One document matched: draft-ietf-mpls-tp-survive-fwk-00.txt
Network Working Group N. Sprecher, Ed.
Internet Draft Nokia Siemens Networks
Category: Informational A. Farrel, Ed.
Created: April 06, 2009 Old Dog Consulting
Expires: October 06, 2009
Multiprotocol Label Switching Transport Profile
Survivability Framework
draft-ietf-mpls-tp-survive-fwk-00.txt
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with
the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
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 transport networks like
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 survivability mechanisms are data plane-driven and are
based on MPLS-TP OAM fault management functions which are used to
trigger protection switching in the absence of a control plane. Other
Sprecher and Farrel MPLS-TP Survivability Framework [Page 1]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
survivability 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 ................................. 7
4. Functional Architecture ........................................ 9
4.1. Elements of Control .......................................... 9
4.1.1. Manual Control ............................................. 9
4.1.2. Failure-Triggered Actions ................................. 10
4.1.3. OAM Signaling ............................................. 10
4.1.4. Control Plane Signaling ................................... 10
4.2. Elements of Recovery ........................................ 11
4.2.1. Span Recovery ............................................. 11
4.2.2. Segment Recovery .......................................... 12
4.2.3. End-to-End Recovery ....................................... 12
4.3. Levels of Recovery .......................................... 12
4.3.1. Dedicated Protection ...................................... 13
4.3.2. Shared Protection ......................................... 13
4.3.3. Extra Traffic ............................................. 13
4.3.4. Restoration and Repair .................................... 14
4.3.5. Reversion ................................................. 15
4.4. Mechanisms for Recovery ..................................... 15
4.4.1. Link-Level Protection ..................................... 15
4.4.2. Alternate Paths and Segments .............................. 16
4.4.3. Bypass Tunnels ............................................ 16
4.5. Protection in Different Topologies .......................... 17
4.5.1. Mesh Networks ............................................. 17
4.5.2. Ring Networks ............................................. 21
4.5.3. Protection and Restoration Domains ........................ 22
4.6. Recovery in Layered Networks .. ............................. 23
4.6.1. Inherited Link-Level Protection ........................... 23
4.6.2. Shared Risk Groups ........................................ 23
4.6.3. Fault Correlation ......................................... 23
5. Mechanisms for Providing Protection in MPLS-TP ................ 24
5.1. Management Plane ............................................ 24
5.1.1. Configuration of Protection Operation ..................... 24
5.1.2. External Manual Commands .................................. 25
5.2. Fault Detection ............................................. 25
5.3. Fault Isolation ............................................. 25
5.4. OAM Signaling ............................................... 25
5.4.1. Fault Detection ........................................... 25
5.4.2. Fault Isolation ........................................... 25
5.4.3. Fault Reporting ........................................... 25
5.4.4. Coordination of Recovery Actions .......................... 26
Sprecher and Farrel MPLS-TP Survivability Framework [Page 2]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
5.5. Control Plane ............................................... 26
5.5.1. Fault Detection ........................................... 26
5.5.2. Testing for Faults ........................................ 27
5.5.3. Fault Isolation ........................................... 28
5.5.4. Fault Reporting ........................................... 28
5.5.5. Coordination of Recovery Actions .......................... 29
5.5.6. Establishment of Protection and Restoration LSPs .......... 29
6. Pseudowire Protection Considerations .......................... 29
6.1. Utilizing Underlying MPLS-TP Protection ..................... 30
6.2. Protection in the Pseudowire Layer .......................... 30
7. Manageability Considerations .................................. 30
8. Security Considerations ....................................... 30
9. IANA Considerations ........................................... 30
10. Acknowledgments .............................................. 30
11. References ................................................... 30
11.1. Normative References ....................................... 30
11.2. Informative References ..................................... 32
12. Editors' Addresses ........................................... 33
13. Author's Address ............................................. 33
14. Intellectual Property Statement .............................. 33
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)
[RFC5317], [MPLS-TP-REQ] is a packet transport technology that
combines the packet experience of MPLS with the operational
experience of transport networks such as 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
Sprecher and Farrel MPLS-TP Survivability Framework [Page 3]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
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 to
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
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; unidirectional meaning that each
direction of a bidirectional connection is protection switched
independently, while bidirectional means that both directions are
switched at the same time even if the fault applies to only one
direction of the connection. 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.
The recovery schemes described in [RFC4427] and evaluated in
[RFC4428] assume some control plane-driven actions that are performed
in the recovery context (such as the configuration of the protection
entities and functions, etc.). 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
survivability 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 across connections that were set up using
management plane configuration. 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
Sprecher and Farrel MPLS-TP Survivability Framework [Page 4]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
to those described 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 Protection State Coordination (PSC) 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 recovery mechanisms may be applied at various nested
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 any or all of MPLS-TP link
recovery, path segment recovery, or end-to-end recovery, where:
- MPLS-TP link recovery refers to the recovery of an individual link
(and hence all or a subset of the LSPs routed over the link)
between two neighboring label switching routers (LSRs).
- Segment recovery refers to the recovery of an LSP segment (i.e.,
segment and concatenated segment in the language of [MPLS-TP-REQ])
between two nodes which are the boundary nodes of the segment
- End-to-end recovery refers to the recovery of an entire LSP from
its ingress to its 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 recovery levels affecting
the directions of an LSP such that both directions of the LSP are
switched to a new MPLS-TP link, segment, or end-to-end path together.
The recovery 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.
Sprecher and Farrel MPLS-TP Survivability Framework [Page 5]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
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].
The framework also describes the qualitative levels of the
survivability functions that can be provided, such as dedicated
recovery, shared protection, restoration, etc. The level of recovery
directly affects the service level provided to the end user in the
event of a network failure. There is a correlation between the level
of recovery provided and the cost to the network.
This framework applies to general recovery schemes, but also for
schemes that are optimized for specific topologies, such as mesh and
ring, 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 (e.g., PSC) and the management and/or the control plane in
order to support the recovery mechanisms. MPLS-TP introduces a tool
kit to enable recovery in MPLS-TP-based transport networks and to
ensure that affected traffic is recovered 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].
Sprecher and Farrel MPLS-TP Survivability Framework [Page 6]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
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].
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 protection and restoration.
- Must be applicable at various nested levels, including link, LSP
segment and LSP end-to-end path, PW segment and end-to-end PW.
- Should be equally applicable to LSPs and pseudowires.
- Must provide appropriate recovery times.
- Should support the configuration of the recovery objectives (such
as BW and QOS) per transport path.
- Must scale when many services are affected by a single fault.
- Must support management plane control.
- Must support control plane control.
- Must be applicable for any topology.
- Must provide coordination between protection mechanisms at
Sprecher and Farrel MPLS-TP Survivability Framework [Page 7]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
different layers.
- Must provide mechanism to prevent recovery operations thrashing.
- Must support Physical layer fault indication as a trigger to the
recovery operation.
- Must support OAM based triggers to the recovery operation.
- Must support administrative commands as triggers to the recovery
operation (e.g. force switch, etc).
- Must support a mechanism to allow the distinction of recovery
actions that are initiated by administrative commands from those
that are initiated by other means.
- Should support control plane triggers when a control plane is
available.
- Must support the management plane configuration of timers used for
the recovery operation.
- Must support the management plane configuration of the elements of
controls (triggers for recovery).
- Must support the control plane configuration of the recovery
entities and functions (if the control plane is present).
- Must support the control plane signaling of an administrative
commands if the control plane is present).
- Must support the control plane signaling of the protection state,
in order to synch the protection state between the edges of the
protection domain.
Restoration:
- Must support soft re-routing (Make-before-break).
- Must support pre-planning of restoration resources.
- May support computation of restoration resources after failure.
- May support shared mesh restoration.
- May support hard LSP restoration (break-before-make).
- Must support restoration priority (under operator configuration)
- Must support preemption priority during restoration (under operator
configuration).
Protection:
- Must support bidirectional 1+1 protection switching (which should
be the default behavior) and 1+1 unidirectional protection
switching for P2P paths.
- Must support bidirectional 1:n protection switching (which should
be the default behavior) for P2P paths.
- Must support 1:1 and 1+1 unidirectional protection switching for
P2MP.
- Must support protection ration of 100%.
- Should support 1:n shared mesh protection. (*** contradict the Must
support above) .
- Must support shared bandwidth.
Sprecher and Farrel MPLS-TP Survivability Framework [Page 8]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
- Must support the definition of shared protection groups (to allow
coordination of protection actions).
- Must support sharing of protection resources.
- Must support revertive (which is the default behavior) and non-
revertive behavior.
- Must support the management plane configuration of the protection
path and the protection group.
- Must provide clear indication of the protection state of the
transport path.
- May provide different mechanisms optimized for specific topologies
(such as ring topologies). Such mechanisms must interoperate with
the mechanisms that are defined for the arbitrary topology). For
the specific requirements for ring topologies, see Section 4.5.2 on
rings.
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 (detecting a network failure), may be enhanced
by in-band (i.e. data-plane based) OAM fault management or
performance monitoring, in-band or out-of-band control plane
signaling, or 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.
Sprecher and Farrel MPLS-TP Survivability Framework [Page 9]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
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. In some
circumstances, a fault may be reported to the operator and the
operator may then select and initiate the appropriate recovery
action.
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, or failure to receive an OAM
continuity message) 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 between the
edges of the recovery domain may require message exchanges for some
qualitative levels of recovery.
4.1.3. OAM Signaling
OAM signaling refers to message exchanges that are 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.
OAM signaling may also be used to coordinate recovery actions within
the network.
4.1.4. Control Plane Signaling
Control plane signaling is responsible for setup, maintenance, 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.
Sprecher and Farrel MPLS-TP Survivability Framework [Page 10]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
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.
Note that the terminology in this section is consistent with
[RFC4427]. Where the terms differ from those in [MPLS-TP-REQ] a
mapping is provided.
4.2.1. Span Recovery
A span is a single hop between neighboring MPLS-TP LSRs 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.
[MPLS-TP-REQ] refers to a span as a "link".
Sprecher and Farrel MPLS-TP Survivability Framework [Page 11]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
4.2.2. Segment Recovery
An LSP segment is one or more hops on the path of the LSP. In some
MPLS-TP documents LSP segment is referred as LSP Tandem Connection
(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.
[MPLS-TP-REQ] defines two terms. A "segment" is a single hop on the
path of an LSP, and a "concatenated segment" is more than one hop on
the path of an LSP. In the context of this document, a segment covers
both of these concepts.
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.
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
Sprecher and Farrel MPLS-TP Survivability Framework [Page 12]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
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,
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
Sprecher and Farrel MPLS-TP Survivability Framework [Page 13]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
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
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.
Both hard or soft LSP restoration may be supported. In hard LSP
restoration, the resources of the LSP are released before the full
establishment of the recovery LSP (i.e., break-before-make). In soft
LSP restoration, the resources of the LSP are released after the full
establishment of an alternate LSP (i.e., make-before-break).
Note that the restoration resources may be pre-calculated and even
pre-signaled before the restoration action starts, but not pre-
Sprecher and Farrel MPLS-TP Survivability Framework [Page 14]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
allocated. This is known as pre-planned LSP restoration. The complete
establishment/activation of the restoration LSP occurs only when the
restoration action starts. The pre-planning may happen periodically
to have the most accurate information about the available resources
in the network.
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
Link-level protection refers to the paradigm whereby protection is
provided in a lower network layer.
Link-level protection offers the following levels of protections:
- Full protection, where a dedicated protection entity (e.g. a link
or span) is pre-established to protect a working entity. When the
working link fails, the protected traffic is switched onto the
protecting entity. In this scenario, all LSPs carried over the
entity are recovered (in one protection operation) when there is a
failure condition at the link-level. This is referred to in
[RFC4427] as 'bulk recovery'.
- Partial protection, where only a subset of the LSPs carried over a
given entity is recovered when there is a failure condition. The
decision as to which LSPs will be protected and which will not
depends on local policy.
When there is no failure on the working link, the protection entity
Sprecher and Farrel MPLS-TP Survivability Framework [Page 15]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
may transport extra traffic which may be preempted when protection
switching occurs.
As with recovery in layered networks, the protection mechanism at the
link-level needs to co-ordinate the timing for switchover, in order
to avoid race conditions and to enable switchover to be performed at
the link level before the upper level.
Note that link-level protection does not protect the nodes at each
end of the entity (e.g. a link or span) that is protected. End-to-end
or segment LSP protection should be used to protect against a failure
of the edge node.
4.4.2. Alternate Paths and Segments
The alternate paths and segments refer to the paradigm whereby the
protection is performed at the same network layer of the protected
LSP/segment-LSP.
Different levels of protection may be provided:
- Dedicated protection, where a dedicated entity (e.g. LSP, segment
LSP) is fully pre-established to protect a working entity (e.g.,
LSP, segment LSP). When there is a failure condition on the working
entity, the normal traffic is switched over into the protection
entity.
Dedicated protection may be accomplished by the 1:1 or 1+1
protection schemes. When the failure condition is eliminated, the
traffic may revert to the working entity. This is subject to local
configuration.
- Shared protection, where one or more protection entities are pre-
established to protect against a failure of one or more working
entities (1:n or m:n).
When the failure condition on the working entity is eliminated, the
traffic should revert back to the working entity.
4.4.3. Bypass Tunnels
A bypass tunnels is a transport entity (LSP) that is pre-provisioned
in order to protect against a failure condition along a network
segment, which may affect one or more LSPs that transmit over the
network segment.
When there is a failure condition in the network segment, one or more
of the protected LSPs are switched over at the ingress point of the
Sprecher and Farrel MPLS-TP Survivability Framework [Page 16]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
network segment and transmitted over the bypass tunnel. The natural
way to realize this is using label stacking. Label mapping may be an
option as well.
Different levels of protection may be provided:
- Dedicated protection, where the bypass tunnel has resource
reservations sufficient to provide protection for all protected
LSPs without service degradation.
- Shared protection, where the bypass tunnel has resources to protect
some of the protected LSPs, but not all of them simultaneously.
4.5. Protection in Different Topologies
As described in the requirements listed in Section 2.8.5 and detailed
in [MPLS-TP-REQ], the recovery techniques used may be optimized for
different network topologies if the performance of those optimized
mechanisms is significantly better than the performance of the
generic ones in the same topology.
It is required that such mechanisms interoperate with the mechanisms
defined for arbitrary topologies to allow end-to-end protection and
to allow consistent protection techniques to be used across the whole
network.
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 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).
Linear protection operates in the context of a Protection Domain
which is composed of the following architectural elements:
- A set of end points which reside at the boundary of the Protection
Domain. In this simple case of a 1:n or 1+1 P2P entity, exactly two
endpoint reside at the boundary of the Protection Domain. In each
transmission direction one of the end points is referred to as a
Sprecher and Farrel MPLS-TP Survivability Framework [Page 17]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
source and the other one is referred to as a sink.
In the case of unidirectional P2MP, three or more endpoints reside
at the boundary of the Protection Domain. One of the endpoints is
referred to as source/root and the other ones are referred to as
sinks/leaves.
- A Protection Group which consists of a Working (primary) entity and
one or more Protection (backup) entities. In order to guarantee
complete protection, a dedicated Protection entity should be pre-
provisioned to protect against a failure of the Working entity.
Also the Working and the Protection entities should be disjoint
entities, i.e., the physical routes of the Working and the
Protection entities should have complete physical diversity. Note
that resources of the Protection entity may be degraded from the
Working entity. In such a case, the Protection entity may not have
sufficient resources to protect the traffic of the Working entity.
As mentioned above in section 4.3.2, the resources of the
Protection entity may be shared as 1:n. In such a case, the
Protection entity might not have sufficient resources to
simultaneously protect all of the Working entities that may be
affected by fault conditions at the same time.
Protection switching occurs at the protection controllers which
reside at the edges of the Protected Domain. The working and
protection entities reside between these endpoints.
[MPLS-TP-REQ] requires that both 1:n linear protection scheme and 1+1
protection schemes are supported. The 1:n protection switching,
bidirectional protection switching should be supported. In 1+1
linear protection switching both unidirectional and bidirectional
protection switching should be supported.
In bidirectional protection switching, in the event of failure, the
recovery actions are taken in both directions (even when the fault is
unidirectional). This requires the synchronization of the recovery
state between the endpoints of the protection domain.
In unidirectional protection switching, the recovery actions are
taken only in the affected direction.
1:1 linear protection:
- In normal conditions the data traffic is transmitted over the
working entity. Normal conditions are defined when there is no
failure or degradation on the 'working' entity and there is no
administrative configuration or requests that cause traffic to
Sprecher and Farrel MPLS-TP Survivability Framework [Page 18]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
transmit over the 'protection' entity. Upon a fault condition
(failure or degradation) or a specific administrative request, the
traffic is switched over to the 'protection' entity.
Note that in the non-revertive behavior (see section 4.3.5), data
traffic can be transmitted over the Protection entity also in
normal conditions. This can happen after a failure condition on the
Working entity (which caused a recovery action) is eliminated.
- 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 bidirectional protection switching, both ends of the protection
domain switch to the 'protection' entity (even when the failure is
unidirectional). This requires a protocol to synchronize the
protection state between the two end points of the Protection
Domain.
- When there is no failure, the resources of the 'idle' entity may be
used for less priority traffic. When protection switching is
performed, the less priority traffic may be pre-empted by the
protected traffic.
1+1 linear protection:
- The data traffic is copied at fed at the source 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).
In 1+1 unidirectional protection switching there is no need to
coordinate the recovery state between the protection controllers at
both ends of the protection domain. In 1+1 bidirectional protection
switching, there is a need for a protocol to coordinate the
protection state between the edges of the Protection Domain.
In both protection schemes when the failure condition is
eliminated, operation, when the failure condition is eliminated,
the protected traffic may revert back into the Working entity. To
verify that the network has stabilized, and to avoid frequent
switching in case of intermittent failures, traffic is not switched
back to the Working entity before the Wait-to-Restore (WTR) timer
Sprecher and Farrel MPLS-TP Survivability Framework [Page 19]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
has expired.
Revertive/non-revertive operations are provided as network operator
options.
The protection switching may be performed when:
- A fault condition ('failed' or 'degraded') is declared on the
active entity and is not declared on the standby entity. OAM CC&V
(Continuity and Connectivity Verification) monitoring of both
Working and Protection entities may be used to enable the fast
detection of a fault condition. For protection switching, it is
common to run a CC&V every 3.33ms. In the absence of three
consecutive CC messages, a 'failed' condition is declared. In
order to monitor the Working and the Protection entities, an OAM
Maintenance Entity should be defined for each of the entities.
OAM information should be provided as input to the protection
switching controllers.
Input from OAM performance monitoring indicating degradation in
the Working entity may also be used as a trigger for protection
switching. In the case of degradation, switching to the
Protection entity is needed only if the Protection entity can
guarantee better conditions.
Note that in bidirectional protection switching, an attempt is
made to coordinate the protection switching state between both
end points of the Protection Domain when a unidirectional failure
is detected or when an external administrative requests is
received. A PSC (Protection State Coordination) protocol may be
used for this purpose. This protocol is also used to detect
mismatches between the provisioned protection switching
configuration and the two ends of a Protection Domain.
Note that in order to achieve 50ms protection switching it is
recommended to use inband signaling protocol to coordinate the
protection states.
- An indication is received from a lower layer server that there is
a network failure.
- An external operator command is received (e.g., 'Forced Switch',
'Manual Switch'). For details see Section 5.1.2.
- A request to switch over is received from the far end (relevant
in case of bidirectional 1:1 protection switching only).
Linear protection provides a clear indication of the protection
Sprecher and Farrel MPLS-TP Survivability Framework [Page 20]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
status.
1:n linear protection:
In 1:n linear protection, one Protection entity is used to protect
n Working entities. The Protection entity might not have sufficient
resources to simultaneously protect all of the Working entities
that may be affected by fault conditions at the same time.
Revertive behavior is recommended when 1:n is supported.
P2MP linear protection:
Linear protection may apply to protect P2MP path using 1+1
protection architecture. The source/root LSR bridges the user
traffic to both the Working and Protected entities. Each sink/leaf
LSR selects the traffic from one entity based on some
predetermined criteria. Note that when there is a fault condition
on one of the branches of the P2MP path, some leaf LSRs may select
the Working entity, while other leaf LSRs may select traffic from
the Protection entity.
In a 1:1 P2MP protection scheme, the source/root LSR needs to
identify the existence of a fault condition on any of the branches
of the network. This requires the sink/leaf LSRs to notify the
source/root LSR of any fault condition. This required also a
return path from the sinks/leaves to the source/root LSR.
When protection switching is triggered, the source/root LSR
selects the recovery transport path to transfer the traffic.
4.5.2. Ring Networks
Several Service Providers have expresses a high level of interest in
operating MPLS-TP in ring topologies and require a high level of
survivability function in these topologies.
Different criteria for optimization are considered in ring
topologies, such as:
1. Simplification of the operation of the Ring in terms of the number
of OAM Maintenance Entities that are needed to trigger the
recovery actions, the number of elements of recovery, the number
of management plane transactions during maintenance operations,
etc.
2. Optimization of resource consumption around the ring, like the
number of labels needed for the protection paths that cross the
Sprecher and Farrel MPLS-TP Survivability Framework [Page 21]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
network, the total bandwidth needed in the ring to ensure the
protection of the paths, etc.
[MPLS-TP-REQ] introduces a list of requirements on ring protection
that cover the recovery mechanisms need to protect traffic in a
single ring and traffic that traverses more than one ring. Note that
configuration and the operation of the recovery mechanisms in a ring
must scale well with the number of transport paths, the number of
nodes, and the number of ring interconnects.
The requirements for ring protection are fully compatible with the
generic requirements for recovery.
The architecture and the mechanisms for ring protection are specified
in separate documents. These mechanisms need to be evaluated against
the requirements specified in [MPLS-TP-REQ]. The principles for the
development of the mechanisms should be:
1. Reuse existing procedures and mechanisms for recovery in ring
topologies as along as their performance is as good as new
potential mechanisms.
2. Ensure complete interoperability with the mechanisms defined for
arbitrary topologies to allow end-to-end protection.
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.
Sprecher and Farrel MPLS-TP Survivability Framework [Page 22]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
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 coordinated 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
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.
Reference should be made to [RFC3386] that presents the near-term and
practical requirements for network survivability and hierarchy in
current service provider environments.
4.6.1. Inherited Link-Level Protection
TBD
4.6.2. Shared Risk Groups
TBD
4.6.3. Fault Correlation
TBD
Sprecher and Farrel MPLS-TP Survivability Framework [Page 23]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
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
bidirectional 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
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
Sprecher and Farrel MPLS-TP Survivability Framework [Page 24]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
'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
TBD
5.3. Fault Isolation
TBD
5.4. OAM Signaling
TBD
5.4.1. Fault Detection
TBD
5.4.2. Fault Isolation
TBD
5.4.3. Fault Reporting
TBD
Sprecher and Farrel MPLS-TP Survivability Framework [Page 25]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
5.4.4. Coordination of Recovery Actions
TBD
5.5. Control Plane
The GMPLS control plane has been proposed as the control plane for
MPLS-TP [RFC5317]. Since GMPLS was designed for use in transport
networks, and has been implemented and deployed in many networks, it
is not surprising that it contains many features to support a high
level of survivability function.
The signaling elements of the GMPLS control plane utilize extensions
to the Resource Reservation Protocol (RSVP) as documented in a series
of documents commencing with [RFC3471] and [RFC3473], but based on
[RFC3209] and [RFC2205]. The architecture for GMPLS is provided in
[RFC3945], and [RFC4426] gives a functional description of the
protocol extensions needed to support GMPLS-based recovery (i.e.,
protection and restoration).
A further control plane protocol called the Link Management Protocol
(LMP) [RFC4204] is part of the GMPLS protocol family and can be used
to coordinate fault isolation and reporting.
Clearly, the control plane techniques described here only apply where
an MPLS-TP control plane is deployed and operated. All mandatory
survivability features must be enabled even in the absence of the
control plane, but where the control plane is present it may provide
alternative mechanisms that may be desirable by virtue of their ease
of automation or richer feature-set.
5.5.1. Fault Detection
The control plane is not able to detect data plane faults. However,
it does provide mechanisms to detect control plane faults and these
can be can be used to deduce data plane faults where it is known that
the control and data planes are fate sharing. Although [MPLS-TP-REQ]
specifies that MPLS-TP must support an out-of-band control channel,
it does not insist that this is used exclusively. That means that
there may be deployments where an in-band (or at least in-fiber)
control channel is used. In this case, the failure of the control
channel can be used to infer a failure of the data channel or at
least to trigger an investigation of the health of the data channel.
Both RSVP and LMP provide a control channel "keep-alive" mechanism
(called the Hello message in both cases). Failure to receive a
message in the configured/negotiated time period indicates a control
plane failure. GMPLS routing protocols ([RFC4203] and [RFC5307]) also
Sprecher and Farrel MPLS-TP Survivability Framework [Page 26]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
include keepalive mechanisms designed to detect routing adjacency
failures and, although these keep-alive mechanisms tend to operate at
a relatively low frequency (order of seconds) it is still possible
that the first indication of a control plane fault will be through
the routing protocol.
Note, however, care must be taken that the failure is not caused by a
problem with the control plane software or processor component at the
far end of a link.
Because of the various issues involved, it is not recommended that
the control plane be relied upon as the primary mechanism for fault
detection in an MPLS-TP network.
5.5.2. Testing for Faults
The control plane may be used to initiate and coordinate testing of
links, LSP segments, or whole LSPs. This is important in some
technologies where it is necessary to halt data transmission while
testing, but may also be useful where testing needs to be
specifically enabled or configured.
LMP provides a control plane mechanism to test the continuity and
connectivity (and naming) of individual links. A single management
operation is required to initiate the test at one end of the link,
and LMP handles the coordination with the other end of the link. The
test mechanism for an MPLS packet link relies on the LMP Test message
inserted into the data stream at one end of the link and extracted at
the other end of the link. This mechanism need not be disruptive to
data flowing on the link.
Note that a link in LMP may in fact be an LSP tunnel used to form a
link in the MPLS-TP network.
GMPLS signaling (RSVP) offers two mechanisms that may also assist
with testing for faults. First, [RFC3473] defines the Admin_Status
object that allows an LSP to be set into "testing mode". The
interpretation of this mode is implementation specific and could be
documented more precisely for MPLS-TP. The mode sets the whole LSP
into a state where it can be tested; this need not be disruptive to
data traffic.
The second mechanism provided by GMPLS to support testing is provided
in [GMPLS-OAM]. This protocol extension supports the configuration
(including enabling and disabling) of OAM mechanisms for a specific
LSP.
Sprecher and Farrel MPLS-TP Survivability Framework [Page 27]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
5.5.3. Fault Isolation
Fault isolation is the process of determining exactly where a fault
has occurred. It is often the case the fault detection only takes
place at key points in the network (such as at LSP end points, or
MEPs). This means that the fault may be located anywhere within a
segment of the LSP concerned.
If segment or end-to-end protection are in use, this level of
information is often sufficient to repair the LSP. However, if a
finer granularity of information is needed (either to implement
optimal recovery actions or to diagnose the fault), it is necessary
to isolate the fault more closely.
LMP provides a cascaded test-and-propagate mechanism specifically
designed for this purpose.
5.5.4. Fault Reporting
GMPLS signaling uses the Notify message to report faults. The Notify
message can apply to a single LSP or can carry fault information for
a set of LSPs to improve the scalability of fault notification.
Since the Notify message is targeted at a specific node it can be
delivered rapidly without requiring hop-by-hop processing. It can be
targeted at LSP end-points, or at segment end-points (such as MEPs).
The target points for Notify messages can be manually configured
within the network or may be signaled as the LSP is set up. This
allows the process to be made consistent with segment protection and
the concept of Maintenance Entities.
GMPLS signaling also provides a slower, hop-by-hop mechanism for
reporting individual LSP faults on a hop-by-hop basis using the
PathErr and ResvErr messages.
[RFC4783] provides a mechanism to coordinate alarms and other event
or fault information through GMPLS signaling. This mechanism is
useful to understand the status of the resources used by an LSP and
to help understand why an LSP is not functioning, but it is not
intended to replace other fault reporting mechanisms.
GMPLS routing protocols ([RFC4203] and [RFC5307]) are used to
advertise link availability and capabilities within a GMPLS-enabled
network. Thus, the routing protocols can also provide indirect
information about network faults. That is, the protocol may stop
advertising or withdraw the advertisement for a failed link, or may
advertise that the link is about to be shut down gracefully. This
mechanisms is, however, not normally considered to be fast enough to
Sprecher and Farrel MPLS-TP Survivability Framework [Page 28]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
be used as a trigger for protection switching.
5.5.5. Coordination of Recovery Actions
Fault coordination is an important feature for certain protection
mechanisms (such as bidirectional 1:1 protection). The use of the
GMPLS Notify message for this purpose is described in [RFC4426],
however, specific message field values remain to be defined or this
operation.
A further piece of work in [GMPLS-REV] allows control and
configuration of reversion behavior for end-to-end and segment
protection.
5.5.6. Establishment of Protection and Restoration LSPs
It should not be forgotten that protection and recovery depend on the
establishment of suitable LSPs. The management plane may be used to
set up these LSPs, but the control plane may be used if it is
present.
Several protocol extensions exist to make this process more simple:
- [RFC4872] provides features in support of end-to-end protection
switching.
- [RFC4873] describes how to establish a single, segment protected
LSP.
- [RFC4874] allows one LSP to be signaled with a request that its
path excludes specified resources (links, nodes, SRLGs). This
allows a disjoint protection path to be requested, or a recovery
path to be set up avoiding failed resources.
Lastly, it should be noted that [RFC5298] provides an overview of the
GMPLS techniques available to achieve protection in multi-domain
environments.
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
Sprecher and Farrel MPLS-TP Survivability Framework [Page 29]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
LSPs that support the pseudowire, or could be a feature of the
pseudowire layer itself.
6.1. Utilizing Underlying MPLS-TP Protection
TBD
6.2. Protection in the Pseudowire Layer
TBD
7. Manageability Considerations
TBD
8. Security Considerations
TBD
9. IANA Considerations
This informational document makes no requests for IANA action.
10. Acknowledgments
TBD
11. References
11.1. Normative References
[RFC2205] Braden, R. (Ed.), Zhang, L., Berson, S., Herzog, S.
and S. Jamin, "Resource ReserVation Protocol --
Version 1 Functional Specification", RFC 2205,
September 1997.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T.,
Srinivasan, V. and G. Swallow, "RSVP-TE: Extensions
to RSVP for LSP Tunnels", RFC 3209, December 2001.
[RFC3471] Berger, L., Editor, "Generalized Multi-Protocol
Label Switching (GMPLS) Signaling Functional
Description", RFC 3471, January 2003.
[RFC3473] Berger, L. (Ed.), "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Resource ReserVation
Protocol-Traffic Engineering (RSVP-TE) Extensions",
RFC 3473, January 2003.
Sprecher and Farrel MPLS-TP Survivability Framework [Page 30]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
[RFC3945] Mannie, E., "Generalized Multi-Protocol Label
Switching (GMPLS) Architecture", RFC 3945, October
2004.
[RFC4203] Kompella, K, and Rekhter, Y., "IS-IS Extensions in
Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 4203, October 2005.
[RFC4204] Lang, J., Ed., "The Link Management Protocol (LMP)",
RFC 4204, September 2005.
[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.
[RFC5307] Kompella, K, and Rekhter, Y., "IS-IS Extensions in
Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 5307, October 2008.
[RFC5317] Bryant, S., and Andersson, L. "Joint Working Team
(JWT) Report on MPLS Architectural Considerations for
a Transport Profile", RFC 5317, February 2009.
[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-REQ] B. Niven-Jenkins, et al., "Requirements for MPLS-TP",
draft-ietf-mpls-tp-requirements, work in progress.
[MPLS-TP-OAM] Vigoureux, M., Betts, M., and Ward, D., "MPLS TP OAM
Requirements (MPLS)", work in progress.
Sprecher and Farrel MPLS-TP Survivability Framework [Page 31]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
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.
[RFC4783] Berger, L., "GMPLS - Communication of Alarm
Information", RFC 4783, December 2006.
[RFC4872] Lang, J., Rekhter, Y., and Papadimitriou, D., "RSVP-TE
Extensions in Support of End-to-End Generalized Multi-
Protocol Label Switching (GMPLS) Recovery", RFC 4872,
May 2007.
[RFC4873] Berger, L., Bryskin, I., Papadimitriou, D., and
Farrel, A., "GMPLS Segment Recovery", RFC 4873, May
2007.
[RFC4874] Lee, CY., Farrel, A., and De Cnodder, S., "Exclude
Routes - Extension to Resource ReserVation Protocol-
Traffic Engineering (RSVP-TE)", RFC 4874, April 2007.
[RFC5298] Takeda, T., Farrel, A., Ikejiri, Y., and Vasseur, JP.,
"Analysis of Inter-Domain Label Switched Path (LSP)
Recovery", RFC 5298, August 2008.
[GMPLS-OAM] Takacs, A., Fedyk, D., and Jia, H., "OAM Configuration
Framework and Requirements for GMPLS RSVP-TE",
draft-ietf-ccamp-oam-configuration-fwk, work in
progress.
[GMPLS-REV] Takacs, A., Fondelli, F., Tremblay, B., "GMPLS RSVP-TE
recovery extension for data plane initiated
reversion", draft-takacs-ccamp-revertive-ps, work in
progress.
Sprecher and Farrel MPLS-TP Survivability Framework [Page 32]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
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
13. Author's Address
Himanshu Shah
Ciena
Email: hshah@ciena.com
14. Intellectual Property Statement
The IETF Trust takes no position regarding the validity or scope of
any Intellectual Property Rights or other rights that might be
claimed to pertain to the implementation or use of the technology
described in any IETF Document or the extent to which any license
under such rights might or might not be available; nor does it
represent that it has made any independent effort to identify any
such rights.
Copies of Intellectual Property disclosures made to the IETF
Secretariat and any assurances of licenses to be made available, or
the result of an attempt made to obtain a general license or
permission for the use of such proprietary rights by implementers or
users of this specification can be obtained from the IETF on-line IPR
repository at http://www.ietf.org/ipr
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
any standard or specification contained in an IETF Document. Please
address the information to the IETF at ietf-ipr@ietf.org.
The definitive version of an IETF Document is that published by, or
under the auspices of, the IETF. Versions of IETF Documents that are
published by third parties, including those that are translated into
other languages, should not be considered to be definitive versions
of IETF Documents. The definitive version of these Legal Provisions
is that published by, or under the auspices of, the IETF. Versions of
Sprecher and Farrel MPLS-TP Survivability Framework [Page 33]
Internet Draft draft-ietf-mpls-tp-survive-fwk-00.txt February 2009
these Legal Provisions that are published by third parties, including
those that are translated into other languages, should not be
considered to be definitive versions of these Legal Provisions.
For the avoidance of doubt, each Contributor to the IETF Standards
Process licenses each Contribution that he or she makes as part of
the IETF Standards Process to the IETF Trust pursuant to the
provisions of RFC 5378. No language to the contrary, or terms,
conditions or rights that differ from or are inconsistent with the
rights and licenses granted under RFC 5378, shall have any effect and
shall be null and void, whether published or posted by such
Contributor, or included with or in such Contribution.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s)
controlling the copyright in such materials, this document may not
be modified outside the IETF Standards Process, and derivative
works of it may not be created outside the IETF Standards Process,
except to format it for publication as an RFC or to translate it
into languages other than English.
Disclaimer of Validity
All IETF Documents and the information contained therein are provided
on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE
IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL
WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
WARRANTY THAT THE USE OF THE INFORMATION THEREIN WILL NOT INFRINGE
ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS
FOR A PARTICULAR PURPOSE.
Full Copyright Statement
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your
rights and restrictions with respect to this document.
Sprecher and Farrel MPLS-TP Survivability Framework [Page 34]
| PAFTECH AB 2003-2026 | 2026-04-23 10:01:28 |