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Versions: 00 01 02 03 04 RFC 4426

CCAMP Working Group                                Jonathan P. Lang, Ed
Internet Draft                                    Bala Rajagopalan, Ed.
Expiration Date: March, 2004








                                                        September, 2003



           Generalized MPLS Recovery Functional Specification

           draft-ietf-ccamp-gmpls-recovery-functional-01.txt


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [RFC2026].

   Internet-Drafts are working documents of the Internet Engineering
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Abstract

   This document presents a functional description of the protocol
   extensions needed to support GMPLS-based recovery (i.e. protection
   and restoration). Protocol specific formats and mechanisms will be
   described in companion documents.






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Contributors

   This document was the product of many individuals working together
   in the CCAMP WG Protection and Restoration design team.  The
   following are the authors that contributed to this document:

   Deborah Brungard (AT&T)
   Rm. D1-3C22 - 200 S. Laurel Ave.
   Middletown, NJ 07748, USA
   E-mail: dbrungard@att.com

   Sudheer Dharanikota (Consult)
   E-mail: sudheer@ieee.org

   Jonathan P. Lang
   Email: jplang@ieee.org

   Guangzhi Li (AT&T)
   180 Park Avenue,
   Florham Park, NJ 07932, USA
   E-mail: gli@research.att.com

   Eric Mannie (Consult)
   E-mail: eric_mannie@hotmail.com

   Dimitri Papadimitriou (Alcatel)
   Francis Wellesplein, 1
   B-2018 Antwerpen, Belgium
   E-mail: dimitri.papadimitriou@alcatel.be

   Bala Rajagopalan
   Tellium
   2 Crescent Place - P.O. Box 901
   Oceanport, NJ 07757-0901, USA
   Email: braja@tellium.com

   Yakov Rekhter (Juniper)
   1194 N. Mathilda Avenue
   Sunnyvale, CA 94089, USA
   E-mail: yakov@juniper.net

1. Introduction

   A requirement for the development of a common control plane for both
   optical and electronic switching equipment is that there must be
   signaling, routing, and link management mechanisms that support data
   plane fault recovery.  In this document, the term "recovery" is
   generically used to denote both protection and restoration; the
   specific terms "protection" and "restoration" are only used when
   differentiation is required.  The subtle distinction between

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   protection and restoration is made based on the resource allocation
   done during the recovery period (see [TERM]).

   A label-switched path (LSP) may be subject to local (span), segment,
   and/or end-to-end recovery. Local span protection refers to the
   protection of the link (and hence all the LSPs marked as required
   for span protection and routed over the link) between two
   neighboring switches. Segment protection refers to the recovery of
   an LSP segment (i.e., an SNC in the ITU-T terminology) between two
   nodes, i.e. the boundary nodes of the segment. End-to-end protection
   refers to the protection of an entire LSP from the ingress to the
   egress port. The end-to-end recovery models discussed in this draft
   apply to segment protection where the source and destination refer
   to the protected segment rather than the entire LSP. Multiple
   recovery levels may be used concurrently by a single LSP for added
   resiliency; however, the interaction between levels becomes
   affecting any one direction of the LSP results in both directions of
   the LSP being switched to a new span, segment, or end-to-end path.

   Unless otherwise stated, all references to ôlinkö in this draft
   indicate a bi-directional link (which may be realized as a pair of
   unidirectional links).

   Consider the control plane message flow during the establishment of
   an LSP. This message flow proceeds from an initiating (or source)
   node to a terminating (or destination) node, via a sequence of
   intermediate nodes. A node along the LSP is said to be UPSTREAM from
   another node if the former occurs first in the sequence. The latter
   node is said to be DOWNSTREAM from the former node. That is, an
   UPSTREAM node is closer to the initiating node than a node further
   DOWNSTREAM. Unless otherwise stated, all references to UPSTREAM and
   DOWNSTREAM are in terms of the control plane message flow.

   The flow of the data traffic is defined from ingress (source node)
   to egress (destination node). Note that for bi-directional LSPs
   there are two different data plane flows, one for each direction of
   the LSP. This document presents a protocol functional description to
   support GMPLS-based recovery (i.e., protection and restoration).
   Protocol specific formats and mechanisms will be described in
   companion documents.

   2. Span Protection

   Consider a (working) link i between two nodes A and B. There are two
   fundamental models for span protection. The first is referred to as
   1+1 protection. Under this model, a dedicated link j is pre-assigned
   to protect link i. LSP traffic is permanently bridged onto both
   links i and j at the ingress node and the egress node selects the
   signal (i.e., normal traffic) from i or j, based on a selection
   function (e.g., signal quality). Under unidirectional 1+1 span
   protection (Section 2.1), each node A and B acts autonomously to
   select the signal from the working link (i) or the protection link

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   (j). Under bi-directional 1+1 span protection (Section 2.2) the two
   nodes A and B coordinate the selection function such that they
   select the signal from the same link, i or j.

   Under the second model, a set of N working links are protected by a
   set of M protection links, usually cwith M <= N. A failure in any of    the N working links results in traffic being switched to one of the
   M protection links that is available. This is typically a three-step
   process: first the data plane failure is detected at the egress node
   and reported (notification), then a protection link is selected, and
   finally, the LSPs on the failed link are moved to the protection
   link. If reversion is supported, a fourth step is included, i.e.
   return of the traffic to the working link (when the working link has
   recovered from the failure). In Section 2.3, 1:1 span protection is
   described. In Section 2.4, M:N span protection is described, where M
   <= N.

   2.1 Unidirectional 1+1 dedicated protection

   Suppose a bi-directional LSP is routed over link i between two nodes
   A and B. Under unidirectional 1+1 protection, a dedicated link j is
   pre-assigned to protect the working link i. LSP traffic is
   permanently bridged on both links at the ingress node and the egress
   node selects the normal traffic from one of the links, i or j. If a
   node (A or B) detects a failure of a span, it autonomously invokes a
   process to receive the traffic from the protection span. Thus, it is
   possible that node A selects the signal from link i in the B to A
   direction of the LSP, and node B selects the signal from link j in
   the A to B direction.

   The following functionality is required for 1+1 unidirectional span
   protection:

     o Routing: A single TE link encompassing both working and
        protection links should be announced with Link Protection Type
        ôDedicated 1+1ö along with the bandwidth parameters for the
        working link. As the resources are consumed/released, the
        bandwidth parameters of the TE link are adjusted accordingly.
        Encoding of the Link Protection Type and bandwidth parameters
        in IS-IS is specified in [GMPLS-ISIS].  Encoding of this
        information in OSPF is specified in [GMPLS-OSPF].

     o Signaling: The Link Protection object/TLV should be used to
        request "Dedicated 1+1" link protection for that LSP. This
        object/TLV is defined in [GMPLS-SIG]. If the Link Protection
        object/TLV is not used, link selection is a matter of local
        policy. No additional signaling is required when a fail-over
        occurs.

     o Link management: Both nodes must have a consistent view of the
        link protection association for the spans. This can be done


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        using the Link Management Protocol (LMP), or if LMP is not
        used, this must be configured manually.


2.2 Bi-directional 1+1 dedicated protection

   Suppose an LSP is routed over link i between two nodes A and B.
   Under bi-directional 1+1 protection, a dedicated link j is pre-
   assigned to protect the working link i. LSP traffic is permanently
   duplicated on both links and under normal conditions, the traffic
   from link i is received by nodes A and B (in the appropriate
   directions).  A failure affecting link i results in both A and B
   switching to the traffic on link j in the respective directions.
   Note that some form of signaling is required to ensure that both A
   and B start receiving from the protection link.

   The basic steps in 1+1 bi-directional span protection are as
   follows:

     1. If a node (A or B) detects the failure of the working link (or
        a degradation of signal quality over the working link), it
        should begin receiving on the protection link and send a
        switchover message reliably to the other node (B or A,
        respectively). This message should indicate the identity of the
        failed working link and other relevant information.

     2. Upon receipt of the switchover message, a node MUST begin
        receiving from the protection link and send a switchover
        response message to the other node (A or B, respectively).
        Since both the working/protect spans are exposed to routing &
        signaling as a single link, the switchover should be
        transparent to routing and signaling.

          o The routing procedures are the same as in 1+1
             unidirectional.

          o The signaling procedures are the same as in 1+1
             unidirectional.

          o In addition to the procedures described in 1+1
             (unidirectional), a switchover request message must be
             used to signal the switchover request. This can be done
             using LMP. Note that GMPLS-based mechanisms may not be
             necessary when the underlying span (transport) technology
             provides such a mechanism.

   2.3 Dedicated 1:1 protection with Extra Traffic

   Consider two adjacent nodes A and B. Under 1:1 protection, a
   dedicated link j between A and B is pre-assigned to protect working
   link i. Link j may be carrying (preemptable) Extra Traffic. A
   failure affecting link i results in the corresponding LSP(s) being

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   restored to link j. Extra Traffic being routed over link j may need
   to be preempted to accommodate the LSPs that have to be restored.

   Once a fault is isolated/localized, the affected LSP(s) must be
   moved to the protection link. The process of moving an LSP from a
   failed (working) link to a protection link must be initiated by one
   of the nodes, A or B. This node is referred to as the ômasterö. The    other node is called the ôslaveö. The determination of the master    and the slave may be based on configured information or protocol
   specific requirements.

   The basic steps in dedicated 1:1 span protection (ignoring
   reversion) are as follows:

         1. If the master detects/localizes a link failure event, it
            invokes a process to allocate the protection link to the
            affected LSP(s).
         2. If the slave detects a link failure event, it informs the
            master of the failure using a failure indication message.
            The master then invokes the same procedure as (1) to move
            the LSPs to the protection link. If the protection link is
            carrying Extra Traffic, the slave stops using the span for
            the Extra Traffic.
         3. Once the span protection procedure is invoked in the
            master, it requests the slave to switch the affected LSP(s)
            to the protection link. Prior to this, if the protection
            link is carrying Extra Traffic, the master stops using the
            span for this traffic (i.e., the traffic is dropped by the
            master and not forwarded into or out of the protection
            link).
         4. The slave sends an acknowledgement to the master. Prior to
            this, the slave stops using the link for Extra
            Traffic(i.e.,the traffic is dropped by the slave and not
            forwarded into or out of the protection link). It then
            starts sending the normal traffic on the selected
            protection link.
         5. When the master receives the acknowledgement, it starts
            sending and receiving the normal traffic over the new link.
            The switchover of the LSPs is thus completed.

   From the description above, it is clear that 1:1 span protection may
   require up to three signaling messages for each failed span: a
   failure indication message, an LSP switchover request message, and
   an LSP switchover response message. Furthermore, it may be possible
   to switch multiple LSPs from the working span to the protection span
   simultaneously.

          o Pre-emption MUST be supported to accommodate Extra
             Traffic.

          o Routing: A single TE link encompassing both working and
             protection links is announced with Link Protection Type

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             "Dedicated 1:1". If Extra Traffic is supported over the
             protection link, then the bandwidth parameters for the
             protection link must also be announced. The
             differentiation between bandwidth for working and protect
             links is made using priority mechanisms. In other words,
             the network must be configured such that bandwidth at
             priority X or lower is considered Extra Traffic.

             If there is a failure on the working link, then the normal
             traffic is switched to the protection link, preempting
             Extra Traffic if necessary. The bandwidth for the
             protection link must be adjusted accordingly.

          o Signaling: To establish an LSP on the working link, the
             Link Protection object/TLV indicating "Dedicated 1:1"
             should be included in the signaling request message for
             that LSP. To establish an LSP on the protection link, the
             appropriate priority (indicating Extra Traffic) should be
             used for that LSP. These objects/TLVs are defined in
             [GMPLS-SIG]. If the Link Protection object/TLV is not
             used, link selection is a matter of local policy.

          o Link management: Both nodes must have a consistent view of
             the link protection association for the spans. This can be
             done using LMP or via manual configuration.

          o When a link failure is detected at the slave, a failure
             indication message must be sent to the master informing
             the node of the link failure.

2.4 Shared M:N protection

   Shared M:N protection is described with respect to two neighboring
   nodes A and B. The scenario considered is as follows:

          o  At any point in time, there are two sets of links between
             A and B, i.e., a working set of N (bi-directional) links
             carrying traffic subject to protection and a protection
             set of M (bi-directional) links. A protection link may be
             carrying Extra Traffic. There is no a priori relationship
             between the two sets of links, but the value of M and N
             may be pre-configured. The specific links in the
             protection set MAY be pre-configured to be physically
             diverse to avoid the possibility that failure events
             affect a large proportion of protection links (along with
             working links).

          o  When a link in the working set is affected by a failure,
             the normal traffic is diverted to a link in the protection
             set, if such a link is available. Note that such a link
             might be carrying more than one LSP, e.g., an OC-192 link
             carrying four STS-48 LSPs.

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          o  More than one link in the working set may be affected by
             the same failure event. In this case, there may not be an
             adequate number of protection links to accommodate all of
             the affected traffic carried by failed working links. The
             set of affected working links that are actually restored
             over available protection links is then subject to
             policies (e.g., based on relative priority of working
             traffic). These policies are not specified in this draft.

          o  When normal traffic must be diverted from a failed link in
             the working set to a protection link, the decision as to
             which protection link is chosen is always made by one of
             the nodes, A or B. This node is considered the "master"
             and it is required to both apply any policies and select
             specific protection links to divert working traffic. The
             other node is considered the "slave". The determination of
             the master and the slave may be based on configured
             information, protocol specific requirements, or as a
             result of running a neighbor discovery procedure.

          o  Failure events themselves are detected by transport layer
             mechanisms if available (e.g., SONET Alarm Indication
             Signal (AIS)/ Remote Defect Indication (RDI)). Since the
             bi-directional links are formed by a pair of
             unidirectional links, a failure in the link from A to B is
             typically detected by B and a failure in the opposite
             direction is detected by A. It is possible that a failure
             simultaneously affects both directions of the bi-
             directional link. In this case, A and B will concurrently
             detect failures, in the B-to-A direction and in the A-to-B
             direction, respectively.

   The basic steps in M:N protection (ignoring reversion) are as
   follows:

      1.   If the master detects a failure of a working link, it
           autonomously invokes a process to allocate a protection link
           to the affected traffic.

      2.   If the slave detects a failure of a working link, it MUST
           inform the master of the failure using a failure indication
           message. The master then invokes the same procedure as above
           to allocate a protection link. (It is possible that the
           master has itself detected the same failure, for example, a
           failure simultaneously affecting both directions of a link).

      3.   Once the master has determined the identity of the
           protection link, it indicates this to the slave and requests
           the switchover of the traffic (using a "switchover request"
           message). Prior to this, if the protection link is carrying
           Extra Traffic, the master stops using the link for this

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           traffic (i.e., the traffic is dropped by the master and not
           forwarded into or out of the protection link).

      4.   The slave sends a "switchover response" message back to the
           master. Prior to this, if the selected protection link is
           carrying traffic that could be preempted, the slave stops
           using the link for this traffic (i.e., the traffic is
           dropped by the slave and not forwarded into or out of the
           protection link). It then starts sending the normal traffic
           on the selected protection link.

      5.   When the master receives the switchover response, it starts
           sending and receiving the traffic that was previously
           carried on the  now-failed link over the new link.

   From the description above, it is clear that M:N span restoration
   (involving LSP local recovery) may require up to three messages for
   each working link being switched: a failure indication message, a
   switchover request message and a switchover response message.

          o  Pre-emption MUST be supported to accommodate Extra
             Traffic.

          o  Routing: A single TE link encompassing both sets of
             working and protect links should be announced with Link
             Protection Type "Shared M:N". If Extra Traffic is
             supported over set of the protection links, then the
             bandwidth parameters for the set of protection links must
             also be announced. The differentiation between bandwidth
             for working and protect links is made using priority
             mechanisms.

        If there is a failure on a working link, then the affected
        LSP(s) must be switched to a protection link, preempting Extra
        Traffic if necessary. The bandwidth for the protection link
        must be adjusted accordingly.

          o  Signaling: To establish an LSP on the working link, the
             Link Protection object/TLV indicating "Shared M:N" should
             be included in the signaling request message for that LSP.
             To establish an LSP on the protection link, the
             appropriate priority (indicating Extra Traffic) should be
             used for that. These objects/TLVs are defined in [GMPLS-
             SIG]. If the Link Protection object/TLV is not used, link
             selection is a matter of local policy.

          o  For link management, both nodes must have a consistent
             view of the link protection association for the links.
             This can be done using LMP or via manual configuration.




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2.6 Messages

   The following messages are used in local span protection procedures.
   All these messages must be transmitted reliably from the message
   source to the message destination.

2.6.1 Failure Indication Message

   This message is sent from the slave to the master to indicate the
   identities of one or more failed working links. (This message may
   not be necessary when the transport plane technology itself provides
   for such a notification).

   The number of links included in the message would depend on the
   number of failures detected within a window of time by the sending
   node. A node may choose to send separate failure indication messages
   in the interest of completing the recovery for a given link within
   an implementation-dependent time constraint.

2.6.2  Switchover Request Message

   Under bi-directional 1+1 span protection, this message is used to
   coordinate the selecting function at both nodes. This message is
   originated at the node that detected the failure.

   Under dedicated 1:1 and shared M:N span protection, this message is
   used as an LSP switchover request. This message is sent from the
   master node to the slave node (reliably) to indicate that the LSP(s)
   on the (failed) working link can be switched to an available
   protection link. If so, the ID of the protection link as well as the
   LSP labels (if necessary) must be indicated. These identifiers used
   must be consistent with those used in GMPLS signaling.

   A working link may carry multiple LSPs. Since the normal traffic
   carried over the working link is switched to the protection link, it
   may be possible for the LSPs on the working link to be mapped to the
   protection link without re-signaling each individual LSP. For
   example, if link bundling [BUNDLE] is used where the working and
   protect links are mapped to component links, and the labels are the
   same on the working and protection links, it may be possible to
   change the component links without needing to re-signal each
   individual LSP. Optionally, the labels may need to be explicitly
   coordinated between the two nodes. In this case, the switchover
   request message should carry the new label mappings.

   The master may not be able to find protection links to accommodate
   all failed working links. Thus, if this message is generated in
   response to a Failure Indication message from the slave then the set
   of failed links in the message may be a sub-set of the links
   received in the Failure Indication message. Depending on time
   constraints, the master may switch the normal traffic from the set
   of failed links in smaller batches. Thus, a single failure
   indication message may result in the master sending more than one
   Switchover Request message to the same slave node.


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2.6.3  Switchover Response Message

   This message is sent from the slave to the master (reliably) to
   indicate the completion (or failure) of switchover at the slave.  In
   this message, the slave may indicate that it cannot switch over to
   the corresponding free link for some reason. The master and slave in
   this case notify the user (operator) of the failed switchover. A
   notification of the failure may also be used as a trigger in an end-
   to-end recovery.

2.7 Preventing Unintended Connections

   An unintended connection occurs when traffic from the wrong source
   is delivered to a receiver. This must be prevented during protection
   switching. This is primarily a concern when the protection link is
   being used to carry Extra Traffic. In this case, it must be ensured
   that the LSP traffic being switched from the (failed) working link
   to the protection link is not delivered to the receiver of the
   preempted traffic. Thus, in the message flow described above, the
   master node MUST disconnect (any) preempted traffic on the selected
   protection link before sending the Switchover Request. The slave
   node MUST also disconnect preempted traffic before sending the
   Switchover Response. In addition, the master node should start
   receiving traffic for the protected LSP from the protection link.
   Finally, the master node should start sending protected traffic on
   the protection link upon receipt of the Switchover Response.

3.0 End-to-End (Path) Protection and Restoration

   End-to-end path protection and restoration refer to the recovery of
   an entire LSP from the initiator to the terminator. Suppose the
   primary path of an LSP is routed from the initiator (Node A) to the
   terminator (Node B) through a set of intermediate nodes. In the
   following subsections, we describe three previously proposed end-
   tend protection schemes and the functional steps needed to implement
   them.

3.1 Unidirectional 1+1 Protection

   A dedicated, resource-disjoint alternate path is pre-established to
   protect the LSP. Traffic is simultaneously sent on both paths and
   received from one of the functional paths by the end nodes A and B.

   There is no explicit signaling involved with this mode of
   protection.

3.2 Bi-directional 1+1 Protection

   A dedicated, resource-disjoint alternate path is pre-established to
   protect the LSP. Traffic is simultaneously sent on both paths; under
   normal conditions, the traffic from the working path is received by
   nodes A and B (in the appropriate directions). A failure affecting

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   the working path results in both A and B switching to the traffic on
   the protection path in the respective directions.

   Note that this requires coordination between the end nodes to switch
   to the protection path.

   The basic steps in bi-directional 1+1 path protection are as
   follows:

          o Failure detection: There are two possibilities for this.

                1.   A node in the working path detects a failure
                     event. Such a node must send a failure indication
                     message towards the upstream or/and downstream end
                     node of the LSP (node A or B). This message may be
                     forwarded along the working path, or routed over a
                     different path if the network has general routing
                     intelligence. Mechanisms provided by the data
                     transport plane may also be used for this, if
                     available.

                2.   The end nodes (A or B) detect the failure
                     themselves (e.g., loss of light).

          o Switchover: The action when an end node detects a failure
             in the working path is as follows: Start receiving from
             the protection path. At the same time, send a switchover
             request message to the other end node to enable switching
             at the other end.

      The action when an end node receives a switchover message is as
      follows:

           - Start receiving from the protection path. At the same
             time, send a switchover response message to the other end
             node.

   GMPLS signaling mechanisms may be used to (reliably) signal the
   switchover request. This message may be forwarded along the
   protection path if no other routing intelligence is available in the
   network.

3.2.1 Identifiers

   LSP Identifier: A unique identifier for each LSP. The LSP Identifier
   is within the scope of the Source ID and Destination ID.

   Source ID: ID of the source (e.g., IP address).

   Destination ID: ID of the destination (e.g., IP address).



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3.2.2  Nodal Information

   Each node that is on the working or protection path of an LSP must
   have knowledge of the LSP identifier as well as the previous and
   next nodes in the LSP. This is so that restoration-related messages
   may be forwarded properly. The optical network may also have general
   routing intelligence. In this case, messages may be forwarded along
   paths different than that of the LSP.

   The nodal information may be assembled when the working and
   protection paths of the LSP are provisioned using signaling, or may
   be configured when LSP provisioning does not involve signaling
   (e.g., provisioning through a management system). This information
   must remain until the LSP is explicitly de-provisioned.

3.2.3  End-to-End Failure Indication Message

   This message is sent (reliably) by an intermediate node towards the
   source of an LSP. For instance, such a node might have attempted
   local span protection and failed. This message may not be necessary
   if the data transport layer provides mechanisms for the notification
   of LSP failure by the endpoints (i.e. if LSP endpoints are co-
   located with a corresponding data (transport) maintenance/recovery
   domain).

   Consider a node that detects a link failure. The node must determine
   the identities of all LSPs that are affected by the failure of the
   link, and send an end-to-end failure indication message to the
   source of each LSP. Each intermediate node receiving such a message
   must forward the message to the appropriate next node such that the
   message would ultimately reach the LSP source. Furthermore, if an
   intermediate node is itself generating a failure indication message,
   there SHOULD be a mechanism to suppress all but one source of
   failure indication messages. Finally, the failure indication message
   must be sent reliably from the node detecting the failure to the LSP
   source. Reliability may be achieved, for example, by re-transmitting
   the message until an acknowledgement is received.

3.2.4  End-to-End Failure Acknowledge Message

   This message is sent by the source node in response to an End-to-End
   failure indication message. This message is sent to the originator
   of the failure indication message. The acknowledge message should be
   sent for each failure indication message received.  Each
   intermediate node receiving the acknowledge message must forward it
   towards the destination of the message.

3.2.5  End-to-End Switchover Request Message

   This message is generated by the source node receiving an indication
   of failure in an LSP. It is sent to the LSP destination, and it
   carries the identifier of LSP being restored. The End-to-End

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   Switchover message must be sent reliably from the source to the
   destination of the LSP.

3.2.6  End-to-End Switchover Response Message

   This message is sent by the destination node receiving an End-to-End
   Switchover Request message towards the source of the LSP. This
   message should identify the LSP being switched over. This message
   must be transmitted in response to each End-to-End Switchover
   Request message received.

3.3 Shared Mesh Restoration

   Shared mesh restoration refers to schemes under which protection
   paths for multiple LSPs share common link and node resources. Under
   these schemes, the protection capacity is pre-reserved, i.e., link
   capacity is allocated to protect one or more LSPs but explicit
   action is required to instantiate a specific protection LSP. This
   requires restoration signaling along the protection path.
   Typically, the protection capacity is shared only amongst LSPs whose
   working paths are physically diverse. This criterion can be enforced
   when provisioning the protection path. Specifically, provisioning-
   related signaling messages may carry information about the working
   path to nodes along the protection path. This can be used as call
   admission control to accept/reject connections along the protection
   path based on the identification of the resources used for the
   primary path.

   Thus, shared mesh restoration is designed to protect an LSP after a
   single failure event, i.e., a failure that affects the working path
   of at most one LSP sharing the protection capacity. It is possible
   that a protection path may not be successfully activated when
   multiple, concurrent failure events occur. In this case, shared mesh
   restoration capacity may be claimed for more than one failed LSP and
   the protection path can be activated only for one of them (at most).
   For implementing shared mesh restoration, the identifier and nodal
   information related to signaling along the control path are as
   defined for 1+1 protection in Sections 3.2.1 and 3.2.2. In addition,
   each node must also keep (local) information needed to establish the
   data plane of the protection path. This information must indicate
   the local resources to be allocated, the fabric cross-connect to be
   established to activate the path, etc. The precise nature of this
   information would depend on the type of node and LSP (the GMPLS
   signaling draft describes different type of switches [GMPLS_SIG]).
   It would also depend on whether the information is fine or coarse-
   grained. For example, fine-grained information would indicate pre-
   selection of all details pertaining to protection path activation,
   such as outgoing link, labels, etc. Coarse-grained information, on
   the other hand, would allow some details to be determined during
   protection path activation. For example, protection resources may be
   pre-selected at the level of a TE link, while the selection of the


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   specific component link and label occurs during protection path
   activation.

   While the coarser specification allows some flexibility in selection
   of the precise resource to activate, it also brings in more
   complexity in decision making and signaling during the time-critical
   restoration phase. Furthermore, the procedures for the assignment of
   bandwidth to protection paths must take into account the total
   resources in a TE link so that single-failure survivability
   requirements are satisfied.

3.3.1  End-to-End Failure Indication and Acknowledgement

   The End-to-End failure indication and acknowledgement procedures and
   messages are as defined in Sections 3.2.3 and 3.2.4.

3.3.2  End-to-End Switchover Request

   This message is generated by the source node receiving an indication
   of failure in an LSP. It is sent to the LSP destination along the
   protection path, and it identifies the LSP being restored. If any
   intermediate node is unable to establish cross-connects for the
   protection path, then it is desirable that no other node in the path
   establishes cross-connects for the path. This would allow shared
   mesh restoration paths to be efficiently utilized.

   The End-to-End Switchover message must be sent reliably from the
   source to the destination of the LSP along the protection path.

3.3.3 End-to-End Switchover Response

   This message is sent by the destination node receiving an End-to-End
   Switchover Request message towards the source of the LSP, along the
   protection path. This message should identify the LSP that is being
   switched over. Prior to activating the secondary bandwidth at each
   hop along the path, Extra Traffic (if used) must be dropped and not
   forwarded

   This message must be transmitted in response to each End-to-End
   Switchover Request message received.

4. Reversion and other Administrative Procedures

   Reversion refers to the process of moving an LSP back to the
   original working path after a failure is cleared and the path is
   repaired. Reversion applies both to local span and end-to-end path
   protected LSPs. Reversion is desired for the following reasons.
   First, the protection path may not be optimal as compared to the
   working path from a routing and resource consumption point of view.
   Second, moving an LSP to its working path allows the protection
   resources to be used to protect other LSPs. Reversion has the
   disadvantage of causing a second service disruption. Use of

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   reversion is at the option of the operator. Reversion implies that a
   working path remains allocated to the LSP that was originally routed
   over it even after a failure. It is
   important to have mechanisms that allow reversion to be performed
   with minimal service disruption to the customer. This can be
   achieved using a ôbridge-and-switchö approach (often referred to as    make-before-break).

   The basic steps involved in bridge-and-switch are:

     1. The source node commences the process by ôbridgingö the signal         onto both the working and the protection paths (or links in the
        case of span protection).
     2. Once the bridging process is complete, the source node sends a
        Bridge and Switch Request message to the destination,
        identifying the LSP and other information necessary to perform
        reversion. Upon receipt of this message, the destination
        selects the signal from the working path. At the same time, it
        bridges the transmitted signal onto both the working and
        protection paths.
     3. The destination then sends a Bridge and Switch Response message
        to the source confirming the completion of the operation.
     4. When the source receives this message, it switches to receive
        from the working path, and stops transmitting traffic on the
        protection path. The source then sends a Bridge and Switch
        Completed message to the destination confirming that the LSP
        has been reverted.
     5. Upon receipt of this message, the destination stops
        transmitting along the protection path and de-activates the LSP
        along this path. The de-activation procedure should remove the
        cross-LSPs along the protection path (and frees the resources
        to be used for restoring other failures.

   Administrative procedures other than reversion include the ability
   to force a switchover (from working to protect or vice versa), and
   locking out switchover, i.e., preventing an LSP from moving from
   working to protect administratively. These administrative conditions
   have to be supported by signaling.


5. Discussion

5.1 LSP Priorities During Protection

   Under span protection, a failure event could affect more than one
   working link and there could be fewer protection links than the
   number of failed working links.  Furthermore, a working link may
   contain multiple LSPs of varying priority.  Under this scenario, a
   decision must be made as to which working links (and therefore LSPs)
   should be protected. This decision may be based on LSP priorities.
   In general, a node might detect failures sequentially, i.e., all
   failed working links may not be detected simultaneously, but only

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   sequentially. In this case, as per the proposed signaling
   procedures, LSPs on a working link may be switched over to a given
   protection link, but another failure (of a working link carrying
   higher priority LSPs) may be detected soon afterwards. In this case,
   the new LSPs may bump the ones previously switched over the
   protection link.

   In the case of end-to-end shared mesh restoration, priorities may be
   implemented for allocating shared link resources under multiple
   failure scenarios. As described in Section 3.3, more than one LSP
   can claim shared resources under multiple failure scenarios. If such
   resources are first allocated to a lower priority LSP, they may have
   to be reclaimed and allocated to a higher priority LSP.

6. Author's Addresses

      Jonathan P. Lang                Bala Rajagopalan
      email: jplang@ieee.org          Tellium, Inc.
                                      2 Crescent Place
                                      P.O. Box 901
                                      Oceanport, NJ 07757-0901
                                      email: braja@tellium.com

7. Intellectual Property Considerations

   This section is taken from Section 10.4 of [RFC2026].

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETFÆs procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication and any assurances
   of licenses to be made available, or the result of an attempt made
   to obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification
   can be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.







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8. References

8.1 Normative References

   [BUNDLE] Kompella, K., Rekhter, Y. and Berger, L., "Link Bundling in
   MPLS Traffic Engineering", draft-ietf-mpls-bundle-04.txt (work in
   progress).

   [GMPLS-ISIS] Kompella, K., Rekhter, Y., Banerjee, A. et al, "IS-IS
   Extensions in Support of Generalized MPLS", draft-ietf-isis-gmpls-
   extensions-16.txt (work in progress).

   [GMPLS-OSPF] Kompella, K., Rekhter, Y., Banerjee, A. et al, "OSPF
   Extensions in Support of Generalized MPLS", draft-ietf-ccamp-ospf-
   gmpls-extensions-09.txt (work in progress).

   [GMPLS-SIG]  Ashwood-Smith, P., Banerjee, A., et al, "Generalized
   MPLS - Signaling Functional Description," RFC 3471.

   [LMP] Lang, P, ed., "Link Management Protocol (LMP) v1.0" Internet
   Draft, Work in progress, draft-ietf-ccamp-lmp-09.

8.2 Informative References

   [RFC2026] Bradner, S., "The Internet Standards Process -- Revision
   3," BCP 9, RFC 2026, October 1996.

   [TERM] Mannie, E., Papadimitriou, D., ed., "Recovery  (Protection
   Internet Draft, draft-mannie-gmpls-recovery-terminology-02.txt,
   (work in progress).

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   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.



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   This document and the information contained herein is provided on an
   ôAS ISö basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING    TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
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   This draft expires in March, 2004.













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