Network Working Group|
Request for Comments: 3469
V. Sharma, Ed.|
F. Hellstrand, Ed.
This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
Copyright © The Internet Society (2003). All Rights Reserved.
Multi-protocol label switching (MPLS) 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 switching routers (LSRs) support fault detection, fault notification, and fault recovery mechanisms, and that MPLS signaling support the configuration of recovery. With these objectives in mind, this document specifies a framework for MPLS based recovery. Restart issues are not included in this framework.
1.2. Motivation for MPLS-Based Recovery
2.1. Recovery Models
2.1.2 Protection Switching
2.2. The Recovery Cycles
2.2.1 MPLS Recovery Cycle Model
2.2.2 MPLS Reversion Cycle Model
2.2.3 Dynamic Re-routing Cycle Model
2.2.4 Example Recovery Cycle
2.3. Definitions and Terminology
2.3.1 General Recovery Terminology
2.3.2 Failure Terminology
3. MPLS-based Recovery Principles
3.1. Configuration of Recovery
3.2. Initiation of Path Setup
3.3. Initiation of Resource Allocation
3.3.1 Subtypes of Protection Switching
3.4. Scope of Recovery
3.4.2 Path Mapping
3.4.3 Bypass Tunnels
3.4.4 Recovery Granularity
3.4.5 Recovery Path Resource Use
3.5. Fault Detection
3.6. Fault Notification
3.7. Switch-Over Operation
3.7.1 Recovery Trigger
3.7.2 Recovery Action
3.8. Post Recovery Operation
3.8.1 Fixed Protection Counterparts
3.8.2 Dynamic Protection Counterparts
3.8.3 Restoration and Notification
3.8.4 Reverting to Preferred Path(or Controlled Rearrangement)
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.
At points in the document, we provide some thoughts about the operation or viability of certain recovery objectives. These should be viewed as the opinions of the authors, and not the consolidated views of the IETF. The document is informational and it is expected that a standards track document will be developed in the future to describe a subset of this document as to meet the needs currently specified by the TE WG.
Network routing deployed today is focused primarily on connectivity, and typically supports only one class of service, the best effort class. Multi-protocol label switching [RFC3031], 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 such media-specific forwarding mechanisms as label swapping. This enables some sophisticated features such as quality-of-service (QoS) and traffic engineering [RFC2702] 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 robust and survivable, the amount of time they take to recover from a fault can be significant, in the order of several seconds (for interior gateway protocols (IGPs)) or minutes (for exterior gateway protocols, such as the Border Gateway Protocol (BGP)), causing disruption of service for some applications in the interim. This is unacceptable in situations where the aim is to provide a highly reliable service, with recovery times that are in the order of seconds down to 10's of milliseconds. IP routing may also not be able to provide bandwidth recovery, where the objective is to provide not only an alternative path, but also bandwidth equivalent to that available on the original path. (For some recent work on bandwidth recovery schemes, the reader is referred to [MPLS- BACKUP].) Examples of such applications are Virtual Leased Line services, Stock Exchange data services, voice traffic, video services etc, i.e., every application that gets a disruption in service long enough to not fulfill service agreements or the required level of quality.
MPLS recovery may be motivated by the notion that there are limitations to improving the recovery times of current routing algorithms. Additional improvement can be obtained by augmenting these algorithms with MPLS recovery mechanisms [MPLS-PATH]. Since MPLS is a possible technology of choice in future IP-based transport networks, it is useful that MPLS be able to provide protection and restoration of traffic. MPLS may facilitate the convergence of network functionality on a common control and management plane. Further, a protection priority could be used as a differentiating
mechanism for premium services that require high reliability, such as Virtual Leased Line services, and high priority voice and video traffic. The remainder of this document provides a framework for MPLS based recovery. It is focused at a conceptual level and is meant to address motivation, objectives and requirements. Issues of mechanism, policy, routing plans and characteristics of traffic carried by recovery paths are beyond the scope of this document.
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 IP layer) approaches alone while still providing the visibility of the network afforded by Layer 3. Furthermore, a protection mechanism using MPLS could enable IP traffic to be put directly over WDM optical channels and provide a recovery option without an intervening SONET layer or optical protection. This would facilitate the construction of IP-over-WDM networks that request a fast recovery ability (Note that what is meant here is the transport of IP traffic over WDM links, not the Generalized MPLS, or GMPLS, control of a WDM link).
The need for MPLS-based recovery arises because of the following:
II. Layer 3 or IP rerouting does not provide the ability to provide bandwidth protection to specific flows (e.g., voice over IP, virtual leased line services).
III. Layer 0 (for example, optical layer) or Layer 1 (for example, SONET) mechanisms may be wasteful use of resources.
IV. 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.
restoration. In networks where the latter class of traffic is dominant, providing fast restoration to all classes of traffic may not be cost effective from a service provider's perspective.
VI. MPLS has desirable attributes when applied to the purpose of recovery for connectionless networks. Specifically that an LSP is source routed and a forwarding path for recovery can be "pinned" and is not affected by transient instability in SPF routing brought on by failure scenarios.
VII. Establishing interoperability of protection mechanisms between routers/LSRs from different vendors in IP or MPLS networks is desired to enable recovery mechanisms to work in a multivendor environment, and to enable the transition of certain protected services to an MPLS core.
The following are some important goals for MPLS-based recovery.
I. MPLS-based recovery mechanisms may be subject to the traffic engineering goal of optimal use of resources.
II. MPLS based recovery mechanisms should aim to facilitate restoration times that are sufficiently fast for the end user application. That is, that better match the end-user's application requirements. In some cases, this may be as short as 10s of milliseconds.
We observe that I and II may be conflicting objectives, and a trade off may exist between them. The optimal choice depends on the end- user application's sensitivity to restoration time and the cost impact of introducing restoration in the network, as well as the end-user application's sensitivity to cost.
III. MPLS-based recovery should aim to maximize network reliability and availability. MPLS-based recovery of traffic should aim to minimize the number of single points of failure in the MPLS protected domain.
IV. MPLS-based recovery should aim to enhance the reliability of the protected traffic while minimally or predictably degrading the traffic carried by the diverted resources.
V. MPLS-based recovery techniques should aim to 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. Note that a path is used as a general term and includes the notion of a link, IP route or LSP.
VI. MPLS-based recovery techniques may be applicable for an entire end-to-end path or for segments of an end-to-end path.
VII. MPLS-based recovery mechanisms should aim to take into consideration the recovery actions of lower layers. MPLS-based mechanisms should not trigger lower layer protection switching nor should MPLS-based mechanisms be triggered when lower layer switching has or may imminently occur.
VIII. MPLS-based recovery mechanisms should aim to minimize the loss of data and packet reordering during recovery operations. (The current MPLS specification itself has no explicit requirement on reordering.)
IX. MPLS-based recovery mechanisms should aim to minimize the state overhead incurred for each recovery path maintained.
X. MPLS-based recovery mechanisms should aim to minimize the signaling overhead to setup and maintain recovery paths and to notify failures.
XI. MPLS-based recovery mechanisms should aim 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.
We observe that some of the above are conflicting goals, and real deployment will often involve engineering compromises based on a variety of factors such as cost, end-user application requirements, network efficiency, complexity involved, and revenue considerations. Thus, these goals are subject to tradeoffs based on the above considerations.
There are several options for providing protection of traffic. The most generic requirement is the specification of whether recovery should be via Layer 3 (or IP) rerouting or via MPLS protection switching or rerouting actions.
Generally network operators aim to provide the fastest, most stable, and the best protection mechanism that can be provided at a reasonable cost. The higher the levels of protection, the more the
resources consumed. Therefore it is expected that network operators will offer a spectrum of service levels. MPLS-based recovery 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 in order to give operators more control over that tradeoff. 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, using approaches outlined below, a Virtual Leased Line (VLL) service or real-time applications like Voice over IP (VoIP) may be supported using link/node protection together with pre- established, pre-reserved path protection. Best effort traffic, on the other hand, may use path protection that is established on demand or may simply rely on IP re-route or higher layer recovery mechanisms. As another example of their range of application, MPLS- based recovery strategies may be used to protect traffic not originally flowing on label switched paths, such as IP traffic that is normally routed hop-by-hop, as well as traffic forwarded on label switched paths.
There are two basic models for path recovery: rerouting and protection switching.
Protection switching and rerouting, as defined below, 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.
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, paths or path segments to bypass the fault are established using signaling.
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 discussed further in Section 3.8.
In terms of the principles defined in section 3, reroute recovery employs paths established-on-demand with resources reserved-on- demand.
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. However if the recovery path shares sources of failure with the working path, the overall reliability of the construct is degraded. When a fault is detected, the protected traffic is switched over to the recovery path(s) and restored.
In terms of the principles in section 3, protection switching employs pre-established recovery paths, and, if resource reservation is required on the recovery path, pre-reserved resources. The various sub-types of protection switching are detailed in Section 4.4 of this document.
There are three defined recovery cycles: the MPLS Recovery Cycle, the MPLS Reversion Cycle and the Dynamic Re-routing Cycle. The first cycle detects a fault and restores traffic onto MPLS-based recovery paths. If the recovery path is non-optimal the cycle may be followed by any of the two latter cycles to achieve an optimized network again. The reversion cycle applies for explicitly routed traffic that does not rely on any dynamic routing protocols to converge. The dynamic re-routing cycle applies for traffic that is forwarded based on hop-by-hop routing.
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 Recovered | | | | | | | | | | | | 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 Fault Hold-off Time T3 Fault Notification Time T4 Recovery Operation Time T5 Traffic Recovery 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.
Fault 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 Fault Hold-off Time may be zero.
Note: The Fault Hold-Off Time may occur after the Fault Notification Time interval if the node responsible for the switchover, the Path Switch LSR (PSL), rather than the detecting LSR, is configured to wait.
Fault Notification Time
The time between initiation of a Fault Indication Signal (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 or infers a fault from such events as an adjacency failure.
Note: If the PSL detects the fault itself, there still may be a Fault 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 (Path Merge LSR) to coordinate recovery actions.
Traffic Recovery 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.
Protection switching, revertive mode, requires the traffic 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 Clear Hold-Off Time T9 Clear Notification Time T10 Reversion Operation Time T11 Traffic Reversion 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.
Clear Hold-Off Time
The configured waiting time between the clearing of a fault and MPLS-based recovery action(s). Waiting time may be needed to ensure that the path is stable and to avoid flapping in cases where a fault is intermittent. The Clear Hold-Off Time may be zero.
Note: The Clear Hold-Off Time may occur after the Clear Notification Time interval if the PSL is configured to wait.
Clear Notification Time
The time between initiation of a Fault Recovery Signal (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 Clear Hold-off 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 Reversion 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 most interesting times in the reversion cycle are the Clear Hold-off Time and the Reversion Operation Time together with Traffic Reversion Time (or some other measure of traffic
disruption). The first interval is to ensure stability of the repaired path and the latter one is to minimize disruption time while the reversion action is in progress.
Given that both paths are available, it is better to wait to have a well-controlled switch-back with minimal disruption than have an immediate operation that may cause new faults to be introduced (except, perhaps, when the recovery path is unable to offer a quality of service comparable to the preferred path).
Dynamic rerouting aims to bring the IP network to a stable state after a network impairment has occurred. A re-optimized network is achieved after the routing protocols have converged, and the traffic is moved 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 be overlaid on 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 occurs if 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 New Working Path | | | | | | | | | | v v v v v ----------------------------------------------------------------- | T12 | T13 | T14 | T15 |
Figure 3. 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.
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.
Traffic Restoration Time
The time between the last restoration action and the time that traffic (if present) is completely restored on the new preferred path.
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.
This document assumes the terminology given in [RFC3031], and, in addition, introduces the following new terms.
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.
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.
The protected path that carries traffic before the occurrence of a fault. The working path can be of different kinds; a hop-by-hop routed path, a trunk, a link, an LSP or part of a multipoint-to- point LSP.
Synonyms for a working path are primary path and active 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 is established by MPLS means. 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.
Synonyms for a recovery path are: back-up path, alternative path, and protection path.
The "other" path when discussing pre-planned protection switching schemes. The protection counterpart for the working path is the recovery path and vice-versa.
Path Switch LSR (PSL)
An LSR that is responsible for switching or replicating the traffic between the working path and the recovery path.
Path Merge LSR (PML)
An LSR that is responsible for receiving the recovery path traffic, and either merging the traffic back onto the working path, or, if it is itself the destination, passing the traffic on to the higher layer protocols.
Point of Repair (POR)
An LSR that is setup for performing MPLS recovery. In other words, an LSR that is responsible for effecting the repair of an LSP. The POR, for example, can be a PSL or a PML, depending on the type of recovery scheme employed.
An LSR on a working or recovery path that is neither a PSL nor a PML for that path.
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.
A path that serves to back up a set of working paths using the label stacking approach [RFC3031]. The working paths and the bypass tunnel must all share the same path switch LSR (PSL) and the path merge LSR (PML).
The process of switching the traffic from the path that the traffic is flowing on 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).
The process of returning the traffic from one or more recovery paths back to the working path(s).
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. This assumes a failed working path does not automatically surrender resources to the network.
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, i.e., is surrendered to the network.)
MPLS Protection Domain
The set of LSRs over which a working path and its corresponding recovery path are routed.
MPLS Protection Plan
The set of all LSP protection paths and the mapping from working to protection paths deployed in an MPLS protection domain at a given time.
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 document. (This could be accomplished, for example, by transmitting a control message along the same links and nodes as the data traffic or similarly could be measured by the absence of traffic and by providing feedback.)
Path Failure (PF)
Path failure is a fault detected by MPLS-based recovery mechanisms, which is defined 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 (the POR). The FIS is transmitted
periodically by the node/nodes closest to the point of failure, for some configurable length of time or until the transmitting node receives an acknowledgement from its neighbor.
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. The FRS is transmitted periodically by the node/nodes closest to the point of failure, for some configurable length of time or until the transmitting node receives an acknowledgement from its neighbor.
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. POR: Point of Repair. PPG: Protected Path Group. PTP: Protected Traffic Portion. PSL: Path Switch LSR.
MPLS-based recovery refers to the ability to effect quick and complete restoration of traffic affected by a fault in an MPLS- enabled network. The fault may be detected on the IP layer or in lower layers over which IP traffic is transported. Fastest MPLS recovery is assumed to be achieved with protection switching and 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. Further, MPLS-based recovery may provide bandwidth protection for paths that require it. This section provides a discussion of the concepts and principles of MPLS-based recovery. The concepts are presented in terms of atomic or primitive terms that may be combined to specify recovery approaches. We do not make any assumptions about the underlying layer 1 or layer 2 transport mechanisms or their recovery mechanisms.
An LSR may support any or all of the following recovery options on a per-path basis:
Default-recovery (No MPLS-based recovery enabled): Traffic on the working path is recovered only via Layer 3 or IP rerouting or by some lower layer mechanism such as SONET APS. 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.
There are three options for the initiation of the recovery path setup. The active and recovery paths may be established by using either RSVP-TE [RFC2205][RFC3209] or CR-LDP [RFC3212], or by any other means including SNMP.
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, 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.
A pre-established path need not be created, it may be pre- qualified. A pre-qualified recovery path is not created expressly for protecting the working path, but instead is a path created for other purposes that is designated as a recovery path after determining that it is an acceptable alternative for carrying the working path traffic. Variants include the case where an optical path or trail is configured, but no switches are set.
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. The recovery path may be pre- computed or computed on demand, which influences recovery times.
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:
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. The resources held by a set of recovery paths may be shared if they protect resources that are not simultaneously subject to failure.
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.
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. This leads to two subtypes of protection switching.
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). In effect the PSL function is deprecated to establishment of the working and recovery paths and a simple replication function. The recovery intelligence is delegated to the PML.
In 1:1 ("one for one") protection, the resources (if any) allocated on the recovery path are fully available to preemptible 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.
This concept can be extended to 1:n (one for n) and m:n (m for n) protection.
The intent of local repair is to protect against a link or neighbor node fault and to minimize the amount of time required for failure propagation. In local repair (also known as local recovery), the node immediately upstream of the fault is the one to initiate recovery (either rerouting or protection switching). Local repair can be of two types:
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, 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. Link recovery is potentially the fastest to perform the switchover, and can be effective in situations where certain path components are much more unreliable than others.
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.
The intent of global repair is to protect against any link or node fault on a path or on a segment of a path, with the obvious exception of the faults occurring at the ingress node of the protected path segment. In global repair, the POR is usually distant from the failure and needs to be notified by a FIS.
In global repair also, end-to-end path recovery/restoration applies. In many cases, 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 (end-to-end path or path segment).
However, it may, in some cases, be slower than local repair since the fault notification message must now travel to the POR to trigger the recovery action.
It is possible to restore service without specifically recovering the faulted path.
For example, for best effort IP service it is possible to select a recovery path that has a different egress point from the working path (i.e., there is no PML). The recovery path egress must simply be a router that is acceptable for forwarding the FEC carried by the working path (without creating looping). In an engineering context, specific alternative FEC/LSP mappings with alternate egresses can be formed.
This may simplify enhancing the reliability of implicitly constructed MPLS topologies. A PSL may qualify LSP/FEC bindings as candidate recovery paths as simply link and node disjoint with the immediate downstream LSR of the working path.
Multi-layer repair broadens the network designer's tool set for those cases where multiple network layers can be managed together to achieve overall network goals. Specific criteria for determining when multi-layer repair is appropriate are beyond the scope of this document.
A given service may cross multiple networks and these may employ different recovery mechanisms. It is possible to concatenate protection domains so that service recovery can be provided end-to- end. It is considered that the recovery mechanisms in different domains may operate autonomously, and that multiple points of attachment may be used between domains (to ensure there is no single point of failure). Alternate egress repair requires management of concatenated domains in that an explicit MPLS point of failure (the PML) is by definition excluded. Details of concatenated protection domains are beyond the scope of this document.
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, as described below. Note that the options below should be viewed as atomic terms that only describe how the working and protection paths are mapped to each other. The issues of resource reservation along these paths, and how switchover is actually performed lead to the more commonly used composite terms, such as 1+1 and 1:1 protection, which were described in Section 4.3.1..
In 1-to-1 protection the working path has a designated recovery path that is only to be used to recover that specific working path.
In n-to-1 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 document.
In n-to-m protection, up to n working paths are protected using m recovery paths. Once again, if the intent is to protect against 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 document. n-to-m protection is for further study.
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(s), and may require the PML(s) 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 document.
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 [RFC2702]. In this case, one LSP (the tunnel) is established between the PSL and PML following an acceptable route and a number of recovery paths can be supported through the tunnel via label stacking. It is not necessary to apply label stacking when using a bypass tunnel. 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 n-to-1 or n-to-m protection cases mentioned in Section 3.4.2.
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.
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.
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.
In the case of pre-reserved recovery paths, there is the question of what use these resources may be put to when the recovery path is not in use. There are two options:
Dedicated-resource: If the recovery path resources are dedicated, they may not be used for anything except carrying the working traffic. For example, in the case of 1+1 protection, the working traffic is always carried on the recovery path. Even if the recovery path is not always carrying the working traffic, it may not be possible or desirable to allow other traffic to use these resources.
Extra-traffic-allowed: If the recovery path only carries the working traffic when the working path fails, then it is possible to allow extra traffic to use the reserved resources at other times. Extra traffic is, by definition, traffic that can be displaced (without violating service agreements) whenever the recovery path resources are needed for carrying the working path traffic.
Shared-resource: A shared recovery resource is dedicated for use by multiple primary resources that (according to SRLGs) are not expected to fail simultaneously.
MPLS recovery is initiated after the detection of either a lower layer fault or a fault at the IP layer or 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 [MPLS-PATH]. 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 path has connectivity, but that the quality of the connection is unacceptable. This may be detected by a path performance monitoring mechanism, or some other 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 path 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 (Loss of Signal)), 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 path 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.
MPLS-based recovery relies on rapid and reliable 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. If the node is not capable of initiating direct action (e.g., as a point of repair, POR) the node should send out a notification of the fault by transmitting a FIS to the POR. This can take several forms:
(i) control plane messaging: relayed hop-by-hop along the path
upstream of the failed LSP until a POR is reached.
(ii) user plane messaging: sent downstream to the PML, which may take corrective action (as a POR for 1+1) or communicate with a POR upstream (for 1:n) by any of several means:
- control plane messaging - user plane return path (either through a bi-directional LSP or via other means)
Since the FIS is a control message, it should be transmitted with
high priority to ensure that it propagates rapidly towards the
affected POR(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 [MPLS-PATH]. The use of a Layer 2-based
notification requires a Layer 2 path direct to the POR. 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
or it can be implicitly denoted by a teardown message.
Alternatively, it could be a separate fault notification packet. The intermediate LSR should identify which of its incoming links to propagate the FIS on.
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 to the POR via the FIS, which serves as the protection switch trigger at the POR. MPLS protection switching in response to external commands results when the operator initiates a protection switch by a command to a POR (or alternatively by a configuration command to an intermediate LSR, which transmits the FIS towards the POR).
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.
After a fault is detected or FIS is received by the POR, 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.
When traffic is flowing on the recovery path, decisions can be made as to whether to let the traffic remain on the recovery path and consider it as a new working path or to do a switch back to the old or to a new working path. This post recovery operation has two styles, one where the protection counterparts, i.e., the working and recovery path, are fixed or "pinned" to their routes, and one in which the PSL or other network entity with real-time knowledge of failure dynamically performs re-establishment or controlled rearrangement of the paths comprising the protected service.
For fixed protection counterparts the PSL will be pre-configured with the appropriate behavior to take when the original fixed path is restored to service. The choices are revertive and non-revertive mode. The choice will typically be dependent on relative costs of the working and protection paths, and the tolerance of the service to the effects of switching paths yet again. These protection modes indicate whether or not there is a preferred path for the protected traffic.
If the working path always is the preferred path, this path will be used whenever it is available. Thus, in the event of a fault on this path, its unused resources will not be reclaimed by the network on failure. Resources here may include assigned labels, links, bandwidth etc. If the working 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.
There are a number of implications to pinned working and recovery paths:
- upon failure and after traffic has been moved to the recovery path, the traffic is unprotected until such time as the path defect in the original working path is repaired and that path restored to service.
- upon failure and after traffic has been moved to the recovery path, the resources associated with the original path remain reserved.
In the non-revertive mode of operation, there is no preferred path or it may be desirable to minimize further disruption of the service brought on by a revertive switching operation. A switch-back to the original working path is not desired or not possible since the original path may no longer exist after the occurrence of a fault on that path. If there is a fault on the working path, traffic is switched to the recovery path. When or if the faulty path (the originally working path) is restored, it may become the recovery path (either by configuration, or, if desired, by management actions).
In the non-revertive mode of operation, the working traffic may or may not be restored to a new optimal working path or to the original working path anyway. 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 on the recovery path, or (c) it may be desirable to move the traffic to a new optimal working path that is calculated based on network topology and network policies. Once a new working path has been defined, an associated recovery path may be setup.
For dynamic protection counterparts when the traffic is switched over to a recovery path, the association between the original working path and the recovery path may no longer exist, since the original path itself may no longer exist after the fault. Instead, when the network reaches a stable state following routing convergence, the recovery path may be switched over to a different preferred path either optimization based on the new network topology and associated information or based on pre-configured information.
Dynamic protection counterparts assume that upon failure, the PSL or other network entity will establish new working paths if another switch-over will be performed.
MPLS restoration deals with returning the working traffic from the recovery path to the original or a new working path. Restoration is performed by the PSL either upon receiving notification, via FRS, that the working path is repaired, or upon receiving notification that a new working path is established.
For fixed counterparts in revertive mode, 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. At the point the PSL receives the FRS, it switches the working traffic back to the original working path.
A similar scheme is used for dynamic counterparts where e.g., an update of topology and/or network convergence may trigger installation or setup of new working paths and may send notification to the PSL to perform a switch over.
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 working 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 4.8.1, without leaving the recovery path unprotected.
In the revertive mode, "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 paths can also be used to satisfy traffic engineering requirements for load balancing across an MPLS domain.
Resource/performance requirements for recovery paths should be specified in terms of the following attributes:
Limited Recovery Class: The recovery path does not have the same performance guarantees as the working path.
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.
The following features are desirable from an operational point of view:
II. Each PSL should be capable of performing MPLS recovery upon the detection of the impairments or upon receipt of notifications of impairments.
III. A MPLS recovery method should 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 may 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 or establishment of a more optimal working path.
Possible criteria to use for comparison of MPLS-based recovery schemes are as follows:
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.
The amount of time that a working path or a set of working paths is left unprotected during such tasks as recovery path computation and recovery path setup may be used to compare schemes. The nature of this vulnerability should be taken into account, e.g., End to End schemes correlate the vulnerability with working paths,
Local Repair schemes have a topological correlation that cuts across working paths and Network Plan approaches have a correlation that impacts the entire network.
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 protection plan selection algorithms as well as the signaling and re-routing methods.
Recovery schemes may introduce additive latency for 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.
Quality of Protection
Recovery schemes can be considered to encompass a spectrum of "packet survivability" which may range from "relative" to "absolute". Relative survivability may mean that the packet is on an equal footing with other traffic of, as an example, the same diff-serv code point (DSCP) in contending for the resources of the portion of the network that survives the failure. Absolute survivability may mean that the survivability of the protected traffic has explicit guarantees.
Recovery schemes may introduce re-ordering of packets. Also the action of putting traffic back on preferred paths might cause packet re-ordering.
As the number of recovery paths in a protection plan 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 for recovery. The state overhead may be a function of several parameters. For example, the number of recovery paths and the number of the protected facilities (links, nodes, or shared link risk groups (SRLGs)).
Recovery schemes may introduce a certain amount of packet loss during switchover to a recovery path. Schemes that 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.
Recovery schemes may offer various types of failover coverage. The total coverage may be defined in terms of several metrics:
II. Number of concurrent faults: dependent on the layout of recovery paths in the protection plan, 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.
The MPLS recovery that is specified herein does not raise any
security issues that are not already present in the MPLS
Confidentiality or encryption of information on the recovery path is outside the scope of this document, but any method designed to do this in other contexts may be used with the methods described in this document.
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.
We would like to thank members of the MPLS WG mailing list for their suggestions on the earlier versions of this document. In particular, Bora Akyol, Dave Allan, Dave Danenberg, Sharam Davari, and Neil Harrison whose suggestions and comments were very helpful in revising the document.
The editors would like to give very special thanks to Curtis Villamizar for his careful and extremely thorough reading of the document and for taking the time to provide numerous suggestions, which were very helpful in the last couple of revisions of the document. Thanks are also due to Adrian Farrel for a through reading of the last version of the document, and to Jean-Phillipe Vasseur and Anna Charny for several useful editorial comments and suggestions, and for input on bandwidth recovery.
[RFC3031] Rosen, E., Viswanathan, A. and R. Callon, "Multiprotocol Label Switching Architecture", RFC 3031, January 2001. [RFC2702] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M. and J. McManus, "Requirements for Traffic Engineering Over MPLS", RFC 2702, September 1999. [RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V. and G. Swallow, "RSVP-TE Extensions to RSVP for LSP Tunnels", RFC 3209, December 2001. [RFC3212] Jamoussi, B. (Ed.), Andersson, L., Callon, R., Dantu, R., Wu, L., Doolan, P., Worster, T., Feldman, N., Fredette, A., Girish, M., Gray, E., Heinanen, J., Kilty, T. and A. Malis, "Constraint-Based LSP Setup using LDP", RFC 3212, January 2002.
[MPLS-BACKUP] Vasseur, J. P., Charny, A., LeFaucheur, F., and Achirica, "MPLS Traffic Engineering Fast reroute: backup tunnel path computation for bandwidth protection", Work in Progress.
[MPLS-PATH] Haung, C., Sharma, V., Owens, K., Makam, V. "Building Reliable MPLS Networks Using a Path Protection Mechanism", IEEE Commun. Mag., Vol. 40, Issue 3, March 2002, pp. 156-162. [RFC2205] Braden, R., Zhang, L., Berson, S., Herzog, S., "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specification", RFC 2205, September 1997.
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