One document matched: draft-sprecher-mpls-tp-survive-fwk-00.txt
Network Working Group N. Sprecher
Internet Draft Nokia Siemens Networks
Category: Informational A. Farrel
Created: July 7, 2008 Old Dog Consulting
Expires: January 7, 2009 V. Kompella
Alcatel-Lucent
Multiprotocol Label Switching Transport Profile
Survivability Framework
draft-sprecher-mpls-tp-survive-fwk-00.txt
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Abstract
Network survivability is the network's ability to restore traffic
following failure or attack; it plays a critical factor in the
delivery of reliable services in transport networks. Guaranteed
services in the form of Service Level Agreements (SLAs) require a
resilient network that detects facility or node failures, very
rapidly, and immediately starts to restore network operations in
accordance with the terms of the SLA.
The Transport Profile of Multiprotocol Label Switching (MPLS-TP) is a
packet transport technology that combines the packet experience of
MPLS with the operational experience of SONET/SDH. It provides
survivability mechanisms such as protection and restoration, with
similar function levels to those found in established transport
networks such as in SONET/SDH networks. Some of the MPLS-TP
protection mechanisms are data plane-driven and are based on MPLS-TP
OAM fault management functions which are used to trigger protection
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switching in the absence of a control plane. Other protection
mechanisms utilize the MPLS-TP control plane.
This document provides a framework for MPLS-TP survivability.
Table of Contents
1. Introduction 3
2. Terminology and References 6
3. Requirements for Survivability 6
4. Functional Architecture 8
4.1. Elements of Control 8
4.1.1. Manual Control 8
4.1.2. Failure-Triggered Actions 9
4.1.3. OAM Signaling 9
4.1.4. 9
4.2. Elements of Recovery 9
4.2.1. Span Recovery 10
4.2.2. Segment Recovery 10
4.2.3. 10
4.3. Levels of Recovery 11
4.3.1. Dedicated Protection 11
4.3.2. Shared Protection 11
4.3.3. Extra Traffic 12
4.3.4. Restoration and Repair 12
4.3.5. 13
4.4. Mechanisms for Recovery 13
4.4.1. Link-Level Protection 13
4.4.2. Alternate Paths and Segments 13
4.4.3. 13
4.5. Protection in Different Topologies 13
4.5.1. Mesh Networks 13
4.5.2. Ring Networks 15
4.5.3. 15
4.6. Recovery in Layered Networks 15
4.6.1. Inherited Link-Level Protection 16
4.6.2. Shared Risk Groups 16
4.6.3. Fault Correlation 16
5. Mechanisms for Providing Protection in MPLS-TP 16
5.1. Management Plane 16
5.1.1. Configuration of Protection Operation 17
5.1.2. Forced Protection Actions 17
5.1.3. Blocked Protection Actions 17
5.2. Fault Detection 17
5.3. Fault Isolation 18
5.4. OAM Signaling 18
5.4.1. Fault Detection 18
5.4.2. Fault Isolation 18
5.4.3. Fault Reporting 18
5.4.4. Coordination of Recovery Actions 18
5.5. Control Plane Signaling 18
5.5.1. Fault Detection 18
5.5.2. Fault Isolation 18
5.5.3. Fault Reporting 18
5.5.4. Coordination of Recovery Actions 18
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6. Pseudowire Protection Considerations 18
6.1. Utilizing Underlying MPLS-TP Protection 18
6.2. Protection in the Pseudowire Layer 18
7. Manageability Considerations 18
8. Security Considerations 18
9. IANA Considerations 18
10. Acknowledgments 18
11. References 19
11.1. Normative References 19
11.2. Informative References 19
12. Editors' Addresses 20
13. Intellectual Property Statement 20
1. Introduction
Network survivability is the network's ability to restore traffic
following failure or attack; it plays a critical factor in the
delivery of reliable services in transport networks. Guaranteed
services in the form of Service Level Agreements (SLAs) require a
resilient network that very rapidly detects facility or node
failures, and immediately starts to restore network operations in
accordance with the terms of the SLA.
The Transport Profile of Multiprotocol Label Switching (MPLS-TP)
[MPLS-TP-JWT] , [MPLS-TP-REQ] is a packet transport technology that
combines the packet experience of MPLS with the operational
experience of SONET/SDH. MPLS-TP is designed to be consistent with
existing transport network operations and management models and
provide survivability mechanisms, such as protection and restoration
with similar function levels to those found in established transport
networks such as the SONET/SDH networks which provided service
providers with a high benchmark for reliability.
This document provides a framework for MPLS-TP-based survivability.
It uses the recovery terminology defined in [RFC4427] which draws
heavily on [G.808.1], and refers to the requirements specified in
[MPLS-TP-REQ].
Various recovery schemes (for protection and restoration) and
processes have been defined and analyzed in [RFC4427] and [RFC4428].
These schemes may also be applied in MPLS-TP networks to re-establish
end-to-end traffic delivery within the agreed service level and so
recover from 'failed' or 'degraded' transport entities (links or
nodes). Such actions are normally initiated by the detection of a
defect or performance degradation, or by an external request (e.g.,
an operator request for manual control of protection switching).
[RFC4427] makes a distinction between protection switching and
restoration mechanisms. Protection switching makes use of
pre-assigned capacity between nodes, where the simplest scheme has
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one dedicated protection entity for each working entity, while the
most complex scheme has m protection entities shared between n
working entities (m:n). Protection switching may be either
unidirectional or bidirectional. Restoration uses any capacity
available between nodes and usually involves re-routing. The
resources used for restoration may be pre-planned and recovery
priority may be used as a differentiation mechanism to determine
which services are recovered and which are not recovered or are
sacrificed in order to achieve recovery of other services.. In
general, protection actions are completed within time frames of tens
of milliseconds, while restoration actions are normally completed in
periods ranging from hundreds of milliseconds to a maximum of a few
seconds.
However, the recovery schemes described in [RFC4427] and evaluated in
[RFC4428] assume some control plane-driven actions that are performed
in the recovery context. As for other transport technologies and
associated transport networks, the presence of a distributed control
plane in support of MPLS-TP network operations is optional, and the
absence of such a control plane does not affect the ability to
operate the network and to use MPLS-TP forwarding, OAM, and
protection capabilities.
Thus, some of the MPLS-TP recovery mechanisms do not depend on a
control plane and rely on MPLS-TP OAM capabilities to trigger
protection switching. These mechanisms are data plane-driven and are
based on MPLS-TP OAM fault management functions. "Fault management"
in this context refers to failure detection, localization, and
notification (where the term "failure" is used to represent both
signal failure and signal degradation).
The principles of MPLS-TP protection switching operation are similar
to those defined in [RFC4427], as the protection mechanism is based
on the ability to detect certain defects in the transport entities
within the protected domain. The protection switching controller does
not care which monitoring method is used, as long as it can be given
information about the status of the transport entities within the
recovery domain (e.g., 'OK', signal failure, signal degradation,
etc.).
An MPLS-TP OAM Automatic Protection Switching (APS) protocol may be
used as an in-band (i.e., data plane-based) control protocol to align
both ends of the protected domain.
The MPLS-TP protection mechanisms may be applied at various levels
throughout the MPLS-TP network, as is the case with the recovery
schemes defined in [RFC4427] and [RFC4873]. A Label Switching Path
(LSP) may be subject to span, segment, and/or end-to-end recovery,
where:
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- span protection refers to the protection of an individual link (and
hence all or a subset of the LSPs routed over the link) between two
neighboring switches;
- segment protection refers to the recovery of an LSP segment (i.e.,
tandem connection in the language of [MPLS-TP-REQ]) between two
nodes which are the boundary nodes of the segment; and
- end-to-end protection refers to the protection of an entire LSP
from the ingress to the egress node.
Multiple recovery levels may be used concurrently by a single LSP for
added resiliency.
It is a basic requirement of MPLS-TP that both directions of a
bidirectional LSP should be co-routed (that is, share the same route
within the network) and be fate-sharing (that is, if one direction
fails, both directions should cease to operate) [MPLS-TP-REQ]. This
causes a direct interaction between the protection levels affecting
the directions of an LSP such that both directions of the LSP are
switched to a new span, segment, or end-to-end path together.
The protection scheme operating at the data plane level can function
in a multi-domain environment; it should also protect against a
failure of a boundary node in the case of inter-domain operation.
The MPLS-TP recovery schemes apply to LSPs and PWE3. This document
focuses on LSPs and handles both point-to-point (P2P) and point-to-
multipoint (P2MP) LSPs.
This framework introduces the architecture of the MPLS-TP recovery
domain and describes the recovery schemes in MPLS-TP (based on the
recovery types defined in [RFC4427]) as well as the principles of
operation, recovery states, recovery triggers, and information
exchanges between the different elements that sustain the reference
model. The reference model is based on the MPLS-TP OAM reference
model which is defined in [MPLS-TP-OAM].
This framework also refers to recovery schemes that are optimized for
specific topologies, such as linear, ring, and mesh, in order to
handle protection switching in a cost-efficient manner.
This document takes into account the timing co-ordination of
protection switches at multiple layers. This prevents races and
allows the protection switching mechanism of the server layer to fix
a problem before switching at the MPLS-TP layer.
This framework also specifies the functions that must be supported by
MPLS-TP OAM (e.g., APS) and the management and/or the control plane
in order to support the recovery mechanisms.
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MPLS-TP introduces a tool kit to enable recovery in MPLS-TP-based
transport networks and to ensure that affected traffic is restored in
the event of a failure.
Different recovery levels may be used concurrently by a single LSP
for added resiliency.
Generally, network operators aim to provide the fastest, most stable,
and the best protection mechanism available at a reasonable cost. The
higher the levels of protection, the greater the number of resources
consumed. It is therefore expected that network operators will offer
a wide spectrum of service levels. MPLS-TP-based recovery offers the
flexibility to select the recovery mechanism, choose the granularity
at which traffic is protected, and also choose the specific types of
traffic that are to be protected. With MPLS-TP-based recovery, it is
possible to provide different levels of protection for different
classes of service, based on their service requirements.
2. Terminology and References
The terminology used in this document is consistent with that defined
in [RFC4427]. That RFC is, itself, consistent with [G.808.1].
However, certain protection concepts (such as ring protection) are
not discussed in [RFC4427], and for those concepts, terminology in
this document is drawn from [G.841].
Readers should refer to those documents for normative definitions.
This document supplies brief summaries of some terms for clarity and
to aid the reader, but does not re-define terms.
In particular, note the distinction and definitions made in [RFC4427]
for the following three terms.
- Protection: re-establishing end-to-end traffic using pre-allocated
resources.
- Restoration: re-establishing end-to-end traffic using resources
allocated at the time of need. Sometimes referred to as "repair".
- Recovery: a generic term covering both Protection and Restoration.
Important background information can be found in [RFC3386],
[RFC3469], [RFC4426], [RFC4427], and [RFC4428].
3. Requirements for Survivability
MPLS-TP requirements are presented in [MPLS-TP-REQ]. Survivability is
presented as a critical factor in the delivery of reliable services,
and the requirements for survivability are set out using the recovery
terminology defined in [RFC4427].
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These requirements are summarized below. This section may be updated
if changes are made to [MPLS-TP-REQ], and that document should be
regarded as normative for the definition of all MPLS-TP requirements
including those for survivability.
General:
- Must support tandem network connection protection.
- Must support LSP protection.
- Must support pseudowire protection.
- Must provide appropriate recovery times.
- Must scale when many services are affected by a single fault.
- Should support span protection.
- Should support tandem connection protection.
- Should support end-to-end protection.
- Must support management plane control.
- Must support control plane control.
Restoration:
- May support pre-planning of restoration resources.
- May support computation of restoration resources after failure.
- May support shared mesh restoration.
- Should support soft LSP restoration (Make-before-break).
- May support hard LSP restoration (break-before-make).
- Must be topology agnostic.
- May support restoration priority.
- May utilize preemption during restoration, but only under operator
configuration.
Protection:
- Should be able to apply protection at different levels in the
network.
- Should operate in conjunction with protection in under-lying
networks.
- Must support data plane triggered recovery.
- Should be equally applicable to LSPs and pseudowires.
- Must include mechanisms to detect, locate, notify, and remedy
network faults.
- May support 1:1 bidirectional protection switching in which case
protection switching must be synchronized.
- May support 1+1 unidirectional protection switching.
- Must be applicable to P2P LSPs
- Should be applicable to P2MP LSPs.
- Must support protection ration of 100%.
- Must support operator's QoS objectives on protection path.
- May support extra traffic in 1:1 protection modes.
- Must provide operator control and protection prioritization.
- Must support revertive and non-revertive behavior.
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- Must provide mechanisms to prevent protection switching thrashing.
- Must provide coordination between protection mechanisms at
different layers.
- May provide different mechanisms optimized for specific topologies.
4. Functional Architecture
This section presents an overview of the elements of the functional
architecture for survivability within an MPLS-TP network. The
intention is to break the components out as separate items so that it
can be seen how they may be combined to provide different levels of
recovery to meet the requirements set out in the previous section.
4.1. Elements of Control
Survivability is achieved through specific actions taken to repair
network resources or to redirect traffic onto paths that avoid
failures in the network. Those actions may be triggered automatically
by the network devices, may be enhanced by data plane (i.e., OAM)
control plane signaling, and may be under direct the control of an
operator.
These different options are explored in the next sections.
4.1.1. Manual Control
Of course, the survivability behavior of the network as a whole, and
the reaction of each LSP when a fault is reported, may be under
operator control. That is, the operator may establish network-wide or
local policies that determine what actions will be taken when
different failures are reported that affect different LSPs. At the
same time, when a service request is made to cause the establishment
of one or more LSPs in the network, the operator (or requesting
application) may express a required or desired level of service, and
this will be mapped to particular survivability actions taken before
and during LSP setup, after the failure of network resources, and
upon recovery of those resources.
The operator can also be given manual control of survivability
actions and events. For example, the operator may force a switchover
from a working path to a recovery path (for network optimization
purposes with minimal disturbance of services, like when modifying
protected or unprotected services, when replacing network elements,
etc.), inhibit survivability actions, enable or disable survivability
function, or induce the simulation of a network fault.
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4.1.2. Failure-Triggered Actions
Survivability actions may be directly triggered by network failures.
That is, the device that detects the failure (for example, Loss of
Light on an optical interface) may immediately perform a
survivability action. Note that the term "failure" is used to
represent both signal failure and signal degradation.
This behavior can be subject to management plane or control plane
control, but does not require any messages exchanges in any of the
management plane, control plane, or data plane to trigger the
recovery action - it is directly triggered by data plane stimuli.
Note, however, that coordination of recovery actions may require
message exchanges.
4.1.3. OAM Signaling
OAM signaling refers to message exchanges in-band or closely coupled
to the data channel. Such messages may be used to detect and isolate
faults, but in this context we are concerned with the use of these
messages to control or trigger survivability actions.
Note that in some cases, it may be the failure to receive an OAM
signaling message that causes the survivability action to be taken.
OAM signaling may also be used to coordinate recovery actions within
the network.
4.1.4. Control Plane Signaling
The control plane signaling is responsible for setup and teardown of
LSPs that are not under management plane control. The control plane
can also be used to detect, isolate, and communicate network failures
pertaining to peer relationships (neighbor-to-neighbor, or end-to-
end). Thus, control plane signaling can initiate and coordinate
survivability actions.
The control plane can also be used to distribute topology and
resource-availability information. In this way, "graceful shutdown"
of resources may be effected by withdrawing them, and this can be
used as a stimulus to survivability action in a similar way to the
reporting or discovery of a fault as described in the previous
sections.
4.2. Elements of Recovery
This section describes the elements of recovery. These are the
quantitative aspects of recovery; that is the pieces of the network
for which recovery can be provided.
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4.2.1. Span Recovery
A span is a single hop between neighboring nodes in the same network
layer. A span is sometimes referred to as a link although this may
cause some confusion between the concept of a data link and a traffic
engineering (TE) link. LSPs traverse TE links between neighboring
label switching routers (LSRs) in the MPLS-TP network, however, a TE
link may be provided by:
- a single data link
- a series of data links in a lower layer established as an LSP and
presented to the upper layer as a single TE link
- a set of parallel data links in the same layer presented either as
a bundle of TE links or a collection of data links that, together,
provide data link layer protection scheme.
Thus, span recovery may be provided by:
- moving the TE link to be supported by a different data link between
the same pair of neighbors
- re-routing the LSP in the lower layer.
Moving the protected LSP to another TE link between the same pair of
neighbors is known as segment recovery and is described in Section
4.2.2.
4.2.2. Segment Recovery
An LSP segment is one or more hops on the path of the LSP. (Note that
recovery of pseudowire segments is discussed in Section 6).
Segment recovery involves redirecting traffic from one end of a
segment of an LSP on an alternate path to the other end of the
segment. This redirection may be on a pre-established LSP segment,
through re-routing of the protected segment, or by tunneling the
protected LSP on a "bypass" LSP.
Note that protecting an LSP against the failure of a node requires
the use of segment recovery, while a link could be protected using
span or segment recovery.
4.2.3. End-to-End Recovery
End-to-end recovery is a special case of segment recovery where the
protected LSP segment is the whole of the LSP. End-to-end recovery
may be provided as link-diverse or node-diverse recovery where the
recovery path shares no links or no nodes with the recovery path.
Note that node-diverse paths are necessarily link-diverse, and that
full, end-to-end node-diversity is required to guarantee recovery.
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4.3. Levels of Recovery
This section describes the qualitative levels of survivability
function that can be provided. The level of recovery offered has a
direct effect on the service level provided to the end-user in the
event of a network fault. This will be observed as the amount of data
lost when a network fault occurs, and the length of time to recovery
connectivity.
In general there is a correlation between the service level (i.e.,
the rapidity of recovery and reduction of data loss) and the cost to
the network; better service levels require pre-allocation of
resources to the recovery paths, and those resources cannot be used
for other purposes if high quality recovery is required.
Sections 6 and 7 of [RFC4427] provide a full break down of protection
and recovery schemes. This section summarizes the qualitative levels
available.
4.3.1. Dedicated Protection
In dedicated protection, the resources for the recovery LSP are
pre-assigned for use only by the protected service. This will clearly
be the case in 1+1 protection, and may also be the case in 1:1
protection where extra traffic (see Section 4.3.3) is not supported.
Note that in the bypass tunnel recovery mechanism (see Section 4.4.3)
resources may also be dedicated to protecting a specific service. In
some cases (one-for-one protection) the whole of the bypass tunnel
may be dedicated to provide recovery for a specific LSP, but in other
cases (such as facility backup) a subset of the resources of the
bypass tunnel may be pre-assigned for use to recover a specific
service. However, as described in Section 4.4.3, the bypass tunnel
approach can also be used for shared protection (Section 4.3.2), to
carry extra traffic (Section 4.3.3), or without reserving resources
to achieve best-effort recovery.
4.3.2. Shared Protection
In shared protection, the resources for the recovery LSPs of several
services are shared. These may be shared as 1:n or m:n, and may be
shared on individual links, on LSP segments, or on end-to-end LSPs.
Where a bypass tunnel is used (Section 4.4.3) the tunnel might not
have sufficient resources to simultaneously protect all of the LSPs
to which it offers protection so that if they were all affected by
network failures at the same time, they would not all be recovered.
Shared protection is a trade-off between expensive network resources
being dedicated to protection that is not required most of the time,
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and the risk of unrecoverable services in the event of multiple
network failures. There is also a trade-off between rapid recovery
(that can be achieved with dedicated protection, but which is delayed
by message exchanges in the management, control, or data planes for
shared protection) and the reduction of network cost by sharing
protection resources. These trade-offs may be somewhat mitigated by
using m:n for some value of m <> 1, and by establishing new
protection paths as each available protection path is put into use.
4.3.3. Extra Traffic
A way to utilize network resources that would otherwise be idle
awaiting use to protect services, is to use them to carry other
traffic. Obviously, this is not practical in dedicated protection
(Section 4.3.1), but is practical in shared protection (Section
4.3.2) and bypass tunnel protection (Section 4.4.3).
When a network resource that is carrying extra traffic is required
for protection, the extra traffic is disrupted - essentially it is
pre-empted by the recovery LSP. This may require some additional
messages exchanges in the management, control, or data planes, with
the consequence that recovery may be delayed somewhat. This provides
an obvious trade-off against the cost reduction (or rather, revenue
increase) achieved by carrying extra traffic.
4.3.4. Restoration and Repair
If resources are not pre-assigned for use by the recovery LSP, the
recovery LSP must be established "on demand" when the network failure
is detected and reported, or upon instruction from the management
plane.
Restoration represents the most cost-effective use of network
resources as no resources are tied up for specific protection usage.
However, restoration requires computation of a new path and
activation of a new LSP (through the management or control plane).
These steps can take much more time than is required for recovery
using protection techniques.
Furthermore, there is no guarantee that restoration will be able to
recover the service. It may be that all suitable network resources
are already in use for other LSPs so that no new path can be found.
This problem can be partially mitigated by the use of LSP setup
priorities so that recovery LSPs can pre-empt other low priority
LSPs.
Additionally, when a network failure occurs, multiple LSPs may be
disrupted by the same event. These LSPs may have been established by
different Network Management Stations (NMSs) or signaled by different
head-end LSRs, and this means that multiple points in the network
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will be trying to compute and establish recovery LSPs at the same
time. This can lead to contention within the network meaning that
some recovery LSPs must be retried resulting in very slow recovery
times for some services.
4.3.5. Reversion
When a service has been recovered so that traffic is flowing on the
recovery LSP, the faulted network resource may be repaired. The
choice must be made about whether to redirect the traffic back on to
the original working LSP, or to leave it where it is on the recovery
LSP. These behaviors are known as "revertive" and "non-revertive",
respectively.
In "revertive" mode, care should be taken to prevent frequent
operation of the recovery operation due to an intermittent defect.
Therefore, when the failure condition of a recovery element has been
handled, a fixed period of time should elapse before normal data
traffic is redirected back onto the original working entity.
4.4. Mechanisms for Recovery
The purpose of this section is to describe in general (MPLS-TP
non-specific) terms the mechanisms that can be used to provide
protection.
4.4.1. Link-Level Protection
4.4.2. Alternate Paths and Segments
4.4.3. Bypass Tunnels
4.5. Protection in Different Topologies
As described in the requirements listed in Section 3 and detailed in
[MPLS-TP-REQ], the recovery techniques used may be optimized for
different network topologies. This section describes two different
topologies and explains how recovery may be markedly different in
those different scenarios. It also introduces the concept of a
recovery domain and shows how end-to-end survivability may be
achieved through a concatenation of recovery domains each providing
some level of recovery in part of the network.
4.5.1. Mesh Networks
Linear protection provides a fast and simple protection switching
mechanism and it fits best in mesh networks. It can protect against a
failure that may happen on an entity (element of recovery that may
constitute a span, LSP segment, PW segment, end-to-end LSP or end-to-
end PW).
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In order to guarantee the protection, two entities are
pre-provisioned. One of the pre-provisioned entities is configured to
be the 'working' entity (primary) and the other is configured as the
'protection' entity (backup).
The Protection switching occurs at the protection controllers which
reside at the edges of the protected entity. Between these endpoints,
there are working and protection entities.
In linear protection, a protection entity is pre-provisioned to
protect the working entity. In order to guarantee protection
switching in case of a 'failed' condition, the physical routes of the
working and the protection entities should have complete physical
diversity.
[MPLS-TP-REQ] requires that both 1:1 linear protection scheme and 1+1
protection schemes are supported. The 1:1 protection switching,
bi-directional protection switching should be supported. In 1+1
linear protection switching unidirectional protection switching
should be supported.
1:1 linear protection:
- In normal conditions the data traffic is transmitted either over
the working entity or the 'protection' entity. Normal conditions
are defined when there is no failure on the 'working' entity and
there is no administrative configuration or requests that cause
traffic to transmit over the 'protection' entity. Upon a failure
condition or a specific administrative request, the traffic is
switched over to the 'protection' entity.
- In each transmission direction, the source of the protection domain
bridges the traffic into the appropriate entity and the sink of the
protected domain selects the traffic from the appropriate entity.
The source and the sink need to be coordinated to ensure that the
bridging and the selection are done to and from the same entity.
For that sake a signaling coordination protocol is needed.
- In bi-directional protection switching, both ends of the protection
domain should switch to the 'protection' entity (even when the
failure is unidirectional).
- When there is no failure, the resources of the 'idle' entity may be
used for less priority traffic, extra traffic. When protection
switching is performed, the extra traffic is required for
protection, the extra traffic is pre-empted by the protected
traffic.
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1+1 linear protection:
- The data traffic is copied at fed to both the 'working' and the
'protection' entities. The traffic on the 'working' and the
'protection' entities is transmitted simultaneously to the sink of
the protected domain, where a selection between the 'working' and
'protection' entities is made (based on some predetermined
criteria). Since only uni-directional protection switching is
supported in the 1+1 linear protection scheme, there is no need to
coordinate between the protection controllers.
4.5.2. Ring Networks
4.5.3. Protection and Restoration Domains
Protection and restoration are performed in the context of a recovery
domain. A recovery domain is defined between two recovery reference
points which are located at the edges of the recovery domain and are
responsible for performing recovery for a 'working' entity (which may
be one of the elements of recovery defined above) when an appropriate
trigger is received. These reference points function as recovery
controllers.
As described in section 4.2 above, the recovery element may
constitute a spam, a tandem connection (i.e. either an LSP segment or
a PW segment), an end-to-end LSP, or an end-to-end PW.
The method used to monitor the health of the recovery element is
unimportant, provided that the recovery controllers receive
information on its condition. The condition of the recovery element
may be OK, 'failed', or degraded.
When the recovery operation is launched by an OAM trigger, the
recovery domain is equivalent to the OAM maintenance entity which is
defined in [MPLS-TP-OAM], and the recovery reference points are
defined at the same location as the OAM MEPs.
4.6. Recovery in Layered Networks
In multi-layer or multi-region networking, recovery may be performed
at multiple layers or across cascaded recovery domains.
The MPLS-TP recovery mechanism must ensure that the timing of
recovery is co-ordinated in order to avoid races, and to allow either
the recovery mechanism of the server layer to fix the problem before
recovery takes place at the MPLS-TP layer, or to allow an upstream
recovery domain to perform recovery before a downstream domain. In
inter-connected rings, for example, it may be preferable to allow the
upstream ring to perform recovery before the downstream ring, in
order to ensure that recovery takes place in the ring in which the
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failure occurred.
A hold-off timer is required to coordinate the timing of recovery at
multiple layers or across cascaded recovery domains. Setting this
configurable timer involves a trade-off between rapid recovery and
the creation of a race condition where multiple layers respond to the
same fault, potentially allocating resources in an inefficient
manner. Thus, the detection of a failure condition in the MPLS-TP
layer should not immediately trigger the recovery process if the
hold-off timer is set to a value other than zero. The hold-off timer
should be started and, on expiry, the recovery element should be
checked to determine whether the failure condition still exists. If
it does exist, the defect triggers the recovery operation.
In other configurations, where the lower layer does not have a
restoration capability, or where it is not expected to provide
protection, the lower layer needs to trigger the higher layer to
immediately perform recovery.
[RFC3386]
4.6.1. Inherited Link-Level Protection
4.6.2. Shared Risk Groups
4.6.3. Fault Correlation
5. Mechanisms for Providing Protection in MPLS-TP
This section describes the existing mechanisms available to provide
protection within MPLS-TP networks and highlights areas where new
work is required. It is expected that, as new protocol extensions and
techniques are developed, this section will be updated to convert the
statements of required work into references to those protocol
extensions and techniques.
5.1. Management Plane
As described above, a fundamental requirement of MPLS-TP is that
recovery mechanisms should be capable of functioning in the absence
of a control plane. Recovery may be triggered by MPLS-TP OAM fault
management functions or by external requests (e.g. an operator
request for manual control of protection switching).
The management plane may be used to configure the recovery domain by
setting the reference points (recovery controllers), the 'working'
and 'protection' entities, and the recovery type (e.g. 1:1
bi-directional linear protection, ring protection, etc.). Additional
parameters associated with the recovery process (such as a hold-off
timer, revertive/non-revertive operation, etc.) may also be
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configured.
In addition, the management plane may initiate manual control of the
protection switching function. Either the fault condition or the
operator request should be prioritized.
Since provisioning the recovery domain involves the selection of a
number of options, mismatches may occur at the different reference
points. The MPLS-TP OAM Automatic Protection Switching (APS) protocol
may be used as an in-band (i.e., data plane-based) control protocol
to align both ends of the protected domain.
It should also be possible for the management plane to monitor the
recovery status.
5.1.1. Configuration of Protection Operation
In order to implement the protection switching mechanism, the
following entities and information should be provisioned:
- The protection controllers (reference points)
- The protection group consisting of a 'working' entity (which may be
one of the recovery elements defined above) and a 'protection'
entity. To guarantee protection, the paths of the 'working' and the
'protection' entities should have complete physical diversity.
- The protection type that should be applied
- Revertive/non-revertive behavior
5.1.2. External manual commands
The following external, manual commands may be applied to a
protection group; they are listed in descending order of priority:
- Blocked protection action - a manual command to prevent data
traffic from switching to the 'protection' entity. This command
actually disables the protection group.
- Force protection action - a manual command that forces a switch of
normal data traffic to the 'protection' entity.
- Manual protection action - a manual command that forces a switch of
data traffic to the 'protection' entity when there is no failure in
the 'working' or the 'protection' entity
5.2. Fault Detection
5.3. Fault Isolation
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5.4. OAM Signaling
5.4.1. Fault Detection
5.4.2. Fault Isolation
5.4.3. Fault Reporting
5.4.4. Coordination of Recovery Actions
5.5. Control Plane Signaling
5.5.1. Fault Detection
5.5.2. Fault Isolation
5.5.3. Fault Reporting
5.5.4. Coordination of Recovery Actions
6. Pseudowire Protection Considerations
The main application for the MPLS-TP network is currently identified
as the pseudowire. Pseudowires provide end-to-end connectivity over
the MPLS-TP network and may be comprised of a single pseudowire
segment, or multiple segments "stitched" together to provide end-to-
end connectivity.
The pseudowire service may, itself, require a level of protection as
part of its SLA. This protection could be provided by the MPLS-TP
LSPs that support the pseudowire, or could be a feature of the
pseudowire layer itself.
6.1. Utilizing Underlying MPLS-TP Protection
6.2. Protection in the Pseudowire Layer
7. Manageability Considerations
8. Security Considerations
9. IANA Considerations
This informational document makes no requests for IANA action.
10. Acknowledgments
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11. References
11.1. Normative References
[RFC4427] Mannie, E., and Papadimitriou, D., "Recovery
(Protection and Restoration) Terminology for
Generalized Multi-Protocol Label Switching (GMPLS)",
RFC 4427, March 2006.
[RFC4428] Papadimitriou D. and E.Mannie, Editors, "Analysis of
Generalized Multi-Protocol Label Switching (GMPLS)-
based Recovery Mechanisms (including Protection and
Restoration)", RFC 4428, March 2006.
[RFC4873] Berger, L., Bryskin, I., Papadimitriou, D., and
Farrel, A., " GMPLS Segment Recovery", RFC 4873, May
2007.
[G.808.1] ITU-T, "Generic Protection Switching - Linear trail
and subnetwork protection,", Recommendation G.808.1,
December 2003.
[G.841] ITU-T, "Types and Characteristics of SDH Network
Protection Architectures," Recommendation G.841,
October 1998.
[MPLS-TP-JWT] Bryant, S., and Andersson, L. "JWT Report on MPLS
Architectural Considerations for a Transport Profile",
draft-bryant-jwt-mplstp-report, work in progress.
[MPLS-TP-REQ] B. Niven-Jenkins, et al., "Requirements for MPLS-TP",
draft-jenkins-mpls-mplstp-requirements, work in
progress.
[MPLS-TP-OAM] Vigoureux, M., Betts, M., and Ward, D., "MPLS TP OAM
Requirements (MPLS)", work in progress.
11.2. Informative References
[RFC3386] Lai, W. and D. McDysan, "Network Hierarchy and
Multilayer Survivability", RFC 3386, November 2002.
[RFC3469] Sharma, V., and Hellstrand, F., "Framework for Multi-
Protocol Label Switching (MPLS)-based Recovery", RFC
3469, February 2003.
[RFC4426] Lang, J., Rajagopalan B., and D. Papadimitriou,
Editors, "Generalized Multiprotocol Label Switching
(GMPLS) Recovery Functional Specification", RFC 4426,
March 2006.
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12. Editors' Addresses
Nurit Sprecher
Nokia Siemens Networks
3 Hanagar St. Neve Ne'eman B
45241 Hod Hasharon, Israel
Tel. +972 9 7751229
Email: nurit.sprecher@nsn.com
Adrian Farrel
Old Dog Consulting
Email: adrian@olddog.co.uk
Vach Kompella
Alcatel-Lucent
701 East Middlefield Rd.
Mountain View, CA 94043
Email: vach.kompella@alcatel.com
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