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

Network Working Group                                        N. Sprecher
Internet Draft                                    Nokia Siemens Networks
Category: Informational                                        A. Farrel
Created: July 7, 2008                                 Old Dog Consulting
Expires: January 7, 2009                                     V. Kompella
                                                          Alcatel-Lucent

               Multiprotocol Label Switching Transport Profile
                           Survivability Framework

                  draft-sprecher-mpls-tp-survive-fwk-00.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 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
   protection mechanisms are data plane-driven and are based on MPLS-TP
   OAM fault management functions which are used to trigger protection

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   switching in the absence of a control plane. Other protection
   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                   6
   4. Functional Architecture                          8
   4.1. Elements of Control                            8
   4.1.1. Manual Control                               8
   4.1.2. Failure-Triggered Actions                    9
   4.1.3. OAM Signaling                                9
   4.1.4.                                              9
   4.2. Elements of Recovery                           9
   4.2.1. Span Recovery                               10
   4.2.2. Segment Recovery                            10
   4.2.3.                                             10
   4.3. Levels of Recovery                            11
   4.3.1. Dedicated Protection                        11
   4.3.2. Shared Protection                           11
   4.3.3. Extra Traffic                               12
   4.3.4. Restoration and Repair                      12
   4.3.5.                                             13
   4.4. Mechanisms for Recovery                       13
   4.4.1. Link-Level Protection                       13
   4.4.2. Alternate Paths and Segments                13
   4.4.3.                                             13
   4.5. Protection in Different Topologies            13
   4.5.1. Mesh Networks                               13
   4.5.2. Ring Networks                               15
   4.5.3.                                             15
   4.6. Recovery in Layered Networks                  15
   4.6.1. Inherited Link-Level Protection             16
   4.6.2. Shared Risk Groups                          16
   4.6.3. Fault Correlation                           16
   5. Mechanisms for Providing Protection in MPLS-TP  16
   5.1. Management Plane                              16
   5.1.1. Configuration of Protection Operation       17
   5.1.2. Forced Protection Actions                   17
   5.1.3. Blocked Protection Actions                  17
   5.2. Fault Detection                               17
   5.3. Fault Isolation                               18
   5.4. OAM Signaling                                 18
   5.4.1. Fault Detection                             18
   5.4.2. Fault Isolation                             18
   5.4.3. Fault Reporting                             18
   5.4.4. Coordination of Recovery Actions            18
   5.5. Control Plane Signaling                       18
   5.5.1. Fault Detection                             18
   5.5.2. Fault Isolation                             18
   5.5.3. Fault Reporting                             18
   5.5.4. Coordination of Recovery Actions            18

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   6. Pseudowire Protection Considerations            18
   6.1. Utilizing Underlying MPLS-TP Protection       18
   6.2. Protection in the Pseudowire Layer            18
   7. Manageability Considerations                    18
   8. Security Considerations                         18
   9. IANA Considerations                             18
   10. Acknowledgments                                18
   11. References                                     19
   11.1. Normative References                         19
   11.2. Informative References                       19
   12. Editors' Addresses                             20
   13. Intellectual Property Statement                20

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)
   [MPLS-TP-JWT] , [MPLS-TP-REQ] is a packet transport technology that
   combines the packet experience of MPLS with the operational
   experience of 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
   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 so
   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

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   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. 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.

   However, the recovery schemes described in [RFC4427] and evaluated in
   [RFC4428] assume some control plane-driven actions that are performed
   in the recovery context. 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
   protection 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. 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
   to those defined 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 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.

   The MPLS-TP protection mechanisms may be applied at various 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 span, segment, and/or end-to-end recovery,
   where:


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   - span protection refers to the protection of an individual link (and
     hence all or a subset of the LSPs routed over the link) between two
     neighboring switches;

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

   - end-to-end protection refers to the protection of an entire LSP
     from the ingress to the 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 protection levels affecting
   the directions of an LSP such that both directions of the LSP are
   switched to a new span, segment, or end-to-end path together.

   The protection 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.

   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].

   This framework also refers to recovery schemes that are optimized for
   specific topologies, such as linear, ring, and mesh, 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 OAM (e.g., APS) and the management and/or the control plane
   in order to support the recovery mechanisms.

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   MPLS-TP introduces a tool kit to enable recovery in MPLS-TP-based
   transport networks and to ensure that affected traffic is restored 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].

   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].


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   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 tandem network connection protection.
   - Must support LSP protection.
   - Must support pseudowire protection.
   - Must provide appropriate recovery times.
   - Must scale when many services are affected by a single fault.
   - Should support span protection.
   - Should support tandem connection protection.
   - Should support end-to-end protection.
   - Must support management plane control.
   - Must support control plane control.

   Restoration:

   - May support pre-planning of restoration resources.
   - May support computation of restoration resources after failure.
   - May support shared mesh restoration.
   - Should support soft LSP restoration (Make-before-break).
   - May support hard LSP restoration (break-before-make).
   - Must be topology agnostic.
   - May support restoration priority.
   - May utilize preemption during restoration, but only under operator
     configuration.

   Protection:

   - Should be able to apply protection at different levels in the
     network.
   - Should operate in conjunction with protection in under-lying
     networks.
   - Must support data plane triggered recovery.
   - Should be equally applicable to LSPs and pseudowires.
   - Must include mechanisms to detect, locate, notify, and remedy
     network faults.
   - May support 1:1 bidirectional protection switching in which case
     protection switching must be synchronized.
   - May support 1+1 unidirectional protection switching.
   - Must be applicable to P2P LSPs
   - Should be applicable to P2MP LSPs.
   - Must support protection ration of 100%.
   - Must support operator's QoS objectives on protection path.
   - May support extra traffic in 1:1 protection modes.
   - Must provide operator control and protection prioritization.
   - Must support revertive and non-revertive behavior.

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   - Must provide mechanisms to prevent protection switching thrashing.
   - Must provide coordination between protection mechanisms at
     different layers.
   - May provide different mechanisms optimized for specific topologies.

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, may be enhanced by data plane (i.e., OAM)
   control plane signaling, and 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.

   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.






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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) 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 may require
   message exchanges.

4.1.3. OAM Signaling

   OAM signaling refers to message exchanges 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.

   Note that in some cases, it may be the failure to receive an OAM
   signaling message that causes the survivability action to be taken.

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

4.1.4. Control Plane Signaling

   The control plane signaling is responsible for setup 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.

   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.


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4.2.1. Span Recovery

   A span is a single hop between neighboring nodes 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.

4.2.2. Segment Recovery

   An LSP segment is one or more hops on the path of the LSP. (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.

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.


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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
   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,

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   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
   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

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   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.

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

4.4.2. Alternate Paths and Segments

4.4.3. Bypass Tunnels

4.5. Protection in Different Topologies

   As described in the requirements listed in Section 3 and detailed in
   [MPLS-TP-REQ], the recovery techniques used may be optimized for
   different network topologies. 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 it 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).

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   In order to guarantee the protection, two entities are
   pre-provisioned. One of the pre-provisioned entities is configured to
   be the 'working' entity (primary) and the other is configured as the
   'protection' entity (backup).

   The Protection switching occurs at the protection controllers which
   reside at the edges of the protected entity. Between these endpoints,
   there are working and protection entities.

   In linear protection, a protection entity is pre-provisioned to
   protect the working entity. In order to guarantee protection
   switching in case of a 'failed' condition, the physical routes of the
   working and the protection entities should have complete physical
   diversity.

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

   1:1 linear protection:

   - In normal conditions the data traffic is transmitted either over
     the working entity or the 'protection' entity. Normal conditions
     are defined when there is no failure on the 'working' entity and
     there is no administrative configuration or requests that cause
     traffic to transmit over the 'protection' entity. Upon a failure
     condition or a specific administrative request, the traffic is
     switched over to the 'protection' entity.

   - 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 bi-directional protection switching, both ends of the protection
     domain should switch to the 'protection' entity (even when the
     failure is unidirectional).

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




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   1+1 linear protection:

   - The data traffic is copied at fed 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). Since only uni-directional protection switching is
     supported in the 1+1 linear protection scheme, there is no need to
     coordinate between the protection controllers.

4.5.2. Ring Networks

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.

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 co-ordinated 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

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   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.

   [RFC3386]

4.6.1. Inherited Link-Level Protection

4.6.2. Shared Risk Groups

4.6.3. Fault Correlation

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
   bi-directional 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

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   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
     '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

5.3. Fault Isolation

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5.4. OAM Signaling

5.4.1. Fault Detection

5.4.2. Fault Isolation

5.4.3. Fault Reporting

5.4.4. Coordination of Recovery Actions

5.5. Control Plane Signaling

5.5.1. Fault Detection

5.5.2. Fault Isolation

5.5.3. Fault Reporting

5.5.4. Coordination of Recovery Actions

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

6.1. Utilizing Underlying MPLS-TP Protection

6.2. Protection in the Pseudowire Layer

7. Manageability Considerations

8. Security Considerations

9. IANA Considerations

   This informational document makes no requests for IANA action.

10. Acknowledgments





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11. References

11.1. Normative References

   [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.

   [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-JWT]  Bryant, S., and Andersson, L. "JWT Report on MPLS
                  Architectural Considerations for a Transport Profile",
                  draft-bryant-jwt-mplstp-report, work in progress.

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

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

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.

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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

   Vach Kompella
   Alcatel-Lucent
   701 East Middlefield Rd.
   Mountain View, CA 94043
   Email: vach.kompella@alcatel.com

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   WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE
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