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:
- Phase 1: Failure Detection
- Phase 2: Failure Localization (and Isolation)
- Phase 3: Failure Notification
- Phase 4: Recovery (Protection or Restoration)
- Phase 5: Reversion (Normalization)
Together, failure detection, localization, and notification phases
are referred to as "fault management". Within a recovery domain, the
entities involved during the recovery operations are defined in
[RFC4427]; these entities include ingress, egress, and intermediate
nodes. The term "recovery mechanism" is used to cover both
protection and restoration mechanisms. Specific terms such as
"protection" and "restoration" are used only when differentiation is
required. Likewise the term "failure" is used to represent both
signal failure and signal degradation.
In addition, when analyzing the different hierarchical recovery
mechanisms including disjointness-related issues, a clear distinction
is made between partitioning (horizontal hierarchy) and layering
(vertical hierarchy). In order to assess the current GMPLS protocol
capabilities and the potential need for further extensions, the
dimensions for analyzing each of the recovery mechanisms detailed in
this document are introduced. This document concludes by detailing
the applicability of the current GMPLS protocol building blocks for
recovery purposes.
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2. Contributors
This document is the result of the CCAMP Working Group Protection and
Restoration design team joint effort. Besides the editors, the
following are the authors that contributed to the present memo:
Deborah Brungard (AT&T)
200 S. Laurel Ave.
Middletown, NJ 07748, USA
EMail: dbrungard@att.com
Sudheer Dharanikota
EMail: sudheer@ieee.org
Jonathan P. Lang (Sonos)
506 Chapala Street
Santa Barbara, CA 93101, USA
EMail: jplang@ieee.org
Guangzhi Li (AT&T)
180 Park Avenue,
Florham Park, NJ 07932, USA
EMail: gli@research.att.com
Eric Mannie
Perceval
Rue Tenbosch, 9
1000 Brussels
Belgium
Phone: +32-2-6409194
EMail: eric.mannie@perceval.net
Dimitri Papadimitriou (Alcatel)
Francis Wellesplein, 1
B-2018 Antwerpen, Belgium
EMail: dimitri.papadimitriou@alcatel.be
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:
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RFC 4428 GMPLS Recovery Mechanisms March 2006
- Continuity: SONET/SDH monitors the integrity of the continuity of a
trail (i.e., section or path). This operation is performed by
monitoring the presence/absence of the signal. Examples are Loss
of Signal (LOS) detection for the physical layer, Unequipped (UNEQ)
Signal detection for the path layer, Server Signal Fail Detection
(e.g., AIS) at the client layer.
- Connectivity: SONET/SDH monitors the integrity of the routing of
the signal between end-points. Connectivity monitoring is needed
if the layer provides flexible connectivity, either automatically
(e.g., cross-connects) or manually (e.g., fiber distribution
frame). An example is the Trail (i.e., section or path) Trace
Identifier used at the different layers and the corresponding Trail
Trace Identifier Mismatch detection.
- Alignment: SONET/SDH checks that the client and server layer frame
start can be correctly recovered from the detection of loss of
alignment. The specific processes depend on the signal/frame
structure and may include: (multi-)frame alignment, pointer
processing, and alignment of several independent frames to a common
frame start in case of inverse multiplexing. Loss of alignment is
a generic term. Examples are loss of frame, loss of multi-frame,
or loss of pointer.
- Payload type: SONET/SDH checks that compatible adaptation functions
are used at the source and the destination. Normally, this is done
by adding a payload type identifier (referred to as the "signal
label") at the source adaptation function and comparing it with the
expected identifier at the destination. For instance, the payload
type identifier is compared with the corresponding mismatch
detection.
- Signal Quality: SONET/SDH monitors the performance of a signal.
For instance, if the performance falls below a certain threshold, a
defect -- excessive errors (EXC) or degraded signal (DEG) -- is
detected.
The most important point is that the supervision processes and the
corresponding failure detection (used to initiate the recovery
phase(s)) result in either:
- Signal Degrade (SD): A signal indicating that the associated data
has degraded in the sense that a degraded defect condition is
active (for instance, a dDEG declared when the Bit Error Rate
exceeds a preset threshold). Or
Papadimitriou & Mannie Informational [Page 6]RFC 4428 GMPLS Recovery Mechanisms March 2006
- Signal Fail (SF): A signal indicating that the associated data has
failed in the sense that a signal interrupting near-end defect
condition is active (as opposed to the degraded defect).
In Optical Transport Networks (OTN), equivalent supervision
capabilities are provided at the optical/digital section layers
(i.e., Optical Transmission Section (OTS), Optical Multiplex Section
(OMS) and Optical channel Transport Unit (OTU)) and at the
optical/digital path layers (i.e., Optical Channel (OCh) and Optical
channel Data Unit (ODU)). Interested readers are referred to the
ITU-T Recommendations [G.798] and [G.709] for more details.
The above are examples that illustrate cases where the failure
detection and reporting entities (see [RFC4427]) are co-located. The
following example illustrates the scenario where the failure
detecting and reporting entities (see [RFC4427]) are not co-located.
In pre-OTN networks, a failure may be masked by intermediate O-E-O
based Optical Line System (OLS), preventing a Photonic Cross-Connect
(PXC) from detecting upstream failures. In such cases, failure
detection may be assisted by an out-of-band communication channel,
and failure condition may be reported to the PXC control plane. This
can be provided by using [RFC4209] extensions that deliver IP
message-based communication between the PXC and the OLS control
plane. Also, since PXCs are independent of the framing format,
failure conditions can only be triggered either by detecting the
absence of the optical signal or by measuring its quality. These
mechanisms are generally less reliable than electrical (digital)
ones. Both types of detection mechanisms are outside the scope of
this document. If the intermediate OLS supports electrical (digital)
mechanisms, using the LMP communication channel, these failure
conditions are reported to
the PXC and subsequent recovery actions are performed as described in
Section 5. As such, from the control plane viewpoint, this mechanism
turns the OLS-PXC-composed system into a single logical entity, thus
having the same failure management mechanisms as any other O-E-O
capable device.
More generally, the following are typical failure conditions in
SONET/SDH and pre-OTN networks:
- Loss of Light (LOL)/Loss of Signal (LOS): Signal Failure (SF)
condition where the optical signal is not detected any longer on
the receiver of a given interface.
- Signal Degrade (SD): detection of the signal degradation over
a specific period of time.
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- For SONET/SDH payloads, all of the above-mentioned supervision
capabilities can be used, resulting in SD or SF conditions.
In summary, the following cases apply when considering the
communication between the detecting and reporting entities:
- Co-located detecting and reporting entities: both the detecting and
reporting entities are on the same node (e.g., SONET/SDH equipment,
Opaque cross-connects, and, with some limitations, Transparent
cross-connects, etc.)
- Non-co-located detecting and reporting entities:
o with in-band communication between entities: entities are
physically separated, but the transport plane provides in-band
communication between them (e.g., Server Signal Failures such as
Alarm Indication Signal (AIS), etc.)
o with out-of-band communication between entities: entities are
physically separated, but an out-of-band communication channel is
provided between them (e.g., using [RFCF4204]).
4.2. Failure Localization and Isolation
Failure localization provides information to the deciding entity
about the location (and so the identity) of the transport plane
entity that detects the LSP(s)/span(s) failure. The deciding entity
can then make an accurate decision to achieve finer grained recovery
switching action(s). Note that this information can also be included
as part of the failure notification (see Section 4.3).
In some cases, this accurate failure localization information may be
less urgent to determine if it requires performing more time-
consuming failure isolation (see also Section 4.4). This is
particularly the case when edge-to-edge LSP recovery is performed
based on a simple failure notification (including the identification
of the working LSPs under failure condition). Note that "edge"
refers to a sub-network end-node, for instance. In this case, a more
accurate localization and isolation can be performed after recovery
of these LSPs.
Failure localization should be triggered immediately after the fault
detection phase. This operation can be performed at the transport
plane and/or (if the operation is unavailable via the transport
plane) the control plane level where dedicated signaling messages can
be used. When performed at the control plane level, a protocol such
as LMP (see [RFC4204], Section 6) can be used for failure
localization purposes.
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4.3. Failure Notification
Failure notification is used 1) to inform intermediate nodes that an
LSP/span failure has occurred and has been detected and 2) to inform
the deciding entities (which can correspond to any intermediate or
end-point of the failed LSP/span) that the corresponding service is
not available. In general, these deciding entities will be the ones
making the appropriate recovery decision. When co-located with the
recovering entity, these entities will also perform the corresponding
recovery action(s).
Failure notification can be provided either by the transport or by
the control plane. As an example, let us first briefly describe the
failure notification mechanism defined at the SONET/SDH transport
plane level (also referred to as maintenance signal supervision):
- AIS (Alarm Indication Signal) occurs as a result of a failure
condition such as Loss of Signal and is used to notify downstream
nodes (of the appropriate layer processing) that a failure has
occurred. AIS performs two functions: 1) inform the intermediate
nodes (with the appropriate layer monitoring capability) that a
failure has been detected and 2) notify the connection end-point
that the service is no longer available.
For a distributed control plane supporting one (or more) failure
notification mechanism(s), regardless of the mechanism's actual
implementation, the same capabilities are needed with more (or less)
information provided about the LSPs/spans under failure condition,
their detailed statuses, etc.
The most important difference between these mechanisms is related to
the fact that transport plane notifications (as defined today) would
directly initiate either a certain type of protection switching (such
as those described in [RFC4427]) via the transport plane or
restoration actions via the management plane.
On the other hand, using a failure notification mechanism through the
control plane would provide the possibility of triggering either a
protection or a restoration action via the control plane. This has
the advantage that a control-plane-recovery-responsible entity does
not necessarily have to be co-located with a transport
maintenance/recovery domain. A control plane recovery domain can be
defined at entities not supporting a transport plane recovery.
Moreover, as specified in [RFC3473], notification message exchanges
through a GMPLS control plane may not follow the same path as the
LSP/spans for which these messages carry the status. In turn, this
ensures a fast, reliable (through acknowledgement and the use of
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either a dedicated control plane network or disjoint control
channels), and efficient (through the aggregation of several LSP/span
statuses within the same message) failure notification mechanism.
The other important properties to be met by the failure notification
mechanism are mainly the following:
- Notification messages must provide enough information such that the
most efficient subsequent recovery action will be taken at the
recovering entities (in most of the recovery types and schemes this
action is even deterministic). Remember here that these entities
can be either intermediate or end-points through which normal
traffic flows. Based on local policy, intermediate nodes may not
use this information for subsequent recovery actions (see for
instance the APS protocol phases as described in [RFC4427]). In
addition, since fast notification is a mechanism running in
collaboration with the existing GMPLS signaling (see [RFC3473])
that also allows intermediate nodes to stay informed about the
status of the working LSP/spans under failure condition.
The trade-off here arises when defining what information the
LSP/span end-points (more precisely, the deciding entities) need in
order for the recovering entity to take the best recovery action:
If not enough information is provided, the decision cannot be
optimal (note that in this eventuality, the important issue is to
quantify the level of sub-optimality). If too much information is
provided, the control plane may be overloaded with unnecessary
information and the aggregation/correlation of this notification
information will be more complex and time-consuming to achieve.
Note that a more detailed quantification of the amount of
information to be exchanged and processed is strongly dependent on
the failure notification protocol.
- If the failure localization and isolation are not performed by one
of the LSP/span end-points or some intermediate points, the points
should receive enough information from the notification message in
order to locate the failure. Otherwise, they would need to (re-)
initiate a failure localization and isolation action.
- Avoiding so-called notification storms implies that 1) the failure
detection output is correlated (i.e., alarm correlation) and
aggregated at the node detecting the failure(s), 2) the failure
notifications are directed to a restricted set of destinations (in
general the end-points), and 3) failure notification suppression
(i.e., alarm suppression) is provided in order to limit flooding in
case of multiple and/or correlated failures detected at several
locations in the network.
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- Alarm correlation and aggregation (at the failure-detecting node)
implies a consistent decision based on the conditions for which a
trade-off between fast convergence (at detecting node) and fast
notification (implying that correlation and aggregation occurs at
receiving end-points) can be found.
4.4. Failure Correlation
A single failure event (such as a span failure) can cause multiple
failure (such as individual LSP failures) conditions to be reported.
These can be grouped (i.e., correlated) to reduce the number of
failure conditions communicated on the reporting channel, for both
in-band and out-of-band failure reporting.
In such a scenario, it can be important to wait for a certain period
of time, typically called failure correlation time, and gather all
the failures to report them as a group of failures (or simply group
failure). For instance, this approach can be provided using LMP-WDM
for pre-OTN networks (see [RFC4209]) or when using Signal
Failure/Degrade Group in the SONET/SDH context.
Note that a default average time interval during which failure
correlation operation can be performed is difficult to provide since
it is strongly dependent on the underlying network topology.
Therefore, providing a per-node configurable failure correlation time
can be advisable. The detailed selection criteria for this time
interval are outside of the scope of this document.
When failure correlation is not provided, multiple failure
notification messages may be sent out in response to a single failure
(for instance, a fiber cut). Each failure notification message
contains a set of information on the failed working resources (for
instance, the individual lambda LSP flowing through this fiber).
This allows for a more prompt response, but can potentially overload
the control plane due to a large amount of failure notifications.
5. Recovery Mechanisms
5.1. Transport vs. Control Plane Responsibilities
When applicable, recovery resources are provisioned, for both
protection and restoration, using GMPLS signaling capabilities.
Thus, these are control plane-driven actions (topological and
resource-constrained) that are always performed in this context.
The following tables give an overview of the responsibilities taken
by the control plane in case of LSP/span recovery:
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1. LSP/span Protection
- Phase 1: Failure Detection Transport plane
- Phase 2: Failure Localization/Isolation Transport/Control plane
- Phase 3: Failure Notification Transport/Control plane
- Phase 4: Protection Switching Transport/Control plane
- Phase 5: Reversion (Normalization) Transport/Control plane
Note: in the context of LSP/span protection, control plane actions
can be performed either for operational purposes and/or
synchronization purposes (vertical synchronization between transport
and control plane) and/or notification purposes (horizontal
synchronization between end-nodes at control plane level). This
suggests the selection of the responsible plane (in particular for
protection switching) during the provisioning phase of the
protected/protection LSP.
2. LSP/span Restoration
- Phase 1: Failure Detection Transport plane
- Phase 2: Failure Localization/Isolation Transport/Control plane
- Phase 3: Failure Notification Control plane
- Phase 4: Recovery Switching Control plane
- Phase 5: Reversion (Normalization) Control plane
Therefore, this document primarily focuses on provisioning of LSP
recovery resources, failure notification mechanisms, recovery
switching, and reversion operations. Moreover, some additional
considerations can be dedicated to the mechanisms associated to the
failure localization/isolation phase.
5.2. Technology-Independent and Technology-Dependent Mechanisms
The present recovery mechanisms analysis applies to any circuit-
oriented data plane technology with discrete bandwidth increments
(like SONET/SDH, G.709 OTN, etc.) being controlled by a GMPLS-based
distributed control plane.
The following sub-sections are not intended to favor one technology
versus another. They list pro and cons for each technology in order
to determine the mechanisms that GMPLS-based recovery must deliver to
overcome their cons and make use of their pros in their respective
applicability context.
5.2.1. OTN Recovery
OTN recovery specifics are left for further consideration.
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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:
- They benefit from a simpler architecture, making it more suitable
for mesh-based recovery types and schemes (on a per-channel basis).
- Failure suppression at intermediate node transponders, e.g., use of
squelching, implies that failures (such as LoL) will propagate to
edge nodes. Thus, edge nodes will have the possibility to initiate
recovery actions driven by upper layers (vs. use of non-standard
masking of upstream failures).
The main disadvantage is the lack of interworking due to the large
amount of failure management (in particular failure notification
protocols) and recovery mechanisms currently available.
Note also, that for all-optical networks, combination of recovery
with optical physical impairments is left for a future release of
this document because corresponding detection technologies are under
specification.
5.2.3. SONET/SDH Recovery
Some of the advantages of SONET [T1.105]/SDH [G.707], and more
generically any Time Division Multiplexing (TDM) transport plane
recovery, are that they provide:
- Protection types operating at the data plane level that are
standardized (see [G.841]) and can operate across protected domains
and interwork (see [G.842]).
- Failure detection, notification, and path/section Automatic
Protection Switching (APS) mechanisms.
- Greater control over the granularity of the TDM LSPs/links that can
be recovered with respect to coarser optical channel (or whole
fiber content) recovery switching
Some of the limitations of the SONET/SDH recovery are:
- Limited topological scope: Inherently the use of ring topologies,
typically, dedicated Sub-Network Connection Protection (SNCP) or
shared protection rings, has reduced flexibility and resource
efficiency with respect to the (somewhat more complex) meshed
recovery.
Papadimitriou & Mannie Informational [Page 13]RFC 4428 GMPLS Recovery Mechanisms March 2006
- Inefficient use of spare capacity: SONET/SDH protection is largely
applied to ring topologies, where spare capacity often remains
idle, making the efficiency of bandwidth usage a real issue.
- Support of meshed recovery requires intensive network management
development, and the functionality is limited by both the network
elements and the capabilities of the element management systems
(thus justifying the development of GMPLS-based distributed
recovery mechanisms.)
5.3. Specific Aspects of Control Plane-Based Recovery Mechanisms
5.3.1. In-Band vs. Out-Of-Band Signaling
The nodes communicate through the use of IP terminating control
channels defining the control plane (transport) topology. In this
context, two classes of transport mechanisms can be considered here:
in-fiber or out-of-fiber (through a dedicated physically diverse
control network referred to as the Data Communication Network or
DCN). The potential impact of the usage of an in-fiber (signaling)
transport mechanism is briefly considered here.
In-fiber transport mechanisms can be further subdivided into in-band
and out-of-band. As such, the distinction between in-fiber in-band
and in-fiber out-of-band signaling reduces to the consideration of a
logically- versus physically-embedded control plane topology with
respect to the transport plane topology. In the scope of this
document, it is assumed that at least one IP control channel between
each pair of adjacent nodes is continuously available to enable the
exchange of recovery-related information and messages. Thus, in
either case (i.e., in-band or out-of-band) at least one logical or
physical control channel between each pair of nodes is always
expected to be available.
Therefore, the key issue when using in-fiber signaling is whether one
can assume independence between the fault-tolerance capabilities of
control plane and the failures affecting the transport plane
(including the nodes). Note also that existing specifications like
the OTN provide a limited form of independence for in-fiber signaling
by dedicating a separate optical supervisory channel (OSC, see
[G.709] and [G.874]) to transport the overhead and other control
traffic. For OTNs, failure of the OSC does not result in failing the
optical channels. Similarly, loss of the control channel must not
result in failing the data channels (transport plane).
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5.3.2. Uni- vs. Bi-Directional Failures
The failure detection, correlation, and notification mechanisms
(described in Section 4) can be triggered when either a uni-
directional or a bi-directional LSP/Span failure occurs (or a
combination of both). As illustrated in Figures 1 and 2, two
alternatives can be considered here:
1. Uni-directional failure detection: the failure is detected on the
receiver side, i.e., it is detected by only the downstream node to
the failure (or by the upstream node depending on the failure
propagation direction, respectively).
2. Bi-directional failure detection: the failure is detected on the
receiver side of both downstream node AND upstream node to the
failure.
Notice that after the failure detection time, if only control-plane-
based failure management is provided, the peering node is unaware of
the failure detection status of its neighbor.
------- ------- ------- -------
| | | |Tx Rx| | | |
| NodeA |----...----| NodeB |xxxxxxxxx| NodeC |----...----| NodeD |
| |----...----| |---------| |----...----| |
------- ------- ------- -------
t0 >>>>>>> F
t1 x <---------------x
Notification
t2 <--------...--------x x--------...-------->
Up Notification Down Notification
Figure 1: Uni-directional failure detection
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------- ------- ------- -------
| | | |Tx Rx| | | |
| NodeA |----...----| NodeB |xxxxxxxxx| NodeC |----...----| NodeD |
| |----...----| |xxxxxxxxx| |----...----| |
------- ------- ------- -------
t0 F <<<<<<< >>>>>>> F
t1 x <-------------> x
Notification
t2 <--------...--------x x--------...-------->
Up Notification Down Notification
Figure 2: Bi-directional failure detection
After failure detection, the following failure management operations
can be subsequently considered:
- Each detecting entity sends a notification message to the
corresponding transmitting entity. For instance, in Figure 1, node
C sends a notification message to node B. In Figure 2, node C
sends a notification message to node B while node B sends a
notification message to node C. To ensure reliable failure
notification, a dedicated acknowledgement message can be returned
back to the sender node.
- Next, within a certain (and pre-determined) time window, nodes
impacted by the failure occurrences may perform their correlation.
In case of uni-directional failure, node B only receives the
notification message from node C, and thus the time for this
operation is negligible. In case of bi-directional failure, node B
has to correlate the received notification message from node C with
the corresponding locally detected information (and node C has to
do the same with the message from node B).
- After some (pre-determined) period of time, referred to as the
hold-off time, if the local recovery actions (see Section 5.3.4)
were not successful, the following occurs. In case of uni-
directional failure and depending on the directionality of the LSP,
node B should send an upstream notification message (see [RFC3473])
to the ingress node A. Node C may send a downstream notification
message (see [RFC3473]) to the egress node D. However, in that
case, only node A would initiate an edge to edge recovery action.
Node A is referred to as the "master", and node D is referred to as
the "slave", per [RFC4427]. Note that the other LSP end-node (node
D in this case) may be optionally notified using a downstream
notification message (see [RFC3473]).
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
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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
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produce large increases in the restoration success rate and ability
to initiate successful LSP restoration attempts, especially at high
loads, while not adding significantly to the long-term average
recovery time. Allowing additional attempts produces only small
additional gains in performance. This suggests using additional
(intermediate) crankback signaling when using dynamic LSP restoration
(described in Section 5.5.2 - case 2). Details on crankback
signaling are outside the scope of this document.
5.4. Difference between Recovery Type and Scheme
[RFC4427] defines the basic LSP/span recovery types. This section
describes the recovery schemes that can be built using these recovery
types. In brief, a recovery scheme is defined as the combination of
several ingress-egress node pairs supporting a given recovery type
(from the set of the recovery types they allow). Several examples
are provided here to illustrate the difference between recovery types
such as 1:1 or M:N, and recovery schemes such as (1:1)^n or (M:N)^n
(referred to as shared-mesh recovery).
1. (1:1)^n with recovery resource sharing
The exponent, n, indicates the number of times a 1:1 recovery type is
applied between at most n different ingress-egress node pairs. Here,
at most n pairs of disjoint working and recovery LSPs/spans share a
common resource at most n times. Since the working LSPs/spans are
mutually disjoint, simultaneous requests for use of the shared
(common) resource will only occur in case of simultaneous failures,
which are less likely to happen.
For instance, in the common (1:1)^2 case, if the 2 recovery LSPs in
the group overlap the same common resource, then it can handle only
single failures; any multiple working LSP failures will cause at
least one working LSP to be denied automatic recovery. Consider for
instance the following topology with the working LSPs A-B-C and F-G-H
and their respective recovery LSPs A-D-E-C and F-D-E-H that share a
common D-E link resource.
A---------B---------C
\ /
\ /
D-------------E
/ \
/ \
F---------G---------H
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2. (M:N)^n with recovery resource sharing
The (M:N)^n scheme is documented here for the sake of completeness
only (i.e., it is not mandated that GMPLS capabilities support this
scheme). The exponent, n, indicates the number of times an M:N
recovery type is applied between at most n different ingress-egress
node pairs. So the interpretation follows from the previous case,
except that here disjointness applies to the N working LSPs/spans and
to the M recovery LSPs/spans while sharing at most n times M common
resources.
In both schemes, it results in a "group" of sum{n=1}^N N{n} working
LSPs and a pool of shared recovery resources, not all of which are
available to any given working LSP. In such conditions, defining a
metric that describes the amount of overlap among the recovery LSPs
would give some indication of the group's ability to handle
simultaneous failures of multiple LSPs.
For instance, in the simple (1:1)^n case, if n recovery LSPs in a
(1:1)^n group overlap, then the group can handle only single
failures; any simultaneous failure of multiple working LSPs will
cause at least one working LSP to be denied automatic recovery. But
if one considers, for instance, a (2:2)^2 group in which there are
two pairs of overlapping recovery LSPs, then two LSPs (belonging to
the same pair) can be simultaneously recovered. The latter case can
be illustrated by the following topology with 2 pairs of working LSPs
A-B-C and F-G-H and their respective recovery LSPs A-D-E-C and
F-D-E-H that share two common D-E link resources.
A========B========C
\\ //
\\ //
D =========== E
// \\
// \\
F========G========H
Moreover, in all these schemes, (working) path disjointness can be
enforced by exchanging information related to working LSPs during the
recovery LSP signaling. Specific issues related to the combination
of shared (discrete) bandwidth and disjointness for recovery schemes
are described in Section 8.4.2.
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5.5. LSP Recovery Mechanisms
5.5.1. Classification
The recovery time and ratio of LSPs/spans depend on proper recovery
LSP provisioning (meaning pre-provisioning when performed before
failure occurrence) and the level of overbooking of recovery
resources (i.e., over-provisioning). A proper balance of these two
operations will result in the desired LSP/span recovery time and
ratio when single or multiple failures occur. Note also that these
operations are mostly performed during the network planning phases.
The different options for LSP (pre-)provisioning and overbooking are
classified below to structure the analysis of the different recovery
mechanisms.
1. Pre-Provisioning
Proper recovery LSP pre-provisioning will help to alleviate the
failure of the working LSPs (due to the failure of the resources that
carry these LSPs). As an example, one may compute and establish the
recovery LSP either end-to-end or segment-per-segment, to protect a
working LSP from multiple failure events affecting link(s), node(s)
and/or SRLG(s). The recovery LSP pre-provisioning options are
classified as follows in the figure below:
(1) The recovery path can be either pre-computed or computed on-
demand.
(2) When the recovery path is pre-computed, it can be either pre-
signaled (implying recovery resource reservation) or signaled
on-demand.
(3) When the recovery resources are pre-signaled, they can be either
pre-selected or selected on-demand.
Recovery LSP provisioning phases:
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(1) Path Computation --> On-demand
|
|
--> Pre-Computed
|
|
(2) Signaling --> On-demand
|
|
--> Pre-Signaled
|
|
(3) Resource Selection --> On-demand
|
|
--> Pre-Selected
Note that these different options lead to different LSP/span recovery
times. The following sections will consider the above-mentioned
pre-provisioning options when analyzing the different recovery
mechanisms.
2. Overbooking
There are many mechanisms available that allow the overbooking of the
recovery resources. This overbooking can be done per LSP (as in the
example mentioned above), per link (such as span protection), or even
per domain. In all these cases, the level of overbooking, as shown
in the below figure, can be classified as dedicated (such as 1+1 and
1:1), shared (such as 1:N and M:N), or unprotected (and thus
restorable, if enough recovery resources are available).
Overbooking levels:
+----- Dedicated (for instance: 1+1, 1:1, etc.)
|
|
+----- Shared (for instance: 1:N, M:N, etc.)
|
Level of |
Overbooking -----+----- Unprotected (for instance: 0:1, 0:N)
Also, when using shared recovery, one may support preemptible extra-
traffic; the recovery mechanism is then expected to allow preemption
of this low priority traffic in case of recovery resource contention
during recovery operations. The following sections will consider the
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above-mentioned overbooking options when analyzing the different
recovery mechanisms.
5.5.2. LSP Restoration
The following times are defined to provide a quantitative estimation
about the time performance of the different LSP restoration
mechanisms (also referred to as LSP re-routing):
- Path Computation Time: Tc
- Path Selection Time: Ts
- End-to-end LSP Resource Reservation Time: Tr (a delta for resource
selection is also considered, the corresponding total time is then
referred to as Trs)
- End-to-end LSP Resource Activation Time: Ta (a delta for
resource selection is also considered, the corresponding total
time is then referred to as Tas)
The Path Selection Time (Ts) is considered when a pool of recovery
LSP paths between a given pair of source/destination end-points is
pre-computed, and after a failure occurrence one of these paths is
selected for the recovery of the LSP under failure condition.
Note: failure management operations such as failure detection,
correlation, and notification are considered (for a given failure
event) as equally time-consuming for all the mechanisms described
below:
1. With Route Pre-computation (or LSP re-provisioning)
An end-to-end restoration LSP is established after the failure(s)
occur(s) based on a pre-computed path. As such, one can define this
as an "LSP re-provisioning" mechanism. Here, one or more (disjoint)
paths for the restoration LSP are computed (and optionally pre-
selected) before a failure occurs.
No reservation or selection of resources is performed along the
restoration path before failure occurrence. As a result, there is no
guarantee that a restoration LSP is available when a failure occurs.
The expected total restoration time T is thus equal to Ts + Trs or to
Trs when a dedicated computation is performed for each working LSP.
2. Without Route Pre-computation (or Full LSP re-routing)
An end-to-end restoration LSP is dynamically established after the
failure(s) occur(s). After failure occurrence, one or more
(disjoint) paths for the restoration LSP are dynamically computed and
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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
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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
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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:
- The lower the layer, the faster the notification and switching.
- The higher the layer, the finer the granularity of the recoverable
entity and therefore the granularity of the recovery resource.
Moreover, in the context of this analysis, a vertical hierarchy
consists of multiple layered transport planes providing different:
- Discrete bandwidth granularities for non-packet LSPs such as OCh,
ODUk, STS_SPE/HOVC, and VT_SPE/LOVC LSPs and continuous bandwidth
granularities for packet LSPs.
- 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.
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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
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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
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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})]
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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:
- ---------- 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.
--------------------------------------------------------------------
| Path Search (computation and selection)
--------------------------------------------------------------------
| Pre-planned (a) | Dynamic (b)
--------------------------------------------------------------------
| | faster recovery | Does not apply
| | less flexible |
| 1 | less robust |
| | most resource-consuming |
Path | | |
Setup ------------------------------------------------------------
| | 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.
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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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