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IETF RFC 5557
Path Computation Element Communication Protocol (PCEP) Requirements and Protocol Extensions in Support of Global Concurrent Optimization
Last modified on Friday, July 17th, 2009
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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.
Lee, et al. Standards Track PAGE 1 
RFC 5557 PCEP Requirements & Protocol Extensions for GCO July 2009
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
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Please review these documents carefully, as they describe your rights
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This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Lee, et al. Standards Track PAGE 2
RFC 5557 PCEP Requirements & Protocol Extensions for GCO July 2009
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
Lee, et al. Standards Track PAGE 3
RFC 5557 PCEP Requirements & Protocol Extensions for GCO July 2009
1. Introduction
[RFC 4655] 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. [RFC 4657] 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 [RFC 4655] 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
Lee, et al. Standards Track PAGE 4
RFC 5557 PCEP Requirements & Protocol Extensions for GCO July 2009
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 [RFC 4674], [RFC 5088], [RFC 5089]; but, that is
out of the scope of this document.
It is to be noted that Backward Recursive Path Computation (BRPC)
[RFC 5441] 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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2. Terminology
Most of the terminology used in this document is explained in
[RFC 4655]. 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 [RFC 2119].
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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3.1. Application of the PCE Architecture
Figure 1 shows the PCE-based network architecture as defined in
[RFC 4655] to which GCO application is applied. It must be observed
that the PCC is not necessarily an LSR [RFC 4655]. 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.
-----------
Application | ----- |
Request | | TED | |
| | ----- |
v | | |
------------- Request/ | v |
| PCC | Response| ----- |
| (NMS/Server)|<--------+> | PCE | |
| | | ----- |
------------- -----------
Service |
Request |
v
---------- Signaling ----------
| Head-End | Protocol | Adjacent |
| Node |<---------->| Node |
---------- ----------
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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3.2. Greenfield Optimization
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.
3.2.1. Single-Layer Traffic Engineering
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.
3.2.2. Multi-Layer Traffic Engineering
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.
3.3. Reoptimization of Existing Networks
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.
Lee, et al. Standards Track PAGE 8
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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 [RFC 5212] [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.
3.3.2. Traffic Migration
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 [RFC 3209], 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).
4. PCECP Requirements
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 [RFC 4657]:
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 [RFC 4657]. For example, Section 5.1.16 in [RFC 4657]
provides a list of generic constraints while Section 5.1.17 in
[RFC 4657] provides a list of generic objective functions that MUST be
supported by the PCECP. While using such generic requirements as the
Lee, et al. Standards Track PAGE 10
RFC 5557 PCEP Requirements & Protocol Extensions for GCO July 2009
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 [RFC 4655] and [RFC 4657].
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 [RFC 4657] and
that [RFC 5541] 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 [RFC 4657] and defined in [RFC 5440]).
* 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 [RFC 4657] and defined in
[RFC 5440]).
* 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 [RFC 5440]. Since the SVEC object as defined in
[RFC 5440] 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 [RFC 4657], 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 [RFC 5440].
* memory overflow.
* PCE too busy. Note that this is already covered in [RFC 5440].
* PCE not capable of concurrent reoptimization.
* no migration path available.
* administrative privileges do not allow global reoptimization.
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RFC 5557 PCEP Requirements & Protocol Extensions for GCO July 2009
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 [RFC 5440].
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
[RFC 5511]. Please see Appendix A for a full set of RBNF fragments
defined in this document and the necessary code license.
<PCReq Message> ::= <Common Header>
[<svec-list>]
<request-list>
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The <svec-list> is changed as follows:
<svec-list> ::= <SVEC>
[<OF>]
[<GC>]
[<XRO>]
[<svec-list>]
Note that three optional objects are added, following the SVEC
object: the OF (Objective Function) object, which is defined in
[RFC 5541], 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 [RFC 5521]. The placement of the OF object
(in which the global objective function is specified) in the SVEC-
list is defined in [RFC 5541]. 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 [RFC 5541]. The OF object includes a
16-bit Objective Function identifier. As discussed in [RFC 5541],
Objective Function identifier code points are managed by IANA.
Three global objective functions defined in [RFC 5541] 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.
5.3. Request for the Order of TE LSP
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.
Lee, et al. Standards Track PAGE 15
RFC 5557 PCEP Requirements & Protocol Extensions for GCO July 2009
5.4. The Order Response
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.
The format of the Order TLV is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Delete Order |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Setup Order |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
Lee, et al. Standards Track PAGE 16
RFC 5557 PCEP Requirements & Protocol Extensions for GCO July 2009
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.
5.5. GLOBAL CONSTRAINTS (GC) Object
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:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MH | MU | mU | OB |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Optional TLV(s) //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: GC Body Object Format
MH (Max Hop: 8 bits): 8-bit integer that indicates the maximum hop
count for all the TE LSPs.
Lee, et al. Standards Track PAGE 17
RFC 5557 PCEP Requirements & Protocol Extensions for GCO July 2009
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.
5.6. Error Indicator
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).
Lee, et al. Standards Track PAGE 18
RFC 5557 PCEP Requirements & Protocol Extensions for GCO July 2009
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 [RFC 5440].
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.
5.7. NO-PATH Indicator
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
[RFC 5440]. 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:
6.1. Control of Function and Policy
In addition to the parameters already listed in Section 8.1 of
[RFC 5440], a PCEP implementation SHOULD allow configuring the
following PCEP session parameters on a PCC:
o The ability to send a GCO request.
Lee, et al. Standards Track PAGE 19
RFC 5557 PCEP Requirements & Protocol Extensions for GCO July 2009
In addition to the parameters already listed in Section 8.1 of
[RFC 5440], 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.
6.3. Liveness Detection and Monitoring
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 [RFC 5440].
6.4. Verifying Correct Operation
Mechanisms defined in this document do not imply any new verification
requirements in addition to those already listed in Section 8.4 of
[RFC 5440]
6.5. Requirements on Other Protocols and Functional Components
The PCE Discovery mechanisms ([RFC 5088] and [RFC 5089]) 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.
6.6. Impact on Network Operation
Mechanisms defined in this document do not imply any new network
operation requirements in addition to those already listed in Section
8.6 of [RFC 5440].
Lee, et al. Standards Track PAGE 20
RFC 5557 PCEP Requirements & Protocol Extensions for GCO July 2009
7. Security Considerations
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 [RFC 5440] to secure a PCEP session can be
used to secure global concurrent path computation requests/responses.
8. IANA Considerations
IANA maintains a registry of PCEP parameters. IANA has made
allocations from the sub-registries as described in the following
sections.
8.1. Request Parameter Bit Flags
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) [RFC 5557]
22 Report the request order (D-bit) [RFC 5557]
8.2. New PCEP TLV
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.
TLV Type Meaning Reference
5 Order TLV [RFC 5557]
Lee, et al. Standards Track PAGE 21
RFC 5557 PCEP Requirements & Protocol Extensions for GCO July 2009
8.3. New Flag in PCE-CAP-FLAGS Sub-TLV in PCED
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.
FLAG Meaning Reference
9 Global Concurrent Optimization (GCO) [RFC 5557]
8.4. New PCEP Object
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 [RFC 5557]
Object-Type
1: Global Constraints [RFC 5557]
8.5. New PCEP Error Codes
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.
8.5.1. New Error-Values for Existing Error-Types
Error-
Type Meaning Reference
5 Policy violation
Error-value=5: [RFC 5557]
Global concurrent optimization not allowed
Lee, et al. Standards Track PAGE 22
RFC 5557 PCEP Requirements & Protocol Extensions for GCO July 2009
8.5.2. New Error-Types and Error-Values
Error-
Type Meaning Reference
15 Global Concurrent Optimization Error [RFC 5557]
Error-value=1:
Insufficient memory [RFC 5557]
Error-value=2:
Global concurrent optimization not supported
[RFC 5557]
8.6. New No-Path Reasons
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 [RFC 5557]
26 No GCO migration path found [RFC 5557]
9. References
9.1. Normative References
[RFC 5441] 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.
[RFC 5541] Le Roux, JL., Vasseur, JP., and Y. Lee, "Encoding of
Objective Functions in Path Computation Element
Communication Protocol (PCEP)", RFC 5541, May 2009.
[RFC 5521] Oki, E., Takeda, T., and A. Farrel, "Extensions to the
Path Computation Element Communication Protocol (PCEP) for
Route Exclusions", RFC 5521, April 2009.
[RFC 5440] Vasseur, JP., Ed., and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
March 2009.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Lee, et al. Standards Track PAGE 23
RFC 5557 PCEP Requirements & Protocol Extensions for GCO July 2009
[RFC 3209] 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.
[RFC 5088] 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.
[RFC 5089] 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.
9.2. Informative References
[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.
[RFC 5511] Farrel, A., "Routing Backus-Naur Form (RBNF): A Syntax
Used to Form Encoding Rules in Various Routing Protocol
Specifications", RFC 5511, April 2009.
[RFC 4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture", RFC 4655,
August 2006.
[RFC 4657] Ash, J., Ed., and J. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol Generic
Requirements", RFC 4657, September 2006.
[RFC 4674] Le Roux, J., Ed., "Requirements for Path Computation
Element (PCE) Discovery", RFC 4674, October 2006.
[RFC 5212] 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.
10. Acknowledgments
We would like to thank Jerry Ash, Adrian Farrel, J-P Vasseur, Ning
So, Lucy Yong, and Fabien Verhaeghe for their useful comments and
suggestions.
Lee, et al. Standards Track PAGE 24
RFC 5557 PCEP Requirements & Protocol Extensions for GCO July 2009
Appendix A. RBNF Code Fragments
Copyright (c) 2009 IETF Trust and the persons identified as authors
of the code. All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
- Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
- Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the
distribution.
- Neither the name of Internet Society, IETF or IETF Trust, nor the
names of specific contributors, may be used to endorse or promote
products derived from this software without specific prior written
permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
<PCReq Message> ::= <Common Header>
[<svec-list>]
<request-list>
<svec-list> ::= <SVEC>
[<OF>]
[<GC>]
[<XRO>]
[<svec-list>]
Lee, et al. Standards Track PAGE 25
RFC 5557 PCEP Requirements & Protocol Extensions for GCO July 2009
Authors' Addresses
Young Lee
Huawei
1700 Alma Drive, Suite 100
Plano, TX 75075
US
Phone: +1 972 509 5599 x2240
Fax: +1 469 229 5397
EMail: ylee@huawei.com
JL Le Roux
France Telecom
2, Avenue Pierre-Marzin
Lannion 22307
FRANCE
EMail: jeanlouis.leroux@orange-ftgroup.com
Daniel King
Old Dog Consulting
United Kingdom
EMail: daniel@olddog.co.uk
Eiji Oki
University of Electro-Communications
1-5-1 Chofugaoka
Chofu, Tokyo 182-8585
JAPAN
EMail: oki@ice.uec.ac.jp
Lee, et al. Standards Track PAGE 26
Path Computation Element Communication Protocol (PCEP) Requirements and Protocol Extensions in Support of Global Concurrent Optimization
RFC TOTAL SIZE: 58888 bytes
PUBLICATION DATE: Friday, July 17th, 2009
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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