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rfc:rfc4428

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

Papadimitriou & Mannie Informational [Page 1] RFC 4428 GMPLS Recovery Mechanisms March 2006

    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

Papadimitriou & Mannie Informational [Page 2] RFC 4428 GMPLS Recovery Mechanisms March 2006

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

Papadimitriou & Mannie Informational [Page 3] RFC 4428 GMPLS Recovery Mechanisms March 2006

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

Papadimitriou & Mannie Informational [Page 4] RFC 4428 GMPLS Recovery Mechanisms March 2006

 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:

Papadimitriou & Mannie Informational [Page 5] RFC 4428 GMPLS Recovery Mechanisms March 2006

  1. 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.
  1. 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.
  1. 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.
  1. 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.
  1. 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:
  1. 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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  1. 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:
  1. 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.
  1. Signal Degrade (SD): detection of the signal degradation over

a specific period of time.

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  1. 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:
  1. 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.)
  1. 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.

Papadimitriou & Mannie Informational [Page 8] RFC 4428 GMPLS Recovery Mechanisms March 2006

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):
  1. 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

Papadimitriou & Mannie Informational [Page 9] RFC 4428 GMPLS Recovery Mechanisms March 2006

 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:
  1. 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.
  1. 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.
  1. 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.

Papadimitriou & Mannie Informational [Page 10] RFC 4428 GMPLS Recovery Mechanisms March 2006

  1. 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
  1. Phase 1: Failure Detection Transport plane
  2. Phase 2: Failure Localization/Isolation Transport/Control plane
  3. Phase 3: Failure Notification Transport/Control plane
  4. Phase 4: Protection Switching Transport/Control plane
  5. 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
  1. Phase 1: Failure Detection Transport plane
  2. Phase 2: Failure Localization/Isolation Transport/Control plane
  3. Phase 3: Failure Notification Control plane
  4. Phase 4: Recovery Switching Control plane
  5. 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.

Papadimitriou & Mannie Informational [Page 12] RFC 4428 GMPLS Recovery Mechanisms March 2006

5.2.2. Pre-OTN Recovery

 Pre-OTN recovery specifics (also referred to as "lambda switching")
 present mainly the following advantages:
  1. They benefit from a simpler architecture, making it more suitable

for mesh-based recovery types and schemes (on a per-channel basis).

  1. 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:
  1. 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]).
  1. Failure detection, notification, and path/section Automatic

Protection Switching (APS) mechanisms.

  1. 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:
  1. 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.

Papadimitriou & Mannie Informational [Page 13] RFC 4428 GMPLS Recovery Mechanisms March 2006

  1. 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.
  1. 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).

Papadimitriou & Mannie Informational [Page 14] RFC 4428 GMPLS Recovery Mechanisms March 2006

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.
  1. —— ——- ——- ——-

| | | |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

Papadimitriou & Mannie Informational [Page 15] RFC 4428 GMPLS Recovery Mechanisms March 2006

  1. —— ——- ——- ——-

| | | |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:
  1. 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.
  1. 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).
  1. 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]).

Papadimitriou & Mannie Informational [Page 16] RFC 4428 GMPLS Recovery Mechanisms March 2006

   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

Papadimitriou & Mannie Informational [Page 17] RFC 4428 GMPLS Recovery Mechanisms March 2006

 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

Papadimitriou & Mannie Informational [Page 18] RFC 4428 GMPLS Recovery Mechanisms March 2006

 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.

Papadimitriou & Mannie Informational [Page 20] RFC 4428 GMPLS Recovery Mechanisms March 2006

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

Papadimitriou & Mannie Informational [Page 22] RFC 4428 GMPLS Recovery Mechanisms March 2006

 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):
  1. Path Computation Time: Tc
  2. Path Selection Time: Ts
  3. 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

Papadimitriou & Mannie Informational [Page 23] RFC 4428 GMPLS Recovery Mechanisms March 2006

 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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6. Reversion

 Reversion (a.k.a. normalization) is defined as the mechanism allowing
 switching of normal traffic from the recovery LSP/span to the working
 LSP/span previously under failure condition.  Use of normalization is
 at the discretion of the recovery domain policy.  Normalization may
 impact the normal traffic (a second hit) depending on the
 normalization mechanism used.
 If normalization is supported, then 1) the LSP/span must be returned
 to the working LSP/span when the failure condition clears and 2) the
 capability to de-activate (turn-off) the use of reversion should be
 provided.  De-activation of reversion should not impact the normal
 traffic, regardless of whether it is currently using the working or
 recovery LSP/span.
 Note: during the failure, the reuse of any non-failed resources
 (e.g., LSP and/or spans) belonging to the working LSP/span is under
 the discretion of recovery domain policy.

6.1. Wait-To-Restore (WTR)

 A specific mechanism (Wait-To-Restore) is used to prevent frequent
 recovery switching operations due to an intermittent defect (e.g.,
 Bit Error Rate (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 on the order of a few
 minutes (for instance, 5 minutes) and should be capable of being set.
 The WTR timer may be either a fixed period, or provide for
 incrementally longer periods before retrying.  An SF or SD condition
 on the previously working LSP/span will override the WTR timer value
 (i.e., the WTR cancels and the WTR timer will restart).

6.2. Revertive Mode Operation

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

Papadimitriou & Mannie Informational [Page 26] RFC 4428 GMPLS Recovery Mechanisms March 2006

 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 if there are higher priority request attempts.  That is, the
 recovery LSP/span usage by the normal traffic may be preempted if a
 higher priority request for this recovery LSP/span is attempted.

6.3. Orphans

 When a reversion operation is requested, normal traffic must be
 switched from the recovery to the recovered working LSP/span.  A
 particular situation occurs when the previously working LSP/span
 cannot be recovered, so normal traffic cannot be switched back.  In
 that case, the LSP/span under failure condition (also referred to as
 "orphan") must be cleared (i.e., removed) from the pool of resources
 allocated for normal traffic.  Otherwise, potential de-
 synchronization between the control and transport plane resource
 usage can appear.  Depending on the signaling protocol capabilities
 and behavior, different mechanisms are expected here.
 Therefore, any reserved or allocated resources for the LSP/span under
 failure condition must be unreserved/de-allocated.  Several ways can
 be used for that purpose: wait for the clear-out time interval to
 elapse, initiate a deletion from the ingress or the egress node, or
 trigger the initiation of deletion from an entity (such as an EMS or
 NMS) capable of reacting upon reception of an appropriate
 notification message.

7. Hierarchies

 Recovery mechanisms are being made available at multiple (if not all)
 transport layers within so-called "IP/MPLS-over-optical" networks.
 However, each layer has certain recovery features, and one needs to
 determine the exact impact of the interaction between the recovery
 mechanisms provided by these layers.
 Hierarchies are used to build scalable complex systems.  By hiding
 the internal details, 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.  Network
 survivability is the set of capabilities that allow a network to
 restore affected traffic in the event of a failure.  Network
 survivability is defined further in [RFC4427].  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

Papadimitriou & Mannie Informational [Page 27] RFC 4428 GMPLS Recovery Mechanisms March 2006

 of recovery actions.  Moreover, recovery hierarchies also can be
 bound to control plane logical partitions (e.g., administrative or
 topological boundaries).  Each logical partition may apply different
 recovery mechanisms.
 In brief, it is commonly accepted that the lower layers can provide
 coarse but faster recovery while the higher layers can provide finer
 but slower recovery.  Moreover, it is also desirable to avoid similar
 layers with functional overlaps in order to optimize network resource
 utilization and processing overhead, since repeating the same
 capabilities at each layer does not create any added value for the
 network as a whole.  In addition, even if a lower layer recovery
 mechanism is enabled, it does not prevent the additional provision of
 a recovery mechanism at the upper layer.  The inverse statement does
 not necessarily hold; that is, enabling an upper layer recovery
 mechanism may prevent the use of a lower layer recovery mechanism.
 In this context, this section analyzes 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 "recovery domain" since each of the LSPs they carry can have
 its own recovery type (or even scheme).  The support of multiple
 recovery types and schemes within a sub-network is referred to as a
 "multi-recovery capable domain" or simply "multi-recovery domain".

7.2. Vertical Hierarchy (Layers)

 It is very challenging to combine 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 network-supported services.

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

entity and therefore the granularity of the recovery resource.

 Moreover, in the context of this analysis, a vertical hierarchy
 consists of multiple layered transport planes providing different:
  1. Discrete bandwidth granularities for non-packet LSPs such as OCh,

ODUk, STS_SPE/HOVC, and VT_SPE/LOVC LSPs and continuous bandwidth

   granularities for packet LSPs.
  1. Potential recovery capabilities with different temporal

granularities: ranging from milliseconds to tens of seconds

 Note: based on the bandwidth granularity, we can determine four
 classes of vertical hierarchies: (1) packet over packet, (2) packet
 over circuit, (3) circuit over packet, and (4) circuit over circuit.
 Below we briefly expand on (4) only. (2) is covered in [RFC3386]. (1)
 is extensively covered by the MPLS Working Group, and (3) by the PWE3
 Working Group.
 In SONET/SDH environments, one typically considers the VT_SPE/LOVC
 and STS SPE/HOVC as independent layers (for example, VT_SPE/LOVC LSP
 uses the underlying STS_SPE/HOVC LSPs as links).  In OTN, the ODUk
 path layers will lie on the OCh path layer, i.e., the ODUk LSPs use
 the underlying OCh LSPs as OTUk links.  Note here that lower layer
 LSPs may simply be provisioned and not necessarily dynamically
 triggered or established (control driven approach).  In this context,
 an LSP at the path layer (i.e., established using GMPLS signaling),
 such as an optical channel LSP, appears at the OTUk layer as a link,
 controlled by a link management protocol such as LMP.
 The first key issue with multi-layer recovery is that achieving
 individual or bulk LSP recovery will be as efficient as the
 underlying link (local span) recovery.  In such a case, the span can
 be either protected or unprotected, but the LSP it carries must be
 (at least locally) recoverable.  Therefore, the span recovery process
 can be either independent when protected (or restorable), or
 triggered by the upper LSP recovery process.  The former case
 requires coordination to achieve subsequent LSP recovery.  Therefore,
 in order to achieve robustness and fast convergence, multi-layer
 recovery requires a fine-tuned coordination mechanism.

Papadimitriou & Mannie Informational [Page 29] RFC 4428 GMPLS Recovery Mechanisms March 2006

 Moreover, in the absence of adequate recovery mechanism coordination
 (for instance, a pre-determined coordination when using a hold-off
 timer), a failure notification may propagate from one layer to the
 next one within a recovery hierarchy.  This can cause "collisions"
 and trigger simultaneous recovery actions that may lead to race
 conditions and, in turn, reduce the optimization of the resource
 utilization and/or generate global instabilities in the network (see
 [MANCHESTER]).  Therefore, a consistent and efficient escalation
 strategy is needed to coordinate recovery across several layers.
 One can expect that the definition of the recovery mechanisms and
 protocol(s) is technology-independent so that they can be
 consistently implemented at different layers; this would in turn
 simplify their global coordination.  Moreover, as mentioned in
 [RFC3386], some looser form of coordination and communication between
 (vertical) layers such as a consistent hold-off timer configuration
 (and setup through signaling during the working LSP establishment)
 can be considered, thereby allowing the synchronization between
 recovery actions performed across these layers.

7.2.1. Recovery Granularity

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

7.3. Escalation Strategies

 There are two types of escalation strategies (see [DEMEESTER]):
 bottom-up and top-down.
 The bottom-up approach assumes that lower layer recovery types and
 schemes are more expedient and faster than upper layer ones.
 Therefore, we can inhibit or hold off higher layer recovery.
 However, this assumption is not entirely true.  Consider for instance
 a SONET/SDH based protection mechanism (with a protection switching
 time of less than 50 ms) lying on top of an OTN restoration mechanism
 (with a restoration time of less than 200 ms).  Therefore, this

Papadimitriou & Mannie Informational [Page 30] RFC 4428 GMPLS Recovery Mechanisms March 2006

 assumption should be (at least) clarified as: the lower layer
 recovery mechanism is expected to be faster than the upper level one,
 if the same type of recovery mechanism is used at each layer.
 Consequently, taking into account the recovery actions at the
 different layers in a bottom-up approach: if lower layer recovery
 mechanisms are provided and sequentially activated in conjunction
 with higher layer ones, the lower layers must have an opportunity to
 recover normal traffic before the higher layers do.  However, if
 lower layer recovery is slower than higher layer recovery, the lower
 layer must either communicate the failure-related information to the
 higher layer(s) (and allow it to perform recovery), or use a hold-off
 timer in order to temporarily set the higher layer recovery action in
 a "standby mode".  Note that the a priori information exchange
 between layers concerning their efficiency is not within the current
 scope of this document.  Nevertheless, the coordination functionality
 between layers must be configurable and tunable.
 For example, coordination between the optical and packet layer
 control plane enables the optical layer to perform the failure
 management operations (in particular, failure detection and
 notification) while giving to the packet layer control plane the
 authority to decide and perform the recovery actions.  If the packet
 layer recovery action is unsuccessful, fallback at the optical layer
 can be performed subsequently.
 The top-down approach attempts service recovery at the higher layers
 before invoking lower layer recovery.  Higher layer recovery is
 service selective, and permits "per-CoS" or "per-connection" re-
 routing.  With this approach, the most important aspect is that the
 upper layer should provide its own reliable and independent failure
 detection mechanism from the lower layer.
 [DEMEESTER] also suggests recovery mechanisms incorporating a
 coordinated effort shared by two adjacent layers with periodic status
 updates.  Moreover, some of these recovery operations can be pre-
 assigned (on a per-link basis) to a certain layer, e.g., a given link
 will be recovered at the packet layer while another will be recovered
 at the optical layer.

7.4. Disjointness

 Having link and node diverse working and recovery LSPs/spans does not
 guarantee their complete disjointness.  Due to the common physical
 layer topology (passive), additional hierarchical concepts, such as
 the Shared Risk Link Group (SRLG), and mechanisms, such as SRLG
 diverse path computation, must be developed to provide complete
 working and recovery LSP/span disjointness (see [IPO-IMP] and

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 [RFC4202]).  Otherwise, a failure affecting the working LSP/span
 would also potentially affect the recovery LSP/span; one refers to
 such an event as "common failure".

7.4.1. SRLG Disjointness

 A Shared Risk Link Group (SRLG) is defined as the set of links
 sharing a common risk (such as a common physical resource such as a
 fiber link or a fiber cable).  For instance, a set of links L belongs
 to the same SRLG s, if they are provisioned over the same fiber link
 f.
 The SRLG properties can be summarized as follows:
 1) A link belongs to more than one SRLG if and only if it crosses one
    of the resources covered by each of them.
 2) Two links belonging to the same SRLG can belong individually to
    (one or more) other SRLGs.
 3) The SRLG set S of an LSP is defined as the union of the individual
    SRLG s of the individual links composing this LSP.
 SRLG disjointness is also applicable to LSPs:
    The LSP SRLG disjointness concept is based on the following
    postulate: an LSP (i.e., a 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 they do not cover simultaneously this SRLG s.
    Whilst the SRLG disjointness for LSPs with respect to a set S of
    SRLGs, is defined as follows: two LSPs are disjoint with respect
    to a set of SRLGs S if and only if the set of SRLGs that are
    common to both LSPs is disjoint from set S.
 The impact on recovery is noticeable: SRLG disjointness is a
 necessary (but not a sufficient) condition to ensure network
 survivability.  With respect to the physical network resources, a
 working-recovery LSP/span pair must be SRLG-disjoint in case of
 dedicated recovery type.  On the other hand, in case of shared
 recovery, a group of working LSP/spans must be mutually SRLG-disjoint
 in order to allow for a (single and common) shared recovery LSP that
 is itself SRLG-disjoint from each of the working LSPs/spans.

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8. Recovery Mechanisms Analysis

 In order to provide a structured analysis of the recovery mechanisms
 detailed in the previous sections, the following dimensions can be
 considered:
 1. Fast convergence (performance): provide a mechanism that
    aggregates multiple failures (implying fast failure detection and
    correlation mechanisms) and fast recovery decision independently
    of the number of failures occurring in the optical network (also
    implying a fast failure notification).
 2. Efficiency (scalability): minimize the switching time required for
    LSP/span recovery independently of the number of LSPs/spans being
    recovered (this implies efficient failure correlation, fast
    failure notification, and time-efficient recovery mechanisms).
 3. Robustness (availability): minimize the LSP/span downtime
    independently of the underlying topology of the transport plane
    (this implies a highly responsive recovery mechanism).
 4. Resource optimization (optimality): minimize the resource
    capacity, including LSPs/spans and nodes (switching capacity),
    required for recovery purposes; this dimension can also be
    referred to as optimizing the sharing degree of the recovery
    resources.
 5. Cost optimization: provide a cost-effective recovery type/scheme.
 However, these dimensions are either outside the scope of this
 document (such as cost optimization and recovery path computational
 aspects) or mutually conflicting.  For instance, it is obvious that
 providing a 1+1 LSP protection minimizes the LSP downtime (in case of
 failure) while being non-scalable and consuming recovery resource
 without enabling any extra-traffic.
 The following sections analyze the recovery phases and mechanisms
 detailed in the previous sections with respect to the dimensions
 described above in order to assess the GMPLS protocol suite
 capabilities and applicability.  In turn, this allows the evaluation
 of the potential need for further GMPLS signaling and routing
 extensions.

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

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

8.2. Efficiency (Recovery Switching Time)

 In general, the more pre-assignment/pre-planning of the recovery
 LSP/span, the more rapid the recovery is.  Because protection implies
 pre-assignment (and cross-connection) of the protection resources, in
 general, protection recovers faster than restoration.
 Span restoration is likely to be slower than most span protection
 types; however this greatly depends on the efficiency of the span
 restoration signaling.  LSP restoration with pre-signaled and pre-
 selected recovery resources is likely to be faster than fully dynamic
 LSP restoration, especially because of the elimination of any
 potential crankback during the recovery LSP establishment.
 If one excludes the crankback issue, the difference between dynamic
 and pre-planned restoration depends on the restoration path
 computation and selection time.  Since computational considerations
 are outside the scope of this document, it is up to the vendor to
 determine the average and maximum path computation time in different
 scenarios and to the operator to decide whether or not dynamic
 restoration is advantageous over pre-planned schemes that depend on
 the network environment.  This difference also depends on the
 flexibility provided by pre-planned restoration versus dynamic
 restoration.  Pre-planned restoration implies a somewhat limited
 number of failure scenarios (that can be due, for instance, to local
 storage capacity limitation).  Dynamic restoration enables on-demand
 path computation based on the information received through failure
 notification message, and as such, it is more robust with respect to
 the failure scenario scope.
 Moreover, LSP segment restoration, in particular, dynamic restoration
 (i.e., no path pre-computation, so none of the recovery resource is
 pre-reserved) will generally be faster than end-to-end LSP
 restoration.  However, local LSP restoration assumes that each LSP
 segment end-point has enough computational capacity to perform this
 operation while end-to-end LSP restoration requires only that LSP
 end-points provide this path computation capability.
 Recovery time objectives for SONET/SDH protection switching (not
 including time to detect failure) are specified in [G.841] at 50 ms,
 taking into account constraints on distance, number of connections

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 involved, and in the case of ring enhanced protection, number of
 nodes in the ring.  Recovery time objectives for restoration
 mechanisms have been proposed through a separate effort [RFC3386].

8.3. Robustness

 In general, the less pre-assignment (protection)/pre-planning
 (restoration) of the recovery LSP/span, the more robust the recovery
 type or scheme is to a variety of single failures, provided that
 adequate resources are available.  Moreover, the pre-selection of the
 recovery resources gives (in the case of multiple failure scenarios)
 less flexibility than no recovery resource pre-selection.  For
 instance, if failures occur that affect two LSPs sharing a common
 link along their restoration paths, then only one of these LSPs can
 be recovered.  This occurs unless the restoration path of at least
 one of these LSPs is re-computed, or the local resource assignment is
 modified on the fly.
 In addition, recovery types and schemes with pre-planned recovery
 resources (in particular, LSP/spans for protection and LSPs for
 restoration purposes) will not be able to recover from failures that
 simultaneously affect both the working and recovery LSP/span.  Thus,
 the recovery resources should ideally be as disjoint as possible
 (with respect to link, node, and SRLG) from the working ones, so that
 any single failure event will not affect both working and recovery
 LSP/span.  In brief, working and recovery resources must be fully
 diverse in order to guarantee that a given failure will not affect
 simultaneously the working and the recovery LSP/span.  Also, the risk
 of simultaneous failure of the working and the recovery LSPs can be
 reduced.  It is reduced by computing a new recovery path whenever a
 failure occurs along one of the recovery LSPs or by computing a new
 recovery path and provision the corresponding LSP whenever a failure
 occurs along a working LSP/span.  Both methods enable the network to
 maintain the number of available recovery path constant.
 The robustness of a recovery scheme is also determined by the amount
 of pre-reserved (i.e., signaled) recovery resources within a given
 shared resource pool: as the sharing degree of recovery resources
 increases, the recovery scheme becomes less robust to multiple
 LSP/span failure occurrences.  Recovery schemes, in particular
 restoration, with pre-signaled resource reservation (with or without
 pre-selection) should be capable of reserving an 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.

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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 the
 working entities.  Moreover, non-permanent bridging protection types
 allow (under normal conditions) for extra-traffic over the recovery
 resources.
 From this perspective, the following statements are true:
 1) 1+1 LSP/Span protection is the most resource-consuming protection
    type because it does not allow for any extra traffic.
 2) 1:1 LSP/span recovery requires dedicated recovery LSP/span
    allowing for extra traffic.
 3) 1:N and M:N LSP/span recovery require 1 (and M, respectively)
    recovery LSP/span (shared between the N working LSP/span) allowing
    for extra traffic.
 Obviously, 1+1 protection precludes, and 1:1 recovery does not allow
 for any recovery LSP/span sharing, whereas 1:N and M:N recovery do
 allow sharing of 1 (M, respectively) recovery LSP/spans between N
 working LSP/spans.  However, despite the fact that 1:1 LSP recovery
 precludes the sharing of the recovery LSP, the recovery schemes that
 can be built from it (e.g., (1:1)^n, see Section 5.4) do allow
 sharing of its recovery resources.  In addition, the flexibility in
 the usage of shared recovery resources (in particular, shared links)
 may be limited because of network topology restrictions, e.g., fixed
 ring topology for traditional enhanced protection schemes.
 On the other hand, when using LSP restoration with pre-signaled
 resource reservation, the amount of reserved restoration capacity is
 determined by the local bandwidth reservation policies.  In LSP
 restoration schemes with re-provisioning, a pool of spare resources
 can be defined from which all resources are selected after failure
 occurrence for the purpose of restoration path computation.  The
 degree to which restoration schemes allow sharing amongst multiple
 independent failures is then directly inferred from the size of the
 resource pool.  Moreover, in all restoration schemes, spare resources
 can be used to carry preemptible traffic (thus over preemptible
 LSP/span) when the corresponding resources have not been committed
 for LSP/span recovery purposes.
 From this, it clearly follows that less recovery resources (i.e.,
 LSP/spans and switching capacity) have to be allocated to a shared

Papadimitriou & Mannie Informational [Page 36] RFC 4428 GMPLS Recovery Mechanisms March 2006

 recovery resource pool if a greater sharing degree is allowed.  Thus,
 the network survivability level is determined by the policy that
 defines the amount of shared recovery resources and by the maximum
 sharing degree allowed for these recovery resources.

8.4.1. Recovery Resource Sharing

 When recovery resources are shared over several LSP/Spans, the use of
 the Maximum Reservable Bandwidth, the Unreserved Bandwidth, and the
 Maximum LSP Bandwidth (see [RFC4202]) provides the information needed
 to obtain the optimization of the network resources allocated for
 shared recovery purposes.
 The Maximum Reservable Bandwidth is defined as the Maximum Link
 Bandwidth but it may be greater in case of link over-subscription.
 The Unreserved Bandwidth (at priority p) is defined as the bandwidth
 not yet reserved on a given TE link (its initial value for each
 priority p corresponds to the Maximum Reservable Bandwidth).  Last,
 the Maximum LSP Bandwidth (at priority p) is defined as the smaller
 of Unreserved Bandwidth (at priority p) and Maximum Link Bandwidth.
 Here, one generally considers a recovery resource sharing degree (or
 ratio) to globally optimize the shared recovery resource usage.  The
 distribution of the bandwidth utilization per TE link can be inferred
 from the per-priority bandwidth pre-allocation.  By using the Maximum
 LSP Bandwidth and the Maximum Reservable Bandwidth, the amount of
 (over-provisioned) resources that can be used for shared recovery
 purposes is known from the IGP.
 In order to analyze this behavior, we define the difference between
 the Maximum Reservable Bandwidth (in the present case, this value is
 greater than the Maximum Link Bandwidth) and the Maximum LSP
 Bandwidth per TE link i as the Maximum Shareable Bandwidth or
 max_R[i].  Within this quantity, the amount of bandwidth currently
 allocated for shared recovery per TE link i is defined as R[i].  Both
 quantities are expressed in terms of discrete bandwidth units (and
 thus, the Minimum LSP Bandwidth is of one bandwidth unit).
 The knowledge of this information available per TE link can be
 exploited in order to optimize the usage of the resources allocated
 per TE link for shared recovery.  If one refers to r[i] as the actual
 bandwidth per TE link i (in terms of discrete bandwidth units)
 committed for shared recovery, then the following quantity must be
 maximized over the potential TE link candidates:
      sum {i=1}^N [(R{i} - r{i})/(t{i} - b{i})]

Papadimitriou & Mannie Informational [Page 37] RFC 4428 GMPLS Recovery Mechanisms March 2006

      or equivalently: sum {i=1}^N [(R{i} - r{i})/r{i}]
      with R{i} >= 1 and r{i} >= 1 (in terms of per component
      bandwidth unit)
 In this formula, N is the total number of links traversed by a given
 LSP, t[i] the Maximum Link Bandwidth per TE link i, and b[i] the sum
 per TE link i of the bandwidth committed for working LSPs and other
 recovery LSPs (thus except "shared bandwidth" LSPs).  The quantity
 [(R{i} - r{i})/r{i}] is defined as the Shared (Recovery) Bandwidth
 Ratio per TE link i.  In addition, TE links for which R[i] reaches
 max_R[i] or for which r[i] = 0 are pruned during shared recovery path
 computation as well as TE links for which max_R[i] = r[i] that can
 simply not be shared.
 More generally, one can draw the following mapping between the
 available bandwidth at the transport and control plane level:
  1. ———- Max Reservable Bandwidth

| —– ^

                              |R -----  |
                              |  -----  |
                               - -----  |max_R
                                 -----  |
 --------  TE link Capacity    - ------ | - Maximum TE Link Bandwidth
 -----                        |r -----  v
 -----     <------ b ------>   - ---------- Maximum LSP Bandwidth
 -----                           -----
 -----                           -----
 -----                           -----
 -----                           -----
 -----                           ----- <--- Minimum LSP Bandwidth
 -------- 0                      ---------- 0
 Note that the above approach does not require the flooding of any per
 LSP information or any detailed distribution of the bandwidth
 allocation per component link or individual ports or even any per-
 priority shareable recovery bandwidth information (using a dedicated
 sub-TLV).  The latter would provide the same capability as the
 already defined Maximum LSP bandwidth per-priority information.  This
 approach is referred to as a Partial (or Aggregated) Information
 Routing as described in [KODIALAM1] and [KODIALAM2].  They show that
 the difference obtained with a Full (or Complete) Information Routing
 approach (where for the whole set of working and recovery LSPs, the
 amount of bandwidth units they use per-link is known at each node and
 for each link) is clearly negligible.  The Full Information Routing

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 approach is detailed in [GLI].  Note also that both approaches rely
 on the deterministic knowledge (at different degrees) of the network
 topology and resource usage status.
 Moreover, extending the GMPLS signaling capabilities can enhance the
 Partial Information Routing approach.  It is enhanced by allowing
 working-LSP-related information and, in particular, its path
 (including link and node identifiers) to be exchanged with the
 recovery LSP request.  This enables more efficient admission control
 at upstream nodes of shared recovery resources, and in particular,
 links (see Section 8.4.3).

8.4.2. Recovery Resource Sharing and SRLG Recovery

 Resource shareability can also be maximized with respect to the
 number of times each SRLG is protected by a recovery resource (in
 particular, a shared TE link) and methods can be considered for
 avoiding contention of the shared recovery resources in case of
 single SRLG failure.  These methods enable the sharing of recovery
 resources between two (or more) recovery LSPs, if their respective
 working LSPs are mutually disjoint with respect to link, node, and
 SRLGs.  Then, a single failure does not simultaneously disrupt
 several (or at least two) working LSPs.
 For instance, [BOUILLET] shows that the Partial Information Routing
 approach can be extended to cover recovery resource shareability with
 respect to SRLG recoverability (i.e., the number of times each SRLG
 is recoverable).  By flooding this aggregated information per TE
 link, path computation and selection of SRLG-diverse recovery LSPs
 can be optimized with respect to the sharing of recovery resource
 reserved on each TE link.  This yields a performance difference of
 less than 5%, which is negligible compared to the corresponding Full
 Information Flooding approach (see [GLI]).
 For this purpose, additional extensions to [RFC4202] in support of
 path computation for shared mesh recovery have been often considered
 in the literature.  TE link attributes would include, among others,
 the current number of recovery LSPs sharing the recovery resources
 reserved on the TE link, and the current number of SRLGs recoverable
 by this amount of (shared) recovery resources reserved on the TE
 link.  The latter is equivalent to the current number of SRLGs that
 will be recovered by the recovery LSPs sharing the recovery resource
 reserved on the TE link.  Then, if explicit SRLG recoverability is
 considered, a TE link attribute would be added that includes the
 explicit list of SRLGs (recoverable by the shared recovery resource
 reserved on the TE link) and their respective shareable recovery
 bandwidths.  The latter information is equivalent to the shareable
 recovery bandwidth per SRLG (or per group of SRLGs), which implies

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

8.4.3. Recovery Resource Sharing, SRLG Disjointness and Admission

      Control
 Admission control is a strict requirement to be fulfilled by nodes
 giving access to shared links.  This can be illustrated using the
 following network topology:
    A ------ C ====== D
    |        |        |
    |        |        |
    |        B        |
    |        |        |
    |        |        |
     ------- E ------ F
 Node A creates a working LSP to D (A-C-D), B creates simultaneously a
 working LSP to D (B-C-D) and a recovery LSP (B-E-F-D) to the same
 destination.  Then, A decides to create a recovery LSP to D (A-E-F-
 D), but since the C-D span carries both working LSPs, node E should
 either assign a dedicated resource for this recovery LSP or reject
 this request if the C-D span has already reached its maximum recovery
 bandwidth sharing ratio.  In the latter case, C-D span failure would
 imply that one of the working LSP would not be recoverable.
 Consequently, node E must have the required information to perform
 admission control for the recovery LSP requests it processes
 (implying for instance, that the path followed by the working LSP is
 carried with the corresponding recovery LSP request).  If node E can
 guarantee that the working LSPs (A-C-D and B-C-D) are SRLG disjoint
 over the C-D span, it may securely accept the incoming recovery LSP
 request and assign to the recovery LSPs (A-E-F-D and B-E-F-D) the

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 same resources on the link E-F.  This may occur if the link E-F has
 not yet reached its maximum recovery bandwidth sharing ratio.  In
 this example, one assumes that the node failure probability is
 negligible compared to the link failure probability.
 To achieve this, the path followed by the working LSP is transported
 with the recovery LSP request and examined at each upstream node of
 potentially shareable links.  Admission control is performed using
 the interface identifiers (included in the path) to retrieve in the
 TE DataBase the list of SRLG IDs associated to each of the working
 LSP links.  If the working LSPs (A-C-D and B-C-D) have one or more
 link or SRLG ID in common (in this example, one or more SRLG id in
 common over the span C-D), node E should not assign the same resource
 over link E-F to the recovery LSPs (A-E-F-D and B-E-F-D).  Otherwise,
 one of these working LSPs would not be recoverable if C-D span
 failure occurred.
 There are some issues related to this method; the major one is the
 number of SRLG IDs that a single link can cover (more than 100, in
 complex environments).  Moreover, when using link bundles, this
 approach may generate the rejection of some recovery LSP requests.
 This occurs when the SRLG sub-TLV corresponding to a link bundle
 includes the union of the SRLG id list of all the component links
 belonging to this bundle (see [RFC4202] and [RFC4201]).
 In order to overcome this specific issue, an additional mechanism may
 consist of querying the nodes where the information would be
 available (in this case, node E would query C).  The main drawback of
 this method is that (in addition to the dedicated mechanism(s) it
 requires) it may become complex when several common nodes are
 traversed by the working LSPs.  Therefore, when using link bundles,
 solving this issue is closely related to the sequence of the recovery
 operations.  Per-component flooding of SRLG identifiers would deeply
 impact the scalability of the link state routing protocol.
 Therefore, one may rely on the usage of an on-line accessible network
 management system.

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9. Summary and Conclusions

 The following table summarizes the different recovery types and
 schemes analyzed throughout this document.
  1. ——————————————————————-

| Path Search (computation and selection)

  1. ——————————————————————-

| Pre-planned (a) | Dynamic (b)

  1. ——————————————————————-

| | faster recovery | Does not apply

        |   | less flexible              |
        | 1 | less robust                |
        |   | most resource-consuming    |
 Path   |   |                            |
 Setup   ------------------------------------------------------------
        |   | relatively fast recovery   | Does not apply
        |   | relatively flexible        |
        | 2 | relatively robust          |
        |   | resource consumption       |
        |   |  depends on sharing degree |
         ------------------------------------------------------------
        |   | relatively fast recovery   | less faster (computation)
        |   | more flexible              | most flexible
        | 3 | relatively robust          | most robust
        |   | less resource-consuming    | least resource-consuming
        |   |  depends on sharing degree |
 --------------------------------------------------------------------
 1a. Recovery LSP setup (before failure occurrence) with resource
     reservation (i.e., signaling) and selection is referred to as LSP
     protection.
 2a. Recovery LSP setup (before failure occurrence) with resource
     reservation (i.e., signaling) and with resource pre-selection is
     referred to as pre-planned LSP re-routing with resource pre-
     selection.  This implies only recovery LSP activation after
     failure occurrence.
 3a. Recovery LSP setup (before failure occurrence) with resource
     reservation (i.e., signaling) and without resource selection is
     referred to as pre-planned LSP re-routing without resource pre-
     selection.  This implies recovery LSP activation and resource
     (i.e., label) selection after failure occurrence.
 3b. Recovery LSP setup after failure occurrence is referred to as to
     as LSP re-routing, which is full when recovery LSP path
     computation occurs after failure occurrence.

Papadimitriou & Mannie Informational [Page 42] RFC 4428 GMPLS Recovery Mechanisms March 2006

 Thus, the term pre-planned refers to recovery LSP path pre-
 computation, signaling (reservation), and a priori resource selection
 (optional), but not cross-connection.  Also, the shared-mesh recovery
 scheme can be viewed as a particular case of 2a) and 3a), using the
 additional constraint described in Section 8.4.3.
 The implementation of these recovery mechanisms requires only
 considering extensions to GMPLS signaling protocols (i.e., [RFC3471]
 and [RFC3473]).  These GMPLS signaling extensions should mainly focus
 in delivering (1) recovery LSP pre-provisioning for the cases 1a, 2a,
 and 3a, (2) LSP failure notification, (3) recovery LSP switching
 action(s), and (4) reversion mechanisms.
 Moreover, the present analysis (see Section 8) shows that no GMPLS
 routing extensions are expected to efficiently implement any of these
 recovery types and schemes.

10. Security Considerations

 This document does not introduce any additional security issue or
 imply any specific security consideration from [RFC3945] to the
 current RSVP-TE GMPLS signaling, routing protocols (OSPF-TE, IS-IS-
 TE) or network management protocols.
 However, the authorization of requests for resources by GMPLS-capable
 nodes should determine whether a given party, presumably already
 authenticated, has a right to access the requested resources.  This
 determination is typically a matter of local policy control, for
 example, by setting limits on the total bandwidth made available to
 some party in the presence of resource contention.  Such policies may
 become quite complex as the number of users, types of resources, and
 sophistication of authorization rules increases.  This is
 particularly the case for recovery schemes that assume pre-planned
 sharing of recovery resources, or contention for resources in case of
 dynamic re-routing.
 Therefore, control elements should match the requests against the
 local authorization policy.  These control elements must be capable
 of making decisions based on the identity of the requester, as
 verified cryptographically and/or topologically.

11. Acknowledgements

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

Papadimitriou & Mannie Informational [Page 43] RFC 4428 GMPLS Recovery Mechanisms March 2006

 Thanks also to Adrian Farrel for the thorough review of the document.

12. References

12.1. Normative References

 [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3471]    Berger, L., "Generalized Multi-Protocol Label Switching
              (GMPLS) Signaling Functional Description", RFC 3471,
              January 2003.
 [RFC3473]    Berger, L., "Generalized Multi-Protocol Label Switching
              (GMPLS) Signaling Resource ReserVation Protocol-Traffic
              Engineering (RSVP-TE) Extensions", RFC 3473, January
              2003.
 [RFC3945]    Mannie, E., "Generalized Multi-Protocol Label Switching
              (GMPLS) Architecture", RFC 3945, October 2004.
 [RFC4201]    Kompella, K., Rekhter, Y., and L. Berger, "Link Bundling
              in MPLS Traffic Engineering (TE)", RFC 4201, October
              2005.
 [RFC4202]    Kompella, K., Ed. and Y. Rekhter, Ed., "Routing
              Extensions in Support of Generalized Multi-Protocol
              Label Switching (GMPLS)", RFC 4202, October 2005.
 [RFC4204]    Lang, J., Ed., "Link Management Protocol (LMP)", RFC
              4204, October 2005.
 [RFC4209]    Fredette, A., Ed. and J. Lang, Ed., "Link Management
              Protocol (LMP) for Dense Wavelength Division
              Multiplexing (DWDM) Optical Line Systems", RFC 4209,
              October 2005.
 [RFC4427]    Mannie E., Ed. and D. Papadimitriou, Ed., "Recovery
              (Protection and Restoration) Terminology for Generalized
              Multi-Protocol Label Switching (GMPLS)", RFC 4427, March
              2006.

12.2. Informative References

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

Papadimitriou & Mannie Informational [Page 44] RFC 4428 GMPLS Recovery Mechanisms March 2006

 [DEMEESTER]  P. Demeester, et al., "Resilience in Multilayer
              Networks," IEEE Communications Magazine, Vol. 37, No. 8,
              pp. 70-76, August 1998.
 [GLI]        G. Li, et al., "Efficient Distributed Path Selection for
              Shared Restoration Connections," IEEE Infocom 2002, New
              York City, June 2002.
 [IPO-IMP]    Strand, J. and A. Chiu, "Impairments and Other
              Constraints on Optical Layer Routing", RFC 4054, May
              2005.
 [KODIALAM1]  M. Kodialam and T.V. Lakshman, "Restorable Dynamic
              Quality of Service Routing," IEEE Communications
              Magazine, pp. 72-81, June 2002.
 [KODIALAM2]  M. Kodialam and T.V. Lakshman, "Dynamic Routing of
              Restorable Bandwidth-Guaranteed Tunnels using Aggregated
              Network Resource Usage Information," IEEE/ ACM
              Transactions on Networking, pp. 399-410, June 2003.
 [MANCHESTER] J. Manchester, P. Bonenfant and C. Newton, "The
              Evolution of Transport Network Survivability," IEEE
              Communications Magazine, August 1999.
 [RFC3386]    Lai, W. and D. McDysan, "Network Hierarchy and
              Multilayer Survivability", RFC 3386, November 2002.
 [T1.105]     ANSI, "Synchronous Optical Network (SONET): Basic
              Description Including Multiplex Structure, Rates, and
              Formats," ANSI T1.105, January 2001.
 [WANG]       J. Wang, L. Sahasrabuddhe, and B. Mukherjee, "Path vs.
              Subpath vs. Link Restoration for Fault Management in
              IP-over-WDM Networks: Performance Comparisons Using
              GMPLS Control Signaling," IEEE Communications Magazine,
              pp. 80-87, November 2002.
 For information on the availability of the following documents,
 please see http://www.itu.int
 [G.707]      ITU-T, "Network Node Interface for the Synchronous
              Digital Hierarchy (SDH)," Recommendation G.707, October
              2000.
 [G.709]      ITU-T, "Network Node Interface for the Optical Transport
              Network (OTN)," Recommendation G.709, February 2001 (and
              Amendment no.1, October 2001).

Papadimitriou & Mannie Informational [Page 45] RFC 4428 GMPLS Recovery Mechanisms March 2006

 [G.783]      ITU-T, "Characteristics of Synchronous Digital Hierarchy
              (SDH) Equipment Functional Blocks," Recommendation
              G.783, October 2000.
 [G.798]      ITU-T, "Characteristics of optical transport network
              hierarchy equipment functional block," Recommendation
              G.798, June 2004.
 [G.806]      ITU-T, "Characteristics of Transport Equipment -
              Description Methodology and Generic Functionality",
              Recommendation G.806, October 2000.
 [G.841]      ITU-T, "Types and Characteristics of SDH Network
              Protection Architectures," Recommendation G.841, October
              1998.
 [G.842]      ITU-T, "Interworking of SDH network protection
              architectures," Recommendation G.842, October 1998.
 [G.874]      ITU-T, "Management aspects of the optical transport
              network element," Recommendation G.874, November 2001.

Editors' Addresses

 Dimitri Papadimitriou
 Alcatel
 Francis Wellesplein, 1
 B-2018 Antwerpen, Belgium
 Phone:  +32 3 240-8491
 EMail: dimitri.papadimitriou@alcatel.be
 Eric Mannie
 Perceval
 Rue Tenbosch, 9
 1000 Brussels
 Belgium
 Phone: +32-2-6409194
 EMail: eric.mannie@perceval.net

Papadimitriou & Mannie Informational [Page 46] RFC 4428 GMPLS Recovery Mechanisms March 2006

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Papadimitriou & Mannie Informational [Page 47]

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