination between the dual-homing PEs for
   dual-homing PW local protection.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

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

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

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

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

   This Internet-Draft will expire on October 28, 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
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Overview of the Proposed Solution . . . . . . . . . . . . . .   3
   3.  Protocol Extensions for Dual-Homing MPLS-TP PW Protection . .   4
     3.1.  Information Exchange Between Dual-Homing PEs  . . . . . .   5
     3.2.  Protection Procedures . . . . . . . . . . . . . . . . . .   9
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   6.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  14
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   [RFC6372], [RFC6378] and [RFC7771] describe the framework and
   mechanism of MPLS Transport Profile (MPLS-TP) linear protection,
   which can provide protection for the MPLS Label Switched Path (LSP)
   and Pseudowires (PWs) between the edge nodes.  These mechanisms
   cannot protect the failure of the Attachment Circuit (AC) or the edge
   nodes.  [RFC6718] and [RFC6870] specifies the PW redundancy framework
   and mechanism for protecting the AC or edge node failure by adding
   one or more edge nodes, but it requires PW switchover in case of an
   AC failure, also PW redundancy relies on Packet Switched Network
   (PSN) protection mechanisms to protect the failure of PW.

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   In some scenarios such as mobile backhauling, the MPLS PWs are
   provisioned with dual-homing topology, in which at least the CE node
   on one side is dual-homed to two Provider Edge (PE) nodes.  If a
   failure occurs in the primary AC, operators usually prefer to perform
   local switchover in the dual-homing PE side and keep the working
   pseudowire unchanged if possible.  This is to avoid massive PW
   switchover in the mobile backhaul network due to the AC failure in
   the mobile core site, which may in turn lead to congestion due to the
   migration of traffic from the paths preferred by the network
   planners.  Similarly, as multiple PWs share the physical AC in the
   mobile core site, it is preferable to keep using the working AC when
   one working PW fails in the PSN network, which could avoid
   unnecessary switchover for other PWs.  A fast dual-homing PW
   protection mechanism is needed to protect the failure in AC, the PE
   node and the PSN network to meet the above requirements.

   [I-D.ietf-pals-mpls-tp-dual-homing-protection] describes a framework
   and several scenarios of dual-homing PW local protection.  This
   document proposes a dual-homing coordination mechanism for static
   MPLS-TP PWs, which is used for information exchange and switchover
   coordination between the dual-homing PEs for the dual-homing PW local
   protection.  The proposed mechanism has been implemented and deployed
   in several mobile backhaul networks which use static MPLS-TP PWs for
   the backhauling of mobile traffic from the radio access sites to the
   core site.

2.  Overview of the Proposed Solution

   Linear protection mechanisms for MPLS-TP network are defined in
   [RFC6378], [RFC7271] and [RFC7324].  When such mechanisms are applied
   to PW linear protection [RFC7771], both the working PW and the
   protection PW are terminated on the same PE node.  In order to
   provide dual-homing protection for MPLS-TP PWs, some additional
   mechanisms are needed.

   In MPLS-TP PW dual-homing protection, the linear protection mechanism
   as defined in [RFC6378] [RFC7271] and [RFC7324] on the single-homing
   PE (e.g.  PE3 in Figure 1) is not changed, while on the dual-homing
   side, the working PW and protection PW are terminated on two dual-
   homing PEs (e.g.  PE1 and PE2 in Figure 1) respectively to protect a
   failure occuring in a PE or a connected AC.  As described in
   [I-D.ietf-pals-mpls-tp-dual-homing-protection], a dedicated Dual-Node
   Interconnection (DNI) PW is used between the two dual-homing PE nodes
   to forward the traffic.  In order to utilize the linear protection
   mechanism [RFC7771] in the dual-homing PEs scenario, coordination
   between the dual-homing PE nodes is needed, so that the dual-homing
   PEs can switch the connection between the AC, the service PW and the
   DNI-PW properly in a coordinated fashion by the forwarder.

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         +----------------------------------+
         |                PE1               |
         +----------------------------------+             +----+
         |                 |                |   Working   |    |
         X    Forwarder    +     Service    X-------------X    |
        /|                 |       PW       | Service PW1 |    |
   AC1 / +--------+--------+                |             |    |
      /  |     DNI PW      |                |             |    |
 +---*   +--------X--------+----------------+             |    |   +---+
 |   |            ^                                       |    |   |   |
 |CE1|            |  DNI PW                               |PE3 +---|CE2|
 |   |            |                                       |    |   |   |
 |   |            V                                       |    |   |   |
 +---*   +--------X--------+----------------+             |    |   +---+
      \  |     DNI PW      |                |             |    |
   AC2 \ +--------+--------+                | Protection  |    |
        \|                 |     Service    X-------------X    |
         X    Forwarder    +       PW       | Service PW2 |    |
         |                 |                |             +----+
         +----------------------------------+
         |                PE2               |
         +----------------------------------+

       Figure 1. Dual-homing Protection with DNI-PW

3.  Protocol Extensions for Dual-Homing MPLS-TP PW Protection

   In dual-homing MPLS-TP PW local protection, the forwarding state of
   the dual-homing PEs are determined by the forwarding state machine in
   Table 1.

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          +-----------+---------+--------+---------------------+
          |Service PW |   AC    | DNI PW | Forwarding Behavior |
          +-----------+---------+--------+---------------------+
          |  Active   | Active  |   Up   |Service PW <-> AC    |
          +-----------+---------+--------+---------------------+
          |  Active   | Standby |   Up   |Service PW <-> DNI PW|
          +-----------+---------+--------+---------------------+
          |  Standby  | Active  |   Up   |    DNI PW <-> AC    |
          +-----------+---------+--------+---------------------+
          |  Standby  | Standby |   Up   |  Drop all packets   |
          +-----------+---------+--------+---------------------+
          |  Active   | Active  |  Down  |Service PW <-> AC    |
          +-----------+---------+--------+---------------------+
          |  Active   | Standby |  Down  |  Drop all packets   |
          +-----------+---------+--------+---------------------+
          |  Standby  | Active  |  Down  |  Drop all packets   |
          +-----------+---------+--------+---------------------+
          |  Standby  | Standby |  Down  |  Drop all packets   |
          +-----------+---------+--------+---------------------+
             Table 1. Dual-homing PE Forwarding State Machine

   In order to achieve the dual-homing MPLS-TP PW protection,
   coordination between the dual-homing PE nodes is needed to exchange
   the PW status and protection coordination requests.

3.1.  Information Exchange Between Dual-Homing PEs

   The coordination information will be sent on the DNI PW over the
   Generic Associated Channel (G-ACh) as described in [RFC5586].  A new
   G-ACh channel type is defined for the dual-homing coordination
   between the dual-homing PEs of MPLS-TP PWs.  This channel type can be
   used for the exchange of different types of information between the
   dual-homing PEs.  This document uses this channel type for the
   exchange of PW status and switchover coordination between the dual-
   homing PEs.  Other potential usages of this channel type are for
   further study and are out of the scope of this document.

   The MPLS-TP Dual-Homing Coordination (DHC) message is sent on the DNI
   PW between the dual-homing PEs.  The format of the MPLS-TP DHC
   message is shown below:

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   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    |         DHC Channel Type      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Dual-Homing PEs Group ID                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         TLV  Length           |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                              TLVs                             ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          Figure 2. MPLS-TP Dual-Homing Coordination Message

   The first 4-octets is the common G-ACh header as specified in
   [RFC5586].  The DHC Channel Type is the G-ACh channel type code point
   to be assigned by IANA.

   The Dual-Homing Group ID is a 4-octet unsigned integer to identify
   the dual-homing group which the dual-homing PEs belong to.  It MUST
   be the same at both PEs in the same group.

   The TLV Length field specifies the total length in octets of the
   subsequent TLVs.

   In this document, two TLVs are defined in MPLS-TP Dual-Homing
   Coordination message for dual-homing MPLS-TP PW protection:

   Type        Description                Length
    1           PW Status                 20 Bytes
    2        Dual-Node Switching         16 Bytes

   The PW Status TLV is used by a dual-homing PE to report its service
   PW status to the other dual-homing PE in the same dual-homing group.

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      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type=1 (PW Status)         |          Length              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Destination Dual-homing PE Node_ID               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                Source Dual-homing PE Node_ID                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         DNI PW-ID                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Flags                               |P|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Service PW Status                    |D|F|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                       Figure 3. PW Status TLV

   - The Length field specifies the length in octets of the value field
   of the TLV.

   - The Destination Dual-homing PE Node_ID is the 32-bit identifier of
   the receiver PE [RFC6370] which supports both IPv4 and IPv6
   environments.  Usually it is the same as the LSR-ID of the receiver
   PE.

   - The Source Dual-homing PE Node_ID is the 32-bit identifier of the
   sending PE [RFC6370] which supports both IPv4 and IPv6 environments.
   Usually it is the same as the LSR-ID of the sending PE.

   - The DNI PW-ID field contains the 32-bit PW ID [RFC4447] of the DNI
   PW.

   - The Flags field contains 32 bit flags, in which:

   o  The P (Protection) bit indicates whether the Source Dual-homing PE
      is the working PE (P=0) or the protection PE (P=1).

   o  Other bits are reserved for future use, which MUST be set to 0 on
      transmission and MUST be ignored upon receipt.

   - The Service PW Status field indicates the status of the Service PW
   between the sending PE and the remote PE.  Currently two bits are
   defined in the Service PW Status field:

   o  F bit: If set, it indicates Signal Fail (SF) [RFC6378] on the
      service PW.  It can be either a local request generated by the PE
      itself or a remote request received from the remote PE.

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   o  D bit: If set, it indicates Signal Degrade (SD) [RFC6378] on the
      service PW.  It can be either a local request or a remote request
      received from the remote PE.

   o  Other bits are reserved for future use, which MUST be set to 0 on
      transmission and MUST be ignored upon receipt.

   The Dual-Node Switching TLV is used by one dual-homing PE to send
   protection state coordination to the other PE in the same dual-homing
   group.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Type=2 (Dual-Node Switching) |          Length               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Destination Dual-homing PE Node_ID               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                Source Dual-homing PE Node_ID                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         DNI PW-ID                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Flags                            |S|P|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      Figure 4. Dual-Node Switching TLV

   - The Length field specifies the length in octets of the value field
   of the TLV.

   - The Destination Dual-homing PE Node_ID is the 32-bit identifier of
   the receiver PE [RFC6370].  Usually it is the same as the LSR-ID of
   the receiver PE.

   - The Source Dual-homing PE Node_ID is the 32-bit identifier of the
   sending PE [RFC6370].  Usually it is the same as the LSR-ID of the
   sending PE.

   - The DNI PW-ID field contains the 32-bit PW-ID [RFC4447] of the DNI
   PW.

   - The Flags field contains 32 bit flags, in which:

   o  The P (Protection) bit indicates whether the Source Dual-homing PE
      is the working PE or the protection PE.  It is set to 1 when the
      Source PE of the dual-node switching request is the protection PE.

   o  The S (PW Switching) bit indicates which service PW is used for
      forwarding traffic.  It is set to 0 when traffic will be

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      transported on the working PW, and is set to 1 if traffic will be
      transported on the protection PW.  The value of the S bit is
      determined by the protection coordination mechanism between the
      dual-homing PEs and the remote PE.

   o  Other bits are reserved for future use, which MUST be set to 0 on
      transmission and MUST be ignored upon receipt.

   When a change of the service PW status is detected by one of the
   dual-homing PEs, it MUST be reflected in the PW Status TLV and sent
   to the other dual-homing PE as quickly as possible to allow for fast
   protection switching using 3 consecutive DHC messages.  This set of
   three messages allows for fast protection switching even if one or
   two of these packets are lost or corrupted.  After the transmission
   of the three rapid messages, the dual-homing PE MUST send the most
   recently transmitted service PW status periodically to the other
   dual-homing PE on a continual basis using the DHC message.

   When one dual-homing PE determines that the active service PW needs
   to be switched from the working PW to the protection PW, It MUST send
   the Dual-Node Switching TLV to the other dual-homing PE as quickly as
   possible to allow for fast protection switching using 3 consecutive
   DHC messages.  After the transmission of the three messages, the
   protection PW would become the active service PW, and the dual-homing
   PE MUST send the most recently transmitted Dual-Node Switching TLV
   periodically to the other dual-homing PE on a continual basis using
   the DHC message.

   It is RECOMMENDED that the default interval of the first three rapid
   DHC messages is 3.3 ms similar to [RFC6378], and the default interval
   of the subsequent messages is 1 second.  Both the default interval of
   the three consecutive messages as well as the default interval of the
   periodical messages SHALL be configurable by the operator.

3.2.  Protection Procedures

   The dual-homing MPLS-TP PW protection mechanism can be deployed with
   the existing AC redundancy mechanisms.  On the PSN network side, PSN
   tunnel protection mechanism is not required, as the dual-homing PW
   protection can also protect if a failure occurs in the PSN network.

   This section uses the one-side dual-homing scenario as an example to
   describe the dual-homing PW protection procedures, the procedures for
   two-side dual-homing scenario would be similar.

   On the dual-homing PE side, the role of working and protection PE are
   set by the management system or local configuration.  The service PW

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   connecting to the working PE is the working PW, and the service PW
   connecting to the protection PE is called the protection PW.

   On the single-homing PE side, it treats the working PW and protection
   PW as if they terminate on the same remote PE node, thus normal MPLS-
   TP protection coordination procedures still apply on the single-
   homing PE.

   The forwarding behavior of the dual-homing PEs is determined by the
   components shown in the figure below:

             +---------------------------------+          +-----+
             |        PE1 (Working PE)         |          |     |
             +---------------------------------+    PW1   |     |
             |                 |               |  Working |     |
             +    Forwarder    +    Service    X<-------->X     |
            /|                 |      PW       |          |     |
           / +--------+--------+               |          |     |
     AC1  /  |     DNI PW      |               |          |     |
         /   +--------X--------+---------------+          |     |
 +-----+/   AC        ^    DNI PW                         |     |  +---+
 | CE1 |redundancy    |                                   | PE3 +--|CE2|
 +-----+ mechanism    |  DHC message                      |     |  +---+
        \             V   exchange                        |     |
     AC2 \   +--------X--------+---------------+          |     |
          \  |     DNI PW      |               |          |     |
           \ +--------+--------+               |    PW2   |     |
            \|                 |     Service   |Protection|     |
             +    Forwarder    +       PW      X<-------->X     |
             |                 |               |   PSC    |     |
             +---------------------------------+  message |     |
             |        PE2 (Protection PE)      | exchange |     |
             +---------------------------------+          +-----+
          Figure 5. Components of one-side dual-homing PW protection

   In Figure 5, for each dual-homing PE, the service PW is the PW used
   to carry service between the dual-homing PE and the remote PE.  The
   state of the service PW is determined by the Operation Administration
   and Maintanence (OAM) mechanisms between the dual-homing PEs and the
   remote PE.

   The DNI PW is provisioned between the two dual-homing PE nodes.  It
   is used to bridge traffic when a failure occurs in the PSN network or
   in the ACs.  The state of the DNI PW is determined by the OAM
   mechanism between the dual-homing PEs.  Since the DNI PW is used to
   carry both the DHC messages and the service traffic during protection
   switching, it is important to ensure the robustness of the DNI PW.
   In order to avoid the DNI PW failure due to the failure of a

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   particular link, it is RECOMMENDED that multiple diverse links be
   deployed between the dual-homing PEs and the underlay LSP protection
   mechanism SHOULD be enabled.

   The AC is the link which connects a dual-homing PE to the dual-homed
   CE.  The status of AC is determined by the existing AC redundancy
   mechanisms, this is out of the scope of this document.

   In order to perform dual-homing PW local protection, the service PW
   status and Dual-node switching coordination requests are exchanged
   between the dual-homing PEs using the DHC message defined in
   Section 3.1.

   Whenever a change of service PW status is detected by a dual-homing
   PE, it MUST be reflected in the PW Status TLV and sent to the other
   dual-homing PE immediately using the 3 consecutive DHC messages.
   After the transmission of the three rapid messages, the dual-homing
   PE MUST send the most recently transmitted service PW status
   periodically to the other dual-homing PE on a continual basis using
   the DHC message.  This way, both dual-homing PEs have the status of
   the working and protection PW consistently.

   When there is a switchover request either generated locally or
   received on the protection PW from the remote PE, based on the status
   of the working and protection service PW, along with the local and
   remote request of the protection coordination between the dual-homing
   PEs and the remote PE, the active/standby state of the service PW can
   be determined by the dual-homing PEs.  As the remote protection
   coordination request is transmitted over the protection path, in this
   case the active/standby status of the service PW is determined by the
   protection PE in the dual-homing group.

   If it is determined on one dual-homing PE that switchover of service
   PW is needed, this dual-homing PE MUST set the S bit in the Dual-Node
   Switching TLV and send it to the other dual-homing PE immediately
   using the 3 consecutive DHC messages.  With the exchange of service
   PW status and the switching request, both dual-homing PEs are
   consistent on the Active/Standby forwarding status of the working and
   protection service PWs.  The status of the DNI PW is determined by PW
   OAM mechanism as defined in [RFC5085], and the status of ACs are
   determined by existing AC redundancy mechanisms, both are out of the
   scope of this document.  The forwarding behavior on the dual-homing
   PE nodes is determined by the forwarding state machine as shown in
   Table 1 .

   Using the topology in Figure 5 as an example, in normal state, the
   working PW (PW1) is in active state, the protection PW (PW2) is in
   standby state, the DNI PW is up, and AC1 is in active state according

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   to the AC redundancy mechanism.  According to the forwarding state
   machine in Table 1, traffic will be forwarded through the working PW
   (PW1) and the primary AC (AC1).  No traffic will go through the
   protection PE (PE2) or the DNI PW, as both the protection PW (PW2)
   and the AC connecting to PE2 are in standby state.

   If a failure occurs in AC1, the state of AC2 changes to active
   according to the AC redundancy mechanism, while there is no change in
   the state of the working and protection PWs.  According to the
   forwarding state machine in Table 1, PE1 starts to forward traffic
   between the working PW and the DNI PW, and PE2 starts to forward
   traffic between AC2 and the DNI PW.  It should be noted that in this
   case only AC switchover takes place, in the PSN network traffic is
   still forwarded using the working PW.

   If a failure in the PSN network brings PW1 down, the failure can be
   detected by PE1 or PE3 using existing OAM mechanisms.  If PE1 detects
   the failure of PW1, it MUST inform PE2 the state of working PW using
   the PW Status TLV in the DHC messages and change the forwarding
   status of PW1 to standby.  On receipt of the DHC message, PE2 SHOULD
   change the forwarding status of PW2 to active.  Then according to the
   forwarding state machine in Table 1, PE1 SHOULD set up the connection
   between the DNI PW and AC1, and PE2 SHOULD set up the connection
   between PW2 and the DNI PW.  According to the linear protection
   mechanism [RFC6378], PE2 also sends an appropriate protection
   coordination message [RFC6378] over the protection PW (PW2) to PE3
   for the remote side to switchover from PW1 to PW2.  If PE3 detects
   the failure of PW1, according to linear protection mechanism
   [RFC6378], it sends a protection coordination message on the
   protection PW (PW2) to inform PE2 of the failure on the working PW.
   Upon receipt of the message, PE2 SHOULD change the forwarding status
   of PW2 to active and set up the connection according to the
   forwarding state machine in Table 1.  PE2 SHOULD send a DHC message
   to PE1 with the S bit set in the Dual-Node Switching TLV to
   coordinate the switchover on PE1 and PE2.  This is useful for a
   unidirectional failure which cannot be detected by PE1.

   If a failure brings the working PE (PE1) down, the failure can be
   detected by both PE2 and PE3 using existing OAM mechanisms.  Both PE2
   and PE3 SHOULD change the forwarding status of PW2 to active, and
   send a protection coordination message [RFC6378] on the protection PW
   (PW2) to inform the remote side to switchover.  According to the
   existing AC redundancy mechanisms, the status of AC1 changes to
   standby, and the state of AC2 changes to active.  According to the
   forwarding state machine in Table 1, PE2 starts to forward traffic
   between the PW2 and AC2.

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4.  IANA Considerations

   This document requests that IANA assigns one new channel type for
   "MPLS-TP Dual-Homing Coordination message" from the "MPLS Generalized
   Associated Channel (G-ACh) Types (including Pseudowire Associated
   Channel Types)" registry of the "Generic Associated Channel (G-ACh)
   Parameters" registry.

   Value  Description                                Reference
   TBD    MPLS-TP Dual-Homing Coordination message   [This document]

   This document requests that IANA creates a new sub-registry called
   "MPLS-TP DHC TLVs" in the "Generic Associated Channel (G-ACh)
   Parameters" registry, with fields and initial allocations as follows:

   Type        Description                 Length       Reference
   0x00        Reserved
   0x01        PW Status                   20 Bytes     [this document]
   0x02        Dual-Node Switching         16 Bytes     [this document]

   The allocation policy for this registry is IETF Review as specified
   in [RFC5226].

5.  Security Considerations

   MPLS-TP is a subset of MPLS and so 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 DHC message defined in this document contains control
   information, if it is injected or modified by an attacker, the dual-
   homing PEs might not agree on which PE should be used to deliver the
   CE traffic, and this could be used as a denial of service attack
   against the CE.  It is important that the DHC message is used within
   a trusted MPLS-TP network domain as described in [RFC6941].

   The DHC 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.

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   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].

   Uniqueness of the identifiers defined in this document is guaranteed
   by the assigner (e.g. the operator).  Failure by an assigner to use
   unique values within the specified scoping for any of the identifiers
   defined herein could result in operational problems.  Please refer to
   [RFC6370] for more details about the uniqueness of the identifiers.

6.  Contributors

   The following individuals substantially contributed to the content of
   this document:

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

   Shahram Davari
   Broadcom Corporation
   davari@broadcom.com

7.  References

7.1.  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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4447]  Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., and
              G. Heron, "Pseudowire Setup and Maintenance Using the
              Label Distribution Protocol (LDP)", RFC 4447,
              DOI 10.17487/RFC4447, April 2006,
              <http://www.rfc-editor.org/info/rfc4447>.

   [RFC5085]  Nadeau, T., Ed. and C. Pignataro, Ed., "Pseudowire Virtual
              Circuit Connectivity Verification (VCCV): A Control
              Channel for Pseudowires", RFC 5085, DOI 10.17487/RFC5085,
              December 2007, <http://www.rfc-editor.org/info/rfc5085>.

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

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   [RFC6370]  Bocci, M., Swallow, G., and E. Gray, "MPLS Transport
              Profile (MPLS-TP) Identifiers", RFC 6370,
              DOI 10.17487/RFC6370, September 2011,
              <http://www.rfc-editor.org/info/rfc6370>.

   [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, <http://www.rfc-editor.org/info/rfc6378>.

   [RFC7271]  Ryoo, J., Ed., Gray, E., Ed., van Helvoort, H.,
              D'Alessandro, A., Cheung, T., and E. Osborne, "MPLS
              Transport Profile (MPLS-TP) Linear Protection to Match the
              Operational Expectations of Synchronous Digital Hierarchy,
              Optical Transport Network, and Ethernet Transport Network
              Operators", RFC 7271, DOI 10.17487/RFC7271, June 2014,
              <http://www.rfc-editor.org/info/rfc7271>.

   [RFC7324]  Osborne, E., "Updates to MPLS Transport Profile Linear
              Protection", RFC 7324, DOI 10.17487/RFC7324, July 2014,
              <http://www.rfc-editor.org/info/rfc7324>.

7.2.  Informative References

   [I-D.ietf-pals-mpls-tp-dual-homing-protection]
              Cheng, W., Wang, L., Li, H., Davari, S., and J. Dong,
              "Dual-Homing Protection for MPLS and MPLS-TP Pseudowires",
              draft-ietf-pals-mpls-tp-dual-homing-protection-05 (work in
              progress), January 2017.

   [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, <http://www.rfc-editor.org/info/rfc4385>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

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

   [RFC6372]  Sprecher, N., Ed. and A. Farrel, Ed., "MPLS Transport
              Profile (MPLS-TP) Survivability Framework", RFC 6372,
              DOI 10.17487/RFC6372, September 2011,
              <http://www.rfc-editor.org/info/rfc6372>.

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   [RFC6718]  Muley, P., Aissaoui, M., and M. Bocci, "Pseudowire
              Redundancy", RFC 6718, DOI 10.17487/RFC6718, August 2012,
              <http://www.rfc-editor.org/info/rfc6718>.

   [RFC6870]  Muley, P., Ed. and M. Aissaoui, Ed., "Pseudowire
              Preferential Forwarding Status Bit", RFC 6870,
              DOI 10.17487/RFC6870, February 2013,
              <http://www.rfc-editor.org/info/rfc6870>.

   [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, <http://www.rfc-editor.org/info/rfc6941>.

   [RFC7771]  Malis, A., Ed., Andersson, L., van Helvoort, H., Shin, J.,
              Wang, L., and A. D'Alessandro, "Switching Provider Edge
              (S-PE) Protection for MPLS and MPLS Transport Profile
              (MPLS-TP) Static Multi-Segment Pseudowires", RFC 7771,
              DOI 10.17487/RFC7771, January 2016,
              <http://www.rfc-editor.org/info/rfc7771>.

Authors' Addresses

   Weiqiang Cheng
   China Mobile
   No.32 Xuanwumen West Street
   Beijing  100053
   China

   Email: chengweiqiang@chinamobile.com

   Lei Wang
   China Mobile
   No.32 Xuanwumen West Street
   Beijing  100053
   China

   Email: Wangleiyj@chinamobile.com

   Han Li
   China Mobile
   No.32 Xuanwumen West Street
   Beijing  100053
   China

   Email: Lihan@chinamobile.com

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   Jie Dong
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing  100095
   China

   Email: jie.dong@huawei.com

   Alessandro D'Alessandro
   Telecom Italia
   via Reiss Romoli, 274
   Torino  10148
   Italy

   Email: alessandro.dalessandro@telecomitalia.it

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