Subject: New IETF internet draft submitted

From: Srinivas Makam <[email-masked]>
Date: Fri, 10 Mar 2000 17:20:49 -0600

Hi,

We have submitted a new draft on Framework for MPLS Based Recovery to
the IETF.
We request the MPLS mailing list group to provide comments on the
attached draft.
We are planning to present this draft at the Adelaide MPLS working group
meeting.

Thanks

Vas Makam

Internet Draft                               Srinivas Makam
Multi-Protocol Label Switching               Vishal Sharma
Expiration Date: September 2000              Ken Owens
                                             Changcheng Huang
                                             Ben Mack-Crane
                                             Tellabs

                                             Fiffi Hellstrand
                                             Jon Weil
                                             Brad Cain
                                             Loa Andersson
                                             Bilel Jamoussi
                                             Nortel Networks

                                             Seyhan Civanlar
                                             Angela Chiu
                                             AT&T Labs

                                             March 2000

                      Framework for MPLS Based Recovery
   
                 <draft-makam-mpls-recovery-frmwrk-00.txt>

Status of this memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.
   
   Internet-Drafts are working documents of the Internet Engineering
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   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt
   
   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Abstract

   Multiprotocol label switching (MPLS) [1] integrates the label
   swapping forwarding paradigm with network layer routing. To deliver
   reliable service, MPLS requires a set of procedures to provide
   protection of the traffic carried on different paths. This requires
   that the label switched routers (LSRs) support fault detection,
   fault notification, and fault recovery mechanisms, and that MPLS
   signaling [2] [3] [4] [5] [6] support the configuration of working
   and recovery paths. With these objectives in mind, this document
   specifies a framework for MPLS based recovery.

Table of Contents

1.0 Introduction
1.1 Background
1.2 Motivations for MPLS-Based Recovery
1.3 Objectives

2.0 Overview
2.1 Recovery Models
2.2 Recovery Cycles
2.3 Terminology
2.4 Abbreviations

3.0 MPLS Recovery Principles
3.1 Recovery Models
3.2 Configuration of Recovery
3.3 Scope of Recovery
3.3.1 Topology
3.3.2 Path Mapping
3.3.3 Bypass Tunnels
3.3.4 Recovery Granularity
3.3.4.1 Selective Traffic Recovery
3.3.4.2 Bundling
3.4 Fault Detection
3.5 Fault Notification
3.6 Switch Over Operation
3.6.1 Recovery Trigger
3.6.2 Recovery Action
3.7 Switch Back Operation
3.7.1 Revertive and Non-revertive Mode
3.7.2 Restoration and Notification
3.7.3 Reverting to Preferred LSP
3.8 Performance

4.0 Recovery Requirements
5.0 MPLS Recovery Options
6.0 Comparison Criteria
7.0 Security Considerations
8.0 Intellectual Property Considerations
9.0 Author's Addresses
10.0 References

1.0 Introduction

   This memo describes a framework for MPLS-based recovery. We provide
   a detailed taxonomy of recovery terminology, and discuss the
   motivation for, the objectives of, and the requirements for MPLS-
   based recovery. We outline principles for MPLS-based recovery, and
   also provide comparison criteria that may serve as a basis for
   comparing and evaluating different recovery schemes.
   
1.1 Background

   Network routing deployed today is focussed primarily on
   connectivity and typically supports only one class of service, the
   best effort class. Multi-protocol label switching, on the other
   hand, by integrating forwarding based on  label-swapping of a link
   local label with network layer routing allows flexibility in the
   delivery of new routing services. MPLS allows for using media
   specific forwarding mechanisms as label swapping. This enables more
   sophisticated features such as quality-of-service (QoS) and traffic
   engineering [7] to be implemented more effectively. An important
   component of providing QoS, however, is the ability to transport
   data reliably and efficiently. Although the current routing
   algorithms are very robust and survivable, the amount of time they
   take to recover from a fault can be significant, on the order of
   several seconds or minutes, causing serious disruption of service
   for some applications in the interim. This is unacceptable to many
   organizations that aim to provide a highly reliable service, and
   thus require recovery times on the order of tens of milliseconds,
   as specified, for example, in the GR253 specification for SONET.
   
   Since MPLS binds packets to a route (or path) via labels and is
   likely to be the technology of choice in the future IP-based
   transport network, it is imperative that MPLS be able to provide
   protection and restoration of traffic. In fact, a protection
   priority could be used as a differentiating mechanism for premium
   services that require high reliability. The remainder of this
   document provides a framework for MPLS based recovery.

1.1 Motivation for MPLS-Based Recovery

   MPLS based protection of traffic (called MPLS-based Recovery) is
   useful for a number of reasons. The most important is its ability
   to increase network reliability by enabling a faster response to
   faults than is possible with traditional Layer 3 (or the IP layer)
   alone. Furthermore, a protection mechanism using could enable IP
   traffic to be put directly over WDM optical channels, without an
   intervening SONET layer, which would facilitate the construction of
   IP-over-WDM networks.
   
   The need for MPLS-based recovery arises because of the following:
   
   I. Layer 3 or IP rerouting may be too slow for a core MPLS network
   that needs to support high reliability/availability.
   
   II. Layer 0 (for example, optical layer) or Layer 1 (for example,
   SONET) mechanisms may be deployed in ring topologies and may not
   always include mesh protection. That is, layer 0 or layer 1
   networks may not be deployed in topologies that meet carriers’
   protection goals.
   
   III. The granularity at which the lower layers may be able to
   protect traffic may be too coarse for traffic that is switched
   using MPLS-based mechanisms.
   
   IV. Layer 0 or Layer 1 mechanisms may have no visibility into
   higher layer operations.  Thus, while they may provide, for
   example, link protection, they cannot easily provide node
   protection.

   Furthermore there is a need for open standards.
   
   V. Establishing interoperability of protection mechanisms between
   routers/LSRs from different vendors in IP or MPLS networks is
   urgently required to enable the adoption of MPLS as a viable core
   transport and traffic engineering technology.

1.3 Objectives/Goals

   We lay down the following objectives for MPLS-based recovery.
   
   I. MPLS-based recovery mechanisms should facilitate fast (10’s of
   ms) recovery times.
   
   II. MPLS-based recovery should maximize network reliability and
   availability.
   
   III. MPLS-based recovery techniques should be applicable for
   protection of traffic at various granularities. For example, it
   should be possible to specify MPLS-based recovery for a portion of
   the traffic on an individual path, for all traffic on an individual
   path, or for all traffic on a group of paths.
   
   IV. MPLS-based recovery techniques may be applicable for an entire
   end-to-end path or for segments of an end-to-end path.
   
   V. MPLS-based recovery actions should not adversely affect other
   network operations.
   
   VI. MPLS-based recovery actions in one MPLS protection domain
   (defined in Section 2.2) should not affect the recovery actions in
   other MPLS protection domains.
   
   VII. MPLS-based recovery mechanisms should be able to take into
   consideration the recovery actions of other layers.
   
   VIII. MPLS-based recovery actions should avoid network-layering
   violations. That is, defects in MPLS-based mechanisms should not
   trigger lower layer protection switching.
   
   IX. MPLS-based recovery mechanisms should minimize the loss of data
   and packet reordering during recovery operations. (The current MPLS
   specification has itself no explicit requirement on reordering).
   
   X. MPLS-based recovery mechanisms should minimize, if required by
   the traffic, the additive latency that may be incurred when a
   recovery path is activated.
   
   XI. MPLS-based recovery mechanisms should minimize the state
   overhead incurred for each recovery path maintained.
   
   XII. MPLS-based recovery mechanisms should be able to preserve the
   constraints on traffic after switchover, if desired.  That is, if
   desired, the recovery path should meet the resource requirements
   of, and achieve the same performance characteristics, as the
   working path.

2.0 Overview

   There are several options for providing protection of traffic using
   MPLS. The most generic requirement is the specification of whether
   recovery should be  via Layer 3 (or IP) rerouting or via protection
   switching actions.
   
   More importantly, MPLS-based protection should give the flexibility
   to select the recovery mechanism, choose the granularity at which
   traffic is protected, and to also choose the specific types of
   traffic that are protected.
   
   Generally network operators aim to provide the fastest and the best
   protection mechanism that can be provided at a reasonable cost. The
   higher the level of protection, the more resources it consumes.
   With MPLS-based recovery, it can be possible to provide different
   levels of protection for different classes of service, based on
   their service requirements. For example, a VLL service that
   supports real-time applications like VoIP may be supported using
   link/node protection together with pre-established, pre-reserved
   path protection, while best effort traffic may use established-on-
   demand path protection or simply rely on – IP re-route or higher
   layer recovery mechanisms.
   
2.1 Recovery Models

   There are two basic models for path recovery: rerouting and
   protection switching.
   
2.1.1 Rerouting

   Recovery by rerouting is defined as establishing new paths or path
   segments on demand for restoring traffic after the occurrence of a
   fault. The new paths may be based upon fault information, network
   routing policies, pre-defined configurations and network topology
   information. Thus, upon detecting a fault, the affected paths are
   re-established using signaling. Reroute mechanisms are inherently
   slower than protection switching mechanisms, since more must be
   done following the detection of a fault.  Once the network routing
   algorithms have converged after a fault, it may be preferable, in
   some cases, to reoptimize the network by performing a reroute based
   on the current state of the network and network policies. This is
   currently discussed further in Section 3.8, but will also be
   clarified further in upcoming revisions of this document.
   
2.1.2 Protection Switching

   Protection switching recovery mechanisms pre-establish a recovery
   path or path segment, based upon network routing policies, the
   restoration requirements of the traffic on the working path, and
   administrative considerations. The recovery path may or may not be
   link and node disjoint with the working path [8].  When a fault is
   detected on the working path, a switch to the recovery path
   restores traffic.  The resources (bandwidth, buffers, processing)
   on the recovery path may be used to carry either a copy of the
   working path traffic or extra traffic that is displaced when a
   protection switch occurs.
   
   Protection switching and rerouting may be used together.  For
   example, protection switching to a recovery path may be used for
   rapid restoration of connectivity while rerouting determines a new
   optimal network configuration, rearranging paths, as needed, at a
   later time [9] [10].
   
   Additional specifications of the recovery actions are found in
   Section 3.
   
2.2 The Recovery Cycles

   The MPLS recovery cycle model is illustrated in Figure 1.
   Definitions and a key to abbreviations follow.
   
     --Network Impairment
     |    --Fault Detected
     |    |    --Start of Notification
     |    |    |    -- Start of Recovery Operation
     |    |    |    |    --Recovery Operation Complete
     |    |    |    |    |    --Path Traffic Restored
      |    |    |    |    |    |
     |    |    |    |    |    |
       v    v    v    v    v    v
    -----------------------------------------------------------------
    ------
     | T1 | T2 | T3 | T4 | T5 |
    
                  Figure 1. MPLS Recovery Cycle Model

   The various timing measures used in the model are described below.
   
    T1   Fault Detection Time
    T2   Hold-off Time
    T3   Notification Time
    T4   Recovery Operation Time
    T5   Traffic Restoration Time

   Definitions of the recovery cycle times are as follows:
   
   Fault Detection Time
   
   The time between the occurrence of a network impairment and the
   moment the fault is detected by MPLS-based recovery mechanisms.
   This time may be highly dependent on lower layer protocols.
   
   Hold-Off Time
   
   The configured waiting time between the detection of a fault and
   taking MPLS-based recovery action, to allow time for lower layer
   protection to take effect. The Hold-off Time may be zero.
   
   Note: The Hold-Off Time may occur after the Notification Time
   interval if the node responsible for the switchover, the Path
   Switch LSR (PSL), rather than the detecting LSR, is configured to
   wait.
   
   Notification Time
   
   The time between initiation of an FIS by the LSR detecting the
   fault and the time at which the Path Switch LSR (PSL) begins the
   recovery operation.  This is zero if the PSL detects the fault
   itself.
   
   Note: If the PSL detects the fault itself, there still may be a
   Hold-Off Time period between detection and the start of the
   recovery operation.
   
   Recovery Operation Time
   
   The time between the first and last recovery actions.  This may
   include message exchanges between the PSL and PML to coordinate
   recovery actions.
   
   Traffic Restoration Time
   
   The time between the last recovery action and the time that the
   traffic (if present) is completely - recovered.  This interval is
   intended to account for the time required for traffic to once again
   –arrive at the point in the network that experienced disrupted or
   degraded service due to the occurrence of the fault (e.g. the PML).
   This time may depend on the location of the fault, the recovery
   mechanism, and the propagation delay along the recovery path.
   
   In protection switching, revertive mode requires the LSP to be
   switched back to a preferred path when the fault on that path is
   cleared.  The MPLS reversion cycle model is illustrated in Figure
   2. Note that the cycle shown below comes after the recovery cycle
   shown in Fig. 1.

       --Network Impairment Repaired
       |    --Fault Cleared
       |    |    -- Path Available
       |    |    |    -- Start of Reversion Operation
       |    |    |    |    --Reversion Operation Complete
       |    |    |    |    |    --Traffic Restored on Preferred Path
       |    |    |    |    |    |
       |    |    |    |    |    |
       v    v    v    v    v    v
    ------------------------------------------------------------------
-----
       | T7 | T8 | T9 | T10| T11|
    
                 Figure 2. MPLS Reversion Cycle Model
                                   
   The various timing measures used in the model are described below.
   
    T7   Fault Clearing Time
    T8   Wait-to-Restore Time
    T9   Notification Time
    T10  Reversion Operation Time
    T11  Traffic Restoration Time
    
   Note that time T6 (not shown above) is the time for which the
   network impairment is not repaired and traffic is flowing on the
   recovery path.
   
   Definitions of the reversion cycle times are as follows:
   
   Fault Clearing Time
   
   The time between the repair of a network impairment and the time
   that MPLS-based mechanisms learn that the fault has been cleared.
   This time may be highly dependent on lower layer protocols.
   
   Wait-to-Restore Time
   
   The configured waiting time between the clearing of a fault and
   MPLS-based recovery action(s).  Waiting time may be needed to
   ensure the path is stable and to avoid flapping in cases where a
   fault is intermittent. The Wait-to-Restore Time may be zero.
   
   Note: The Wait-to-Restore Time may occur after the Notification
   Time interval if the PSL is configured to wait.
   
   Notification Time
   
   The time between initiation of an FRS by the LSR clearing the fault
   and the time at which the path switch LSR begins the reversion
   operation.  This is zero if the PSL clears the fault itself.
   
   Note: If the PSL clears the fault itself, there still may be a Wait-
   to-Restore Time period between fault clearing and the start of the
   reversion operation.
   
   Reversion Operation Time
   
   The time between the first and last reversion actions.  This may
   include message exchanges between the PSL and PML to coordinate
   reversion actions.
   
   Traffic Restoration Time
   
   The time between the last reversion action and the time that
   traffic (if present) is completely restored on the preferred path.
   This interval is expected to be quite small since both paths are
   working and care may be taken to limit the traffic disruption
   (e.g., using “make before break” techniques and synchronous switch-
   over).

   In practice, the only interesting times in the reversion cycle are
   the Wait-to-Restore Time and the Traffic Restoration Time (or some
   other measure of traffic disruption).  Given that both paths are
   available, there is no need for rapid operation, and a well-
   controlled switch-back with minimal disruption is desirable.
   
   Recovery based on dynamic rerouting requires the MPLS network to be
   in a stable state after a network impairment occurs. The goal is to
   reoptimize the network after the routing protocols converge, and
   move the traffic from a recovery path to a (possibly) new working
   path. The steps involved in this mode are illustrated in Figure 3.
   Note that the cycle shown below may follow the recovery cycle shown
   in Fig. 1 or the reversion cycle shown in Fig. 2, or both (in the
   event that both the recovery cycle and the reversion cycle take
   place before the routing protocols converge, and after the
   convergence of the routing protocols it is determined (based on on-
   line algorithms or off-line traffic engineering tools, network
   configuration, or a variety of other possible criteria) that there
   is a better route for the working path).
   
       --Network Enters a Semi-stable State after an Impairment
       |     --Dynamic Routing Protocols Converge
       |     |     -- Initiate Setup of New Working Path between PSL
and PML
       |     |     |     -- –Switchover Operation Complete
       |     |     |     |     --Traffic -Moved to Preferred Path
       |     |     |     |     |
       |     |     |     |     |
       v     v     v     v     v
    ------------------------------------------------------------------
-----
       | T12 | T13 | T14 | T15 |
    
             Figure 3. MPLS Dynamic Rerouting Cycle Model
                                   
   The various timing measures used in the model are described below.
   
    T12  Network Route Convergence Time
    T13  Hold-down Time (optional)
    T14  Switchover Operation Time
    T15  Traffic Restoration Time

   Network Route Convergence Time
   
   We define the network route convergence time as the time taken for
   the network routing protocols to converge and for the network to
   reach a stable state.
   
   Holddown Time
   
   We define the holddown period as a bounded time for which a
   recovery path must be used. In some scenarios it may be difficult
   to determine if the working path is stable. In these cases a
   holddown time may be used to prevent excess flapping of traffic
   between a working and a recovery path.
   
   Switchover Operation Time
   
   The time between the first and last switchover actions.  This may
   include message exchanges between the PSL and PML to coordinate the
   switchover actions.

   As an example of the recovery cycle, we present a sequence of
   events that occur after a network impairment occurs and when a
   protection switch is followed by dynamic rerouting.

   I. Link or path fault occurs
   
   II. Signaling initiated (FIS) for the fault detected
   
   III. FIS arrives at the PSL
   
   IV. The PSL initiates a protection switch to a pre-configured
   recovery path
   
   V. The PSL switches over the traffic from the working path to the
   recovery path
   
   VI. The network enters a semi-stable state
   
   VII. Dynamic routing protocols converge after the fault, and a new
   working path is calculated (based, for example, on some of the
   criteria mentioned earlier in Section 2.1.1).
   
   VIII. A new working path is established between the PSL and the PML
   (assumption is that PSL and PML have not changed)
   
   IX. Traffic is switched over to the new working path.

2.2 Definitions and Terminology

   This document assumes the terminology given in Error! Reference
   source not found., and, in addition, introduces the following new
   terms.
   
2.2.1 General Recovery Terminology

   Rerouting
   
   A recovery mechanism in which the recovery path or path segments
   are created dynamically after the detection of a fault on the
   working path. In other words, a recovery mechanism in which the
   recovery path is not pre-established.
   
   Protection Switching
   
   A recovery mechanism in which the recovery path or path segments
   are created prior to the detection of a fault on the working path.
   In other words, a recovery mechanism in which the recovery path is
   pre-established.
   
   Working Path
   
   The protected path that carries traffic before the occurrence of a
   fault.
   
   Recovery Path
   
   The path by which traffic is restored after the occurrence of a
   fault. In other words, the path on which the traffic is directed by
   the recovery mechanism. The recovery path can either be an
   equivalent recovery path and ensure no reduction in quality of
   service, or be a limited recovery path and thereby not guarantee
   the same quality of service (or some other criteria of performance)
   as the working path. A limited recovery path is not expected to be
   used for an extended period of time.
   
   Path Group (PG)
   
   A logical bundling of multiple working paths, each of which is
   routed identically between a Path Switch LSR and a Path Merge LSR.
   
   Protected Path Group (PPG)
   
   A path group that requires protection.
   
   Protected Traffic Portion (PTP)
   
   The portion of the traffic on an individual path that requires
   protection.  For example, code points in the EXP bits of the shim
   header may identify a protected portion.
   
   Path Switch LSR (PSL)
   
   An LSR that is the transmitter of both the working path traffic and
   its corresponding recovery path traffic. The PSL is responsible for
   switching of the traffic between the working path and the recovery
   path.
   
   Path Merge LSR (PML)
   
   An LSR that receives both working path traffic and its
   corresponding recovery path traffic, and either merges their
   traffic into a single outgoing path, or, if it is itself the
   destination, passes the traffic on to the higher layer protocols.
   
   Intermediate LSR
   
   An LSR on a working or recovery path that is neither a PSL nor a
   PML for that path.
   
   Bypass Tunnel
   
   A path that serves to backup a set of working paths using the label
   stacking approach. The working paths and the bypass tunnel must all
   share the same path switch LSR (PSL) and the path merge LSR (PML).
   
   Switch-Over
   
   The process of switching the traffic from a working path onto one
   or more alternate path(s). This may involve moving traffic from a
   working path onto one or more recovery paths, or may involve moving
   traffic from a recovery path(s) on to a more optimal working
   path(s).
   
   Switch-Back
   
   The process of -returning the traffic from one or more recovery
   paths back to –the working path(s).
   
   Revertive Mode
   
   A recovery mode in which traffic is automatically switched back
   from the recovery path to the original working path upon the
   restoration of the working path to a fault-free condition.
   
   Non-revertive Mode
   
   A recovery mode in which traffic is not automatically switched back
   to the original working path after this path is restored to a fault-
   free condition. (Depending on the configuration, the original
   working path may, upon moving to a fault free condition, become the
   recovery path, or it may be used for new working traffic, and be no
   longer associated with its original recovery path).
   
   MPLS Protection Domain
   
   The set of LSRs over which a working path and its corresponding
   recovery path are routed.
   
   Liveness Message
   
   A message exchanged periodically between two adjacent LSRs that
   serves as a link probing mechanism. It provides an integrity check
   of the forward and the backward directions of the link between the
   two LSRs as well as a check of neighbor aliveness.
   
   Path Continuity Test
   
   A test that verifies the integrity and continuity of a path or path
   segment. The details of such a test are beyond the scope of this
   draft.(This could be accomplished, for example, by the transmitting
   a control message along the same links and nodes as the data
   traffic.)
   
2.2.2 Failure Terminology

   Path Failure (PF)
   
   Path failure is fault detected by MPLS-based recovery mechanisms,
   which is define as the failure of the liveness message test or a
   path continuity test, which indicates that path connectivity is
   lost.
   
   Path Degraded (PD)
   
   Path degraded is a fault detected by MPLS-based recovery mechanisms
   that indicates that the quality of the path is unacceptable.
   
   Link Failure (LF)
   
   A lower layer fault indicating that link continuity is lost. This
   may be communicated to the MPLS-based recovery mechanisms by the
   lower layer.
   
   Link Degraded (LD)
   
   A lower layer indication to MPLS-based recovery mechanisms that the
   link is performing below an acceptable level.
   
   Fault Indication Signal (FIS)
   
   A signal that indicates that a fault along a path has occurred. It
   is relayed by each intermediate LSR to its upstream or downstream
   neighbor, until it reaches an LSR that is setup to perform MPLS
   recovery.
   
   Fault Recovery Signal (FRS)
   
   A signal that indicates a fault along a working path has been
   repaired. Again, like the FIS, it is relayed by each intermediate
   LSR to its upstream or downstream neighbor, until is reaches the
   LSR that performs recovery of the original path.

2.3 Abbreviations

     FIS: Fault Indication Signal.
     FRS: Fault Recovery Signal.
     LD:  Link Degraded.
     LF: Link Failure.
     PD: Path Degraded.
     PF: Path Failure.
     PML: Path Merge LSR.
     PG: Path Group.
     PPG: Protected Path Group.
     PTP: Protected Traffic Portion.
     PSL: Path Switch LSR.

3.0 MPLS-based Recovery Principles

   MPLS-based recovery refers to the ability to effect quick and
   complete restoration of traffic affected by a fault in MPLS-based
   transport mechanisms or in or lower layers over which MPLS is
   transported. Fast MPLS protection may be viewed as the MPLS LSR
   switch completion time that is comparable to, or equivalent to, the
   50 ms switch-over completion time of the SONET layer. This section
   provides a discussion of the concepts and principles of MPLS-based
   recovery. We do not make any assumptions about the underlying layer
   1 or layer 2 transport mechanisms or their recovery mechanisms.

3.1 Initiation of Path Setup

   As explained in Section 2.2, there are two options for the
   initiation of the recovery path setup.
   
   Pre-established:
   This is the same as the protection switching option. Here a
   recovery path(s) is established prior to any failure on the working
   path. The path selection can either be determined by an
   administrative centralized tool (online or offline), or chosen
   based on some algorithm implemented at the PSL and possibly
   intermediate nodes. To guard against the situation when the pre-
   established recovery path fails before or at the same time as the
   working path, the recovery path should have secondary configuration
   options as explained in Section 3.3 below.
   
   Established-on-Demand:
   This is the same as the rerouting option. Here, a recovery path is
   established after a failure on its working path has been detected
   and notified to the PSL.

3.2 Initiation of Resource Allocation

   A recovery path may support the same traffic contract as the
   working path, or it may not. We will distinguish these two
   situations by using different additive terms. If the recovery path
   is capable of replacing the working path without degrading service,
   it will be called an equivalent recovery path. If the recovery path
   lacks the resources (or resource reservations) to replace the
   working path without degrading service, it will be called a limited
   recovery path. Based on this, there are two options for the
   initiation of resource allocation:
   
   Pre-reserved:
   
   This option applies only to protection switching. Here a pre-
   established recovery path reserves required resources on all hops
   along its route during its establishment. Although the reserved
   resources (e.g., bandwidth and/or buffers) at each node cannot be
   used to admit more working paths, they are available to be used by
   all traffic that is present at the node before a failure occurs,
   which results in better resource usage than SONET APS.

   Reserved-on-Demand:
   
   This option may apply either to rerouting or to protection
   switching. Here a recovery path reserves the required resources
   after a failure on the working path has been detected and notified
   to the PSL and before the traffic on the working path is switched
   over to the recovery path.
   
   Note that under both the options above, depending on the amount of
   resources reserved on the recovery path, it could either be an
   equivalent recovery path or a limited recovery path.

3.3 Configuration of Recovery

   The recovery path should allow for configuration of the following
   recovery options:
   
   Default-recovery (No MPLS-based recovery enabled): Traffic on the
   working path is recovered only via Layer 3 or IP rerouting. This is
   equivalent to having no MPLS-based recovery. This option may be
   used for low priority traffic or for traffic that is “recovered” in
   another way (for example load shared traffic on parallel working
   paths, may be automatically “recovered” upon a fault along one of
   the working paths by distributing it among the remaining working
   paths)
   
   Recoverable (MPLS-based recovery enabled): This working path is
   recovered using one or more recovery paths, either via rerouting or
   via protection switching.

3.4 Scope of Recovery

3.4.1 Topology

   Local Repair
   
   The intent of local repair is to protect against a single link or
   neighbor node fault. In local repair (also known as local recovery
   [11] [9]), the node detecting the fault is the one to initiate
   recovery (either rerouting or protection switching). Local repair
   can be of two types:
   
   Link Recovery/Restoration
   
   In this case, the recovery path may be configured to route around a
   certain link deemed to be unreliable. If protection switching is
   used, several recovery paths may be configured for one working
   path, depending on the specific faulty link that each protects
   against. Alternatively, if rerouting is used then, upon the
   occurrence of a fault on the specified link, each path is rebuilt
   such that it detours around the faulty link.
   
   In this case, the recovery path need only be disjoint from its
   working path at a particular link on the working path, and may have
   overlapping segments with the working path. Traffic on the working
   path is switched over to an alternate path at the upstream LSR that
   connects to the failed link. This method is potentially the
   fastest, and can be effective in situations where certain path
   components are much more unreliable than others.
   
   Node Recovery/Restoration
   
   In this case, the recovery path may be configured to route around a
   neighbor node deemed to be unreliable. Thus the recovery path is
   disjoint from the working path only at a particular node and at
   links associated with the working path at that node. Once again,
   the traffic on the primary path is switched over to the recovery
   path at the upstream LSR that directly connects to the failed node,
   and the recovery path shares overlapping portions with the working
   path.
   
   Global Repair
   
   The intent of global repair is to protect against any link or node
   fault on the entire path or on a segment of a path (with the
   obvious exception of the ingress and egress nodes). In global
   repair (also known as path recovery/restoration) the node that
   initiates the recovery may be distant from the faulty link or node.
   In some cases, a fault notification (in the form of a FIS) must be
   sent from the node detecting the fault to the node responsible for
   initiating the recovery action. The recovery path can be made
   completely link and node disjoint with its working path. This has
   the advantage of protecting against all link and node fault(s) on
   the working path (or path segment), and being more efficient than
   per-hop link or node recovery.
   
   In addition, it can be potentially more optimal in resource usage
   than the link or node recovery. However, it is in some cases slower
   than local repair since it takes longer for the fault notification
   message to get to the PSL to trigger the recovery action.

3.4.2 Path Mapping

   Path mapping refers to the methods of mapping traffic from a faulty
   working path on to the recovery path. There are several options for
   this. The first four require standard path semantics, while the
   fifth requires extended path semantics, and is for further study.
   
   i) 1+1 Protection
   
   In 1+1 (“one plus one”) protection, the resources (bandwidth,
   buffers, processing capacity) on the recovery path are fully
   reserved and carry the same traffic as the working path. Selection
   between the traffic on the working and recovery paths is made at
   the path merge LSR (PML).
   
   ii) 1:1 Protection
   
   In 1:1 (“one for one”) protection, the resources (bandwidth,
   buffers, and processing capacity) allocated on the recovery path
   are fully available to preemptable low priority traffic except when
   the recovery path is in use due to a fault on the working path. In
   other words, in 1:1 protection, the protected traffic normally
   travels only on the working path, and is switched to the recovery
   path only when the working path has a fault. Once the protection
   switch is initiated, the low priority traffic being carried on the
   recovery path may be displaced by the protected traffic. This
   method affords a way to make efficient use of the recovery path
   resources.
   
   iii) 1:n Protection
   
   In 1:n protection, up to n working paths are protected using only
   one recovery path. If the intent is to protect against any single
   fault on any of the working paths, the n working paths should be
   diversely routed between the same PSL and PML. In some cases,
   handshaking between PSL and PML may be required to complete the
   recovery, the details of which are beyond the scope of this draft.
   
   iv)  m:n Protection
   
   In m:n protection, up to n working paths are protected using  m
   recovery paths. Once again, if the intent is to protect again any
   single fault on any of the n working paths, the n working paths and
   the m recovery paths should be diversely routed between the same
   PSL and PML. In some cases, handshaking between PSL and PML may be
   required to complete the recovery, the details of which are beyond
   the scope of this draft. m:n protection is for further study.
   
   v) Split Path Protection
   
   In split path protection, multiple recovery paths are allowed to
   carry the traffic of a working path based on a certain configurable
   load splitting ratio.  This is especially useful when no single
   recovery path can be found that can carry the entire traffic of the
   working path in case of a fault. Split path protection may require
   handshaking between the PSL and the PML, and may require the PML to
   correlate the traffic arriving on multiple recovery paths with the
   working path. Although this is an attractive option, the details of
   split path protection are beyond the scope of this draft, and are
   for further study.

3.4.3 Bypass Tunnels

   It may be convenient, in some cases, to create a “bypass tunnel”
   for a PPG between a PSL and PML, thereby allowing multiple recovery
   paths to be transparent to intervening LSRs [11].  In this case,
   one LSP (the tunnel) is established between the PSL and PML
   following an acceptable route and a number of recovery paths are
   supported through the tunnel via label stacking. A bypass tunnel
   can be used with any of the path mapping options discussed in the
   previous section.
   
   As with recovery paths, the bypass tunnel may or may not have
   resource reservations sufficient to provide recovery without
   service degradation.  It is possible that the bypass tunnel may
   have sufficient resources to recover some number of working paths,
   but not all at the same time.  If the number of recovery paths
   carrying traffic in the tunnel at any given time is restricted,
   this is similar to the 1:n or m:n protection cases mentioned in
   Section 3.3.2.

3.4.4 Recovery Granularity

   Another dimension of recovery considers the amount of traffic
   requiring protection. This may range from a fraction of a path to a
   bundle of paths.

3.4.4.1 Selective Traffic Recovery

   This option allows for the protection of a fraction of traffic
   within the same path. The portion of the traffic on an individual
   path that requires protection is called a protected traffic portion
   (PTP). A single path may carry different classes of traffic, with
   different protection requirements. The protected portion of this
   traffic may be identified by its class, as for example, via the EXP
   bits in the MPLS shim header or via the priority bit in the ATM
   header.

3.4.4.2 Bundling

   Bundling is a technique used to group multiple working paths
   together in order to recover them simultaneously. The logical
   bundling of multiple working paths requiring protection, each of
   which is routed identically between a PSL and a PML, is called a
   protected path group (PPG). When a fault occurs on the working path
   carrying the PPG, the PPG as a whole can be protected either by
   being switched to a bypass tunnel or by being switched to a
   recovery path.

3.5 Fault Detection

   MPLS recovery is initiated after the detection of either a lower
   layer fault or a fault in the operation of MPLS-based mechanisms.
   We consider four classes of impairments: Path Failure, Path
   Degraded, Link Failure, and Link Degraded.
   
   Path Failure (PF) is a fault that indicates to an MPLS-based
   recovery scheme that the connectivity of the path is lost.  This
   may be detected by a path continuity test between the PSL and PML.
   Some, and perhaps the most common, path failures may be detected
   using a link probing mechanism between neighbor LSRs. An example of
   a probing mechanism is a liveness message that is exchanged
   periodically along the working path between peer LSRs.  For either
   a link probing mechanism or path continuity test to be effective,
   the test message must be guaranteed to follow the same route as the
   working or recovery path, over the segment being tested. In
   addition, the path continuity test must take the path merge points
   into consideration. In the case of a bi-directional link
   implemented as two unidirectional links, path failure could mean
   that either one or both unidirectional links are damaged.
   
   Path Degraded (PD) is a fault that indicates to MPLS-based recovery
   schemes/mechanisms that the LSP has connectivity, but that the
   quality of the connection is unacceptable.  This may be detected by
   a path performance monitoring mechanism, or some other MPLS-based
   mechanism for determining the error rate on the path or some
   portion of the path. This is local to the LSR and consists of
   excessive discarding of packets at an interface, either due to
   label mismatch or due to TTL errors, for example.

   Link Failure (LF) is an indication from a lower layer that the link
   over which the LSP is carried has failed.  If the lower layer
   supports detection and reporting of this fault (that is, any fault
   that indicates link failure e.g., SONET LOS), this may be used by
   the MPLS recovery mechanism. In some cases, using LF indications
   may provide faster fault detection than using only MPLS –based
   fault detection mechanisms.
   
   Link Degraded (LD) is an indication from a lower layer that the
   link over which the LSP is carried is performing below an
   acceptable level.  If the lower layer supports detection and
   reporting of this fault, it may be used by the MPLS recovery
   mechanism. In some cases, using LD indications may provide faster
   fault detection than using only MPLS-based fault detection
   mechanisms.

3.6 Fault Notification

   Protection switching relies on rapid notification of faults. Once a
   fault is detected, the node that detected the fault must determine
   if the fault is severe enough to require path recovery. Then the
   node should send out a notification of the fault by transmitting a
   FIS to those of its upstream LSRs that were sending traffic on the
   working path that is affected by the fault. This notification is
   relayed hop-by-hop by each subsequent LSR to its upstream neighbor,
   until it eventually reaches a PSL. A PSL is the only LSR that can
   terminate the FIS and initiate a protection switch of the working
   path to a recovery path. Since the FIS is a control message, it
   should be transmitted with high priority to ensure that it
   propagates rapidly towards the affected PSL(s). Depending on how
   fault notification is configured in the LSRs of an MPLS domain, the
   FIS could be sent either as a Layer 2 or Layer 3 packet. An example
   of a FIS could be the liveness message sent by a downstream LSR to
   its upstream neighbor, with an optional fault notification field
   set. Alternatively, it could be a separate fault notification
   packet. The intermediate LSR should identify which of its incoming
   links (upstream LSRs) to propagate the FIS on. In the case of 1+1
   protection, the FIS should also be sent downstream to the PML where
   the recovery action is taken.

3.7 Switch-Over Operation

3.7.1 Recovery Trigger

   The activation of an MPLS protection switch following the detection
   or notification of a fault requires a trigger mechanism at the PSL.
   MPLS protection switching may be initiated due to automatic inputs
   or external commands. The automatic activation of an MPLS
   protection switch results from a response to a defect or fault
   conditions detected at the PSL or to fault notifications received
   at the PSL. It is possible that the fault detection and trigger
   mechanisms may be combined, as is the case when a PF, PD, LF, or LD
   is detected at a PSL and triggers a protection switch to the
   recovery path. In most cases, however, the detection and trigger
   mechanisms are distinct, involving the detection of fault at some
   intermediate LSR followed by the propagation of a fault
   notification back to the PSL via the FIS, which serves as the
   protection switch trigger at the PSL. MPLS protection switching in
   response to external commands results when the operator initiates a
   protection switch by a command to a PSL (or alternatively by a
   configuration command to an intermediate LSR, which transmits the
   FIS towards the PSL).

   Note that the PF fault applies to hard failures (fiber cuts,
   transmitter failures, or LSR fabric failures), as does the LF
   fault, with the difference that the LF is a lower layer impairment
   that may be communicated to - MPLS-based recovery mechanisms. The
   PD (or LD) fault, on the other hand, applies to soft defects
   (excessive errors due to noise on the link, for instance). The PD
   (or LD) results in a fault declaration only when the percentage of
   lost packets exceeds a given threshold, which is provisioned and
   may be set based on the service level agreement(s) in effect
   between a service provider and a customer.
   
3.7.2 Recovery Action

   After a fault is detected or FIS is received by the PSL, the
   recovery action involves either a rerouting or protection switching
   operation. In both scenarios, the next hop label forwarding entry
   for a recovery path is bound to the working path.

3.8 Switch-Back Operation

3.8.1 Revertive and Non-Revertive Modes

   These protection modes indicate whether or not there is a
   “preferred” path for the protected traffic.

   If there is a preferred path, this path will be used whenever it is
   available.  If the preferred path has a fault, traffic is switched
   to the recovery path.  In the revertive mode of operation, when the
   preferred path is restored the traffic is automatically switched
   back to it.

   In the non-revertive mode of operation, there is no preferred path.
   If there is a fault on the working path, traffic is switched to the
   recovery path.  When or if the faulty path is restored, it may
   become the recovery path (either by configuration, or by management
   action, if desired). On the other hand, once the traffic is
   switched over to a recovery path, the association between the
   original working path and the recovery path may no longer exist.
   Instead, when the network reaches a stable state following routing
   convergence, the recovery path may be switched over to a different
   preferred path based either on pre-configured information or
   optimization based on the new network topology and associated
   information.

3.8.2 Restoration and Notification

   MPLS restoration deals with returning the working traffic from the
   recovery path to the original working path.  Reversion is performed
   by the PSL upon receiving notification, via FRS, that the working
   path is repaired.

   As before, an LSR that detected the fault on the working path also
   detects the restoration of the working path. If the working path
   had experienced a LF defect, the LSR detects a return to normal
   operation via the receipt of a liveness message from its peer. If
   the working path had experienced a LD defect at an LSR interface,
   the LSR could detect a return to normal operation via the
   resumption of error-free packet reception on that interface.
   Alternatively, a lower layer that no longer detects a LF defect may
   inform the MPLS-based recovery mechanisms at the LSR that the link
   to its peer LSR is operational. The LSR then transmits FRS to its
   upstream LSR(s) that were transmitting traffic on the working path.
   This is relayed hop-by-hop until it reaches the PSL(s), at which
   point the PSL switches the working traffic back to the original
   working path.

   In the non-revertive mode of operation, the working traffic may or
   may not be restored to the original working path. This is because
   it might be useful, in some cases, to either: (a) administratively
   perform a protection switch back to the original working path after
   gaining further assurances about the integrity of the path, or (b)
   it may be acceptable to continue operation without the recovery
   path being protected, or (c) it may be desirable to move the
   traffic to a new working path that is calculated based on network
   topology and network policies, after the dynamic routing protocols
   have converged. We note that if there is a way to transmit fault
   information back along a recovery path towards a PSL and if the
   recovery path is an equivalent recovery path, it is possible for
   the working path and its recovery path to exchange roles once the
   original working path is repaired following a fault. This is
   because, in that case, the recovery path effectively becomes the
   working path, and the restored working path functions as a recovery
   path for the original recovery path. This is important, since it
   affords the benefits of non-revertive switch operation outlined in
   Section 3.8.1, without leaving the recovery path unprotected.

3.8.3 Reverting to Preferred LSP (or Controlled Rearrangement)

   In the revertive mode, a “make before break” restoration switching
   can be used, which is less disruptive than performing protection
   switching upon the occurrence of network impairments. This will
   minimize both packet loss and packet reordering. The controlled
   rearrangement of LSPs can also be used to satisfy traffic
   engineering requirements for load balancing across an MPLS domain.

3.9 Performance

   Resource/performance requirements for recovery paths should be
   specified in terms of the following attributes:

   I. Resource class attribute:
   
   Equivalent Recovery Class: The recovery path has the same resource
   reservations and performance guarantees as the working path. In
   other words, the recovery path meets the same SLAs as the working
   path.
   
   Limited Recovery Class: The recovery path does not have the same
   resource reservations and performance guarantees as the working
   path.
   
   A. Lower Class: The recovery path has lower resource requirements
   or less stringent performance requirements than the working path.
   
   B. Best Effort Class: The recovery path is best effort.

   II. Priority Attribute:
   
   The recovery path has a priority attribute just like the working
   path (i.e., the priority attribute of the associated traffic
   trunks). It can have the same priority as the working path or lower
   priority.

   III. Preemption Attribute:
   
   The recovery path can have the same preemption attribute as the
   working path or a lower one.

4.0 MPLS Recovery Requirements

   The following are the MPLS recovery requirements:

   I. MPLS recovery SHALL provide an option to identify protection
   groups (PPGs) and protection portions (PTPs).
   
   II. Each PSL SHALL be capable of performing MPLS recovery upon the
   detection of the impairments or upon receipt of notifications of
   impairments.
   
   III. A MPLS recovery method SHALL not preclude manual protection
   switching commands. This implies that it would be possible under
   administrative commands to transfer traffic from a working path to
   a recovery path, or to transfer traffic from a recovery path to a
   working path, once the working path becomes operational following a
   fault.
   
   IV. A PSL SHALL be capable of performing either a switch back to
   –the original working path after the fault is corrected or a
   switchover to a new working path, upon the discovery of a more
   optimal working path.
   
   V. The recovery model should take into consideration path merging
   at intermediate LSRs. If a fault affects the merged segment, all
   the paths sharing that merged segment should be able to recover.
   Similarly, if a fault affects a non-merged segment, only the path
   that is affected by the fault should be recovered.

5.0 MPLS Recovery Options

   There SHOULD be an option for:
   
   I. Configuration of the recovery path as excess or reserved, with
   excess as the default. The recovery path that is configured as
   excess SHALL provide lower priority preemptable traffic access to
   the protection bandwidth, while the recovery path configured as
   reserved SHALL not provide any other traffic access to the
   protection bandwidth.
   
   II. Each protected path SHALL provide an option for configuring the
   protection alternatives as either rerouting or protection
   switching.
   
   III. Each protected path SHALL provide a configuration option for
   enabling restoration as either non-revertive or revertive, with
   revertive as the default.
   
   IV. Each LSR supporting protection switching SHALL provide an
   option for fault notification to the PSL.
   
6.0 Comparison Criteria

   Possible criteria to use for comparison of MPLS-based recovery
   schemes are as follows:
   
   Recovery Time
   
   We define recovery time as the time required for a recovery path to
   be activated (and traffic flowing) after a fault. Recovery Time is
   the sum of the Fault Detection Time, Hold-off Time, Notification
   Time, Recovery Operation Time, and the Traffic Restoration Time. In
   other words, it is the time between a failure of a node or link in
   the network and the time before a recovery path is installed and
   the traffic starts flowing on it.
   
   Full Restoration Time
   
   We define full restoration time as the time required for a
   permanent restoration. This is the time required for traffic to be
   routed onto links which are capable of or have been engineered
   sufficiently to handle traffic in recovery scenarios. Note that
   this time may or may not be different from the "Recovery Time"
   depending on whether equivalent or limited recovery paths are used.
   
   Backup Capacity
   
   Recovery schemes may require differing amounts of "backup capacity"
   in the event of a fault. This capacity will be dependent on the
   traffic characteristics of the network. However, it may also be
   dependent on the particular recovery path selection algorithms as
   well as the signaling and re-routing methods.
   
   Additive Latency
   
   Recovery schemes may introduce additive latency to traffic. For
   example, a recovery path may take many more hops than the working
   path. This may be dependent on the recovery path selection
   algorithms.
   
   Re-ordering
   
   Recovery schemes may introduce re-ordering of packets. Also the
   action of putting traffic back on preferred paths might cause
   packet re-ordering.
   
   State Overhead
   
   As the number of recovery paths grows, the state required to
   maintain them also grows. Schemes may require differing numbers of
   paths to maintain certain levels of coverage, etc. The state
   required may also depend on the particular scheme used to recover.
   In many cases the state overhead will be in proportion to the
   number of recovery paths.
   
   Loss
   
   Recovery schemes may introduce a certain amount of packet loss
   during switchover to a recovery path. Schemes which introduce loss
   during recovery can measure this loss by evaluating recovery times
   in proportion to the link speed.
   
   In case of link or node failure a certain packet loss is
   inevitable.
   
   Coverage
   
   Recovery schemes may offer various types of failover coverage. The
   total coverage may be defined in terms of several metrics:

   I. Fault Types: Recovery schemes may account for only link faults
   or both node and link faults or also degraded service. For example,
   a scheme may require more recovery paths to take node faults into
   account.
   
   II. Number of concurrent faults: dependent on the layout of
   recovery paths, multiple fault scenarios may be able to be
   restored.
   
   III. Number of recovery paths: for a given fault, there may be one
   or more recovery paths.
   
   IV. Percentage of coverage: dependent on a scheme and its
   implementation, a certain percentage of faults may be covered. This
   may be subdivided into percentage of link faults and percentage of
   node faults.
   
   V. The number of protected paths will highly effect how fast the
   total set of paths affected by a fault could be recovered. The
   ratio of protected is n/N, where n is the number of protected paths
   and N is the total number of paths.

7.0 Security Considerations

   The MPLS recovery that is specified herein does not raise any
   security issues that are not already present in the MPLS
   architecture.
   
8.0 Intellectual Property Considerations

   The IETF has been notified of intellectual property rights claimed
   in regard to some or all of the specification contained in this
   document. For more information consult the online list of claimed
   rights.

   
   
9.0 Authors’ Addresses

Srinivas Makam
Tellabs
4951 Indiana Avenue
Lisle, IL 60532
Ph: 630-512-7217
Email: [email-masked]

Vishal Sharma
Tellabs Research Center
One Kendall Square
Cambridge, MA 02139
Ph: 617-577-8760
Email: [email-masked]

Ken Owens
Tellabs
4951 Indiana Avenue
Lisle, IL 60532
Ph: 314-918-1579825-7009
Email: [email-masked]

Changcheng Huang
Tellabs
4951 Indiana Avenue
Lisle, IL 60532
Ph: 630-512-7754
Email: [email-masked]

Ben Mack-Crane
Tellabs
4951 Indiana Avenue
Lisle, IL 60532
Email: [email-masked]
Ph: 630-512-7255

Fiffi Hellstrand
Nortel Networks
St Eriksgatan 115, PO Box 6701
113 85 Stockholm, Sweden
Ph: +46 8 5088 3687
e-mail: [email-masked]

Jon Weil
Nortel Networks
Harlow Laboratories London Road
Harlow Essex CM17 9NA, UK
Phone: +44 (0)1279 403935
e-mail: [email-masked]

Brad Cain
Nortel Networks
3 Federal Street, BL3-03
Billerica, MA 01821, USA
Email: [email-masked]

Loa Andersson
Nortel Networks
St Eriksgatan 115, PO Box 6701
113 85 Stockholm, Sweden
phone: +46 8 50 88 36 34
e-mail: [email-masked]

Bilel Jamoussi
Nortel Networks
3 Federal Street, BL3-03
Billerica, MA 01821, USA
Email: [email-masked]

Seyhan Civanlar
AT&T Labs
Room C4-3A25
200 Laurel Ave.
Middletown, NJ 07748
Phone: (732) 420-2640
Email: [email-masked]

Angela Chiu
AT&T Labs
Room C4-3A22
200 Laurel Ave.
Middletown, NJ 07748
Phone: (732) 420-2290
Email: [email-masked]

10.0 References

_______________________________
   1 Rosen, E., Viswanathan, A., and Callon, R., "Multiprotocol Label
      Switching Architecture", Work in Progress, Internet Draft <draft-
      ietf-mpls-arch-06.txt>, August 1999.
      
   2 Andersson, L., Doolan, P., Feldman, N., Fredette, A., Thomas, B.,
      "LDP Specification", Work in Progress, Internet Draft <draft-
      ietf-mpls-ldp-06.txt>, September 1999.
      
   3 Awduche, D. Hannan, A., and Xiao, X., “Applicability Statement
      for Extensions to RSVP for LSP-Tunnels”, draft-ietf-mpls-rsvp-
      tunnel-applicability-00.txt”, work in progress, Sept. 1999.
      
   4 Jamoussi, B. "Constraint-Based LSP Setup using LDP", Work in
      Progress, Internet Draft <draft-ietf-mpls-cr-ldp-03.txt>,
      September 1999.
      
   5 Braden, R., Zhang, L., Berson, S., Herzog, S., "Resource
      ReSerVation Protocol (RSVP) -- Version 1 Functional
      Specification", RFC 2205, September 1997.
      
   6 Awduche, D. et al "Extensions to RSVP for LSP Tunnels", Work in
      Progress, Internet Draft <draft-ietf-mpls-rsvp-lsp-tunnel-
      04.txt, September 1999.
      
   7 Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., McManus, J.,
      "Requirements for Traffic Engineering Over MPLS", RFC 2702,
      September 1999.
      
   8 Makam, S., Sharma, V., Owens, K., Huang, C.,
      “Protection/restoration of MPLS Networks”, draft-makam-mpls-
      protection-00.txt, work in progress, October 1999.
      
   9 Andersson, L., Cain B., Jamoussi, B., “Requirement Framework for
      Fast Re-route with MPLS”, draft-andersson-reroute-frmwrk-00.txt,
      work in progress, October 1999.
      
   10 Goguen, R. and Swallow, G., “RSVP Label Allocation for Backup
      Tunnels”, draft-swallow-rsvp-bypass-label-00.txt, work in
      progress, October 1999.
      
   11 Haskin, D. and Krishnan R., “A Method for Setting an Alternative
      Label Switched Path to Handle Fast Reroute”, draft-haskin-mpls-
      fast-reroute-01.txt, 1999, Work in progress.