Network Working Group Y. Lee Request for Comments: 5557 Huawei Category: Standards Track JL. Le Roux France Telecom D. King Old Dog Consulting E. Oki University of Electro Communications July 2009
Path Computation Element Communication Protocol (PCEP) Requirements and Protocol Extensions in Support of Global Concurrent Optimization
Abstract
The Path Computation Element Communication Protocol (PCEP) allows Path Computation Clients (PCCs) to request path computations from Path Computation Elements (PCEs), and lets the PCEs return responses. When computing or reoptimizing the routes of a set of Traffic Engineering Label Switched Paths (TE LSPs) through a network, it may be advantageous to perform bulk path computations in order to avoid blocking problems and to achieve more optimal network-wide solutions. Such bulk optimization is termed Global Concurrent Optimization (GCO). A GCO is able to simultaneously consider the entire topology of the network and the complete set of existing TE LSPs, and their respective constraints, and look to optimize or reoptimize the entire network to satisfy all constraints for all TE LSPs. A GCO may also be applied to some subset of the TE LSPs in a network. The GCO application is primarily a Network Management System (NMS) solution.
This document provides application-specific requirements and the PCEP extensions in support of GCO applications.
Status of This Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
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Copyright Notice
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Table of Contents
1. Introduction ....................................................4 2. Terminology .....................................................6 3. Applicability of Global Concurrent Optimization (GCO) ...........6 3.1. Application of the PCE Architecture ........................7 3.2. Greenfield Optimization ....................................8 3.2.1. Single-Layer Traffic Engineering ....................8 3.2.2. Multi-Layer Traffic Engineering .....................8 3.3. Reoptimization of Existing Networks ........................8 3.3.1. Reconfiguration of the Virtual Network Topology (VNT) ......................................9 3.3.2. Traffic Migration ...................................9 4. PCECP Requirements .............................................10 5. Protocol Extensions for Support of Global Concurrent Optimization ...................................................13 5.1. Global Objective Function (GOF) Specification .............14 5.2. Indication of Global Concurrent Optimization Requests .....15 5.3. Request for the Order of TE LSP ...........................15 5.4. The Order Response ........................................16 5.5. GLOBAL CONSTRAINTS (GC) Object ............................17 5.6. Error Indicator ...........................................18 5.7. NO-PATH Indicator .........................................19 6. Manageability Considerations ...................................19 6.1. Control of Function and Policy ............................19 6.2. Information and Data Models (e.g., MIB Module) ............20 6.3. Liveness Detection and Monitoring .........................20 6.4. Verifying Correct Operation ...............................20 6.5. Requirements on Other Protocols and Functional Components ................................................20 6.6. Impact on Network Operation ...............................20 7. Security Considerations ........................................21 8. IANA Considerations ............................................21 8.1. Request Parameter Bit Flags ...............................21 8.2. New PCEP TLV ..............................................21 8.3. New Flag in PCE-CAP-FLAGS Sub-TLV in PCED .................22 8.4. New PCEP Object ...........................................22 8.5. New PCEP Error Codes ......................................22 8.5.1. New Error-Values for Existing Error-Types ..........22 8.5.2. New Error-Types and Error-Values ...................23 8.6. New No-Path Reasons .......................................23 9. References .....................................................23 9.1. Normative References ......................................23 9.2. Informative References ....................................24 10. Acknowledgments ...............................................24 Appendix A. RBNF Code Fragments ...................................25
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[RFC4655] defines the Path Computation Element (PCE)-based architecture and explains how a PCE may compute Label Switched Paths (LSPs) in Multiprotocol Label Switching Traffic Engineering (MPLS-TE) and Generalized MPLS (GMPLS) networks at the request of Path Computation Clients (PCCs). A PCC is shown to be any network component that makes such a request and may be, for instance, a Label Switching Router (LSR) or a Network Management System (NMS). The PCE, itself, is shown to be located anywhere within the network, and it may be within an LSR, an NMS or Operational Support System (OSS), or may be an independent network server.
The PCE Communication Protocol (PCEP) is the communication protocol used between PCC and PCE, and it may also be used between cooperating PCEs. [RFC4657] sets out generic protocol requirements for PCEP. Additional application-specific requirements for PCEP are defined in separate documents.
This document provides a set of requirements and PCEP extensions in support of concurrent path computation applications. A concurrent path computation is a path computation application where a set of TE paths are computed concurrently in order to efficiently utilize network resources. The computation method involved with a concurrent path computation is referred to as "global concurrent optimization" in this document. Appropriate computation algorithms to perform this type of optimization are out of the scope of this document.
The Global Concurrent Optimization (GCO) application is primarily an NMS or a PCE-Server-based solution. Owing to complex synchronization issues associated with GCO applications, the management-based PCE architecture defined in Section 5.5 of [RFC4655] is considered as the most suitable usage to support GCO application. This does not preclude other architectural alternatives to support GCO application, but they are NOT RECOMMENDED. For instance, GCO might be enabled by distributed LSRs through complex synchronization mechanisms. However, this approach might suffer from significant synchronization overhead between the PCE and each of the PCCs. It would likely affect the network stability and hence significantly diminish the benefits of deploying PCEs.
The need for global concurrent path computation may also arise when network operators need to establish a set of TE LSPs in their network planning process. It is also envisioned that network operators might require global concurrent path computation in the event of catastrophic network failures, where a set of TE LSPs need to be
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optimally rerouted. The nature of this work promotes the use of such systems for off-line processing. Online application of this work should only be considered with proven empirical validation.
As new TE LSPs are added or removed from the network over time, the global network resources become fragmented and the existing placement of TE LSPs within the network no longer provides optimal use of the available capacity. A global concurrent path computation is able to simultaneously consider the entire topology of the network and the complete set of existing TE LSPs and their respective constraints, and is able to look to reoptimize the entire network to satisfy all constraints for all TE LSPs. Alternatively, the application may consider a subset of the TE LSPs and/or a subset of the network topology. Note that other preemption can also help reduce the fragmentation issues.
While GCO is applicable to any simultaneous request for multiple TE LSPs (for example, a request for end-to-end protection), it is NOT RECOMMENDED that global concurrent reoptimization would be applied in a network (such as an MPLS-TE network) that contains a very large number of very low bandwidth or zero bandwidth TE LSPs since the large scope of the problem and the small benefit of concurrent reoptimization relative to single TE LSP reoptimization is unlikely to make the process worthwhile. Further, applying global concurrent reoptimization in a network with a high rate of change of TE LSPs (churn) is NOT RECOMMENDED because of the likelihood that TE LSPs would change before they could be globally reoptimized. Global reoptimization is more applicable to stable networks such as transport networks or those with long-term TE LSP tunnels.
The main focus of this document is to highlight the PCC-PCE communication needs in support of a concurrent path computation application and to define protocol extensions to meet those needs.
The PCC-PCE requirements addressed herein are specific to the context where the PCE is a specialized PCE that is capable of performing computations in support of GCO. Discovery of such capabilities might be desirable and could be achieved through extensions to the PCE discovery mechanisms [RFC4674], [RFC5088], [RFC5089]; but, that is out of the scope of this document.
It is to be noted that Backward Recursive Path Computation (BRPC) [RFC5441] is a multi-PCE path computation technique used to compute a shortest constrained inter-domain path, whereas this ID specifies a technique where a set of path computation requests are bundled and sent to a PCE with the objective of "optimizing" the set of computed paths.
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Most of the terminology used in this document is explained in [RFC4655]. A few key terms are repeated here for clarity.
PCC: Path Computation Client. Any client application requesting a path computation to be performed by a Path Computation Element.
PCE: Path Computation Element. An entity (component, application, or network node) that is capable of computing a network path or route based on a network graph and applying computational constraints.
TED: Traffic Engineering Database. The TED contains the topology and resource information of the domain. The TED may be fed by IGP extensions or potentially by other means.
PCECP: The PCE Communication Protocol. PCECP is the generic abstract idea of a protocol that is used to communicate path computation requests from a PCC to a PCE and to return computed paths from the PCE to the PCC. The PCECP can also be used between cooperating PCEs.
PCEP: The PCE communication Protocol. PCEP is the actual protocol that implements the PCECP idea.
GCO: Global Concurrent Optimization. A concurrent path computation application where a set of TE paths are computed concurrently in order to optimize network resources. A GCO path computation is able to simultaneously consider the entire topology of the network and the complete set of existing TE LSPs, and their respective constraints, and look to optimize or reoptimize the entire network to satisfy all constraints for all TE LSPs. A GCO path computation can also provide an optimal way to migrate from an existing set of TE LSPs to a reoptimized set (Morphing Problem).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. These terms are used to specify requirements in this document.
3. Applicability of Global Concurrent Optimization (GCO)
This section discusses the PCE architecture to which GCO is applied. It also discusses various application scenarios for which global concurrent path computation may be applied.
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Figure 1 shows the PCE-based network architecture as defined in [RFC4655] to which GCO application is applied. It must be observed that the PCC is not necessarily an LSR [RFC4655]. The GCO application is primarily an NMS-based solution in which an NMS plays the function of the PCC. Although Figure 1 shows the PCE as remote from the NMS, it might be collocated with the NMS. Note that in the collocated case, there is no need for a standard communication protocol; this can rely on internal APIs.
Figure 1: PCE-Based Architecture for Global Concurrent Optimization
Upon receipt of an application request (e.g., a traffic demand matrix is provided to the NMS by the operator's network planning procedure), the NMS requests a global concurrent path computation from the PCE. The PCE then computes the requested paths concurrently applying some algorithms. Various algorithms and computation techniques have been proposed to perform this function. Specification of such algorithms or techniques is outside the scope of this document.
When the requested path computation completes, the PCE sends the resulting paths back to the NMS. The NMS then supplies the head-end LSRs with a fully computed explicit path for each TE LSP that needs to be established.
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Greenfield optimization is a special case of GCO application when there are no TE LSPs already set up in the network. The need for greenfield optimization arises when the network planner wants to make use of a computation server to plan the TE LSPs that will be provisioned in the network. Note that greenfield operation is a one-time optimization. When network conditions change due to failure or other changes, then the reoptimization mode of operation will kick in.
When a new TE network needs to be provisioned from a greenfield perspective, a set of TE LSPs needs to be created based on traffic demand, network topology, service constraints, and network resources. In this scenario, the ability to perform concurrent computation is desirable, or required, to utilize network resources in an optimal manner and avoid blocking.
Greenfield optimization can be applied when layer-specific TE LSPs need to be created from a greenfield perspective. For example, an MPLS-TE network can be planned based on Layer 3 specific traffic demands, the network topology, and available network resources. Greenfield optimization for single-layer traffic engineering can be applied to optical transport networks such as Synchronous Digital Hierarchy/Synchronous Optical Network (SDH/SONET), Ethernet Transport, Wavelength Division Multiplexing (WDM), etc.
Greenfield optimization is not limited to single-layer traffic engineering. It can also be applied to multi-layer traffic engineering [PCE-MLN]. The network resources and topology (of both the client and server layers) can be considered simultaneously in setting up a set of TE LSPs that traverse the layer boundary.
The need for global concurrent path computation may arise in existing networks. When an existing TE LSP network experiences sub-optimal use of its resources, the need for reoptimization or reconfiguration may arise. The scope of reoptimization and reconfiguration may vary depending on particular situations. The scope of reoptimization may be limited to bandwidth modification to an existing TE LSP. However, it could well be that a set of TE LSPs may need to be reoptimized concurrently. In an extreme case, the TE LSPs may need to be globally reoptimized.
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In loaded networks, with large size TE LSPs, a sequential reoptimization may not produce substantial improvements in terms of overall network optimization. Sequential reoptimization refers to a path computation method that computes the reoptimized path of one TE LSP at a time without giving any consideration to the other TE LSPs that need to be reoptimized in the network. The potential for network-wide gains from reoptimization of TE LSPs sequentially is dependent upon the network usage and size of the TE LSPs being optimized. However, the key point remains: computing the reoptimized path of one TE LSP at a time without giving any consideration to the other TE LSPs in the network could result in sub-optimal use of network resources. This may be far more visible in an optical network with a low ratio of potential TE LSPs per link, and far less visible in packet networks with micro-flow TE LSPs.
With regards to applicability of GCO in the event of catastrophic failures, there may be a real benefit in computing the paths of the TE LSPs as a set rather than computing new paths from the head-end LSRs in a distributed manner. Distributed jittering is a technique that could prevent race condition (i.e., competing for the same resource from different head-end LSRs) with a distributed computation. GCO provides an alternative way that could also prevent race condition in a centralized manner. However, a centralized system will typically suffer from a slower response time than a distributed system.
3.3.1. Reconfiguration of the Virtual Network Topology (VNT)
Reconfiguration of the VNT [RFC5212] [PCE-MLN] is a typical application scenario where global concurrent path computation may be applicable. Triggers for VNT reconfiguration, such as traffic demand changes, network failures, and topological configuration changes may require a set of existing TE LSPs to be re-computed.
When migrating from one set of TE LSPs to a reoptimized set of TE LSPs, it is important that the traffic be moved without causing disruption. Various techniques exist in MPLS and GMPLS, such as make-before-break [RFC3209], to establish the new TE LSPs before tearing down the old TE LSPs. When multiple TE LSP routes are changed according to the computed results, some of the TE LSPs may be disrupted due to the resource constraints. In other words, it may prove to be impossible to perform a direct migration from the old TE LSPs to the new optimal TE LSPs without disrupting traffic because there are insufficient network resources to support both sets of TE LSPs when make-before-break is used. However, a PCE may be able to determine a sequence of make-before-break replacement of individual
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TE LSPs or small sets of TE LSPs so that the full set of TE LSPs can be migrated without any disruption. This scenario assumes that the bandwidth of existing TE LSP is kept during the migration, which is required in optical networks. In packet networks, this assumption can be relaxed as the bandwidth of temporary TE LSPs during migration can be zeroed.
It may be the case that the reoptimization is radical. This could mean that it is not possible to apply make-before-break in any order to migrate from the old TE LSPs to the new TE LSPs. In this case, a migration strategy is required that may necessitate TE LSPs being rerouted using make-before-break onto temporary paths in order to make space for the full reoptimization. A PCE might indicate the order in which reoptimized TE LSPs must be established and take over from the old TE LSPs, and it may indicate a series of different temporary paths that must be used. Alternatively, the PCE might perform the global reoptimization as a series of sub-reoptimizations by reoptimizing subsets of the total set of TE LSPs.
The benefit of this multi-step rerouting includes minimization of traffic disruption and optimization gain. However, this approach may imply some transient packets desequencing, jitter, as well as control plane stress.
Note also that during reoptimization, traffic disruption may be allowed for some TE LSPs carrying low priority services (e.g., Internet traffic) and not allowed for some TE LSPs carrying mission critical services (e.g., voice traffic).
This section provides the PCECP requirements to support global concurrent path computation applications. The requirements specified here should be regarded as application-specific requirements and are justifiable based on the extensibility clause found in Section 6.1.14 of [RFC4657]:
The PCECP MUST support the requirements specified in the application-specific requirements documents. The PCECP MUST also allow extensions as more PCE applications will be introduced in the future.
It is also to be noted that some of the requirements discussed in this section have already been discussed in the PCECP requirement document [RFC4657]. For example, Section 5.1.16 in [RFC4657] provides a list of generic constraints while Section 5.1.17 in [RFC4657] provides a list of generic objective functions that MUST be supported by the PCECP. While using such generic requirements as the
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baseline, this section provides application-specific requirements in the context of global concurrent path computation and in a more detailed level than the generic requirements.
The PCEP SHOULD support the following capabilities either via creation of new objects and/or modification of existing objects where applicable.
o An indicator to convey that the request is for a global concurrent path computation. This indicator is necessary to ensure consistency in applying global objectives and global constraints in all path computations. Note: This requirement is covered by "synchronized path computation" in [RFC4655] and [RFC4657]. However, an explicit indicator to request a global concurrent optimization is a new requirement.
o A Global Objective Function (GOF) field in which to specify the global objective function. The global objective function is the overarching objective function to which all individual path computation requests are subjected in order to find a globally optimal solution. Note that this requirement is covered by "synchronized objective functions" in Section 5.1.7 [RFC4657] and that [RFC5541] defined three global objective functions as follows. A list of available global objective functions SHOULD include the following objective functions at the minimum and SHOULD be expandable for future addition:
* Minimize aggregate Bandwidth Consumption (MBC)
* Minimize the load of the Most Loaded Link (MLL)
* Minimize Cumulative Cost of a set of paths (MCC)
o A Global Constraints (GC) field in which to specify the list of global constraints to which all the requested path computations should be subjected. This list SHOULD include the following constraints at the minimum and SHOULD be expandable for future addition:
* Maximum link utilization value -- This value indicates the highest possible link utilization percentage set for each link. (Note: to avoid floating point numbers, the values should be integer values.)
* Minimum link utilization value -- This value indicates the lowest possible link utilization percentage set for each link. (Note: same as above.)
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* Overbooking factor -- The overbooking factor allows the reserved bandwidth to be overbooked on each link beyond its physical capacity limit.
* Maximum number of hops for all the TE LSPs -- This is the largest number of hops that any TE LSP can have. Note that this constraint can also be provided on a per-TE-LSP basis (as requested in [RFC4657] and defined in [RFC5440]).
* Exclusion of links/nodes in all TE LSP path computation (i.e., all TE LSPs should not include the specified links/nodes in their paths). Note that this constraint can also be provided on a per-TE-LSP basis (as requested in [RFC4657] and defined in [RFC5440]).
* An indication should be available in a path computation response that further reoptimization may only become available once existing traffic has been moved to the new TE LSPs.
o A Global Concurrent Vector (GCV) field in which to specify all the individual path computation requests that are subject to concurrent path computation and subject to the global objective function and all of the global constraints. Note that this requirement is entirely fulfilled by the SVEC object in the PCEP specification [RFC5440]. Since the SVEC object as defined in [RFC5440] allows identifying a set of concurrent path requests, the SVEC can be reused to specify all the individual concurrent path requests for a global concurrent optimization.
o An indicator field in which to indicate the outcome of the request. When the PCE cannot find a feasible solution with the initial request, the reason for failure SHOULD be indicated. This requirement is partially covered by [RFC4657], but not in this level of detail. The following indicators SHOULD be supported at the minimum:
* no feasible solution found. Note that this is already covered in [RFC5440].
* memory overflow.
* PCE too busy. Note that this is already covered in [RFC5440].
* PCE not capable of concurrent reoptimization.
* no migration path available.
* administrative privileges do not allow global reoptimization.
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o In order to minimize disruption associated with bulk path provisioning, the following requirements MUST be supported:
* The request message MUST allow requesting the PCE to provide the order in which TE LSPs should be reoptimized (i.e., the migration path) in order to minimize traffic disruption during the migration. That is, the request message MUST allow indicating to the PCE that the set of paths that will be provided in the response message (PCRep) has to be ordered.
* In response to the "ordering" request from the PCC, the PCE MUST be able to indicate in the response message (PCRep) the order in which TE LSPs should be reoptimized so as to minimize traffic disruption. It should indicate for each request the order in which the old TE LSP should be removed and the order in which the new TE LSP should be setup. If the removal order is lower than the setup order, this means that make-before- break cannot be done for this request. It MAY also be desirable to have the PCE indicate whether ordering is in fact required or not.
* During a migration, it may not be possible to do a make-before- break for all existing TE LSPs. The request message MUST allow indicating for each request whether make-before-break is required (e.g., voice traffic) or break-before-make is acceptable (e.g., Internet traffic). The response message must allow indicating TE LSPs for which make-before-break reoptimization is not possible (this will be deduced from the TE LSP setup and deletion orders).
5. Protocol Extensions for Support of Global Concurrent Optimization
This section provides protocol extensions for support of global concurrent optimization. Protocol extensions discussed in this section are built on [RFC5440].
The format of a PCReq message after incorporating new requirements for support of global concurrent optimization is as follows. The message format uses Reduced Backus-Naur Format as defined in [RFC5511]. Please see Appendix A for a full set of RBNF fragments defined in this document and the necessary code license.
Note that three optional objects are added, following the SVEC object: the OF (Objective Function) object, which is defined in [RFC5541], the GC (Global Constraints) object, which is defined in this document (Section 5.5), as well as the eXclude Route Object (XRO), which is defined in [RFC5521]. The placement of the OF object (in which the global objective function is specified) in the SVEC- list is defined in [RFC5541]. Details of this change will be discussed in the following sections.
Note also that when the XRO is global to an SVEC, and F-bit is set, it SHOULD be allowed to specify multiple Record Route Objects in the PCReq message.
5.1. Global Objective Function (GOF) Specification
The global objective function can be specified in the PCEP Objective Function (OF) object, defined in [RFC5541]. The OF object includes a 16-bit Objective Function identifier. As discussed in [RFC5541], Objective Function identifier code points are managed by IANA.
Three global objective functions defined in [RFC5541] are used in the context of GCO.
Function Code Description
4 Minimize aggregate Bandwidth Consumption (MBC)
5 Minimize the load of the Most Loaded Link (MLL)*
6 Minimize the Cumulative Cost of a set of paths (MCC)
* Note: This can be achieved by the following objective function: minimize max over all links {A(i)/C(i)} where C(i) is the link capacity for link i, and A(i) is the total bandwidth allocated on link i.
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5.2. Indication of Global Concurrent Optimization Requests
All the path requests in this application should be indicated so that the global objective function and all of the global constraints are applied to each of the requested path computation. This can be indicated implicitly by placing the GCO related objects (OF, GC, or XRO) after the SVEC object. That is, if any of these objects follows the SVEC object in the PCReq message, all of the requested path computations specified in the SVEC object are subject to OF, GC, or XRO.
In order to minimize disruption associated with bulk path provisioning, the PCC may indicate to the PCE that the response MUST be ordered. That is, the PCE has to include the order in which TE LSPs MUST be moved so as to minimize traffic disruption. To support such indication a new flag, the D flag, is defined in the RP object as follows:
D-bit (orDer - 1 bit): when set, in a PCReq message, the requesting PCC requires the PCE to specify in the PCRep message the order in which this particular path request is to be provisioned relative to other requests.
To support the determination of whether make-before-break optimization is required, a new flag, the M flag, is defined in the RP object as follows.
M-bit (Make-before-break - 1 bit): when set, this indicates that a make-before-break reoptimization is required for this request.
When the M-bit is not set, this implies that a break-before-make reoptimization is allowed for this request. Note that the M-bit can be set only if the R (Reoptimization) flag is set.
Two new bit flags are defined to be carried in the Flags field in the RP object.
Bit 21 (M-bit): When set, make-before-break is required. Bit 22 (D-bit): When set, report of the request order is required.
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The PCE MUST specify the order number in response to the Order Request made by the PCC in the PCReq message if so requested by the setting of the D-bit in the RP object in the PCReq message. To support such an ordering indication, a new optional TLV, the Order TLV, is defined in the RP object.
The Order TLV is an optional TLV in the RP object, that indicates the order in which the old TE LSP must be removed and the new TE LSP must be setup during a reoptimization. It is carried in the PCRep message in response to a reoptimization request.
The Order TLV MUST be included in the RP object in the PCRep message if the D-bit is set in the RP object in the PCReq message.
Figure 2: The Order TLV in the RP Object in the PCRep Message
Type: 5 Length: Variable
Delete Order: 32-bit integer that indicates the order in which the old TE LSP should be removed.
Setup Order: 32-bit integer that indicates the order in which the new TE LSP should be setup.
The delete order SHOULD NOT be equal to the setup order. If the delete order is higher than the setup order, this means that the reoptimization can be done in a make-before-break manner, else it cannot be done in a make-before-break manner.
For a new TE LSP, the delete order is not applicable. The value 0 is designated to specify this case. When the value of the delete order is 0, it implies that the resulting TE LSP is a new TE LSP.
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To illustrate this, consider a network with two established TE LSPs: R1 with path P1, and R2 with path P2. During a reoptimization, the PCE may provide the following ordered reply:
R1, path P1', remove order 1, setup order 4 R2, path P2', remove order 3, setup order 2
This indicates that the NMS should do the following sequence of tasks:
1: Remove path P1 2: Set up path P2' 3: Remove path P2 4: Set up path P1'
That is, R1 is reoptimized in a break-before-make manner and R2 in a make-before-break manner.
The GLOBAL CONSTRAINTS (GC) Object is used in a PCReq message to specify the necessary global constraints that should be applied to all individual path computations for a global concurrent path optimization request.
GLOBAL-CONSTRAINTS Object-Class is 24.
Global Constraints Object-Type is 1.
The format of the GC object body that includes the global constraints is as follows:
MH (Max Hop: 8 bits): 8-bit integer that indicates the maximum hop count for all the TE LSPs.
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MU (Max Utilization Percentage: 8 bits) : 8-bit integer that indicates the upper-bound utilization percentage by which all links should be bound. Utilization = (Link Capacity - Allocated Bandwidth on the Link)/ Link Capacity. MU is intended to be an integer that can only be between 0 and 100.
mU (minimum Utilization Percentage: 8 bits) : 8-bit integer that indicates the lower-bound utilization percentage by which all links should be bound. mU is intended to be an integer that can only be between 0 and 100.
OB (Over Booking factor Percentage: 8 bits) : 8-bit integer that indicates the overbooking percentage that allows the reserved bandwidth to be overbooked on each link beyond its physical capacity limit. The value, for example, 10% means that 110 Mbps can be reserved on a 100 Mbps link.
The exclusion of the list of nodes/links from a global path computation can be done by including the XRO object following the GC object in the new SVEC-list definition.
Optional TLVs may be included within the GC object body to specify additional global constraints. The TLV format and processing is consistent with Section 7.1 of RFC 5440. Any TLVs will be allocated from the "PCEP TLV Type Indicators" registry. Note that no TLVs are defined in this document.
To indicate errors associated with the global concurrent path optimization request, a new Error-Type (14) and subsequent error- values are defined as follows for inclusion in the PCEP-ERROR Object:
A new Error-Type (15) and subsequent error-values are defined as follows:
Error-Type=15; Error-value=1: if a PCE receives a global concurrent path optimization request and the PCE is not capable of processing the request due to insufficient memory, the PCE MUST send a PCErr message with a PCEP-ERROR Object (Error-Type=15) and an Error-value (Error-value=1). The PCE stops processing the request. The corresponding global concurrent path optimization request MUST be cancelled at the PCC.
Error-Type=15; Error-value=2: if a PCE receives a global concurrent path optimization request and the PCE is not capable of global concurrent optimization, the PCE MUST send a PCErr message with a PCEP-ERROR Object (Error-Type=15) and an Error-value (Error-value=2).
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The PCE stops processing the request. The corresponding global concurrent path optimization MUST be cancelled at the PCC.
To indicate an error associated with policy violation, a new error value "global concurrent optimization not allowed" should be added to an existing error code for policy violation (Error-Type=5) as defined in [RFC5440].
Error-Type=5; Error-value=5: if a PCE receives a global concurrent path optimization request that is not compliant with administrative privileges (i.e., the PCE policy does not support global concurrent optimization), the PCE sends a PCErr message with a PCEP-ERROR Object (Error-Type=5) and an Error-value (Error-value=5). The PCE stops the processing the request. The corresponding global concurrent path computation MUST be cancelled at the PCC.
To communicate the reason(s) for not being able to find global concurrent path computation, the NO-PATH object can be used in the PCRep message. The format of the NO-PATH object body is defined in [RFC5440]. The object may contain a NO-PATH-VECTOR TLV to provide additional information about why a path computation has failed.
Two new bit flags are defined to be carried in the Flags field in the NO-PATH-VECTOR TLV carried in the NO-PATH Object.
Bit 6: When set, the PCE indicates that no migration path was found.
Bit 7: When set, the PCE indicates no feasible solution was found that meets all the constraints associated with global concurrent path optimization in the PCRep message.
6. Manageability Considerations
Manageability of global concurrent path computation with PCE must address the following considerations:
In addition to the parameters already listed in Section 8.1 of [RFC5440], a PCEP implementation SHOULD allow configuring the following PCEP session parameters on a PCC:
o The ability to send a GCO request.
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In addition to the parameters already listed in Section 8.1 of [RFC5440], a PCEP implementation SHOULD allow configuring the following PCEP session parameters on a PCE:
o The support for Global Concurrent Optimization.
o The maximum number of synchronized path requests per request message.
o A set of GCO specific policies (authorized sender, request rate limiter, etc.).
These parameters may be configured as default parameters for any PCEP session the PCEP speaker participates in, or may apply to a specific session with a given PCEP peer or a specific group of sessions with a specific group of PCEP peers.
6.2. Information and Data Models (e.g., MIB Module)
Extensions to the PCEP MIB module defined in [PCEP-MIB] should be defined, so as to cover the GCO information introduced in this document.
Mechanisms defined in this document do not imply any new liveness detection and monitoring requirements in addition to those already listed in Section 8.3 of [RFC5440].
Mechanisms defined in this document do not imply any new verification requirements in addition to those already listed in Section 8.4 of [RFC5440]
6.5. Requirements on Other Protocols and Functional Components
The PCE Discovery mechanisms ([RFC5088] and [RFC5089]) may be used to advertise global concurrent path computation capabilities to PCCs. A new flag (value=9) in PCE-CAP-FLAGs Sub-TLV has been assigned to be able to indicate GCO capability.
Mechanisms defined in this document do not imply any new network operation requirements in addition to those already listed in Section 8.6 of [RFC5440].
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When global reoptimization is applied to an active network, it could be extremely disruptive. Although the real security and policy issues apply at the NMS, if the wrong results are returned to the NMS, the wrong actions may be taken in the network. Therefore, it is very important that the operator issuing the commands has sufficient authority and is authenticated, and that the computation request is subject to appropriate policy.
The mechanism defined in [RFC5440] to secure a PCEP session can be used to secure global concurrent path computation requests/responses.
As described in Section 5.3, two new bit flags are defined for inclusion in the Flags field of the RP object. IANA has made the following allocations from the "RP Object Flag Field" sub-registry.
Bit Description Reference
21 Make-before-break (M-bit) [RFC5557] 22 Report the request order (D-bit) [RFC5557]
As described in Section 5.4, a new PCEP TLV is defined to indicate the setup and delete order of TE LSPs in a GCO. IANA has made the following allocation from the "PCEP TLV Type Indicators" sub- registry.
As described in Section 6.5, a new PCE-CAP-FLAGS Sub-TLV is defined to indicate a GCO capability. IANA has made the following allocation from the "Path Computation Element (PCE) Capability Flags" sub- registry, which was created by Section 7.2 of RFC 5088. It is an OSPF registry.
As descried in Section 5.5, a new PCEP object is defined to carry global constraints. IANA has made the following allocation from the "PCEP Objects" sub-registry.
Object Name Reference Class
24 GLOBAL-CONSTRAINTS [RFC5557] Object-Type 1: Global Constraints [RFC5557]
As described in Section 5.6, new PCEP error codes are defined for GCO errors. IANA has made allocations from the "PCEP-ERROR Object Error Types and Values" sub-registry as set out in the following sections.
IANA has made the following allocations from the "NO-PATH-VECTOR TLV Flag Field" sub-registry for bit flags carried in the NO-PATH-VECTOR TLV in the PCEP NO-PATH object as described in Section 5.7.
Bit Number Name Reference
25 No GCO solution found [RFC5557] 26 No GCO migration path found [RFC5557]
[RFC5441] Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux, "A Backward-Recursive PCE-Based Computation (BRPC) Procedure to Compute Shortest Constrained Inter-Domain Traffic Engineering Label Switched Paths", RFC 5441, April 2009.
[RFC5541] Le Roux, JL., Vasseur, JP., and Y. Lee, "Encoding of Objective Functions in Path Computation Element Communication Protocol (PCEP)", RFC 5541, May 2009.
[RFC5521] Oki, E., Takeda, T., and A. Farrel, "Extensions to the Path Computation Element Communication Protocol (PCEP) for Route Exclusions", RFC 5521, April 2009.
[RFC5440] Vasseur, JP., Ed., and JL. Le Roux, Ed., "Path Computation Element (PCE) Communication Protocol (PCEP)", RFC 5440, March 2009.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
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[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.
[RFC5088] Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and R. Zhang, "OSPF Protocol Extensions for Path Computation Element (PCE) Discovery", RFC 5088, January 2008.
[RFC5089] Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and R. Zhang, "IS-IS Protocol Extensions for Path Computation Element (PCE) Discovery", RFC 5089, January 2008.
[PCE-MLN] Oki, E., Takeda, T., Le Roux, JL., and A. Farrel, "Framework for PCE-Based Inter-Layer MPLS and GMPLS Traffic Engineering", Work in Progress, March 2009.
[PCEP-MIB] Koushik, K. and E. Stephan, "PCE communication protocol (PCEP) Management Information Base", Work in Progress, November 2008.
[RFC5511] Farrel, A., "Routing Backus-Naur Form (RBNF): A Syntax Used to Form Encoding Rules in Various Routing Protocol Specifications", RFC 5511, April 2009.
[RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path Computation Element (PCE)-Based Architecture", RFC 4655, August 2006.
[RFC4657] Ash, J., Ed., and J. Le Roux, Ed., "Path Computation Element (PCE) Communication Protocol Generic Requirements", RFC 4657, September 2006.
[RFC4674] Le Roux, J., Ed., "Requirements for Path Computation Element (PCE) Discovery", RFC 4674, October 2006.
[RFC5212] Shiomoto, K., Papadimitriou, D., Le Roux, JL., Vigoureux, M., and D. Brungard, "Requirements for GMPLS-Based Multi- Region and Multi-Layer Networks (MRN/MLN)", RFC 5212, July 2008.
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