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Versions: (draft-sprecher-mpls-tp-survive-fwk) 00 01 02 03 04 05 06 RFC 6372

Network Working Group                                        N. Sprecher
Internet-Draft                                    Nokia Siemens Networks
Intended status: Informational                          A. Adrian Farrel
Expires: April 28, 2010                               Old Dog Consulting
                                                            Oct 25, 2009


Multiprotocol Label Switching Transport Profile Survivability Framework
                 draft-ietf-mpls-tp-survive-fwk-01.txt

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
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   Please review these documents carefully, as they describe your rights
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Abstract

   Network survivability is the network's ability to restore traffic
   delivery following failure of network resources or an attack on the
   network.  It plays a critical role in the delivery of guaranteed
   services in transport networks to meet the requirements expressed in
   Service Level Agreements (SLAs).

   The Transport Profile of Multiprotocol Label Switching (MPLS-TP) is a
   packet transport technology based on the MPLS data plane and re-using
   many aspects of the MPLS management and control planes.

   This document provides a framework for the provision of survivability
   functions in the data plane of an MPLS-TP network using tools
   provided by the management plane and the control plane as well as
   techniques inherent in the data plane itself.

   This document is a product of a joint International
   Telecommunications Union (ITU)-IETF effort to include an MPLS
   Transport Profile within the IETF MPLS and PWE3 architectures to
   support the capabilities and functionalities of a packet transport
   network as defined by International Telecommunications Union -
   Telecommunications Standardization Sector (ITU-T).






















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Terminology and References . . . . . . . . . . . . . . . . . .  9
   3.  Requirements for Survivability . . . . . . . . . . . . . . . . 10
     3.1.  General Requirements . . . . . . . . . . . . . . . . . . . 10
     3.2.  Requirements for Restoration . . . . . . . . . . . . . . . 10
     3.3.  Requirements for Protection  . . . . . . . . . . . . . . . 11
     3.4.  Requirements for Survivability in Ring Topologies  . . . . 11
     3.5.  Triggers for Protection, Restoration, and Reversion  . . . 12
     3.6.  Management Plane Operation . . . . . . . . . . . . . . . . 13
     3.7.  Control Plane and In-band OAM  . . . . . . . . . . . . . . 13
   4.  Functional Architecture  . . . . . . . . . . . . . . . . . . . 13
     4.1.  Elements of Control  . . . . . . . . . . . . . . . . . . . 13
       4.1.1.  Manual Control . . . . . . . . . . . . . . . . . . . . 14
       4.1.2.  Failure-Triggered Actions  . . . . . . . . . . . . . . 14
       4.1.3.  OAM Signaling  . . . . . . . . . . . . . . . . . . . . 15
       4.1.4.  Control Plane Signaling  . . . . . . . . . . . . . . . 15
     4.2.  Elements of Recovery . . . . . . . . . . . . . . . . . . . 15
       4.2.1.  Span Recovery  . . . . . . . . . . . . . . . . . . . . 16
       4.2.2.  Segment Recovery . . . . . . . . . . . . . . . . . . . 16
       4.2.3.  End-to-end Recovery  . . . . . . . . . . . . . . . . . 17
     4.3.  Levels of Recovery . . . . . . . . . . . . . . . . . . . . 17
       4.3.1.  Dedicated Protection . . . . . . . . . . . . . . . . . 17
       4.3.2.  Shared Protection  . . . . . . . . . . . . . . . . . . 18
       4.3.3.  Extra Traffic  . . . . . . . . . . . . . . . . . . . . 18
       4.3.4.  Restoration and Repair . . . . . . . . . . . . . . . . 19
       4.3.5.  Reversion  . . . . . . . . . . . . . . . . . . . . . . 20
     4.4.  Mechanisms for Recovery  . . . . . . . . . . . . . . . . . 20
       4.4.1.  Link-Level Protection  . . . . . . . . . . . . . . . . 20
       4.4.2.  Alternate Paths and Segments . . . . . . . . . . . . . 21
       4.4.3.  Protection Tunnels . . . . . . . . . . . . . . . . . . 22
     4.5.  Protection in Different Topologies . . . . . . . . . . . . 22
       4.5.1.  Mesh Networks  . . . . . . . . . . . . . . . . . . . . 23
       4.5.2.  Ring Networks  . . . . . . . . . . . . . . . . . . . . 29
       4.5.3.  Protection and Restoration Domains . . . . . . . . . . 30
     4.6.  Recovery in Layered Networks . . . . . . . . . . . . . . . 31
       4.6.1.  Inherited Link-Level Protection  . . . . . . . . . . . 32
       4.6.2.  Shared Risk Groups . . . . . . . . . . . . . . . . . . 33
       4.6.3.  Fault Correlation  . . . . . . . . . . . . . . . . . . 33
   5.  Mechanisms for Providing Protection of MPLS-TP LSPs  . . . . . 33
     5.1.  Management Plane . . . . . . . . . . . . . . . . . . . . . 34
       5.1.1.  Configuration of Protection Operation  . . . . . . . . 34
       5.1.2.  External Manual Commands . . . . . . . . . . . . . . . 35
     5.2.  Fault Detection  . . . . . . . . . . . . . . . . . . . . . 36
     5.3.  Fault Isolation  . . . . . . . . . . . . . . . . . . . . . 37
     5.4.  OAM Signaling  . . . . . . . . . . . . . . . . . . . . . . 37
       5.4.1.  Fault Detection  . . . . . . . . . . . . . . . . . . . 38



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       5.4.2.  Fault Isolation  . . . . . . . . . . . . . . . . . . . 39
       5.4.3.  Fault Reporting  . . . . . . . . . . . . . . . . . . . 39
       5.4.4.  Coordination of Recovery Actions . . . . . . . . . . . 40
     5.5.  Control Plane  . . . . . . . . . . . . . . . . . . . . . . 40
       5.5.1.  Fault Detection  . . . . . . . . . . . . . . . . . . . 41
       5.5.2.  Testing for Faults . . . . . . . . . . . . . . . . . . 41
       5.5.3.  Fault Isolation  . . . . . . . . . . . . . . . . . . . 42
       5.5.4.  Fault Reporting  . . . . . . . . . . . . . . . . . . . 42
       5.5.5.  Coordination of Recovery Actions . . . . . . . . . . . 43
       5.5.6.  Establishment of Protection and Restoration LSPs . . . 43
   6.  Pseudowire Protection Considerations . . . . . . . . . . . . . 44
     6.1.  Utilizing Underlying MPLS-TP Recovery  . . . . . . . . . . 44
     6.2.  Recovery in the Pseudowire Layer . . . . . . . . . . . . . 45
   7.  Manageability Considerations . . . . . . . . . . . . . . . . . 45
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 45
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 45
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 45
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 45
     11.1. Normatve References  . . . . . . . . . . . . . . . . . . . 45
     11.2. Informative References . . . . . . . . . . . . . . . . . . 47
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 48






























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1.  Introduction

   Network survivability is the network's ability to restore traffic
   delivery following a failure or degradation of traffic delivery
   caused by a network fault or an attack on the network; it plays a
   critical role 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 MPLS Transport Profile (MPLS-TP) is described in [RFC5654] and
   [MPLS-TP-FWK] .  MPLS-TP is designed to be consistent with existing
   transport network operations and management models, and to provide
   survivability mechanisms, such as protection and restoration.  The
   function provided is intended to be similar or better to that found
   in established transport networks which set 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 it refers to the requirements specified in
   [RFC5654].

   Various recovery schemes (for protection and restoration) and
   processes have been defined and analyzed in [RFC4427] and [RFC4428].
   These schemes can 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



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   recovery of other services.  In general, protection actions are
   completed within time frames of tens of milliseconds, while automated
   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] are presented in the context of control plane-driven
   actions (such as the configuration of the protection entities and
   functions, etc.).  The presence of a distributed control plane in an
   MPLS-TP network 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, Operations, Administration and Maintenance (OAM), and
   survivability capabilities.

   Thus, some of the MPLS-TP recovery mechanisms do not depend on a
   control plane and use MPLS-TP OAM mechanisms or management actions to
   trigger protection switching across connections that were set up
   using management plane configuration.  These OAM mechanisms may be
   triggered by data plane events or by operator actions, 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 term 'trigger' is used
   to indicate any event that may be used to cause an implementation to
   consider taking protection action.

   The principles of MPLS-TP protection switching operation are similar
   to those described in [RFC4427] as the protection mechanism is based
   on the ability to detect certain defects in the transport entities
   within the recovery domain.  In the context of MPLS-TP, transport
   entities are nodes, links, concatenated segments, and transport
   paths.  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.).

   The protection switching operation is basically a data-plane
   capability and in the context of MPLS-TP it needs to be ensured that
   it is possible to switch over independent of the way the network is
   configured and managed.  All the MPLS and GMPLS protection mechanisms
   are applicable in MPLS-TP environment, and it should be possible also
   to provision and manage the related protection entities and functions
   defined in MPLS and GMPLS using a management plane.

   In some protection switching schemes (such as bidirectional
   protection switching), it is necessary to coordinate the protection
   state between the edges of the recovery domain.  An MPLS-TP
   Protection State Coordination (PSC) protocol may be used as an in-



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   band (i.e. data plane-based) control protocol to align both ends of
   the protected domain.  Control plane-based mechanism can also be used
   to synchronize the protection states between the edges of the
   protection 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:

   o  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 MPLS-TP nodes.

   o  Segment recovery refers to the recovery of an LSP segment (i.e.
      segment and concatenated segment in the language of [RFC5654])
      between two nodes which are the boundary nodes of the segment.

   o  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.

   Co-routed bidirectional MPLS-TP LSPs are defined such that both
   directions of the LSP follow the same route through the network.  In
   this case the directions are often required by the operator to fate-
   share (that is, if one direction fails, both directions should cease
   to operate).  This may also be the case for associated bidirectional
   LSPs where the two directions of the LSP take different paths through
   the network.  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.

   MPLS-TP recovery schemes are intended to protect client traffic as it
   is sent across the MPLS-TP network.  This document introduces
   protection and restoration techniques in general terms and then
   describes how they may be applied in the LSP layer and in the
   pseudowire layer to meet the requirements of the MPLS-TP recovery
   schemes [RFC5654].  A description of the MPLS-TP LSP and pseudowire
   layers can be found in [MPLS-TP-FWK].




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   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.

   The general description of the functional architecture is applicable
   for both LSPs and pseudowires (PWs).

   This framework applies to general LSP recovery schemes, but also to
   schemes that are optimized for specific topologies in order to handle
   protection switching in a cost-efficient manner.  Recovery schemes
   for PWs are introduced in Section 6, but the details are for further
   study and will be addressed in a separate document in the PWE3
   working group.

   This document takes into account the need for co-ordination of
   protection switches at multiple layers.  This allows an operator to
   prevent races and allows the protection switching mechanism of one
   layer to fix a problem before switching at another layer.

   This framework also specifies the functions that must be supported by
   MPLS-TP to support the recovery mechanisms.  MPLS-TP introduces a
   tool kit to enable recovery in MPLS-TP-based networks and to ensure
   that affected traffic is recovered in the event of a failure.

   Generally, network operators aim to provide the fastest, most stable,
   and the best protection mechanism at a reasonable cost according to
   the requirements of the customers.  The higher the levels of
   protection, the greater the number of resources consumed and so the
   higher the likely cost both to the operator and to the customer.  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.




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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.

   o  Protection: re-establishing end-to-end traffic using pre-allocated
      resources.

   o  Restoration: re-establishing end-to-end traffic using resources
      allocated at the time of need.  Sometimes referred to as "repair".

   o  Recovery: a generic term covering both Protection and Restoration.

   Important background information on survivability can be found in
   [RFC3386], [RFC3469] , [RFC4426], [RFC4427], and [RFC4428].

   In this document, the following additional terminology is applied:

   o  Fault Management refers to the combination of failure detection,
      localization, and notification mechanisms.

   o  Failure is used to indicate both signal failure and signal
      degradation event.

   o  Trigger indicates any event that may be used to cause an
      implementation to consider taking protection action.

   o  The acronym OAM is defined as Operations, Administration and
      Maintenance consistent with [OAM-SOUP].

   General terminology for MPLS-TP is found in [MPLS-TP-FWK] and
   [ROSETTA].  Background information on MPLS-TP can be found in
   [RFC5654].







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3.  Requirements for Survivability

   MPLS-TP requirements are presented in [RFC5654] and serve as a
   normative reference for the definition of all MPLS-TP function
   including survivability.  Survivability is presented in [RFC5654] 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.  Reference numbers refer to
   the requirements as presented in [RFC5654].  Readers should refer to
   [RFC5654] for the definitive list of requirements which is not
   replaced or superseded by the list provided here.

3.1.  General Requirements

   o  Protection and restoration mechanisms must be provided (56).

   o  Recovery techniques should be as similar as possible to those in
      existing transport networks (56A).

   o  Point-to-point (P2P) and point-to-multipoint (P2MP) recovery
      techniques should be the same if possible (56B).

   o  Recovery must be applicable to links, transport paths, segments,
      concatenated segments, and end-to-end LSPs and PWs (57).

   o  Recovery objectives must be configurable to meet the SLA
      objectives of the services offered including rapid (sub-50ms)
      recovery, protection of all traffic on a path, and protection
      across multiple domains (58, 59).

   o  The recovery mechanisms should be applicable to any topology
      (60).See also Section 3.4.

   o  Recovery must be coordinated across network layers (61).

   o  Recovery and reversion must not 'flap' (62).

   Note that there is no requirement for support for extra traffic
   [RFC4427] except in a ring where MPLS-TP must support the sharing of
   protection bandwidth in a ring by allowing best-effort traffic (108).

3.2.  Requirements for Restoration

   o  The restored and protected paths must be able to share resources
      (70).




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   o  Priorities must be available to control the order of restoration
      and to facilitate preemption during restoration (71, 72).

   o  Reversion must be supported (73).

3.3.  Requirements for Protection

   o  MPLS-TP data plane protection must operate without regard to
      payload content (63).

   o  The following protection schemes must be supported:

      *  reversion (64).

      *  unidirectional and bidirectional 1+1 protection for P2P (65A,
         65B)).

      *  unidirectional 1+1 protection for P2MP (65C).

      *  bidirectional 1:n protection for P2P (67A).

      *  unidirectional 1:n protection for P2MP (67B).

   o  It must be possible to share protection resources (66).  This
      includes:

      *  1:n mesh recovery should be supported (68).

      *  sharing of resources between protection paths that will not be
         required to protect the same fault (69).

3.4.  Requirements for Survivability in Ring Topologies

   o  MPLS-TP recovery mechanisms may be optimized for specific
      topologies provided such optimizations interoperate with, and be
      as similar as possible to, standard techniques to provided end-to-
      end recovery (91, 100).

   o  Ring topologies support must include:

      *  single ring (92)

      *  interconnected rings (93)

      *  connection of rings to arbitrary networks (99)

      *  logical and physical rings (101)




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   o  Traffic protection in rings must include:

      *  unidirectional and bidirectional P2P paths (94)

      *  unidirectional P2MP paths (95)

   o  Ring recovery techniques:

      *  must default to bidirectional (102)

      *  must support reversion as the default behavior (103)

      *  must distinguish (to the operator) trigger mechanisms (104)

      *  should protect against multiple failures (106B)

      *  must support sharing of protection resources (109)

      *  must prevent recovery flapping (107)

   o  Ring protection mechanism scaling must include:

      *  1+1 and 1:1 protection switching 50 ms from the moment of fault
         detection in a network with a 16-node ring with less than
         1200km of fiber (96)

      *  independence from the number of LSPs crossing the ring (97)

      *  good performance with increases in the number of transport
         paths, the number of nodes on the ring, and the number of ring
         interconnects (98)

   o  It must be possible to disable protection mechanisms on selected
      links in a ring (105).

   o  MPLS-TP recovery mechanisms in a ring must support prioritization
      of recovery actions arising from different commands or triggers
      and for different protected entities (106A).

3.5.  Triggers for Protection, Restoration, and Reversion

   o  Triggers must be supported from:

      *  lower network layers (74)

      *  MPLS-TP OAM (75)





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      *  the management plane (76)

      *  the control plane (if present) (78)

   o  It must be possible to distinguish trigger sources and to
      prioritize recovery action requests (77, 79).

3.6.  Management Plane Operation

   o  Support is required for preplanning, pre-calculation, and pre-
      provisioning of recovery paths and groups of paths (80, 81, 82,
      85).

   o  External commands (controls) must allow the operator to effect,
      prevent, or test without effecting, any recovery operation (83,
      84).

   o  It must be possible to configure all aspects of recovery (86).

   o  It must be possible to monitor all aspects of recovery (87, 88).

3.7.  Control Plane and In-band OAM

   o  If a control plane is used, it must be possible operate all
      aspects of recovery (89).

   o  In-band OAM must support administrative control and protection
      state coordination (90).


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 MPLS-TP network nodes (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.



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   These different options are explored in the next sections.

4.1.1.  Manual Control

   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 requested 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.

   It should be noted that it is unusual to present a user or customer
   with options directly related to recovery actions.  Instead, the
   user/customer enters into an SLA with the network provider, and the
   network operator maps the terms of the SLA (for example for
   guaranteed delivery, availability, or reliability) onto recovery
   schemes within the network.

   The operator can also be given manual control of survivability
   actions and events.  For example, the operator may perform the
   following actions:

   o  inhibit survivability actions

   o  enable or disable survivability function

   o  induce the simulation of a network fault or force a switchover
      from a working path to a recovery path (for network optimization
      purposes with minimal disturbance of services, such as when
      modifying protected or unprotected services, when replacing
      MPLS-TP network nodes, etc.)  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, detection
   of Loss of Light on an optical interface, a failure to receive an OAM
   Continuity message, or a reception of OAM Alarm Report) may
   immediately perform a survivability action.  Recall that the term
   "failure" is used to represent both signal failure and signal
   degradation.



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   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 or when performing a bidirectional
   recovery action.

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 or indicate a degradation in the operation of the
   network, 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 protection domain.

4.1.4.  Control Plane Signaling

   Control plane signaling is responsible for setup, maintenance, and
   teardown of transport paths that are not under management plane
   control.  The control plane may 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
   may 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.

   Note that the terminology in this section is consistent with
   [RFC4427].  Where the terms differ from those in [RFC5654] a mapping
   is provided.






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4.2.1.  Span Recovery

   A span is a single hop between neighboring MPLS-TP 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 MPLS-TP nodes in the MPLS-TP network, however, a TE link
   may be provided by:

   o  a single data link

   o  a series of data links in a lower layer established as an LSP and
      presented to the upper layer as a single TE link

   o  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:

   o  selecting a different TE link from a bundle

   o  moving the TE link so that it is supported by a different data
      link between the same pair of neighbors

   o  re-routing the LSP in the lower layer.

   Moving the protected LSP to another TE link between the same pair of
   neighbors is a form of segment recovery and is described in Section
   4.2.2.

   [RFC5654] refers to a span as a "link".

4.2.2.  Segment Recovery

   An LSP segment is one or more continuous hops on the path of the LSP.
   [RFC5654] 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.

   A PW segment refers to a Single Segment PW (SS-PW) or to a single
   segment out of multi-segment PW (MS-PW) that is set up between two PE
   devices (i.e.  T-PE and S-PE, S-PE and S-PE, or S-PE and T-PE).  As
   indicated in Section 1, the recovery of PWs and PW segments is for
   further study and will be described in a separate document in the
   PWE3 working group.  See also Section 6 of this document.




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   LSP segment recovery involves redirecting of traffic at one end of a
   segment of an LSP onto an alternate path to the other end of the
   segment.  According to the required level of recovery (described in
   Section 4.3), this redirection may be onto a pre-established LSP
   segment, through re-routing of the protected segment, or by tunneling
   the protected LSP through a "bypass" LSP.  For details on recovery
   mechanisms, see Sections 4.4 and 4.5 below.

   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.

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
   recover 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.  Thus, 'cost' in this
   case may be measured as the financial cost of providing resources for
   the recovery scheme, or the financial loss from dedicating resources
   to the recovery scheme such that they cannot be used to draw new
   revenue.

   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



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   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, on PW segments, or on
   end-to-end transport path (LSP or PW).  Note that as indicated in
   Section 3 and [RFC5654], m:n recovery is not required in MPLS-TP.

   Where a bypass tunnel is used (Section 4.4.3), the tunnel might not
   have sufficient resources to simultaneously protect all of the paths
   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
   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



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   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 provide
   an obvious trade-off against the cost reduction (or rather, revenue
   increase) achieved by carrying extra traffic.

   Note that in MPLS-TP support for extra traffic is not required except
   in ring topologies (Section 3 and [RFC5654].

4.3.4.  Restoration and Repair

   This section refers to LSP restoration and repair.  Restoration for
   PWs is for further study and will be described in a separate document
   in the PWE3 working group (see also Section 6).  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 MPLS-TP nodes, 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 even slower
   recovery times for some services.

   Both hard and 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



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   pre-signaled before the restoration action starts, but not pre-
   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 predetermined period of time should elapse before normal
   data traffic is redirected back onto the original working entity.  It
   should be possible for an operator to configure this period of time
   per LSP.  A default value should be defined.

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.  As indicated above, while the functional architecture
   applies to both LSPs and PWs, the mechanism for recovery described in
   this document refers to LSPs and LSP segments only.  Recovery
   mechanisms for pseudowires and pseudowire segment are for further
   study and will be described in a separate document in the PWE3
   working group (see also Section 6).

4.4.1.  Link-Level Protection

   Link-level protection refers to two paradigms: (1) where the
   protection is provided in a lower network layer, and (2) the
   protection is provided by the MPLS-TP link layer.

   Note that link-level protection mechanisms do not protect the nodes
   at each end of the entity (e.g. a link or span) that is protected.
   End-to-end or segment protection should be used in conjunction to
   link-level protection to protect against a failure of the edge nodes.

   Link-level protection offers the following levels of protections:




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   o  Full protection, where a dedicated protection entity (e.g. a link
      or span) is pre-established to protect a working entity.  When the
      working entity 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.  This is referred to in [RFC4427] as 'bulk
      recovery'.

   o  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 recovered and which will not
      depends on local policy.

   When there is no failure on the working entity, the protection entity
   may transport extra traffic which may be preempted when protection
   switching occurs.

   As with recovery in layered networks, a protection mechanism at the
   lower layer needs to be coordinated with protection actions at the
   upper layer in order to avoid race conditions.  In general, this is
   arranged to allow protection actions to be performed in the lower
   layer before any attempt is made to perform protection actions in the
   upper layer.

   A protection mechanism may be provided at the MPLS-TP link layer
   (which connects two MPLS-TP nodes).  Such a mechanism can make use of
   the procedures defined in [RFC5586] to set up in-band communication
   channels at the MPLS-TP link level and use these channels to monitor
   the health of the MPLS-TP link and coordinate the protection states
   between the ends of the MPLS-TP link.

4.4.2.  Alternate Paths and Segments

   The use of alternate paths and segments refers to the paradigm
   whereby protection is performed in the same network layer as the
   protected LSP either for the entire end-to-end LSP or for a segment
   of the LSP.  In this case, hierarchical LSPs are not used - compare
   with Section 4.4.3.

   Different levels of protection may be provided:

   o  Dedicated protection, where a dedicated entity (e.g.  LSP or LSP
      segment) is fully pre-established to protect a working entity
      (e.g.  LSP or LSP segment).  When there is a failure condition on
      the working entity, the traffic is switched onto the protection
      entity.  Dedicated protection may be performed using 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



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      local configuration.

   o  Shared protection, where one or more protection entity is pre-
      established to protect against a failure of one or more working
      entities (1:n or m:n).

   When the fault condition on the working entity is eliminated, the
   traffic should revert back to the working entity in order to allow
   other related working entities to be protected by the shared
   protection resource.

4.4.3.  Protection Tunnels

   A protection tunnel is a hierarchical 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
   network segment and transmitted over the protection tunnel.  The way
   to realize this is using label stacking.  Label mapping may be an
   option as well.

   Different levels of protection may be provided:

   o  Dedicated protection, where the protection tunnel has resource
      reservations sufficient to provide protection for all protected
      LSPs without service degradation.

   o  Shared protection, where the protection 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 3 and detailed in
   [RFC5654], 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



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   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 node, a span, an LSP segment, or an
   end-to-end LSP.  Linear protection provides a clear indication of the
   protection status.

   Linear protection operates in the context of a Protection Domain.  A
   Protection Domain is a special case of a Recovery Domain [RFC4427]
   that applies to the protection function.  A Protection Domain is
   composed of the following architectural elements:

   o  A set of end points which reside at the boundary of the Protection
      Domain.  In this simple case of 1:n or 1+1 P2P protection, exactly
      two endpoints reside at the boundary of the Protection Domain.  In
      each transmission direction one of the end points is referred to
      as a source and the other one is referred to as a sink.  In the
      case of unidirectional P2MP protection, 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.

   o  A Protection Group which consists of a working (primary) path and
      one or more recovery (backup) paths which run between the
      endpoints of the Protection Domain.  In order to guarantee
      protection in all situations, a dedicated recovery path should be
      pre-provisioned to protect against a failure of a working path
      (i.e. 1:1 or 1+1 protection schemes).  Also the working and the
      recovery paths should be disjoint, i.e., the physical routes of
      the working and the recovery paths should have complete physical
      diversity.

   Note that if the resources of the protection path are less that those
   of the working path, the protection path may not have sufficient
   resources to protect the traffic of the working path.

   As mentioned in Section 4.3.2, the resources of the protection path
   may be shared as 1:n.  In such a case, the protection path might not
   have sufficient resources to simultaneously protect all of the
   working paths that may be affected by fault conditions at the same
   time.



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   For P2P paths, both unidirectional and bidirectional protection
   switching is 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
   some level of 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.

   Revertive and non-revertive operations are provided as network
   operator options.

4.5.1.1.  Protection Schemes in Mesh Topologies

   Linear protection supports the protection schemes described in the
   following sub-sections:

4.5.1.1.1.  1:n Linear Protection

   In the 1:1 scheme, a recovery path is allocated to protect against a
   failure of degradation in a working path.  As described above, in
   order to guarantee protection, the recovery entity should support the
   full capacity and bandwidth, but it may be degraded from the normal
   working entity.

   Figure 1 presents 1:1 protection architecture.  In normal conditions
   the data traffic is transmitted over the working entity and the
   recovery entity is in an idle state.  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 transmit over the recovery entity.

                  |-------------Protection Domain-----------------|
                             ==============================
                          /**********Working path***********\
                +--------+   ==============================   +--------+
                | Node  /|                                    |\  Node |
                |  A {<  |                                    | >}  B  |
                |       \|                                    |/       |
                +--------+   ==============================   +--------+
                                     Recovery path
                             ==============================


                   Figure 1: 1:1 protection architecture

   Upon a fault condition (failure or degradation) along the working



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   entity or a specific administrative request, the traffic is switched
   over to the recovery entity.

   Note that in the non-revertive behavior (see section 4.3.5), data
   traffic can be transmitted over the recovery entity also in normal
   conditions.  This can happen after the condition(s) causing the
   switchover has/have been cleared.

   In each transmission direction, the source of the protection domain
   bridges the traffic into the appropriate entity and the sink selects
   the traffic from the appropriate entity.  The source and the sink
   need to coordinate the protection states to ensure that the bridging
   and the selection are done to and from the same entity.  For that
   sake a signaling coordination protocol (either data-plane in-band
   signaling protocol or a control-plane based signaling protocol) is
   needed.

   In bidirectional protection switching, both ends of the protection
   domain switch to the recovery entity (even when the fault is
   unidirectional).  This requires a protocol to try and 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.

   In the general case of 1:n linear protection, one recovery entity is
   allocated 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.

   In case of failures along multiple working entities, priority should
   be set as to which entity is protected.  The protection states
   between the edges of the Protection Domain should be fully
   synchronized to ensure consistent behavior.  As explained above in
   section Revertive behavior is recommended when 1:n is supported.

4.5.1.1.2.  1+1 Linear Protection

   In the 1+1 protection scheme, a fully dedicated recovery path is
   allocated.

   As depicted in figure 2, data traffic is copied at fed at the source
   to both the working and the recovery entities.  The traffic on the
   working and the recovery entities is transmitted simultaneously to
   the sink of the Protection Domain, where the selection between the



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   working and recovery entities is made (based on some predetermined
   criteria).

                   |-------------Protection Domain-----------------|
                             ==============================
                          /**********Working path************\
                +--------+   ==============================   +--------+
                | Node  /|                                    |\  Node |
                |  A {<  |                                    | >}  Z  |
                |       \|                                    |/       |
                +--------+   ==============================   +--------+
                         \***********Recovery path***********/
                             ==============================



                   Figure 2: 1+1 protection architecture

   Note that control traffic between the edges of the Protection Domain
   (such as OAM or control protocol to synchronies the protection state,
   etc.) may be transmitted on a different entity than the one used for
   the protected traffic.  These packets should not be discarded by the
   sink.

   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 traffic is restored to the working entity
   after the condition(s) causing the switchover has/have been cleared.
   To avoid frequent switching in case of intermittent failures when the
   network is not stabilized, traffic is not switched back to the
   working entity before the Wait-to-Restore (WTR) timer has expired.

4.5.1.1.3.  P2MP linear protection

   Linear protection may apply to protect unidirectional P2MP entity
   using 1+1 protection architecture.  The source/root MPLS-TP node
   bridges the user traffic to both the Working and Protected entities.
   Each sink/leaf MPLS-TP node 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 MPLS-TP
   nodes may select the Working entity, while other leaf MPLS-TP nodes
   may select traffic from the Protection entity.

   In a 1:1 P2MP protection scheme, the source/root MPLS-TP node needs



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   to identify the existence of a fault condition on any of the branches
   of the network.  This requires the sink/leaf MPLS-TP nodes to notify
   the source/root MPLS-TP node of any fault condition.  This required
   also a return path from the sinks/leaves to the source/root MPLS-TP
   node.

   When protection switching is triggered, the source/root MPLS-TP node
   selects the recovery transport path to transfer the traffic.

   Note that such a mechanism does not exist and its exact behavior if
   for further study.

4.5.1.2.  Triggers for the Linear Protection Switching Action

   The protection switching may be performed when:

   o  A fault condition ('failed' or 'degraded') is declared on the
      working entity and is not declared on the recovery entity.
      Proactive in-band OAM CC$amp;V (Continuity and Connectivity
      Verification) monitoring of both the working and the recovery
      entities may be used to enable the fast detection of a fault
      condition.  For protection switching, it is common to run a
      CC$amp;V every 3.33ms.  In the absence of three consecutive
      CC$amp;V messages, a fault condition is declared.  In order to
      monitor the working and the recovery entities, an OAM Maintenance
      Entity should be defined for each of the entities.  OAM
      indications of fault conditions should be provided to the edges of
      the Protection Domain which are responsible for the protection
      switching operation.  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 recovery entity is needed only if the recovery
      entity can guarantee better conditions.

   o  An indication is received from a lower layer server that there is
      a network failure.

   o  An external operator command is received (e.g.  'Forced Switch',
      'Manual Switch').  For details see Section 5.1.2.

   o  A request to switch over is received from the far end.  The far
      end may initiate this request for example when it gets an
      administrative request o switch over, or when bidirectional 1:1
      protection switching is supported and there was a fault that could
      be detected only by the far end, etc.

   As described above, in some cases an attempt should be done to
   coordinate the protection states between the end points of the



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   Protection Domain.  Control message should be exchanged between the
   edges of the Protection Domain to synchronize the protection state of
   the edge nodes.  The control messages can be delivered using in-band
   data-plane driven control protocol or a control plane based protocol.

   In order to achieve 50ms protection switching it is recommended to
   use in-band data-plane driven signaling protocol to coordinate the
   protection states.  An in-band data-plane PSC (Protection State
   Coordination) protocol is defined in [MPLS-TP-Linear-Protection] for
   this purpose.  This protocol is also used to detect mismatches
   between the configuration provisioned at the ends of the Protection
   Domain.

   As described below in section 5.5, GMPLS already defines procedures
   and messages' elements to synchronize the protection states between
   the edges of the protection domain.  These procedures and protocols
   messages are specifies in [RFC4426], [RFC4872] and [RFC4873].
   However, these messages lack the capability to synchronize the
   revertive/non-revertive behavior and the consistency of configured
   timers at the edges of the Protection Domain (timers such as Wait to
   Restore (WTR), Hold-off timer, etc.).

4.5.1.3.  Applicability of linear protection for LSP segments

   In order to implement data-plane based linear protection on LSP
   segments, there is a need to support the MPLS-TP architectural
   element PST (Path Segment Tunnel).  Maintenance operations (e.g.
   monitoring, protection or management) engage with a transmission of
   messages (e.g.  OAM, Protection Path Synchronization, etc.) in the
   maintained domain.  According to the MPLS architecture which is
   defined in [RFC3031], such messages can be initiated and terminated
   at the edges of a path where push and pop operations are enabled.  As
   an exception, these messages may be terminated at an intermediate
   node when the TTL value is expired.  In order to support the option
   to monitor, protect and manage a portion of an LSP, a new
   architectural element is defined, Path Segment Tunnel (PST).  A Path
   Segment Tunnel is an LSP which is basically defined and used for the
   purposes of OAM monitoring, protection or management of LSP segments.
   PST makes use of the MPLS construct of hierarchical nested LSP which
   is defined in [RFC3031].

   For linear protection operation, PSTs should be defined over the
   working and recovery entities between the edges of a Protection
   Domain.  OAM and PSC messages can be initiated at the edge of the PST
   and sent to the peer edge of the PST.  Note that these messages are
   sent over G-ACH channels, within the PST and use two labels stack,
   the PST label at the bottom of stack and the G-ACH label.




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   The end-to-end traffic of the LSP, including data-traffic and control
   traffic (OAM, PSC, management and signaling messages) is tunneled
   within the PSTs by means of label stacking as defined in [RFC3031].

   The mapping between an LSP and a PST can be 1:1 which is similar to
   the ITU-T Tandem Connection element which defines a sub layer
   corresponding to a segment of a path.  The mapping can also be 1:n to
   allow scalable protection of a set of LSPs' segments traversing the
   portion of the network in which a Protection Domain is defined.  Note
   that each of these LSPs can be initiated or terminated at different
   endpoints in the network, but they all traverse the Protection Domain
   and share similar constraints (such as requiremtns for QoS, terms of
   protection ,etc.).  In case of 1:n mapping PSTs can be referred to
   also as TE links.

   Note that in the context of segment recovery, the PSTs serve as the
   working and protection entities.

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
       network, the total bandwidth needed in the ring to ensure the
       protection of the paths, etc.

   [RFC5654] 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.




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   The architecture and the mechanisms for ring protection are specified
   in separate documents.  These mechanisms need to be evaluated against
   the requirements specified in [RFC5654].  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 or more recovery
   reference endpoints which are located at the edges of the recovery
   domain and bounds the element on which recovery can be provided (as
   described in section 4.2 above).  This element can be end-to-end
   path, a portion of a path or a span.

   The case of an end-to-end path can be observed as a special case of a
   portion of a path, and the ingress and the egress LERs serve as the
   recover reference end-points.

   In this simple case of a P2P protected entity, exactly two endpoints
   reside at the boundary of the Protection Domain.  An LSP can enter
   through exactly one reference endpoint and exit the recovery domain
   through another reference endpoint.

   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.  An LSP can enter the recover domain through the root
   point and exit the recovery domain through the leaves points.

   The recovery mechanism should restore interrupted traffic due to a
   facility (link or node) fault within the recovery domain.  Note that
   a single link may part of several recovery domains.  If two recovery
   domains have any links in common, then one recovery domain must be
   contained with the other.  This can be referred to as nested recovery
   domains.  However recovery domains must not overlap.

   Note that the edges of a recovery domain are not protected and unless
   contained in another recovery domain, they form a single point of
   failure.

   A recovery group is defined within a recovery domain and it consists



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   of a working (primary) entity and one or more recovery (backup)
   entities which reside between the endpoints of the recovery Domain.
   In order to guarantee protection in all situations, a dedicated
   recovery entity should be pre-provisioned using disjoint resources in
   the recovery domain in order to protect against a failure of a
   working entity.

   The method used to monitor the health of the recovery element is
   unimportant, provided that the endpoints which are responsible for
   the recovery action receive the information on its condition.  The
   condition of the recovery element may be 'OK', 'failed', or
   'degraded'.

   When the recovery operation is triggered by an OAM FM or PM
   indication, an OAM Maintenance Entity Group is defined for each of
   the working and protection entities.

   The recovery entities and functions in a recovery domain can be
   provisioned using a management plane or a control plane.  A
   management plane may be used to configure the recovery domain by
   setting the reference points, the working and recovery entities, and
   the recovery type (e.g. 1:1 bidirectional linear protection, ring
   protection, etc.).  Additional parameters associated with the
   recovery process may also be configured.  For more details, see
   section 5.1.

   When a control plane is used, the ingress LERs may communicate with
   the recovery reference points requesting protection or restoration
   across a recovery domain.  For details, see section 5.5.

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



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   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.

   The hold-off timer should be configurable.

   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

   Where a link in the MPLS-TP network is formed from connectivity (i.e.
   a packet or non-packet LSP) in a lower layer network, that
   connectivity may itself be protected.  For example, the LSP in the
   lower layer network may be provisioned with 1+1 protection.  In this
   case the link in the MPLS-TP network as an inherited level of
   protection.

   An LSP in the MPLS-TP network may be provisioned with protection in
   the MPLS-TP network as already described, or it may be provisioned to
   utilize only links that themselves have inherited protection.

   By classifying the links in the MPLS-TP network according to the
   level of underlying protection that they have, it is possible to
   compute an end-to-end path in the MPLS-TP network that uses only
   links with a specific or better level of inherited protection.  This
   means that the end-to-end MPLS-TP LSP can be protected at the level
   necessary to conform with the SLA without the need to provide any
   additional protection in the MPLS-TP layer.  This saves complexity
   and network resources, and reduces issues of protection switching
   coordination.

   Where the requisite level of inherited protection is not available
   along the whole path in the MPLS-TP network, it can be "topped up"
   using protection in the MPLS-TP layer.  Segment protection would be
   particularly suitable.




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   It should be noted, however, that inherited protection only applies
   to links.  Nodes cannot be protected in this way.  An operator will
   need to perform an analysis of the relative likelihood and
   consequences of node failure if this approach is taken without
   providing any protection in the MPLS-TP layer to handle node failure.

4.6.2.  Shared Risk Groups

   When an MPLS-TP protection scheme is established, it is essential
   that the working and protection paths do not share resources in the
   network.  If this is not achieved, a single failure may affect both
   the working and the protection path with the result that the traffic
   cannot be delivered - it was, in fact, not protected.

   Note that this restriction does not apply for restoration as this
   takes place after the fault has arisen meaning that the point of
   failure can be avoided.

   When planning a recovery scheme it is possible to select paths that
   use diverse links and nodes within the MPLS-TP network using a
   topology map of the network.  However, this does not guarantee that
   the paths are truly diverse.  For example, two separate links in an
   MPLS-TP network may be provided by two lambdas in the same optical
   fiber, or by two fibers that cross the same bridge.  And two
   completely separate MPLS-TP nodes might be situated in the same
   building with a shared power supply.

   Thus, in order to achieve proper recovery planning, the MPLS-TP
   network must have an understanding of the groups of lower layer
   resources that share a common risk of failure.  From this, MPLS-TP
   shared risk groups can be constructed that show which MPLS-TP
   resources share a common risk of failure.  The working and protection
   paths can be planned to be not only node and link diverse, but to not
   use any resources from the same shared risk groups.

4.6.3.  Fault Correlation

   TBD.  This is about correlating multiple faults from the lower layers
   to observe that they all represent the same fault in the MPLS-TP
   layer.


5.  Mechanisms for Providing Protection of MPLS-TP LSPs

   This section describes the existing mechanisms available to provide
   protection of LSPs 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



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   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 endpoints points (which controls the recovery
   actions), the working and the recovery 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
   WTR and hold-off timers, revertive/non-revertive operation, etc.) may
   also be configured.

   In addition, the management plane may initiate manual control of the
   recovery function.  A priority should be set between fault conditions
   and operator's requests.

   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 PSC (protection State Coordination) which is
   specified in [MPLS-TP-Linear-Protection] may be used as an in-band
   (i.e. data plane-based) control protocol to coordinate the protection
   states between the endpoints of the recovery domain and to check
   consistency of configured parameters (such as timers, revertive/
   non-revertive behavior, etc.)

   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 mechanisms, the
   following entities and information should be configured and
   provisioned:

   o  The endpoints of a recovery domain.  As described above, these
      endpoints bound the element of recovery for which recovery is
      applied.





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   o  The protection group which depending on the required protection
      scheme, consists of a recovery entity and one or more working
      entities.  In 1:1 or 1+1 P2P protection, in order to guarantee
      protection, the paths of the working entity and the recovery
      entities should have complete physical diversity.

   o  As defined in section 4.5.2, in order to implement data-plane
      based LSP segment recovery, there is a need to support the MPLS-TP
      architectural element PST (Path Segment Tunnel), since related
      control messages (e.g. for OAM, Protection Path Synchronization,
      etc.) can be initiated and terminated at the edges of a path where
      push and pop operations are enabled.  PST is an end-to-end LSP
      which corresponds in this context to the recovery entities
      (working and protection) and makes use of the MPLS construct of
      hierarchical nested LSP which is defined in [RFC3031].  OAM and
      PSC messages can be initiated at the edge of the PST and sent to
      the peer edge of the PST, over G-ACH.  There is a need to
      configure the related PSTs and map between the LSP segment(s)
      being protected and the PST.  The mapping can be 1:1 or 1:N to
      allow scalable protection of a set of LSPs' segments traversing
      the portion of the network in which a Protection Domain is
      defined.  Note that each of these LSPs can be initiated or
      terminated at different endpoints in the network, but they all
      traverse the Protection Domain and share similar constraints (such
      as requirements for QoS, terms of protection ,etc.).

   o  The protection type that should be defined (e.g. unidirectional
      1:1, bidirectional 1+1, etc.).

   o  Revertive/non-revertive behavior should be configured.

   o  timers (such as WTR, hold-off timer, etc.) should be set.

5.1.2.  External Manual Commands

   The following external, manual commands may be provide for manual
   control of the protection switching operation.  These commands apply
   to a protection group and they are listed in descending order of
   priority:

   o  Blocked protection action - a manual command to prevent data
      traffic from switching to the recovery entity.  This command
      actually disables the protection group.

   o  Force protection action - a manual command that forces a switch of
      normal data traffic to the recovery entity.





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   o  Manual protection action - a manual command that forces a switch
      of data traffic to the recovery entity when there is no failure in
      the working or the recovery entity.

   o  Clear switching command - the operator may request to clear
      previous administrative command to switch over (manual or force
      switch).

5.2.  Fault Detection

   Fault detection is a fundamental part of recovery and survivability.
   In all schemes except for some forms of 1+1 protection, the necessary
   actions for recovery of traffic delivery rely on discovering that
   there is some kind of fault.

   Faults may be detected in a number of ways depending on the traffic
   pattern and the underlying hardware.  End-to-end faults may be
   reported by the application or by knowledge of the application's data
   pattern, but this is an unusual approach.  There are two more common
   mechanisms for detecting faults in the MPLS-TP layer:

   o  faults reported by the lower layers

   o  faults detected by protocols within the MPLS-TP layer.

   In an IP/MPLS network, the second of these may utilize control plane
   protocols (such as the routing protocols) to detect a failure of
   adjacency between neighboring nodes.  In an MPLS-TP network, there is
   no certainty that a control plane will be present.  Even if a control
   plane is present, it will be a GMPLS control plane [RFC3945] that
   makes a logical separation between control channels and data channels
   with the result that no conclusion about the health of a data channel
   can be drawn from the failure of an associated control channel.
   MPLS-TP layer faults are, therefore, only detected through the use of
   OAM protocols as described in Section 5.4.1.

   Faults may, however, be reported by lower layer.  These generally
   show up as interface failures or link failures within the MPLS-TP
   network.  For example, an underlying optical link may detect loss of
   light and report a failure of the MPLS-TP link that uses it.
   Alternatively, and interface card failure may be reported to the
   MPLS-TP layer.

   Such failures will only be reported after link level protection has
   been attempted (Section 4.6.1) and it is important that any lower
   layer recovery actions are coordinated with the MPLS-TP recovery
   actions (Section 4.6).




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   Faults reported by lower layers are only visible at specific nodes
   within the MPLS-TP network (i.e. at the adjacent end-points of the
   MPLS-TP link).  This only allows recovery to be performed locally.

   In order that recovery can be performed by nodes that are not
   immediately local to the fault, the fault must be reported (Sections
   5.4.3 and 5.5.4).

5.3.  Fault Isolation

   If an MPLS-TP node detects that there is a fault in an LSP (that is,
   not a network fault reported from a lower layer, but a fault detected
   by examining the LSP) it can immediately perform a recovery action.
   However, unless the location of the fault is known, the only
   practical options are:

   o  perform end-to-end recovery

   o  perform some other recovery as a speculative act.

   Since speculative acts are not guaranteed to achieve the desired
   results and could be costly, and since end-to-end recovery is a
   costly option, it is important to be able to isolate the fault.

   Fault isolation may be achieved by dividing the network into
   protection domains.  End-to-end protection is thereby operated on an
   LSP segments depending on the domain in which the fault is
   discovered.  This requires that the LSP can be monitored at the
   domain edges.

   Alternatively, a proactive mechanism of fault isolation through OAM
   (Section 5.4.2) or through the control plane (Section 5.5.3) is
   required.

5.4.  OAM Signaling

   MPLS-TP provides comprehensive set of OAM tools for fault management
   and performance monitoring at different nested levels (end-to-end, a
   portion of a path (LSP or PW) and at the link level).

   These tools support proactive and on-demand fault management (for
   fault detection and fault localization) and for performance
   monitoring (to measure the quality of the signals and detect
   degradation).

   To support fast recovery, it is useful to use some of the proactive
   tools to detect fault conditions (e.g. link/node failure or
   degradation) and trigger the recovery action.



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   The MPLS-TP OAM messages run in-band with the traffic and support
   unidirectional and bidirectional P2P paths as well as P2MP paths.

   As described in [MPLS-TP-OAM-Framework], MPLS-TP OAM operates in the
   context of a Maintenance Entity which bounds the OAM responsibilities
   and represents the portion of a path between two points which is
   being monitored and maintained, and in which OAM messages are
   exchanged.  [MPLS-TP-OAM-Framework] refers also to a Maintenance
   Entity Group (MEG), which is a collection of one or more MEs that
   belongs to the same transport path (e.g.  P2MP transport path) and
   that are maintained and monitored as a group.

   An ME includes two MEPs (Maintenance Group End Points) which reside
   at the boundaries of an ME, and a set of zero or more MIPS
   (Maintenance Group Intermediate Points) which reside within the
   Maintenance Entity along the path.  A MEP is capable of initiating
   and terminating OAM messages, and as such can only located at the
   edges of a path where push and pop operations are supported.  In
   order to be define an ME over a portion of path there is a need to
   support the MPLS-TP architectural element PST (Path Segment Tunnel).
   PST is an end-to-end LSP which corresponds in this context to the ME
   and makes use of the MPLS construct of hierarchical nested LSP which
   is defined in [RFC3031].  OAM messages can be initiated at the edge
   of the PST and sent to the peer edge of the PST, over G-ACH.

   There is a need to configure the related PSTs and map between the LSP
   segment(s) being monitored and the PST.  The mapping can be 1:1 or
   1:N to allow scalable operation.  Note that each of these LSPs can be
   initiated or terminated at different endpoints in the network and
   share similar constraints (such as requirements for QoS, terms of
   protection ,etc.).

   In the context of recovery where MPLS-TP OAM is supported, an OAM
   Maintenance Entity Group is defined for each of the working and
   protection entities.

   MIP is capable of reacting to OAM messages.

5.4.1.  Fault Detection

   MPLS-TP OAM tools may be used proactively to detect the following
   fault conditions between MEPs:

   o  Loss of continuity and misconnectivity - the proactive Continuity
      Check (CC) function is used to detect loss of continuity between
      two MEPs in an MEG.  The proactive misconnectivity (CV) allows a
      sink MEP can detect misconnectivity defect (e.g. mismerge or
      misconnection) with its peer source MEP when the received packet



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      carries an incorrect ME identifier.  For protection switching, it
      is common to run CC&V (Continuity & Connectivity Verification)
      message every 3.33ms.  In the absence of three consecutive CC&V
      messages, Loss of Continuity is declared and locally notified to
      the edge of the recovery domain to trigger a recovery action.  In
      some cases, when a slower recovery time is acceptable, it is also
      possible to lengthen the transmission rate.

   o  Signal degradation - notification from the 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 recovery entity is needed only if
      the recovery entity can guarantee better conditions.  Degradation
      can be measured activating proactively the MPLS-TP OAM packet loss
      measurement or delay measurement.

   o  A MEP can get an indication from its sink MEP of a Remote Defect
      Indication and locally notify the endpoint of the recovery domain
      of fault condition to trigger the recovery action.

5.4.2.  Fault Isolation

   MPLS-TP provides OAM tools to isolate a fault and 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.  Finer granularity of
   information is needed to implement optimal recovery actions or to
   diagnose the fault.  On-demand tools like trace-route, loopback and
   on-demand CC&V can be used to isolate a fault.

   The information may be locally notified to the endpoint of the
   recovery domain to allow him implementing optimal recovery action.
   This may be useful in case of re-calculation of a recovery path.

   The information should also be reported to the network management for
   diagnostics purposes.

5.4.3.  Fault Reporting

   The endpoints of a recovery domain should be able to report a network
   management on fault conditions detected in the recovery domain.

   In addition, a node within a recovery domain detecting a fault
   condition should also be able to report the fault condition to the
   network management.  The network management should be capable to
   correlate the fault reports and identify the source of the fault.




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   MPLS-TP OAM tools support a function where an intermediate node along
   a path can send an alarm report message to the MEP indicating of a
   fault condition in the server layer connecting it to its adjacent
   node.  The purpose of this capability is to allow a MEP to suppress
   alarms that may be generated as a result of the failure condition in
   the server layer.

5.4.4.  Coordination of Recovery Actions

   As described above, in some cases (such as in bidirectional
   protection switching, etc.) there is a need to coordinate the
   protection states between the edges of the recovery domain.
   [MPLS-TP-Linear-Protection] defines procedures and protocol messages
   and elements to support the PSC (Protection State Coordination)
   function.

   The protocol is also used to signal administrative requests (e.g.
   manual switch, etc.) when these are provisioned only at on edge of
   the recovery domain.

   The protocol also allow to detect mismatches between the
   configuration provisioned at the ends of the Protection Domain (such
   as timers, revertive/non-revertive behavior).

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



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   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 [RFC5654]
   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
   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



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   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.

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



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   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 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 for this
   operation.

   A further piece of work in needed to allow control and configuration
   of reversion behavior for end-to-end and segment protection, and the
   coordination of timers' values.

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:

   o  [RFC4872] provides features in support of end-to-end protection
      switching.

   o  [RFC4873] describes how to establish a single, segment protected
      LSP.  Note that end-to-end protection is a sub case of segment
      protection and [RFC4872] can be used also to provide end-to-end



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      protection.

   o  [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.

   o  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

   Pseudowire is one of the clients of MPLS-TP.  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.

   As indicated above, the functional architecture described in this
   document applies to both LSPs and pseudowires.  However the recovery
   mechanisms for pseudowires are for further study and will be defined
   in a separate document in the PWE3 working group.

6.1.  Utilizing Underlying MPLS-TP Recovery

   MPLS-TP PWs are carried across the network inside MPLS-TP LSPs.
   Therefore, an obvious way to protect a PW is to protect the LSP that
   carries it.  Such protection can take any of the forms described in
   this document.  The choice of recovery scheme will depend on the
   speed of recovery necessary and the traffic loss that is acceptable
   for the SLA that the PW is providing.

   If the PW is a multi-segment PW, then LSP recovery can only protect
   the PW on individual segments.  That is, LSP recovery cannot protect
   against a failure of a PW switching point (an S-PE), nor can it
   protect more than one segment at a time since the LSP tunnel is
   terminated at each S-PE.  In this respect, the LSP protection of a PW
   is very much like the link-level protection offered to the MPLS-TP
   LSP layer by an underlying network layer (see Section 4.6).






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6.2.  Recovery in the Pseudowire Layer

   Recovery in the PW layer can be provided simply by running separate
   PWs either end-to-end or between S-PEs.

   As with any recovery mechanism, it is important to coordinate between
   layers.  This coordination is necessary to ensure that recovery
   mechanisms are only actioned in one layer at a time (that is, the
   recovery of an underlying LSP needs to be coordinated with the
   recovery of the PW itself), and to make sure that the working and
   protection PWs do not both use the same MPLS resources within the
   network (for example, by running over the same LSP tunnel - compare
   with Section 4.6.2).


7.  Manageability Considerations

   TBD


8.  Security Considerations

   TBD


9.  IANA Considerations

   This informational document makes no requests for IANA action.


10.  Acknowledgments

   Thanks for useful comments and discussions to Italo Busi, David
   McWalter, Lou Berger and Yaacov Weingarten.


11.  References

11.1.  Normatve References

   [RFC2205]  Bradner, S., Ed., Zhang, L., Berson, S., Herzog, S., and
              J. Jamin, "Resource ReserVation Protocol &mdash 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.




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   [RFC3471]  Berger, L., Ed., "Generalized Multi-Protocol Label
              Switching (GMPLS) Signaling Functional Description",
              RFC 3471, January 2003.

   [RFC3473]  Berger, L., "Generalized Multi-Protocol Label Switching
              (GMPLS) Signaling Resource ReserVation Protocol-Traffic
              Engineering (RSVP-TE) Extensions", RFC 3473, January 2003.

   [RFC3945]  Mannie, E., "Generalized Multi-Protocol Label Switching
              (GMPLS) Architecture", RFC 3945, October 2004.

   [RFC4203]  Kompella, K. and Y. Rekhter, "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 D. Papadimitriou, "Recovery (Protection and
              Restoration) Terminology for Generalized Multi-Protocol
              Label Switching (GMPLS)", RFC 4427, March 2006.

   [RFC4428]  Papadimitriou, D. and E. Mannie, "Analysis of Generalized
              Multi-Protocol Label Switching (GMPLS) - based  Recovery
              Mechanisms (including Protection and Restoration) Recovery
              (Protection and Restoration) Terminology for Generalized
              Multi-Protocol Label Switching (GMPLS)", RFC 4428,
              March 2006.

   [RFC4873]  Berger, L., Bryskin, I., Papadimitriou, D., and A. Farrel,
              "GMPLS Segment Recovery", RFC 4873, May 2007.

   [RFC5307]  Kompella, K. and Y. Rekhter, "IS-IS Extensions in Support
              of Generalized Multi-Protocol Label Switching (GMPLS)",
              RFC 5307, October 2008.

   [RFC5317]  Bryant, S. and L. Andersson, "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.




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   [RFC5654]  Niven-Jenkins, B., Ed., Brungard, D., Ed., Betts, M., Ed.,
              Sprecher, N., and S. Ueno, "Requirements of an MPLS
              Transport Profile", RFC 5317, February 2009.

   [RFC5586]  Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
              "MPLS Generic Associated Channel", RFC 5586, June 2009.

   [MPLS-TP-FWK]
              Vigoureux, M., Ed., Ward, D., Ed., and Betts, "A Framework
              for MPLS in Transport Networks", MPLS-TP-FWK, Work in
              Progress.

   [MPLS-TP-OAM]
              Buci, I., Ed. and B. Niven-Jenkins, Ed., "Requirements for
              OAM in MPLS Transport Networks",
              draft-ietf-mpls-tp-oam-requirements, Work in Progress.

   [MPLS-TP-OAM-Framework]
              Buci, I., Ed. and B. Niven-Jenkins, Ed., "A Framework for
              MPLS in Transport Networks",
              draft-ietf-mpls-tp-oam-framework, Work in Progress.

11.2.  Informative References

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3386, November 2002.

   [RFC3386]  Lai, W. and D. McDysan, "Network Hierarchy and Multilayer
              Survivability", RFC 3386, November 2002.

   [RFC3469]  Sharma, V. and F. Hellstrand, "Framework for Multi-
              Protocol Label Switching (MPLS)-based Recovery", RFC 3469,
              February 2003.

   [RFC4426]  Lang, J., Ed., Rajagopalan, B., and D. Papadimitriou,
              "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 D. Papadimitriou, "RSVP-TE
              Extensions in Support of End-to-End Generalized Multi-
              Protocol Label Switching (GMPLS) Recovery", RFC 4872,
              May 2007.

   [RFC4874]  Lee, CY., Farrel, A., and S. De Cnodder, "Exclude Routes -
              Extension to Resource ReserVation Protocol- Traffic



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              Engineering (RSVP-TE)", RFC 4874, April 2007.

   [RFC5298]  Takeda, T., Farrel, A., Ikejiri, Y., and JP. Vasseur,
              "Analysis of Inter-Domain Label Switched Path (LSP)
              Recovery", RFC 5298, August 2008.

   [MPLS-TP-Linear-Protection]
              Weingarten, Y., Bryant, S., Ed., Sprecher, N., Ed., Van
              Helvoort, H., Ed., and A. Fulignoli, "MPLS-TP Linear
              Protection",
              draft-weingarten-mpls-tp-linear-protection, Work in
              Progress.

   [GMPLS-OAM]
              Takacs, A., Fedyk, D., and H. Jia, "OAM Configuration
              Framework and Requirements for GMPLS RSVP-TE",
              draft-ietf-ccamp-oam-configuration-fwk, Work in Progress.

   [OAM-SOUP]
              Andersson, L., Betts, M., Van Helvoort, H., Bonica, R.,
              and D. Romascanu, "MPLS-TP Linear Protection",
              draft-ietf-opsawg-mpls-tp-oam-def, Work in Progress.

   [ROSETTA]  Van Helvoort, H., Ed., Andersson, L., and N. Sprecher, "A
              Thesaurus for the Terminology used in Multiprotocol Label
              Switching Transport Profile (MPLS-TP) drafts/RFCs and
              ITU-T's Transport Network Recommendations",
              draft-ietf-mpls-tp-rosetta-stone, Work in Progress.


Authors' Addresses

   Nurit Sprecher
   Nokia Siemens Networks
   3 Hanagar St. Neve Ne'eman B
   Hod Hasharon,   45241
   Israel

   Email: nurit.sprecher@nsn.com


   Adrian Farrel
   Old Dog Consulting

   Email: adrian@olddog.co.uk






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