<pre>Network Working Group                                     E. Mannie, Ed.
Request for Comments: 4427                                      Perceval
Category: Informational                            D. Papadimitriou, Ed.
                                                                 Alcatel
                                                              March 2006


           <span class="h1">Recovery (Protection and Restoration) Terminology</span>
        <span class="h1">for Generalized Multi-Protocol Label Switching (GMPLS)</span>

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 defines a common terminology for Generalized Multi-
   Protocol Label Switching (GMPLS)-based recovery mechanisms (i.e.,
   protection and restoration).  The terminology is independent of the
   underlying transport technologies covered by GMPLS.

























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Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
   <a href="#section-2">2</a>. Contributors ....................................................<a href="#page-4">4</a>
   <a href="#section-3">3</a>. Conventions Used in this Document ...............................<a href="#page-5">5</a>
   <a href="#section-4">4</a>. Recovery Terminology Common to Protection and Restoration .......<a href="#page-5">5</a>
      <a href="#section-4.1">4.1</a>. Working and Recovery LSP/Span ..............................<a href="#page-6">6</a>
      <a href="#section-4.2">4.2</a>. Traffic Types ..............................................<a href="#page-6">6</a>
      <a href="#section-4.3">4.3</a>. LSP/Span Protection and Restoration ........................<a href="#page-6">6</a>
      <a href="#section-4.4">4.4</a>. Recovery Scope .............................................<a href="#page-7">7</a>
      <a href="#section-4.5">4.5</a>. Recovery Domain ............................................<a href="#page-8">8</a>
      <a href="#section-4.6">4.6</a>. Recovery Types .............................................<a href="#page-8">8</a>
      <a href="#section-4.7">4.7</a>. Bridge Types ..............................................<a href="#page-10">10</a>
      <a href="#section-4.8">4.8</a>. Selector Types ............................................<a href="#page-10">10</a>
      <a href="#section-4.9">4.9</a>. Recovery GMPLS Nodes ......................................<a href="#page-11">11</a>
      <a href="#section-4.10">4.10</a>. Switch-over Mechanism ....................................<a href="#page-11">11</a>
      <a href="#section-4.11">4.11</a>. Reversion operations .....................................<a href="#page-11">11</a>
      <a href="#section-4.12">4.12</a>. Failure Reporting ........................................<a href="#page-12">12</a>
      <a href="#section-4.13">4.13</a>. External commands ........................................<a href="#page-12">12</a>
      <a href="#section-4.14">4.14</a>. Unidirectional versus Bi-Directional Recovery Switching ..13
      <a href="#section-4.15">4.15</a>. Full versus Partial Span Recovery Switching ..............<a href="#page-14">14</a>
      <a href="#section-4.16">4.16</a>. Recovery Schemes Related Time and Durations ..............<a href="#page-14">14</a>
      <a href="#section-4.17">4.17</a>. Impairment ...............................................<a href="#page-15">15</a>
      <a href="#section-4.18">4.18</a>. Recovery Ratio ...........................................<a href="#page-15">15</a>
      <a href="#section-4.19">4.19</a>. Hitless Protection Switch-over ...........................<a href="#page-15">15</a>
      <a href="#section-4.20">4.20</a>. Network Survivability ....................................<a href="#page-15">15</a>
      <a href="#section-4.21">4.21</a>. Survivable Network .......................................<a href="#page-16">16</a>
      <a href="#section-4.22">4.22</a>. Escalation ...............................................<a href="#page-16">16</a>
   <a href="#section-5">5</a>. Recovery Phases ................................................<a href="#page-16">16</a>
      <a href="#section-5.1">5.1</a>. Entities Involved During Recovery .........................<a href="#page-17">17</a>
   <a href="#section-6">6</a>. Protection Schemes .............................................<a href="#page-17">17</a>
      <a href="#section-6.1">6.1</a>. 1+1 Protection ............................................<a href="#page-18">18</a>
      <a href="#section-6.2">6.2</a>. 1:N (N &gt;= 1) Protection ...................................<a href="#page-18">18</a>
      <a href="#section-6.3">6.3</a>. M:N (M, N &gt; 1, N &gt;= M) Protection .........................<a href="#page-18">18</a>
      <a href="#section-6.4">6.4</a>. Notes on Protection Schemes ...............................<a href="#page-19">19</a>
   <a href="#section-7">7</a>. Restoration Schemes ............................................<a href="#page-19">19</a>
      <a href="#section-7.1">7.1</a>. Pre-Planned LSP Restoration ...............................<a href="#page-19">19</a>
           <a href="#section-7.1.1">7.1.1</a>. Shared-Mesh Restoration ............................<a href="#page-19">19</a>
      <a href="#section-7.2">7.2</a>. LSP Restoration ...........................................<a href="#page-20">20</a>
           <a href="#section-7.2.1">7.2.1</a>. Hard LSP Restoration ...............................<a href="#page-20">20</a>
           <a href="#section-7.2.2">7.2.2</a>. Soft LSP Restoration ...............................<a href="#page-20">20</a>
   <a href="#section-8">8</a>. Security Considerations ........................................<a href="#page-20">20</a>
   <a href="#section-9">9</a>. References .....................................................<a href="#page-20">20</a>
      <a href="#section-9.1">9.1</a>. Normative References ......................................<a href="#page-20">20</a>
      <a href="#section-9.2">9.2</a>. Informative References ....................................<a href="#page-20">20</a>
   <a href="#section-10">10</a>. Acknowledgements ..............................................<a href="#page-21">21</a>





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<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   This document defines a common terminology for Generalized Multi-
   Protocol Label Switching (GMPLS)-based recovery mechanisms (i.e.,
   protection and restoration).

   The terminology proposed in this document is independent of the
   underlying transport technologies and borrows from the G.808.1 ITU-T
   Recommendation [<a href="#ref-G.808.1" title="&quot;Generic Protection Switching - Linear trail and subnetwork protection,&quot;">G.808.1</a>] and from the G.841 ITU-T Recommendation
   [<a href="#ref-G.841" title="&quot;Types and Characteristics of SDH Network Protection Architectures,&quot;">G.841</a>].  The restoration terminology and concepts have been gathered
   from numerous sources including IETF documents.

   In the context of this document, the term "recovery" denotes both
   protection and restoration.  The specific terms "protection" and
   "restoration" will only be used when differentiation is required.

   This document focuses on the terminology for the recovery of Label
   Switched Paths (LSPs) controlled by a GMPLS control plane.  The
   proposed terminology applies to end-to-end, segment, and span (i.e.,
   link) recovery.  Note that the terminology for recovery of the
   control plane itself is not in the scope of this document.

   Protection and restoration of switched LSPs under tight time
   constraints is a challenging problem.  This is particularly relevant
   to optical networks that consist of Time Division Multiplex (TDM)
   and/or all-optical (photonic) cross-connects referred to as GMPLS
   nodes (or simply nodes, or even sometimes "Label Switching Routers,
   or LSRs") connected in a general topology [<a href="./rfc3945" title="&quot;Generalized Multi-Protocol Label Switching (GMPLS) Architecture&quot;">RFC3945</a>].

   Recovery typically involves the activation of a recovery (or
   alternate) LSP when a failure is encountered in the working LSP.

   A working or recovery LSP is characterized by an ingress interface,
   an egress interface, and a set of intermediate nodes and spans
   through which the LSP is routed.  The working and recovery LSPs are
   typically resource disjoint (e.g., node and/or span disjoint).  This
   ensures that a single failure will not affect both the working and
   recovery LSPs.

   A bi-directional span between neighboring nodes is usually realized
   as a pair of unidirectional spans.  Therefore, the end-to-end path
   for a bi-directional LSP consists of a series of bi-directional
   segments (i.e., Sub-Network Connections, or SNCs, in the ITU-T
   terminology) between the source and destination nodes, traversing
   intermediate nodes.






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<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Contributors</span>

   This document is the result of a joint effort by the CCAMP Working
   Group Protection and Restoration design team.  The following are the
   authors that contributed to the present document:

   Deborah Brungard (AT&amp;T)
   Rm. D1-3C22 - 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&amp;T)
   180 Park Avenue,
   Florham Park, NJ 07932, USA

   EMail: gli@research.att.com


   Eric Mannie
   Perceval
   Rue Tenbosch, 9
   1000 Brussels
   Belgium

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


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

   EMail: dimitri.papadimitriou@alcatel.be




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   Bala Rajagopalan
   Microsoft India Development Center
   Hyderabad, India

   EMail: balar@microsoft.com


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

   EMail: yakov@juniper.net

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Conventions Used in this Document</span>

   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 [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Recovery Terminology Common to Protection and Restoration</span>

   This section defines the following general terms common to both
   protection and restoration (i.e., recovery).  In addition, most of
   these terms apply to end-to-end, segment, and span LSP recovery.
   Note that span recovery does not protect the nodes at each end of the
   span, otherwise end-to-end or segment LSP recovery should be used.

   The terminology and the definitions were originally taken from
   [<a href="#ref-G.808.1" title="&quot;Generic Protection Switching - Linear trail and subnetwork protection,&quot;">G.808.1</a>].  However, for generalization, the following language,
   which is not directly related to recovery, has been adapted to GMPLS
   and the common IETF terminology:

   An LSP is used as a generic term to designate either an SNC (Sub-
   Network Connection) or an NC (Network Connection) in ITU-T
   terminology.  The ITU-T uses the term transport entity to designate
   either a link, an SNC, or an NC.  The term "Traffic" is used instead
   of "Traffic Signal".  The term protection or restoration "scheme" is
   used instead of protection or restoration "architecture".

   The reader is invited to read [<a href="#ref-G.841" title="&quot;Types and Characteristics of SDH Network Protection Architectures,&quot;">G.841</a>] and [<a href="#ref-G.808.1" title="&quot;Generic Protection Switching - Linear trail and subnetwork protection,&quot;">G.808.1</a>] for references to
   SDH protection and Generic Protection Switching terminology,
   respectively.  Note that restoration is not in the scope of
   [<a href="#ref-G.808.1" title="&quot;Generic Protection Switching - Linear trail and subnetwork protection,&quot;">G.808.1</a>].








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<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Working and Recovery LSP/Span</span>

   A working LSP/span is an LSP/span transporting "normal" user traffic.
   A recovery LSP/span is an LSP/span used to transport "normal" user
   traffic when the working LSP/span fails.  Additionally, the recovery
   LSP/span may transport "extra" user traffic (i.e., pre-emptable
   traffic) when normal traffic is carried over the working LSP/span.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Traffic Types</span>

   The different types of traffic that can be transported over an
   LSP/span, in the context of this document, are defined hereafter:

   A. Normal traffic:

   User traffic that may be protected by two alternative LSPs/spans (the
   working and recovery LSPs/spans).

   B. Extra traffic:

   User traffic carried over recovery resources (e.g., a recovery
   LSP/span) when these resources are not being used for the recovery of
   normal traffic (i.e., when the recovery resources are in standby
   mode).  When the recovery resources are required to recover normal
   traffic from the failed working LSP/span, the extra traffic is pre-
   empted.  Extra traffic is not protected by definition, but may be
   restored.  Moreover, extra traffic does not need to commence or be
   terminated at the ends of the LSPs/spans that it uses.

   C. Null traffic:

   Traffic carried over the recovery LSP/span if it is not used to carry
   normal or extra traffic.  Null traffic can be any kind of traffic
   that conforms to the signal structure of the specific layer, and it
   is ignored (not selected) at the egress of the recovery LSP/span.

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  LSP/Span Protection and Restoration</span>

   The following subtle distinction is generally made between the terms
   "protection" and "restoration", even though these terms are often
   used interchangeably [<a href="./rfc3386" title="&quot;Network Hierarchy and Multilayer Survivability&quot;">RFC3386</a>].

   The distinction between protection and restoration is made based on
   the resource allocation done during the recovery LSP/span
   establishment.  The distinction between different types of
   restoration is made based on the level of route computation,
   signaling, and resource allocation during the restoration LSP/span
   establishment.



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   A. LSP/Span Protection

   LSP/span protection denotes the paradigm whereby one or more
   dedicated protection LSP(s)/span(s) is/are fully established to
   protect one or more working LSP(s)/span(s).

   For a protection LSP, this implies that route computation took place,
   that the LSP was fully signaled all the way, and that its resources
   were fully selected (i.e., allocated) and cross-connected between the
   ingress and egress nodes.

   For a protection span, this implies that the span has been selected
   and reserved for protection.

   Indeed, it means that no signaling takes place to establish the
   protection LSP/span when a failure occurs.  However, various other
   kinds of signaling may take place between the ingress and egress
   nodes for fault notification, to synchronize their use of the
   protection LSP/span, for reversion, etc.

   B. LSP/Span Restoration

   LSP/span restoration denotes the paradigm whereby some restoration
   resources may be pre-computed, signaled, and selected a priori, but
   not cross-connected to restore a working LSP/span.  The complete
   establishment of the restoration LSP/span occurs only after a failure
   of the working LSP/span, and requires some additional signaling.

   Both protection and restoration require signaling.  Signaling to
   establish the recovery resources and signaling associated with the
   use of the recovery LSP(s)/span(s) are needed.

<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>.  Recovery Scope</span>

   Recovery can be applied at various levels throughout the network.  An
   LSP may be subject to local (span), segment, and/or end-to-end
   recovery.

   Local (span) recovery refers to the recovery of an LSP over a link
   between two nodes.

   End-to-end recovery refers to the recovery of an entire LSP from its
   source (ingress node end-point) to its destination (egress node end-
   point).

   Segment recovery refers to the recovery over a portion of the network
   of a segment LSP (i.e., an SNC in the ITU-T terminology) of an end-
   to-end LSP.  Such recovery protects against span and/or node failure



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   over a particular portion of the network that is traversed by an
   end-to-end LSP.

<span class="h3"><a class="selflink" id="section-4.5" href="#section-4.5">4.5</a>.  Recovery Domain</span>

   A recovery domain is defined as a set of nodes and spans, over which
   one or more recovery schemes are provided.  A recovery domain served
   by one single recovery scheme is referred to as a "single recovery
   domain", while a recovery domain served by multiple recovery schemes
   is referred to as a "multi recovery domain".

   The recovery operation is contained within the recovery domain.  A
   GMPLS recovery domain must be entirely contained within a GMPLS
   domain.  A GMPLS domain (defined as a set of nodes and spans
   controlled by GMPLS) may contain multiple recovery domains.

<span class="h3"><a class="selflink" id="section-4.6" href="#section-4.6">4.6</a>.  Recovery Types</span>

   The different recovery types can be classified depending on the
   number of recovery LSPs/spans that are protecting a given number of
   working LSPs/spans.  The definitions given hereafter are from the
   point of view of a working LSP/span that needs to be protected by a
   recovery scheme.

   A. 1+1 type: dedicated protection

   One dedicated protection LSP/span protects exactly one working
   LSP/span, and the normal traffic is permanently duplicated at the
   ingress node on both the working and protection LSPs/spans.  No extra
   traffic can be carried over the protection LSP/span.

   This type is applicable to LSP/span protection, but not to LSP/span
   restoration.

   B. 0:1 type: unprotected

   No specific recovery LSP/span protects the working LSP/span.
   However, the working LSP/span can potentially be restored through any
   alternate available route/span, with or without any pre-computed
   restoration route.  Note that no resources are pre-established for
   this recovery type.

   This type is applicable to LSP/span restoration, but not to LSP/span
   protection.  Span restoration can be achieved, for instance, by
   moving all the LSPs transported over a failed span to a dynamically
   selected span.





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   C. 1:1 type: dedicated recovery with extra traffic

   One specific recovery LSP/span protects exactly one specific working
   LSP/span, but the normal traffic is transmitted over only one LSP
   (working or recovery) at a time.  Extra traffic can be transported
   using the recovery LSP/span resources.

   This type is applicable to LSP/span protection and LSP restoration,
   but not to span restoration.

   D. 1:N (N &gt; 1) type: shared recovery with extra traffic

   A specific recovery LSP/span is dedicated to the protection of up to
   N working LSPs/spans.  The set of working LSPs/spans is explicitly
   identified.  Extra traffic can be transported over the recovery
   LSP/span.  All these LSPs/spans must start and end at the same nodes.

   Sometimes, the working LSPs/spans are assumed to be resource disjoint
   in the network so that they do not share any failure probability, but
   this is not mandatory.  Obviously, if more than one working LSP/span
   in the set of N are affected by some failure(s) at the same time, the
   traffic on only one of these failed LSPs/spans may be recovered over
   the recovery LSP/span.  Note that N can be arbitrarily large (i.e.,
   infinite).  The choice of N is a policy decision.

   This type is applicable to LSP/span protection and LSP restoration,
   but not to span restoration.

   Note: a shared recovery where each recovery resource can be shared by
   a maximum of X LSPs/spans is not defined as a recovery type but as a
   recovery scheme.  The choice of X is a network resource management
   policy decision.

   E. M:N (M, N &gt; 1, N &gt;= M) type:

   A set of M specific recovery LSPs/spans protects a set of up to N
   specific working LSPs/spans.  The two sets are explicitly identified.
   Extra traffic can be transported over the M recovery LSPs/spans when
   available.  All the LSPs/spans must start and end at the same nodes.

   Sometimes, the working LSPs/spans are assumed to be resource disjoint
   in the network so that they do not share any failure probability, but
   this is not mandatory.  Obviously, if several working LSPs/spans in
   the set of N are concurrently affected by some failure(s), the
   traffic on only M of these failed LSPs/spans may be recovered.  Note
   that N can be arbitrarily large (i.e., infinite).  The choice of N
   and M is a policy decision.




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   This type is applicable to LSP/span protection and LSP restoration,
   but not to span restoration.

<span class="h3"><a class="selflink" id="section-4.7" href="#section-4.7">4.7</a>.  Bridge Types</span>

   A bridge is the function that connects the normal traffic and extra
   traffic to the working and recovery LSP/span.

   A. Permanent bridge

   Under a 1+1 type, the bridge connects the normal traffic to both the
   working and protection LSPs/spans.  This type of bridge is not
   applicable to restoration types.  There is, of course, no extra
   traffic connected to the recovery LSP/span.

   B. Broadcast bridge

   For 1:N and M:N types, the bridge permanently connects the normal
   traffic to the working LSP/span.  In the event of recovery switching,
   the normal traffic is additionally connected to the recovery
   LSP/span.  Extra traffic is either not connected or connected to the
   recovery LSP/span.

   C. Selector bridge

   For 1:N and M:N types, the bridge connects the normal traffic to
   either the working or the recovery LSP/span.  Extra traffic is either
   not connected or connected to the recovery LSP/span.

<span class="h3"><a class="selflink" id="section-4.8" href="#section-4.8">4.8</a>.  Selector Types</span>

   A selector is the function that extracts the normal traffic from
   either the working or the recovery LSP/span.  Extra traffic is either
   extracted from the recovery LSP/span, or is not extracted.

   A. Selective selector

   Is a selector that extracts the normal traffic from either the
   working LSP/span output or the recovery LSP/span output.

   B. Merging selector

   For 1:N and M:N protection types, the selector permanently extracts
   the normal traffic from both the working and recovery LSP/span

   outputs.  This alternative works only in combination with a selector
   bridge.




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<span class="h3"><a class="selflink" id="section-4.9" href="#section-4.9">4.9</a>.  Recovery GMPLS Nodes</span>

   This section defines the GMPLS nodes involved during recovery.

   A. Ingress GMPLS node of an end-to-end LSP/segment LSP/span

   The ingress node of an end-to-end LSP/segment LSP/span is where the
   normal traffic may be bridged to the recovery end-to-end LSP/segment
   LSP/span.  Also known as source node in the ITU-T terminology.

   B. Egress GMPLS node of an end-to-end LSP/segment LSP/span

   The egress node of an end-to-end LSP/segment LSP/span is where the
   normal traffic may be selected from either the working or the
   recovery end-to-end LSP/segment LSP/span.  Also known as sink node in
   the ITU-T terminology.

   C. Intermediate GMPLS node of an end-to-end LSP/segment LSP

   A node along either the working or recovery end-to-end LSP/segment
   LSP route between the corresponding ingress and egress nodes.  Also
   known as intermediate node in the ITU-T terminology.

<span class="h3"><a class="selflink" id="section-4.10" href="#section-4.10">4.10</a>.  Switch-over Mechanism</span>

   A switch-over is an action that can be performed at both the bridge
   and the selector.  This action is as follows:

   A. For the selector:

   The action of selecting normal traffic from the recovery LSP/span
   rather than from the working LSP/span.

   B. For the bridge:

   In case of permanent connection to the working LSP/span, the action
   of connecting or disconnecting the normal traffic to or from the
   recovery LSP/span.  In case of non-permanent connection to the
   working LSP/span, the action of connecting the normal traffic to the
   recovery LSP/span.

<span class="h3"><a class="selflink" id="section-4.11" href="#section-4.11">4.11</a>.  Reversion operations</span>

   A revertive recovery operation refers to a recovery switching
   operation, where the traffic returns to (or remains on) the working
   LSP/span when the switch-over requests are terminated (i.e., when the
   working LSP/span has recovered from the failure).




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   Therefore, a non-revertive recovery switching operation is when the
   traffic does not return to the working LSP/span when the switch-over
   requests are terminated.

<span class="h3"><a class="selflink" id="section-4.12" href="#section-4.12">4.12</a>.  Failure Reporting</span>

   This section gives (for information) several signal types commonly
   used in transport planes to report a failure condition.  Note that
   fault reporting may require additional signaling mechanisms.

   A. Signal Degrade (SD): a signal indicating that the associated data
   has degraded.

   B. Signal Fail (SF): a signal indicating that the associated data has
   failed.

   C. Signal Degrade Group (SDG): a signal indicating that the
   associated group data has degraded.

   D. Signal Fail Group (SFG): a signal indicating that the associated
   group has failed.

   Note: SDG and SFG definitions are under discussion at the ITU-T.

<span class="h3"><a class="selflink" id="section-4.13" href="#section-4.13">4.13</a>.  External commands</span>

   This section defines several external commands, typically issued by
   an operator through the Network Management System (NMS)/Element
   Management System (EMS), that can be used to influence or command the
   recovery schemes.

   A. Lockout of recovery LSP/span:

   A configuration action, initiated externally, that results in the
   recovery LSP/span being temporarily unavailable to transport traffic
   (either normal or extra traffic).

   B. Lockout of normal traffic:

   A configuration action, initiated externally, that results in the
   normal traffic being temporarily not allowed to be routed over its
   recovery LSP/span.  Note that in this case extra-traffic is still
   allowed on the recovery LSP/span.








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   C. Freeze:

   A configuration action, initiated externally, that prevents any
   switch-over action from being taken, and, as such, freezes the
   current state.

   D. Forced switch-over for normal traffic:

   A switch-over action, initiated externally, that switches normal
   traffic to the recovery LSP/span, unless an equal or higher priority
   switch-over command is in effect.

   E. Manual switch-over for normal traffic:

   A switch-over action, initiated externally, that switches normal
   traffic to the recovery LSP/span, unless a fault condition exists on
   other LSPs/spans (including the recovery LSP/span) or an equal or
   higher priority switch-over command is in effect.

   F. Manual switch-over for recovery LSP/span:

   A switch-over action, initiated externally, that switches normal
   traffic to the working LSP/span, unless a fault condition exists on
   the working LSP/span or an equal or higher priority switch-over
   command is in effect.

   G. Clear:

   An action, initiated externally, that clears the active external
   command.

<span class="h3"><a class="selflink" id="section-4.14" href="#section-4.14">4.14</a>.  Unidirectional versus Bi-Directional Recovery Switching</span>

   A. Unidirectional recovery switching:

   A recovery switching mode in which, for a unidirectional fault (i.e.,
   a fault affecting only one direction of transmission), only the
   normal traffic transported in the affected direction (of the LSP or
   span) is switched to the recovery LSP/span.

   B. Bi-directional recovery switching:

   A recovery switching mode in which, for a unidirectional fault, the
   normal traffic in both directions (of the LSP or span), including the
   affected direction and the unaffected direction, are switched to the
   recovery LSP/span.





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<span class="h3"><a class="selflink" id="section-4.15" href="#section-4.15">4.15</a>.  Full versus Partial Span Recovery Switching</span>

   Bulk LSP recovery is initiated upon reception of either span failure
   notification or bulk failure notification of the S LSPs carried by
   this span.  In either case, the corresponding recovery switching
   actions are performed at the LSP level, such that the ratio between
   the number of recovery switching messages and the number of recovered
   LSP (in one given direction) is minimized.  If this ratio equals 1,
   one refers to full span recovery; otherwise, if this ratio is greater
   than 1, one refers to partial span recovery.

   A. Full Span Recovery

   All the S LSP carried over a given span are recovered under span
   failure condition.  Full span recovery is also referred to as "bulk
   recovery".

   B. Partial Span Recovery

   Only a subset s of the S LSP carried over a given span is recovered
   under span failure condition.  Both selection criteria of the
   entities belonging to this subset, and the decision concerning the
   recovery of the remaining (S - s) LSP, are based on local policy.

<span class="h3"><a class="selflink" id="section-4.16" href="#section-4.16">4.16</a>.  Recovery Schemes Related Time and Durations</span>

   This section gives several typical timing definitions that are of
   importance for recovery schemes.

   A. Detection time:

   The time between the occurrence of the fault or degradation and its
   detection.  Note that this is a rather theoretical time because, in
   practice, this is difficult to measure.

   B. Correlation time:

   The time between the detection of the fault or degradation and the
   reporting of the signal fail or degrade.  This time is typically used
   in correlating related failures or degradations.

   C. Notification time:

   The time between the reporting of the signal fail or degrade and the
   reception of the indication of this event by the entities that decide
   on the recovery switching operation(s).





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   D. Recovery Switching time:

   The time between the initialization of the recovery switching
   operation and the moment the normal traffic is selected from the
   recovery LSP/span.

   E. Total Recovery time:

   The total recovery time is defined as the sum of the detection, the
   correlation, the notification, and the recovery switching time.

   F. Wait To Restore time:

   A period of time that must elapse after a recovered fault before an
   LSP/span can be used again to transport the normal traffic and/or to
   select the normal traffic from.

   Note: the hold-off time is defined as the time between the reporting
   of signal fail or degrade, and the initialization of the recovery
   switching operation.  This is useful when multiple layers of recovery
   are being used.

<span class="h3"><a class="selflink" id="section-4.17" href="#section-4.17">4.17</a>.  Impairment</span>

   A defect or performance degradation, which may lead to SF or SD
   trigger.

<span class="h3"><a class="selflink" id="section-4.18" href="#section-4.18">4.18</a>.  Recovery Ratio</span>

   The quotient of the actual recovery bandwidth divided by the traffic
   bandwidth that is intended to be protected.

<span class="h3"><a class="selflink" id="section-4.19" href="#section-4.19">4.19</a>.  Hitless Protection Switch-over</span>

   Protection switch-over, which does not cause data loss, data
   duplication, data disorder, or bit errors upon recovery switching
   action.

<span class="h3"><a class="selflink" id="section-4.20" href="#section-4.20">4.20</a>.  Network Survivability</span>

   The set of capabilities that allows a network to restore affected
   traffic in the event of a failure.  The degree of survivability is
   determined by the network's capability to survive single and multiple
   failures.







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<span class="h3"><a class="selflink" id="section-4.21" href="#section-4.21">4.21</a>.  Survivable Network</span>

   A network that is capable of restoring traffic in the event of a
   failure.

<span class="h3"><a class="selflink" id="section-4.22" href="#section-4.22">4.22</a>.  Escalation</span>

   A network survivability action caused by the impossibility of the
   survivability function in lower layers.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Recovery Phases</span>

   It is commonly accepted that recovery implies that the following
   generic operations need to be performed when an LSP/span or a node
   failure occurs:

   - Phase 1: Failure Detection

   The action of detecting the impairment (defect of performance
   degradation) as a defect condition and the consequential activation
   of SF or SD trigger to the control plane (through internal interface
   with the transport plane).  Thus, failure detection (which should
   occur at the transport layer closest to the failure) is the only
   phase that cannot be achieved by the control plane alone.

   - Phase 2: Failure Localization (and Isolation)

   Failure localization provides, to the deciding entity, information
   about the location (and thus the identity) of the transport plane
   entity that causes the LSP(s)/span(s) failure.  The deciding entity
   can then make an accurate decision to achieve finer grained recovery
   switching action(s).

   - Phase 3: Failure Notification

   Failure notification phase is used 1) to inform intermediate nodes
   that LSP(s)/span(s) failure has occurred and has been detected and 2)
   to inform the recovery deciding entities (which can correspond to any
   intermediate or end-point of the failed LSP/span) that the
   corresponding LSP/span is not available.

   - Phase 4: Recovery (Protection or Restoration)

   See <a href="#section-4.3">Section 4.3</a>.

   - Phase 5: Reversion (Normalization)

   See <a href="#section-4.11">Section 4.11</a>.



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   The combination of Failure Detection and Failure Localization and
   Notification is referred to as Fault Management.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Entities Involved During Recovery</span>

   The entities involved during the recovery operations can be defined
   as follows; these entities are parts of ingress, egress, and
   intermediate nodes, as defined previously:

   A. Detecting Entity (Failure Detection):

   An entity that detects a failure or group of failures; thus providing
   a non-correlated list of failures.

   B. Reporting Entity (Failure Correlation and Notification):

   An entity that can make an intelligent decision on fault correlation
   and report the failure to the deciding entity.  Fault reporting can
   be automatically performed by the deciding entity detecting the
   failure.

   C. Deciding Entity (part of the failure recovery decision process):

   An entity that makes the recovery decision or selects the recovery
   resources.  This entity communicates the decision to the impacted
   LSPs/spans with the recovery actions to be performed.

   D. Recovering Entity (part of the failure recovery activation
   process):

   An entity that participates in the recovery of the LSPs/spans.

   The process of moving failed LSPs from a failed (working) span to a
   protection span must be initiated by one of the nodes that terminates
   the span, e.g., A or B.  The deciding (and recovering) entity is
   referred to as the "master", while the other node is called the
   "slave" and corresponds to a recovering only entity.

   Note: The determination of the master and the slave may be based on
   configured information or protocol-specific requirements.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Protection Schemes</span>

   This section clarifies the multiple possible protection schemes and
   the specific terminology for the protection.






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<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>.  1+1 Protection</span>

   1+1 protection has one working LSP/span, one protection LSP/span, and
   a permanent bridge.  At the ingress node, the normal traffic is
   permanently bridged to both the working and protection LSP/span.  At
   the egress node, the normal traffic is selected from the better of
   the two LSPs/spans.

   Due to the permanent bridging, the 1+1 protection does not allow an
   unprotected extra traffic signal to be provided.

<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>.  1:N (N &gt;= 1) Protection</span>

   1:N protection has N working LSPs/spans that carry normal traffic and
   1 protection LSP/span that may carry extra-traffic.

   At the ingress, the normal traffic is either permanently connected to
   its working LSP/span and may be connected to the protection LSP/span
   (case of broadcast bridge), or is connected to either its working
   LSP/span or the protection LSP/span (case of selector bridge).  At
   the egress node, the normal traffic is selected from either its
   working or protection LSP/span.

   Unprotected extra traffic can be transported over the protection
   LSP/span whenever the protection LSP/span is not used to carry a
   normal traffic.

<span class="h3"><a class="selflink" id="section-6.3" href="#section-6.3">6.3</a>.  M:N (M, N &gt; 1, N &gt;= M) Protection</span>

   M:N protection has N working LSPs/spans carrying normal traffic and M
   protection LSP/span that may carry extra-traffic.

   At the ingress, the normal traffic is either permanently connected to
   its working LSP/span and may be connected to one of the protection
   LSPs/spans (case of broadcast bridge), or is connected to either its
   working LSP/span or one of the protection LSPs/spans (case of
   selector bridge).  At the egress node, the normal traffic is selected
   from either its working or one of the protection LSP/span.

   Unprotected extra traffic can be transported over the M protection
   LSP/span whenever the protection LSPs/spans is not used to carry a
   normal traffic.









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<span class="h3"><a class="selflink" id="section-6.4" href="#section-6.4">6.4</a>.  Notes on Protection Schemes</span>

   All protection types are either uni- or bi-directional; obviously,
   the latter applies only to bi-directional LSPs/spans and requires
   coordination between the ingress and egress node during protection
   switching.

   All protection types except 1+1 unidirectional protection switching
   require a communication channel between the ingress and the egress
   node.

   In the GMPLS context, span protection refers to the full or partial
   span recovery of the LSPs carried over that span (see <a href="#section-4.15">Section 4.15</a>).

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Restoration Schemes</span>

   This section clarifies the multiple possible restoration schemes and
   the specific terminology for the restoration.

<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>.  Pre-Planned LSP Restoration</span>

   Also referred to as pre-planned LSP re-routing.  Before failure
   detection and/or notification, one or more restoration LSPs are
   instantiated between the same ingress-egress node pair as the working
   LSP.  Note that the restoration resources must be pre-computed, must
   be signaled, and may be selected a priori, but may not cross-
   connected.  Thus, the restoration LSP is not able to carry any
   extra-traffic.

   The complete establishment of the restoration LSP (i.e., activation)
   occurs only after failure detection and/or notification of the
   working LSP and requires some additional restoration signaling.
   Therefore, this mechanism protects against working LSP failure(s) but
   requires activation of the restoration LSP after failure occurrence.
   After the ingress node has activated the restoration LSP, the latter
   can carry the normal traffic.

   Note: when each working LSP is recoverable by exactly one restoration
   LSP, one refers also to 1:1 (pre-planned) re-routing without extra-
   traffic.

<span class="h4"><a class="selflink" id="section-7.1.1" href="#section-7.1.1">7.1.1</a>.  Shared-Mesh Restoration</span>

   "Shared-mesh" restoration is defined as a particular case of pre-
   planned LSP re-routing that reduces the restoration resource
   requirements by allowing multiple restoration LSPs (initiated from
   distinct ingress nodes) to share common resources (including links
   and nodes.)



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<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>.  LSP Restoration</span>

   Also referred to as LSP re-routing.  The ingress node switches the
   normal traffic to an alternate LSP that is signaled and fully
   established (i.e., cross-connected) after failure detection and/or
   notification.  The alternate LSP path may be computed after failure
   detection and/or notification.  In this case, one also refers to
   "Full LSP Re-routing."

   The alternate LSP is signaled from the ingress node and may reuse the
   intermediate node's resources of the working LSP under failure
   condition (and may also include additional intermediate nodes.)

<span class="h4"><a class="selflink" id="section-7.2.1" href="#section-7.2.1">7.2.1</a>.  Hard LSP Restoration</span>

   Also referred to as hard LSP re-routing.  A re-routing operation
   where the LSP is released before the full establishment of an
   alternate LSP (i.e., break-before-make).

<span class="h4"><a class="selflink" id="section-7.2.2" href="#section-7.2.2">7.2.2</a>.  Soft LSP Restoration</span>

   Also referred to as soft LSP re-routing.  A re-routing operation
   where the LSP is released after the full establishment of an
   alternate LSP (i.e., make-before-break).

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Security Considerations</span>

   Security considerations are detailed in [<a href="./rfc4428" title="&quot;Analysis of Generalized Multi-Protocol Label Switching (GMPLS)-based Recovery Mechanisms (including Protection and Restoration)&quot;">RFC4428</a>] and [<a href="./rfc4426" title="&quot;Generalized Multiprotocol Label Switching (GMPLS) Recovery Functional Specification&quot;">RFC4426</a>].

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  References</span>

<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>.  Normative References</span>

   [<a id="ref-RFC2119">RFC2119</a>]    Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.

<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>.  Informative References</span>

   [<a id="ref-RFC3386">RFC3386</a>]    Lai, W. and D.  McDysan, "Network Hierarchy and
                Multilayer Survivability", <a href="./rfc3386">RFC 3386</a>, November 2002.

   [<a id="ref-RFC3945">RFC3945</a>]    Mannie, E., "Generalized Multi-Protocol Label Switching
                (GMPLS) Architecture", <a href="./rfc3945">RFC 3945</a>, October 2004.

   [<a id="ref-RFC4426">RFC4426</a>]    Lang, J., Rajagopalan B., and D.Papadimitriou, Editors,
                "Generalized Multiprotocol Label Switching (GMPLS)
                Recovery Functional Specification", <a href="./rfc4426">RFC 4426</a>, March
                2006.



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   [<a id="ref-RFC4428">RFC4428</a>]    Papadimitriou D. and E.Mannie, Editors, "Analysis of
                Generalized Multi-Protocol Label Switching (GMPLS)-based
                Recovery Mechanisms (including Protection and
                Restoration)", <a href="./rfc4428">RFC 4428</a>, March 2006.

   For information on the availability of the following documents,
   please see <a href="http://www.itu.int">http://www.itu.int</a>

   [<a id="ref-G.808.1">G.808.1</a>]    ITU-T, "Generic Protection Switching - Linear trail and
                subnetwork protection," Recommendation G.808.1, December
                2003.

   [<a id="ref-G.841">G.841</a>]      ITU-T, "Types and Characteristics of SDH Network
                Protection Architectures," Recommendation G.841, October
                1998.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  Acknowledgements</span>

   Many thanks to Adrian Farrel for having thoroughly review this
   document.

Editors' Addresses

   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

   Phone: +32 3 240-8491
   EMail: dimitri.papadimitriou@alcatel.be











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Full Copyright Statement

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Acknowledgement

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