Network Working Group                       CCAMP GMPLS P&R Design Team
Internet Draft
Category: Informational                  Dimitri Papadimitriou (Editor)
Expiration Date: October 2005                      Eric Mannie (Editor)

                                                             April 2005

  Analysis of Generalized Multi-Protocol Label Switching (GMPLS)-based
       Recovery Mechanisms (including Protection and Restoration)


Status of this Memo

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   RFC 3668.

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Copyright Notice

   Copyright (C) The Internet Society (2005). All Rights Reserved.


   This document provides an analysis grid to evaluate, compare and
   contrast the Generalized Multi-Protocol Label Switching (GMPLS)
   protocol suite capabilities with respect to the recovery mechanisms
   currently proposed at the IETF CCAMP Working Group. A detailed
   analysis of each of the recovery phases is provided using the
   terminology defined in a companion document. This document focuses
   on transport plane survivability and recovery issues and not on
   control plane resilience and related aspects.

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Table of Content

   Status of this Memo .............................................. 1
   Abstract ......................................................... 1
   Table of Content ................................................. 2
   1. Contributors .................................................. 3
   2. Conventions used in this Document ............................. 4
   3. Introduction .................................................. 4
   4. Fault Management .............................................. 5
   4.1 Failure Detection ............................................ 5
   4.2 Failure Localization and Isolation ........................... 7
   4.3 Failure Notification ......................................... 8
   4.4 Failure Correlation ......................................... 10
   5. Recovery Mechanisms .......................................... 10
   5.1 Transport vs. Control Plane Responsibilities ................ 10
   5.2 Technology In/dependent Mechanisms .......................... 11
   5.2.1 OTN Recovery .............................................. 11
   5.2.2 Pre-OTN Recovery .......................................... 11
   5.2.3 SONET/SDH Recovery ........................................ 12
   5.3 Specific Aspects of Control Plane-based Recovery Mechanisms . 12
   5.3.1 In-band vs. Out-of-band Signaling ......................... 12
   5.3.2 Uni- vs. Bi-directional Failures .......................... 13
   5.3.3 Partial vs. Full Span Recovery ............................ 15
   5.3.4 Difference between LSP, LSP Segment and Span Recovery ..... 16
   5.4 Difference between Recovery Type and Scheme ................. 16
   5.5 LSP Recovery Mechanisms ..................................... 18
   5.5.1 Classification ............................................ 18
   5.5.2 LSP Restoration ........................................... 20
   5.5.3 Pre-planned LSP Restoration ............................... 21
   5.5.4 LSP Segment Restoration ................................... 22
   6. Reversion .................................................... 23
   6.1 Wait-To-Restore (WTR) ....................................... 23
   6.2 Revertive Mode Operation .................................... 23
   6.3 Orphans ..................................................... 24
   7. Hierarchies .................................................. 24
   7.1 Horizontal Hierarchy (Partitions) ........................... 25
   7.2 Vertical Hierarchy (Layers) ................................. 25
   7.2.1 Recovery Granularity ...................................... 27
   7.3 Escalation Strategies ....................................... 27
   7.4 Disjointness ................................................ 28
   7.4.1 SRLG Disjointness ......................................... 28
   8. Recovery Mechanisms Analysis ................................. 29
   8.1 Fast Convergence (Detection/Correlation and Hold-off Time) .. 30
   8.2 Efficiency (Recovery Switching Time) ........................ 30
   8.3 Robustness .................................................. 31
   8.4 Resource Optimization ....................................... 31
   8.4.1 Recovery Resource Sharing ................................. 32
   8.4.2 Recovery Resource Sharing and SRLG Recovery ............... 34
   8.4.3 Recovery Resource Sharing, SRLG Disjointness and Admission. 35
   9. Summary and Conclusions ...................................... 36
   10. Security Considerations ..................................... 38
   11. IANA Considerations ......................................... 38
   12. Acknowledgments ............................................. 38

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   13. References .................................................. 39
   13.1 Normative References ....................................... 39
   13.2 Informative References ..................................... 40
   14. Editor's Address ............................................ 41
   Intellectual Property Statement ................................. 42
   Disclaimer of Validity .......................................... 42
   Copyright Statement ............................................. 42

1. Contributors

   This document is the result of the CCAMP Working Group Protection
   and Restoration design team joint effort. Besides the editors, the
   following are the authors that contributed to the present memo:

   Deborah Brungard (AT&T)
   200 S. Laurel Ave.
   Middletown, NJ 07748, USA

   Sudheer Dharanikota

   Jonathan P. Lang (Sonos)
   506 Chapala Street
   Santa Barbara, CA 93101, USA

   Guangzhi Li (AT&T)
   180 Park Avenue,
   Florham Park, NJ 07932, USA

   Eric Mannie

   Dimitri Papadimitriou (Alcatel)
   Francis Wellesplein, 1
   B-2018 Antwerpen, Belgium

   Bala Rajagopalan (Intel Broadband Wireless Division)
   2111 NE 25th Ave.
   Hillsboro, OR 97124, USA

   Yakov Rekhter (Juniper)
   1194 N. Mathilda Avenue
   Sunnyvale, CA 94089, USA

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2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   this document are to be interpreted as described in [RFC2119].

   Any other recovery-related terminology used in this document
   conforms to the one defined in [TERM]. The reader is also assumed to
   be familiar with the terminology developed in [RFC3945], [RFC3471],
   [RFC3473], [GMPLS-RTG] and [LMP].

3. Introduction

   This document provides an analysis grid to evaluate, compare and
   contrast the Generalized MPLS (GMPLS) protocol suite capabilities
   with respect to the recovery mechanisms proposed at the IETF CCAMP
   Working Group. The focus is on transport plane survivability and
   recovery issues and not on control plane resilience related aspects.
   Although the recovery mechanisms described in this document impose
   different requirements on GMPLS-based recovery protocols, the
   protocol(s) specifications will not be covered in this document.
   Though the concepts discussed are technology independent, this
   document implicitly focuses on SONET [T1.105]/SDH [G.707], Optical
   Transport Networks (OTN) [G.709] and pre-OTN technologies except
   when specific details need to be considered (for instance, in the
   case of failure detection).

   A detailed analysis is provided for each of the recovery phases as
   identified in [TERM]. These phases define the sequence of generic
   operations that need to be performed when a LSP/Span failure (or any
   other event generating such failures) occurs:

      - Phase 1: Failure detection
      - Phase 2: Failure localization and isolation
      - Phase 3: Failure notification
      - Phase 4: Recovery (Protection/Restoration)
      - Phase 5: Reversion (normalization)

   Failure detection, localization and notification phases together are
   referred to as fault management. Within a recovery domain, the
   entities involved during the recovery operations are defined in
   [TERM]; these entities include ingress, egress and intermediate
   nodes. The term "recovery mechanism" is used to cover both
   protection and restoration mechanisms. Specific terms such as
   protection and restoration are only used when differentiation is
   required. Likewise the term "failure" is used to represent both
   signal failure and signal degradation.

   In addition, a clear distinction is made between partitioning
   (horizontal hierarchy) and layering (vertical hierarchy) when
   analyzing the different hierarchical recovery mechanisms including
   disjointness related issues. The dimensions from which each of the
   recovery mechanisms detailed in this document can be analyzed are

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   introduced to assess the current GMPLS protocol capabilities and the
   potential need for further extensions. This document concludes by
   detailing the applicability of the current GMPLS protocol building
   blocks for recovery purposes.

4. Fault Management

4.1 Failure Detection

   Transport failure detection is the only phase that can not be
   achieved by the control plane alone since the latter needs a hook to
   the transport plane to collect the related information. It has to be
   emphasized that even if failure events themselves are detected by
   the transport plane, the latter, upon a failure condition, must
   trigger the control plane for subsequent actions through the use of
   GMPLS signalling capabilities (see [RFC3471] and [RFC3473]) or Link
   Management Protocol capabilities (see [LMP], Section 6).

   Therefore, by definition, transport failure detection is transport
   technology dependent (and so exceptionally, we keep here the
   "transport plane" terminology). In transport fault management,
   distinction is made between a defect and a failure. Here, the
   discussion addresses failure detection (persistent fault cause). In
   the technology-dependent descriptions, a more precise specification
   will be provided.

   As an example, SONET/SDH (see [G.707], [G.783] and [G.806]) provides
   supervision capabilities covering:

   - Continuity: monitors the integrity of the continuity of a trail
     (i.e. section or path). This operation is performed by monitoring
     the presence/absence of the signal. Examples are Loss of Signal
     (LOS) detection for the physical layer, Unequipped (UNEQ) Signal
     detection for the path layer, Server Signal Fail Detection (e.g.
     AIS) at the client layer.

   - Connectivity: monitors the integrity of the routing of the signal
     between end-points. Connectivity monitoring is needed if
     the layer provides flexible connectivity, either automatically
     (e.g. cross-connects) or manually (e.g. fiber distribution frame).
     An example is the Trail (i.e. section or path) Trace Identifier
     used at the different layers and the corresponding Trail Trace
     Identifier Mismatch detection.

   - Alignment: checks that the client and server layer frame start can
     be correctly recovered from the detection of loss of alignment.
     The specific processes depend on the signal/frame structure and
     may include: (multi-)frame alignment, pointer processing and
     alignment of several independent frames to a common frame start in
     case of inverse multiplexing. Loss of alignment is a generic term.
     Examples are loss of frame, loss of multi-frame, or loss of

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   - Payload type: checks that compatible adaptation functions are used
     at the source and the destination. This is normally done by adding
     a payload type identifier (referred to as the "signal label") at
     the source adaptation function and comparing it with the expected
     identifier at the destination. For instance, the payload type
     identifier and the corresponding mismatch detection.

   - Signal Quality: monitors the performance of a signal. For
     instance, if the performance falls below a certain threshold a
     defect - excessive errors (EXC) or degraded signal (DEG) - is

   The most important point is that the supervision processes and the
   corresponding failure detection (used to initiate the recovery
   phase(s)) result in either:

   - Signal Degrade (SD): A signal indicating that the associated data
     has degraded in the sense that a degraded defect condition is
     active (for instance, a dDEG declared when the Bit Error Rate
     exceeds a preset threshold).

   - Signal Fail (SF): A signal indicating that the associated data has
     failed in the sense that a signal interrupting near-end defect
     condition is active (as opposed to the degraded defect).

   In Optical Transport Networks (OTN) equivalent supervision
   capabilities are provided at the optical/digital section layers
   (i.e. Optical Transmission Section (OTS), Optical Multiplex Section
   (OMS) and Optical channel Transport Unit (OTU)) and at the optical/
   digital path layers (i.e. Optical Channel (OCh) and Optical channel
   Data Unit (ODU)). Interested readers are referred to the ITU-T
   Recommendations [G.798] and [G.709] for more details.

   The above are examples that illustrate cases where the failure
   detection, and reporting entities (see [TERM]) are co-located. The
   following example illustrates the scenario where the failure
   detecting and reporting entities (see [TERM]) are not co-located.

   In pre-OTN networks, a failure may be masked by intermediate O-E-O
   based Optical Line System (OLS), preventing a Photonic Cross-Connect
   (PXC) from detecting upstream failures. In such cases, failure
   detection may be assisted by an out-of-band communication channel
   and failure condition reported to the PXC control plane. This can be
   provided by using [LMP-WDM] extensions that delivers IP message-
   based communication between the PXC and the OLS control plane. Also,
   since PXCs are independent of the framing format, failure conditions
   can only be triggered either by detecting the absence of the optical
   signal or by measuring its quality. These mechanisms are generally
   less reliable than electrical (digital) ones. Both types of
   detection mechanisms are outside the scope of this document. If the
   intermediate OLS supports electrical (digital) mechanisms, using the
   LMP communication channel, these failure conditions are reported to

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   the PXC and subsequent recovery actions performed as described in
   Section 5. As such from the control plane viewpoint, this mechanism
   turn the OLS-PXC composed system into a single logical entity
   allowing the consideration of the same failure management mechanisms
   for such entity as for any other O-E-O capable device.

   More generally, the following are typical failure conditions in
   SONET/SDH and pre-OTN networks:
   - Loss of Light (LOL)/Loss of Signal (LOS): Signal Failure (SF)
     condition where the optical signal is not detected any longer on
     the receiver of a given interface.
   - Signal Degrade (SD): detection of the signal degradation over
     a specific period of time.
   - For SONET/SDH payloads, all of the above-mentioned supervision
     capabilities can be used, resulting in SD or SF condition.

   In summary, the following cases apply when considering the
   communication between the detecting and reporting entities:

   - Co-located detecting and reporting entities: both the detecting
     and reporting entities are on the same node (e.g., SONET/SDH
     equipment, Opaque cross-connects, and, with some limitations,
     Transparent cross-connects, etc.)

   - Non co-located detecting and reporting entities:
     o with in-band communication between entities: entities are
       physically separated but the transport plane provides in-band
       communication between them (e.g., Server Signal Failures (Alarm
       Indication Signal (AIS)), etc.)
     o with out-of-band communication between entities: entities are
       physically separated but an out-of-band communication channel is
       provided between them (e.g., using [LMP]).

4.2 Failure Localization and Isolation

   Failure localization provides to the deciding entity information
   about the location (and so the identity) of the transport plane
   entity that detects the LSP(s)/span(s) failure. The deciding entity
   can then make an accurate decision to achieve finer grained recovery
   switching action(s). Note that this information can also be included
   as part of the failure notification (see Section 4.3).

   In some cases, this accurate failure localization information may be
   less urgent to determine if it requires performing more time
   consuming failure isolation (see also Section 4.4). This is
   particularly the case when edge-to-edge LSP recovery (edge referring
   to a sub-network end-node for instance) is performed based on a
   simple failure notification (including the identification of the
   working LSPs under failure condition). In this case, a more accurate
   localization and isolation can be performed after recovery of these

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   Failure localization should be triggered immediately after the fault
   detection phase. This operation can be performed at the transport
   plane and/or, if unavailable (via the transport plane), the control
   plane level where dedicated signaling messages can be used. When
   performed at the control plane level, a protocol such as LMP (see
   [LMP], Section 6) can be used for failure localization purposes.

4.3 Failure Notification

   Failure notification is used 1) to inform intermediate nodes that an
   LSP/span failure has occurred and has been detected 2) to inform the
   deciding entities (which can correspond to any intermediate or end-
   point of the failed LSP/span) that the corresponding service is not
   available. In general, these deciding entities will be the ones
   taking the appropriate recovery decision. When co-located with the
   recovering entity, these entities will also perform the
   corresponding recovery action(s).

   Failure notification can be either provided by the transport or by
   the control plane. As an example, let us first briefly describe the
   failure notification mechanism defined at the SONET/SDH transport
   plane level (also referred to as maintenance signal supervision):

   - AIS (Alarm Indication Signal) occurs as a result of a failure
     condition such as Loss of Signal and is used to notify downstream
     nodes (of the appropriate layer processing) that a failure has
     occurred. AIS performs two functions 1) inform the intermediate
     nodes (with the appropriate layer monitoring capability) that a
     failure has been detected 2) notify the connection end-point that
     the service is no longer available.

   For a distributed control plane supporting one (or more) failure
   notification mechanism(s), regardless of the mechanism's actual
   implementation, the same capabilities are needed with more (or less)
   information provided about the LSPs/spans under failure condition,
   their detailed status, etc.

   The most important difference between these mechanisms is related to
   the fact that transport plane notifications (as defined today) would
   directly initiate either a certain type of protection switching
   (such as those described in [TERM]) via the transport plane or
   restoration actions via the management plane.

   On the other hand, using a failure notification mechanism through
   the control plane would provide the possibility to trigger either a
   protection or a restoration action via the control plane. This has
   the advantage that a control plane recovery responsible entity does
   not necessarily have to be co-located with a transport
   maintenance/recovery domain. A control plane recovery domain can be
   defined at entities not supporting a transport plane recovery.

   Moreover, as specified in [RFC3473], notification message exchanges
   through a GMPLS control plane may not follow the same path as the

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   LSP/spans for which these messages carry the status. In turn, this
   ensures a fast, reliable (through acknowledgement and the use of
   either a dedicated control plane network or disjoint control
   channels) and efficient (through the aggregation of several LSP/span
   status within the same message) failure notification mechanism.

   The other important properties to be met by the failure notification
   mechanism are mainly the following:

   - Notification messages must provide enough information such that
     the most efficient subsequent recovery action will be taken (in
     most of the recovery types and schemes this action is even
     deterministic) at the recovering entities. Remember here that
     these entities can be either intermediate or end-points through
     which normal traffic flows. Based on local policy, intermediate
     nodes may not use this information for subsequent recovery actions
     (see for instance the APS protocol phases as described in [TERM]).
     In addition, since fast notification is a mechanism running in
     collaboration with the existing GMPLS signalling (see [RFC3473])
     that also allows intermediate nodes to stay informed about the
     status of the working LSP/spans under failure condition.

     The trade-off here is to define what information the LSP/span end-
     points (more precisely, the deciding entity) needs in order for
     the recovering entity to take the best recovery action: if not
     enough information is provided, the decision can not be optimal
     (note that in this eventuality, the important issue is to quantify
     the level of sub-optimality), if too much information is provided
     the control plane may be overloaded with unnecessary information
     and the aggregation/correlation of this notification information
     will be more complex and time consuming to achieve. Note that a
     more detailed quantification of the amount of information to be
     exchanged and processed is strongly dependent on the failure
     notification protocol.

   - If the failure localization and isolation is not performed by one
     of the LSP/span end-points or some intermediate points, they
     should receive enough information from the notification message in
     order to locate the failure otherwise they would need to (re-)
     initiate a failure localization and isolation action.

   - Avoiding so-called notification storms implies that 1) the failure
     detection output is correlated (i.e. alarm correlation) and
     aggregated at the node detecting the failure(s) 2) the failure
     notifications are directed to a restricted set of destinations (in
     general the end-points) and that 3) failure notification
     suppression (i.e. alarm suppression) is provided in order to limit
     flooding in case of multiple and/or correlated failures appearing
     at several locations in the network.

   - Alarm correlation and aggregation (at the failure detecting
     node) implies a consistent decision based on the conditions for
     which a trade-off between fast convergence (at detecting node) and

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     fast notification (implying that correlation and aggregation
     occurs at receiving end-points) can be found.

4.4 Failure Correlation

   A single failure event (such as a span failure) can result into
   reporting multiple failures (such as individual LSP failures)
   conditions. These can be grouped (i.e. correlated) to reduce the
   number of failure conditions communicated on the reporting channel,
   for both in-band and out-of-band failure reporting.

   In such a scenario, it can be important to wait for a certain period
   of time, typically called failure correlation time, and gather all
   the failures to report them as a group of failures (or simply group
   failure). For instance, this approach can be provided using LMP-WDM
   for pre-OTN networks (see [LMP-WDM]) or when using Signal Failure/
   Degrade Group in the SONET/SDH context.

   Note that a default average time interval during which failure
   correlation operation can be performed is difficult to provide since
   it is strongly dependent on the underlying network topology.
   Therefore, it can be advisable to provide a per-node configurable
   failure correlation time. The detailed selection criteria for this
   time interval are outside of the scope of this document.

   When failure correlation is not provided, multiple failure
   notification messages may be sent out in response to a single
   failure (for instance, a fiber cut), each one containing a set of
   information on the failed working resources (for instance, the
   individual lambda LSP flowing through this fiber). This allows for a
   more prompt response but can potentially overload the control plane
   due to a large amount of failure notifications.

5. Recovery Mechanisms

5.1 Transport vs. Control Plane Responsibilities

   For both protection and restoration, and when applicable, recovery
   resources are provisioned using GMPLS signalling capabilities. Thus,
   these are control plane-driven actions (topological and resource-
   constrained) that are always performed in this context.

   The following tables give an overview of the responsibilities taken
   by the control plane in case of LSP/span recovery:

   1. LSP/span Protection

   - Phase 1: Failure detection                 Transport plane
   - Phase 2: Failure localization/isolation    Transport/Control plane
   - Phase 3: Failure notification              Transport/Control plane
   - Phase 4: Protection switching              Transport/Control plane
   - Phase 5: Reversion (normalization)         Transport/Control plane

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   Note: in the context of LSP/span protection, control plane actions
   can be performed either for operational purposes and/or
   synchronization purposes (vertical synchronization between transport
   and control plane) and/or notification purposes (horizontal
   synchronization between end-nodes at control plane level). This
   suggests the selection of the responsible plane (in particular for
   protection switching) during the provisioning phase of the
   protected/protection LSP.

   2. LSP/span Restoration

   - Phase 1: Failure detection                 Transport plane
   - Phase 2: Failure localization/isolation    Transport/Control plane
   - Phase 3: Failure notification              Control plane
   - Phase 4: Recovery switching                Control plane
   - Phase 5: Reversion (normalization)         Control plane

   Therefore, this document primarily focuses on provisioning of LSP
   recovery resources, failure notification mechanisms, recovery
   switching, and reversion operations. Moreover some additional
   considerations can be dedicated to the mechanisms associated to the
   failure localization/isolation phase.

5.2 Technology in/dependent mechanisms

   The present recovery mechanisms analysis applies in fact to any
   circuit oriented data plane technology with discrete bandwidth
   increments (like SONET/SDH, G.709 OTN, etc.) being controlled by a
   GMPLS-based distributed control plane.

   The following sub-sections are not intended to favor one technology
   versus another. They just list pro and cons for each of them in
   order to determine the mechanisms that GMPLS-based recovery must
   deliver to overcome their cons and take benefits of their pros in
   their respective applicability context.

5.2.1 OTN Recovery

   OTN recovery specifics are left for further considerations.

5.2.2 Pre-OTN Recovery

   Pre-OTN recovery specifics (also referred to as "lambda switching")
   present mainly the following advantages:

   - benefits from a simpler architecture making it more suitable for
     mesh-based recovery types and schemes (on a per channel basis).

   - when providing suppression of intermediate node transponders (vs.
     use of non-standard masking of upstream failures) e.g. use of
     squelching, implies that failures (such as LoL) will propagate to
     edge nodes giving the possibility to initiate recovery actions
     driven by upper layers.

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   The main disadvantage comes from the lack of interworking due to the
   large amount of failure management (in particular failure
   notification protocols) and recovery mechanisms currently available.

   Note also, that for all-optical networks, combination of recovery
   with optical physical impairments is left for a future release of
   this document since corresponding detection technologies are under

5.2.3 SONET/SDH Recovery

   Some of the advantages of SONET [T1.105]/SDH [G.707] and more
   generically any TDM transport plane recovery are that they provide:

   - Protection types operating at the data plane level are
     standardized (see [G.841]) and can operate across protected
     domains and interwork (see [G.842]).

   - Failure detection, notification and path/section Automatic
     Protection Switching (APS) mechanisms.

   - Greater control over the granularity of the TDM LSPs/links that
     can be recovered with respect to coarser optical channel (or whole
     fiber content) recovery switching

   Some of the limitations of the SONET/SDH recovery are:

   - Limited topological scope: Inherently the use of ring topologies,
     typically, dedicated Sub-Network Connection Protection (SNCP) or
     shared protection rings, has reduced flexibility and resource
     efficiency with respect to the (somewhat more complex) meshed

   - Inefficient use of spare capacity: SONET/SDH protection is largely
     applied to ring topologies, where spare capacity often remains
     idle, making the efficiency of bandwidth usage a real issue.

   - Support of meshed recovery requires intensive network management
     development and the functionality is limited by both the network
     elements and the capabilities of the element management systems
     (justifying thus the development of GMPLS-based distributed
     recovery mechanisms.)

5.3 Specific Aspects of Control Plane-based Recovery Mechanisms

5.3.1 In-band vs Out-of-band Signalling

   The nodes communicate through the use of IP terminating control
   channels defining the control plane (transport) topology. In this
   context, two classes of transport mechanisms can be considered here
   i.e. in-fiber or out-of-fiber (through a dedicated physically
   diverse control network referred to as the Data Communication

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   Network or DCN). The potential impact of the usage of an in-fiber
   (signalling) transport mechanism is briefly considered here.

   In-fiber transport mechanism can be further subdivided into in-band
   and out-of-band. As such, the distinction between in-fiber in-band
   and in-fiber out-of-band signalling reduces to the consideration of
   a logically versus physically embedded control plane topology with
   respect to the transport plane topology. In the scope of this
   document, it is assumed that at least one IP control channel between
   each pair of adjacent nodes is continuously available to enable the
   exchange of recovery-related information and messages. Thus, in
   either case (i.e. in-band or out-of-band) at least one logical or
   physical control channel between each pair of nodes is always
   expected to be available.

   Therefore, the key issue when using in-fiber signalling is whether
   one can assume independence between the fault-tolerance capabilities
   of control plane and the failures affecting the transport plane
   (including the nodes). Note also that existing specifications like
   the OTN provide a limited form of independence for in-fiber
   signaling by dedicating a separate optical supervisory channel (OSC,
   see [G.709] and [G.874]) to transport the overhead and other control
   traffic. For OTNs, failure of the OSC does not result in failing the
   optical channels. Similarly, loss of the control channel must not
   result in failing the data channels (transport plane).

5.3.2 Uni- versus Bi-directional Failures

   The failure detection, correlation and notification mechanisms
   (described in Section 4) can be triggered when either a
   unidirectional or a bi-directional LSP/Span failure occurs (or a
   combination of both). As illustrated in Figure 1 and 2, two
   alternatives can be considered here:

   1. Uni-directional failure detection: the failure is detected on the
      receiver side i.e. it is only is detected by the downstream node
      to the failure (or by the upstream node depending on the failure
      propagation direction, respectively).

   2. Bi-directional failure detection: the failure is detected on the
      receiver side of both downstream node AND upstream node to the

   Notice that after the failure detection time, if only control plane
   based failure management is provided, the peering node is unaware of
   the failure detection status of its neighbor.

    -------             -------           -------             -------
   |       |           |       |Tx     Rx|       |           |       |
   | NodeA |----...----| NodeB |xxxxxxxxx| NodeC |----...----| NodeD |
   |       |----...----|       |---------|       |----...----|       |
    -------             -------           -------             -------

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   t0                                >>>>>>> F

   t1                      x <---------------x
   t2  <--------...--------x                 x--------...-------->
          Up Notification                      Down Notification

    -------             -------           -------             -------
   |       |           |       |Tx     Rx|       |           |       |
   | NodeA |----...----| NodeB |xxxxxxxxx| NodeC |----...----| NodeD |
   |       |----...----|       |xxxxxxxxx|       |----...----|       |
    -------             -------           -------             -------

   t0                      F <<<<<<< >>>>>>> F

   t1                      x <-------------> x
   t2  <--------...--------x                 x--------...-------->
          Up Notification                      Down Notification

   After failure detection, the following failure management operations
   can be subsequently considered:

   - Each detecting entity sends a notification message to the
     corresponding transmitting entity. For instance, in Fig. 1 (Fig.
     2), node C sends a notification message to node B (while node B
     sends a notification message to node C). To ensure reliable
     failure notification, a dedicated acknowledgment message can be
     returned back to the sender node.

   - Next, within a certain (and pre-determined) time window, nodes
     impacted by the failure occurrences may perform their correlation.
     In case of unidirectional failure, node B only receives the
     notification message from node C and thus the time for this
     operation is negligible. In case of bi-directional failure, node B
     (and node C) has to correlate the received notification message
     from node C (node B, respectively) with the corresponding locally
     detected information.

   - After some (pre-determined) period of time, referred to as the
     hold-off time, after which the local recovery actions (see Section
     5.3.4) were not successful, the following occurs. In case of
     unidirectional failure and depending on the directionality of the
     LSP, node B should send an upstream notification message (see
     [RFC3473]) to the ingress node A and node C may send a downstream
     notification message (see [RFC3473]) to the egress node D.
     However, in such a case only node A referred to as the "master"
     (node D being then referred to as the "slave" per [TERM]), would
     initiate an edge to edge recovery action. Note that the other LSP
     end-node (i.e. node D in this case) may be optionally notified
     using a downstream notification message (see [RFC3473]).

     In case of bi-directional failure, node B should send an upstream

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     notification message (see [RFC3473]) to the ingress node A and
     node C may send a downstream notification message (see [RFC3473])
     to the egress node D. However, due to the dependence on the LSP
     directionality, only ingress node A would initiate an edge to edge
     recovery action. Note that the other LSP end-node (i.e. node D in
     this case) should also be notified of this event using a
     downstream notification message (see [RFC3473]). For instance, if
     an LSP directed from D to A is under failure condition, only the
     notification message sent from node C to D would initiate a
     recovery action and, in this case, per [TERM], the deciding and
     recovering node D is referred to as the "master" while node A is
     referred to as the "slave" (i.e. recovering only entity).

     Note: The determination of the master and the slave may be based
     either on configured information or dedicated protocol capability.

   In the above scenarios, the path followed by the upstream and
   downstream notification messages does not have to be the same as the
   one followed by the failed LSP (see [RFC3473] for more details on
   the notification message exchange). The important point, concerning
   this mechanism, is that either the detecting/reporting entity (i.e.
   the nodes B and C) is also the deciding/recovery entity or the
   detecting/reporting entity is simply an intermediate node in the
   subsequent recovery process. One refers to local recovery in the
   former case and to edge-to-edge recovery in the latter one (see also
   Section 5.3.4).

5.3.3 Partial versus Full Span Recovery

   When a given span carries more than one LSPs or LSP segments, an
   additional aspect must be considered. In case of span failure, the
   LSPs it carries can be either individually recovered or recovered as
   a group (aka bulk LSP recovery) or independent sub-groups. The
   selection of this mechanism would be triggered independently of the
   failure notification granularity when correlation time windows are
   used and simultaneous recovery of several LSPs can be performed
   using a single request. Moreover, criteria by which such sub-groups
   can be formed are outside of the scope of this document.

   Additional complexity arises in the case of (sub-)group LSP
   recovery. Between a given pair of nodes, the LSPs that a given (sub-
   )group contains may have been created from different source nodes
   (i.e. initiator) and directed toward different destinations nodes.
   Consequently the failure notification messages sub-sequent to a bi-
   directional span failure affecting several LSPs (or the whole group
   of LSPs it carries) are not necessarily directed toward the same
   initiator nodes. In particular these messages may be directed to
   both the upstream and downstream nodes to the failure. Therefore,
   such span failure may trigger recovery actions to be performed from
   both sides (i.e. both from the upstream and the downstream node to
   the failure). In order to facilitate the definition of the
   corresponding recovery mechanisms (and their sequence), one assumes
   here as well, that per [TERM] the deciding (and recovering) entity,

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   referred to as the "master" is the only initiator of the recovery of
   the whole LSP (sub-)group.

5.3.4 Difference between LSP, LSP Segment and Span Recovery

   The recovery definitions given in [TERM] are quite generic and apply
   for link (or local span) and LSP recovery. The major difference
   between LSP, LSP Segment and span recovery is related to the number
   of intermediate nodes that the signalling messages have to travel.
   Since nodes are not necessarily adjacent in case of LSP (or LSP
   Segment) recovery, signalling message exchanges from the reporting
   to the deciding/recovery entity may have to cross several
   intermediate nodes. In particular, this applies for the notification
   messages due to the number of hops separating the location of a
   failure occurrence from its destination. This results in an
   additional propagation and forwarding delay. Note that the former
   delay may in certain circumstances be non-negligible; e.g. in case
   of copper out-of-band network, approximately 1 ms per 200km.

   Moreover, the recovery mechanisms applicable to end-to-end LSPs and
   to the segments that may compose an end-to-end LSP (i.e. edge-to-
   edge recovery) can be exactly the same. However, one expects in the
   latter case, that the destination of the failure notification
   message will be the ingress/egress of each of these segments.
   Therefore, using the mechanisms described in Section 5.3.2, failure
   notification messages can be first exchanged between terminating
   points of the LSP segment and after expiration of the hold-off time
   be directed toward terminating points of the end-to-end LSP.

   Note: Several studies provide quantitative analysis of the relative
   performance of LSP/span recovery techniques. [WANG] for instance,
   provides an analysis grid for these techniques showing that dynamic
   LSP restoration (see Section 5.5.2) performs well under medium
   network loads but suffers performance degradations at higher loads
   due to greater contention for recovery resources. LSP restoration
   upon span failure, as defined in [WANG], degrades at higher loads
   because paths around failed links tend to increase the hop count of
   the affected LSPs and thus consume additional network resources.
   Also, performance of LSP restoration can be enhanced by a failed
   working LSP's source node initiating a new recovery attempt if an
   initial attempt fails. A single retry attempt is sufficient to
   produce large increases in the restoration success rate and ability
   to initiate successful LSP restoration attempts, especially at high
   loads, while not adding significantly to the long-term average
   recovery time. Allowing additional attempts produces only small
   additional gains in performance. This suggests using additional
   (intermediate) crankback signalling when using dynamic LSP
   restoration (described in Section 5.5.2 - case 2). Details on
   crankback signalling are outside the scope of the present document.

5.4 Difference between Recovery Type and Scheme

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   [TERM] defines the basic LSP/span recovery types. This section
   describes the recovery schemes that can be built using these
   recovery types. In brief, a recovery scheme is defined as the
   combination of several ingress-egress node pairs supporting a given
   recovery type (from the set of the recovery types they allow).
   Several examples are provided here to illustrate the difference
   between recovery types such as 1:1 or M:N and recovery schemes such
   as (1:1)^n or (M:N)^n referred to as shared-mesh recovery.

   1. (1:1)^n with recovery resource sharing

   The exponent, n, indicates the number of times a 1:1 recovery type
   is applied between at most n different ingress-egress node pairs.
   Here, at most n pairs of disjoint working and recovery LSPs/spans
   share at most n times a common resource. Since the working LSPs/
   spans are mutually disjoint, simultaneous requests for use of the
   shared (common) resource will only occur in case of simultaneous
   failures, which is less likely to happen.

   For instance, in the common (1:1)^2 case, if the 2 recovery LSPs in
   the group overlap the same common resource, then it can handle only
   single failures; any multiple working LSP failures will cause at
   least one working LSP to be denied automatic recovery. Consider for
   instance the following topology with the working LSPs A-B-C and F-G-
   H and their respective recovery LSPs A-D-E-C and F-D-E-H that share
   a common D-E link resource.

                           \                 /
                            \               /
                            /               \
                           /                 \

   2. (M:N)^n with recovery resource sharing

   The (M:N)^n scheme is documented here for the sake of completeness
   only (i.e. it is not mandated that GMPLS capabilities would support
   this scheme). The exponent, n, indicates the number of times an M:N
   recovery type is applied between at most n different ingress-egress
   node pairs. So the interpretation follows from the previous case,
   except that here disjointness applies to the N working LSPs/spans
   and to the M recovery LSPs/spans while sharing at most n times M
   common resources.

   In both schemes, it results in a "group" of sum{n=1}^N N{n} working
   LSPs and a pool of shared recovery resources, not all of which are
   available to any given working LSP. In such conditions, defining a
   metric that describes the amount of overlap among the recovery LSPs
   would give some indication of the group's ability to handle
   simultaneous failures of multiple LSPs.

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   For instance, in the simple (1:1)^n case, if n recovery LSPs in a
   (1:1)^n group overlap, then it can handle only single failures; any
   simultaneous failure of multiple working LSPs will cause at least
   one working LSP to be denied automatic recovery. But if one
   considers for instance, a (2:2)^2 group in which there are two pairs
   of overlapping recovery LSPs, then two LSPs (belonging to the same
   pair) can be simultaneously recovered. The latter case can be
   illustrated by the following topology with 2 pairs of working LSPs
   A-B-C and F-G-H and their respective recovery LSPs A-D-E-C and F-D-
   E-H that share two common D-E link resources.

                           \\               //
                            \\             //
                             D =========== E
                            //             \\
                           //               \\

   Moreover, in all these schemes, (working) path disjointness can be
   enforced by exchanging information related to working LSPs during
   the recovery LSP signaling. Specific issues related to the
   combination of shared (discrete) bandwidth and disjointness for
   recovery schemes are described in Section 8.4.2.

5.5 LSP Recovery Mechanisms

5.5.1 Classification

   The recovery time and ratio of LSPs/spans depend on proper recovery
   LSP provisioning (meaning pre-provisioning when performed before
   failure occurrence) and the level of overbooking of recovery
   resources (i.e. over-provisioning). A proper balance of these two
   operations will result in the desired LSP/span recovery time and
   ratio when single or multiple failure(s) occur(s). Note also that
   these operations are mostly performed during the network planning

   The different options for LSP (pre-)provisioning and overbooking are
   classified below to structure the analysis of the different recovery

   1. Pre-Provisioning

   Proper recovery LSP pre-provisioning will help to alleviate the
   failure of the working LSPs (due to the failure of the resources
   that carry these LSPs). As an example, one may compute and establish
   the recovery LSP either end-to-end or segment-per-segment, to
   protect a working LSP from multiple failure events affecting

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   link(s), node(s) and/or SRLG(s). The recovery LSP pre-provisioning
   options can be classified (in the below figure) as follows:

   (1) the recovery path can be either pre-computed or computed

   (2) when the recovery path is pre-computed, it can be either pre-
       signaled (implying recovery resource reservation) or signaled

   (3) when the recovery resources are pre-signaled, they can be either
       pre-selected or selected on-demand.

   Recovery LSP provisioning phases:

   (1) Path Computation --> On-demand
            --> Pre-Computed
                   (2) Signalling --> On-demand
                            --> Pre-Signaled
                                   (3) Resource Selection --> On-demand
                                                 --> Pre-Selected

   Note that these different options lead to different LSP/span
   recovery times. The following sections will consider the above-
   mentioned pre-provisioning options when analyzing the different
   recovery mechanisms.

   2. Overbooking

   There are many mechanisms available that allow the overbooking of
   the recovery resources. This overbooking can be done per LSP (such
   as the example mentioned above), per link (such as span protection)
   or even per domain. In all these cases, the level of overbooking, as
   shown in the below figure, can be classified as dedicated (such as
   1+1 and 1:1), shared (such as 1:N and M:N) or unprotected (and thus
   restorable if enough recovery resources are available).

   Overbooking levels:

                    +----- Dedicated (for instance: 1+1, 1:1, etc.)

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                    +----- Shared (for instance: 1:N, M:N, etc.)
   Level of         |
   Overbooking -----+----- Unprotected (for instance: 0:1, 0:N)

   Also, when using shared recovery, one may support preemptible extra-
   traffic; the recovery mechanism is then expected to allow preemption
   of this low priority traffic in case of recovery resource contention
   during recovery operations. The following sections will consider the
   above-mentioned overbooking options when analyzing the different
   recovery mechanisms.

5.5.2 LSP Restoration

   The following times are defined to provide a quantitative estimation
   about the time performance of the different LSP restoration
   mechanisms (also referred to as LSP re-routing):

   - Path Computation Time: Tc
   - Path Selection Time: Ts
   - End-to-end LSP Resource Reservation Time: Tr (a delta for resource
     selection is also considered, the corresponding total time is then
     referred to as Trs)
   - End-to-end LSP Resource Activation Time: Ta (a delta for
     resource selection is also considered, the corresponding total
     time is then referred to as Tas)

   The Path Selection Time (Ts) is considered when a pool of recovery
   LSP paths between a given pair of source/destination end-points is
   pre-computed and after a failure occurrence one of these paths is
   selected for the recovery of the LSP under failure condition.

   Note: failure management operations such as failure detection,
   correlation and notification are considered (for a given failure
   event) as equally time consuming for all the mechanisms described
   here below:

   1. With Route Pre-computation (or LSP re-provisioning)

   An end-to-end restoration LSP is established after the failure(s)
   occur(s) based on a pre-computed path. As such, one can define this
   as an "LSP re-provisioning" mechanism. Here, one or more (disjoint)
   paths for the restoration LSP are computed (and optionally pre-
   selected) before a failure occurs.

   No reservation or selection of resources is performed along the
   restoration path before failure occurrence. As a result, there is no
   guarantee that a restoration LSP is available when a failure occurs.

   The expected total restoration time T is thus equal to Ts + Trs or
   to Trs when a dedicated computation is performed for each working

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   2. Without Route Pre-computation (or Full LSP re-routing)

   An end-to-end restoration LSP is dynamically established after the
   failure(s) occur(s). Here, after failure occurrence, one or more
   (disjoint) paths for the restoration LSP are dynamically computed
   and one is selected. As such, one can define this as a complete "LSP
   re-routing" mechanism.

   No reservation or selection of resources is performed along the
   restoration path before failure occurrence. As a result, there is no
   guarantee that a restoration LSP is available when a failure occurs.

   The expected total restoration time T is thus equal to Tc (+ Ts) +
   Trs. Therefore, time performance between these two approaches
   differs by the time required for route computation Tc (and its
   potential selection time, Ts).

5.5.3 Pre-planned LSP Restoration

   Pre-planned LSP restoration (also referred to as pre-planned LSP re-
   routing) implies that the restoration LSP is pre-signaled. This in
   turn implies the reservation of recovery resources along the
   restoration path. Two cases can be defined based on whether the
   recovery resources are pre-selected or not.

   1. With resource reservation and without resource pre-selection

   Before failure occurrence, an end-to-end restoration path is pre-
   selected from a set of pre-computed (disjoint) paths. The
   restoration LSP is signaled along this pre-selected path to reserve
   resources at each node but these resources are not selected.

   In this case, the resources reserved for each restoration LSP may be
   dedicated or shared between multiple restoration LSPs whose working
   LSPs are not expected to fail simultaneously. Local node policies
   can be applied to define the degree to which these resources can be
   shared across independent failures. Also, since a restoration scheme
   is considered, resource sharing should not be limited to restoration
   LSPs starting and ending at the same ingress and egress nodes.
   Therefore, each node participating to this scheme is expected to
   receive some feedback information on the sharing degree of the
   recovery resource(s) that this scheme involves.

   Upon failure detection/notification message reception, signaling is
   initiated along the restoration path to select the resources, and to
   perform the appropriate operation at each node crossed by the
   restoration LSP (e.g. cross-connections). If lower priority LSPs
   were established using the restoration resources, they must be
   preempted when the restoration LSP is activated.

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   The expected total restoration time T is thus equal to Tas (post-
   failure activation) while operations performed before failure
   occurrence takes Tc + Ts + Tr.

   2. With both resource reservation and resource pre-selection

   Before failure occurrence, an end-to-end restoration path is pre-
   selected from a set of pre-computed (disjoint) paths. The
   restoration LSP is signaled along this pre-selected path to reserve
   AND select resources at each node but these resources are not
   committed at the data plane level. Such that the selection of the
   recovery resources is committed at the control plane level only, no
   cross-connections are performed along the restoration path.

   In this case, the resources reserved and selected for each
   restoration LSP may be dedicated or even shared between multiple
   restoration LSPs whose associated working LSPs are not expected to
   fail simultaneously. Local node policies can be applied to define
   the degree to which these resources can be shared across independent
   failures. Also, since a restoration scheme is considered, resource
   sharing should not be limited to restoration LSPs starting and
   ending at the same ingress and egress nodes. Therefore, each node
   participating to this scheme is expected to receive some feedback
   information on the sharing degree of the recovery resource(s) that
   this scheme involves.

   Upon failure detection/notification message reception, signaling is
   initiated along the restoration path to activate the reserved and
   selected resources, and to perform the appropriate operation at each
   node crossed by the restoration LSP (e.g. cross-connections). If
   lower priority LSPs were established using the restoration
   resources, they must be preempted when the restoration LSP is

   The expected total restoration time T is thus equal to Ta (post-
   failure activation) while operations performed before failure
   occurrence takes Tc + Ts + Trs. Therefore, time performance between
   these two approaches differs only by the time required for resource
   selection during the activation of the recovery LSP (i.e. Tas - Ta).

5.5.4 LSP Segment Restoration

   The above approaches can be applied on an edge-to-edge LSP basis
   rather than end-to-end LSP basis (i.e. to reduce the global recovery
   time) by allowing the recovery of the individual LSP segments
   constituting the end-to-end LSP.

   Also, by using the horizontal hierarchy approach described in
   Section 7.1, an end-to-end LSP can be recovered by multiple recovery
   mechanisms applied on an LSP segment basis (e.g. 1:1 edge-to-edge
   LSP protection in a metro network and M:N edge-to-edge protection in
   the core). These mechanisms are ideally independent and may even use
   different failure localization and notification mechanisms.

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6. Reversion

   Reversion (a.k.a. normalization) is defined as the mechanism
   allowing switching of normal traffic from the recovery LSP/span to
   the working LSP/span previously under failure condition. Use of
   normalization is at the discretion of the recovery domain policy.
   Normalization (also referred to as reversion) may impact the normal
   traffic (a second hit) depending on the normalization mechanism

   If normalization is supported 1) the LSP/span must be returned to
   the working LSP/span when the failure condition clears 2) the
   capability to de-activate (turn-off) the use of reversion should be
   provided. De-activation of reversion should not impact the normal
   traffic regardless of whether currently using the working or
   recovery LSP/span.

   Note: during the failure, the reuse of any non-failed resources
   (e.g. LSP and/or spans) belonging to the working LSP/span is under
   the discretion of recovery domain policy.

6.1 Wait-To-Restore (WTR)

   A specific mechanism (Wait-To-Restore) is used to prevent frequent
   recovery switching operations due to an intermittent defect (e.g.
   BER fluctuating around the SD threshold).

   First, an LSP/span under failure condition must become fault-free,
   e.g. a BER less than a certain recovery threshold. After the
   recovered LSP/span (i.e. the previously working LSP/span) meets this
   criterion, a fixed period of time shall elapse before normal traffic
   uses the corresponding resources again. This duration called Wait-
   To-Restore (WTR) period or timer is generally of the order of a few
   minutes (for instance, 5 minutes) and should be capable of being
   set. The WTR timer may be either a fixed period, or provide for
   incrementally longer periods before retrying. An SF or SD condition
   on the previously working LSP/span will override the WTR timer value
   (i.e. the WTR cancels and the WTR timer will restart).

6.2 Revertive Mode Operation

   In revertive mode of operation, when the recovery LSP/span is no
   longer required, i.e. the failed working LSP/span is no longer in SD
   or SF condition, a local Wait-to-Restore (WTR) state will be
   activated before switching the normal traffic back to the recovered
   working LSP/span.

   During the reversion operation, since this state becomes the highest
   in priority, signalling must maintain the normal traffic on the
   recovery LSP/span from the previously failed working LSP/span.
   Moreover, during this WTR state, any null traffic or extra traffic
   (if applicable) request is rejected.

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   However, deactivation (cancellation) of the wait-to-restore timer
   may occur in case of higher priority request attempts. That is the
   recovery LSP/span usage by the normal traffic may be preempted if a
   higher priority request for this recovery LSP/span is attempted.

6.3 Orphans

   When a reversion operation is requested normal traffic must be
   switched from the recovery to the recovered working LSP/span. A
   particular situation occurs when the previously working LSP/span
   cannot be recovered such that normal traffic can not be switched
   back. In such a case, the LSP/span under failure condition (also
   referred to as "orphan") must be cleared i.e. removed from the pool
   of resources allocated for normal traffic. Otherwise, potential de-
   synchronization between the control and transport plane resource
   usage can appear. Depending on the signalling protocol capabilities
   and behavior different mechanisms are expected here.

   Therefore any reserved or allocated resources for the LSP/span under
   failure condition must be unreserved/de-allocated. Several ways can
   be used for that purpose: either wait for the elapsing of the clear-
   out time interval, or initiate a deletion from the ingress or the
   egress node, or trigger the initiation of deletion from an entity
   (such as an EMS or NMS) capable to react on the reception of an
   appropriate notification message.

7. Hierarchies

   Recovery mechanisms are being made available at multiple (if not
   each) transport layers within so-called "IP/MPLS-over-optical"
   networks. However, each layer has certain recovery features and one
   needs to determine the exact impact of the interaction between the
   recovery mechanisms provided by these layers.

   Hierarchies are used to build scalable complex systems. Abstraction
   is used as a mechanism to build large networks or as a technique for
   enforcing technology, topological or administrative boundaries by
   hiding the internal details. The same hierarchical concept can be
   applied to control the network survivability. Network survivability
   is the set of capabilities that allow a network to restore affected
   traffic in the event of a failure. Network survivability is defined
   further in [TERM]. In general, it is expected that the recovery
   action is taken by the recoverable LSP/span closest to the failure
   in order to avoid the multiplication of recovery actions. Moreover,
   recovery hierarchies can be also bound to control plane logical
   partitions (e.g. administrative or topological boundaries). Each of
   them may apply different recovery mechanisms.

   In brief, the commonly accepted ideas are generally that the lower
   layers can provide coarse but faster recovery while the higher
   layers can provide finer but slower recovery. Moreover, it is also
   desirable to avoid similar layers with functional overlaps to

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   optimize network resource utilization and processing overhead, since
   repeating the same capabilities at each layer does not create any
   added value for the network as a whole. In addition, even if a lower
   layer recovery mechanism is enabled, doing so does not prevent the
   additional provision of a recovery mechanism at the upper layer. The
   inverse statement does not necessarily hold; that is, enabling an
   upper layer recovery mechanism may prevent the use of a lower layer
   recovery mechanism. In this context, this section intends to analyze
   these hierarchical aspects including the physical (passive)

7.1 Horizontal Hierarchy (Partitioning)

   A horizontal hierarchy is defined when partitioning a single-layer
   network (and its control plane) into several recovery domains.
   Within a domain, the recovery scope may extend over a link (or
   span), LSP segment or even an end-to-end LSP. Moreover, an
   administrative domain may consist of a single recovery domain or can
   be partitioned into several smaller recovery domains. The operator
   can partition the network into recovery domains based on physical
   network topology, control plane capabilities or various traffic
   engineering constraints.

   An example often addressed in the literature is the metro-core-metro
   application (sometimes extended to a metro-metro/core-core) within a
   single transport layer (see Section 7.2). For such a case, an end-
   to-end LSP is defined between the ingress and egress metro nodes,
   while LSP segments may be defined within the metro or core sub-
   networks. Each of these topological structures determines a so-
   called "recovery domain" since each of the LSPs they carry can have
   its own recovery type (or even scheme). The support of multiple
   recovery types and schemes within a sub-network is referred to as a
   multi-recovery capable domain or simply multi-recovery domain.

7.2 Vertical Hierarchy (Layers)

   It is a very challenging task to combine in a coordinated manner the
   different recovery capabilities available across the path (i.e.
   switching capable) and section layers to ensure that certain network
   survivability objectives are met for the different services
   supported by the network.

   As a first analysis step, one can draw the following guidelines for
   a vertical coordination of the recovery mechanisms:
   - The lower the layer the faster the notification and switching
   - The higher the layer the finer the granularity of the recoverable
     entity and therefore the granularity of the recovery resource

   Moreover, in the context of this analysis, a vertical hierarchy
   consists of multiple layered transport planes providing different:
   - Discrete bandwidth granularities for non-packet LSPs such as OCh,
     ODUk, STS_SPE/HOVC and VT_SPE/LOVC LSPs and continuous bandwidth
     granularities for packet LSPs

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   - Potential recovery capabilities with different temporal
     granularities: ranging from milliseconds to tens of seconds

   Note: based on the bandwidth granularity we can determine four
   classes of vertical hierarchies (1) packet over packet (2) packet
   over circuit (3) circuit over packet and (4) circuit over circuit.
   Below we briefly expand on (4) only. (2) is covered in [RFC3386].
   (1) is extensively covered by the MPLS Working Group, and (3) by the
   PWE3 Working Group.

   In SONET/SDH environments, one typically considers the VT_SPE/LOVC
   and STS SPE/HOVC as independent layers, VT_SPE/LOVC LSP using the
   underlying STS_SPE/HOVC LSPs as links, for instance. In OTN, the
   ODUk path layers will lie on the OCh path layer i.e. the ODUk LSPs
   using the underlying OCh LSPs as OTUk links. Note here that lower
   layer LSPs may simply be provisioned and not necessarily dynamically
   triggered or established (control driven approach). In this context,
   an LSP at the path layer (i.e. established using GMPLS signalling),
   for instance an optical channel LSP, appears at the OTUk layer as a
   link, controlled by a link management protocol such as LMP.

   The first key issue with multi-layer recovery is that achieving
   individual or bulk LSP recovery will be as efficient as the
   underlying link (local span) recovery. In such a case, the span can
   be either protected or unprotected, but the LSP it carries must be
   (at least locally) recoverable. Therefore, the span recovery process
   can be either independent when protected (or restorable), or
   triggered by the upper LSP recovery process. The former case
   requires coordination to achieve subsequent LSP recovery. Therefore,
   in order to achieve robustness and fast convergence, multi-layer
   recovery requires a fine-tuned coordination mechanism.

   Moreover, in the absence of adequate recovery mechanism coordination
   (for instance, a pre-determined coordination when using a hold-off
   timer), a failure notification may propagate from one layer to the
   next one within a recovery hierarchy. This can cause "collisions"
   and trigger simultaneous recovery actions that may lead to race
   conditions and in turn, reduce the optimization of the resource
   utilization and/or generate global instabilities in the network (see
   [MANCHESTER]). Therefore, a consistent and efficient escalation
   strategy is needed to coordinate recovery across several layers.

   Therefore, one can expect that the definition of the recovery
   mechanisms and protocol(s) is technology-independent such that they
   can be consistently implemented at different layers; this would in
   turn simplify their global coordination. Moreover, as mentioned in
   [RFC3386], some looser form of coordination and communication
   between (vertical) layers such a consistent hold-off timer
   configuration (and setup through signalling during the working LSP
   establishment) can be considered, allowing the synchronization
   between recovery actions performed across these layers.

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7.2.1 Recovery Granularity

   In most environments, the design of the network and the vertical
   distribution of the LSP bandwidth are such that the recovery
   granularity is finer at higher layers. The OTN and SONET/SDH layers
   can only recover the whole section or the individual connections it
   transports whereas the IP/MPLS control plane can recover individual
   packet LSPs or groups of packet LSPs and this independently of their
   granularity. On the other side, the recovery granularity at the sub-
   wavelength level (i.e. SONET/SDH) can be provided only when the
   network includes devices switching at the same granularity (and thus
   not with optical channel level). Therefore, the network layer can
   deliver control-plane driven recovery mechanisms on a per-LSP basis
   if and only if these LSPs have their corresponding switching
   granularity supported at the transport plane level.

7.3 Escalation Strategies

   There are two types of escalation strategies (see [DEMEESTER]):
   bottom-up and top-down.

   The bottom-up approach assumes that lower layer recovery types and
   schemes are more expedient and faster than the upper layer one.
   Therefore we can inhibit or hold-off higher layer recovery. However
   this assumption is not entirely true. Consider for instance a
   Sonet/SDH based protection mechanism (with a less than 50 ms
   protection switching time) lying on top of an OTN restoration
   mechanism (with a less than 200 ms restoration time). Therefore,
   this assumption should be (at least) clarified as: lower layer
   recovery mechanism is expected to be faster than upper level one if
   the same type of recovery mechanism is used at each layer.

   Consequently, taking into account the recovery actions at the
   different layers in a bottom-up approach, if lower layer recovery
   mechanisms are provided and sequentially activated in conjunction
   with higher layer ones, the lower layers must have an opportunity to
   recover normal traffic before the higher layers do. However, if
   lower layer recovery is slower than higher layer recovery, the lower
   layer must either communicate the failure related information to the
   higher layer(s) (and allow it to perform recovery), or use a hold-
   off timer in order to temporarily set the higher layer recovery
   action in a "standby mode". Note that the a priori information
   exchange between layers concerning their efficiency is not within
   the current scope of this document. Nevertheless, the coordination
   functionality between layers must be configurable and tunable.

   An example of coordination between the optical and packet layer
   control plane enables for instance the optical layer performing the
   failure management operations (in particular, failure detection and
   notification) while giving to the packet layer control plane the
   authority to decide and perform the recovery actions. In case the
   packet layer recovery action is unsuccessful, fallback at the
   optical layer can be subsequently performed.

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   The top-down approach attempts service recovery at the higher layers
   before invoking lower layer recovery. Higher layer recovery is
   service selective, and permits "per-CoS" or "per-connection" re-
   routing. With this approach, the most important aspect is that the
   upper layer should provide its own reliable and independent failure
   detection mechanism from the lower layer.

   The same reference also suggests recovery mechanisms incorporating a
   coordinated effort shared by two adjacent layers with periodic
   status updates. Moreover, some of these recovery operations can be
   pre-assigned (on a per-link basis) to a certain layer, e.g. a given
   link will be recovered at the packet layer while another will be
   recovered at the optical layer.

7.4 Disjointness

   Having link and node diverse working and recovery LSPs/spans does
   not guarantee their complete disjointness. Due to the common
   physical layer topology (passive), additional hierarchical concepts
   such as the Shared Risk Link Group (SRLG) and mechanisms such as
   SRLG diverse path computation must be developed to provide complete
   working and recovery LSP/span disjointness (see [IPO-IMP] and
   [GMPLS-RTG]). Otherwise, a failure affecting the working LSP/span
   would also potentially affect the recovery LSP/span; one refers to
   such an event as "common failure".

7.4.1 SRLG Disjointness

   A Shared Risk Link Group (SRLG) is defined as the set of links
   sharing a common risk (for instance, a common physical resource such
   as a fiber link or a fiber cable). For instance, a set of links L
   belongs to the same SRLG s, if they are provisioned over the same
   fiber link f.

   The SRLG properties can be summarized as follows:

   1) A link belongs to more than one SRLG if and only if it crosses
      one of the resources covered by each of them.

   2) Two links belonging to the same SRLG can belong individually to
      (one or more) other SRLGs.

   3) The SRLG set S of an LSP is defined as the union of the
      individual SRLG s of the individual links composing this LSP.

   SRLG disjointness is also applicable to LSPs:

      The LSP SRLG disjointness concept is based on the following
      postulate: an LSP (i.e. sequence of links and nodes) covers an
      SRLG if and only if it crosses one of the links or nodes
      belonging to that SRLG.

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      Therefore, the SRLG disjointness for LSPs can be defined as
      follows: two LSPs are disjoint with respect to an SRLG s if and
      only if they do not cover simultaneously this SRLG s.

      Whilst the SRLG disjointness for LSPs with respect to a set S of
      SRLGs is defined as follows: two LSPs are disjoint with respect
      to a set of SRLGs S if and only if the common SRLGs between the
      sets of SRLGs they individually cover is disjoint from set S.

   The impact on recovery is noticeable: SRLG disjointness is a
   necessary (but not a sufficient) condition to ensure network
   survivability. With respect to the physical network resources, a
   working-recovery LSP/span pair must be SRLG disjoint in case of
   dedicated recovery type. On the other hand, in case of shared
   recovery, a group of working LSP/span must be mutually SRLG-disjoint
   in order to allow for a (single and common) shared recovery LSP
   itself SRLG-disjoint from each of the working LSPs/spans.

8. Recovery Mechanisms Analysis

   In order to provide a structured analysis of the recovery mechanisms
   detailed in the previous sections, the following dimensions can be

   1. Fast convergence (performance): provide a mechanism that
      aggregates multiple failures (this implies fast failure
      detection and correlation mechanisms) and fast recovery decision
      independently of the number of failures occurring in the optical
      network (implying also a fast failure notification).

   2. Efficiency (scalability): minimize the switching time required
      for LSP/span recovery independently of the number of LSPs/spans
      being recovered (this implies an efficient failure correlation, a
      fast failure notification and time-efficient recovery

   3. Robustness (availability): minimize the LSP/span downtime
      independently of the underlying topology of the transport plane
      (this implies a highly responsive recovery mechanism).

   4. Resource optimization (optimality): minimize the resource
      capacity, including LSPs/spans and nodes (switching capacity),
      required for recovery purposes; this dimension can also be
      referred to as optimizing the sharing degree of the recovery

   5. Cost optimization: provide a cost-effective recovery type/scheme.

   However, these dimensions are either outside the scope of this
   document such as cost optimization and recovery path computational
   aspects or mutually conflicting. For instance, it is obvious that
   providing a 1+1 LSP protection minimizes the LSP downtime (in case

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   of failure) while being non-scalable and consuming recovery resource
   without enabling any extra-traffic.

   The following sections provide an analysis of the recovery phases
   and mechanisms detailed in the previous sections with respect to the
   dimensions described here above to assess the GMPLS protocol suite
   capabilities and applicability. In turn, this allows the evaluation
   of the potential need for further GMPLS signaling and routing

8.1 Fast Convergence (Detection/Correlation and Hold-off Time)

   Fast convergence is related to the failure management operations. It
   refers to the elapsing time between the failure detection/
   correlation and hold-off time, point at which the recovery switching
   actions are initiated. This point has been detailed in Section 4.

8.2 Efficiency (Recovery Switching Time)

   In general, the more pre-assignment/pre-planning of the recovery
   LSP/span, the more rapid the recovery is. Since protection implies
   pre-assignment (and cross-connection) of the protection resources,
   in general, protection recover faster than restoration.

   Span restoration is likely to be slower than most span protection
   types; however this greatly depends on the efficiency of the span
   restoration signalling. LSP restoration with pre-signaled and pre-
   selected recovery resources is likely to be faster than fully
   dynamic LSP restoration, especially because of the elimination of
   any potential crankback during the recovery LSP establishment.

   If one excludes the crankback issue, the difference between dynamic
   and pre-planned restoration depends on the restoration path
   computation and selection time. Since computational considerations
   are outside the scope of this document, it is up to the vendor to
   determine the average and maximum path computation time in different
   scenarios and to the operator to decide whether or not dynamic
   restoration is advantageous over pre-planned schemes depending on
   the network environment. This difference depends also on the
   flexibility provided by pre-planned restoration versus dynamic
   restoration: the former implies a somewhat limited number of failure
   scenarios (that can be due, for instance, to local storage capacity
   limitation). The latter enables on-demand path computation based on
   the information received through failure notification message and as
   such is more robust with respect to the failure scenario scope.

   Moreover, LSP segment restoration, in particular, dynamic
   restoration (i.e. no path pre-computation so none of the recovery
   resource is pre-reserved) will generally be faster than end-to-end
   LSP restoration. However, local LSP restoration assumes that each
   LSP segment end-point has enough computational capacity to perform
   this operation while end-to-end LSP restoration requires only that
   LSP end-points provides this path computation capability.

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   Recovery time objectives for SONET/SDH protection switching (not
   including time to detect failure) are specified in [G.841] at 50 ms,
   taking into account constraints on distance, number of connections
   involved, and in the case of ring enhanced protection, number of
   nodes in the ring. Recovery time objectives for restoration
   mechanisms have been proposed through a separate effort [RFC3386].

8.3 Robustness

   In general, the less pre-assignment (protection)/pre-planning
   (restoration) of the recovery LSP/span, the more robust the recovery
   type or scheme is to a variety of single failures, provided that
   adequate resources are available. Moreover, the pre-selection of the
   recovery resources gives in the case of multiple failure scenarios
   less flexibility than no recovery resource pre-selection. For
   instance, if failures occur that affect two LSPs sharing a common
   link along their restoration paths, then only one of these LSPs can
   be recovered. This occurs unless the restoration path of at least
   one of these LSPs is re-computed or the local resource assignment is
   modified on the fly.

   In addition, recovery types and schemes with pre-planned recovery
   resources, in particular LSP/spans for protection and LSPs for
   restoration purposes, will not be able to recover from failures that
   simultaneously affect both the working and recovery LSP/span. Thus,
   the recovery resources should ideally be as disjoint as possible
   (with respect to link, node and SRLG) from the working ones, so that
   any single failure event will not affect both working and recovery
   LSP/span. In brief, working and recovery resource must be fully
   diverse in order to guarantee that a given failure will not affect
   simultaneously the working and the recovery LSP/span. Also, the risk
   of simultaneous failure of the working and the recovery LSP can be
   reduced. This, by computing a new recovery path whenever a failure
   occurs along one of the recovery LSPs or by computing a new recovery
   path and provision the corresponding LSP whenever a failure occurs
   along a working LSP/span. Both methods enable the network to
   maintain the number of available recovery path constant.

   The robustness of a recovery scheme is also determined by the amount
   of pre-reserved (i.e. signaled) recovery resources within a given
   shared resource pool: as the sharing degree of recovery resources
   increases, the recovery scheme becomes less robust to multiple
   LSP/span failure occurrences. Recovery schemes, in particular
   restoration, with pre-signaled resource reservation (with or without
   pre-selection) should be capable to reserve the adequate amount of
   resource to ensure recovery from any specific set of failure events,
   such as any single SRLG failure, any two SRLG failures etc.

8.4 Resource Optimization

   It is commonly admitted that sharing recovery resources provides
   network resource optimization. Therefore, from a resource

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   utilization perspective, protection schemes are often classified
   with respect to their degree of sharing recovery resources with
   respect to the working entities. Moreover, non-permanent bridging
   protection types allow (under normal conditions) for extra-traffic
   over the recovery resources.

   From this perspective 1) 1+1 LSP/Span protection is the most
   resource consuming protection type since not allowing for any extra-
   traffic 2) 1:1 LSP/span recovery requires dedicated recovery
   LSP/span allowing for extra-traffic 3) 1:N and M:N LSP/span recovery
   require 1 (M, respectively) recovery LSP/span (shared between the N
   working LSP/span) allowing for extra-traffic. Obviously, 1+1
   protection precludes and 1:1 recovery does not allow for any
   recovery LSP/span sharing whereas 1:N and M:N recovery do allow
   sharing of 1 (M, respectively) recovery LSP/spans between N working
   LSP/spans. However, despite the fact that 1:1 LSP recovery precludes
   the sharing of the recovery LSP, the recovery schemes (see Section
   5.4) that can be built from it (e.g. (1:1)^n) do allow sharing of
   its recovery resources. In addition, the flexibility in the usage of
   shared recovery resources (in particular, shared links) may be
   limited because of network topology restrictions, e.g. fixed ring
   topology for traditional enhanced protection schemes.

   On the other hand, when using LSP restoration with pre-signaled
   resource reservation, the amount of reserved restoration capacity is
   determined by the local bandwidth reservation policies. In LSP
   restoration schemes with re-provisioning, a pool of spare resources
   can be defined from which all resources are selected after failure
   occurrence for the purpose of restoration path computation. The
   degree to which restoration schemes allow sharing amongst multiple
   independent failures is then directly inferred from the size of the
   resource pool. Moreover, in all restoration schemes, spare resources
   can be used to carry preemptible traffic (thus over preemptible
   LSP/span) when the corresponding resources have not been committed
   for LSP/span recovery purposes.

   From this, it clearly follows that less recovery resources (i.e.
   LSP/spans and switching capacity) have to be allocated to a shared
   recovery resource pool if a greater sharing degree is allowed. Thus,
   the network survivability level is determined by the policy that
   defines the amount of shared recovery resources and by the maximum
   sharing degree allowed for these recovery resources.

8.4.1. Recovery Resource Sharing

   When recovery resources are shared over several LSP/Spans, the use
   of the Maximum Reservable Bandwidth, the Unreserved Bandwidth and
   the Maximum LSP Bandwidth (see [GMPLS-RTG]) provides the information
   needed to obtain the optimization of the network resources allocated
   for shared recovery purposes.

   The Maximum Reservable Bandwidth is defined as the Maximum Link
   Bandwidth but it may be greater in case of link over-subscription.

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   The Unreserved Bandwidth (at priority p) is defined as the bandwidth
   not yet reserved on a given TE link (its initial value for each
   priority p corresponds to the Maximum Reservable Bandwidth). Last,
   the Maximum LSP Bandwidth (at priority p) is defined as the smaller
   of Unreserved Bandwidth (at priority p) and Maximum Link Bandwidth.

   Here, one generally considers a recovery resource sharing degree (or
   ratio) to globally optimize the shared recovery resource usage. The
   distribution of the bandwidth utilization per TE link can be
   inferred from the per-priority bandwidth pre-allocation. By using
   the Maximum LSP Bandwidth and the Maximum Reservable Bandwidth, the
   amount of (over-provisioned) resources that can be used for shared
   recovery purposes is known from the IGP.

   In order to analyze this behavior, we define the difference between
   the Maximum Reservable Bandwidth (in the present case, this value is
   greater than the Maximum Link Bandwidth) and the Maximum LSP
   Bandwidth per TE link i as the Maximum Shareable Bandwidth or
   max_R[i]. Within this quantity, the amount of bandwidth currently
   allocated for shared recovery per TE link i is defined as R[i]. Both
   quantities are expressed in terms of discrete bandwidth units (and
   thus, the Minimum LSP Bandwidth is of one bandwidth unit).

   The knowledge of this information available per TE link can be
   exploited in order to optimize the usage of the resources allocated
   per TE link for shared recovery. If one refers to r[i] as the actual
   bandwidth per TE link i (in terms of discrete bandwidth units)
   committed for shared recovery, then the following quantity must be
   maximized over the potential TE link candidates:

        sum {i=1}^N [(R{i} - r{i})/(t{i} - b{i})]

        or equivalently: sum {i=1}^N [(R{i} - r{i})/r{i}]

        with R{i} >= 1 and r{i} >= 1 (in terms of per component
        bandwidth unit)

   In this formula, N is the total number of links traversed by a given
   LSP, t[i] the Maximum Link Bandwidth per TE link i and b[i] the sum
   per TE link i of the bandwidth committed for working LSPs and other
   recovery LSPs (thus except "shared bandwidth" LSPs). The quantity
   [(R{i} - r{i})/r{i}] is defined as the Shared (Recovery) Bandwidth
   Ratio per TE link i. In addition, TE links for which R[i] reaches
   max_R[i] or for which r[i] = 0 are pruned during shared recovery
   path computation as well as TE links for which max_R[i] = r[i] which
   can simply not be shared.

   More generally, one can draw the following mapping between the
   available bandwidth at the transport and control plane level:

                                 - ---------- Max Reservable Bandwidth
                                |  -----  ^
                                |R -----  |

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                                |  -----  |
                                 - -----  |max_R
                                   -----  |
   --------  TE link Capacity    - ------ | - Maximum TE Link Bandwidth
   -----                        |r -----  v
   -----     <------ b ------>   - ---------- Maximum LSP Bandwidth
   -----                           -----
   -----                           -----
   -----                           -----
   -----                           -----
   -----                           ----- <--- Minimum LSP Bandwidth
   -------- 0                      ---------- 0

   Note that the above approach does not require the flooding of any
   per LSP information or any detailed distribution of the bandwidth
   allocation per component link or individual ports or even any per-
   priority shareable recovery bandwidth information (using a dedicated
   sub-TLV). The latter would provide the same capability than the
   already defined Maximum LSP bandwidth per-priority information. Such
   approach is referred to as a Partial (or Aggregated) Information
   Routing as described for instance in [KODIALAM1] and [KODIALAM2].
   They show that the difference obtained with a Full (or Complete)
   Information Routing approach (where for the whole set of working and
   recovery LSPs, the amount of bandwidth units they use per-link is
   known at each node and for each link) is clearly negligible. The
   latter approach is detailed in [GLI], for instance. Note also that
   both approaches rely on the deterministic knowledge (at different
   degrees) of the network topology and resource usage status.

   Moreover, extending the GMPLS signalling capabilities can enhance
   the Partial Information Routing approach. This, by allowing working
   LSP related information and in particular, its path (including link
   and node identifiers) to be exchanged with the recovery LSP request
   to enable more efficient admission control at upstream nodes of
   shared recovery resources, in particular links (see Section 8.4.3).

8.4.2 Recovery Resource Sharing and SRLG Recovery

   Resource shareability can also be maximized with respect to the
   number of times each SRLG is protected by a recovery resource (in
   particular, a shared TE link) and methods can be considered for
   avoiding contention of the shared recovery resources in case of
   single SRLG failure. These methods enable for the sharing of
   recovery resources between two (or more) recovery LSPs if their
   respective working LSPs are mutually disjoint with respect to link,
   node and SRLGs. A single failure then does not simultaneously
   disrupt several (or at least two) working LSPs.

   For instance, [BOUILLET] shows that the Partial Information Routing
   approach can be extended to cover recovery resource shareability
   with respect to SRLG recoverability (i.e. the number of times each
   SRLG is recoverable). By flooding this aggregated information per TE

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   link, path computation and selection of SRLG-diverse recovery LSPs
   can be optimized with respect to the sharing of recovery resource
   reserved on each TE link giving a performance difference of less
   than 5% (and so negligible) compared to the corresponding Full
   Information Flooding approach (see [GLI]).

   For this purpose, additional extensions to [GMPLS-RTG] in support of
   path computation for shared mesh recovery have been often considered
   in the literature. TE link attributes would include, among other,
   the current number of recovery LSPs sharing the recovery resources
   reserved on the TE link and the current number of SRLGs recoverable
   by this amount of (shared) recovery resources reserved on the TE
   link. The latter is equivalent to the current number of SRLGs that
   the recovery LSPs sharing the recovery resource reserved on the TE
   link shall recover. Then, if explicit SRLG recoverability is
   considered an additional TE link attribute including the explicit
   list of SRLGs recoverable by the shared recovery resource reserved
   on the TE link and their respective shareable recovery bandwidth.
   The latter information is equivalent to the shareable recovery
   bandwidth per SRLG (or per group of SRLGs) which implies to consider
   a decreasing amount of shareable bandwidth and SRLG list over time.

   Compared to the case of recovery resource sharing only (regardless
   of SRLG recoverability, as described in Section 8.4.1), this
   additional TE link attributes would potentially deliver better path
   computation and selection (at distinct ingress node) for shared mesh
   recovery purposes. However, due to the lack of results of evidence
   for better efficiency and due to the complexity that such extensions
   would generate, they are not further considered in the scope of the
   present analysis. For instance, a per-SRLG group minimum/maximum
   shareable recovery bandwidth is restricted by the length that the
   corresponding (sub-)TLV may take and thus the number of SRLGs that
   it can include. Therefore, the corresponding parameter should not be
   translated into GMPLS routing (or even signalling) protocol
   extensions in the form of TE link sub-TLV.

8.4.3 Recovery Resource Sharing, SRLG Disjointness and Admission

   Admission control is a strict requirement to be fulfilled by nodes
   giving access to shared links. This can be illustrated using the
   following network topology:

      A ------ C ====== D
      |        |        |
      |        |        |
      |        B        |
      |        |        |
      |        |        |
       ------- E ------ F

   Node A creates a working LSP to D (A-C-D), B creates simultaneously
   a working LSP to D (B-C-D) and a recovery LSP (B-E-F-D) to the same

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   destination. Then, A decides to create a recovery LSP to D (A-E-F-
   D), but since the C-D span carries both working LSPs, node E should
   either assign a dedicated resource for this recovery LSP or reject
   this request if the C-D span has already reached its maximum
   recovery bandwidth sharing ratio. Otherwise, in the latter case, C-D
   span failure would imply that one of the working LSP would not be

   Consequently, node E must have the required information to perform
   admission control for the recovery LSP requests it processes
   (implying for instance, that the path followed by the working LSP is
   carried with the corresponding recovery LSP request). If node E can
   guarantee that the working LSPs (A-C-D and B-C-D) are SRLG disjoint
   over the C-D span, it may securely accept the incoming recovery LSP
   request and assign to the recovery LSPs (A-E-F-D and B-E-F-D) the
   same resources on the link E-F. This, if the link E-F has not yet
   reached its maximum recovery bandwidth sharing ratio. In this
   example, one assumes that the node failure probability is negligible
   compared to the link failure probability.

   To achieve this, the path followed by the working LSP is transported
   with the recovery LSP request and examined at each upstream node of
   potentially shareable links. Admission control is performed using
   the interface identifiers (included in the path) to retrieve in the
   TE DataBase the list of SRLG Ids associated to each of the working
   LSP links. If the working LSPs (A-C-D and B-C-D) have one or more
   link or SRLG id in common (in this example, one or more SRLG id in
   common over the span C-D) node E should not assign the same resource
   over link E-F to the recovery LSPs (A-E-F-D and B-E-F-D). Otherwise,
   one of these working LSPs would not be recoverable in case of C-D
   span failure.

   There are some issues related to this method, the major one being
   the number of SRLG Ids that a single link can cover (more than 100,
   in complex environments). Moreover, when using link bundles, this
   approach may generate the rejection of some recovery LSP requests.
   This occurs when the SRLG sub-TLV corresponding to a link bundle
   includes the union of the SRLG id list of all the component links
   belonging to this bundle (see [GMPLS-RTG] and [BUNDLE]).

   In order to overcome this specific issue, an additional mechanism
   may consist of querying the nodes where such an information would be
   available (in this case, node E would query C). The main drawback of
   this method is that, in addition to the dedicated mechanism(s) it
   requires, it may become complex when several common nodes are
   traversed by the working LSPs. Therefore, when using link bundles,
   solving this issue is tightly related to the sequence of the
   recovery operations. Per component flooding of SRLG identifiers
   would deeply impact the scalability of the link state routing
   protocol. Therefore, one may rely on the usage of an on-line
   accessible network management system.

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9. Summary and Conclusions

   The following table summarizes the different recovery types and
   schemes analyzed throughout this document.

              |       Path Search (computation and selection)
              |       Pre-planned (a)      |         Dynamic (b)
          |   | faster recovery            | Does not apply
          |   | less flexible              |
          | 1 | less robust                |
          |   | most resource consuming    |
   Path   |   |                            |
   Setup   ------------------------------------------------------------
          |   | relatively fast recovery   | Does not apply
          |   | relatively flexible        |
          | 2 | relatively robust          |
          |   | resource consumption       |
          |   |  depends on sharing degree |
          |   | relatively fast recovery   | less faster (computation)
          |   | more flexible              | most flexible
          | 3 | relatively robust          | most robust
          |   | less resource consuming    | least resource consuming
          |   |  depends on sharing degree |

   1a. Recovery LSP setup (before failure occurrence) with resource
       reservation (i.e. signalling) and selection is referred to as
       LSP protection.

   2a. Recovery LSP setup (before failure occurrence) with resource
       reservation (i.e. signalling) and with resource pre-selection is
       referred to as pre-planned LSP re-routing with resource pre-
       selection. This implies only recovery LSP activation after
       failure occurrence.

   3a. Recovery LSP setup (before failure occurrence) with resource
       reservation (i.e. signalling) and without resource selection is
       referred to as pre-planned LSP re-routing without resource pre-
       selection. This implies recovery LSP activation and resource
       (i.e. label) selection after failure occurrence.

   3b. Recovery LSP setup after failure occurrence is referred to as
       to as LSP re-routing, which is full when recovery LSP path
       computation occurs after failure occurrence.

   The term pre-planned refers thus to recovery LSP path pre-
   computation, signaling (reservation), and a priori resource
   selection (optional), but not cross-connection. Also, the shared-

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   mesh recovery scheme can be viewed as a particular case of 2a) and
   3a) using the additional constraint described in Section 8.4.3.

   The implementation of these recovery mechanisms requires only
   considering extensions to GMPLS signalling protocols (i.e. [RFC3471]
   and [RFC3473]). These GMPLS signalling extensions should mainly
   focus in delivering (1) recovery LSP pre-provisioning for the cases
   1a, 2a and 3a, (2) LSP failure notification, (3) recovery LSP
   switching action(s), and (4) reversion mechanisms.

   Moreover, the present analysis (see Section 8) shows that no GMPLS
   routing extensions are expected to efficiently implement any of
   these recovery types and schemes.

10. Security Considerations

   This document does not introduce any additional security issue or
   imply any specific security consideration from [RFC3945] to the
   current RSVP-TE GMPLS signaling, routing protocols (OSPF-TE, IS-IS-
   TE) or network management protocols.

   However, the authorization of requests for resources by GMPLS-
   capable nodes should determining whether a given party, presumable
   already authenticated, has a right to access the requested
   resources. This determination is typically a matter of local policy
   control, for example by setting limits on the total bandwidth made
   available to some party in the presence of resource contention. Such
   policies may become quite complex as the number of users, types of
   resources and sophistication of authorization rules increases. This
   is particularly the case for recovery schemes that assume pre-
   planned sharing of recovery resources, or contention for resources
   in case of dynamic re-routing.

   Therefore, control elements should match them against the local
   authorization policy. These control elements must be capable of
   making decisions based on the identity of the requester, as verified
   cryptographically and/or topologically.

11. IANA Considerations

   This document defines no new code points and requires no action by

12. Acknowledgments

   The authors would like to thank Fabrice Poppe (Alcatel) and Bart
   Rousseau (Alcatel) for their revision effort, Richard Rabbat
   (Fujitsu Labs), David Griffith (NIST) and Lyndon Ong (Ciena) for
   their useful comments.

   Thanks also to Adrian Farrel for the thorough review of the

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13. References

13.1 Normative References

   [BUNDLE]     K.Kompella et al., "Link Bundling in MPLS Traffic
                Engineering," Work in progress, draft-ietf-mpls-bundle-
                06.txt, December 2004.

   [GMPLS-RTG]  K.Kompella (Editor) et al., "Routing Extensions in
                Support of Generalized Multi-Protocol Label Switching,"
                Work in Progress, draft-ietf-ccamp-gmpls-routing-
                09.txt, October 2003.

   [LMP]        J.P.Lang (Editor) et al., "Link Management Protocol
                (LMP)," Work in progress, draft-ietf-ccamp-lmp-10.txt,
                October 2003.

   [LMP-WDM]    A.Fredette and J.P.Lang (Editors), "Link Management
                Protocol (LMP) for Dense Wavelength Division
                Multiplexing (DWDM) Optical Line Systems," Work in
                progress, draft-ietf-ccamp-lmp-wdm-03.txt, October

   [RFC2026]    S.Bradner, "The Internet Standards Process -- Revision
                3," BCP 9, RFC 2026, October 1996.

   [RFC2119]    S.Bradner, "Key words for use in RFCs to Indicate
                Requirement Levels," BCP 14, RFC 2119, March 1997.

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

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

   [RFC3667]    S.Bradner, "IETF Rights in Contributions", BCP 78,
                RFC 3667, February 2004.

   [RFC3668]    S.Bradner, Ed., "Intellectual Property Rights in IETF
                Technology", BCP 79, RFC 3668, February 2004.

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

   [TERM]       E.Mannie and D.Papadimitriou (Editors), "Recovery
                (Protection and Restoration) Terminology for
                Generalized Multi-Protocol Label Switching (GMPLS),"
                Work in progress, draft-ietf-ccamp-gmpls-recovery-
                terminology-06.txt, April 2005.

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13.2 Informative References

   [BOUILLET]   E.Bouillet et al., "Stochastic Approaches to Compute
                Shared Meshed Restored Lightpaths in Optical Network
                Architectures," IEEE Infocom 2002, New York City, June

   [DEMEESTER]  P.Demeester et al., "Resilience in Multilayer
                Networks," IEEE Communications Magazine, Vol. 37, No.
                8, pp. 70-76, August 1998.

   [GLI]        G.Li et al., "Efficient Distributed Path Selection for
                Shared Restoration Connections," IEEE Infocom 2002, New
                York City, June 2002.

   [IPO-IMP]    J.Strand and A.Chiu, "Impairments and Other Constraints
                On Optical Layer Routing," Work in Progress, draft-
                ietf-ipo-impairments-05.txt, May 2003.

   [KODIALAM1]  M.Kodialam and T.V.Lakshman, "Restorable Dynamic
                Quality of Service Routing," IEEE Communications
                Magazine, pp. 72-81, June 2002.

   [KODIALAM2]  M.Kodialam and T.V.Lakshman, "Dynamic Routing of
                Restorable Bandwidth-Guaranteed Tunnels using
                Aggregated Network Resource Usage Information," IEEE/
                ACM Transactions on Networking, pp. 399-410, June 2003.

   [MANCHESTER] J.Manchester, P.Bonenfant and C.Newton, "The Evolution
                of Transport Network Survivability," IEEE
                Communications Magazine, August 1999.

   [RFC3386]    W.Lai, D.McDysan, J.Boyle, et al., "Network Hierarchy
                and Multi-layer Survivability," RFC 3386, November 2002

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

   [T1.105]     ANSI, "Synchronous Optical Network (SONET): Basic
                Description Including Multiplex Structure, Rates, and
                Formats," ANSI T1.105, January 2001.

   [WANG]       J.Wang, L.Sahasrabuddhe, and B.Mukherjee, "Path vs.
                Subpath vs. Link Restoration for Fault Management in
                IP-over-WDM Networks: Performance Comparisons Using
                GMPLS Control Signaling," IEEE Communications Magazine,
                pp. 80-87, November 2002.

   For information on the availability of the following documents,
   please see

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   [G.707]      ITU-T, "Network Node Interface for the Synchronous
                Digital Hierarchy (SDH)," Recommendation G.707, October

   [G.709]      ITU-T, "Network Node Interface for the Optical
                Transport Network (OTN)," Recommendation G.709,
                February 2001 (and Amendment no.1, October 2001).

   [G.783]      ITU-T, "Characteristics of Synchronous Digital
                Hierarchy (SDH) Equipment Functional Blocks,"
                Recommendation G.783, October 2000.

   [G.806]      ITU-T, "Characteristics of Transport Equipment -
                Description Methodology and Generic Functionality",
                Recommendation G.806, October 2000.

   [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.

   [G.842]      ITU-T, "Interworking of SDH network protection
                architectures," Recommendation G.842, October 1998.

14. Editor's Addresses

   Eric Mannie

   Dimitri Papadimitriou
   Francis Wellesplein, 1
   B-2018 Antwerpen, Belgium
   Phone:  +32 3 240-8491

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