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Versions: (draft-makam-mpls-recovery-frmwrk) 00 01 02 03 04 05 06 07 RFC 3469

   MPLS Working Group                     Vishal Sharma (Metanoia, Inc.)
   Informational Track                Fiffi Hellstrand (Nortel Networks)
   Expires: Januray 2003                                       (Editors)


                                                              July  2002

                    Framework for MPLS-based Recovery
                <draft-ietf-mpls-recovery-frmwrk-06.txt>



   Status of this memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.
   Internet-Drafts are working documents of the Internet Engineering
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   Abstract

   Multi-protocol label switching (MPLS) integrates the label swapping
   forwarding paradigm with network layer routing. To deliver reliable
   service, MPLS requires a set of procedures to provide protection of
   the traffic carried on different paths. This requires that the label
   switched routers (LSRs) support fault detection, fault notification,
   and fault recovery mechanisms, and that MPLS signaling, support the
   configuration of recovery. With these objectives in mind, this
   document specifies a framework for MPLS based recovery.

   Table of Contents
1.    Introduction....................................................2
1.1.  Background......................................................3
1.2.  Motivation for MPLS-Based Recovery..............................3
1.3.  Objectives/Goals................................................4
2.    Contributing Authors............................................6
3.    Overview........................................................6
3.1.  Recovery Models.................................................7
3.1.1   Rerouting.....................................................7
3.1.2   Protection Switching..........................................7

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3.2.  The Recovery Cycles.............................................8
3.2.1   MPLS Recovery Cycle Model.....................................8
3.2.2   MPLS Reversion Cycle Model....................................9
3.2.3   Dynamic Re-routing Cycle Model...............................11
3.3.  Definitions and Terminology....................................12
3.3.1   General Recovery Terminology.................................13
3.3.2   Failure Terminology..........................................15
3.4.  Abbreviations..................................................16
4.    MPLS-based Recovery Principles.................................16
4.1.  Configuration of Recovery......................................17
4.2.  Initiation of Path Setup.......................................17
4.3.  Initiation of Resource Allocation..............................18
4.4.  Scope of Recovery..............................................18
4.4.1   Topology.....................................................18
4.4.1.1   Local Repair................................................18
4.4.1.2   Global Repair...............................................19
4.4.1.3   Alternate Egress Repair.....................................19
4.4.1.4   Multi-Layer Repair..........................................20
4.4.1.5   Concatenated Protection Domains.............................20
4.4.2   Path Mapping.................................................20
4.4.3   Bypass Tunnels...............................................21
4.4.4   Recovery Granularity.........................................21
4.4.4.1   Selective Traffic Recovery..................................22
4.4.4.2   Bundling....................................................22
4.4.5   Recovery Path Resource Use...................................22
4.5.  Fault Detection................................................22
4.6.  Fault Notification.............................................23
4.7.  Switch-Over Operation..........................................24
4.7.1   Recovery Trigger.............................................24
4.7.2   Recovery Action..............................................25
4.8.  Post Recovery Operation........................................25
4.8.1   Fixed Protection Counterparts................................25
4.8.1.1   Revertive Mode..............................................25
4.8.1.2   Non-revertive Mode..........................................25
4.8.2   Dynamic Protection Counterparts..............................26
4.8.3   Restoration and Notification.................................26
4.8.4   Reverting to Preferred Path (or Controlled Rearrangement)....27
4.9.  Performance....................................................27
5.    MPLS Recovery Features.........................................28
6.    Comparison Criteria............................................28
7.    Security Considerations........................................30
8.    Intellectual Property Considerations...........................30
9.    Acknowledgements...............................................31
10.   EditorsÆ Addresses.............................................31
11.   References.....................................................31


1. Introduction

   This memo describes a framework for MPLS-based recovery. We provide a
   detailed taxonomy of recovery terminology, and discuss the motivation

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   for, the objectives of, and the requirements for MPLS-based recovery.
   We outline principles for MPLS-based recovery, and also provide
   comparison criteria that may serve as a basis for comparing and
   evaluating different recovery schemes.

   At points in the document, we provide some thoughts about the
   operation or viability of certain recovery objectives. These should
   be viewed as the opinions of the authors, and not the consolidated
   views of the IETF.

1.1. Background

   Network routing deployed today is focused primarily on connectivity,
   and typically supports only one class of service, the best effort
   class. Multi-protocol label switching [1], on the other hand, by
   integrating forwarding based on label-swapping of a link local label
   with network layer routing allows flexibility in the delivery of new
   routing services. MPLS allows for using such media specific
   forwarding mechanisms as label swapping. This enables some
   sophisticated features such as quality-of-service (QoS) and traffic
   engineering [2] to be implemented more effectively. An important
   component of providing QoS, however, is the ability to transport data
   reliably and efficiently. Although the current routing algorithms are
   robust and survivable, the amount of time they take to recover from a
   fault can be significant, on the order of several seconds or minutes,
   causing disruption of service for some applications in the interim.
   This is unacceptable is situations where the aim to provide a highly
   reliable service, with recovery times that are on the order of
   seconds down to 10's of milliseconds.

   MPLS recovery may be motivated by the notion that there are
   limitations to improving the recovery times of current routing
   algorithms. Additional improvement can be obtained by augmenting
   these algorithms with MPLS recovery mechanisms [3]. Since MPLS is a
   possible technology of choice in future IP-based transport networks,
   it is useful that MPLS be able to provide protection and restoration
   of traffic.  MPLS may facilitate the convergence of network
   functionality on a common control and management plane. Further, a
   protection priority could be used as a differentiating mechanism for
   premium services that require high reliability. The remainder of this
   document provides a framework for MPLS based recovery.  It is focused
   at a conceptual level and is meant to address motivation, objectives
   and requirements.  Issues of mechanism, policy, routing plans and
   characteristics of traffic carried by recovery paths are beyond the
   scope of this document.

1.2. Motivation for MPLS-Based Recovery

   MPLS based protection of traffic (called MPLS-based Recovery) is
   useful for a number of reasons. The most important is its ability to
   increase network reliability by enabling a faster response to faults
   than is possible with traditional Layer 3 (or IP layer) approaches
   alone while still providing the visibility of the network afforded by

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   Layer 3. Furthermore, a protection mechanism using MPLS could enable
   IP traffic to be put directly over WDM optical channels and provide a
   recovery option without an intervening SONET layer.  This would
   facilitate the construction of IP-over-WDM networks that request a
   fast recovery ability.

   The need for MPLS-based recovery arises because of the following:

   I. Layer 3 or IP rerouting may be too slow for a core MPLS network
   that needs to support recovery times that are smaller than the
   convergence times of IP routing protocols.

   II. Layer 0 (for example, optical layer) or Layer 1 (for example,
   SONET) mechanisms may be wasteful use of resources.

   III. The granularity at which the lower layers may be able to protect
   traffic may be too coarse for traffic that is switched using MPLS-
   based mechanisms.

   IV. Layer 0 or Layer 1 mechanisms may have no visibility into higher
   layer operations.  Thus, while they may provide, for example, link
   protection, they cannot easily provide node protection or protection
   of traffic transported at layer 3. Further, this may prevent the
   lower layers from providing restoration based on the trafficÆs needs.
   For example, fast restoration for traffic that needs it, and slower
   restoration (with possibly more optimal use of resources) for traffic
   that does not require fast restoration. In networks where the latter
   class of traffic is dominant, providing fast restoration to all
   classes of traffic may not be cost effective from a service
   providerÆs perspective.

   V. MPLS has desirable attributes when applied to the purpose of
   recovery for connectionless networks. Specifically that an LSP is
   source routed and a forwarding path for recovery can be "pinned" and
   is not affected by transient instability in SPF routing brought on by
   failure scenarios.

   VI. Establishing interoperability of protection mechanisms between
   routers/LSRs from different vendors in IP or MPLS networks is desired
   to enable recovery mechanisms to work in a multivendor environment,
   and to enable the transition of certain protected services to an MPLS
   core.

1.3. Objectives/Goals

   The following are some important goals for MPLS-based recovery.

   Ia. MPLS-based recovery mechanisms may be subject to the traffic
   engineering goal of optimal use of resources.

   Ib. MPLS based recovery mechanisms should aim to facilitate
   restoration times that are sufficiently fast for the end user
   application. That is, that better match the end-userÆs application

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   requirements. In some cases, this may be as short as 10s of
   milliseconds.

   We observe that Ia and Ib are conflicting objectives, and a trade off
   exists between them. The optimal choice depends on the end-user
   applicationÆs sensitivity to restoration time and the cost impact of
   introducing restoration in the network, as well as the end-user
   applicationÆs sensitivity to cost.

   II. MPLS-based recovery should aim to maximize network reliability
   and availability. MPLS-based recovery of traffic should aim to
   minimize the number of single points of failure in the MPLS protected
   domain.

   III. MPLS-based recovery should aim to enhance the reliability of the
   protected traffic while minimally or predictably degrading the
   traffic carried by the diverted resources.

   IV. MPLS-based recovery techniques should aim to be applicable for
   protection of traffic at various granularities. For example, it
   should be possible to specify MPLS-based recovery for a portion of
   the traffic on an individual path, for all traffic on an individual
   path, or for all traffic on a group of paths. Note that a path is
   used as a general term and includes the notion of a link, IP route or
   LSP.

   V. MPLS-based recovery techniques may be applicable for an entire
   end-to-end path or for segments of an end-to-end path.

   VI. MPLS-based recovery mechanisms should aim to take into
   consideration the recovery actions of lower layers. MPLS-based
   mechanisms should not trigger lower layer protection switching.

   VII. MPLS-based recovery mechanisms should aim to minimize the loss
   of data and packet reordering during recovery operations. (The
   current MPLS specification itself has no explicit requirement on
   reordering).

   VIII. MPLS-based recovery mechanisms should aim to minimize the state
   overhead incurred for each recovery path maintained.

   IX. MPLS-based recovery mechanisms should aim to preserve the
   constraints on traffic after switchover, if desired.  That is, if
   desired, the recovery path should meet the resource requirements of,
   and achieve the same performance characteristics as, the working
   path.

   We observe that some of the above are conflicting goals, and real
   deployment will often involve engineering compromises based on a
   variety of factors such as cost, end-user application requirements,
   network efficiency, and revenue considerations. Thus, these goals are
   subject to tradeoffs based on the above considerations.


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2. Contributing Authors

   This document was the collective work of several individuals over a
   period of two and a half years. The text and content of this document
   was contributed by the editors and the co-authors listed below. (The
   contact information for the editors appears in Section 10, and is not
   repeated below.)

   Ben Mack-Crane                       Srinivas Makam
   Tellabs Operations, Inc.             Eshernet, Inc.
   4951 Indiana Avenue                  1712 Ada Ct.
   Lisle, IL 60532                      Naperville, IL 60540
   Phone: (630) 512-7255                Phone: (630) 308-3213
   Ben.Mack-Crane@tellabs.com           Smakam60540@yahoo.com

   Ken Owens                            Changcheng Huang
   Erlang Technology, Inc.              Carleton University
   345 Marshall Ave., Suite 300         Minto Center, Rm. 3082
   St. Louis, MO 63119                  1125 Colonial By Drive
   Phone: (314) 918-1579                Ottawa, Ont. K1S 5B6 Canada
   keno@erlangtech.com                  Phone: (613) 520-2600 x2477
                                        Changcheng.Huang@sce.carleton.ca

   Jon Weil                             Brad Cain
   Nortel Networks                      Storigen Systems
   Harlow Laboratories London Road      650 Suffolk Street
   Harlow Essex CM17 9NA, UK            Lowell, MA 01854
   Phone: +44 (0)1279 403935            Phone: (978) 323-4454
   jonweil@nortelnetworks.com           bcain@storigen.com

   Loa Andersson                        Bilel Jamoussi
   Utfors AB                            Nortel Networks
   R…sundav„gen 12, Box 525             3 Federal Street, BL3-03
   169 29 Solna, Sweden                 Billerica, MA 01821, USA
   Phone: +46 8 5270 5038               Phone:(978) 288-4506
   loa.andersson@utfors.se              jamoussi@nortelnetworks.com

   Angela Chiu                          Seyhan Civanlar
   Celion Networks, Inc.                Lemur Networks, Inc.
   One Shiela Drive, Suite 2            135 West 20th Street, 5th Floor
   Tinton Falls, NJ 07724               New York, NY 10011
   Phone: (732) 345-3441                                         Phone: (212) 367-7676
   angela.chiu@celion.com                                             scivanlar@lemurnetworks.com


3. Overview

   There are several options for providing protection of traffic. The
   most generic requirement is the specification of whether recovery
   should be via Layer 3 (or IP) rerouting or via MPLS protection
   switching or rerouting actions.


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   Generally network operators aim to provide the fastest and the best
   protection mechanism that can be provided at a reasonable cost. The
   higher the levels of protection, the more the resources consumed.
   Therefore it is expected that network operators will offer a spectrum
   of service levels. MPLS-based recovery should give the flexibility to
   select the recovery mechanism, choose the granularity at which
   traffic is protected, and to also choose the specific types of
   traffic that are protected in order to give operators more control
   over that tradeoff.  With MPLS-based recovery, it can be possible to
   provide different levels of protection for different classes of
   service, based on their service requirements. For example, using
   approaches outlined below, a Virtual Leased Line (VLL) service or
   real-time applications like Voice over IP (VoIP) may be supported
   using link/node protection together with pre-established, pre-
   reserved path protection. Best effort traffic, on the other hand, may
   use path protection that is established on demand or may simply rely
   on IP re-route or higher layer recovery mechanisms.  As another
   example of their range of application, MPLS-based recovery strategies
   may be used to protect traffic not originally flowing on label
   switched paths, such as IP traffic that is normally routed hop-by-
   hop, as well as traffic forwarded on label switched paths.

3.1. Recovery Models

   There are two basic models for path recovery: rerouting and
   protection switching.

   Protection switching and rerouting, as defined below, may be used
   together.  For example, protection switching to a recovery path may
   be used for rapid restoration of connectivity while rerouting
   determines a new optimal network configuration, rearranging paths, as
   needed, at a later time.

3.1.1     Rerouting

   Recovery by rerouting is defined as establishing new paths or path
   segments on demand for restoring traffic after the occurrence of a
   fault. The new paths may be based upon fault information, network
   routing policies, pre-defined configurations and network topology
   information. Thus, upon detecting a fault, paths or path segments to
   bypass the fault are established using signaling.

   Once the network routing algorithms have converged after a fault, it
   may be preferable, in some cases, to reoptimize the network by
   performing a reroute based on the current state of the network and
   network policies. This is discussed further in Section 3.8.

   In terms of the principles defined in section 3, reroute recovery
   employs paths established-on-demand with resources reserved-on-
   demand.

3.1.2     Protection Switching


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   Protection switching recovery mechanisms pre-establish a recovery
   path or path segment, based upon network routing policies, the
   restoration requirements of the traffic on the working path, and
   administrative considerations. The recovery path may or may not be
   link and node disjoint with the working path. However if the recovery
   path shares sources of failure with the working path, the overall
   reliability of the construct is degraded. When a fault is detected,
   the protected traffic is switched over to the recovery path(s) and
   restored.

   In terms of the principles in section 3, protection switching employs
   pre-established recovery paths, and, if resource reservation is
   required on the recovery path, pre-reserved resources. The various
   sub-types of protection switching are detailed in Section 4.4 of this
   document.


3.2. The Recovery Cycles

   There are three defined recovery cycles: the MPLS Recovery Cycle, the
   MPLS Reversion Cycle and the Dynamic Re-routing Cycle. The first
   cycle detects a fault and restores traffic onto MPLS-based recovery
   paths. If the recovery path is non-optimal the cycle may be followed
   by any of the two latter cycles to achieve an optimized network
   again. The reversion cycle applies for explicitly routed traffic that
   that does not rely on any dynamic routing protocols to be converged.
   The dynamic re-routing cycle applies for traffic that is forwarded
   based on hop-by-hop routing.

3.2.1     MPLS Recovery Cycle Model

   The MPLS recovery cycle model is illustrated in Figure 1.
   Definitions and a key to abbreviations follow.

    --Network Impairment
    |    --Fault Detected
    |    |    --Start of Notification
    |    |    |    -- Start of Recovery Operation
    |    |    |    |    --Recovery Operation Complete
    |    |    |    |    |    --Path Traffic Restored
    |    |    |    |    |    |
    |    |    |    |    |    |
    v    v    v    v    v    v
   ----------------------------------------------------------------
    | T1 | T2 | T3 | T4 | T5 |

   Figure 1. MPLS Recovery Cycle Model

   The various timing measures used in the model are described below.
   T1   Fault Detection Time
   T2   Hold-off Time
   T3   Notification Time

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   T4   Recovery Operation Time
   T5   Traffic Restoration Time

   Definitions of the recovery cycle times are as follows:

   Fault Detection Time

   The time between the occurrence of a network impairment and the
   moment the fault is detected by MPLS-based recovery mechanisms. This
   time may be highly dependent on lower layer protocols.

   Hold-Off Time

   The configured waiting time between the detection of a fault and
   taking MPLS-based recovery action, to allow time for lower layer
   protection to take effect. The Hold-off Time may be zero.

   Note: The Hold-Off Time may occur after the Notification Time
   interval if the node responsible for the switchover, the Path Switch
   LSR (PSL), rather than the detecting LSR, is configured to wait.

   Notification Time

   The time between initiation of a fault indication signal (FIS) by the
   LSR detecting the fault and the time at which the Path Switch LSR
   (PSL) begins the recovery operation.  This is zero if the PSL detects
   the fault itself or infers a fault from such events as an adjacency
   failure.

   Note: If the PSL detects the fault itself, there still may be a Hold-
   Off Time period between detection and the start of the recovery
   operation.

   Recovery Operation Time

   The time between the first and last recovery actions.  This may
   include message exchanges between the PSL and PML to coordinate
   recovery actions.

   Traffic Restoration Time

   The time between the last recovery action and the time that the
   traffic (if present) is completely recovered.  This interval is
   intended to account for the time required for traffic to once again
   arrive at the point in the network that experienced disrupted or
   degraded service due to the occurrence of the fault (e.g. the PML).
   This time may depend on the location of the fault, the recovery
   mechanism, and the propagation delay along the recovery path.

3.2.2     MPLS Reversion Cycle Model




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   Protection switching, revertive mode, requires the traffic to be
   switched back to a preferred path when the fault on that path is
   cleared.  The MPLS reversion cycle model is illustrated in Figure 2.
   Note that the cycle shown below comes after the recovery cycle shown
   in Fig. 1.

          --Network Impairment Repaired
          |    --Fault Cleared
          |    |    --Path Available
          |    |    |    --Start of Reversion Operation
          |    |    |    |    --Reversion Operation Complete
          |    |    |    |    |    --Traffic Restored on Preferred Path
          |    |    |    |    |    |
          |    |    |    |    |    |
          v    v    v    v    v    v
       -----------------------------------------------------------------
          | T7 | T8 | T9 | T10| T11|

   Figure 2. MPLS Reversion Cycle Model

   The various timing measures used in the model are described below.
   T7   Fault Clearing Time
   T8   Wait-to-Restore Time
   T9   Notification Time
   T10  Reversion Operation Time
   T11  Traffic Restoration Time

   Note that time T6 (not shown above) is the time for which the network
   impairment is not repaired and traffic is flowing on the recovery
   path.

   Definitions of the reversion cycle times are as follows:

   Fault Clearing Time

   The time between the repair of a network impairment and the time that
   MPLS-based mechanisms learn that the fault has been cleared. This
   time may be highly dependent on lower layer protocols.

   Wait-to-Restore Time

   The configured waiting time between the clearing of a fault and MPLS-
   based recovery action(s).  Waiting time may be needed to ensure that
   the path is stable and to avoid flapping in cases where a fault is
   intermittent. The Wait-to-Restore Time may be zero.

   Note: The Wait-to-Restore Time may occur after the Notification Time
   interval if the PSL is configured to wait.

   Notification Time



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   The time between initiation of a fault recovery signal (FRS) by the
   LSR clearing the fault and the time at which the path switch LSR
   begins the reversion operation.  This is zero if the PSL clears the
   fault itself.
   Note: If the PSL clears the fault itself, there still may be a Wait-
   to-Restore Time period between fault clearing and the start of the
   reversion operation.

   Reversion Operation Time

   The time between the first and last reversion actions.  This may
   include message exchanges between the PSL and PML to coordinate
   reversion actions.

   Traffic Restoration Time

   The time between the last reversion action and the time that traffic
   (if present) is completely restored on the preferred path.  This
   interval is expected to be quite small since both paths are working
   and care may be taken to limit the traffic disruption (e.g., using
   "make before break" techniques and synchronous switch-over).

   In practice, the only interesting times in the reversion cycle are
   the Wait-to-Restore Time and the Traffic Restoration Time (or some
   other measure of traffic disruption).  Given that both paths are
   available, there is no need for rapid operation, and a well-
   controlled switch-back with minimal disruption is desirable.

3.2.3     Dynamic Re-routing Cycle Model

   Dynamic rerouting aims to bring the IP network to a stable state
   after a network impairment has occurred. A re-optimized network is
   achieved after the routing protocols have converged, and the traffic
   is moved from a recovery path to a (possibly) new working path. The
   steps involved in this mode are illustrated in Figure 3.

   Note that the cycle shown below may be overlaid on the recovery cycle
   shown in Fig. 1 or the reversion cycle shown in Fig. 2, or both (in
   the event that both the recovery cycle and the reversion cycle take
   place before the routing protocols converge), and after the
   convergence of the routing protocols it is determined (based on on-
   line algorithms or off-line traffic engineering tools, network
   configuration, or a variety of other possible criteria) that there is
   a better route for the working path.

          --Network Enters a Semi-stable State after an Impairment
          |     --Dynamic Routing Protocols Converge
          |     |     --Initiate Setup of New Working Path between PSL
          |     |     |                                         and PML
          |     |     |     --Switchover Operation Complete
          |     |     |     |     --Traffic Moved to New Working Path
          |     |     |     |     |
          |     |     |     |     |

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          v     v     v     v     v
       -----------------------------------------------------------------
          | T12 | T13 | T14 | T15 |

   Figure 3. Dynamic Rerouting Cycle Model
   The various timing measures used in the model are described below.
   T12  Network Route Convergence Time
   T13  Hold-down Time (optional)
   T14  Switchover Operation Time
   T15  Traffic Restoration Time

   Network Route Convergence Time

   We define the network route convergence time as the time taken for
   the network routing protocols to converge and for the network to
   reach a stable state.

   Holddown Time

   We define the holddown period as a bounded time for which a recovery
   path must be used. In some scenarios it may be difficult to determine
   if the working path is stable. In these cases a holddown time may be
   used to prevent excess flapping of traffic between a working and a
   recovery path.

   Switchover Operation Time

   The time between the first and last switchover actions.  This may
   include message exchanges between the PSL and PML to coordinate the
   switchover actions.

   As an example of the recovery cycle, we present a sequence of events
   that occur after a network impairment occurs and when a protection
   switch is followed by dynamic rerouting.

   I. Link or path fault occurs
   II. Signaling initiated (FIS) for the detected fault
   III. FIS arrives at the PSL
   IV. The PSL initiates a protection switch to a pre-configured
   recovery path
   V. The PSL switches over the traffic from the working path to the
   recovery path
   VI. The network enters a semi-stable state
   VII. Dynamic routing protocols converge after the fault, and a new
   working path is calculated (based, for example, on some of the
   criteria mentioned in Section 2.1.1).
   VIII. A new working path is established between the PSL and the PML
   (assumption is that PSL and PML have not changed)
   IX. Traffic is switched over to the new working path.

3.3. Definitions and Terminology



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   This document assumes the terminology given in [1], and, in addition,
   introduces the following new terms.

3.3.1     General Recovery Terminology

   Rerouting

   A recovery mechanism in which the recovery path or path segments are
   created dynamically after the detection of a fault on the working
   path. In other words, a recovery mechanism in which the recovery path
   is not pre-established.

   Protection Switching

   A recovery mechanism in which the recovery path or path segments are
   created prior to the detection of a fault on the working path. In
   other words, a recovery mechanism in which the recovery path is pre-
   established.

   Working Path

   The protected path that carries traffic before the occurrence of a
   fault.  The working path exists between a PSL and PML. The working
   path can be of different kinds; a hop-by-hop routed path, a trunk, a
   link, an LSP or part of a multipoint-to-point LSP.

   Synonyms for a working path are primary path and active path.

   Recovery Path

   The path by which traffic is restored after the occurrence of a
   fault. In other words, the path on which the traffic is directed by
   the recovery mechanism. The recovery path is established by MPLS
   means. The recovery path can either be an equivalent recovery path
   and ensure no reduction in quality of service, or be a limited
   recovery path and thereby not guarantee the same quality of service
   (or some other criteria of performance) as the working path. A
   limited recovery path is not expected to be used for an extended
   period of time.

   Synonyms for a recovery path are: back-up path, alternative path, and
   protection path.

   Protection Counterpart

   The "other" path when discussing pre-planned protection switching
   schemes. The protection counterpart for the working path is the
   recovery path and vice-versa.

   Path Group (PG)

   A logical bundling of multiple working paths, each of which is routed
   identically between a Path Switch LSR and a Path Merge LSR.

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   Protected Path Group (PPG)

   A path group that requires protection.

   Protected Traffic Portion (PTP)

   The portion of the traffic on an individual path that requires
   protection.  For example, code points in the EXP bits of the shim
   header may identify a protected portion.

   Path Switch LSR (PSL)

   An LSR that is responsible for switching or replicating the traffic
   between the working path and the recovery path.

   Path Merge LSR (PML)

   An LSR that is responsible for receiving the recovery path traffic,
   and either merging the traffic back onto the working path, or, if it
   is itself the destination, passing the traffic on to the higher layer
   protocols.

   Point of Repair (POR)

   An LSR that is setup for performing MPLS recovery. In other words, an
   LSR that is responsible for effecting the repair of an LSP. The POR,
   for example, can be a PSL or a PML, depending on the type of recovery
   scheme employed.

   Intermediate LSR

   An LSR on a working or recovery path that is neither a PSL nor a PML
   for that path.

   Bypass Tunnel

   A path that serves to back up a set of working paths using the label
   stacking approach [1]. The working paths and the bypass tunnel must
   all share the same path switch LSR (PSL) and the path merge LSR
   (PML).

   Switch-Over

   The process of switching the traffic from the path that the traffic
   is flowing on onto one or more alternate path(s). This may involve
   moving traffic from a working path onto one or more recovery paths,
   or may involve moving traffic from a recovery path(s) on to a more
   optimal working path(s).

   Switch-Back



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   The process of returning the traffic from one or more recovery paths
   back to the working path(s).

   Revertive Mode

   A recovery mode in which traffic is automatically switched back from
   the recovery path to the original working path upon the restoration
   of the working path to a fault-free condition. This assumes a failed
   working path does not automatically surrender resources to the
   network.

   Non-revertive Mode

   A recovery mode in which traffic is not automatically switched back
   to the original working path after this path is restored to a fault-
   free condition. (Depending on the configuration, the original working
   path may, upon moving to a fault-free condition, become the recovery
   path, or it may be used for new working traffic, and be no longer
   associated with its original recovery path).

   MPLS Protection Domain

   The set of LSRs over which a working path and its corresponding
   recovery path are routed.

   MPLS Protection Plan

   The set of all LSP protection paths and the mapping from working to
   protection paths deployed in an MPLS protection domain at a given
   time.

   Liveness Message

   A message exchanged periodically between two adjacent LSRs that
   serves as a link probing mechanism. It provides an integrity check of
   the forward and the backward directions of the link between the two
   LSRs as well as a check of neighbor aliveness.

   Path Continuity Test

   A test that verifies the integrity and continuity of a path or path
   segment. The details of such a test are beyond the scope of this
   draft. (This could be accomplished, for example, by transmitting a
   control message along the same links and nodes as the data traffic or
   similarly could be measured by the absence of traffic and by
   providing feedback.)

3.3.2     Failure Terminology

   Path Failure (PF)
   Path failure is fault detected by MPLS-based recovery mechanisms,
   which is define as the failure of the liveness message test or a path
   continuity test, which indicates that path connectivity is lost.

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   Path Degraded (PD)
   Path degraded is a fault detected by MPLS-based recovery mechanisms
   that indicates that the quality of the path is unacceptable.

   Link Failure (LF)
   A lower layer fault indicating that link continuity is lost. This may
   be communicated to the MPLS-based recovery mechanisms by the lower
   layer.

   Link Degraded (LD)
   A lower layer indication to MPLS-based recovery mechanisms that the
   link is performing below an acceptable level.

   Fault Indication Signal (FIS)
   A signal that indicates that a fault along a path has occurred. It is
   relayed by each intermediate LSR to its upstream or downstream
   neighbor, until it reaches an LSR that is setup to perform MPLS
   recovery (the POR).  The FIS is transmitted periodically by the
   node/nodes closest to the point of failure, for some configurable
   length of time.

   Fault Recovery Signal (FRS)
   A signal that indicates a fault along a working path has been
   repaired. Again, like the FIS, it is relayed by each intermediate LSR
   to its upstream or downstream neighbor, until is reaches the LSR that
   performs recovery of the original path. The FRS is transmitted
   periodically by the node/nodes closest to the point of failure, for
   some configurable length of time.


3.4. Abbreviations

   FIS:   Fault Indication Signal.
   FRS:   Fault Recovery Signal.
   LD:    Link Degraded.
   LF:    Link Failure.
   PD:    Path Degraded.
   PF:    Path Failure.
   PML:   Path Merge LSR.
   PG:    Path Group.
   POR:   Point of Repair
   PPG:   Protected Path Group.
   PTP:   Protected Traffic Portion.
   PSL:   Path Switch LSR.

4. MPLS-based Recovery Principles

   MPLS-based recovery refers to the ability to effect quick and
   complete restoration of traffic affected by a fault in an MPLS-
   enabled network. The fault may be detected on the IP layer or in
   lower layers over which IP traffic is transported. Fastest MPLS

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   recovery is assumed to be achieved with protection switching and may
   be viewed as the MPLS LSR switch completion time that is comparable
   to, or equivalent to, the 50 ms switch-over completion time of the
   SONET layer. This section provides a discussion of the concepts and
   principles of MPLS-based recovery. The concepts are presented in
   terms of atomic or primitive terms that may be combined to specify
   recovery approaches.  We do not make any assumptions about the
   underlying layer 1 or layer 2 transport mechanisms or their recovery
   mechanisms.


4.1. Configuration of Recovery

   An LSR may support any or all of the following recovery options:

   Default-recovery (No MPLS-based recovery enabled):
   Traffic on the working path is recovered only via Layer 3 or IP
   rerouting or by some lower layer mechanism such as SONET APS.  This
   is equivalent to having no MPLS-based recovery. This option may be
   used for low priority traffic or for traffic that is recovered in
   another way (for example load shared traffic on parallel working
   paths may be automatically recovered upon a fault along one of the
   working paths by distributing it among the remaining working paths).

   Recoverable (MPLS-based recovery enabled):
   This working path is recovered using one or more recovery paths,
   either via rerouting or via protection switching.

4.2. Initiation of Path Setup

   There are three options for the initiation of the recovery path
   setup. The active and recovery paths may be established by using
   either RSVP-TE [4][5] or CR-LDP [6].

   Pre-established:

   This is the same as the protection switching option. Here a recovery
   path(s) is established prior to any failure on the working path. The
   path selection can either be determined by an administrative
   centralized tool, or chosen based on some algorithm implemented at
   the PSL and possibly intermediate nodes. To guard against the
   situation when the pre-established recovery path fails before or at
   the same time as the working path, the recovery path should have
   secondary configuration options as explained in Section 3.3 below.

   Pre Qualified:

   A pre-established path need not be created, it may be pre-qualified.
   A pre-qualified recovery path is not created expressly for protecting
   the working path, but instead is a path created for other purposes
   that is designated as a recovery path after determining that it is an
   acceptable alternative for carrying the working path traffic.


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   Variants include the case where an optical path or trail is
   configured, but no switches are set.

   Established-on-Demand:

   This is the same as the rerouting option. Here, a recovery path is
   established after a failure on its working path has been detected and
   notified to the PSL.

4.3. Initiation of Resource Allocation

   A recovery path may support the same traffic contract as the working
   path, or it may not. We will distinguish these two situations by
   using different additive terms. If the recovery path is capable of
   replacing the working path without degrading service, it will be
   called an equivalent recovery path. If the recovery path lacks the
   resources (or resource reservations) to replace the working path
   without degrading service, it will be called a limited recovery path.
   Based on this, there are two options for the initiation of resource
   allocation:

   Pre-reserved:

   This option applies only to protection switching. Here a pre-
   established recovery path reserves required resources on all hops
   along its route during its establishment. Although the reserved
   resources (e.g., bandwidth and/or buffers) at each node cannot be
   used to admit more working paths, they are available to be used by
   all traffic that is present at the node before a failure occurs.

   Reserved-on-Demand:

   This option may apply either to rerouting or to protection switching.
   Here a recovery path reserves the required resources after a failure
   on the working path has been detected and notified to the PSL and
   before the traffic on the working path is switched over to the
   recovery path.

   Note that under both the options above, depending on the amount of
   resources reserved on the recovery path, it could either be an
   equivalent recovery path or a limited recovery path.

4.4. Scope of Recovery

4.4.1     Topology

4.4.1.1  Local Repair

   The intent of local repair is to protect against a link or neighbor
   node fault and to minimize the amount of time required for failure
   propagation. In local repair (also known as local recovery), the node
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   (either rerouting or protection switching). Local repair can be of
   two types:

   Link Recovery/Restoration

   In this case, the recovery path may be configured to route around a
   certain link deemed to be unreliable. If protection switching is
   used, several recovery paths may be configured for one working path,
   depending on the specific faulty link that each protects against.

   Alternatively, if rerouting is used, upon the occurrence of a fault
   on the specified link, each path is rebuilt such that it detours
   around the faulty link.
   In this case, the recovery path need only be disjoint from its
   working path at a particular link on the working path, and may have
   overlapping segments with the working path. Traffic on the working
   path is switched over to an alternate path at the upstream LSR that
   connects to the failed link. This method is potentially the fastest
   to perform the switchover, and can be effective in situations where
   certain path components are much more unreliable than others.

   Node Recovery/Restoration

   In this case, the recovery path may be configured to route around a
   neighbor node deemed to be unreliable. Thus the recovery path is
   disjoint from the working path only at a particular node and at links
   associated with the working path at that node. Once again, the
   traffic on the primary path is switched over to the recovery path at
   the upstream LSR that directly connects to the failed node, and the
   recovery path shares overlapping portions with the working path.

4.4.1.2 Global Repair

   The intent of global repair is to protect against any link or node
   fault on a path or on a segment of a path, with the obvious exception
   of the faults occurring at the ingress node of the protected path
   segment. In global repair, the POR is usually distant from the
   failure and needs to be notified by a FIS.
   In global repair also, end-to-end path recovery/restoration applies.
   In many cases, the recovery path can be made completely link and node
   disjoint with its working path. This has the advantage of protecting
   against all link and node fault(s) on the working path (end-to-end
   path or path segment).
   However, it may, in some cases, be slower than local repair since the
   fault notification message must now travel to the POR to trigger the
   recovery action.

4.4.1.3 Alternate Egress Repair

   It is possible to restore service without specifically recovering the
   faulted path.
   For example, for best effort IP service it is possible to select a
   recovery path that has a different egress point from the working path

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   (i.e., there is no PML).  The recovery path egress must simply be a
   router that is acceptable for forwarding the FEC carried by the
   working path (without creating looping).  In an engineering context,
   specific alternative FEC/LSP mappings with alternate egresses can be
   formed.

   This may simplify enhancing the reliability of implicitly constructed
   MPLS topologies. A PSL may qualify LSP/FEC bindings as candidate
   recovery paths as simply link and node disjoint with the immediate
   downstream LSR of the working path.

4.4.1.4 Multi-Layer Repair

   Multi-layer repair broadens the network designerÆs tool set for those
   cases where multiple network layers can be managed together to
   achieve overall network goals.  Specific criteria for determining
   when multi-layer repair is appropriate are beyond the scope of this
   draft.

4.4.1.5 Concatenated Protection Domains

   A given service may cross multiple networks and these may employ
   different recovery mechanisms.  It is possible to concatenate
   protection domains so that service recovery can be provided end-to-
   end.  It is considered that the recovery mechanisms in different
   domains may operate autonomously, and that multiple points of
   attachment may be used between domains (to ensure there is no single
   point of failure).  Alternate egress repair requires management of
   concatenated domains in that an explicit MPLS point of failure (the
   PML) is by definition excluded.  Details of concatenated protection
   domains are beyond the scope of this draft.

4.4.2     Path Mapping

   Path mapping refers to the methods of mapping traffic from a faulty
   working path on to the recovery path. There are several options for
   this, as described below. Note that the options below should be
   viewed as atomic terms that only describe how the working and
   protection paths are mapped to each other. The issues of resource
   reservation along these paths, and how switchover is actually
   performed lead to the more commonly used composite terms, such as 1+1
   and 1:1 protection, which were described in Section 2.1.

   1-to-1 Protection

   In 1-to-1 protection the working path has a designated recovery path
   that is only to be used to recover that specific working path.

   n-to-1 Protection

   In n-to-1 protection, up to n working paths are protected using only
   one recovery path. If the intent is to protect against any single
   fault on any of the working paths, the n working paths should be

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   diversely routed between the same PSL and PML. In some cases,
   handshaking between PSL and PML may be required to complete the
   recovery, the details of which are beyond the scope of this draft.

   n-to-m Protection

   In n-to-m protection, up to n working paths are protected using m
   recovery paths. Once again, if the intent is to protect against any
   single fault on any of the n working paths, the n working paths and
   the m recovery paths should be diversely routed between the same PSL
   and PML. In some cases, handshaking between PSL and PML may be
   required to complete the recovery, the details of which are beyond
   the scope of this draft. n-to-m protection is for further study.

   Split Path Protection

   In split path protection, multiple recovery paths are allowed to
   carry the traffic of a working path based on a certain configurable
   load splitting ratio.  This is especially useful when no single
   recovery path can be found that can carry the entire traffic of the
   working path in case of a fault. Split path protection may require
   handshaking between the PSL and the PML(s), and may require the
   PML(s) to correlate the traffic arriving on multiple recovery paths
   with the working path. Although this is an attractive option, the
   details of split path protection are beyond the scope of this draft,
   and are for further study.

4.4.3     Bypass Tunnels

   It may be convenient, in some cases, to create a "bypass tunnel" for
   a PPG between a PSL and PML, thereby allowing multiple recovery paths
   to be transparent to intervening LSRs [2].  In this case, one LSP
   (the tunnel) is established between the PSL and PML following an
   acceptable route and a number of recovery paths are supported through
   the tunnel via label stacking. A bypass tunnel can be used with any
   of the path mapping options discussed in the previous section.

   As with recovery paths, the bypass tunnel may or may not have
   resource reservations sufficient to provide recovery without service
   degradation.  It is possible that the bypass tunnel may have
   sufficient resources to recover some number of working paths, but not
   all at the same time.  If the number of recovery paths carrying
   traffic in the tunnel at any given time is restricted, this is
   similar to the n-to-1 or n-to-m protection cases mentioned in Section
   3.4.2.

4.4.4     Recovery Granularity

   Another dimension of recovery considers the amount of traffic
   requiring protection. This may range from a fraction of a path to a
   bundle of paths.



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4.4.4.1 Selective Traffic Recovery

   This option allows for the protection of a fraction of traffic within
   the same path. The portion of the traffic on an individual path that
   requires protection is called a protected traffic portion (PTP). A
   single path may carry different classes of traffic, with different
   protection requirements. The protected portion of this traffic may be
   identified by its class, as for example, via the EXP bits in the MPLS
   shim header or via the priority bit in the ATM header.

4.4.4.2 Bundling

   Bundling is a technique used to group multiple working paths together
   in order to recover them simultaneously. The logical bundling of
   multiple working paths requiring protection, each of which is routed
   identically between a PSL and a PML, is called a protected path group
   (PPG). When a fault occurs on the working path carrying the PPG, the
   PPG as a whole can be protected either by being switched to a bypass
   tunnel or by being switched to a recovery path.

4.4.5     Recovery Path Resource Use

   In the case of pre-reserved recovery paths, there is the question of
   what use these resources may be put to when the recovery path is not
   in use.  There are two options:

   Dedicated-resource:
   If the recovery path resources are dedicated, they may not be used
   for anything except carrying the working traffic.  For example, in
   the case of 1+1 protection, the working traffic is always carried on
   the recovery path.  Even if the recovery path is not always carrying
   the working traffic, it may not be possible or desirable to allow
   other traffic to use these resources.

   Extra-traffic-allowed:
   If the recovery path only carries the working traffic when the
   working path fails, then it is possible to allow extra traffic to use
   the reserved resources at other times.  Extra traffic is, by
   definition, traffic that can be displaced (without violating service
   agreements) whenever the recovery path resources are needed for
   carrying the working path traffic.

   Shared-resource:
   A shared recovery resource is dedicated for use by multiple primary
   resources that (according to SRLGs) are not expected to fail
   simultaneously.

4.5. Fault Detection

   MPLS recovery is initiated after the detection of either a lower
   layer fault or a fault at the IP layer or in the operation of MPLS-
   based mechanisms. We consider four classes of impairments: Path
   Failure, Path Degraded, Link Failure, and Link Degraded.

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   Path Failure (PF) is a fault that indicates to an MPLS-based recovery
   scheme that the connectivity of the path is lost.  This may be
   detected by a path continuity test between the PSL and PML.  Some,
   and perhaps the most common, path failures may be detected using a
   link probing mechanism between neighbor LSRs. An example of a probing
   mechanism is a liveness message that is exchanged periodically along
   the working path between peer LSRs [3].  For either a link probing
   mechanism or path continuity test to be effective, the test message
   must be guaranteed to follow the same route as the working or
   recovery path, over the segment being tested. In addition, the path
   continuity test must take the path merge points into consideration.
   In the case of a bi-directional link implemented as two
   unidirectional links, path failure could mean that either one or both
   unidirectional links are damaged.

   Path Degraded (PD) is a fault that indicates to MPLS-based recovery
   schemes/mechanisms that the path has connectivity, but that the
   quality of the connection is unacceptable.  This may be detected by a
   path performance monitoring mechanism, or some other mechanism for
   determining the error rate on the path or some portion of the path.
   This is local to the LSR and consists of excessive discarding of
   packets at an interface, either due to label mismatch or due to TTL
   errors, for example.

   Link Failure (LF) is an indication from a lower layer that the link
   over which the path is carried has failed.  If the lower layer
   supports detection and reporting of this fault (that is, any fault
   that indicates link failure e.g., SONET LOS), this may be used by the
   MPLS recovery mechanism. In some cases, using LF indications may
   provide faster fault detection than using only MPLSûbased fault
   detection mechanisms.

   Link Degraded (LD) is an indication from a lower layer that the link
   over which the path is carried is performing below an acceptable
   level.  If the lower layer supports detection and reporting of this
   fault, it may be used by the MPLS recovery mechanism. In some cases,
   using LD indications may provide faster fault detection than using
   only MPLS-based fault detection mechanisms.

4.6. Fault Notification

   MPLS-based recovery relies on rapid and reliable notification of
   faults. Once a fault is detected, the node that detected the fault
   must determine if the fault is severe enough to require path
   recovery. If the node is not capable of initiating direct action
   (e.g. as a point of repair, POR) the node should send out a
   notification of the fault by transmitting a FIS to the POR. This can
   take several forms:

   (i) control plane messaging: relayed hop-by-hop along the path of the
   failed LSP until a POR is reached.


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   (ii) user plane messaging: sent to the PML, which may take corrective
   action (as a POR for 1+1) or then communicate with a POR (for 1:n) by
   any of several means:
   - control plane messaging
   - user plane return path (either through a bi-directional LSP
   or via other means)

   Since the FIS is a control message, it should be transmitted with
   high priority to ensure that it propagates rapidly towards the
   affected POR(s). Depending on how fault notification is configured in
   the LSRs of an MPLS domain, the FIS could be sent either as a Layer 2
   or Layer 3 packet [3]. The use of a Layer 2-based notification
   requires a Layer 2 path direct to the POR. An example of a FIS could
   be the liveness message sent by a downstream LSR to its upstream
   neighbor, with an optional fault notification field set or it can be
   implicitly denoted by a teardown message. Alternatively, it could be
   a separate fault notification packet. The intermediate LSR should
   identify which of its incoming links to propagate the FIS on.

4.7. Switch-Over Operation

4.7.1     Recovery Trigger

   The activation of an MPLS protection switch following the detection
   or notification of a fault requires a trigger mechanism at the PSL.
   MPLS protection switching may be initiated due to automatic inputs or
   external commands. The automatic activation of an MPLS protection
   switch results from a response to a defect or fault conditions
   detected at the PSL or to fault notifications received at the PSL. It
   is possible that the fault detection and trigger mechanisms may be
   combined, as is the case when a PF, PD, LF, or LD is detected at a
   PSL and triggers a protection switch to the recovery path. In most
   cases, however, the detection and trigger mechanisms are distinct,
   involving the detection of fault at some intermediate LSR followed by
   the propagation of a fault notification to the POR via the FIS, which
   serves as the protection switch trigger at the POR. MPLS protection
   switching in response to external commands results when the operator
   initiates a protection switch by a command to a POR (or alternatively
   by a configuration command to an intermediate LSR, which transmits
   the FIS towards the POR).

   Note that the PF fault applies to hard failures (fiber cuts,
   transmitter failures, or LSR fabric failures), as does the LF fault,
   with the difference that the LF is a lower layer impairment that may
   be communicated to - MPLS-based recovery mechanisms. The PD (or LD)
   fault, on the other hand, applies to soft defects (excessive errors
   due to noise on the link, for instance). The PD (or LD) results in a
   fault declaration only when the percentage of lost packets exceeds a
   given threshold, which is provisioned and may be set based on the
   service level agreement(s) in effect between a service provider and a
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4.7.2     Recovery Action

   After a fault is detected or FIS is received by the POR, the recovery
   action involves either a rerouting or protection switching operation.
   In both scenarios, the next hop label forwarding entry for a recovery
   path is bound to the working path.

4.8. Post Recovery Operation

   When traffic is flowing on the recovery path decisions can be made to
   whether let the traffic remain on the recovery path and consider it
   as a new working path or do a switch to the old or a new working
   path. This post recovery operation has two styles, one where the
   protection counterparts, i.e. the working and recovery path, are
   fixed or "pinned" to its route and one in which the PSL or other
   network entity with real time knowledge of failure dynamically
   performs re-establishment or controlled rearrangement of the paths
   comprising the protected service.

4.8.1     Fixed Protection Counterparts

   For fixed protection counterparts the PSL will be pre-configured with
   the appropriate behavior to take when the original fixed path is
   restored to service. The choices are revertive and non-revertive
   mode. The choice will typically be depended on relative costs of the
   working and protection paths, and the tolerance of the service to the
   effects of switching paths yet again. These protection modes indicate
   whether or not there is a preferred path for the protected traffic.

4.8.1.1   Revertive Mode

   If the working path always is the preferred path, this path will be
   used whenever it is available. Thus, in the event of a fault on this
   path, its unused resources will not be reclaimed by the network on
   failure.  If the working path has a fault, traffic is switched to the
   recovery path.  In the revertive mode of operation, when the
   preferred path is restored the traffic is automatically switched back
   to it.

   There are a number of implications to pinned working and recovery
   paths:
   - upon failure and traffic moved to recovery path, the traffic is
   unprotected until such time as the path defect in the original
   working path is repaired and that path restored to service.
   - upon failure and traffic moved to recovery path, the resources
   associated with the original path remain reserved.

4.8.1.2 Non-revertive Mode

   In the non-revertive mode of operation, there is no preferred path or
   it may be desirable to minimize further disruption of the service
   brought on by a revertive switching operation. A switch-back to the
   original working path is not desired or not possible since the

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   original path may no longer exist after the occurrence of a fault on
   that path.
   If there is a fault on the working path, traffic is switched to the
   recovery path. When or if the faulty path (the originally working
   path) is restored, it may become the recovery path (either by
   configuration, or, if desired, by management actions).

   In the non-revertive mode of operation, the working traffic may or
   may not be restored to a new optimal working path or to the original
   working path anyway. This is because it might be useful, in some
   cases, to either: (a) administratively perform a protection switch
   back to the original working path after gaining further assurances
   about the integrity of the path, or (b) it may be acceptable to
   continue operation on the recovery path, or (c) it may be desirable
   to move the traffic to a new optimal working path that is calculated
   based on network topology and network policies.

4.8.2     Dynamic Protection Counterparts

   For dynamic protection counterparts when the traffic is switched over
   to a recovery path, the association between the original working path
   and the recovery path may no longer exist, since the original path
   itself may no longer exist after the fault. Instead, when the network
   reaches a stable state following routing convergence, the recovery
   path may be switched over to a different preferred path either
   optimization based on the new network topology and associated
   information or based on pre-configured information.

   Dynamic protection counterparts assume that upon failure, the PSL or
   other network entity will establish new working paths if another
   switch-over will be performed.

4.8.3     Restoration and Notification

   MPLS restoration deals with returning the working traffic from the
   recovery path to the original or a new working path.  Reversion is
   performed by the PSL either upon receiving notification, via FRS,
   that the working path is repaired, or upon receiving notification
   that a new working path is established.

   For fixed counterparts in revertive mode, an LSR that detected the
   fault on the working path also detects the restoration of the working
   path. If the working path had experienced a LF defect, the LSR
   detects a return to normal operation via the receipt of a liveness
   message from its peer. If the working path had experienced a LD
   defect at an LSR interface, the LSR could detect a return to normal
   operation via the resumption of error-free packet reception on that
   interface. Alternatively, a lower layer that no longer detects a LF
   defect may inform the MPLS-based recovery mechanisms at the LSR that
   the link to its peer LSR is operational.
   The LSR then transmits FRS to its upstream LSR(s) that were
   transmitting traffic on the working path. At the point the PSL


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   receives the FRS, it switches the working traffic back to the
   original working path.

   A similar scheme is for dynamic counterparts where e.g. an update of
   topology and/or network convergence may trigger installation or setup
   of new working paths and may send notification to the PSL to perform
   a switch over.

   We note that if there is a way to transmit fault information back
   along a recovery path towards a PSL and if the recovery path is an
   equivalent working path, it is possible for the working path and its
   recovery path to exchange roles once the original working path is
   repaired following a fault. This is because, in that case, the
   recovery path effectively becomes the working path, and the restored
   working path functions as a recovery path for the original recovery
   path. This is important, since it affords the benefits of non-
   revertive switch operation outlined in Section 3.8.1, without leaving
   the recovery path unprotected.

4.8.4     Reverting to Preferred Path (or Controlled Rearrangement)

   In the revertive mode, a "make before break" restoration switching
   can be used, which is less disruptive than performing protection
   switching upon the occurrence of network impairments. This will
   minimize both packet loss and packet reordering. The controlled
   rearrangement of paths can also be used to satisfy traffic
   engineering requirements for load balancing across an MPLS domain.

4.9. Performance

   Resource/performance requirements for recovery paths should be
   specified in terms of the following attributes:

   I. Resource class attribute:
   Equivalent Recovery Class: The recovery path has the same resource
   reservations and performance guarantees as the working path. In other
   words, the recovery path meets the same SLAs as the working path.
   Limited Recovery Class: The recovery path does not have the same
   resource reservations and performance guarantees as the working path.

   A. Lower Class: The recovery path has lower resource requirements or
   less stringent performance requirements than the working path.

   B. Best Effort Class: The recovery path is best effort.

   II. Priority Attribute:
   The recovery path has a priority attribute just like the working path
   (i.e., the priority attribute of the associated traffic trunks). It
   can have the same priority as the working path or lower priority.

   III. Preemption Attribute:
   The recovery path can have the same preemption attribute as the
   working path or a lower one.

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5.  MPLS Recovery Features

   The following features are desirable from an operational point of
   view:

   I. It is desirable that MPLS recovery provides an option to identify
   protection groups (PPGs) and protection portions (PTPs).

   II. Each PSL should be capable of performing MPLS recovery upon the
   detection of the impairments or upon receipt of notifications of
   impairments.

   III. A MPLS recovery method should not preclude manual protection
   switching commands. This implies that it would be possible under
   administrative commands to transfer traffic from a working path to a
   recovery path, or to transfer traffic from a recovery path to a
   working path, once the working path becomes operational following a
   fault.

   IV. A PSL may be capable of performing either a switch back to the
   original working path after the fault is corrected or a switchover to
   a new working path, upon the discovery or establishment of a more
   optimal working path.

   V. The recovery model should take into consideration path merging at
   intermediate LSRs. If a fault affects the merged segment, all the
   paths sharing that merged segment should be able to recover.
   Similarly, if a fault affects a non-merged segment, only the path
   that is affected by the fault should be recovered.

6.  Comparison Criteria

   Possible criteria to use for comparison of MPLS-based recovery
   schemes are as follows:

   Recovery Time

   We define recovery time as the time required for a recovery path to
   be activated (and traffic flowing) after a fault. Recovery Time is
   the sum of the Fault Detection Time, Hold-off Time, Notification
   Time, Recovery Operation Time, and the Traffic Restoration Time. In
   other words, it is the time between a failure of a node or link in
   the network and the time before a recovery path is installed and the
   traffic starts flowing on it.

   Full Restoration Time

   We define full restoration time as the time required for a permanent
   restoration. This is the time required for traffic to be routed onto
   links, which are capable of or have been engineered sufficiently to
   handle traffic in recovery scenarios. Note that this time may or may

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   not be different from the "Recovery Time" depending on whether
   equivalent or limited recovery paths are used.

   Setup vulnerability

   The amount of time that a working path or a set of working paths is
   left unprotected during such tasks as recovery path computation and
   recovery path setup may be used to compare schemes.  The nature of
   this vulnerability should be taken into account, e.g.:  End to End
   schemes correlate the vulnerability with working paths, Local Repair
   schemes have a topological correlation that cuts across working paths
   and Network Plan approaches have a correlation that impacts the
   entire network.

   Backup Capacity

   Recovery schemes may require differing amounts of "backup capacity"
   in the event of a fault. This capacity will be dependent on the
   traffic characteristics of the network. However, it may also be
   dependent on the particular protection plan selection algorithms as
   well as the signaling and re-routing methods.

   Additive Latency

   Recovery schemes may introduce additive latency to traffic. For
   example, a recovery path may take many more hops than the working
   path. This may be dependent on the recovery path selection
   algorithms.

   Quality of Protection

   Recovery schemes can be considered to encompass a spectrum of "packet
   survivability" which may range from "relative" to "absolute".
   Relative survivability may mean that the packet is on an equal
   footing with other traffic of, as an example, the same diff-serv code
   point (DSCP) in contending for the resources of the portion of the
   network that survives the failure. Absolute survivability may mean
   that the survivability of the protected traffic has explicit
   guarantees.

   Re-ordering

   Recovery schemes may introduce re-ordering of packets. Also the
   action of putting traffic back on preferred paths might cause packet
   re-ordering.

   State Overhead

   As the number of recovery paths in a protection plan grows, the state
   required to maintain them also grows. Schemes may require differing
   numbers of paths to maintain certain levels of coverage, etc. The
   state required may also depend on the particular scheme used to


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   recover. In many cases the state overhead will be in proportion to
   the number of recovery paths.

   Loss

   Recovery schemes may introduce a certain amount of packet loss during
   switchover to a recovery path. Schemes that introduce loss during
   recovery can measure this loss by evaluating recovery times in
   proportion to the link speed.

   In case of link or node failure a certain packet loss is inevitable.

   Coverage

   Recovery schemes may offer various types of failover coverage. The
   total coverage may be defined in terms of several metrics:

   I. Fault Types: Recovery schemes may account for only link faults or
   both node and link faults or also degraded service. For example, a
   scheme may require more recovery paths to take node faults into
   account.

   II. Number of concurrent faults: dependent on the layout of recovery
   paths in the protection plan, multiple fault scenarios may be able to
   be restored.

   III. Number of recovery paths: for a given fault, there may be one or
   more recovery paths.

   IV. Percentage of coverage: dependent on a scheme and its
   implementation, a certain percentage of faults may be covered. This
   may be subdivided into percentage of link faults and percentage of
   node faults.

   V. The number of protected paths may effect how fast the total set of
   paths affected by a fault could be recovered. The ratio of protected
   is n/N, where n is the number of protected paths and N is the total
   number of paths.

7. Security Considerations

   The MPLS recovery that is specified herein does not raise any
   security issues that are not already present in the MPLS
   architecture.

8. Intellectual Property Considerations

   The IETF has been notified of intellectual property rights claimed in
   regard to some or all of the specification contained in this
   document. For more information consult the online list of claimed
   rights.


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9. Acknowledgements

   We would like to thank members of the MPLS WG mailing list for their
   suggestions on the earlier versions of this draft. In particular,
   Bora Akyol, Dave Allan, Dave Danenberg, Sharam Davari, and Neil
   Harrison whose suggestions and comments were very helpful in revising
   the document.

   The editors would like to give very special thanks to Curtis
   Villamizar for his careful and extremely thorough reading of the
   document and for taking the time to provide numerous suggestions,
   which were very helpful in the last couple of revisions of the
   document.


10.  EditorsÆ Addresses

   Vishal Sharma                        Fiffi Hellstrand
   Metanoia, Inc.                       Nortel Networks
   1600 Villa Street, Unit 352          St Eriksgatan 115
   Mountain View, CA 94041-1174         PO Box 6701
   Phone: (650) 386-6723                113 85 Stockholm, Sweden
   v.sharma@ieee.org                    Phone: +46 8 5088 3687
                                        Fiffi@nortelnetworks.com



11.  References

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

   [2] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., McManus, J.,
      "Requirements for Traffic Engineering Over MPLS", RFC 2702,
      September 1999.

   [3] Haung, C., Sharma, V., Owens, K., Makam, V. "Building Reliable
      MPLS Networks Using a Path Protection Mechanism", IEEE Commun.
      Mag., Vol. 40, Issue 3, March 2002, pp. 156-162.

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

   [5] Awduche, D., et al "RSVP-TE Extensions to RSVP for LSP Tunnels",
      RFC 3209, December 2001.

   [6] Jamoussi, B., et al "Constraint-Based LSP Setup using LDP", RFC
      3212, January 2002.




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