RFC 6372

Internet Engineering Task Force (IETF)                  N. Sprecher, Ed.
Request for Comments: 6372                        Nokia Siemens Networks
Category: Informational                                   A. Farrel, Ed.
ISSN: 2070-1721                                         Juniper Networks
                                                          September 2011

        MPLS Transport Profile (MPLS-TP) Survivability Framework


   Network survivability is the ability of a network to recover traffic
   delivery following failure or degradation of network resources.
   Survivability is critical for the delivery of guaranteed network
   services, such as those subject to strict Service Level Agreements
   (SLAs) that place maximum bounds on the length of time that services
   may be degraded or unavailable.

   The Transport Profile of Multiprotocol Label Switching (MPLS-TP) is a
   packet-based transport technology based on the MPLS data plane that
   reuses many aspects of the MPLS management and control planes.

   This document comprises a framework for the provision of
   survivability in an MPLS-TP network; it describes recovery elements,
   types, methods, and topological considerations.  To enable data-plane
   recovery, survivability may be supported by the control plane,
   management plane, and by Operations, Administration, and Maintenance
   (OAM) functions.  This document describes mechanisms for recovering
   MPLS-TP Label Switched Paths (LSPs).  A detailed description of
   pseudowire recovery in MPLS-TP networks is beyond the scope of this

   This document is a product of a joint Internet Engineering Task Force
   (IETF) / International Telecommunication Union Telecommunication
   Standardization Sector (ITU-T) effort to include an MPLS Transport
   Profile within the IETF MPLS and Pseudowire Emulation Edge-to-Edge
   (PWE3) architectures to support the capabilities and functionalities
   of a packet-based transport network as defined by the ITU-T.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents

Sprecher & Farrel             Informational                     [Page 1]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1. Introduction ....................................................4
      1.1. Recovery Schemes ...........................................4
      1.2. Recovery Action Initiation .................................5
      1.3. Recovery Context ...........................................6
      1.4. Scope of This Framework ....................................7
   2. Terminology and References ......................................8
   3. Requirements for Survivability .................................10
   4. Functional Architecture ........................................10
      4.1. Elements of Control .......................................10
           4.1.1. Operator Control ...................................11
           4.1.2. Defect-Triggered Actions ...........................12
           4.1.3. OAM Signaling ......................................12
           4.1.4. Control-Plane Signaling ............................12
      4.2. Recovery Scope ............................................13
           4.2.1. Span Recovery ......................................13
           4.2.2. Segment Recovery ...................................13
           4.2.3. End-to-End Recovery ................................14
      4.3. Grades of Recovery ........................................15
           4.3.1. Dedicated Protection ...............................15
           4.3.2. Shared Protection ..................................16
           4.3.3. Extra Traffic ......................................17
           4.3.4. Restoration ........................................19
           4.3.5. Reversion ..........................................20
      4.4. Mechanisms for Protection .................................20

Sprecher & Farrel             Informational                     [Page 2]

RFC 6372             MPLS-TP Survivability Framework      September 2011

           4.4.1. Link-Level Protection ..............................20
           4.4.2. Alternate Paths and Segments .......................21
           4.4.3. Protection Tunnels .................................22
      4.5. Recovery Domains ..........................................23
      4.6. Protection in Different Topologies ........................24
      4.7. Mesh Networks .............................................25
           4.7.1. 1:n Linear Protection ..............................26
           4.7.2. 1+1 Linear Protection ..............................28
           4.7.3. P2MP Linear Protection .............................29
           4.7.4. Triggers for the Linear Protection
                  Switching Action ...................................30
           4.7.5. Applicability of Linear Protection for LSP
                  Segments ...........................................31
           4.7.6. Shared Mesh Protection .............................32
      4.8. Ring Networks .............................................33
      4.9. Recovery in Layered Networks ..............................34
           4.9.1. Inherited Link-Level Protection ....................35
           4.9.2. Shared Risk Groups .................................35
           4.9.3. Fault Correlation ..................................36
   5. Applicability and Scope of Survivability in MPLS-TP ............37
   6. Mechanisms for Providing Survivability for MPLS-TP LSPs ........39
      6.1. Management Plane ..........................................39
           6.1.1. Configuration of Protection Operation ..............40
           6.1.2. External Manual Commands ...........................41
      6.2. Fault Detection ...........................................41
      6.3. Fault Localization ........................................42
      6.4. OAM Signaling .............................................43
           6.4.1. Fault Detection ....................................44
           6.4.2. Testing for Faults .................................44
           6.4.3. Fault Localization .................................45
           6.4.4. Fault Reporting ....................................45
           6.4.5. Coordination of Recovery Actions ...................46
      6.5. Control Plane .............................................46
           6.5.1. Fault Detection ....................................47
           6.5.2. Testing for Faults .................................47
           6.5.3. Fault Localization .................................48
           6.5.4. Fault Status Reporting .............................48
           6.5.5. Coordination of Recovery Actions ...................49
           6.5.6. Establishment of Protection and Restoration LSPs ...49
   7. Pseudowire Recovery Considerations .............................50
      7.1. Utilization of Underlying MPLS-TP Recovery ................50
      7.2. Recovery in the Pseudowire Layer ..........................51
   8. Manageability Considerations ...................................51
   9. Security Considerations ........................................52
   10. Acknowledgments ...............................................52
   11. References ....................................................53
      11.1. Normative References .....................................53
      11.2. Informative References ...................................54

Sprecher & Farrel             Informational                     [Page 3]

RFC 6372             MPLS-TP Survivability Framework      September 2011

1. Introduction

   Network survivability is the network's ability to recover traffic
   delivery following the failure or degradation of traffic delivery
   caused by a network fault or a denial-of-service attack on the
   network.  Survivability plays a critical role in the delivery of
   reliable services in transport networks.  Guaranteed services in the
   form of Service Level Agreements (SLAs) require a resilient network
   that very rapidly detects facility or node degradation or failures,
   and immediately starts to recover network operations in accordance
   with the terms of the SLA.

   The MPLS Transport Profile (MPLS-TP) is described in [RFC5921].
   MPLS-TP is designed to be consistent with existing transport network
   operations and management models, while providing survivability
   mechanisms, such as protection and restoration.  The functionality
   provided is intended to be similar to or better than that found in
   established transport networks that set a high benchmark for
   reliability.  That is, it is intended to provide the operator with
   functions with which they are familiar through their experience with
   other transport networks, although this does not preclude additional

   This document provides a framework for MPLS-TP-based survivability
   that meets the recovery requirements specified in [RFC5654].  It uses
   the recovery terminology defined in [RFC4427], which draws heavily on
   [G.808.1], and it refers to the requirements specified in [RFC5654].

   This document is a product of a joint Internet Engineering Task Force
   (IETF) / International Telecommunication Union Telecommunication
   Standardization Sector (ITU-T) effort to include an MPLS Transport
   Profile within the IETF MPLS and PWE3 architectures to support the
   capabilities and functionalities of a packet-based transport network,
   as defined by the ITU-T.

1.1.  Recovery Schemes

   Various recovery schemes (for protection and restoration) and
   processes have been defined and analyzed in [RFC4427] and [RFC4428].
   These schemes can also be applied in MPLS-TP networks to re-establish
   end-to-end traffic delivery according to the agreed service
   parameters, and to trigger recovery from "failed" or "degraded"
   transport entities.  In the context of this document, transport
   entities are nodes, links, transport path segments, concatenated
   transport path segments, and entire transport paths.  Recovery
   actions are initiated by the detection of a defect, or by an external
   request (e.g., an operator's request for manual control of protection

Sprecher & Farrel             Informational                     [Page 4]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   [RFC4427] makes a distinction between protection switching and
   restoration mechanisms.

   - Protection switching uses pre-assigned capacity between nodes,
     where the simplest scheme has a single, dedicated protection entity
     for each working entity, while the most complex scheme has m
     protection entities shared between n working entities (m:n).

   - Restoration uses any capacity available between nodes and usually
     involves rerouting.  The resources used for restoration may be pre-
     planned (i.e., predetermined, but not yet allocated to the recovery
     path), and recovery priority may be used as a differentiation
     mechanism to determine which services are recovered and which are
     not recovered.

   Both protection switching and restoration may be either
   unidirectional or bidirectional; unidirectional implies that
   protection switching is performed independently for each direction of
   a bidirectional transport path, while bidirectional means that both
   directions are switched simultaneously using appropriate
   coordination, even if the fault applies to only one direction of the

   Both protection and restoration mechanisms may be either revertive or
   non-revertive as described in Section 4.11 of [RFC4427].

   Preemption priority may be used to determine which services are
   sacrificed to enable the recovery of other services.  Restoration may
   also be either unidirectional or bidirectional.  In general,
   protection actions are completed within time frames amounting to tens
   of milliseconds, while automated restoration actions are normally
   completed within periods ranging from hundreds of milliseconds to a
   maximum of a few seconds.  Restoration is not guaranteed (for
   example, because network resources may not be available at the time
   of the defect).

1.2.  Recovery Action Initiation

   The recovery schemes described in [RFC4427] and evaluated in
   [RFC4428] are presented in the context of control-plane-driven
   actions (such as the configuration of the protection entities and
   functions, etc.).  The presence of a distributed control plane in an
   MPLS-TP network is optional.  However, the absence of such a control
   plane does not affect the operation of the network and the use of
   MPLS-TP forwarding, Operations, Administration, and Maintenance
   (OAM), and survivability capabilities.  In particular, the concepts

Sprecher & Farrel             Informational                     [Page 5]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   discussed in [RFC4427] and [RFC4428] refer to recovery actions
   effected in the data plane; they are equally applicable in MPLS-TP,
   with or without the use of a control plane.

   Thus, some of the MPLS-TP recovery mechanisms do not depend on a
   control plane and use MPLS-TP OAM mechanisms or management actions to
   trigger recovery actions.

   The principles of MPLS-TP protection-switching actions are similar to
   those described in [RFC4427], since the protection mechanism is based
   on the capability to detect certain defects in the transport entities
   within the recovery domain.  The protection-switching controller does
   not care which initiation method is used, provided that it can be
   given information about the status of the transport entities within
   the recovery domain (e.g., OK, signal failure, signal degradation,

   In the context of MPLS-TP, it is imperative to ensure that performing
   switchovers is possible, regardless of the way in which the network
   is configured and managed (for example, regardless of whether a
   control-plane, management-plane, or OAM initiation mechanism is

   All MPLS and GMPLS protection mechanisms [RFC4428] are applicable in
   an MPLS-TP environment.  It is also possible to provision and manage
   the related protection entities and functions defined in MPLS and
   GMPLS using the management plane [RFC5654].  Regardless of whether an
   OAM, management, or control plane initiation mechanism is used, the
   protection-switching operation is a data-plane operation.

   In some recovery schemes (such as bidirectional protection
   switching), it is necessary to coordinate the protection state
   between the edges of the recovery domain to achieve initiation of
   recovery actions for both directions.  An MPLS-TP protocol may be
   used as an in-band (i.e., data-plane based) control protocol in order
   to coordinate the protection state between the edges of the
   protection domain.  When the MPLS-TP control plane is in use, a
   control-plane-based mechanism can also be used to coordinate the
   protection states between the edges of the protection domain.

1.3.  Recovery Context

   An MPLS-TP Label Switched Path (LSP) may be subject to any part of or
   all of MPLS-TP link recovery, path-segment recovery, or end-to-end
   recovery, where:

Sprecher & Farrel             Informational                     [Page 6]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   o  MPLS-TP link recovery refers to the recovery of an individual link
      (and hence all or a subset of the LSPs routed over the link)
      between two MPLS-TP nodes.  For example, link recovery may be
      provided by server-layer recovery.

   o  Segment recovery refers to the recovery of an LSP segment (i.e.,
      segment and concatenated segment in the language of [RFC5654])
      between two nodes and is used to recover from the failure of one
      or more links or nodes.

   o  End-to-end recovery refers to the recovery of an entire LSP, from
      its ingress to its egress node.

   For additional resiliency, more than one of these recovery techniques
   may be configured concurrently for a single path.

   Co-routed bidirectional MPLS-TP LSPs are defined in a way that allows
   both directions of the LSP to follow the same route through the
   network.  In this scenario, the operator often requires the
   directions to fate-share (that is, if one direction fails, both
   directions should cease to operate).

   Associated bidirectional MPLS-TP LSPs exist where the two directions
   of a bidirectional LSP follow different paths through the network.
   An operator may also request fate-sharing for associated
   bidirectional LSPs.

   The requirement for fate-sharing causes a direct interaction between
   the recovery processes affecting the two directions of an LSP, so
   that both directions of the bidirectional LSP are recovered at the
   same time.  This mode of recovery is termed bidirectional recovery
   and may be seen as a consequence of fate-sharing.

   The recovery scheme operating at the data-plane level can function in
   a multi-domain environment (in the wider sense of a "domain"
   [RFC4726]).  It can also protect against a failure of a boundary node
   in the case of inter-domain operation.  MPLS-TP recovery schemes are
   intended to protect client services when they are sent across the
   MPLS-TP network.

1.4.  Scope of This Framework

   This framework introduces the architecture of the MPLS-TP recovery
   domain and describes the recovery schemes in MPLS-TP (based on the
   recovery types defined in [RFC4427]) as well as the principles of
   operation, recovery states, recovery triggers, and information
   exchanges between the different elements that support the reference

Sprecher & Farrel             Informational                     [Page 7]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   The framework also describes the qualitative grades of the
   survivability functions that can be provided, such as dedicated
   recovery, shared protection, restoration, etc.  In the event of a
   network failure, the grade of recovery directly affects the service
   grade provided to the end-user.

   The general description of the functional architecture is applicable
   to both LSPs and pseudowires (PWs); however, PW recovery is only
   introduced in Section 7, and the relevant details are beyond the
   scope of this document and are for further study.

   This framework applies to general recovery schemes as well as to
   mechanisms that are optimized for specific topologies and are
   tailored to efficiently handle protection switching.

   This document addresses the need for the coordination of protection
   switching across multiple layers and at sub-layers (for clarity, we
   use the term "layer" to refer equally to layers and sub-layers).
   This allows an operator to prevent race conditions and allows the
   protection-switching mechanism of one layer to recover from a failure
   before switching is invoked at another layer.

   This framework also specifies the functions that must be supported by
   MPLS-TP to provide the recovery mechanisms.  MPLS-TP introduces a
   tool kit to enable recovery in MPLS-TP-based networks and to ensure
   that affected services are recovered in the event of a failure.

   Generally, network operators aim to provide the fastest, most stable,
   and best protection mechanism at a reasonable cost in accordance with
   customer requirements.  The greater the grade of protection required,
   the greater the number of resources will be consumed.  It is
   therefore expected that network operators will offer a wide spectrum
   of service grade.  MPLS-TP-based recovery offers the flexibility to
   select a recovery mechanism, define the granularity at which traffic
   delivery is to be protected, and choose the specific traffic types
   that are to be protected.  With MPLS-TP-based recovery, it should be
   possible to provide different grades of protection for different
   traffic classes within the same path based on the service

2.  Terminology and References

   The terminology used in this document is consistent with that defined
   in [RFC4427].  The latter is consistent with [G.808.1].

   However, certain protection concepts (such as ring protection) are
   not discussed in [RFC4427]; for those concepts, the terminology used
   in this document is drawn from [G.841].

Sprecher & Farrel             Informational                     [Page 8]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   Readers should refer to those documents for normative definitions.

   This document supplies brief summaries of a number of terms for
   reasons of clarity and to assist the reader, but it does not redefine

   Note, in particular, the distinction and definitions made in
   [RFC4427] for the following three terms:

   o  Protection: re-establishing end-to-end traffic delivery using pre-
      allocated resources.

   o  Restoration: re-establishing end-to-end traffic delivery using
      resources allocated at the time of need; sometimes referred to as
      "repair" of a service, LSP, or the traffic.

   o  Recovery: a generic term covering both Protection and Restoration.

   Note that the term "survivability" is used in [RFC5654] to cover the
   functional elements of "protection" and "restoration", which are
   collectively known as "recovery".

   Important background information on survivability can be found in
   [RFC3386], [RFC3469], [RFC4426], [RFC4427], and [RFC4428].

   In this document, the following additional terminology is applied:

   o  "Fault Management", as defined in [RFC5950].

   o  The terms "defect" and "failure" are used interchangeably to
      indicate any defect or failure in the sense that they are defined
      in [G.806].  The terms also include any signal degradation event
      as defined in [G.806].

   o  A "fault" is a fault or fault cause as defined in [G.806].

   o  "Trigger" indicates any event that may initiate a recovery action.
      See Section 4.1 for a more detailed discussion of triggers.

   o  The acronym "OAM" is defined as Operations, Administration, and
      Maintenance, consistent with [RFC6291].

   o  A "Transport Entity" is a node, link, transport path segment,
      concatenated transport path segment, or entire transport path.

   o  A "Working Entity" is a transport entity that carries traffic
      during normal network operation.

Sprecher & Farrel             Informational                     [Page 9]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   o  A "Protection Entity" is a transport entity that is pre-allocated
      and used to protect and transport traffic when the working entity

   o  A "Recovery Entity" is a transport entity that is used to recover
      and transport traffic when the working entity fails.

   o  "Survivability Actions" are the steps that may be taken by network
      nodes to communicate faults and to switch traffic from faulted or
      degraded paths to other paths.  This may include sending messages
      and establishing new paths.

   General terminology for MPLS-TP is found in [RFC5921] and [ROSETTA].
   Background information on MPLS-TP requirements can be found in

3.  Requirements for Survivability

   MPLS-TP requirements are presented in [RFC5654] and serve as
   normative references for the definition of all MPLS-TP functionality,
   including survivability.  Survivability is presented in [RFC5654] as
   playing a critical role in the delivery of reliable services, and the
   requirements for survivability are set out using the recovery
   terminology defined in [RFC4427].

4.  Functional Architecture

   This section presents an overview of the elements relating to the
   functional architecture for survivability within an MPLS-TP network.
   The components are presented separately to demonstrate the way in
   which they may be combined to provide the different grades of
   recovery needed to meet the requirements set out in the previous

4.1.  Elements of Control

   Recovery is achieved by implementing specific actions.  These actions
   aim to repair network resources or redirect traffic along paths that
   avoid failures in the network.  They may be triggered automatically
   by the MPLS-TP network nodes upon detection of a network defect, or
   they may be triggered by an operator.  Automated actions may be
   enhanced by in-band (i.e., data-plane-based) OAM mechanisms, or by
   in-band or out-of-band control-plane signaling.

Sprecher & Farrel             Informational                    [Page 10]

RFC 6372             MPLS-TP Survivability Framework      September 2011

4.1.1.  Operator Control

   The survivability behavior of the network as a whole, and the
   reaction of each transport path when a fault is reported, may be
   controlled by the operator.  This control can be split into two sets
   of functions: policies and actions performed when the transport path
   is set up, and commands used to control or force recovery actions for
   established transport paths.

   The operator may establish network-wide or local policies that
   determine the actions that will be taken when various defects are
   reported that affect different transport paths.  Also, when a service
   request is made that causes the establishment of one or more
   transport paths in the network, the operator (or requesting
   application) may define a particular grade of service, and this will
   be mapped to specific survivability actions taken before and during
   transport path setup, after the discovery of a failure of network
   resources, and upon recovery of those resources.

   It should be noted that it is unusual to present a user or customer
   with options directly related to recovery actions.  Instead, the
   user/customer enters into an SLA with the network provider, and the
   network operator maps the terms of the SLA (for example, for
   guaranteed delivery, availability, or reliability) to recovery
   schemes within the network.

   The operator can also issue commands to control recovery actions and
   events.  For example, the operator may perform the following actions:

   o  Enable or disable the survivability function.

   o  Invoke the simulation of a network fault.

   o  Force a switchover from a working path to a recovery path or vice

   Forced switchover may be performed for network optimization purposes
   with minimal service interruption, such as when modifying protected
   or unprotected services, when replacing MPLS-TP network nodes, etc.
   In some circumstances, a fault may be reported to the operator, and
   the operator may then select and initiate the appropriate recovery
   action.  A description of the different operator commands is found in
   Section 4.12 of [RFC4427].

Sprecher & Farrel             Informational                    [Page 11]

RFC 6372             MPLS-TP Survivability Framework      September 2011

4.1.2.  Defect-Triggered Actions

   Survivability actions may be directly triggered by network defects.
   This means that the device that detects the defect (for example,
   notification of an issue reported from equipment in a lower layer,
   failure to receive an OAM Continuity message, or receipt of an OAM
   message reporting a failure condition) may immediately perform a
   survivability action.

   The action is directly triggered by events in the data plane.  Note,
   however, that coordination of recovery actions between the edges of
   the recovery domain may require message exchanges for some recovery
   functions or for performing a bidirectional recovery action.

4.1.3.  OAM Signaling

   OAM signaling refers to data-plane OAM message exchange.  Such
   messages may be used to detect and localize faults or to indicate a
   degradation in the operation of the network.  However, in this
   context these messages are used to control or trigger survivability
   actions.  The mechanisms to achieve this are discussed in [RFC6371].

   OAM signaling may also be used to coordinate recovery actions within
   the protection domain.

4.1.4.  Control-Plane Signaling

   Control-plane signaling is responsible for setup, maintenance, and
   teardown of transport paths that do not fall under management-plane
   control.  The control plane may also be used to coordinate the
   detection, localization, and reaction to network defects pertaining
   to peer relationships (neighbor-to-neighbor or end-to-end).  Thus,
   control-plane signaling may initiate and coordinate survivability

   The control plane can also be used to distribute topology and
   information relating to resource availability.  In this way, the
   "graceful shutdown" [RFC5817] of resources may be affected by
   withdrawing them; this can be used to invoke a survivability action
   in a similar way to that used when reporting or discovering a fault,
   as described in the previous sections.

   The use of a control plane for MPLS-TP is discussed in [RFC6373].

Sprecher & Farrel             Informational                    [Page 12]

RFC 6372             MPLS-TP Survivability Framework      September 2011

4.2.  Recovery Scope

   This section describes the elements of recovery.  These are the
   quantitative aspects of recovery, that is, the parts of the network
   for which recovery can be provided.

   Note that the terminology in this section is consistent with
   [RFC4427].  Where the terms differ from those in [RFC5654], mapping
   is provided.

4.2.1.  Span Recovery

   A span is a single hop between neighboring MPLS-TP nodes in the same
   network layer.  A span is sometimes referred to as a link, and this
   may cause some confusion between the concept of a data link and a
   traffic engineering (TE) link.  LSPs traverse TE links between
   neighboring MPLS-TP nodes in the MPLS-TP network layer.  However, a
   TE link may be provided by any of the following:

   o  A single data link.

   o  A series of data links in a lower layer, established as an LSP and
      presented to the upper layer as a single TE link.

   o A set of parallel data links in the same layer, presented either as
      a bundle of TE links, or as a collection of data links that
      together provide a data-link-layer protection scheme.

   Thus, span recovery may be provided by any of the following:

   o  Selecting a different TE link from a bundle.

   o  Moving the TE link so that it is supported by a different data
      link between the same pair of neighbors.

   o  Rerouting the LSP in the lower layer.

   Moving the protected LSP to another TE link between the same pair of
   neighbors is a form of segment recovery and not a form of span
   recovery.  Segment Recovery is described in Section 4.2.2.

4.2.2.  Segment Recovery

   An LSP segment comprises one or more continuous hops on the path of
   the LSP.  [RFC5654] defines two terms.  A "segment" is a single hop
   along the path of an LSP, while a "concatenated segment" is more than
   one hop along the path of an LSP.  In the context of this document, a
   segment covers both of these concepts.

Sprecher & Farrel             Informational                    [Page 13]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   A PW segment refers to a Single-Segment PW (SS-PW) or to a single
   segment of a Multi-Segment PW (MS-PW) that is set up between two PE
   devices that may be Terminating PEs (T-PEs) or Switching PEs (S-PEs)
   so that the full set of possibilities is T-PE to S-PE, S-PE to S-PE,
   S-PE to T-PE, or T-PE to T-PE (for the SS-PW case).  As indicated in
   Section 1, the recovery of PWs and PW segments is beyond the scope of
   this document; however, see Section 7.

   Segment recovery involves redirecting or copying traffic at the
   source end of a segment onto an alternate path leading to the other
   end of the segment.  According to the required grade of recovery
   (described in Section 4.3), traffic may be either redirected to a
   pre-established segment, through rerouting the protected segment, or
   tunneled to the far end of the protected segment through a "bypass"
   LSP.  For details on recovery mechanisms, see Section 4.4.

   Note that protecting a transport path against node failure requires
   the use of segment recovery or end-to-end recovery, while a link
   failure can be protected using span, segment, or end-to-end recovery.

4.2.3.  End-to-End Recovery

   End-to-end recovery is a special case of segment recovery where the
   protected segment comprises the entire transport path.  End-to-end
   recovery may be provided as link-diverse or node-diverse recovery
   where the recovery path shares no links or no nodes with the working

   Note that node-diverse paths are necessarily link-diverse and that
   full, end-to-end node-diversity is required to guarantee recovery.

   Two observations need to be made about end-to-end recovery.

   - Firstly, there may be circumstances where node-diverse end-to-end
     paths do not guarantee recovery.  The ingress and egress nodes will
     themselves be single points of failure.  Additionally, there may be
     shared risks of failure (for example, geographic collocation,
     shared resources, etc.) between diverse nodes as described in
     Section 4.9.2.

   - Secondly, it is possible to use end-to-end recovery techniques even
     when there is not full diversity and the working and protection
     paths share links or nodes.

Sprecher & Farrel             Informational                    [Page 14]

RFC 6372             MPLS-TP Survivability Framework      September 2011

4.3.  Grades of Recovery

   This section describes the qualitative grades of survivability that
   can be provided.  In the event of a network failure, the grade of
   recovery offered directly affects the service grade provided to the
   end-user.  This will be observed as the amount of data lost when a
   network fault occurs, and the length of time required to recover

   In general, there is a correlation between the recovery service grade
   (i.e., the speed of recovery and reduction of data loss) and the
   amount of resources used in the network; better service grades
   require the pre-allocation of resources to the recovery paths, and
   those resources cannot be used for other purposes if high-quality
   recovery is required.  An operator will consider how providing
   different grades of recovery may require that network resources be
   provisioned and allocated for exclusive use of the recovery paths
   such that the resources cannot be used to support other customer

   Sections 6 and 7 of [RFC4427] provide a full breakdown of the
   protection and recovery schemes.  This section summarizes the
   qualitative grades available.

   Note that, in the context of recovery, a useful discussion of the
   term "resource" and its interpretation in both the IETF and ITU-T
   contexts may be found in Section 3.2 of [RFC4397].

   The selection of the recovery grade and schemes to satisfy the
   service grades for an LSP using available network resources is
   subject to network and local policy and may be pre-designated through
   network planning or may be dynamically determined by the network.

4.3.1.  Dedicated Protection

   In dedicated protection, the resources for the recovery entity are
   pre-assigned for the sole use of the protected transport path.  This
   will clearly be the case in 1+1 protection, and may also be the case
   in 1:1 protection where extra traffic (see Section 4.3.3) is not

   Note that when using protection tunnels (see Section 4.4.3),
   resources may also be dedicated to the protection of a specific
   transport path.  In some cases (1:1 protection), the entire bypass
   tunnel may be dedicated to providing recovery for a specific
   transport path, while in other cases (such as facility backup), a
   subset of the resources associated with the bypass tunnel may be pre-
   assigned for the recovery of a specific service.

Sprecher & Farrel             Informational                    [Page 15]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   However, as described in Section 4.4.3, the bypass tunnel method can
   also be used for shared protection (Section 4.3.2), either to carry
   extra traffic (Section 4.3.3) or to achieve best-effort recovery
   without the need for resource reservation.

4.3.2.  Shared Protection

   In shared protection, the resources for the recovery entities of
   several services are shared.  These may be shared as 1:n or m:n and
   are shared on individual links.  Link-by-link resource sharing may be
   managed and operated along LSP segments, on PW segments, or on end-
   to-end transport paths (LSP or PW).  Note that there is no
   requirement for m:n recovery in the list of MPLS-TP requirements
   documented in [RFC5654].  Shared protection can be applied in
   different topologies (mesh, ring, etc.) and can utilize different
   protection mechanisms (linear, ring, etc.).

   End-to-end shared protection shares resources between a number of
   paths that have common end points.  Thus, a number of paths (n paths)
   are all protected by one or more protection paths (m paths, where m
   may equal 1).  When there have been m failures, there are no more
   available protection paths, and the n paths are no longer protected.
   Thus, in 1:n protection, one fault can be protected against before
   all the n paths are unprotected.  The fact that the paths have become
   unprotected needs to be conveyed to the path end points since they
   may need to report the change in service grade or may need to take
   further action to increase their protection.  In end-to-end shared
   protection, this communication is simple since the end points are

   In shared mesh protection (see Section 4.7.6), the paths that share
   the protection resources do not necessarily have the same end points.
   This provides a more flexible resource-sharing scheme, but the
   network planning and the coordination of protection state after a
   recovery action are more complex.

   Where a bypass tunnel is used (Section 4.4.3), the tunnel might not
   have sufficient resources to simultaneously protect all of the paths
   for which it offers protection; in the event that all paths were
   affected by network defects and failures at the same time, not all of
   them would be recovered.  Policy would dictate how this situation
   should be handled: some paths might be protected, while others would
   simply fail; the traffic for some paths would be guaranteed, while
   traffic on other paths would be treated as best-effort with the risk
   of dropped packets.  Alternatively, it is possible that protection
   would not be attempted according to local policy at the nodes that
   perform the recovery actions.

Sprecher & Farrel             Informational                    [Page 16]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   Shared protection is a trade-off between assigning network resources
   to protection (which is not required most of the time) and risking
   unrecoverable services in the event that multiple network defects or
   failures occur.  Rapid recovery can be achieved with dedicated
   protection, but it is delayed by message exchanges in the management,
   control, or data planes for shared protection.  This means that there
   is also a trade-off between rapid recovery and resource sharing.  In
   some cases, shared protection might not meet the speed required for
   protection, but it may still be faster than restoration.

   These trade-offs may be somewhat mitigated by the following:

   o  Adjusting the value of n in 1:n protection.

   o  Using m:n protection for a value of m > 1.

   o  Establishing new protection paths as each available protection
      path is put into use.

   In an MPLS-TP network, the degree to which a resource is shared
   between LSPs is a policy issue. This policy may be applied to the
   resource or to the LSPs, and may be pre-configured, configured per
   LSP and installed during LSP establishment, or may be dynamically

4.3.3.  Extra Traffic

   Section of [RFC5654] says: "Support for extra traffic (as
   defined in [RFC4427]) is not required in MPLS-TP and MAY be omitted
   from the MPLS-TP specifications".  This document observes that extra
   traffic facilities may therefore be provided as part of the MPLS-TP
   survivability toolkit depending upon the development of suitable
   solution specifications.  The remainder of this section explains the
   concepts of extra traffic without prejudging the decision to specify
   or not specify such solutions.

   Network resources allocated for protection represent idle capacity
   during the time that recovery is not actually required, and can be
   utilized by carrying other traffic, referred to as "extra traffic".

   Note that extra traffic does not need to start or terminate at the
   ends of the entity (e.g., LSP) that it uses.

   When a network resource carrying extra traffic is required for the
   recovery of protected traffic from the failed working path, the extra
   traffic is disrupted.  This disruption make take one of two forms:

Sprecher & Farrel             Informational                    [Page 17]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   - In "hard preemption", the extra traffic is excluded from the
     protection resource.  The disruption of the extra traffic is total,
     and the service supported by the extra traffic must be dropped, or
     some form of rerouting or restoration must be applied to the extra
     traffic LSP in order to recover the service.

     Hard preemption is achieved by "setting a switch" on the path of
     the extra traffic such that it no longer flows.  This situation may
     be detected by OAM and reported as a fault, or may be proactively
     reported through OAM or control-plane signaling.

   - In "soft preemption", the extra traffic is not explicitly excluded
     from the protection resource, but is given lower priority than the
     protected traffic.  In a packet network (such as MPLS-TP), this can
     result in oversubscription of the protection resource with the
     result that the extra traffic receives "best-effort" delivery.
     Depending on the volume of protection and extra traffic, and the
     level of oversubscription, the extra traffic may be slightly or
     heavily impacted.

     The event of soft preemption may be detected by OAM and reported as
     a degradation of traffic delivery or as a fault.  It may also be
     proactively reported through OAM or control-plane signaling.

   Note that both hard and soft preemption may utilize additional
   message exchanges in the management, control, or data planes.  These
   messages do not necessarily mean that recovery is delayed, but may
   increase the complexity of the protection system.  Thus, the benefits
   of carrying extra traffic must be weighed against the disadvantages
   of delayed recovery, additional network overhead, and the impact on
   the services that support the extra traffic according to the details
   of the solutions selected.

   Note that extra traffic is not protected by definition, but may be

   Extra traffic is not supported on dedicated protection resources,
   which, by definition, are used for 1+1 protection (Section 4.3.1),
   but it can be supported in other protection schemes, including shared
   protection (Section 4.3.2) and tunnel protection (Section 4.4.3).

   Best-effort traffic should not be confused with extra traffic.  For
   best-effort traffic, the network does not guarantee data delivery,
   and the user does not receive guaranteed quality of service (e.g., in
   terms of jitter, packet loss, delay, etc.).  Best-effort traffic
   depends on the current traffic load.  However, for extra traffic,
   quality can only be guaranteed until resources are required for
   recovery.  At this point, the extra traffic may be completely

Sprecher & Farrel             Informational                    [Page 18]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   displaced, may be treated as best effort, or may itself be recovered
   (for example, by restoration techniques).

4.3.4.  Restoration

   This section refers to LSP restoration.  Restoration for PWs is
   beyond the scope of this document (but see Section 7).

   Restoration represents the most effective use of network resources,
   since no resources are reserved for recovery.  However, restoration
   requires the computation of a new path and the activation of a new
   LSP (through the management or control plane).  It may be more time-
   consuming to perform these steps than to implement recovery using
   protection techniques.

   Furthermore, there is no guarantee that restoration will be able to
   recover the service.  It may be that all suitable network resources
   are already in use for other LSPs, so that no new path can be found.
   This problem can be partially mitigated by using LSP setup
   priorities, so that recovery LSPs can preempt existing LSPs with
   lower priorities.

   Additionally, when a network defect occurs, multiple LSPs may be
   disrupted by the same event.  These LSPs may have been established by
   different Network Management Stations (NMSes) or they may have been
   signaled by different head-end MPLS-TP nodes, meaning that multiple
   points in the network will try to compute and establish recovery LSPs
   at the same time.  This can lead to a lack of resources within the
   network and cause recovery failures; some recovery actions will need
   to be retried, resulting in even slower recovery times for some

   Both hard and soft LSP restoration may be supported.  For hard LSP
   restoration, the resources of the working LSP are released before the
   recovery LSP is fully established (i.e., break-before-make).  For
   soft LSP restoration, the resources of the working LSP are released
   after an alternate LSP is fully established (i.e., make-before-
   break).  Note that in the case of reversion (Section 4.3.5), the
   resources associated with the working LSP are not released.

   The restoration resources may be pre-calculated and even pre-signaled
   before the restoration action starts, but not pre-allocated.  This is
   known as pre-planned LSP restoration.  The complete
   establishment/activation of the restoration LSP occurs only when the
   restoration action starts.  Pre-planning may occur periodically and
   provides the most accurate information about the available resources
   in the network.

Sprecher & Farrel             Informational                    [Page 19]

RFC 6372             MPLS-TP Survivability Framework      September 2011

4.3.5.  Reversion

   After a service has been recovered and traffic is flowing along the
   recovery LSP, the defective network resource may be replaced.
   Traffic can be redirected back onto the original working LSP (known
   as "reversion"), or it can be left where it is on the recovery LSP
   ("non-revertive" behavior).

   It should be possible to specify the reversion behavior of each
   service; this might even be configured for each recovery instance.

   In non-revertive mode, an additional operational option is possible
   where protection roles are switched, so that the recovery LSP becomes
   the working LSP, while the previous working path (or the resources
   used by the previous working path) are used for recovery in the event
   of an additional fault.

   In revertive mode, it is important to prevent excessive swapping
   between the working and recovery paths in the case of an intermittent
   defect.  This can be addressed by using a reversion delay timer (the
   Wait-To-Restore timer), which controls the length of time to wait
   before reversion following the repair of a fault on the original
   working path.  It should be possible for an operator to configure
   this timer per LSP, and a default value should be defined.

4.4.  Mechanisms for Protection

   This section provides general descriptions (MPLS-TP non-specific) of
   the mechanisms that can be used for protection purposes.  As
   indicated above, while the functional architecture applies to both
   LSPs and PWs, the mechanism for recovery described in this document
   refers to LSPs and LSP segments only.  Recovery mechanisms for
   pseudowires and pseudowire segments are for further study and will be
   described in a separate document (see also Section 7).

4.4.1.  Link-Level Protection

   Link-level protection refers to two paradigms: (1) where protection
   is provided in a lower network layer and (2) where protection is
   provided by the MPLS-TP link layer.

   Note that link-level protection mechanisms do not protect the nodes
   at each end of the entity (e.g., a link or span) that is protected.
   End-to-end or segment protection should be used in conjunction with
   link-level protection to protect against a failure of the edge nodes.

Sprecher & Farrel             Informational                    [Page 20]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   Link-level protection offers the following grades of protection:

   o  Full protection where a dedicated protection entity (e.g., a link
      or span) is pre-established to protect a working entity.  When the
      working entity fails, the protected traffic is switched to the
      protecting entity.  In this scenario, all LSPs carried over the
      working entity are recovered (in one protection operation) when
      there is a failure condition.  This is referred to in [RFC4427] as
      "bulk recovery".

   o  Partial protection where only a subset of the LSPs or traffic
      carried over a selected entity is recovered when there is a
      failure condition.  The decision as to which LSPs will be
      recovered and which will not depends on local policy.

   When there is no failure on the working entity, the protection entity
   may transport extra traffic that may be preempted when protection
   switching occurs.

   If link-level protection is available, it may be desirable to allow
   this to be attempted before attempting other recovery mechanisms for
   the transport paths affected by the fault because link-level
   protection may be faster and more conservative of network resources.
   This can be achieved both by limiting the propagation of fault
   condition notifications and by delaying the other recovery actions.
   This consideration of other protection can be compared with the
   discussion of recovery domains (Section 4.5) and recovery in multi-
   layer networks (Section 4.9).

   A protection mechanism may be provided at the MPLS-TP link layer
   (which connects two MPLS-TP nodes).  Such a mechanism can make use of
   the procedures defined in [RFC5586] to set up in-band communication
   channels at the MPLS-TP Section level, to use these channels to
   monitor the health of the MPLS-TP link, and to coordinate the
   protection states between the ends of the MPLS-TP link.

4.4.2.  Alternate Paths and Segments

   The use of alternate paths and segments refers to the paradigm
   whereby protection is performed in the network layer in which the
   protected LSP is located; this applies either to the entire end-to-
   end LSP or to a segment of the LSP.  In this case, hierarchical LSPs
   are not used (compare with Section 4.4.3).

   Different grades of protection may be provided:

   o  Dedicated protection where a dedicated entity (e.g., LSP or LSP
      segment) is (fully) pre-established to protect a working entity

Sprecher & Farrel             Informational                    [Page 21]

RFC 6372             MPLS-TP Survivability Framework      September 2011

      (e.g., LSP or LSP segment).  When a failure condition occurs on
      the working entity, traffic is switched onto the protection
      entity.  Dedicated protection may be performed using 1:1 or 1+1
      linear protection schemes.  When the failure condition is
      eliminated, the traffic may revert to the working entity.  This is
      subject to local configuration.

   o  Shared protection where one or more protection entities is pre-
      established to protect against a failure of one or more working
      entities (1:n or m:n).

   When the fault condition on the working entity is eliminated, the
   traffic should revert back to the working entity in order to allow
   other related working entities to be protected by the shared
   protection resource.

4.4.3.  Protection Tunnels

   A protection tunnel is pre-provisioned in order to protect against a
   failure condition along a sequence of spans in the network.  This may
   be achieved using LSP heirarchy.  We call such a sequence a network
   segment.  A failure of a network segment may affect one or more LSPs
   that transit the network segment.

   When a failure condition occurs in the network segment (detected
   either by OAM on the network segment, or by OAM on a concatenated
   segment of one of the LSPs transiting the network segment), one or
   more of the protected LSPs are switched over at the ingress point of
   the network segment and are transmitted over the protection tunnel.
   This is implemented through label stacking.  Label mapping may be an
   option as well.

   Different grades of protection may be provided:

   o  Dedicated protection where the protection tunnel reserves
      sufficient resources to provide protection for all protected LSPs
      without causing service degradation.

   o  Partial protection where the protection tunnel has enough
      resources to protect some of the protected LSPs, but not all of
      them simultaneously.  Policy dictates how this situation should be
      handled: it is possible that some LSPs would be protected, while
      others would simply fail; it is possible that traffic would be
      guaranteed for some LSPs, while for other LSPs it would be treated
      as best effort with the risk of packets being dropped.
      Alternatively, it is possible that protection would not be

Sprecher & Farrel             Informational                    [Page 22]

RFC 6372             MPLS-TP Survivability Framework      September 2011

4.5.  Recovery Domains

   Protection and restoration are performed in the context of a recovery
   domain.  A recovery domain is defined between two or more recovery
   reference end points that are located at the edges of the recovery
   domain and that border on the element on which recovery can be
   provided (as described in Section 4.2).  This element can be an end-
   to-end path, a segment, or a span.

   An end-to-end path can be observed as a special segment case where
   the ingress and egress Label Edge Routers (LERs) serve as the
   recovery reference end points.

   In this simple case of a point-to-point (P2P) protected entity, two
   end points reside at the boundary of the protection domain.  An LSP
   can enter through one reference end point and exit the recovery
   domain through another reference end point.

   In the case of unidirectional point-to-multipoint (P2MP), three or
   more end points reside at the boundary of the protection domain.  One
   of the end points is referred to as the source/root, while the others
   are referred to as sinks/leaves.  An LSP can enter the recovery
   domain through the root point and exit the recovery domain through
   the leaf points.

   The recovery mechanism should restore traffic that was interrupted by
   a facility (link or node) fault within the recovery domain.  Note
   that a single link may be part of several recovery domains.  If two
   recovery domains have common links, one recovery domain must be
   contained within the other.  This can be referred to as nested
   recovery domains.  The boundaries of recovery domains may coincide,
   but recovery domains must not overlap.

   Note that the edges of a recovery domain are not protected, and
   unless the whole domain is contained within another recovery domain,
   the edges form a single point of failure.

   A recovery group is defined within a recovery domain and consists of
   a working (primary) entity and one or more recovery (backup) entities
   that reside between the end points of the recovery domain.  To
   guarantee protection in all situations, a dedicated recovery entity
   should be pre-provisioned using disjoint resources in the recovery
   domain, in order to protect against a failure of a working entity.
   Of course, mechanisms to detect faults and to trigger protection
   switching are also needed.

   The method used to monitor the health of the recovery element is
   beyond the scope of this document.  The end points that are

Sprecher & Farrel             Informational                    [Page 23]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   responsible for the recovery action must receive information on its
   condition.  The condition of the recovery element may be 'OK',
   'failed', or 'degraded'.

   When the recovery operation is to be triggered by OAM mechanisms, an
   OAM Maintenance Entity Group must be defined for each of the working
   and protection entities.

   The recovery entities and functions in a recovery domain can be
   configured using a management plane or a control plane.  A management
   plane may be used to configure the recovery domain by setting the
   reference points, the working and recovery entities, and the recovery
   type (e.g., 1:1 bidirectional linear protection, ring protection,
   etc.).  Additional parameters associated with the recovery process
   may also be configured.  For more details, see Section 6.1.

   When a control plane is used, the ingress LERs may communicate with
   the recovery reference points that request that protection or
   restoration be configured across a recovery domain.  For details, see
   Section 6.5.

   Cases of multiple interconnections between distinct recovery domains
   create a hierarchical arrangement of recovery domains, since a single
   top-level recovery domain is created from the concatenation of two
   recovery domains with multiple interconnections.  In this case,
   recovery actions may be taken both in the individual, lower-level
   recovery domains to protect any LSP segment that crosses the domain,
   and within the higher-level recovery domain to protect the longer LSP
   segment that traverses the higher-level domain.

   The MPLS-TP recovery mechanism can be arranged to ensure coordination
   between domains.  In interconnected rings, for example, it may be
   preferable to allow the upstream ring to perform recovery before the
   downstream ring, in order to ensure that recovery takes place in the
   ring in which the defect occurred.  Coordination of recovery actions
   is particularly important in nested domains and is discussed further
   in Section 4.9.

4.6.  Protection in Different Topologies

   As described in the requirements listed in Section 3 and detailed in
   [RFC5654], the selected recovery techniques may be optimized for
   different network topologies if the optimized mechanisms perform
   significantly better than the generic mechanisms in the same

   These mechanisms are required (R91 of [RFC5654]) to interoperate with
   the mechanisms defined for arbitrary topologies, in order to allow

Sprecher & Farrel             Informational                    [Page 24]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   end-to-end protection and to ensure that consistent protection
   techniques are used across the entire network.  In this context,
   'interoperate' means that the use of one technique must not inhibit
   the use of another technique in an adjacent part of the network for
   use on the same end-to-end transport path, and must not prohibit the
   use of end-to-end protection mechanisms.

   The next sections (4.7 and 4.8) describe two different topologies and
   explain how recovery may be markedly different in those different
   scenarios.  They also develop the concept of a recovery domain and
   show how end-to-end survivability may be achieved through a
   concatenation of recovery domains, each providing some grade of
   recovery in part of the network.

4.7.  Mesh Networks

   A mesh network is any network where there is arbitrary
   interconnectivity between nodes in the network.  Mesh networks are
   usually contrasted with more specific topologies such as hub-and-
   spoke or ring (see Section 4.8), although such networks are actually
   examples of mesh networks.  This section is limited to the discussion
   of protection techniques in the context of mesh networks.  That is,
   it does not include optimizations for specific topologies.

   Linear protection is a protection mechanism that provides rapid and
   simple protection switching.  In a mesh network, linear protection
   provides a very suitable protection mechanism because it can operate
   between any pair of points within the network.  It can protect
   against a defect in a node, a span, a transport path segment, or an
   end-to-end transport path.  Linear protection gives a clear
   indication of the protection status.

   Linear protection operates in the context of a protection domain.  A
   protection domain is a special type of recovery domain (see Section
   4.5) associated with the protection function.  A protection domain is
   composed of the following architectural elements:

   o  A set of end points that reside at the boundary of the protection
      domain.  In the simple case of 1:n or 1+1 P2P protection, two end
      points reside at the boundary of the protection domain.  In each
      transmission direction, one of the end points is referred to as
      the source, and the other is referred to as the sink.  For
      unidirectional P2MP protection, three or more end points reside at
      the boundary of the protection domain.  One of the end points is
      referred to as the source/root, while the others are referred to
      as sinks/leaves.

Sprecher & Farrel             Informational                    [Page 25]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   o  A Protection Group consists of one or more working (primary) paths
      and one or more protection (backup) paths that run between the end
      points belonging to the protection domain.  To guarantee
      protection in all scenarios, a dedicated protection path should be
      pre-provisioned to protect against a defect of a working path
      (i.e., 1:1 or 1+1 protection schemes).  In addition, the working
      and the protection paths should be disjoint; i.e., the physical
      routes of the working and the protection paths should be
      physically diverse in every respect.

   Note that if the resources of the protection path are less than those
   of the working path, the protection path may not have sufficient
   resources to protect the traffic of the working path.

   As mentioned in Section 4.3.2, the resources of the protection path
   may be shared as 1:n.  In this scenario, the protection path will not
   have sufficient resources to protect all the working paths at a
   specific time.

   For bidirectional P2P paths, both unidirectional and bidirectional
   protection switching are supported.  If a defect occurs when
   bidirectional protection switching is defined, the protection actions
   are performed in both directions (even if the defect is
   unidirectional).  The protection state is required to operate with a
   level of coordination between the end points of the protection

   In unidirectional protection switching, the protection actions are
   only performed in the affected direction.

   Revertive and non-revertive operations are provided as options for
   the network operator.

   Linear protection supports the protection schemes described in the
   following sub-sections.

4.7.1.  1:n Linear Protection

   In the 1:1 scheme, a protection path is allocated to protect against
   a defect, failure, or a degradation in a working path.  As described
   above, to guarantee protection, the protection entity should support
   the full capacity and bandwidth, although it may be configured (for
   example, because of limited network resource availability) to offer a
   degraded service when compared with the working entity.

   Figure 1 presents 1:1 protection architecture.  In normal conditions,
   data traffic is transmitted over the working entity, while the
   protection entity functions in the idle state.  (OAM may run on the

Sprecher & Farrel             Informational                    [Page 26]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   protection entity to verify its state.)  Normal conditions are
   defined when there is no defect, failure, or degradation on the
   working entity, and no administrative configuration or request causes
   traffic to flow over the protection entity.

           |-----------------Protection Domain---------------|

                   /**********Working path***********\
         +--------+   ==============================   +--------+
         | Node  /|                                    |\  Node |
         |  A {<  |                                    | >}  B  |
         |        |                                    |        |
         +--------+   ==============================   +--------+
                              Protection path

                  Figure 1: 1:1 Protection Architecture

   If there is a defect on the working entity or a specific
   administrative request, traffic is switched to the protection entity.

   Note that when operating with non-revertive behavior (see Section
   4.3.5), after the conditions causing the switchover have been
   cleared, the traffic continues to flow on the protection path, but
   the working and protection roles are not switched.

   In each transmission direction, the protection domain source bridges
   traffic onto the appropriate entity, while the sink selects traffic
   from the appropriate entity.  The source and the sink need to
   coordinate the protection states to ensure that bridging and
   selection are performed to and from the same entity.  For this
   reason, a signaling coordination protocol (either a data-plane in-
   band signaling protocol or a control-plane-based signaling protocol)
   is required.

   In bidirectional protection switching, both ends of the protection
   domain are switched to the protection entity (even when the fault is
   unidirectional).  This requires a protocol to coordinate the
   protection state between the two end points of the protection domain.

   When there is no defect, the bandwidth resources of the idle entity
   may be used for traffic with lower priority.  When protection
   switching is performed, the traffic with lower priority may be
   preempted by the protected traffic through tearing down the LSP with
   lower priority, reporting a fault on the LSP with lower priority, or
   by treating the traffic with lower priority as best effort and
   discarding it when there is congestion.

Sprecher & Farrel             Informational                    [Page 27]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   In the general case of 1:n linear protection, one protection entity
   is allocated to protect n working entities.  The protection entity
   might not have sufficient resources to protect all the working
   entities that may be affected by fault conditions at a specific time.
   In this case, in order to guaranteed protection, the protection
   entity should support enough capacity and bandwidth to protect any of
   the n working entities.

   When defects or failures occur along multiple working entities, the
   entity to be protected should be prioritized.  The protection states
   between the edges of the protection domain should be fully
   coordinated to ensure consistent behavior.  As explained in Section
   4.3.5, revertive behavior is recommended when 1:n is supported.

4.7.2.  1+1 Linear Protection

   In the 1+1 protection scheme, a fully dedicated protection entity is

   As depicted in Figure 2, data traffic is copied and fed at the source
   to both the working and the protection entities.  The traffic on the
   working and the protection entities is transmitted simultaneously to
   the sink of the protection domain, where selection between the
   working and protection entities is performed (based on some
   predetermined criteria).

            |---------------Protection Domain---------------|

                   /**********Working path************\
         +--------+   ==============================   +--------+
         | Node  /|                                    |\  Node |
         |  A {<  |                                    | >}  Z  |
         |       \|                                    |/       |
         +--------+   ==============================   +--------+
                   \**********Protection path*********/

                 Figure 2: 1+1 Protection Architecture

   Note that control traffic between the edges of the protection domain
   (such as OAM or a control protocol to coordinate the protection
   state, etc.) may be transmitted on an entity that differs from the
   one used for the protected traffic.  These packets should not be
   discarded by the sink.

Sprecher & Farrel             Informational                    [Page 28]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   In 1+1 unidirectional protection switching, there is no need to
   coordinate the protection state between the protection controllers at
   both ends of the protection domain.  In 1+1 bidirectional protection
   switching, a protocol is required to coordinate the protection state
   between the edges of the protection domain.

   In both protection schemes, traffic flows end-to-end on the working
   entity after the conditions causing the switchover have been cleared.
   Data selection may return to selecting traffic from the working
   entity if reversion is enabled, and it will require coordination of
   the protection state between the edges of the protection domain.  To
   avoid frequent switching caused by intermittent defects or failures
   when the network is not stable, traffic is not selected from the
   working entity before the Wait-To-Restore (WTR) timer has expired.

4.7.3.  P2MP Linear Protection

   Linear protection may be applied to protect unidirectional P2MP
   entities using 1+1 protection architecture.  The source/root MPLS-TP
   node bridges the user traffic to both the working and protection
   entities.  Each sink/leaf MPLS-TP node selects the traffic from one
   entity according to some predetermined criteria.  Note that when
   there is a fault condition on one of the branches of the P2MP path,
   some leaf MPLS-TP nodes may select the working entity, while other
   leaf MPLS-TP nodes may select traffic from the protection entity.

   In a 1:1 P2MP protection scheme, the source/root MPLS-TP node needs
   to identify the existence of a fault condition on any of the branches
   of the network.  This means that the sink/leaf MPLS-TP nodes need to
   notify the source/root MPLS-TP node of any fault condition.  This
   also necessitates a return path from the sinks/leaves to the
   source/root MPLS-TP node.  When protection switching is triggered,
   the source/root MPLS-TP node selects the protection transport path
   for traffic transfer.

   A form of "segment recovery for P2MP LSPs" could be constructed.
   Given a P2MP LSP, one can protect any possible point of failure (link
   or node) using N backup P2MP LSPs.  Each backup P2MP LSP originates
   from the upstream node with respect to a different possible failure
   point and terminates at all of the destinations downstream of the
   potential failure point.  In case of a failure, traffic is redirected
   to the backup P2MP path.

   Note that such mechanisms do not yet exist, and their exact behavior
   is for further study.

   A 1:n protection scheme for P2MP transport paths is also required by
   [RFC5654].  Such a mechanism is for future study.

Sprecher & Farrel             Informational                    [Page 29]

RFC 6372             MPLS-TP Survivability Framework      September 2011

4.7.4.  Triggers for the Linear Protection Switching Action

  Protection switching may be performed when:

   o  A defect condition is detected on the working entity, and the
      protection entity has "no" or an inferior condition.  Proactive
      in-band OAM Continuity Check and Connectivity Verification (CC-V)
      monitoring of both the working and the protection entities may be
      used to enable the rapid detection of a fault condition.  For
      protection switching, it is common to run a CC-V every 3.33 ms.
      In the absence of three consecutive CC-V messages, a fault
      condition is declared.  In order to monitor the working and the
      protection entities, an OAM Maintenance Entity Group should be
      defined for each entity.  OAM indications associated with fault
      conditions should be provided at the edges of the protection
      domain that is responsible for the protection-switching operation.
      Input from OAM performance monitoring that indicates degradation
      in the working entity may also be used as a trigger for protection
      switching.  In the case of degradation, switching to the
      protection entity is needed only if the protection entity can
      exhibit better operating conditions.

   o  An indication is received from a lower-layer server that there is
      a defect in the lower layer.

   o  An external operator command is received (e.g., 'Forced Switch',
      'Manual Switch').  For details, see Section 6.1.2.

   o  A request to switch over is received from the far end.  The far
      end may initiate this request, for example, on receipt of an
      administrative request to switch over, or when bidirectional 1:1
      protection switching is supported and a defect occurred that could
      only be detected by the far end, etc.

   As described above, the protection state should be coordinated
   between the end points of the protection domain.  Control messages
   should be exchanged between the edges of the protection domain to
   coordinate the protection state of the edge nodes.  Control messages
   can be delivered using an in-band, data-plane-driven control protocol
   or a control-plane-based protocol.

   For 50-ms protection switching, it is recommended that an in-band,
   data-plane-driven signaling protocol be used in order to coordinate
   the protection states.  An in-band, data-plane protocol for use in
   MPLS-TP networks is documented in [MPLS-TP-LP] for linear protection
   (ring protection is discussed in Section 4.8 of this document).  This
   protocol is also used to detect mismatches between the configurations
   provisioned at the ends of the protection domain.

Sprecher & Farrel             Informational                    [Page 30]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   As described in Section 6.5, the GMPLS control plane already includes
   procedures and message elements to coordinate the protection states
   between the edges of the protection domain.  These procedures and
   protocol messages are specified in [RFC4426], [RFC4872], and
   [RFC4873].  However, these messages lack the capability to coordinate
   the revertive/non-revertive behavior and the consistency of
   configured timers at the edges of the protection domain (timers such
   as WTR, hold-off timer, etc.).

4.7.5.  Applicability of Linear Protection for LSP Segments

   In order to implement data-plane-based linear protection on LSP
   segments, use is made of the Sub-Path Maintenance Element (SPME), an
   MPLS-TP architectural element defined in [RFC5921].  Maintenance
   operations (e.g., monitoring, protection, or management) engage with
   message transmission (e.g., OAM, Protection Path Coordination, etc.)
   in the maintained domain.  Further discussion of the architecture for
   OAM and SPME is found in [RFC5921] and [RFC6371].  An SPME is an LSP
   that is basically defined and used for the purposes of OAM
   monitoring, protection, or management of LSP segments.  The SPME uses
   the MPLS construct of a hierarchical, nested LSP, as defined in

   For linear protection, SPMEs should be defined over the working and
   protection entities between the edges of a protection domain.  OAM
   messages and messages used to coordinate protection state can be
   initiated at the edge of the SPME and sent to the peer edge of the
   SPME.  Note that these messages are sent over the Generic Associated
   Channel (G-ACh) within the SPME, and that they use a two-label stack,
   the SPME label, and, at the bottom of the stack, the G-ACh label
   (GAL) [RFC5586].

   The end-to-end traffic of the LSP, which includes data traffic and
   control traffic (messages for OAM, management, signaling, and to
   coordinate protection state), is tunneled within the SPMEs by means
   of label stacking, as defined in [RFC3031].

   Mapping between an LSP and an SPME can be 1:1; this is similar to the
   ITU-T Tandem Connection element that defines a sub-layer
   corresponding to a segment of a path.  Mapping can also be 1:n to
   allow the scalable protection of a set of LSP segments traversing the
   part of the network in which a protection domain is defined.  Note
   that each of these LSPs can be initiated or terminated at different
   end points in the network, but that they all traverse the protection
   domain and share similar constraints (such as requirements for
   quality of service (QoS), terms of protection, etc.).

Sprecher & Farrel             Informational                    [Page 31]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   Note also that in the context of segment protection, the SPMEs serve
   as the working and protection entities.

4.7.6.  Shared Mesh Protection

   For shared mesh protection, the protection resources are used to
   protect multiple LSPs that do not all share the same end points; for
   example, in Figure 3 there are two paths, ABCDE and VWXYZ.  These
   paths do not share end points and cannot, therefore, make use of 1:n
   linear protection, even though they do not have any common points of

   ABCDE may be protected by the path APQRE, while VWXYZ can be
   protected by the path VPQRZ.  In both cases, 1:1 or 1+1 protection
   may be used.  However, it can be seen that if 1:1 protection is used
   for both paths, the PQR network segment does not carry traffic when
   no failures affect either of the two working paths.  Furthermore, in
   the event of only one failure, the PQR segment carries traffic from
   only one of the working paths.

   Thus, it is possible for the network resources on the PQR segment to
   be shared by the two recovery paths.  In this way, mesh protection
   can substantially reduce the number of network resources that have to
   be reserved in order to provide 1:n protection.

              \                 /
               \               /
                \             /
                /             \
               /               \
              /                 \

       Figure 3: A Shared Mesh Protection Topology

   As the network becomes more complex and the number of LSPs increases,
   the potential for shared mesh protection also increases.  However,
   this can quickly become unmanageable owing to the increased
   complexity.  Therefore, shared mesh protection is normally pre-
   planned and configured by the operator, although an automated system
   cannot be ruled out.

   Note that shared mesh protection operates as 1:n linear protection
   (see Section 4.7.1).  However, the protection state needs to be
   coordinated between a larger number of nodes: the end points of the
   shared concatenated protection segment (nodes P and R in the example)

Sprecher & Farrel             Informational                    [Page 32]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   as well as the end points of the protected LSPs (nodes A, E, V, and Z
   in the example).

   Additionally, note that the shared-protection resources could be used
   to carry extra traffic.  For example, in Figure 4, an LSP JPQRK could
   be a preemptable LSP that constitutes extra traffic over the PQR
   hops; it would be displaced in the event of a protection event.  In
   this case, it should be noted that the protection state must also be
   coordinated with the ends of the extra-traffic LSPs.

              \                 /
               \               /
                \             /
                /             \
               /               \
              /                 \

       Figure 4: Shared Mesh Protection with Extra Traffic

4.8.  Ring Networks

   Several service providers have expressed great interest in the
   operation of MPLS-TP in ring topologies; they demand a high degree of
   survivability functionality in these topologies.

   Various criteria for optimization are considered in ring topologies,
   such as:

   1.  Simplification in ring operation in terms of the number of OAM
       Maintenance Entities that are needed to trigger the recovery
       actions, the number of recovery elements, the number of
       management-plane transactions during maintenance operations, etc.

   2.  Optimization of resource consumption around the ring, such as the
       number of labels needed for the protection paths that traverse
       the network, the total bandwidth required in the ring to ensure
       path protection, etc. (see R91 of [RFC5654]).

   [RFC5654] introduces a list of requirements for ring protection
   covering the recovery mechanisms needed to protect traffic in a
   single ring as well as traffic that traverses more than one ring.
   Note that configuration and the operation of the recovery mechanisms
   in a ring must scale well with the number of transport paths, the
   number of nodes, and the number of ring interconnects.

Sprecher & Farrel             Informational                    [Page 33]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   The requirements for ring protection are fully compatible with the
   generic requirements for recovery.

   The architecture and the mechanisms for ring protection are specified
   in separate documents.  These mechanisms need to be evaluated against
   the requirements specified in [RFC5654], which includes guidance on
   the principles for the development of new mechanisms.

4.9.  Recovery in Layered Networks

   In multi-layer or multi-regional networking [RFC5212], recovery may
   be performed at multiple layers or across nested recovery domains.

   The MPLS-TP recovery mechanism must ensure that the timing of
   recovery is coordinated in order to avoid race scenarios.  This also
   allows the recovery mechanism of the server layer to fix the problem
   before recovery takes place in the MPLS-TP layer, or the MPLS-TP
   layer to perform recovery before a client network.

   A hold-off timer is required to coordinate recovery timing in
   multiple layers or across nested recovery domains.  Setting this
   configurable timer involves a trade-off between rapid recovery and
   the creation of a race condition where multiple layers respond to the
   same fault, potentially allocating resources in an inefficient
   manner.  Thus, the detection of a defect condition in the MPLS-TP
   layer should not immediately trigger the recovery process if the
   hold-off timer is configured as a value other than zero.  Instead,
   the hold-off timer should be started when the defect is detected and,
   on expiry, the recovery element should be checked to determine
   whether the defect condition still exists.  If it does exist, the
   defect triggers the recovery operation.

   The hold-off timer should be configurable.

   In other configurations, where the lower layer does not have a
   restoration capability, or where it is not expected to provide
   protection, the lower layer needs to trigger the higher layer to
   immediately perform recovery.  Although this can be forced by
   configuring the hold-off timer as zero, it may be that because of
   layer independence, the higher layer does not know whether the lower
   layer will perform restoration.  In this case, the higher layer will
   configure a non-zero hold-off timer and rely on the receipt of a
   specific notification from the lower layer if the lower layer cannot
   perform restoration.  Since layer boundaries are always within nodes,
   such coordination is implementation-specific and does not need to be
   covered here.

Sprecher & Farrel             Informational                    [Page 34]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   Reference should be made to [RFC3386], which discusses the
   interaction between layers in survivable networks.

4.9.1.  Inherited Link-Level Protection

   Where a link in the MPLS-TP network is formed through connectivity
   (i.e., a packet or non-packet LSP) in a lower-layer network, that
   connectivity may itself be protected; for example, the LSP in the
   lower-layer network may be provisioned with 1+1 protection.  In this
   case, the link in the MPLS-TP network has an inherited grade of

   An LSP in the MPLS-TP network may be provisioned with protection in
   the MPLS-TP network, as already described, or it may be provisioned
   to utilize only those links that have inherited protection.

   By classifying the links in the MPLS-TP network according to the
   grade of protection that they inherited from the server network, it
   is possible to compute an end-to-end path in the MPLS-TP network that
   uses only those links with a specific or superior grade of inherited
   protection.  This means that the end-to-end MPLS-TP LSP can be
   protected at the grade necessary to conform to the SLA without
   needing to provide any additional protection in the MPLS-TP layer.
   This reduces complexity, saves network resources, and eliminates
   protection-switching coordination problems.

   When the requisite grade of inherited protection is not available on
   all segments along the path in the MPLS-TP network, segment
   protection may be used to achieve the desired protection grade.

   It should be noted, however, that inherited protection only applies
   to links.  Nodes cannot be protected in this way.  An operator will
   need to perform an analysis of the relative likelihood and
   consequences of node failure if this approach is taken without
   providing protection in the MPLS-TP LSP or PW layer to handle node

4.9.2.  Shared Risk Groups

   When an MPLS-TP protection scheme is established, it is important
   that the working and protection paths do not share resources in the
   network.  If this is not achieved, a single defect may affect both
   the working and the protection paths with the result that traffic
   cannot be delivered -- since under such a condition the traffic was
   not protected.

Sprecher & Farrel             Informational                    [Page 35]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   Note that this restriction does not apply to restoration, since this
   takes place after the fault has occurred, which means that the point
   of failure can be avoided if an available path exists.

   When planning a recovery scheme, it is possible to use a topology map
   of the MPLS-TP layer to select paths that use diverse links and nodes
   within the MPLS-TP network.  However, this does not guarantee that
   the paths are truly diverse; for example, two separate links in an
   MPLS-TP network may be provided by two lambdas in the same optical
   fiber, or by two fibers that cross the same bridge.  Moreover, two
   completely separate MPLS-TP nodes might be situated in the same
   building with a shared power supply.

   Thus, in order to achieve proper recovery planning, the MPLS-TP
   network must have an understanding of the groups of lower-layer
   resources that share a common risk of failure.  From this, MPLS-TP
   shared risk groups can be constructed that show which MPLS-TP
   resources share a common risk of failure.  Diversity of working and
   protection paths can be planned, not only with regard to nodes and
   links but also in order to refrain from using resources from the same
   shared risk groups.

4.9.3.  Fault Correlation

   In a layered network, a low-layer fault may be detected and reported
   by multiple layers and may sometimes lead to the generation of
   multiple fault reports from the same layer.  For example, a failure
   of a data link may be reported by the line cards in an MPLS-TP node,
   but it could also be detected and reported by the MPLS-TP OAM.

   Section 4.6 explains how it is important to coordinate the
   survivability actions configured and operated in a multi-layer
   network in a way that will avoid over-equipping the survivability
   resources in the network, while ensuring that recovery actions are
   performed in only one layer at a time.

   Fault correlation is about understanding which single event has
   generated a set of fault reports, so that recovery actions can be
   coordinated, and so that the fault logging system does not become
   overloaded.  Fault correlation depends on understanding resource use
   at lower layers, shared risk groups, and a wider view with regard to
   the way in which the layers are interrelated.

   Fault correlation is most easily performed at the point of fault
   detection; for example, an MPLS-TP node that receives a fault
   notification from the lower layer, and detects a fault on an LSP in
   the MPLS-TP layer, can easily correlate these two events.
   Furthermore, if the same node detects multiple faults on LSPs that

Sprecher & Farrel             Informational                    [Page 36]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   share the same faulty data link, it can easily correlate them.  Such
   a node may use correlation to perform group-based recovery actions
   and can reduce the number of alarm events that it generates to its
   management station.

   Fault correlation may also be performed at a management station that
   receives fault reports from different layers and different nodes in
   the network.  This enables the management station to coordinate
   management-originated recovery actions and to present consolidated
   fault information to the user and automated management systems.

   It is also necessary to correlate fault information detected and
   reported through OAM.  This function would enable a fault detected at
   a lower layer, and reported at a transit node of an MPLS-TP LSP, to
   be correlated with an MPLS-TP-layer fault detected at a Maintenance
   End Point (MEP) -- for example, the egress of the MPLS-TP LSP.  Such
   correlation allows the coordination of recovery actions performed at
   the MEP, but it also requires that the lower-layer fault information
   is propagated to the MEP, which is most easily achieved using a
   control plane, management plane, or OAM message.

5.  Applicability and Scope of Survivability in MPLS-TP

   The MPLS-TP network can be viewed as two layers (the MPLS LSP layer
   and the PW layer).  The MPLS-TP network operates over data-link
   connections and data-link networks whereby the MPLS-TP links are
   provided by individual data links or by connections in a lower-layer
   network.  The MPLS LSP layer is a mandatory part of the MPLS-TP
   network, while the PW layer is an optional addition for supporting
   specific services.

   MPLS-TP survivability provides recovery from failure of the links and
   nodes in the MPLS-TP network.  The link defects and failures are
   typically caused by defects or failures in the underlying data-link
   connections and networks, but this section is only concerned with
   recovery actions performed in the MPLS-TP network, which must recover
   from the manifestation of any problem as a defect failure in the
   MPLS-TP network.

   This section lists the recovery elements (see Section 1) supported in
   each of the two layers that can recover from defects or failures of
   nodes or links in the MPLS-TP network.

Sprecher & Farrel             Informational                    [Page 37]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   | Recovery     | MPLS LSP Layer      | PW Layer                     |
   | Element      |                     |                              |
   | Link         | MPLS LSP recovery   | The PW layer is not aware of |
   | Recovery     | can be used to      | the underlying network.      |
   |              | survive the failure | This function is not         |
   |              | of an MPLS-TP link. | supported.                   |
   | Segment/Span | An individual LSP   | For an SS-PW, segment        |
   | Recovery     | segment can be      | recovery is the same as      |
   |              | recovered to        | end-to-end recovery.         |
   |              | survive the failure | Segment recovery for an MS-PW|
   |              | of an MPLS-TP link. | is for future study, and     |
   |              |                     | this function is now         |
   |              |                     | provided using end-to-end    |
   |              |                     | recovery.                    |
   | Concatenated | A concatenated LSP  | Concatenated segment         |
   | Segment      | segment can be      | recovery (in an MS-PW) is for|
   | Recovery     | recovered to        | future study, and this       |
   |              | survive the failure | function is now provided     |
   |              | of an MPLS-TP link  | using end-to-end recovery.   |
   |              | or node.            |                              |
   | End-to-End   | An end-to-end LSP   | End-to-end PW recovery can   |
   | Recovery     | can be recovered to | be applied to survive any    |
   |              | survive any node or | node (including S-PE) or     |
   |              | link failure,       | link failure, except for     |
   |              | except for the      | failure of the ingress or    |
   |              | failure of the      | egress T-PE.                 |
   |              | ingress or egress   |                              |
   |              | node.               |                              |
   | Service      | The MPLS LSP layer  | PW-layer service recovery    |
   | Recovery     | is service-         | requires surviving faults in |
   |              | agnostic.  This     | T-PEs or on Attachment       |
   |              | function is not     | Circuits (ACs).  This is     |
   |              | supported.          | currently out of scope for   |
   |              |                     | MPLS-TP.                     |

                 Table 1: Recovery Elements Supported
                  by the MPLS LSP Layer and PW Layer

   Section 6 provides a description of mechanisms for MPLS-TP-LSP
   survivability.  Section 7 provides a brief overview of mechanisms for
   MPLS-TP-PW survivability.

Sprecher & Farrel             Informational                    [Page 38]

RFC 6372             MPLS-TP Survivability Framework      September 2011

6.  Mechanisms for Providing Survivability for MPLS-TP LSPs

   This section describes the existing mechanisms that provide LSP
   protection within MPLS-TP networks and highlights areas where new
   work is required.

6.1.  Management Plane

   As described above, a fundamental requirement of MPLS-TP is that
   recovery mechanisms should be capable of functioning in the absence
   of a control plane.  Recovery may be triggered by MPLS-TP OAM fault
   management functions or by external requests (e.g., an operator's
   request for manual control of protection switching).  Recovery LSPs
   (and in particular Restoration LSPs) may be provisioned through the
   management plane.

   The management plane may be used to configure the recovery domain by
   setting the reference end-point points (which control the recovery
   actions), the working and the recovery entities, and the recovery
   type (e.g., 1:1 bidirectional linear protection, ring protection,

   Additional parameters associated with the recovery process (such as
   WTR and hold-off timers, revertive/non-revertive operation, etc.) may
   also be configured.

   In addition, the management plane may initiate manual control of the
   recovery function.  A priority should be set for the fault conditions
   and the operator's requests.

   Since provisioning the recovery domain involves the selection of a
   number of options, mismatches may occur at the different reference
   points.  The MPLS-TP protocol to coordinate protection state, which
   is specified in [MPLS-TP-LP], may be used as an in-band (i.e., data-
   plane-based) control protocol to coordinate the protection states
   between the end points of the recovery domain, and to check the
   consistency of configured parameters (such as timers, revertive/non-
   revertive behavior, etc.) with discovered inconsistencies that are
   reported to the operator.

   It should also be possible for the management plane to track the
   recovery status by receiving reports or by issuing polls.

Sprecher & Farrel             Informational                    [Page 39]

RFC 6372             MPLS-TP Survivability Framework      September 2011

6.1.1.  Configuration of Protection Operation

   To implement the protection-switching mechanisms, the following
   entities and information should be configured and provisioned:

   o  The end points of a recovery domain.  As described above, these
      end points border on the element of recovery to which recovery is

   o  The protection group, which, depending on the required protection
      scheme, consists of a recovery entity and one or more working
      entities.  In 1:1 or 1+1 P2P protection, the paths of the working
      entity and the recovery entities must be physically diverse in
      every respect (i.e., not share any resources or physical
      locations), in order to guarantee protection.

   o  As defined in Section 4.8, the SPME must be supported in order to
      implement data-plane-based LSP segment recovery, since related
      control messages (e.g., for OAM, Protection Path Coordination,
      etc.) can be initiated and terminated at the edges of a path where
      push and pop operations are enabled.  The SPME is an end-to-end
      LSP that in this context corresponds to the recovery entities
      (working and protection) and makes use of the MPLS construct of
      hierarchical nested LSP, as defined in [RFC3031].  OAM messages
      and messages to coordinate protection state can be initiated at
      the edge of the SPME and sent over G-ACH to the peer edge of the
      SPME.  It is necessary to configure the related SPMEs and map
      between the LSP segments being protected and the SPME.  Mapping
      can be 1:1 or 1:N to allow scalable protection of a set of LSP
      segments traversing the part of the network in which a protection
      domain is defined.

      Note that each of these LSPs can be initiated or terminated at
      different end points in the network, but that they all traverse
      the protection domain and share similar constraints (such as
      requirements for QoS, terms of protection, etc.).

   o  The protection type that should be defined (e.g., unidirectional
      1:1, bidirectional 1+1, etc.)

   o  Revertive/non-revertive behavior should be configured.

   o  Timers (such as WTR, hold-off timer, etc.) should be set.

Sprecher & Farrel             Informational                    [Page 40]

RFC 6372             MPLS-TP Survivability Framework      September 2011

6.1.2.  External Manual Commands

   The following external, manual commands may be provided for manual
   control of the protection-switching operation.  These commands apply
   to a protection group; they are listed in descending order of

   o  Blocked protection action - a manual command to prevent data
      traffic from switching to the recovery entity.  This command
      actually disables the protection group.

   o  Force protection action - a manual command that forces a switch of
      normal data traffic to the recovery entity.

   o Manual protection action - a manual command that forces a switch of
      data traffic to the recovery entity only when there is no defect
      in the recovery entity.

   o Clear switching command - the operator may request that a previous
      administrative switch command (manual or force switch) be cleared.

6.2.  Fault Detection

   Fault detection is a fundamental part of recovery and survivability.
   In all schemes, with the exception of some types of 1+1 protection,
   the actions required for the recovery of traffic delivery depend on
   the discovery of some kind of fault.  In 1+1 protection, the selector
   (at the receiving end) may simply be configured to choose the better
   signal; thus, it does not detect a fault or degradation of itself,
   but simply identifies the path that is better for data delivery.

   Faults may be detected in a number of ways depending on the traffic
   pattern and the underlying hardware.  End-to-end faults may be
   reported by the application or by knowledge of the application's data
   pattern, but this is an unusual approach.  There are two more common
   mechanisms for detecting faults in the MPLS-TP layer:

   o  Faults reported by the lower layers.

   o  Faults detected by protocols within the MPLS-TP layer.

   In an IP/MPLS network, the second mechanism may utilize control-plane
   protocols (such as the routing protocols) to detect a failure of
   adjacency between neighboring nodes.  In an MPLS-TP network, it is
   possible that no control plane will be present.  Even if a control
   plane is present, it will be a GMPLS control plane [RFC3945], which
   logically separates control channels from data channels; thus, no
   conclusion about the health of a data channel can be drawn from the

Sprecher & Farrel             Informational                    [Page 41]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   failure of an associated control channel.  MPLS-TP-layer faults are,
   therefore, only detected through the use of OAM protocols, as
   described in Section 6.4.1.

   Faults may, however, be reported by a lower layer.  These generally
   show up as interface failures or data-link failures (sometimes known
   as connectivity failures) within the MPLS-TP network, for example, an
   underlying optical link may detect loss of light and report a failure
   of the MPLS-TP link that uses it.  Alternatively, an interface card
   failure may be reported to the MPLS-TP layer.

   Faults reported by lower layers are only visible in specific nodes
   within the MPLS-TP network (i.e., at the adjacent end points of the
   MPLS-TP link).  This would only allow recovery to be performed
   locally, so, to enable recovery to be performed by nodes that are not
   immediately local to the fault, the fault must be reported (Sections
   6.4.3 and 6.5.4).

6.3.  Fault Localization

   If an MPLS-TP node detects that there is a fault in an LSP (that is,
   not a network fault reported from a lower layer, but a fault detected
   by examining the LSP), it can immediately perform a recovery action.
   However, unless the location of the fault is known, the only
   practical options are:

   o  Perform end-to-end recovery.

   o  Perform some other recovery as a speculative act.

   Since the speculative acts are not guaranteed to achieve the desired
   results and could consume resources unnecessarily, and since end-to-
   end recovery can require a lot of network resources, it is important
   to be able to localize the fault.

   Fault localization may be achieved by dividing the network into
   protection domains.  End-to-end protection is thereby operated on LSP
   segments, depending on the domain in which the fault is discovered.
   This necessitates monitoring of the LSP at the domain edges.

   Alternatively, a proactive mechanism of fault localization through
   OAM (Section 6.4.3) or through the control plane (Section 6.5.3) is

   Fault localization is particularly important for restoration because
   a new path must be selected that avoids the fault.  It may not be
   practical or desirable to select a path that avoids the entire failed

Sprecher & Farrel             Informational                    [Page 42]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   working path, and it is therefore necessary to isolate the fault's

6.4.  OAM Signaling

   MPLS-TP provides a comprehensive set of OAM tools for fault
   management and performance monitoring at different nested levels
   (end-to-end, a portion of a path (LSP or PW), and at the link level)

   These tools support proactive and on-demand fault management (for
   fault detection and fault localization) as well as performance
   monitoring (to measure the quality of the signals and detect

   To support fast recovery, it is useful to use some of the proactive
   tools to detect fault conditions (e.g., link/node failure or
   degradation) and to trigger the recovery action.

   The MPLS-TP OAM messages run in-band with the traffic and support
   unidirectional and bidirectional P2P paths as well as P2MP paths.

   As described in [RFC6371], MPLS-TP OAM operates in the context of a
   Maintenance Entity that borders on the OAM responsibilities and
   represents the portion of a path between two points that is monitored
   and maintained, and along which OAM messages are exchanged.
   [RFC6371] refers also to a Maintenance Entity Group (MEG), which is a
   collection of one or more Maintenance Entities (MEs) that belong to
   the same transport path (e.g., P2MP transport path) and which are
   maintained and monitored as a group.

   An ME includes two MEPs (Maintenance Entity Group End Points) that
   reside at the boundaries of an ME, and a set of zero or more MIPs
   (Maintenance Entity Group Intermediate Points) that reside within the
   Maintenance Entity along the path.  A MEP is capable of initiating
   and terminating OAM messages, and as such can only be located at the
   edges of a path where push and pop operations are supported.  In
   order to define an ME over a portion of path, it is necessary to
   support SPMEs.

   The SPME is an end-to-end LSP that in this context corresponds to the
   ME; it uses the MPLS construct of hierarchical nested LSPs, which is
   defined in [RFC3031].  OAM messages can be initiated at the edge of
   the SPME and sent over G-ACH to the peer edge of the SPME.

   The related SPMEs must be configured, and mapping must be performed
   between the LSP segments being monitored and the SPME.  Mapping can
   be 1:1 or 1:N to allow scalable operation.  Note that each of these

Sprecher & Farrel             Informational                    [Page 43]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   LSPs can be initiated or terminated at different end points in the
   network and can share similar constraints (such as requirements for
   QoS, terms of protection, etc.).

   With regard to recovery, where MPLS-TP OAM is supported, an OAM
   Maintenance Entity Group is defined for each of the working and
   protection entities.

6.4.1.  Fault Detection

   MPLS-TP OAM tools may be used proactively to detect the following
   fault conditions between MEPs:

   o  Loss of continuity and misconnectivity - the proactive Continuity
      Check (CC) function is used to detect loss of continuity between
      two MEPs in an MEG.  The proactive Connectivity Verification (CV)
      allows a sink MEP to detect a misconnectivity defect (e.g.,
      mismerge or misconnection) with its peer source MEP when the
      received packet carries an incorrect ME identifier.  For
      protection switching, it is common to run a CC-V (Continuity Check
      and Connectivity Verification) message every 3.33 ms.  In the
      absence of three consecutive CC-V messages, loss of continuity is
      declared and is notified locally to the edge of the recovery
      domain in order to trigger a recovery action.  In some cases, when
      a slower recovery time is acceptable, it is also possible to
      lengthen the transmission rate.

   o  Signal degradation - notification from OAM performance monitoring
      indicating degradation in the working entity may also be used as a
      trigger for protection switching.  In the event of degradation,
      switching to the recovery entity is necessary only if the recovery
      entity can guarantee better conditions.  Degradation can be
      measured by proactively activating MPLS-TP OAM packet loss
      measurement or delay measurement.

   o  A MEP can receive an indication from its sink MEP of a Remote
      Defect Indication and locally notify the end point of the recovery
      domain regarding the fault condition, in order to trigger the
      recovery action.

6.4.2.  Testing for Faults

   The management plane may be used to initiate the testing of links,
   LSP segments, or entire LSPs.

   MPLS-TP provides OAM tools that may be manually invoked on-demand for
   a limited period, in order to troubleshoot links, LSP segments, or
   entire LSPs (e.g., diagnostics, connectivity verification, packet

Sprecher & Farrel             Informational                    [Page 44]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   loss measurements, etc.).  On-demand monitoring covers a combination
   of "in-service" and "out-of-service" monitoring functions.  Out-of-
   service testing is supported by the OAM on-demand lock operation.
   The lock operation temporarily disables the transport entity (LSP,
   LSP segment, or link), preventing the transmission of all types of
   traffic, with the exceptions of test traffic and OAM (dedicated to
   the locked entity).

   [RFC6371] describes the operations of the OAM functions that may be
   initiated on-demand and provides some considerations.

   MPLS-TP also supports in-service and out-of-service testing of the
   recovery (protection and restoration) mechanism, the integrity of the
   protection/recovery transport paths, and the coordination protocol
   between the end points of the recovery domain.  The testing operation
   emulates a protection-switching request but does not perform the
   actual switching action.

6.4.3.  Fault Localization

   MPLS-TP provides OAM tools to locate a fault and determine its
   precise location.  Fault detection often only takes place at key
   points in the network (such as at LSP end points or at MEPs).  This
   means that a fault may be located anywhere within a segment of the
   relevant LSP.  Finer information granularity is needed to implement
   optimal recovery actions or to diagnose the fault.  On-demand tools
   like trace-route, loopback, and on-demand CC-V can be used to
   localize a fault.

   The information may be notified locally to the end point of the
   recovery domain to allow implementation of optimal recovery action.
   This may be useful for the re-calculation of a recovery path.

   The information should also be reported to network management for
   diagnostic purposes.

6.4.4.  Fault Reporting

   The end points of a recovery domain should be able to detect fault
   conditions in the recovery domain and to notify the management plane.

   In addition, a node within a recovery domain that detects a fault
   condition should also be able to report this to network management.
   Network management should be capable of correlating the fault reports
   and identifying the source of the fault.

   MPLS-TP OAM tools support a function where an intermediate node along
   a path is able to send an alarm report message to the MEP, indicating

Sprecher & Farrel             Informational                    [Page 45]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   the presence of a fault condition in the server layer that connects
   it to its adjacent node.  This capability allows a MEP to suppress
   alarms that may be generated as a result of a failure condition in
   the server layer.

6.4.5.  Coordination of Recovery Actions

   As described above, in some cases (such as in bidirectional
   protection switching, etc.) it is necessary to coordinate the
   protection states between the edges of the recovery domain.
   [MPLS-TP-LP] defines procedures, protocol messages, and elements for
   this purpose.

   The protocol is also used to signal administrative requests (e.g.,
   manual switch, etc.), but only when these are provisioned at the edge
   of the recovery domain.

   The protocol also enables mismatches to be detected between the
   configurations at the ends of the protection domain (such as timers,
   revertive/non-revertive behavior); these mismatches can subsequently
   be reported to the management plane.

   In the absence of suitable coordination (owing to failures in the
   delivery or processing of the coordination protocol messages),
   protection switching will fail.  This means that the operation of the
   protocol that coordinates the protection state is a fundamental part
   of protection switching.

6.5.  Control Plane

   The GMPLS control plane has been proposed as the control plane for
   MPLS-TP [RFC5317].  Since GMPLS was designed for use in transport
   networks, and since it has been implemented and deployed in many
   networks, it is not surprising that it contains many features that
   support a high degree of survivability.

   The signaling elements of the GMPLS control plane utilize extensions
   to the Resource Reservation Protocol (RSVP) (as described in a series
   of documents commencing with [RFC3471] and [RFC3473]), although it is
   based on [RFC3209] and [RFC2205].  The architecture for GMPLS is
   provided in [RFC3945], while [RFC4426] gives a functional description
   of the protocol extensions needed to support GMPLS-based recovery
   (i.e., protection and restoration).

   A further control-plane protocol called the Link Management Protocol
   (LMP) [RFC4204] is part of the GMPLS protocol family and can be used
   to coordinate fault localization and reporting.

Sprecher & Farrel             Informational                    [Page 46]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   Clearly, the control-plane techniques described here only apply where
   an MPLS-TP control plane is deployed and operated.  All mandatory
   MPLS-TP survivability features must be enabled, even in the absence
   of the control plane.  However, when present, the control plane may
   be used to provide alternative mechanisms that may be desirable,
   since they offer simple automation or a richer feature set.

6.5.1.  Fault Detection

   The control plane is unable to detect data-plane faults.  However, it
   does provide mechanisms that detect control-plane faults, and these
   can be used to recognize data-plane faults when it is evident that
   the control and data planes are fate-sharing.  Although [RFC5654]
   specifies that MPLS-TP must support an out-of-band control channel,
   it does not insist that it be used exclusively.  This means that
   there may be deployments where an in-band (or at least an in-fiber)
   control channel is used.  In this scenario, failure of the control
   channel can be used to infer that there is a failure of the data
   channel, or, at least, it can be used to trigger an investigation of
   the health of the data channel.

   Both RSVP and LMP provide a control channel "keep-alive" mechanism
   (called the Hello message in both cases).  Failure to receive a
   message in the configured/negotiated time period indicates a control-
   plane failure.  GMPLS routing protocols ([RFC4203] and [RFC5307])
   also include keep-alive mechanisms designed to detect routing
   adjacency failures.  Although these keep-alive mechanisms tend to
   operate at a relatively low frequency (on the order of seconds), it
   is still possible that the first indication of a control-plane fault
   will be received through the routing protocol.

   Note, however, that care must be taken to ascertain that a specific
   failure is not caused by a problem in the control-plane software or
   in a processor component at the far end of a link.

   Because of the various issues involved, it is not recommended that
   the control plane be used as the primary mechanism for fault
   detection in an MPLS-TP network.

6.5.2.  Testing for Faults

   The control plane may be used to initiate and coordinate the testing
   of links, LSP segments, or entire LSPs.  This is important in some
   technologies where it is necessary to halt data transmission while
   testing, but it may also be useful where testing needs to be
   specifically enabled or configured.

Sprecher & Farrel             Informational                    [Page 47]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   LMP provides a control-plane mechanism to test the continuity and
   connectivity (and naming) of individual links.  A single management
   operation is required to initiate the test at one end of the link,
   while the LMP handles the coordination with the other end of the
   link.  The test mechanism for an MPLS packet link relies on the LMP
   Test message inserted into the data stream at one end of the link and
   extracted at the other end of the link.  This mechanism need not
   disrupt data flowing over the link.

   Note that a link in the LMP may, in fact, be an LSP tunnel used to
   form a link in the MPLS-TP network.

   GMPLS signaling (RSVP) offers two mechanisms that may also assist
   with fault testing.  The first mechanism [RFC3473] defines the
   Admin_Status object that allows an LSP to be set into "testing mode".
   The interpretation of this mode is implementation-specific and could
   be documented more precisely for MPLS-TP.  The mode sets the whole
   LSP into a state where it can be tested; this need not be disruptive
   to data traffic.

   The second mechanism provided by GMPLS to support testing is
   described in [GMPLS-OAM].  This protocol extension supports the
   configuration (including enabling and disabling) of OAM mechanisms
   for a specific LSP.

6.5.3.  Fault Localization

   Fault localization is the process whereby the exact location of a
   fault is determined.  Fault detection often only takes place at key
   points in the network (such as at LSP end points or at MEPs).  This
   means that a fault may be located anywhere within a segment of the
   relevant LSP.

   If segment or end-to-end protection is in use, this level of
   information is often sufficient to repair the LSP.  However, if finer
   information granularity is required (either to implement optimal
   recovery actions or to diagnose a fault), it is necessary to localize
   the specific fault.

   LMP provides a cascaded test-and-propagate mechanism that is designed
   specifically for this purpose.

6.5.4.  Fault Status Reporting

   GMPLS signaling uses the Notify message to report fault status
   [RFC3473].  The Notify message can apply to a single LSP or can carry
   fault information for a set of LSPs, in order to improve the
   scalability of fault notification.

Sprecher & Farrel             Informational                    [Page 48]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   Since the Notify message is targeted at a specific node, it can be
   delivered rapidly without requiring hop-by-hop processing.  It can be
   targeted at LSP end points or at segment end points (such as MEPs).
   The target points for Notify messages can be manually configured
   within the network, or they may be signaled when the LSP is set up.

   This enables the process to be made consistent with segment
   protection as well as with the concept of Maintenance Entities.

   GMPLS signaling also provides a slower, hop-by-hop mechanism for
   reporting individual LSP faults on a hop-by-hop basis using PathErr
   and ResvErr messages.

   [RFC4783] provides a mechanism to coordinate alarms and other event
   or fault information through GMPLS signaling.  This mechanism is
   useful for understanding the status of the resources used by an LSP
   and for providing information as to why an LSP is not functioning;
   however, it is not intended to replace other fault-reporting

   GMPLS routing protocols [RFC4203] and [RFC5307] are used to advertise
   link availability and capabilities within a GMPLS-enabled network.
   Thus, the routing protocols can also provide indirect information
   about network faults; that is, the protocol may stop advertising or
   may withdraw the advertisement for a failed link, or it may advertise
   that the link is about to be shut down gracefully [RFC5817].  This
   mechanisms is, however, not normally considered to be fast enough for
   use as a trigger for protection switching.

6.5.5.  Coordination of Recovery Actions

   Fault coordination is an important feature for certain protection
   mechanisms (such as bidirectional 1:1 protection).  The use of the
   GMPLS Notify message for this purpose is described in [RFC4426];
   however, specific message field values have not yet been defined for
   this operation.

   Further work is needed in GMPLS for control and configuration of
   reversion behavior for end-to-end and segment protection, and the
   coordination of timer values.

6.5.6.  Establishment of Protection and Restoration LSPs

   The management plane may be used to set up protection and recovery
   LSPs, but, when present, the control plane may be used.

Sprecher & Farrel             Informational                    [Page 49]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   Several protocol extensions exist that simplify this process:

   o  [RFC4872] provides features that support end-to-end protection

   o  [RFC4873] describes the establishment of a single, segment-
      protected LSP.  Note that end-to-end protection is a special case
      of segment protection, and [RFC4872] can also be used to provide
      end-to-end protection.

   o  [RFC4874] allows an LSP to be signaled with a request that its
      path exclude specified resources such as links, nodes, and shared
      risk link groups (SRLGs).  This allows a disjoint protection path
      to be requested or a recovery path to be set up to avoid failed

   o  Lastly, it should be noted that [RFC5298] provides an overview of
      the GMPLS techniques available to achieve protection in multi-
      domain environments.

7.  Pseudowire Recovery Considerations

   Pseudowires provide end-to-end connectivity over the MPLS-TP network
   and may comprise a single pseudowire segment, or multiple segments
   "stitched" together to provide end-to-end connectivity.

   The pseudowire may, itself, require protection, in order to meet the
   service-level guarantees of its SLA.  This protection could be
   provided by the MPLS-TP LSPs that support the pseudowire, or could be
   a feature of the pseudowire layer itself.

   As indicated above, the functional architecture described in this
   document applies to both LSPs and pseudowires.  However, the recovery
   mechanisms for pseudowires are for further study and will be defined
   in a separate document by the PWE3 working group.

7.1.  Utilization of Underlying MPLS-TP Recovery

   MPLS-TP PWs are carried across the network inside MPLS-TP LSPs.
   Therefore, an obvious way to provide protection for a PW is to
   protect the LSP that carries it.  Such protection can take any of the
   forms described in this document.  The choice of recovery scheme will
   depend on the required speed of recovery and the traffic loss that is
   acceptable for the SLA that the PW is providing.

   If the PW is a Multi-Segment PW, then LSP recovery can only protect
   the PW in individual segments.  This means that a single LSP recovery
   action cannot protect against a failure of a PW switching point (an

Sprecher & Farrel             Informational                    [Page 50]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   S-PE), nor can it protect more than one segment at a time, since the
   LSP tunnel is terminated at each S-PE.  In this respect, LSP
   protection of a PW is very similar to link-level protection offered
   to the MPLS-TP LSP layer by an underlying network layer (see Section

7.2.  Recovery in the Pseudowire Layer

   Recovery in the PW layer can be provided by simply running separate
   PWs end-to-end.  Other recovery mechanisms in the PW layer, such as
   segment or concatenated segment recovery, or service-level recovery
   involving survivability of T-PE or AC faults will be described in a
   separate document.

   As with any recovery mechanism, it is important to coordinate between
   layers.  This coordination is necessary to ensure that actions
   associated with recovery mechanisms are only performed in one layer
   at a time (that is, the recovery of an underlying LSP needs to be
   coordinated with the recovery of the PW itself).  It also makes sure
   that the working and protection PWs do not both use the same MPLS
   resources within the network (for example, by running over the same
   LSP tunnel; see also Section 4.9).

8.  Manageability Considerations

   Manageability of MPLS-TP networks and their functions is discussed in
   [RFC5950].  OAM features are discussed in [RFC6371].

   Survivability has some key interactions with management, as described
   in this document.  In particular:

   o  Recovery domains may be configured in a way that prevents one-to-
      one correspondence between the MPLS-TP network and the recovery

   o  Survivability policies may be configured per network, per recovery
      domain, or per LSP.

   o  Configuration of OAM may involve the selection of MEPs; enabling
      OAM on network segments, spans, and links; and the operation of
      OAM on LSPs, concatenated LSP segments, and LSP segments.

   o  Manual commands may be used to control recovery functions,
      including forcing recovery and locking recovery actions.

   See also the considerations regarding security for management and OAM
   in Section 9 of this document.

Sprecher & Farrel             Informational                    [Page 51]

RFC 6372             MPLS-TP Survivability Framework      September 2011

9.  Security Considerations

   This framework does not introduce any new security considerations;
   general issues relating to MPLS security can be found in [RFC5920].

   However, several points about MPLS-TP survivability should be noted

   o  If an attacker is able to force a protection switch-over, this may
      result in a small perturbation to user traffic and could result in
      extra traffic being preempted or displaced from the protection
      resources.  In the case of 1:n protection or shared mesh
      protection, this may result in other traffic becoming unprotected.
      Therefore, it is important that OAM protocols for detecting or
      notifying faults use adequate security to prevent them from being
      used (through the insertion of bogus messages or through the
      capture of legitimate messages) to falsely trigger a recovery

   o  If manual commands are modified, captured, or simulated (including
      replay), it might be possible for an attacker to perform forced
      recovery actions or to impose lock-out.  These actions could
      impact the capability to provide the recovery function and could
      also affect the normal operation of the network for other traffic.
      Therefore, management protocols used to perform manual commands
      must allow the operator to use appropriate security mechanisms.
      This includes verification that the user who performs the commands
      has appropriate authorization.

   o  If the control plane is used to configure or operate recovery
      mechanisms, the control-plane protocols must also be capable of
      providing adequate security.

10.  Acknowledgments

   Thanks to the following people for useful comments and discussions:
   Italo Busi, David McWalter, Lou Berger, Yaacov Weingarten, Stewart
   Bryant, Dan Frost, Lievren Levrau, Xuehui Dai, Liu Guoman, Xiao Min,
   Daniele Ceccarelli, Scott Bradner, Francesco Fondelli, Curtis
   Villamizar, Maarten Vissers, and Greg Mirsky.

   The Editors would like to thank the participants in ITU-T Study Group
   15 for their detailed review.

   Some figures and text on shared mesh protection were borrowed from
   [MPLS-TP-MESH] with thanks to Tae-sik Cheung and Jeong-dong Ryoo.

Sprecher & Farrel             Informational                    [Page 52]

RFC 6372             MPLS-TP Survivability Framework      September 2011

11.  References

11.1.  Normative References

   [G.806]        ITU-T, "Characteristics of transport equipment -
                  Description methodology and generic functionality",
                  Recommendation G.806, January 2009.

   [G.808.1]      ITU-T, "Generic Protection Switching - Linear trail
                  and subnetwork protection", Recommendation G.808.1,
                  December 2003.

   [G.841]        ITU-T, "Types and Characteristics of SDH Network
                  Protection Architectures", Recommendation G.841,
                  October 1998.

   [RFC2205]      Braden, R., Ed., Zhang, L., Berson, S., Herzog, S.,
                  and S. Jamin, "Resource ReSerVation Protocol (RSVP) --
                  Version 1 Functional Specification", RFC 2205,
                  September 1997.

   [RFC3209]      Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan,
                  V., and G. Swallow, "RSVP-TE: Extensions to RSVP for
                  LSP Tunnels", RFC 3209, December 2001.

   [RFC3471]      Berger, L., Ed., "Generalized Multi-Protocol Label
                  Switching (GMPLS) Signaling Functional Description",
                  RFC 3471, January 2003.

   [RFC3473]      Berger, L., Ed., "Generalized Multi-Protocol Label
                  Switching (GMPLS) Signaling Resource ReserVation
                  Protocol-Traffic Engineering (RSVP-TE) Extensions",
                  RFC 3473, January 2003.

   [RFC3945]      Mannie, E., Ed., "Generalized Multi-Protocol Label
                  Switching (GMPLS) Architecture", RFC 3945, October

   [RFC4203]      Kompella, K., Ed., and Y. Rekhter, Ed., "OSPF
                  Extensions in Support of Generalized Multi-Protocol
                  Label Switching (GMPLS)", RFC 4203, October 2005.

   [RFC4204]      Lang, J., Ed., "Link Management Protocol (LMP)", RFC
                  4204, October 2005.

Sprecher & Farrel             Informational                    [Page 53]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   [RFC4427]      Mannie, E., Ed., and D. Papadimitriou, Ed., "Recovery
                  (Protection and Restoration) Terminology for
                  Generalized Multi-Protocol Label Switching (GMPLS)",
                  RFC 4427, March 2006.

   [RFC4428]      Papadimitriou, D., Ed., and E. Mannie, Ed., "Analysis
                  of Generalized Multi-Protocol Label Switching
                  (GMPLS)-based Recovery Mechanisms (including
                  Protection and Restoration)", RFC 4428, March 2006.

   [RFC4873]      Berger, L., Bryskin, I., Papadimitriou, D., and A.
                  Farrel, "GMPLS Segment Recovery", RFC 4873, May 2007.

   [RFC5307]      Kompella, K., Ed., and Y. Rekhter, Ed., "IS-IS
                  Extensions in Support of Generalized Multi-Protocol
                  Label Switching (GMPLS)", RFC 5307, October 2008.

   [RFC5317]      Bryant, S., Ed., and L. Andersson, Ed., "Joint Working
                  Team (JWT) Report on MPLS Architectural Considerations
                  for a Transport Profile", RFC 5317, February 2009.

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

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

   [RFC5921]      Bocci, M., Ed., Bryant, S., Ed., Frost, D., Ed.,
                  Levrau, L., and L. Berger, "A Framework for MPLS in
                  Transport Networks", RFC 5921, July 2010.

   [RFC5950]      Mansfield, S., Ed., Gray, E., Ed., and K. Lam, Ed.,
                  "Network Management Framework for MPLS-based Transport
                  Networks", RFC 5950, September 2010.

   [RFC6371]      Buci, I., Ed. and B. Niven-Jenkins, Ed., "A Framework
                  for MPLS in Transport Networks", RFC 6371, September

11.2.  Informative References

   [GMPLS-OAM]    Takacs, A., Fedyk, D., and J. He, "GMPLS RSVP-TE
                  extensions for OAM Configuration", Work in Progress,
                  July 2011.

Sprecher & Farrel             Informational                    [Page 54]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   [MPLS-TP-LP]   Weingarten, Y., Osborne, E., Sprecher, N., Fulignoli,
                  A., Ed., and Y. Weingarten, Ed., "MPLS-TP Linear
                  Protection", Work in Progress, August 2011.

   [MPLS-TP-MESH] Cheung, T. and J. Ryoo, "MPLS-TP Shared Mesh
                  Protection", Work in Progress, April 2011.

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

   [RFC3386]      Lai, W., Ed., and D. McDysan, Ed., "Network Hierarchy
                  and Multilayer Survivability", RFC 3386, November

   [RFC3469]      Sharma, V., Ed., and F. Hellstrand, Ed., "Framework
                  for Multi-Protocol Label Switching (MPLS)-based
                  Recovery", RFC 3469, February 2003.

   [RFC4397]      Bryskin, I. and A. Farrel, "A Lexicography for the
                  Interpretation of Generalized Multiprotocol Label
                  Switching (GMPLS) Terminology within the Context of
                  the ITU-T's Automatically Switched Optical Network
                  (ASON) Architecture", RFC 4397, February 2006.

   [RFC4426]      Lang, J., Ed., Rajagopalan, B., Ed., and D.
                  Papadimitriou, Ed., "Generalized Multi-Protocol Label
                  Switching (GMPLS) Recovery Functional Specification",
                  RFC 4426, March 2006.

   [RFC4726]      Farrel, A., Vasseur, J.-P., and A. Ayyangar, "A
                  Framework for Inter-Domain Multiprotocol Label
                  Switching Traffic Engineering", RFC 4726, November

   [RFC4783]      Berger, L., Ed., "GMPLS - Communication of Alarm
                  Information", RFC 4783, December 2006.

   [RFC4872]      Lang, J., Ed., Rekhter, Y., Ed., and D. Papadimitriou,
                  Ed., "RSVP-TE Extensions in Support of End-to-End
                  Generalized Multi-Protocol Label Switching (GMPLS)
                  Recovery", RFC 4872, May 2007.

   [RFC4874]      Lee, CY., Farrel, A., and S. De Cnodder, "Exclude
                  Routes - Extension to Resource ReserVation Protocol-
                  Traffic Engineering (RSVP-TE)", RFC 4874, April 2007.

Sprecher & Farrel             Informational                    [Page 55]

RFC 6372             MPLS-TP Survivability Framework      September 2011

   [RFC5212]      Shiomoto, K., Papadimitriou, D., Le Roux, JL.,
                  Vigoureux, M., and D. Brungard, "Requirements for
                  GMPLS-Based Multi-Region and Multi-Layer Networks
                  (MRN/MLN)", RFC 5212, July 2008.

   [RFC5298]      Takeda, T., Ed., Farrel, A., Ed., Ikejiri, Y., and JP.
                  Vasseur, "Analysis of Inter-Domain Label Switched Path
                  (LSP) Recovery", RFC 5298, August 2008.

   [RFC5817]      Ali, Z., Vasseur, JP., Zamfir, A., and J. Newton,
                  "Graceful Shutdown in MPLS and Generalized MPLS
                  Traffic Engineering Networks", RFC 5817, April 2010.

   [RFC5920]      Fang, L., Ed., "Security Framework for MPLS and GMPLS
                  Networks", RFC 5920, July 2010.

   [RFC6373]      Andersson, L., Ed., Berger, L., Ed., Fang, L., Ed.,
                  and Bitar, N., Ed, and E. Gray, Ed., "MPLS-TP Control
                  Plane Framework", RFC 6373, September 2011.

   [RFC6291]      Andersson, L., van Helvoort, H., Bonica, R.,
                  Romascanu, D., and S. Mansfield, "Guidelines for the
                  Use of the "OAM" Acronym in the IETF", BCP 161, RFC
                  6291, June 2011.

   [ROSETTA]      Van Helvoort, H., Ed., Andersson, L., Ed., and N.
                  Sprecher, Ed., "A Thesaurus for the Terminology used
                  in Multiprotocol Label Switching Transport Profile
                  (MPLS-TP) drafts/RFCs and ITU-T's Transport Network
                  Recommendations", Work in Progress, June 2011.

Authors' Addresses

   Nurit Sprecher (editor)
   Nokia Siemens Networks
   3 Hanagar St.
   Neve Ne'eman B Hod
   Hasharon, 45241 Israel

   EMail: nurit.sprecher@nsn.com

   Adrian Farrel (editor)
   Juniper Networks

   EMail: adrian@olddog.co.uk

Sprecher & Farrel             Informational                    [Page 56]