CCAMP Working Group CCAMP GMPLS P&R Design Team
Internet Draft
Expiration Date: August 2002 Eric Mannie (Ebone)- Editor
Dimitri Papadimitriou (Alcatel) - Editor
Deborah Brungard (AT&T)
Sudheer Dharanikota (Nayna)
Jonathan Lang (Calient)
Guangzhi Li (AT&T)
Bala Rajagopalan (Tellium)
Yakov Rekhter (Juniper)
February 2002
Recovery (Protection and Restoration) Terminology for GMPLS
draft-mannie-gmpls-recovery-terminology-00.txt
Status of this Memo
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1. Abstract
This document defines a common terminology for GMPLS based recovery
mechanisms (i.e. protection and restoration) that are under
consideration by the CCAMP Working Group.
2. 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 RFC-2119 [1].
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3. Introduction
This document defines a common terminology for GMPLS based recovery
mechanisms (i.e. protection and restoration) that are under
consideration by the CCAMP Working Group.
The terminology proposed in this document is intended to be
independent of the underlying transport technologies and borrows
from an ITU-T ongoing effort (G.gps û Generic Protection Switching
[G.gps]) and from the G.841 ITU-T Recommendation. The restoration
terminology and concepts have been gathered from numerous sources
including IETF drafts.
In the context of this document we will use the term "recovery" to
denote both protection and restoration. The specific terms
"protection" and "restoration" will only be used when
differentiation is required.
Note that this document focuses on the terminology for the recovery
of LSPs controlled by a GMPLS control plane. We focus on end-to-end,
segment and span (i.e. link) LSP recovery. Terminology for control
plane recovery 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 TDM and/or all-optical
(photonic) cross-connects referred to as GMPLS nodes (or simply
nodes, or even sometimes "LSRs") connected in a general topology
[GMPLS-ARCH].
Recovery typically involves the activation of a recovery (or
alternate) LSP when a failure is encountered in the working (or
primary) 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. The end-to-end path for a bi-
directional LSP therefore 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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4. Recovery Terminology Common to Protection and Restoration
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 assumes that the nodes at each end of the span
did not fail, otherwise end-to-end or segment LSP recovery is used.
The terminology and the definitions have been originally taken from
G.gps. However, for generalization, the following language that 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 G.841 and G.gps for references to SDH
protection and ITU-T generic protection terminology. Note that
restoration is not in the scope of G.gps.
4.1 Working and Recovery LSP/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.
4.2 Traffic Types
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.
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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.
4.3 LSP/Span Protection and Restoration
The following subtle distinction is generally made between the terms
"protection" and "restoration", even though these terms are often
used interchangeably [TEWG].
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 done during the restoration
LSP/span establishment.
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 ore 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
protecting 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
protecting 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.
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4.4 Recovery Scope
Recovery can be applied at various levels throughout the network.
Local (span) recovery refers to the recovery of an LSP over a link
between two nodes. Segment recovery refers to the recovery of an LSP
segment (i.e. an SNC in the ITU-T terminology) between two nodes,
i.e. the boundary nodes of the segment. End-to-end recovery refers
to the recovery of an entire LSP from its source to its destination.
An LSP may be subject to local (span), segment, and/or end-to-end
recovery.
4.5 Recovery Domain
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".
4.6 Recovery Types
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 there are no resources pre-established
for this recovery type.
This type is applicable to LSP/span restoration, but not to LSP/span
protection. Span restoration can be for instance achieved by moving
all the LSPs transported over of a failed span to a dynamically
selected span.
C. 1:1 type: dedicated recovery with extra traffic
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One specific recovery LSP/span protects exactly one specific working
LSP/span but the normal traffic is transmitted only over 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>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 > 1; M <= N) 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.
This type is applicable to LSP/span protection and LSP restoration,
but not to span restoration.
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4.7 Bridge Types
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.
4.8 Selector Types
A selector is the function that extracts the normal traffic either
from 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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4.9 Recovery GMPLS Nodes
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
working 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
working 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.
4.10 Switching Mechanism
A switch 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 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.
4.11 Reversion operations
A revertive recovery operation refers to a recovery switching
operation, where the traffic returns to (or remains on) the working
LSP/span if the switch requests are terminated; i.e. when the
working LSP/span has recovered from the failure.
Therefore a non-revertive recovery switching operation is when the
traffic does not return to the working LSP/span if the switch
requests are terminated.
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4.12 Failure Reporting
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 (under discussion at the ITU-T).
D. Signal Fail Group (SFG): a signal indicating that the associated
group has failed (under discussion at the ITU-T).
4.13 External commands
This section defines several external commands, typically issued by
an operator through the NMS/EMS, which 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.
C. Freeze:
A configuration action initiated externally that prevents any switch
action to be taken, and as such freezes the current state.
D. Forced switch for normal traffic:
A switch action initiated externally that switches normal traffic to
the recovery LSP/span, unless an equal or higher priority switch
command is in effect.
E. Manual switch for normal traffic:
A switch 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 command is in effect.
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4.14 Unidirectional versus Bi-Directional Recovery Switching
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.
4.15 Recovery Schemes Related Time and Durations
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 since in
practice this is difficult to measure.
B. Correlation time:
The time between 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. Hold-off time:
The time between reporting of signal fail or degrade, and the
initialization of the recovery switching algorithm. This is useful
when multiple layers of recovery are being used.
D. Wait To Restore time:
A period of time that must elapse from a recovered fault before an
LSP/span can be used again to transport the normal traffic and/or to
select the normal traffic from.
E. Switching time:
The time between the initialization of the recovery switching
algorithm and the moment the traffic is selected from the recovery
LSP/span.
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F. Recovery time:
The sum of the detection, correlation, hold-off and switching times.
4.16 Impairment
A defect or performance degradation, which may lead to SF or SD
trigger.
4.17 Recovery Ratio
The quotient of the actually recovery bandwidth divided by the
traffic bandwidth which is intended to be protected.
4.18 Hitless Protection Switch
Protection switch, which does not cause data loss, data duplication,
data disorder, or bit errors upon recovery switching action.
4.19 Network Survivability
The set of capabilities that allow a network to restore affected
traffic in the event of a failure. The degree of survivability is
determined by the networkÆs capability to survive single and
multiple failures.
4.20 Survivable Network
A network that is capable of restoring traffic in the event of a
failure.
4.21 Escalation
A network survivability action caused by the impossibility of the
survivability function in lower layers.
5. Recovery Phases
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
TBD.
- Phase 2: Failure Localization and Isolation
TBD.
- Phase 3: Failure Notification
TBD.
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- Phase 4: Recovery (Protection or Restoration)
See above.
- Phase 5: Reversion (Normalization)
See above.
The combination of Failure Detection and Failure Localization and
Notification is referred to as Fault Management.
5.1 Entities Involved During Recovery
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; providing
thus 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 select 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.
6. Protection Schemes
This section clarifies the multiple possible protection schemes and
the specific terminology for the protection.
To be completed with references to ITU-T protection schemes and a
table summarizing the multiple ITU-T protection schemes.
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7. Restoration Schemes
This section clarifies the multiple possible restoration schemes and
the specific terminology for the restoration.
To be completed when an agreement on restoration scheme definitions
and mechanisms has been achieved in other drafts.
8. Security Considerations
This document does not introduce or imply any specific security
consideration.
9. References
[G.707] ITU-T Recommendation G.707, ôNetwork Node Interface for
the Synchronous Digital Hierarchy (SDH)ö, April 2000.
[G.709] ITU-T Recommendation G.709, ôNetwork Node Interface for
the Optical Transport Network (OTN)ö, February 2001 (+
revision 1, OctÆ01).
[G.783] ITU-T Recommendation G.783,öCharacteristics of
Synchronous Digital Hierarchy (SDH) Equipment
Functional Blocksö
[G.798] ITU-T Recommendation G.798, ôCharacteristics of Optical
Transport Network (OTN) Equipment Functional Blocksö
[G.806] ITU-T Recommendation G.806, ôCharacteristics of
Transport Equipment û Description Methodology and
Generic Functionalityö
[G.841] ITU-T Recommendation G.841, ôTypes and Characteristics
of SDH Network Protection Architecturesö
[G.842] ITU-T Recommendation G.842, ôInterworking of SDH
network protection architecturesö
[G.gps] ITU-T on-going work G.gps, ôGeneric Protection
Switchingö, ITU-T Draft (October, 2001).
[ANSI-T1.105]"Synchronous Optical Network (SONET): Basic
Description Including Multiplex Structure, Rates, and
Formats" ANSI T1.105, 2000.
[BALA] Rajagopalan, Bala et al, "Signaling for Protection and
Restoration in Optical Mesh Networks", Internet Draft,
Draft-bala-protection-restoration-signaling-00.txt.
[GMPLS-ARCH] E.Mannie Editor, "GMPLS Architecture", Internet Draft,
draft-ietf-ccamp-gmpls-architecture-01.txt, November
2001.
E. Mannie et. al. Internet Draft - August 2002 13
Draft-design-team-gmpls-recovery-terminology-00.txt February 2002
[TEWG] W.S Lai, et al., "Network Hierarchy and Multilayer
Survivability", Internet Draft, draft-team-tewg-
restore-hierarchy-00.txt, July 2001.
[SUDHEER] Sudheer Dharanikota et al., ôNNI Protection and
restoration requirements,ö OIF 2001.507.
10. Acknowledgments
Valuable comments and input were received from many people.
11. Author's Addresses
Deborah Brungard
AT&T
Rm. D1-3C22
200 S. Laurel Ave.
Middletown, NJ 07748
USA
Email: dbrungard@att.com
Sudheer Dharanikota
Nayna Networks Inc
481 Sycamore Drive
Milpitas, CA 95035
USA
Email: sudheer@nayna.com
Jonathan P. Lang
Calient Networks
25 Castilian
Goleta, CA 93117
Email: jplang@calient.net
Guangzhi Li
AT&T
180 Park Avenue,
Florham Park, NJ 07932
gli@research.att.com
973-360-7376
Eric Mannie
Ebone
Terhulpsesteenweg 6A
1560 Hoeilaart
Belgium
Phone: +32 2 658 56 52
Email: eric.mannie@ebone.com
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Dimitri Papadimitriou
Alcatel
Francis Wellesplein, 1
B-2018 Antwerpen
Belgium
Phone: +32 3 240-84-91
Email: dimitri.papadimitriou@alcatel.be
Bala Rajagopalan
Tellium, Inc.
2 Crescent Place
P.O. Box 901
Oceanport, NJ 07757-0901
USA
Phone: +1 732 923 4237
Email: braja@tellium.com
Yakov Rekhter
Juniper
Email: yakov@juniper.net
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