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Versions: (draft-papadimitriou-ccamp-gmpls-recovery-analysis) 00 01 02 03 04 05 RFC 4428

CCAMP Working Group                         CCAMP GMPLS P&R Design Team
Internet Draft
Expiration Date: June 2003               Dimitri Papadimitriou (Editor)
                                                   Eric Mannie (Editor)

                                                           January 2003

         Analysis of Generalized MPLS-based Recovery Mechanisms
                 (including Protection and Restoration)


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [1].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-
   Drafts. Internet-Drafts are draft documents valid for a maximum of
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   documents at any time. It is inappropriate to use Internet- Drafts
   as reference material or to cite them other than as "work in

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   For potential updates to the above required-text see:

1. Abstract

   This document provides an analysis grid that can be used to
   evaluate, compare and contrast the numerous Generalized MPLS
   (GMPLS)-based recovery mechanisms currently proposed at the CCAMP
   Working Group. A detailed analysis of each of the recovery phases is
   provided using the terminology defined in [CCAMP-TERM]. Also, this
   document focuses on transport plane survivability and recovery
   issues and not on control plane resilience and related aspects.

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2. 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)
   Rm. D1-3C22 - 200 S. Laurel Ave.
   Middletown, NJ 07748, USA
   E-mail: dbrungard@att.com

   Sudheer Dharanikota (Consult)
   E-mail: sudheer@ieee.org

   Jonathan P. Lang (Calient)
   25 Castilian
   Goleta, CA 93117, USA
   E-mail: jplang@calient.net

   Guangzhi Li (AT&T)
   180 Park Avenue,
   Florham Park, NJ 07932, USA
   E-mail: gli@research.att.com

   Bala Rajagopalan (Tellium)
   2 Crescent Place - P.O. Box 901
   Oceanport, NJ 07757-0901, USA
   E-mail: braja@tellium.com

   Yakov Rekhter (Juniper)
   E-mail: yakov@juniper.net

3. Introduction

   This document provides an analysis grid to evaluate, compare and
   contrast the numerous Generalized MPLS (GMPLS) based recovery
   mechanisms currently proposed in the CCAMP Working Group. Here, the
   focus will only be 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 recovery protocols, the protocol(s) specifications
   will not be covered in this document. Though the concepts discussed
   here are technology independent, this document will implicitly focus
   on SONET/SDH and pre-OTN technologies except when specific details
   need to be considered (for instance, in the case of failure
   detection). Details for applicability to other technologies such as
   Optical Transport Networks (OTN) [ITUT-G709] will be covered in a
   future release of this document.

   In the present release, a detailed analysis is provided for each of
   the recovery phases as identified in [CCAMP-TERM]. These phases
   define the sequence of generic operations that need to be performed

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   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
   [CCAMP-TERM]; these entities include ingress, egress and
   intermediate nodes.

   In this document the term ôrecovery mechanismö 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 ôfailureö 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 hierarchical recovery
   mechanisms including disjointness related issues. We also introduce
   the dimensions from which each of the recovery mechanisms described
   in this document can be further analyzed and provide an analysis
   grid with respect to these dimensions. Last, we conclude by
   detailing the applicability of the current GMPLS protocol building
   blocks for recovery purposes.

   Any other recovery-related terminology used in this document
   conforms to the one defined in [CCAMP-TERM].

   Conventions used in this document:

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   this document are to be interpreted as described in RFC-2119 [2].

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 failure condition, MUST
   trigger the control plane for subsequent actions through the use of
   GMPLS signalling capabilities (see [GMPLS-SIG]) or Link Management
   Protocol (see [LMP], Section 6) capabilities.

   Therefore, by definition, transport failure detection is transport
   technology dependent (and so exceptionally, we keep here the

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   ôtransport planeö 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 controlled by the TMN) 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

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

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

   The most important point to keep in mind 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).

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   - 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
   (OTS, OMS and OTUk) and at optical/digital path layers (OCh and
   ODUk). Interested readers are referred to the ITU-T Recommendations
   [G.798] and [G.709] for more details.

   The above are examples illustrate cases where the failure detection,
   reporting and recovery responsible entities are co-located.

   On the other hand, 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 framing format independent,
   failure conditions can only be triggered either by detecting the
   absence of the optical signal or by measuring its optical quality,
   mechanisms which are less reliable than electrical (digital)
   mechanisms. Both types of detection mechanisms are out of the scope
   of this document. If the intermediate OLS supports electrical
   (digital) mechanisms, using the LMP communication channel, these
   failure conditions are reported to the PXC and subsequent recovery
   actions performed as described in Section 5. As such from the
   control plane viewpoint, this mechanism makes the OLS-PXC composed
   system appearing as a single logical entity allowing considering for
   such entity the same failure management mechanisms as for any other
   O/E/O capable device.

   This example is to illustrate the scenario where the failure
   detection, reporting and recovery responsible entities are not co-

   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 anymore on a
     given interfaceÆs receiver.
   - 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 are considered to illustrate the
   communication between the detecting and reporting (also recovery
   responsible) entities:

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   - 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, for
     Transparent cross-connects, etc.).

   - Non co-located detecting and reporting entities:
     - with In-band communication between entities:
       Entities are separated but transport plane (in-band)
       communication is provided between them (e.g., Server Signal
       Failures (AIS), etc.)
     - with Out-of-band communication between entities:
       Entities are separated but out-of-band communication is provided
       between them (e.g., using [LMP]).

4.2 Failure Localization and Isolation

   Failure localization provides the required information in order to
   perform the subsequent recovery action(s) at the LSP/span end-

   In some cases, accurate failure localization may be less urgent; the
   need is to identify the failure as occurring within the recovery
   domain. 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 failed working LSPs) so that a more accurate localization can
   be performed after LSP recovery.

   Failure localization should be triggered immediately after the fault
   detection phase. This operation can be performed at the transport
   management plane and/or, if unavailable (via the transport plane),
   the control plane level where dedicated signaling messages can be

   When performed at the control plane level, a protocol such as LMP
   (see [LMP], Section 6) can be used for failure localization and
   isolation purposes.

4.3 Failure Notification

   Failure notification is used 1) to inform intermediate nodes that a
   LSP/span failure has occurred and has been detected 2) to inform the
   recovery 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 SDH/SONET transport
   plane level (also referred to as maintenance signal supervision):

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   - 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 mechanismÆs 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
   initiate a protection scheme directly (such as those defined in
   [CCAMP-TERM]) or a restoration scheme 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 [GMPLS-SIG], notification message
   exchanges through a GMPLS control plane may not follow the same path
   as the LSP/spans for which these messages carry the status. In turn,
   this ensures a fast, reliable (through 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 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 [CCAMP-TERM]). In addition,
     since fast notification is a mechanism running in collaboration
     with the existing signalling (see for instance, [GMPLS-RSVP-TE])
     allowing 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

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     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. Notice that
     more detailed quantification of the amount of information to be
     exchanged and processed is strongly dependent on the failure
     notification protocol specification.

   - 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 failure
     detection output is correlated (i.e. alarm correlation) and
     aggregated at the node detecting the failure(s), failure
     notifications are directed to a restricted set of destinations (in
     general the end-points) and 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
     fast notification (implying that correlation and aggregation
     occurs at receiving end-points) can be found.

4.5 Correlating Failure Conditions

   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.

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   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 and Schemes

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 table gives an overview of the responsibilities taken
   by the control plane in case of LSP/Span recovery:

   1. LSP/span Protection Schemes

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

   Note: in the LSP/span protection context 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 nodes at control plane level).

   2. LSP/span Restoration Schemes

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

   Therefore, this document is primarily focused on provisioning of
   recovery resources, failure notification, LSP/span recovery 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

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   increments (like Sonet/SDH, G.709 OTN, etc.) being controlled by an
   IP-centric distributed control plane.

   The following sub-sections are not intended to favor one technology
   versus another. They just lists 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 ôlambda switchingö)
   presents mainly the following advantages:

   - benefits from a simpler architecture making it more suitable for
     meshed-based recovery 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 upper layer driven
     recovery actions.

   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/SDH and more generically any TDM
   transport plane are:

   - Protection schemes are standardized (see [G.841]) and can operate
     across protected domains and interwork (see [G.842]).

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

   - Provides 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 current limitations of the Sonet/SDH layer recovery are:

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   - Limited topological scope: Inherently the use of ring topologies
     (Dedicated SNCP or Shared Protection Rings) has a reduced
     flexibility with respect to the somewhat more complex but
     potentially more resource efficient mesh-based recovery schemes.

   - Inefficient use of spare capacity: Sonet/SDH protection is largely
     applied for ring topologies, where spare capacity often remains
     idle, making the efficiency of bandwidth usage an issue.

   - Support of meshed recovery requires intensive network management
     development, and the functionality is limited by both the network
     elements and the element management systems capabilities.

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
   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, since we assume that (IP terminating) channels between
   nodes must be continuously available in order to enable the exchange
   of recovery-related information and messages, one considers that in
   either case (i.e. in-band or out-of-band) at least one logical
   channel or one physical channel between nodes is available.

   Therefore, the key issue when using in-fiber signalling is whether
   we 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 [ITU-T G.709] and [ITU-T 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 (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

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   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 |
   |       |----...----|       |---------|       |----...----|       |
    -------             -------           -------             -------

   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

    Fig. 1 & 2. Uni- and Bi-directional Failure Detection/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

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     sends a notification message to node A). 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 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. However, in case of bi-directional
     failure, node B (and node C) must 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 local recovery actions were not
     successful, the following occurs. In case of unidirectional
     failure and depending on the directionality of the connection,
     node B should send an upstream notification message to the ingress
     node A or node C should send a downstream notification to the
     egress node D. However, in such a case only node A (node D,
     respectively) referred to as the master and node D, to as the
     slave per [CCAMP-TERM], would initiate a edge to edge recovery
     action. Note that the connection terminating node (i.e. node D or
     node A) may be optionally notified.

     In case of bi-directional failure, node B may send an upstream
     notification message to the ingress node A or node C a downstream
     notification to the egress node D. However, due to the dependence
     on the connection directionality, only ingress node A or egress
     node D would initiate an edge to edge recovery action. Note that
     the connection terminating node (i.e. node D or node A) should be
     also notified of this event using upstream and downstream fast
     notification (see [GMPLS-SIG]). For instance, if a connection
     directed from D to A is under failure condition, only the
     notification sent by from node C to D would initiate a recovery
     action. Here as well, per [CCAMP-TERM], the deciding (and
     recovering) node D is referred to as the "master" while the 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 notification
   messages does not have to be the same as the one followed by the
   failed LSP (see [GMPLS-SIG], 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) are also the deciding/recovery entity or the detecting/reporting
   entities are simply intermediate nodes in the subsequent recovery
   process. One refers to local recovery in the former case and to
   edge-to-edge recovery in the latter one.

5.3.3 Partial versus Full Span Recovery

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   When given span carries more than one LSPs or LSP segments, an
   additional aspect must be considered during span failure carrying
   several LSPs. These LSPs 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 single request. Moreover,
   criteria by which such sub-groups can be formed are outside of the
   scope of this document.

   An additional complexity arises in case of (sub-)group LSP recovery.
   Between a given node pair, the LSPs a given (sub-)group contains may
   have been created from different source (i.e. initiator) nodes
   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
   [CCAMP-TERM] the deciding (and recovering) entity, referred to as
   the "master" is the only initiator of the recovery of the whole LSP

5.3.4 Difference between LSP, LSP Segment and Span Recovery

   The recovery definitions given in [CCAMP-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 will have to cross several
   intermediate nodes. In particular, this applies for the notification
   messages due to the number of hops separating the failure occurrence
   location from their 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 one has to consider approximately 1 ms per 200km.

   Moreover, the recovery mechanisms applicable to end-to-end LSP and
   to the segments (i.e. edge-to-edge) that may compose an end-to-end
   LSP can be exactly the same. However, one expects in the latter
   case, that the destination of the failure notification message will
   be the ingress of each of these segments. Therefore, taking into
   account the mechanism described in Section 5.3.2, failure
   notification can be first exchanged between the LSP segments

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   terminating points and after expiration of the hold-off time
   directed toward end-to-end LSP terminating points.

5.4 Difference between Recovery Type and Scheme

   Section 4.6 of [CCAMP-TERM] defines the basic recovery types. The
   purpose of this section is to describe the schemes that can be built
   using these recovery types. In brief, a recovery scheme is defined
   as the combination between different ingress-egress node pairs of a
   set of identical recovery types. Several examples are provided in
   order to illustrate the difference between a recovery type such as
   1:1 and a recovery scheme such as (1:1)^n.

   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 (1:1)^2 common 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 example, with working LSPs A-B and E-F and
   recovery LSPs A-C-D-B and E-C-D-F sharing a common C-D resource.

                          A ----------------- B
                           \                 /
                            \               /
                             C ----------- D
                            /               \
                           /                 \
                          E ----------------- F

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

   The exponent, n, indicates the number of times a M:N recovery type
   is applied between at most n different ingress-egress node pairs.
   So the interpretation follows from the previous case, expect 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

   In both schemes, one may see the following at the LSP level: we have
   a ôgroupö of sum{n=1}^N N{n} working LSPs and a pool of shared
   backup resources, not all of which are available to any given
   working path. In such conditions, defining a metric that describes
   the amount of overlap among the recovery LSPs would give some

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   indication of the groupÆs ability to handle multiple simultaneous

   For instance, in the simple (1:1)^n case situation if n recovery
   LSPs in a (1:1)^n group overlap, then it can handle only single
   failures; any multiple working LSP failures will cause at least one
   working LSP to be denied automatic recovery. But if one consider 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 as follows: 2 working LSPs A-B and E-F and 2 recovery
   LSPs A-C-D-B and E-C-D-F sharing the two common C-D resources.

                          A ================ B
                           \\               //
                            \\             //
                             C =========== D
                            //             \\
                           //               \\
                          E ================ F

   Moreover, in all these schemes, (working) path disjointness can be
   reinforced by exchanging working LSP related information during the
   recovery LSP signalling. 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 Restoration Schemes

5.5.1 Classification

   LSPs/spans recovery time and ratio depend on the proper recovery LSP
   (soft) provisioning and the level of recovery resources overbooking
   (i.e. over-provisioning). A proper balance of these two mechanisms
   will result in a desired LSP/span recovery time and ratio when
   single or multiple failure(s) occur(s).

   Recovery LSP Provisioning phases:

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

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                                                 --> Pre-Selected

   Overbooking Levels:

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

          Fig 3. LSP Provisioning and Overbooking Classification

   In this figure, we present a classification of different options
   under LSP provisioning and overbooking. Although we acknowledge
   these operations are run mostly during planning (using network
   planning) and provisioning time (using signaling and routing)
   activities, we keep them in analyzing the recovery schemes.

   Proper LSP/span provisioning will help in alleviating many of the
   failures. As an example, one may compute primary and secondary
   paths, either end-to-end or segment-per-segment, to recover an LSP
   from multiple failure events affecting link(s), node(s), SRLG(s)
   and/or SRG(s). Such primary and secondary LSP/span provisioning can
   be categorized, as shown in the above figure, based on:
   (1) the recovery path (i.e. route) can be either pre-computed or
       computed on demand.
   (2) when the recovery path is pre-computed: pre-signaled (implying
       recovery resource reservation) or signaled on demand.
   (3) and when the recovery resources are reserved, they can be either
       pre-selected or selection on-demand.

   Note that these different options give rise to different LSP/span
   recovery times. The following subsections will consider all these
   cases in analyzing the schemes.

   There are many mechanisms available allowing 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
   per domain (such as ring topologies). In all these cases the level
   of overbooking, as shown in the above figure, can be classified as
   dedicated (such as 1+1 and 1:1), shared (such as 1:N and M:N) or
   unprotected (i.e. restorable if enough recovery resources are

   Under a shared restoration scheme one may support preemptable
   (preempt low priority connections in case of resource contention)

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   extra-traffic. In this document we keep in mind all the above-
   mentioned overbooking mechanisms in analyzing the recovery schemes.

5.5.2 Dynamic LSP Restoration

   We first define the following times in order to provide a
   quantitative estimation about the time performance of the different
   dynamic and pre-signaled LSP restoration schemes (note: restoration
   is also referred to as re-routing):
   - Path Computation Time - Tpc
   - Path Selection Time - Tps
   - End-to-end LSP Resource Reservation û Trr (a delta for resource
     selection is also considered, the corresponding total time is then
     referred to as Trrs)
   - End-to-end LSP Resource Activation Time û Tra (a delta for
     resource selection is also considered, the corresponding total
     time is then referred to as Tras)

   Path Selection Time (Tps) is considered when a pool of recovery
   LSPÆs paths between a given source/destination is pre-computed and
   after 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 as equivalently 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 (i.e. route). As such, one can
   define this as an ôLSP re-provisioningö mechanism. Here, one or more
   (disjoint) routes for the restoration path are computed (and
   optionally pre-selected) before a failure occurs.

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

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

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

   An end-to-end restoration LSP is established after the failure(s)
   occur(s). Here, one or more (disjoint) explicit routes for the
   restoration path are dynamically computed and one is selected after
   failure. As such, one can define this as an ôLSP re-routingö

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

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

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

   An end-to-end restoration path is pre-selected from a set of one or
   more pre-computed (disjoint) explicit route before failure. The
   restoration LSP is signaled along this pre-selected path to reserve
   resources (i.e. signaled) at each node but resources are not

   In this case, the resources reserved for each restoration LSP may be
   dedicated or shared between different working LSP that are not
   expected to fail simultaneously. Local node policies can be applied
   to define the degree to which these resources are shared across
   independent failures.

   Upon failure detection, signaling is initiated along the restoration
   path to select the resources, and to perform the appropriate
   operation at each node entity involved in the restoration connection
   (e.g. cross-connections).

   The expected total restoration time T is thus equal to Tras (post-
   failure activation) while operations performed before failure
   occurrence takes Tpc + Tps + Trr.

   2. With both resource reservation and resource pre-selection

   An end-to-end restoration path is pre-selected from a set of one or
   more pre-computed (disjoint) explicit route before failure. The
   restoration LSP is signaled along this pre-selected path to reserve
   AND select resources at each node but not cross-connected. Such that
   the selection of the recovery resources is fixed at the control
   plane level. However, no cross-connections are performed along the
   restoration path.

   In this case, the resources reserved for each restoration LSP may
   only be shared between different working LSPs that are not expected
   to fail simultaneously. Since one considers restoration schemes

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   here, the sharing degree should not be limited to working (and
   recovery) LSPs starting and ending at the same ingress and egress
   nodes. Therefore, one expects to receive some feedback information
   on the recovery resource sharing degree at each node participating
   to the recovery scheme.

   Upon failure detection, signaling is initiated along the restoration
   path to activate the reserved and selected resources and to perform
   the appropriate operation at each node involved in the restoration
   connection (e.g. cross-connections).

   The expected total restoration time T is thus equal to Tra (post-
   failure activation) while operations performed before failure
   occurrence takes Tpc + Tps + Trrs. 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.
   Tras û Tra).

5.5.4 LSP Segment Restoration

   The above approaches can be applied on a sub-network basis rather
   than end-to-end basis (in order to reduce the global recovery time).

   It should be also noted that using the horizontal hierarchical
   approach described in Section 7.1, that a given end-to-end LSP can
   be recovered by multiple recovery mechanisms (e.g. 1:1 protection in
   a metro edge network but M:N protection in the core). These
   mechanisms are ideally independent and may even use different
   failure localization and notification mechanisms.

6. Normalization

   Normalization is defined as the mechanism allowing switching normal
   traffic from the recovery LSP/span to the working LSP/span
   previously under failure condition.

   Use of normalization is under the discretion of the recovery domain
   policy. Normalization (reversion) may impact the normal traffic (a
   second hit) depending on the normalization mechanism used.

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

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

6.1 Wait-To-Restore

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   A specific mechanism (Wait-To-Restore) is used to prevent frequent
   recovery switching operation 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
   incremental 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.

   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 can
   not 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 ôorphanö) 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 to be 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: 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

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   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 ôIP-over-opticalö 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. The
   same hierarchical concept can be applied to control the network
   survivability. 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, 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
   more than desirable to avoid too many layers with functional
   overlaps. In this context, this section intends to analyze these
   hierarchical aspects including the physical (passive) layer(s).

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 ôrecovery domainö since each of the LSPs they carry can have
   its own recovery type (or even scheme). The support of multiple
   recovery schemes within a sub-network is referred to as a multi-
   recovery capable domain or simply multi-recovery domain.

7.2 Vertical Hierarchy (Layers)

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   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
     (and subsequently its sharing ratio)

   Therefore, in the scope 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, HOVC/STS-SPE and LOVC/VT-SPE LSPs and continuous bandwidth
     granularities for packet LSPs
   - Potentially, 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 hierarchiesÆ (1) packet over packet (2) packet
   over circuit (3) circuit over packet and (4) circuit over circuit.
   Here below we extend a little bit more on (4), (2) being covered in
   [TE-RH] on the other hand (1) is extensively covered at the MPLS
   Working Group, and (3) at the PWE3 Working Group.

   In SDH/Sonet environments, one typically considers the LOVC/VT and
   HOVC/STS SPE as independent layers, LOVC/VT LSP using the underlying
   HOVC/STS SPE 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 links. Notice here that server layer LSPs may
   simply be provisioned and not dynamically triggered or established
   (control driven approach).

   The following figure (including only the path layers) illustrates
   the hierarchy that can be covered by the recovery architecture of a
   network comprising a SDH/Sonet and an OTN part:

    LOVC <------------------------------------------------------> LOVC
     |                                                            |
    HOVC ---- HOVC <----------------------------------> HOVC ---- HOVC
               |                                        |
    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
               |                                        |
              ODUk ---- ODUk <--------------> ODUk ---- ODUk
                         |                    |
                        OTUk <--------------> OTUk
                         |                    |
                        OCh -- OCh -..- OCh -- OCh

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   In this context, the important points are the following:
   - these layers are path layers; i.e. the ones controlled by
     the GMPLS (in particular, signalling) protocol suite.
   - an LSP at the lower layer for instance an optical channel (=
     network connection) appears as a section (= link) for the OTUk
     layer i.e. the links that are typically controlled by link
     management protocols such as LMP.

   The first key issue with multi-layer recovery is that achieving
   control plane 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 either be independent when protected (or restorable), or
   triggered by the upper LSP recovery process. The former requires
   coordination in order 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
   (pre-determined for instance by the hold-off timer), a failure
   notification may propagate from one layer to the next 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
   [TE-RH], 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 in this context, allowing synchronization between
   recovery actions performed across these layers.

   Note: 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 for higher layers. The OTN and SDH/Sonet layers
   can only recover the whole section or the individual connections it
   transports whereas IP/MPLS layer(s) can recover individual packet
   LSPs or groups of packet LSPs.

   Obviously, the recovery granularity at the sub-wavelength (i.e.
   SDH/Sonet) level can be provided only when the network includes
   devices switching at the same granularity level (and thus not with
   optical channel switching capable devices). Therefore, the network
   layer can deliver control-plane driven recovery mechanisms on a per-

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   LSP basis if and only if the LSPs class has the corresponding
   switching capability 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 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. Imagine a SDH/Sonet 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 schemes are faster than upper level one but
   only if the same type of recovery mechanism is used at each layer
   (assuming that the lower layer one is faster).

   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 ôstandby modeö. 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 letting 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 perform the recovery actions. In case of
   packet layer unsuccessful recovery action, fallback at the optical
   layer can be subsequently performed.

   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 must provide its own reliable and independent failure
   detection mechanism from the lower layer.

   The same reference suggests also recovery mechanisms incorporating a
   coordinated effort shared by two adjacent layers with periodic
   status updates. Moreover, at certain layers, some of these recovery
   operations can be pre-assigned, e.g. a particular link will be

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   handled by the packet layer while another will be handled by the
   optical layer.

7.4 Disjointness

   Having link and node diverse working and recovery LSPs/spans does
   not guarantee working and recovery LSPs/Spans 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 a complete working and recovery LSP/span disjointness
   (see [IPO-IMP], [GMPLS-RTG] and [CCAMP-SRLG]). Otherwise, a failure
   affecting the working LSP/span would also potentially affect the
   recovery LSP/span resources, one refers to such event as a common

7.4.1 SRLG Disjointness

   A Shared Risk Link Group (SRLG) is defined as the set of optical
   spans (or links or optical lines) sharing a common physical resource
   (for instance, fiber links, fiber trunks or cables) i.e. sharing a
   common risk. 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 for LSP:

      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.

      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 none of them covers simultaneously this SRLG.

      Whilst the SRLG disjointness for LSPs with respect of 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 set of SRLGs they
      individually cover are mutually disjoint from the set S.

   The impact on recovery is obvious: SRLG disjointness is a necessary
   (but not a sufficient) condition to ensure optical network

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   survivability. With respect to the physical network resources, a
   working-recovery LSP/span pair must be SRLG disjoint in case of
   dedicated recovery type while a working-recovery LSP/span group must
   be SRLG disjoint in case of shared recovery.

8. Recovery Scheme/Strategy Selection

   In order to provide a structured selection and analysis of the
   recovery scheme/strategy, the following dimensions can be defined:

   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 number of LSPs/spans being
      recovered (this implies an efficient failure correlation, a fast
      failure notification and timely efficient recovery mechanism(s)).

   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 LSP/span and nodes (switching capacity),
      required for recovery purposes; this dimension can also be
      referred to as optimize the sharing degree of the recovery

   5. Cost optimization: provide a cost-effective recovery strategy.

   However, these dimensions are either out of the scope of this
   document such as cost optimization and recovery path computational
   aspects or going in opposite directions. For instance, it is obvious
   that providing a 1+1 recovery type for each LSP minimizes the LSP
   downtime (in case of failure) while being non-scalable and recovery
   resource consuming without enabling any extra-traffic.

   The following sections try to provide a first response in order to
   select a recovery strategy with respect to the dimensions described
   above and the recovery schemes proposed in [CCAMP-TERM].

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 already discussed in
   Section 4.

8.2 Efficiency (Switching Time)

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

   Span restoration (since using control plane) is also likely to be
   slower than most span protection types; however this greatly depends
   on the span restoration signalling efficiency. 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 crank-back during the recovery LSP

   If one excludes the crank-back issue, the difference between dynamic
   and pre-planned restoration depends on the restoration path
   computation and path selection time. Since computational
   considerations are outside of the scope of this document, it is up
   to the vendor to determine the average 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 with respect
   to dynamic one: the former implies a limited number of failure
   scenarios (that can be due for instance to local storage
   limitation). This, while the latter enables an on-demand path
   computation based on the information received through failure
   notification and as such 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-signaled) will generally be faster than end-to-end
   LSP schemes. However, local LSP restoration assumes that each LSP
   segment end-point has enough computational capacity to perform this
   operation while end-to-end requires only that LSP end-points
   provides this path computation capability.

   Recovery time objectives for SDH/Sonet 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 [TE-RH].

8.3 Robustness

   In general, the less pre-assignment (protection)/pre-planning
   (restoration) of the recovery LSP/span, the more robust the recovery
   type/scheme is to a variety of (single) failures, provided that
   adequate resources are available. Moreover, the pre-selection of the
   recovery resources gives less flexibility for multiple failure

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   scenarios 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 schemes with pre-planned recovery resources,
   in particular spans for protection and LSP 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 chosen to 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 restoration LSP can be reduced by re-
   computing a restoration path whenever a failure occurs along the
   corresponding recovery LSP or by re-computing a restoration path and
   re-provisioning the corresponding recovery LSP whenever a failure
   occurs along a working LSP/span. This method enables to maintain the
   number of available recovery path constant.

   The robustness of a recovery scheme is also determined by the amount
   of reserved (i.e. signaled) recovery resources within a given shared
   resource pool: as the amount of recovery resources sharing degree
   increases, the recovery scheme becomes less robust to multiple
   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
   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 more
   resource consuming protection type since it doesnÆt allow for any
   extra-traffic 2) 1:1 LSP/span protection type requires dedicated
   recovery LSP/span allowing carrying extra preemptible traffic 3) 1:N
   and M:N LSP/span recovery types require 1 (or M, respectively)
   recovery LSP/span (shared between the N working LSP/span) while
   allowing carrying extra preemptible traffic. Obviously, 1+1
   protection precludes and 1:1 recovery type does not allow for
   recovery LSP/span sharing whereas 1:N and M:N recovery types do

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   allow sharing of 1 (M, respectively) recovery LSP/spans between N
   working LSP/spans.

   However, despite the fact that the 1:1 recovery type does not allow
   recovery LSP/span sharing, the recovery schemes (see Section 5.4)
   that can be built from them (e.g.(1:1)^n) do allow for sharing of
   recovery resources these entities includes. 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, in restoration with pre-signaled resource
   reservation, the amount of reserved restoration capacity is
   determined by the local bandwidth reservation policies. In
   restoration schemes with re-provisioning, a pool of restoration
   resource can be defined from which all (spare) restoration resources
   are selected after failure occurrence for recovery path computation
   purpose. The degree to which restoration schemes allow sharing
   amongst multiple independent failures is then directly dictated by
   the size of the restoration 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 degree to which the network is survivable is determined by the
   policy that defines the amount of reserved (shared) recovery
   resources and the maximum sharing degree allowed.

8.4.1. Recovery Resource Sharing

   When recovery resources are shared over several LSP/Spans, [GMPLS-
   RTG], the use of the Maximum Reservable Bandwidth, the Unreserved
   Bandwidth and the Maximum LSP Bandwidth Link sub-TLVs 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
   capacity but may be greater in case of link over-subscription. The
   Unreserved Bandwidth (per priority) is defined as the bandwidth not
   yet reserved on a given TE link (initial value at each priority
   level corresponds to the Maximum Reservable Bandwidth). Last, the
   Maximum LSP Bandwidth (per priority) is defined as the smaller of
   Unreserved Bandwidth and Maximum Reservable Bandwidth.

   Here, one generally considers a recovery resource sharing ratio (or
   degree) in order to globally optimize the shared recovery resource
   usage. The distribution of the bandwidth utilization per (bundled)
   TE link can be inferred from the per-priority bandwidth pre-
   allocation. This by using the Maximum LSP Bandwidth and the

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   Unreserved Bandwidth (see [GMPLS-RTG]), the amount of resources
   (over-provisioned) for shared recovery purposes is known from the

   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 capacity) and the Maximum LSP
   Bandwidth (in the present case, this value corresponds to the
   Unreserved Bandwidth) per TE link i as the Maximum Sharable
   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 component link
   bandwidth unit (and thus equivalently, the Min LSP Bandwidth is of
   one bandwidth unit).

   From these definitions, it results that the usage of this
   information available per TE link can be considered 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 per component bandwidth unit) 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} û
   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 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 ôshared bandwidthö 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 -----  |
                                |  -----  |
                                 - -----  |max_R
                                   -----  |
   --------  TE link Capacity    - ------ | - Maximum Bandwidth
   -----                        |r -----  v
   -----     <------ b ------>   - ---------- Unreserved Bandwidth
   -----                           -----
   -----                           -----
   -----                           -----
   -----                           -----
   -----                           ----- <--- Min LSP Bandwidth
   -------- 0                      ---------- 0

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   Note that the above approach does not require the flooding of any
   per LSP information or a detailed distribution of the bandwidth
   allocation per component link (or individual ports). Moreover, it
   has been demonstrated that this Partial Information Routing approach
   can also be extended to resource shareability with respect to the
   number of times each SRLG is protected by a recovery resource, in
   particular an LSP (see also Section 8.4.2). This method also
   referred to as stochastic approach is described in [BOUILLET]. By
   flooding this summarized information using a link-state protocol,
   recovery path computation and selection for SRLG diverse recovery
   LSPs can be optimized with respect to resource sharing giving a
   performance difference of less than 5% compared to a Full
   Information Flooding approach. The latter can be found in [GLI] for
   instance. Note that strictly speaking both methods rely on
   deterministic knowledge of the network topology and resource (usage)

   For GMPLS-based recovery purposes, the Partial Information Routing
   approach can be further enhanced by extending GMPLS signalling
   capabilities. This, by allowing the working LSP related information
   and in particular, its explicit route to be exchanged over the
   recovery LSP in order to enable more efficient admission control at
   shared (link) resource upstream nodes.

8.4.2 Recovery Resource Sharing and SRLG Recovery

   As stated in the previous section, resource shareability can also be
   maximized with respect to the number of times each SRLG is protected
   by a recovery resource.

   Methods can be considered for avoiding contention for the shared
   recovery resources during a single SRLG failure (see Section 5).
   These allow the sharing of common reserved recovery resource between
   two (or more) recovery LSPs (only) if their respective working LSPs
   are mutually disjoint with respect to link, node or SRLG. A single
   failure then does not disrupt several (at least two) working LSPs

   For this purpose, additional extensions to [GMPLS-RTG] in support of
   path computation for shared mesh restoration would potentially be
   considered. First, the information about the recovery resource
   sharing on a TE link such as the current number of recovery LSPs
   sharing the recovery resources (pre-)allocated on the TE link (see
   also Section 8.4.1) and the current number of SRLGs recoverable by
   this amount of shared recovery resource on this TE link, may be
   considered. The latter is equivalent to the total number of SRLGs
   that the (recovery) LSPs sharing the recovery resources shall
   recover. Then, if SRLG recoverability is considered, the explicit
   list of SRLGs recoverable by the recovery resources shared on the TE
   link together with their respective sharable recovery bandwidth (see
   also Section 8.4.1) may be considered. The latter information is
   equivalent to the maximum sharable recovery bandwidth per SRLG or

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   per group of SRLG implying to consider a decreasing amount of
   sharable bandwidth and SRLG list over time.

   Compared to the case of simple recovery resource sharing regardless
   of SRLG recoverability (as described in Section 8.4.1), the
   additional TE link information considered here would potentially
   allow for better path computation and selection (at distinct ingress
   node) during SRLG-disjoint LSP provisioning in a shared meshed
   recovery scheme. However, due to the lack of results of evidence for
   better efficiency and due to the complexity that such extensions
   would in turn generate, these are not considered in the scope of
   the present analysis. For instance, a per (group of) SRLG maximum
   sharable 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, they SHOULD not be translated into GMPLS
   routing or signalling protocol extensions for recovery purposes.

   The next section will demonstrate that such extensions complements
   the exchange of the explicit route of working LSP over the recovery
   LSP path in order to achieve shared recovery resources contention

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

   Consequently, node E must have the required information (implying
   for instance that the explicit route followed by the primary LSPs to
   be carried with the corresponding recovery LSP request) in order to
   perform an admission control for the recovery LSP requests.

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   Moreover, node E may securely (if its maximum shared recovery
   bandwidth ratio has not been reached yet for this link) accept the
   recovery LSP request and logically assign the same resource to these
   LSPs. This if and only if it can guarantee that A-C-D and B-C-D are
   SRLG disjoint over the C-D span (one considers here in the scope of
   this example, node failure probability as negligible). To achieve
   this, the explicit route of the primary LSP (and transported over
   the recovery path) is examined at each shared link ingress node. The
   latter uses the interface identifier as index to retrieve in the TE
   link State DataBase (TE LSDB) the SRLG id list associated to the
   links of the working LSPs. If these LSPs have one or more SRLG id in
   common (in this example, one or more SRLG id in common over C-D),
   then node E should not assign the same resource to the recovery
   LSPs. 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 because 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 [MPLS-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 major drawback
   of this method is, in addition to the dedicated mechanism it
   requires, that it may become very complex when several common nodes
   are traversed by the working LSPÆs. Therefore, when using link
   bundles, a potential way of solving this issue tightly related to
   the sequence of the recovery operations (at least in a first step,
   since per component flooding of SRLG identifiers would impact the
   link state routing protocol scalability), is to rely on the usage of
   an on-line accessible network management system.

9. Summary

   One can summarize by the following table the selection of a recovery
   scheme/strategy, using the recovery types proposed in [CCAMP-TERM]
   and their detailed analysis proposed in this memo.

              |       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   ------------------------------------------------------------

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          |   | 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 |

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

   2. Recovery LSP setup (before failure occurrence) with resource
      reservation (i.e. signalling) 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) 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 (full) setup after failure occurrence is referred
       to as dynamic LSP re-routing.

   Thus, the term pre-planned refers to recovery resource pre-
   computation, signaling (reservation) and a priori selection
   (optional), but not cross-connection.

10. Security Considerations

   This document does not introduce or imply any specific security

11. References

11.1 Normative References

   [GMPLS-ARCH] E.Mannie (Editor), ôGeneralized MPLS Architectureö,
                Internet Draft, Work in progress, draft-ietf-ccamp-
                gmpls-architecture-03.txt, August 2002.

   [GMPLS-RTG]  K.Kompella (Editor), ôRouting Extensions in Support of
                Generalized MPLS,ö Internet Draft, Work in Progress,
                draft-ietf-ccamp-gmpls-routing-05.txt, August 2002.

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   [GMPLS-SIG]  L.Berger (Editor), ôGeneralized MPLS û Signaling
                Functional Descriptionö, Internet Draft, Work in
                progress, draft-ietf-mpls-generalized-signaling-09.txt,
                October 2002.

   [LMP]        J.Lang (Editor), ôLink Management Protocol (LMP) v1.0ö
                Internet Draft, Work in progress, draft-ietf-ccamp-lmp-
                07, October 2002.

   [LMP-WDM]    A.Fredette and J.Lang (Editors), ôLink Management
                Protocol (LMP) for DWDM Optical Line Systems,ö Internet
                Draft, Work in progress, draft-ietf-ccamp-lmp-wdm-
                01.txt, September 2002.

11.2 Informative References

   [RFC-2026]   Bradner, S., ôThe Internet Standards Process --
                Revision 3ö, BCP 9, RFC 2026, October 1996.

   [RFC-2119]   Bradner, S., ôKey words for use in RFCs to Indicate
                Requirement Levelsö, BCP 14, RFC 2119, March 1997.

   [BOUILLET]   E.Bouillet et al., ôStochastic Approaches to Compute
                Shared Meshed Restored Lightpaths in Optical Network
                Architecturesö, INFOCOM 2002, New York City, June 2002.

   [CCAMP-LI]   G.Li et al. ôRSVP-TE Extensions For Shared-Mesh
                Restoration in Transport Networksö, Internet Draft,
                Work in progress, draft-li-shared-mesh-restoration-
                01.txt, November 2001.

   [CCAMP-LIU]  H.Liu et al. ôOSPF-TE Extensions in Support of Shared
                Mesh Restorationö, Internet Draft, Work in progress,
                draft-liu-gmpls-ospf-restoration-00.txt, October 2002.

   [CCAMP-SRLG] D.Papadimitriou et al., ôShared Risk Link Groups
                Encoding and Processing,ö Internet Draft, Work in
                progress, draft-papadimitriou-ccamp-srlg-processing-
                01.txt, November 2002.

   [CCAMP-TERM] E.Mannie and D.Papadimitriou (Editors), ôRecovery
                (Protection and Restoration) Terminology for GMPLS,ö
                Internet Draft, Work in progress, draft-ietf-ccamp-
                gmpls-recovery-terminology-00.txt, June 2002.

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

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

D.Papadimitriou et al. - Internet Draft û June 2003                 36

draft-ietf-ccamp-gmpls-recovery-analysis-00.txt           January 2003

   [G.709]      ITU-T, ôNetwork Node Interface for the Optical
                Transport Network (OTN)ö, Recommendation G.709,
                February 2001 (and Amendment n—1, October 2001).

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

   [G.798]      ITU-T, ôCharacteristics of Optical Transport Network
                (OTN) Equipment Functional Blocksö, Recommendation
                G.798, January 2002.

   [G.806]      ITU-T, ôCharacteristics of Transport Equipment û
                Description Methodology and Generic Functionalityö,
                Recommendation G.806, October 2000.

   [G.826]      ITU-T, ôPerformance Monitoringö, Recommendation G.826,
                February 1999.

   [G.841]      ITU-T, ôTypes and Characteristics of SDH Network
                Protection Architecturesö, Recommendation G.841,
                October 1998.

   [G.842]      ITU-T, ôInterworking of SDH network protection
                architecturesö, Recommendation G.842, October 1998.

   [G.GPS]      ITU-T Draft Recommendation G.GPS, Version 2, ôGeneric
                Protection Switchingö, Work in progress, May 2002.

   [GLI]        Guangzhi Li et al., ôEfficient Distributed Path
                Selection for Shared Restoration Connectionsö, IEEE
                Infocom, New York, June 2002.

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

   [MPLS-REC]   V.Sharma and F.Hellstrand (Editors) et al., ôA
                Framework for MPLS Recoveryö, Internet Draft, Work in
                Progress, draft-ietf-mpls-recovery-frmwrk-06.txt, July

   [MPLS-OSU]   S.Seetharaman et al, ôIP over Optical Networks: A
                Summary of Issuesö, Internet Draft, Work in Progress,
                draft-osu-ipo-mpls-issues-02.txt, April 2001.

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

   [TE-NS]      K.Owens et al, ôNetwork Survivability Considerations
                for Traffic Engineered IP Networksö, Internet Draft,

D.Papadimitriou et al. - Internet Draft û June 2003                 37

draft-ietf-ccamp-gmpls-recovery-analysis-00.txt           January 2003

                Work in Progress, draft-owens-te-network-survivability-
                01.txt, July 2001.

   [TE-RH]      W.Lai, D.McDysan, J.Boyle, et al, ôNetwork Hierarchy
                and Multi-layer Survivabilityö, Internet Draft, Work in
                Progress, draft-ietf-tewg-restore-hierarchy-01.txt,
                June 2002.

12. Acknowledgments

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

13. Author's Addresses

   Eric Mannie (Consult)
   E-mail: eric_mannie@hotmail.com

   Dimitri Papadimitriou (Alcatel)
   Francis Wellesplein, 1
   B-2018 Antwerpen, Belgium
   Phone : +32 3 240-8491
   E-mail: dimitri.papadimitriou@alcatel.be

D.Papadimitriou et al. - Internet Draft û June 2003                 38

draft-ietf-ccamp-gmpls-recovery-analysis-00.txt           January 2003

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D.Papadimitriou et al. - Internet Draft û June 2003                 39

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