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Versions: 00 01 draft-ietf-mpls-tp-survive-fwk

Network Working Group                                   N. Sprecher, Ed.
Internet Draft                                    Nokia Siemens Networks
Category: Informational                                   A. Farrel, Ed.
Created: February 25, 2009                            Old Dog Consulting
Expires: August 25, 2009

               Multiprotocol Label Switching Transport Profile
                           Survivability Framework

                  draft-sprecher-mpls-tp-survive-fwk-01.txt

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Abstract

   Network survivability is the network's ability to restore traffic
   following failure or attack; it plays a critical factor in the
   delivery of reliable services in transport networks. Guaranteed
   services in the form of Service Level Agreements (SLAs) require a
   resilient network that detects facility or node failures very
   rapidly, and immediately starts to restore network operations in
   accordance with the terms of the SLA.

   The Transport Profile of Multiprotocol Label Switching (MPLS-TP) is a
   packet transport technology that combines the packet experience of
   MPLS with the operational experience of transport networks like
   SONET/SDH. It provides survivability mechanisms such as protection
   and restoration, with similar function levels to those found in
   established transport networks such as in SONET/SDH networks. Some of
   the MPLS-TP survivability mechanisms are data plane-driven and are
   based on MPLS-TP OAM fault management functions which are used to
   trigger protection switching in the absence of a control plane. Other


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   survivability mechanisms utilize the MPLS-TP control plane.

   This document provides a framework for MPLS-TP survivability.

Table of Contents

   1. Introduction ................................................... 3
   2. Terminology and References ..................................... 6
   3. Requirements for Survivability ................................. 7
   4. Functional Architecture ........................................ 9
   4.1. Elements of Control .......................................... 9
   4.1.1. Manual Control ............................................. 9
   4.1.2. Failure-Triggered Actions ................................. 10
   4.1.3. OAM Signaling ............................................. 10
   4.1.4. Control Plane Signaling ................................... 10
   4.2. Elements of Recovery ........................................ 11
   4.2.1. Span Recovery ............................................. 11
   4.2.2. Segment Recovery .......................................... 12
   4.2.3. End-to-End Recovery ....................................... 12
   4.3. Levels of Recovery .......................................... 12
   4.3.1. Dedicated Protection ...................................... 13
   4.3.2. Shared Protection ......................................... 13
   4.3.3. Extra Traffic ............................................. 13
   4.3.4. Restoration and Repair .................................... 14
   4.3.5. Reversion ................................................. 15
   4.4. Mechanisms for Recovery ..................................... 15
   4.4.1. Link-Level Protection ..................................... 15
   4.4.2. Alternate Paths and Segments .............................. 16
   4.4.3. Bypass Tunnels ............................................ 16
   4.5. Protection in Different Topologies .......................... 17
   4.5.1. Mesh Networks ............................................. 17
   4.5.2. Ring Networks ............................................. 21
   4.5.3. Protection and Restoration Domains ........................ 22
   4.6. Recovery in Layered Networks .. ............................. 23
   4.6.1. Inherited Link-Level Protection ........................... 23
   4.6.2. Shared Risk Groups ........................................ 23
   4.6.3. Fault Correlation ......................................... 23
   5. Mechanisms for Providing Protection in MPLS-TP ................ 24
   5.1. Management Plane ............................................ 24
   5.1.1. Configuration of Protection Operation ..................... 24
   5.1.2. External Manual Commands .................................. 25
   5.2. Fault Detection ............................................. 25
   5.3. Fault Isolation ............................................. 25
   5.4. OAM Signaling ............................................... 25
   5.4.1. Fault Detection ........................................... 25
   5.4.2. Fault Isolation ........................................... 25
   5.4.3. Fault Reporting ........................................... 25
   5.4.4. Coordination of Recovery Actions .......................... 26


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   5.5. Control Plane ............................................... 26
   5.5.1. Fault Detection ........................................... 26
   5.5.2. Testing for Faults ........................................ 27
   5.5.3. Fault Isolation ........................................... 28
   5.5.4. Fault Reporting ........................................... 28
   5.5.5. Coordination of Recovery Actions .......................... 29
   5.5.6. Establishment of Protection and Restoration LSPs .......... 29
   6. Pseudowire Protection Considerations .......................... 29
   6.1. Utilizing Underlying MPLS-TP Protection ..................... 30
   6.2. Protection in the Pseudowire Layer .......................... 30
   7. Manageability Considerations .................................. 30
   8. Security Considerations ....................................... 30
   9. IANA Considerations ........................................... 30
   10. Acknowledgments .............................................. 30
   11. References ................................................... 30
   11.1. Normative References ....................................... 30
   11.2. Informative References ..................................... 32
   12. Editors' Addresses ........................................... 33
   13. Author's Address ............................................. 33
   14. Intellectual Property Statement .............................. 33

1. Introduction

   Network survivability is the network's ability to restore traffic
   following failure or attack; it plays a critical factor 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
   failures, and immediately starts to restore network operations in
   accordance with the terms of the SLA.

   The Transport Profile of Multiprotocol Label Switching (MPLS-TP)
   [RFC5317], [MPLS-TP-REQ] is a packet transport technology that
   combines the packet experience of MPLS with the operational
   experience of transport networks such as SONET/SDH. MPLS-TP is
   designed to be consistent with existing transport network operations
   and management models and provide survivability mechanisms, such as
   protection and restoration, with similar function levels to those
   found in established transport networks (such as the SONET/SDH
   networks which provided service providers with a high benchmark for
   reliability).

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

   Various recovery schemes (for protection and restoration) and


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   processes have been defined and analyzed in [RFC4427] and [RFC4428].
   These schemes may also be applied in MPLS-TP networks to re-establish
   end-to-end traffic delivery within the agreed service level and to
   recover from 'failed' or 'degraded' transport entities (links or
   nodes). Such actions are normally initiated by the detection of a
   defect or performance degradation, or by an external request (e.g.,
   an operator request for manual control of protection switching).

   [RFC4427] makes a distinction between protection switching and
   restoration mechanisms. Protection switching makes use of
   pre-assigned capacity between nodes, where the simplest scheme has
   one dedicated protection entity for each working entity, while the
   most complex scheme has m protection entities shared between n
   working entities (m:n). Protection switching may be either
   unidirectional or bidirectional; unidirectional meaning that each
   direction of a bidirectional connection is protection switched
   independently, while bidirectional means that both directions are
   switched at the same time even if the fault applies to only one
   direction of the connection. Restoration uses any capacity available
   between nodes and usually involves re-routing. The resources used for
   restoration may be pre-planned and recovery priority may be used as a
   differentiation mechanism to determine which services are recovered
   and which are not recovered or are sacrificed in order to achieve
   recovery of other services. In general, protection actions are
   completed within time frames of tens of milliseconds, while
   restoration actions are normally completed in periods ranging from
   hundreds of milliseconds to a maximum of a few seconds.

   The recovery schemes described in [RFC4427] and evaluated in
   [RFC4428] assume some control plane-driven actions that are performed
   in the recovery context (such as the configuration of the protection
   entities and functions, etc.). As for other transport technologies
   and associated transport networks, the presence of a distributed
   control plane in support of MPLS-TP network operations is optional,
   and the absence of such a control plane does not affect the ability
   to operate the network and to use MPLS-TP forwarding, OAM, and
   survivability capabilities.

   Thus, some of the MPLS-TP recovery mechanisms do not depend on a
   control plane and rely on MPLS-TP OAM capabilities to trigger
   protection switching across connections that were set up using
   management plane configuration. These mechanisms are data plane-
   driven and are based on MPLS-TP OAM fault management functions.
   "Fault management" in this context refers to failure detection,
   localization, and notification (where the term "failure" is used to
   represent both signal failure and signal degradation).

   The principles of MPLS-TP protection switching operation are similar


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   to those described in [RFC4427] as the protection mechanism is based
   on the ability to detect certain defects in the transport entities
   within the protected domain. The protection switching controller does
   not care which monitoring method is used, as long as it can be given
   information about the status of the transport entities within the
   recovery domain (e.g., 'OK', signal failure, signal degradation,
   etc.).

   An MPLS-TP Protection State Coordination (PSC) protocol may be
   used as an in-band (i.e., data plane-based) control protocol to align
   both ends of the protected domain.

   The MPLS-TP recovery mechanisms may be applied at various nested
   levels throughout the MPLS-TP network, as is the case with the
   recovery schemes defined in [RFC4427] and [RFC4873]. A Label
   Switching Path (LSP) may be subject to any or all of MPLS-TP link
   recovery, path segment recovery, or end-to-end recovery, where:

   - 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 neighboring label switching routers (LSRs).

   - Segment recovery refers to the recovery of an LSP segment (i.e.,
     segment and concatenated segment in the language of [MPLS-TP-REQ])
     between two nodes which are the boundary nodes of the segment

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

   Multiple recovery levels may be used concurrently by a single LSP for
   added resiliency.

   It is a basic requirement of MPLS-TP that both directions of a
   bidirectional LSP should be co-routed (that is, share the same route
   within the network) and be fate-sharing (that is, if one direction
   fails, both directions should cease to operate) [MPLS-TP-REQ]. This
   causes a direct interaction between the recovery levels affecting
   the directions of an LSP such that both directions of the LSP are
   switched to a new MPLS-TP link, segment, or end-to-end path together.

   The recovery scheme operating at the data plane level can function
   in a multi-domain environment; it should also protect against a
   failure of a boundary node in the case of inter-domain operation.

   The MPLS-TP recovery schemes apply to LSPs and PWE3. This document
   focuses on LSPs and handles both point-to-point (P2P) and point-to-
   multipoint (P2MP) LSPs.



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   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 sustain the reference
   model. The reference model is based on the MPLS-TP OAM reference
   model which is defined in [MPLS-TP-OAM].

   The framework also describes the qualitative levels of the
   survivability functions that can be provided, such as dedicated
   recovery, shared protection, restoration, etc. The level of recovery
   directly affects the service level provided to the end user in the
   event of a network failure. There is a correlation between the level
   of recovery provided and the cost to the network.

   This framework applies to general recovery schemes, but also for
   schemes that are optimized for specific topologies, such as mesh and
   ring, in order to handle protection switching in a cost-efficient
   manner.

   This document takes into account the timing co-ordination of
   protection switches at multiple layers. This prevents races and
   allows the protection switching mechanism of the server layer to fix
   a problem before switching at the MPLS-TP layer.

   This framework also specifies the functions that must be supported by
   MPLS-TP (e.g., PSC) and the management and/or the control plane in
   order to support the recovery mechanisms. MPLS-TP introduces a tool
   kit to enable recovery in MPLS-TP-based transport networks and to
   ensure that affected traffic is recovered in the event of a failure.
   Different recovery levels may be used concurrently by a single LSP
   for added resiliency.

   Generally, network operators aim to provide the fastest, most stable,
   and the best protection mechanism available at a reasonable cost. The
   higher the levels of protection, the greater the number of resources
   consumed. It is therefore expected that network operators will offer
   a wide spectrum of service levels. MPLS-TP-based recovery offers the
   flexibility to select the recovery mechanism, choose the granularity
   at which traffic is protected, and also choose the specific types of
   traffic that are to be protected. With MPLS-TP-based recovery, it is
   possible to provide different levels of protection for different
   classes of service, based on their service requirements.

2. Terminology and References

   The terminology used in this document is consistent with that defined
   in [RFC4427]. That RFC is, itself, consistent with [G.808.1].


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

   Readers should refer to those documents for normative definitions.
   This document supplies brief summaries of some terms for clarity and
   to aid the reader, but does not re-define terms.

   In particular, note the distinction and definitions made in [RFC4427]
   for the following three terms.

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

   - Restoration: re-establishing end-to-end traffic using resources
     allocated at the time of need. Sometimes referred to as "repair".

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

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

3. Requirements for Survivability

   MPLS-TP requirements are presented in [MPLS-TP-REQ]. Survivability is
   presented as a critical factor in the delivery of reliable services,
   and the requirements for survivability are set out using the recovery
   terminology defined in [RFC4427].

   These requirements are summarized below. This section may be updated
   if changes are made to [MPLS-TP-REQ], and that document should be
   regarded as normative for the definition of all MPLS-TP requirements
   including those for survivability.

   General:

   - Must support protection and restoration.
   - Must be applicable at various nested levels, including link, LSP
     segment and LSP end-to-end path, PW segment and end-to-end PW.
   - Should be equally applicable to LSPs and pseudowires.
   - Must provide appropriate recovery times.
   - Should support the configuration of the recovery objectives (such
     as BW and QOS) per transport path.
   - Must scale when many services are affected by a single fault.
   - Must support management plane control.
   - Must support control plane control.
   - Must be applicable for any topology.
   - Must provide coordination between protection mechanisms at


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     different layers.
   - Must provide mechanism to prevent recovery operations thrashing.
   - Must support Physical layer fault indication as a trigger to the
     recovery operation.
   - Must support OAM based triggers to the recovery operation.
   - Must support administrative commands as triggers to the recovery
     operation (e.g. force switch, etc).
   - Must support a mechanism to allow the distinction of recovery
     actions that are initiated by administrative commands from those
     that are initiated by other means.
   - Should support control plane triggers when a control plane is
     available.
   - Must support the management plane configuration of timers used for
     the recovery operation.
   - Must support the management plane configuration of the elements of
     controls (triggers for recovery).
   - Must support the control plane configuration of the recovery
     entities and functions (if the control plane is present).
   - Must support the control plane signaling of an administrative
     commands if the control plane is present).
   - Must support the control plane signaling of the protection state,
     in order to synch the protection state between the edges of the
     protection domain.

   Restoration:

   - Must support soft re-routing (Make-before-break).
   - Must support pre-planning of restoration resources.
   - May support computation of restoration resources after failure.
   - May support shared mesh restoration.
   - May support hard LSP restoration (break-before-make).
   - Must support restoration priority (under operator configuration)
   - Must support preemption priority during restoration (under operator
     configuration).

   Protection:

   - Must support bidirectional 1+1 protection switching (which should
     be the default behavior) and 1+1 unidirectional protection
     switching for P2P paths.
   - Must support bidirectional 1:n protection switching (which should
     be the default behavior) for P2P paths.
   - Must support 1:1 and 1+1 unidirectional protection switching for
     P2MP.
   - Must support protection ration of 100%.
   - Should support 1:n shared mesh protection. (*** contradict the Must
     support above) .
   - Must support shared bandwidth.


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   - Must support the definition of shared protection groups (to allow
     coordination of protection actions).
   - Must support sharing of protection resources.
   - Must support revertive (which is the default behavior) and non-
     revertive behavior.
   - Must support the management plane configuration of the protection
     path and the protection group.
   - Must provide clear indication of the protection state of the
     transport path.
   - May provide different mechanisms optimized for specific topologies
     (such as ring topologies). Such mechanisms must interoperate with
     the mechanisms that are defined for the arbitrary topology). For
     the specific requirements for ring topologies, see Section 4.5.2 on
     rings.

4. Functional Architecture

   This section presents an overview of the elements of the functional
   architecture for survivability within an MPLS-TP network. The
   intention is to break the components out as separate items so that it
   can be seen how they may be combined to provide different levels of
   recovery to meet the requirements set out in the previous section.

4.1. Elements of Control

   Survivability is achieved through specific actions taken to repair
   network resources or to redirect traffic onto paths that avoid
   failures in the network. Those actions may be triggered automatically
   by the network devices (detecting a network failure), may be enhanced
   by in-band (i.e. data-plane based) OAM fault management or
   performance monitoring, in-band or out-of-band control plane
   signaling, or may be under direct the control of an operator.

   These different options are explored in the next sections.

4.1.1. Manual Control

   Of course, the survivability behavior of the network as a whole, and
   the reaction of each LSP when a fault is reported, may be under
   operator control. That is, the operator may establish network-wide or
   local policies that determine what actions will be taken when
   different failures are reported that affect different LSPs. At the
   same time, when a service request is made to cause the establishment
   of one or more LSPs in the network, the operator (or requesting
   application) may express a required or desired level of service, and
   this will be mapped to particular survivability actions taken before
   and during LSP setup, after the failure of network resources, and
   upon recovery of those resources.


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   The operator can also be given manual control of survivability
   actions and events. For example, the operator may force a switchover
   from a working path to a recovery path (for network optimization
   purposes with minimal disturbance of services, like when modifying
   protected or unprotected services, when replacing network elements,
   etc.), inhibit survivability actions, enable or disable survivability
   function, or induce the simulation of a network fault. In some
   circumstances, a fault may be reported to the operator and the
   operator may then select and initiate the appropriate recovery
   action.

4.1.2. Failure-Triggered Actions

   Survivability actions may be directly triggered by network failures.
   That is, the device that detects the failure (for example, Loss of
   Light on an optical interface, or failure to receive an OAM
   continuity message) may immediately perform a survivability action.
   Note that the term "failure" is used to represent both signal failure
   and signal degradation.

   This behavior can be subject to management plane or control plane
   control, but does not require any messages exchanges in any of the
   management plane, control plane, or data plane to trigger the
   recovery action - it is directly triggered by data plane stimuli.
   Note, however, that coordination of recovery actions between the
   edges of the recovery domain may require message exchanges for some
   qualitative levels of recovery.

4.1.3. OAM Signaling

   OAM signaling refers to message exchanges that are in-band or closely
   coupled to the data channel. Such messages may be used to detect and
   isolate faults, but in this context we are concerned with the use of
   these messages to control or trigger survivability actions.

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

4.1.4. Control Plane Signaling

   Control plane signaling is responsible for setup, maintenance, and
   teardown of LSPs that are not under management plane control. The
   control plane can also be used to detect, isolate, and communicate
   network failures pertaining to peer relationships (neighbor-to-
   neighbor, or end-to-end). Thus, control plane signaling can initiate
   and coordinate survivability actions.



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   The control plane can also be used to distribute topology and
   resource-availability information. In this way, "graceful shutdown"
   of resources may be effected by withdrawing them, and this can be
   used as a stimulus to survivability action in a similar way to the
   reporting or discovery of a fault as described in the previous
   sections.

4.2. Elements of Recovery

   This section describes the elements of recovery. These are the
   quantitative aspects of recovery; that is the pieces 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 [MPLS-TP-REQ] a
   mapping is provided.

4.2.1. Span Recovery

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

   - a single data link

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

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

   Thus, span recovery may be provided by:

   - moving the TE link to be supported by a different data link between
     the same pair of neighbors

   - re-routing the LSP in the lower layer.

   Moving the protected LSP to another TE link between the same pair of
   neighbors is known as segment recovery and is described in Section
   4.2.2.

   [MPLS-TP-REQ] refers to a span as a "link".



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4.2.2. Segment Recovery

   An LSP segment is one or more hops on the path of the LSP. In some
   MPLS-TP documents LSP segment is referred as LSP Tandem Connection
   (Note that recovery of pseudowire segments is discussed in Section
   6.)

   Segment recovery involves redirecting traffic from one end of a
   segment of an LSP on an alternate path to the other end of the
   segment. This redirection may be on a pre-established LSP segment,
   through re-routing of the protected segment, or by tunneling the
   protected LSP on a "bypass" LSP.

   Note that protecting an LSP against the failure of a node requires
   the use of segment recovery, while a link could be protected using
   span or segment recovery.

   [MPLS-TP-REQ] defines two terms. A "segment" is a single hop on the
   path of an LSP, and a "concatenated segment" is more than one hop on
   the path of an LSP. In the context of this document, a segment covers
   both of these concepts.

4.2.3. End-to-End Recovery

   End-to-end recovery is a special case of segment recovery where the
   protected LSP segment is the whole of the LSP. 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 recovery path.
   Note that node-diverse paths are necessarily link-diverse, and that
   full, end-to-end node-diversity is required to guarantee recovery.

4.3. Levels of Recovery

   This section describes the qualitative levels of survivability
   function that can be provided. The level of recovery offered has a
   direct effect on the service level provided to the end-user in the
   event of a network fault. This will be observed as the amount of data
   lost when a network fault occurs, and the length of time to recovery
   connectivity.

   In general there is a correlation between the service level (i.e.,
   the rapidity of recovery and reduction of data loss) and the cost to
   the network; better service levels require pre-allocation of
   resources to the recovery paths, and those resources cannot be used
   for other purposes if high quality recovery is required.

   Sections 6 and 7 of [RFC4427] provide a full break down of protection
   and recovery schemes. This section summarizes the qualitative levels


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   available.

4.3.1. Dedicated Protection

   In dedicated protection, the resources for the recovery LSP are
   pre-assigned for use only by the protected service. 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 supported.

   Note that in the bypass tunnel recovery mechanism (see Section 4.4.3)
   resources may also be dedicated to protecting a specific service. In
   some cases (one-for-one protection) the whole of the bypass tunnel
   may be dedicated to provide recovery for a specific LSP, but in other
   cases (such as facility backup) a subset of the resources of the
   bypass tunnel may be pre-assigned for use to recover a specific
   service. However, as described in Section 4.4.3, the bypass tunnel
   approach can also be used for shared protection (Section 4.3.2), to
   carry extra traffic (Section 4.3.3), or without reserving resources
   to achieve best-effort recovery.

4.3.2. Shared Protection

   In shared protection, the resources for the recovery LSPs of several
   services are shared. These may be shared as 1:n or m:n, and may be
   shared on individual links, on LSP segments, or on end-to-end LSPs.

   Where a bypass tunnel is used (Section 4.4.3) the tunnel might not
   have sufficient resources to simultaneously protect all of the LSPs
   to which it offers protection so that if they were all affected by
   network failures at the same time, they would not all be recovered.

   Shared protection is a trade-off between expensive network resources
   being dedicated to protection that is not required most of the time,
   and the risk of unrecoverable services in the event of multiple
   network failures. There is also a trade-off between rapid recovery
   (that can be achieved with dedicated protection, but which is delayed
   by message exchanges in the management, control, or data planes for
   shared protection) and the reduction of network cost by sharing
   protection resources. These trade-offs may be somewhat mitigated by
   using m:n for some value of m <> 1, and by establishing new
   protection paths as each available protection path is put into use.

4.3.3. Extra Traffic

   A way to utilize network resources that would otherwise be idle
   awaiting use to protect services, is to use them to carry other
   traffic. Obviously, this is not practical in dedicated protection
   (Section 4.3.1), but is practical in shared protection (Section


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   4.3.2) and bypass tunnel protection (Section 4.4.3).

   When a network resource that is carrying extra traffic is required
   for protection, the extra traffic is disrupted - essentially it is
   pre-empted by the recovery LSP. This may require some additional
   messages exchanges in the management, control, or data planes, with
   the consequence that recovery may be delayed somewhat. This provides
   an obvious trade-off against the cost reduction (or rather, revenue
   increase) achieved by carrying extra traffic.

4.3.4. Restoration and Repair

   If resources are not pre-assigned for use by the recovery LSP, the
   recovery LSP must be established "on demand" when the network failure
   is detected and reported, or upon instruction from the management
   plane.

   Restoration represents the most cost-effective use of network
   resources as no resources are tied up for specific protection usage.
   However, restoration requires computation of a new path and
   activation of a new LSP (through the management or control plane).
   These steps can take much more time than is required for 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 the use of LSP setup
   priorities so that recovery LSPs can pre-empt other low priority
   LSPs.

   Additionally, when a network failure occurs, multiple LSPs may be
   disrupted by the same event. These LSPs may have been established by
   different Network Management Stations (NMSs) or signaled by different
   head-end LSRs, and this means that multiple points in the network
   will be trying to compute and establish recovery LSPs at the same
   time. This can lead to contention within the network meaning that
   some recovery LSPs must be retried resulting in very slow recovery
   times for some services.

   Both hard or soft LSP restoration may be supported. In hard LSP
   restoration, the resources of the LSP are released before the full
   establishment of the recovery LSP (i.e., break-before-make). In soft
   LSP restoration, the resources of the LSP are released after the full
   establishment of an alternate LSP (i.e., make-before-break).

   Note that the restoration resources may be pre-calculated and even
   pre-signaled before the restoration action starts, but not pre-


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   allocated. This is known as pre-planned LSP restoration. The complete
   establishment/activation of the restoration LSP occurs only when the
   restoration action starts. The pre-planning may happen periodically
   to have the most accurate information about the available resources
   in the network.

4.3.5. Reversion

   When a service has been recovered so that traffic is flowing on the
   recovery LSP, the faulted network resource may be repaired. The
   choice must be made about whether to redirect the traffic back on to
   the original working LSP, or to leave it where it is on the recovery
   LSP. These behaviors are known as "revertive" and "non-revertive",
   respectively.

   In "revertive" mode, care should be taken to prevent frequent
   operation of the recovery operation due to an intermittent defect.
   Therefore, when the failure condition of a recovery element has been
   handled, a fixed period of time should elapse before normal data
   traffic is redirected back onto the original working entity.

4.4. Mechanisms for Recovery

   The purpose of this section is to describe in general (MPLS-TP non-
   specific) terms the mechanisms that can be used to provide
   protection.

4.4.1. Link-Level Protection

   Link-level protection refers to the paradigm whereby protection is
   provided in a lower network layer.

   Link-level protection offers the following levels of protections:

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

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

   When there is no failure on the working link, the protection entity


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   may transport extra traffic which may be preempted when protection
   switching occurs.

   As with recovery in layered networks, the protection mechanism at the
   link-level needs to co-ordinate the timing for switchover, in order
   to avoid race conditions and to enable switchover to be performed at
   the link level before the upper level.

   Note that link-level protection does not protect the nodes at each
   end of the entity (e.g. a link or span) that is protected. End-to-end
   or segment LSP protection should be used to protect against a failure
   of the edge node.

4.4.2. Alternate Paths and Segments

   The alternate paths and segments refer to the paradigm whereby the
   protection is performed at the same network layer of the protected
   LSP/segment-LSP.

   Different levels of protection may be provided:

   - Dedicated protection, where a dedicated entity (e.g. LSP, segment
     LSP) is fully pre-established to protect a working entity (e.g.,
     LSP, segment LSP). When there is a failure condition on the working
     entity, the normal traffic is switched over into the protection
     entity.

     Dedicated protection may be accomplished by the 1:1 or 1+1
     protection schemes. When the failure condition is eliminated, the
     traffic may revert to the working entity. This is subject to local
     configuration.

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

     When the failure condition on the working entity is eliminated, the
     traffic should revert back to the working entity.

4.4.3. Bypass Tunnels

   A bypass tunnels is a transport entity (LSP) that is pre-provisioned
   in order to protect against a failure condition along a network
   segment, which may affect one or more LSPs that transmit over the
   network segment.

   When there is a failure condition in the network segment, one or more
   of the protected LSPs are switched over at the ingress point of the


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   network segment and transmitted over the bypass tunnel. The natural
   way to realize this is using label stacking. Label mapping may be an
   option as well.

   Different levels of protection may be provided:

   - Dedicated protection, where the bypass tunnel has resource
     reservations sufficient to provide protection for all protected
     LSPs without service degradation.

   - Shared protection, where the bypass tunnel has resources to protect
     some of the protected LSPs, but not all of them simultaneously.

4.5. Protection in Different Topologies

   As described in the requirements listed in Section 2.8.5 and detailed
   in [MPLS-TP-REQ], the recovery techniques used may be optimized for
   different network topologies if the performance of those optimized
   mechanisms is significantly better than the performance of the
   generic ones in the same topology.

   It is required that such mechanisms interoperate with the mechanisms
   defined for arbitrary topologies to allow end-to-end protection and
   to allow consistent protection techniques to be used across the whole
   network.

   This section describes two different topologies and explains how
   recovery may be markedly different in those different scenarios. It
   also introduces the concept of a recovery domain and shows how end-
   to-end survivability may be achieved through a concatenation of
   recovery domains each providing some level of recovery in part of the
   network.

4.5.1. Mesh Networks

   Linear protection provides a fast and simple protection switching
   mechanism and fits best in mesh networks. It can protect against a
   failure that may happen on an entity (element of recovery that may
   constitute a span, LSP segment, PW segment, end-to-end LSP or end-to-
   end PW).

   Linear protection operates in the context of a Protection Domain
   which is composed of the following architectural elements:

   - A set of end points which reside at the boundary of the Protection
     Domain. In this simple case of a 1:n or 1+1 P2P entity, exactly two
     endpoint reside at the boundary of the Protection Domain. In each
     transmission direction one of the end points is referred to as a


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     source and the other one is referred to as a sink.

     In the case of unidirectional P2MP, three or more endpoints reside
     at the boundary of the Protection Domain. One of the endpoints is
     referred to as source/root and the other ones are referred to as
     sinks/leaves.

   - A Protection Group which consists of a Working (primary) entity and
     one or more Protection (backup) entities. In order to guarantee
     complete protection, a dedicated Protection entity should be pre-
     provisioned to protect against a failure of the Working entity.
     Also the Working and the Protection entities should be disjoint
     entities, i.e., the physical routes of the Working and the
     Protection entities should have complete physical diversity. Note
     that resources of the Protection entity may be degraded from the
     Working entity. In such a case, the Protection entity may not have
     sufficient resources to protect the traffic of the Working entity.

     As mentioned above in section 4.3.2, the resources of the
     Protection entity may be shared as 1:n. In such a case, the
     Protection entity might not have sufficient resources to
     simultaneously protect all of the Working entities that may be
     affected by fault conditions at the same time.

   Protection switching occurs at the protection controllers which
   reside at the edges of the Protected Domain. The working and
   protection entities reside between these endpoints.

   [MPLS-TP-REQ] requires that both 1:n linear protection scheme and 1+1
   protection schemes are supported. The 1:n protection switching,
   bidirectional protection switching should be supported. In 1+1
   linear protection switching both unidirectional and bidirectional
   protection switching should be supported.

   In bidirectional protection switching, in the event of failure, the
   recovery actions are taken in both directions (even when the fault is
   unidirectional). This requires the synchronization of the recovery
   state between the endpoints of the protection domain.

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

   1:1 linear protection:

   - In normal conditions the data traffic is transmitted over the
     working entity. Normal conditions are defined when there is no
     failure or degradation on the 'working' entity and there is no
     administrative configuration or requests that cause traffic to


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     transmit over the 'protection' entity. Upon a fault condition
     (failure or degradation) or a specific administrative request, the
     traffic is switched over to the 'protection' entity.

     Note that in the non-revertive behavior (see section 4.3.5), data
     traffic can be transmitted over the Protection entity also in
     normal conditions. This can happen after a failure condition on the
     Working entity (which caused a recovery action) is eliminated.

   - In each transmission direction, the source of the protection domain
     bridges the traffic into the appropriate entity and the sink of the
     protected domain selects the traffic from the appropriate entity.
     The source and the sink need to be coordinated to ensure that the
     bridging and the selection are done to and from the same entity.
     For that sake a signaling coordination protocol is needed.

   - In bidirectional protection switching, both ends of the protection
     domain switch to the 'protection' entity (even when the failure is
     unidirectional). This requires a protocol to synchronize the
     protection state between the two end points of the Protection
     Domain.

   - When there is no failure, the resources of the 'idle' entity may be
     used for less priority traffic. When protection switching is
     performed, the less priority traffic may be pre-empted by the
     protected traffic.

   1+1 linear protection:

   - The data traffic is copied at 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 protected domain, where a
     selection between the 'working' and 'protection' entities is made
     (based on some predetermined criteria).

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

     In both protection schemes when the failure condition is
     eliminated, operation, when the failure condition is eliminated,
     the protected traffic may revert back into the Working entity. To
     verify that the network has stabilized, and to avoid frequent
     switching in case of intermittent failures, traffic is not switched
     back to the Working entity before the Wait-to-Restore (WTR) timer


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     has expired.

     Revertive/non-revertive operations are provided as network operator
     options.

     The protection switching may be performed when:

     - A fault condition ('failed' or 'degraded') is declared on the
       active entity and is not declared on the standby entity. OAM CC&V
       (Continuity and Connectivity Verification) monitoring of both
       Working and Protection entities may be used to enable the fast
       detection of a fault condition. For protection switching, it is
       common to run a CC&V every 3.33ms. In the absence of three
       consecutive CC messages, a 'failed' condition is declared. In
       order to monitor the Working and the Protection entities, an OAM
       Maintenance Entity should be defined for each of the entities.
       OAM information should be provided as input to the protection
       switching controllers.

       Input from OAM performance monitoring indicating 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
       guarantee better conditions.

       Note that in bidirectional protection switching, an attempt is
       made to coordinate the protection switching state between both
       end points of the Protection Domain when a unidirectional failure
       is detected or when an external administrative requests is
       received. A PSC (Protection State Coordination) protocol may be
       used for this purpose. This protocol is also used to detect
       mismatches between the provisioned protection switching
       configuration and the two ends of a Protection Domain.

       Note that in order to achieve 50ms protection switching it is
       recommended to use inband signaling protocol to coordinate the
       protection states.

     - An indication is received from a lower layer server that there is
       a network failure.

     - An external operator command is received (e.g., 'Forced Switch',
       'Manual Switch'). For details see Section 5.1.2.

     - A request to switch over is received from the far end (relevant
       in case of bidirectional 1:1 protection switching only).

     Linear protection provides a clear indication of the protection


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     status.

   1:n linear protection:

     In 1:n linear protection, one Protection entity is used to protect
     n Working entities. The Protection entity might not have sufficient
     resources to simultaneously protect all of the Working entities
     that may be affected by fault conditions at the same time.

     Revertive behavior is recommended when 1:n is supported.

   P2MP linear protection:

      Linear protection may apply to protect P2MP path using 1+1
      protection architecture. The source/root LSR bridges the user
      traffic to both the Working and Protected entities. Each sink/leaf
      LSR selects the traffic from one entity based on some
      predetermined criteria.  Note that when there is a fault condition
      on one of the branches of the P2MP path, some leaf LSRs may select
      the Working entity, while other leaf LSRs may select traffic from
      the Protection entity.

      In a 1:1 P2MP protection scheme, the source/root LSR needs to
      identify the existence of a fault condition on any of the branches
      of the network.  This requires the sink/leaf LSRs to notify the
      source/root LSR of any fault condition. This required also a
      return path from the sinks/leaves to the source/root LSR.

      When protection switching is triggered, the source/root LSR
      selects the recovery transport path to transfer the traffic.

4.5.2. Ring Networks

   Several Service Providers have expresses a high level of interest in
   operating MPLS-TP in ring topologies and require a high level of
   survivability function in these topologies.

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

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

   2. Optimization of resource consumption around the ring, like the
      number of labels needed for the protection paths that cross the


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      network, the total bandwidth needed in the ring to ensure the
      protection of the paths, etc.

   [MPLS-TP-REQ] introduces a list of requirements on ring protection
   that cover the recovery mechanisms need to protect traffic in a
   single ring and 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.

   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 [MPLS-TP-REQ]. The principles for the
   development of the mechanisms should be:

   1. Reuse existing procedures and mechanisms for recovery in ring
      topologies as along as their performance is as good as new
      potential mechanisms.

   2. Ensure complete interoperability with the mechanisms defined for
      arbitrary topologies to allow end-to-end protection.

4.5.3. Protection and Restoration Domains

   Protection and restoration are performed in the context of a recovery
   domain. A recovery domain is defined between two recovery reference
   points which are located at the edges of the recovery domain and are
   responsible for performing recovery for a 'working' entity (which may
   be one of the elements of recovery defined above) when an appropriate
   trigger is received. These reference points function as recovery
   controllers.

   As described in section 4.2 above, the recovery element may
   constitute a spam, a tandem connection (i.e. either an LSP segment or
   a PW segment), an end-to-end LSP, or an end-to-end PW.

   The method used to monitor the health of the recovery element is
   unimportant, provided that the recovery controllers receive
   information on its condition. The condition of the recovery element
   may be OK, 'failed', or degraded.

   When the recovery operation is launched by an OAM trigger, the
   recovery domain is equivalent to the OAM maintenance entity which is
   defined in [MPLS-TP-OAM], and the recovery reference points are
   defined at the same location as the OAM MEPs.


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4.6. Recovery in Layered Networks

   In multi-layer or multi-region networking, recovery may be performed
   at multiple layers or across cascaded recovery domains.

   The MPLS-TP recovery mechanism must ensure that the timing of
   recovery is coordinated in order to avoid races, and to allow either
   the recovery mechanism of the server layer to fix the problem before
   recovery takes place at the MPLS-TP layer, or to allow an upstream
   recovery domain to perform recovery before a downstream domain. In
   inter-connected 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
   failure occurred.

   A hold-off timer is required to coordinate the timing of recovery at
   multiple layers or across cascaded 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 failure condition in the MPLS-TP
   layer should not immediately trigger the recovery process if the
   hold-off timer is set to a value other than zero. The hold-off timer
   should be started and, on expiry, the recovery element should be
   checked to determine whether the failure condition still exists. If
   it does exist, the defect triggers the recovery operation.

   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.

   Reference should be made to [RFC3386] that presents the near-term and
   practical requirements for network survivability and hierarchy in
   current service provider environments.

4.6.1. Inherited Link-Level Protection

   TBD

4.6.2. Shared Risk Groups

   TBD

4.6.3. Fault Correlation

   TBD



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5. Mechanisms for Providing Protection in MPLS-TP

   This section describes the existing mechanisms available to provide
   protection within MPLS-TP networks and highlights areas where new
   work is required. It is expected that, as new protocol extensions and
   techniques are developed, this section will be updated to convert the
   statements of required work into references to those protocol
   extensions and techniques.

5.1. 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
   request for manual control of protection switching).

   The management plane may be used to configure the recovery domain by
   setting the reference points (recovery controllers), the 'working'
   and 'protection' entities, and the recovery type (e.g. 1:1
   bidirectional linear protection, ring protection, etc.). Additional
   parameters associated with the recovery process (such as a hold-off
   timer, revertive/non-revertive operation, etc.) may also be
   configured.

   In addition, the management plane may initiate manual control of the
   protection switching function. Either the fault condition or the
   operator request should be prioritized.

   Since provisioning the recovery domain involves the selection of a
   number of options, mismatches may occur at the different reference
   points. The MPLS-TP OAM Automatic Protection Switching (APS) protocol
   may be used as an in-band (i.e., data plane-based) control protocol
   to align both ends of the protected domain.

   It should also be possible for the management plane to monitor the
   recovery status.

5.1.1. Configuration of Protection Operation

   In order to implement the protection switching mechanism, the
   following entities and information should be provisioned:

   - The protection controllers (reference points)

   - The protection group consisting of a 'working' entity (which may be
     one of the recovery elements defined above) and a 'protection'
     entity. To guarantee protection, the paths of the 'working' and the


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     'protection' entities should have complete physical diversity.

   - The protection type that should be applied

   - Revertive/non-revertive behavior

5.1.2. External Manual Commands

   The following external, manual commands may be applied to a
   protection group; they are listed in descending order of priority:

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

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

   - Manual protection action - a manual command that forces a switch of
     data traffic to the 'protection' entity when there is no failure in
     the 'working' or the 'protection' entity

5.2. Fault Detection

   TBD

5.3. Fault Isolation

   TBD

5.4. OAM Signaling

   TBD

5.4.1. Fault Detection

   TBD

5.4.2. Fault Isolation

   TBD

5.4.3. Fault Reporting

   TBD





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5.4.4. Coordination of Recovery Actions

   TBD

5.5. 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 has been implemented and deployed in many networks, it
   is not surprising that it contains many features to support a high
   level of survivability function.

   The signaling elements of the GMPLS control plane utilize extensions
   to the Resource Reservation Protocol (RSVP) as documented in a series
   of documents commencing with [RFC3471] and [RFC3473], but based on
   [RFC3209] and [RFC2205]. The architecture for GMPLS is provided in
   [RFC3945], and [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 isolation and reporting.

   Clearly, the control plane techniques described here only apply where
   an MPLS-TP control plane is deployed and operated. All mandatory
   survivability features must be enabled even in the absence of the
   control plane, but where the control plane is present it may provide
   alternative mechanisms that may be desirable by virtue of their ease
   of automation or richer feature-set.

5.5.1. Fault Detection

   The control plane is not able to detect data plane faults. However,
   it does provide mechanisms to detect control plane faults and these
   can be can be used to deduce data plane faults where it is known that
   the control and data planes are fate sharing. Although [MPLS-TP-REQ]
   specifies that MPLS-TP must support an out-of-band control channel,
   it does not insist that this is used exclusively. That means that
   there may be deployments where an in-band (or at least in-fiber)
   control channel is used. In this case, the failure of the control
   channel can be used to infer a failure of the data channel or at
   least 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


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   include keepalive mechanisms designed to detect routing adjacency
   failures and, although these keep-alive mechanisms tend to operate at
   a relatively low frequency (order of seconds) it is still possible
   that the first indication of a control plane fault will be through
   the routing protocol.

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

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

5.5.2. Testing for Faults

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

   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,
   and 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 be disruptive to
   data flowing on the link.

   Note that a link in 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 testing for faults. First, [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 provided
   in [GMPLS-OAM]. This protocol extension supports the configuration
   (including enabling and disabling) of OAM mechanisms for a specific
   LSP.




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5.5.3. Fault Isolation

   Fault isolation is the process of determining exactly where a fault
   has occurred. It is often the case the fault detection only takes
   place at key points in the network (such as at LSP end points, or
   MEPs). This means that the fault may be located anywhere within a
   segment of the LSP concerned.

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

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

5.5.4. Fault Reporting

   GMPLS signaling uses the Notify message to report faults. The Notify
   message can apply to a single LSP or can carry fault information for
   a set of LSPs to improve the scalability of fault notification.

   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 may be signaled as the LSP is set up. This
   allows the process to be made consistent with segment protection and
   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 the
   PathErr and ResvErr messages.

   [RFC4783] provides a mechanism to coordinate alarms and other event
   or fault information through GMPLS signaling. This mechanism is
   useful to understand the status of the resources used by an LSP and
   to help understand why an LSP is not functioning, but it is not
   intended to replace other fault reporting mechanisms.

   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 withdraw the advertisement for a failed link, or may
   advertise that the link is about to be shut down gracefully. This
   mechanisms is, however, not normally considered to be fast enough to


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   be used as a trigger for protection switching.

5.5.5. 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 remain to be defined or this
   operation.

   A further piece of work in [GMPLS-REV] allows control and
   configuration of reversion behavior for end-to-end and segment
   protection.

5.5.6. Establishment of Protection and Restoration LSPs

   It should not be forgotten that protection and recovery depend on the
   establishment of suitable LSPs. The management plane may be used to
   set up these LSPs, but the control plane may be used if it is
   present.

   Several protocol extensions exist to make this process more simple:

   - [RFC4872] provides features in support of end-to-end protection
     switching.

   - [RFC4873] describes how to establish a single, segment protected
     LSP.

   - [RFC4874] allows one LSP to be signaled with a request that its
     path excludes specified resources (links, nodes, SRLGs). This
     allows a disjoint protection path to be requested, or a recovery
     path to be set up avoiding failed resources.

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

6. Pseudowire Protection Considerations

   The main application for the MPLS-TP network is currently identified
   as the pseudowire. Pseudowires provide end-to-end connectivity over
   the MPLS-TP network and may be comprised of a single pseudowire
   segment, or multiple segments "stitched" together to provide end-to-
   end connectivity.

   The pseudowire service may, itself, require a level of protection as
   part of its SLA. This protection could be provided by the MPLS-TP


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   LSPs that support the pseudowire, or could be a feature of the
   pseudowire layer itself.

6.1. Utilizing Underlying MPLS-TP Protection

   TBD

6.2. Protection in the Pseudowire Layer

   TBD

7. Manageability Considerations

   TBD

8. Security Considerations

   TBD

9. IANA Considerations

   This informational document makes no requests for IANA action.

10. Acknowledgments

   TBD

11. References

11.1. Normative References

   [RFC2205]      Braden, R. (Ed.), Zhang, L., Berson, S., Herzog, S.
                  and S. Jamin, "Resource ReserVation Protocol --
                  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., Editor, "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.


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   [RFC3945]      Mannie, E., "Generalized Multi-Protocol Label
                  Switching (GMPLS) Architecture", RFC 3945, October
                  2004.

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

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

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

   [RFC4428]      Papadimitriou D. and E.Mannie, Editors, "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
                  Farrel, A., " GMPLS Segment Recovery", RFC 4873, May
                  2007.

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

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

   [MPLS-TP-REQ]  B. Niven-Jenkins, et al., "Requirements for MPLS-TP",
                  draft-ietf-mpls-tp-requirements, work in progress.

   [MPLS-TP-OAM]  Vigoureux, M., Betts, M., and Ward, D., "MPLS TP OAM
                  Requirements (MPLS)", work in progress.



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11.2. Informative References

   [RFC3386]      Lai, W. and D. McDysan, "Network Hierarchy and
                  Multilayer Survivability", RFC 3386, November 2002.

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

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

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

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

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

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

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

   [GMPLS-OAM]    Takacs, A., Fedyk, D., and Jia, H., "OAM Configuration
                  Framework and Requirements for GMPLS RSVP-TE",
                  draft-ietf-ccamp-oam-configuration-fwk, work in
                  progress.

   [GMPLS-REV]    Takacs, A., Fondelli, F., Tremblay, B., "GMPLS RSVP-TE
                  recovery extension for data plane initiated
                  reversion", draft-takacs-ccamp-revertive-ps, work in
                  progress.







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12. Editors' Addresses

   Nurit Sprecher
   Nokia Siemens Networks
   3 Hanagar St. Neve Ne'eman B
   45241 Hod Hasharon, Israel
   Tel. +972 9 7751229
   Email: nurit.sprecher@nsn.com

   Adrian Farrel
   Old Dog Consulting
   Email: adrian@olddog.co.uk

13. Author's Address

   Himanshu Shah
   Ciena
   Email: hshah@ciena.com

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