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RFC 4428

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

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Network Working Group                              D. Papadimitriou, Ed.
Request for Comments: 4428                                       Alcatel
Category: Informational                                   E. Mannie, Ed.
                                                                Perceval
                                                              March 2006


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

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document provides an analysis grid to evaluate, compare, and
   contrast the Generalized Multi-Protocol Label Switching (GMPLS)
   protocol suite capabilities with the recovery mechanisms currently
   proposed at the IETF CCAMP Working Group.  A detailed analysis of
   each of the recovery phases is provided using the terminology defined
   in RFC 4427.  This document focuses on transport plane survivability
   and recovery issues and not on control plane resilience and related
   aspects.

Table of Contents

   1. Introduction ....................................................3
   2. Contributors ....................................................4
   3. Conventions Used in this Document ...............................5
   4. Fault Management ................................................5
      4.1. Failure Detection ..........................................5
      4.2. Failure Localization and Isolation .........................8
      4.3. Failure Notification .......................................9
      4.4. Failure Correlation .......................................11
   5. Recovery Mechanisms ............................................11
      5.1. Transport vs. Control Plane Responsibilities ..............11
      5.2. Technology-Independent and Technology-Dependent
           Mechanisms ................................................12
           5.2.1. OTN Recovery .......................................12
           5.2.2. Pre-OTN Recovery ...................................13
           5.2.3. SONET/SDH Recovery .................................13

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      5.3. Specific Aspects of Control Plane-Based Recovery
           Mechanisms ................................................14
           5.3.1. In-Band vs. Out-Of-Band Signaling ..................14
           5.3.2. Uni- vs. Bi-Directional Failures ...................15
           5.3.3. Partial vs. Full Span Recovery .....................17
           5.3.4. Difference between LSP, LSP Segment and
                  Span Recovery ......................................18
      5.4. Difference between Recovery Type and Scheme ...............19
      5.5. LSP Recovery Mechanisms ...................................21
           5.5.1. Classification .....................................21
           5.5.2. LSP Restoration ....................................23
           5.5.3. Pre-Planned LSP Restoration ........................24
           5.5.4. LSP Segment Restoration ............................25
   6. Reversion ......................................................26
      6.1. Wait-To-Restore (WTR) .....................................26
      6.2. Revertive Mode Operation ..................................26
      6.3. Orphans ...................................................27
   7. Hierarchies ....................................................27
      7.1. Horizontal Hierarchy (Partitioning) .......................28
      7.2. Vertical Hierarchy (Layers) ...............................28
           7.2.1. Recovery Granularity ...............................30
      7.3. Escalation Strategies .....................................30
      7.4. Disjointness ..............................................31
           7.4.1. SRLG Disjointness ..................................32
   8. Recovery Mechanisms Analysis ...................................33
      8.1. Fast Convergence (Detection/Correlation and
           Hold-off Time) ............................................34
      8.2. Efficiency (Recovery Switching Time) ......................34
      8.3. Robustness ................................................35
      8.4. Resource Optimization .....................................36
           8.4.1. Recovery Resource Sharing ..........................37
           8.4.2. Recovery Resource Sharing and SRLG Recovery ........39
           8.4.3. Recovery Resource Sharing, SRLG
                  Disjointness and Admission Control .................40
   9. Summary and Conclusions ........................................42
   10. Security Considerations .......................................43
   11. Acknowledgements ..............................................43
   12. References ....................................................44
      12.1. Normative References .....................................44
      12.2. Informative References ...................................44

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1.  Introduction

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

   A detailed analysis is provided for each of the recovery phases as
   identified in [RFC4427].  These phases define the sequence of generic
   operations that need to be performed when a LSP/Span failure (or any
   other event generating such failures) occurs:

      - Phase 1: Failure Detection
      - Phase 2: Failure Localization (and Isolation)
      - Phase 3: Failure Notification
      - Phase 4: Recovery (Protection or Restoration)
      - Phase 5: Reversion (Normalization)

   Together, failure detection, localization, and notification phases
   are referred to as "fault management".  Within a recovery domain, the
   entities involved during the recovery operations are defined in
   [RFC4427]; these entities include ingress, egress, and intermediate
   nodes.  The term "recovery mechanism" is used to cover both
   protection and restoration mechanisms.  Specific terms such as
   "protection" and "restoration" are used only when differentiation is
   required.  Likewise the term "failure" is used to represent both
   signal failure and signal degradation.

   In addition, when analyzing the different hierarchical recovery
   mechanisms including disjointness-related issues, a clear distinction
   is made between partitioning (horizontal hierarchy) and layering
   (vertical hierarchy).  In order to assess the current GMPLS protocol
   capabilities and the potential need for further extensions, the
   dimensions for analyzing each of the recovery mechanisms detailed in
   this document are introduced.  This document concludes by detailing
   the applicability of the current GMPLS protocol building blocks for
   recovery purposes.

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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)
   200 S. Laurel Ave.
   Middletown, NJ 07748, USA

   EMail: dbrungard@att.com


   Sudheer Dharanikota

   EMail: sudheer@ieee.org


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

   EMail: jplang@ieee.org


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

   EMail: gli@research.att.com


   Eric Mannie
   Perceval
   Rue Tenbosch, 9
   1000 Brussels
   Belgium

   Phone: +32-2-6409194
   EMail: eric.mannie@perceval.net


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

   EMail: dimitri.papadimitriou@alcatel.be

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   Bala Rajagopalan
   Microsoft India Development Center
   Hyderabad, India

   EMail: balar@microsoft.com


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

   EMail: yakov@juniper.net

3.  Conventions Used in this Document

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

   Any other recovery-related terminology used in this document conforms
   to that defined in [RFC4427].  The reader is also assumed to be
   familiar with the terminology developed in [RFC3945], [RFC3471],
   [RFC3473], [RFC4202], and [RFC4204].

4.  Fault Management

4.1.  Failure Detection

   Transport failure detection is the only phase that cannot be achieved
   by the control plane alone because the latter needs a hook to the
   transport plane in order to collect the related information.  It has
   to be emphasized that even if failure events themselves are detected
   by the transport plane, the latter, upon a failure condition, must
   trigger the control plane for subsequent actions through the use of
   GMPLS signaling capabilities (see [RFC3471] and [RFC3473]) or Link
   Management Protocol capabilities (see [RFC4204], Section 6).

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

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

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

   - Alignment: SONET/SDH 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 pointer.

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

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

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

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

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

   The above are examples that illustrate cases where the failure
   detection and reporting entities (see [RFC4427]) are co-located.  The
   following example illustrates the scenario where the failure
   detecting and reporting entities (see [RFC4427]) are not co-located.

   In pre-OTN networks, a failure may be masked by intermediate O-E-O
   based Optical Line System (OLS), preventing a Photonic Cross-Connect
   (PXC) from detecting upstream failures.  In such cases, failure
   detection may be assisted by an out-of-band communication channel,
   and failure condition may be reported to the PXC control plane.  This
   can be provided by using [RFC4209] extensions that deliver IP
   message-based communication between the PXC and the OLS control
   plane.  Also, since PXCs are independent of the framing format,
   failure conditions can only be triggered either by detecting the
   absence of the optical signal or by measuring its quality.  These
   mechanisms are generally less reliable than electrical (digital)
   ones.  Both types of detection mechanisms are outside the scope of
   this document.  If the intermediate OLS supports electrical (digital)
   mechanisms, using the LMP communication channel, these failure
   conditions are reported to

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

   More generally, the following are typical failure conditions in
   SONET/SDH and pre-OTN networks:

   - Loss of Light (LOL)/Loss of Signal (LOS): Signal Failure (SF)
     condition where the optical signal is not detected any longer on
     the receiver of a given interface.

   - Signal Degrade (SD): detection of the signal degradation over
     a specific period of time.

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   - For SONET/SDH payloads, all of the above-mentioned supervision
     capabilities can be used, resulting in SD or SF conditions.

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

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

   - Non-co-located detecting and reporting entities:

     o with in-band communication between entities: entities are
       physically separated, but the transport plane provides in-band
       communication between them (e.g., Server Signal Failures such as
       Alarm Indication Signal (AIS), etc.)

     o with out-of-band communication between entities: entities are
       physically separated, but an out-of-band communication channel is
       provided between them (e.g., using [RFCF4204]).

4.2.  Failure Localization and Isolation

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

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

   Failure localization should be triggered immediately after the fault
   detection phase.  This operation can be performed at the transport
   plane and/or (if the operation is unavailable via the transport
   plane) the control plane level where dedicated signaling messages can
   be used.  When performed at the control plane level, a protocol such
   as LMP (see [RFC4204], Section 6) can be used for failure
   localization purposes.

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4.3.  Failure Notification

   Failure notification is used 1) to inform intermediate nodes that an
   LSP/span failure has occurred and has been detected and 2) to inform
   the deciding entities (which can correspond to any intermediate or
   end-point of the failed LSP/span) that the corresponding service is
   not available.  In general, these deciding entities will be the ones
   making 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 provided either by the transport or by
   the control plane.  As an example, let us first briefly describe the
   failure notification mechanism defined at the SONET/SDH transport
   plane level (also referred to as maintenance signal supervision):

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

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

   The most important difference between these mechanisms is related to
   the fact that transport plane notifications (as defined today) would
   directly initiate either a certain type of protection switching (such
   as those described in [RFC4427]) via the transport plane or
   restoration actions via the management plane.

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

   Moreover, as specified in [RFC3473], notification message exchanges
   through a GMPLS control plane may not follow the same path as the
   LSP/spans for which these messages carry the status.  In turn, this
   ensures a fast, reliable (through acknowledgement and the use of

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   either a dedicated control plane network or disjoint control
   channels), and efficient (through the aggregation of several LSP/span
   statuses 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 at the
     recovering entities (in most of the recovery types and schemes this
     action is even deterministic).  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 [RFC4427]).  In
     addition, since fast notification is a mechanism running in
     collaboration with the existing GMPLS signaling (see [RFC3473])
     that also allows intermediate nodes to stay informed about the
     status of the working LSP/spans under failure condition.

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

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

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

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   - 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.

4.4.  Failure Correlation

   A single failure event (such as a span failure) can cause multiple
   failure (such as individual LSP failures) conditions to be reported.
   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 [RFC4209]) 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, providing a per-node configurable failure correlation time
   can be advisable.  The detailed selection criteria for this time
   interval are outside of the scope of this document.

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

5.  Recovery Mechanisms

5.1.  Transport vs. Control Plane Responsibilities

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

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

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   1. LSP/span Protection

   - Phase 1: Failure Detection                  Transport plane
   - Phase 2: Failure Localization/Isolation     Transport/Control plane
   - Phase 3: Failure Notification               Transport/Control plane
   - Phase 4: Protection Switching               Transport/Control plane
   - Phase 5: Reversion (Normalization)          Transport/Control plane

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

   2. LSP/span Restoration

   - Phase 1: Failure Detection                  Transport plane
   - Phase 2: Failure Localization/Isolation     Transport/Control plane
   - Phase 3: Failure Notification               Control plane
   - Phase 4: Recovery Switching                 Control plane
   - Phase 5: Reversion (Normalization)          Control plane

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

5.2.  Technology-Independent and Technology-Dependent Mechanisms

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

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

5.2.1.  OTN Recovery

   OTN recovery specifics are left for further consideration.

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5.2.2.  Pre-OTN Recovery

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

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

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

   The main disadvantage is 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 because corresponding detection technologies are under
   specification.

5.2.3.  SONET/SDH Recovery

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

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

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

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

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

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

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

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

5.3.  Specific Aspects of Control Plane-Based Recovery Mechanisms

5.3.1.  In-Band vs. Out-Of-Band Signaling

   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:
   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 (signaling)
   transport mechanism is briefly considered here.

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

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

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5.3.2.  Uni- vs. Bi-Directional Failures

   The failure detection, correlation, and notification mechanisms
   (described in Section 4) can be triggered when either a uni-
   directional or a bi-directional LSP/Span failure occurs (or a
   combination of both).  As illustrated in Figures 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 detected by only 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
      failure.

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

              Figure 1: Uni-directional failure detection

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

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

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

               Figure 2: Bi-directional failure detection

   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 Figure 1, node
     C sends a notification message to node B.  In Figure 2, node C
     sends a notification message to node B while node B sends a
     notification message to node C.  To ensure reliable failure
     notification, a dedicated acknowledgement message can be returned
     back to the sender node.

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

   - After some (pre-determined) period of time, referred to as the
     hold-off time, if the local recovery actions (see Section 5.3.4)
     were not successful, the following occurs.  In case of uni-
     directional failure and depending on the directionality of the LSP,
     node B should send an upstream notification message (see [RFC3473])
     to the ingress node A.  Node C may send a downstream notification
     message (see [RFC3473]) to the egress node D.  However, in that
     case, only node A would initiate an edge to edge recovery action.
     Node A is referred to as the "master", and node D is referred to as
     the "slave", per [RFC4427].  Note that the other LSP end-node (node
     D in this case) may be optionally notified using a downstream
     notification message (see [RFC3473]).

Top      ToC       Page 17 
     In case of bi-directional failure, node B should send an upstream
     notification message (see [RFC3473]) to the ingress node A.  Node C
     may send a downstream notification message (see [RFC3473]) to the
     egress node D.  However, due to the dependence on the LSP
     directionality, only ingress node A would initiate an edge-to-edge
     recovery action.  Note that the other LSP end-node (node D in this
     case) should also be notified of this event using a downstream
     notification message (see [RFC3473]).  For instance, if an LSP
     directed from D to A is under failure condition, only the
     notification message sent from node C to D would initiate a
     recovery action.  In this case, per [RFC4427], the deciding and
     recovering node D is referred to as the "master", while node A is
     referred to as the "slave" (i.e., recovering only entity).

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

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

5.3.3.  Partial vs. Full Span Recovery

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

   Additional complexity arises in the case of (sub-)group LSP recovery.
   Between a given pair of nodes, the LSPs that a given (sub-)group
   contains may have been created from different source nodes (i.e.,
   initiator) and directed toward different destination nodes.
   Consequently the failure notification messages following a bi-
   directional span failure that affects 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

Top      ToC       Page 18 
   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., from both the upstream and the downstream nodes to
   the failure).  In order to facilitate the definition of the
   corresponding recovery mechanisms (and their sequence), one assumes
   here as well that, per [RFC4427], the deciding (and recovering)
   entity (referred to as the "master") is the only initiator of the
   recovery of the whole LSP (sub-)group.

5.3.4.  Difference between LSP, LSP Segment and Span Recovery

   The recovery definitions given in [RFC4427] 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 signaling messages have to
   travel.  Since nodes are not necessarily adjacent in the case of LSP
   (or LSP Segment) recovery, signaling message exchanges from the
   reporting to the deciding/recovery entity may have to cross several
   intermediate nodes.  In particular, this applies to the notification
   messages due to the number of hops separating the location of a
   failure occurrence from its destination.  This results in an
   additional propagation and forwarding delay.  Note that the former
   delay may in certain circumstances be non-negligible; e.g., in a
   copper out-of-band network, the delay is approximately 1 ms per
   200km.

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

   Note: Several studies provide quantitative analysis of the relative
   performance of LSP/span recovery techniques. [WANG] for instance,
   provides an analysis grid for these techniques showing that dynamic
   LSP restoration (see Section 5.5.2) performs well under medium
   network loads, but suffers performance degradations at higher loads
   due to greater contention for recovery resources.  LSP restoration
   upon span failure, as defined in [WANG], degrades at higher loads
   because paths around failed links tend to increase the hop count of
   the affected LSPs and thus consume additional network resources.
   Also, performance of LSP restoration can be enhanced by a failed
   working LSP's source node that initiates a new recovery attempt if an
   initial attempt fails.  A single retry attempt is sufficient to

Top      ToC       Page 19 
   produce large increases in the restoration success rate and ability
   to initiate successful LSP restoration attempts, especially at high
   loads, while not adding significantly to the long-term average
   recovery time.  Allowing additional attempts produces only small
   additional gains in performance.  This suggests using additional
   (intermediate) crankback signaling when using dynamic LSP restoration
   (described in Section 5.5.2 - case 2).  Details on crankback
   signaling are outside the scope of this document.

5.4.  Difference between Recovery Type and Scheme

   [RFC4427] defines the basic LSP/span recovery types.  This section
   describes the recovery schemes that can be built using these recovery
   types.  In brief, a recovery scheme is defined as the combination of
   several ingress-egress node pairs supporting a given recovery type
   (from the set of the recovery types they allow).  Several examples
   are provided here to illustrate the difference between recovery types
   such as 1:1 or M:N, and recovery schemes such as (1:1)^n or (M:N)^n
   (referred to as shared-mesh recovery).

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

   The exponent, n, indicates the number of times a 1:1 recovery type is
   applied between at most n different ingress-egress node pairs.  Here,
   at most n pairs of disjoint working and recovery LSPs/spans share a
   common resource at most n times.  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 are less likely to happen.

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

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

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

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

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

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

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

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

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5.5.  LSP Recovery Mechanisms

5.5.1.  Classification

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

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

   1. Pre-Provisioning

   Proper recovery LSP pre-provisioning will help to alleviate the
   failure of the working LSPs (due to the failure of the resources that
   carry these LSPs).  As an example, one may compute and establish the
   recovery LSP either end-to-end or segment-per-segment, to protect a
   working LSP from multiple failure events affecting link(s), node(s)
   and/or SRLG(s).  The recovery LSP pre-provisioning options are
   classified as follows in the figure below:

   (1) The recovery path can be either pre-computed or computed on-
       demand.

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

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

   Recovery LSP provisioning phases:

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

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

   2. Overbooking

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

   Overbooking levels:

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

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

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

Top      ToC       Page 23 
   above-mentioned overbooking options when analyzing the different
   recovery mechanisms.

5.5.2.  LSP Restoration

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

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

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

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

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

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

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

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

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

   An end-to-end restoration LSP is dynamically established after the
   failure(s) occur(s).  After failure occurrence, one or more
   (disjoint) paths for the restoration LSP are dynamically computed and

Top      ToC       Page 24 
   one is selected.  As such, one can define this as a complete "LSP
   re-routing" mechanism.

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

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

5.5.3.  Pre-Planned LSP Restoration

   Pre-planned LSP restoration (also referred to as pre-planned LSP re-
   routing) implies that the restoration LSP is pre-signaled.  This in
   turn implies the reservation of recovery resources along the
   restoration path.  Two cases can be defined based on whether the
   recovery resources are pre-selected.

   1. With resource reservation and without resource pre-selection

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

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

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

   Thus, the expected total restoration time T is equal to Tas (post-
   failure activation), while operations performed before failure
   occurrence take Tc + Ts + Tr.

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   2. With both resource reservation and resource pre-selection

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

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

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

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

5.5.4.  LSP Segment Restoration

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

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


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