Network Working Group I. Nishioka
Internet Draft NEC
Intended status: Informational Daniel King
Expires: March 2010 Old Dog Consulting
September 23, 2009
The use of SVEC (Synchronization VECtor) list for Synchronized
dependent path computations
draft-ietf-pce-pcep-svec-list-03.txt
Abstract
A Path Computation Element (PCE) performing dependent path
computations, for instance calculating a diverse working and
protected path not sharing common network points, would need to
synchronize the computations in order to increase the probability of
meeting the working and protected path diversity (or disjointness)
objective and network resource optimization objective. When a PCE
computes multiple sets of dependent path computation requests
concurrently, it is required to use Synchronization VECtor (SVEC)
list for association among the sets of dependent path computation
requests. SVEC is also applicable to end-to-end diverse path
computation across multiple domains. This document describes the
usage of SVECs in the SVEC list and diverse path computation
guideline, for the synchronized computation of dependent paths.
Status of this Memo
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Table of Contents
1. Terminology...................................................3
2. Introduction..................................................3
3. SVEC association scenarios....................................5
3.1. Synchronized computation for diverse path requests.......5
3.2. Synchronized computation for point-to-multipoint path
requests......................................................6
4. SVEC association..............................................7
4.1. Associated SVECs.........................................7
4.2. Non-associated SVECs.....................................7
5. Processing of SVEC list.......................................8
5.1. Single PCE, single domain environments...................8
5.2. Multi-PCE, single domain environments....................9
5.3. Single PCE, multi-domain environments....................9
6. End-to-end diverse path computation...........................9
6.1. Disjoint VSPT...........................................10
6.2. Disjoint VSPT encoding..................................11
6.3. Path computation procedure..............................12
7. Manageability considerations.................................12
7.1. Control of Function and Policy..........................12
7.2. Information and Data Models, e.g. MIB modules...........12
7.3. Liveness Detection and Monitoring.......................12
7.4. Verifying Correct Operation.............................13
7.5. Requirements on Other Protocols and Functional Components13
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7.6. Impact on Network Operation.............................13
8. Security Considerations......................................13
9. IANA Considerations..........................................13
10. References..................................................13
10.1. Normative References...................................13
10.2. Informative References.................................14
11 Acknowledgements .............................................14
1. Terminology
This document uses PCE terminology defined in [RFC4655],[RFC4875],
and [RFC5440].
GCO (Global Concurrent Optimization): A concurrent path computation
application, defined in [RFC5557], where a set of TE paths is
computed concurrently in order to efficiently utilize network
resources.
Associated SVECs: A group of multiple SVECs (Synchronization
VECtors), defined in this document, to indicate a set of
synchronized or concurrent path computations.
VSPT: Virtual Shortest Path Tree defined in [RFC5441].
Disjoint VSPT : A set of VSPTs, defined in this document, to
indicate a set of virtual diverse path tree.
2. Introduction
[RFC5440] describes the specifications for PCEP (Path Computation
Element communication Protocol). PCEP specifies the communication
between a Path Computation Client (PCC) and a Path Computation
Element (PCE), or between two PCEs based on the PCE architecture
[RFC4655]. PCEP interactions include path computation requests and
path computation replies.
[RFC5557] specifies the Global Concurrent Optimization (GCO) path
computation mechanism. The GCO application provides the capability
to re-optimize a set of services within the network, in order to
maximize efficient use of network resources. A single or set of
objective functions (OFs) can be applied to a GCO. To compute a set
of such traffic-engineered paths for the GCO application, PCEP
supports the synchronous and dependent path computation requests
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required in [RFC4657]. When a PCC or PCE sends such path computation
requests to a PCE, Synchronization VECtor (SVEC) allows the PCC or
PCE to specify a list of multiple path computation requests that
must be synchronized along with a potential dependency. [RFC5440]
defines two synchronous path computation modes using SVEC.
o Bundle a set of independent and synchronized path computation
requests,
o Bundle a set of dependent and synchronized path computation
requests.
These are exclusive modes in a single SVEC. If one of the dependency
flags (i.e. Node, Link or Shared Risk Link Groups (SRLG) diverse
flags) in a SVEC is set, the SVEC indicates a set of synchronous
path computation requests with a dependency. In order to be
synchronized among multiple sets of path computation requests with a
dependency, it is necessary to use other SVECs.
It is important for the PCE, when performing path computations, to
synchronize any path computation requests with a dependency. For
example, consider a protected end-to-end service. Two diverse path
computation requests are needed to compute the disjointed working
and protected paths. If the diverse path requests are computed
sequentially, fulfillment of the initial diverse path computation
without consideration of the second diverse path computation and
disjoint constraint may result in the PCE providing sub-optimal
results for the second one, or may fail to meet the disjoint
requirement altogether.
Additionally, SVEC can be applied to end-to-end diverse path
computations that traverse multiple domains. [RFC5441] describes two
approaches, synchronous (i.e. simultaneous) and 2-step approaches,
for the end-to-end diverse path computation across a chain of
domains. The path computation procedure is specified for the 2-step
approaches in [RFC5521], but no guidelines are provided for a
synchronous approach.
This document defines the handling of synchronous path computation
for PCE and multiple set of path computation request with a
dependency, based on the PCE architecture [RFC4655]. The following
scenarios are specifically described:
o Single domain, single PCE, dependent and synchronized path
computation request.
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o Single domain, multi-PCE, dependent and synchronized path
computation request.
o Multi-domain, dependent and synchronized path computation request,
including end-to-end diverse path computation.
The association among multiple SVECs for multiple sets of
synchronized dependent path computation is also described in this
document, as well as disjoint Virtual Shortest Path Tree (VSPT)
encoding rule for end-to-end diverse path computation across domains.
Path computation algorithms for these path computation scenarios are
out of the scope of this document.
The SVEC association and the disjoint VSPT described in this
document do not require any extension to PCEP message and object
formats, when computing a GCO for multiple or end-to-end diverse
paths. In addition, the use of multiple SVECs is not restricted to
only SRLG, Node and Link diversity currently defined in the SVEC
object [RFC5440], but is also available for other dependent path
computation requests.
The SVEC association and disjoint VSPT are available to both single
PCE path computation and multi-PCE path computation.
3. SVEC association scenarios
This section clarifies several path computation scenarios, in which
SVEC association can be applied. Also, any combination of scenarios
described in this section could be applicable.
3.1. Synchronized computation for diverse path requests
A PCE may compute two or more point-to-point diverse paths,
concurrently, in order to increase the probability of meeting
primary and secondary path diversity (or disjointness) objective and
network resource optimization objective.
Two scenarios can be considered for the SVEC association of point-
to-point diverse paths.
o Two or more end-to-end diverse paths
When concurrent path computation of two or more end-to-end diverse
paths is requested, SVEC association is needed among diverse path
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requests. Note here that each diverse path request consists of
primary, secondary, and tertiary and beyond path requests, in which
all path requests are grouped with one SVEC association.
Example of this scenario: When there are two associated end-to-end
diverse path requests with primary and secondary, all requests must
be computed in a synchronized manner.
o End-to-end primary path and its segmented secondary paths
When concurrent path computation of an end-to-end primary path and
several segmented secondary paths is requested, SVEC association is
needed among primary/segmented secondary-1 request,
primary/segmented secondary-2 request, and etc.
In this scenario, we assume that the primary path may be pre-
computed, which is used for specifying the segment for secondary
paths. Otherwise, segment for secondary path requests are specified
in advance, by using Exclude Route Object (XRO) and/or Include Route
Object (IRO) constraints in the primary request.
3.2. Synchronized computation for point-to-multipoint path requests
For point-to-multipoint path requests, SVEC association can be
applied.
o Two or more point-to-multipoint paths
If a point-to-multipoint paths request is represented as a set of
point-to-point paths [ID.pce-p2mp-ext], two or more point-to-
multipoint path computation requests can be associated for
concurrent path computation, in order to optimize network resources.
o Point-to-multipoint paths and their secondary paths
When concurrent path computation of a point-to-multipoint path and
its point-to-point secondary paths [RFC4875], or a point-to-
multipoint path and its point-to-multipoint secondary paths is
requested, SVEC association is needed among these requests. In this
scenario, we use the same assumption as "end-to-end primary path and
its segmented secondary paths scenario" in section 3.1
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4. SVEC association
This section describes the associations among SVECs in a SVEC list.
4.1. Associated SVECs
"Associated SVECs" means that there are relationships among multiple
SVECs. Request-IDs in the SVEC objects are used to indicate the
association among SVEC objects. If the same request-IDs exist in
more than two SVECs, this indicates associated SVECs. When
associating among SVECs, only one request-ID in the SVEC object may
be contained in the other SVEC object. This contributes to reducing
the message size of PCEP request. Even in this case, all the path
computation requests are synchronized.
Below is an example of associated SVECs. In this example, the first
SVEC is associated with the other SVECs, and path computation
requests from Request-ID#1 to Request-ID#Z must be synchronized.
<SVEC-list>
<SVEC> without dependency flags
Request-ID #1, Request-ID #3, ..., Request-ID #X
<SVEC> with one or more dependency flags
Request-ID #1, Request-ID #2
<SVEC> with one or more dependency flags
Request-ID #3, Request-ID #4
........
<SVEC> without dependency flag
Request-ID #X, Request-ID #Y, Request-ID #Z
4.2. Non-associated SVECs
Non-associated SVECs mean that there are no relationships among
SVECs. If SVEC objects in PECP request messages do not have the same
request-ID, the relationship among these SVECs is not associated.
Below is an example of non-associated SVECs that does not contain
any same request-IDs.
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<SVEC-list>
<SVEC> with one or more dependency flags
Request-ID #1, Request-ID #2
<SVEC> with one or more dependency flags
Request-ID #3, Request-ID #4
........
<SVEC> without dependency flags
Request-ID #X, Request-ID #Y, Request-ID #Z
5. Processing of SVEC list
5.1. Single PCE, single domain environments
When a PCE receives PCReq messages with more than two SVEC objects
in the SVEC list, PCEP has to first check the request-IDs in all
SVEC objects in order to identify any associations among them. The
SVEC objects may be received in a single or multiple PCReq
message(s). In the latter case, the PCE may start a SyncTimer as
recommended in [RFC5440]. After receiving the entire set of path
computation requests, the analysis for associated SVECs has to be
started.
If there are no matching request-IDs in the different SVEC objects,
these SVEC objects are not associated, and then each set of path
computation requests in the non-associated SVEC objects has to be
computed separately.
If there are matching request-IDs in the different SVEC objects,
these SVEC objects are associated, and then all path computation
requests in the associated SVEC objects are treated in a synchronous
manner for GCO application.
If the PCE does not have capability to handle the associated SVEC
objects, it may send a PCErr message with Error-Type="Capability not
supported".
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5.2. Multi-PCE, single domain environments
Currently no mechanisms exist to manage co-ordination of dependent
SVEC requests between multiple PCEs in the same domain. If a PCC
sends a path computation request to a PCE and then sends a second
service path computation request, which is required to be disjoint
from the first service, and this request is sent to a different PCE
in the domain, no SVEC object correlation function between the PCEs
is currently available. Equally, associated SVECs are not sent to
the different PCEs in the domain.
5.3. Multi-PCE, multi-domain environments
When multiple PCEs located in separate domains are used to
concurrently compute an end-to-end diverse path across multiple
domains, additional processing may be required. The path computation
process for the end-to-end diverse path is described in Section 6.
Furthermore, if the PCReq message contains multiple associated SVEC
objects and these SVEC objects contain path computation requests
that will be sent to the next PCE along the path computation chain,
the following procedure is applied. Intermediate PCEs receiving such
PCReq messages may re-construct associations among SVEC objects, and
then send PCReq messages to corresponding PCEs located in
neighboring domains. If the associated SVECs are re-constructed at
the intermediate PCE, the PCE must not start path computation until
all PCRep messages have been received from all neighbor PCEs. In
addition, it is not recommended that SVEC objects coming from
different PCReq messages are re-constructed. This may contribute to
resource optimization from network operator's point of view, but it
is unrealistic in the case of multiple PCE path computation
scenarios.
6. End-to-end diverse path computation
End-to-end diverse path is a set of primary path and secondary paths,
which do not share common network resources across domains. To
compute the end-to-end diverse path, the BRPC procedure can be used.
[RFC5441] describes two approaches, synchronous (i.e. simultaneous)
and 2-step approaches, for the end-to-end diverse path computation
across a chain of domains. The 2-step approach computes primary and
secondary paths sequentially, using XRO, and its procedure is
described in [RFC5521]. In this section, the synchronous approach is
provided to compute primary and secondary paths simultaneously.
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6.1. Disjoint VSPT
The BRPC procedure constructs a VSPT to inform the enquiring PCE of
potential paths to the destination node.
In the end-to-end diverse path computation, diversity (or
disjointness) information among the potential paths must be
preserved in the VSPT to ensure end-to-end disjoint path. In order
to preserve diversity (or disjointness) information, disjoint VSPTs
are sent in the PCEP PCRep message.
A definition of the disjoint VSPT is a collection of VSPTs, in which
each VSPT contains a potential set of primary and secondary paths.
Figure-1 shows an example network. Here, transit nodes in domains
are not depicted, and PCE1 and PCE2 may be located in border nodes.
In this network, there are three VSPTs for the potential set of
diverse paths shown in Figure 2, when the primary path and secondary
path are requested from S1 to D1. These VSPTs consist of a disjoint
VSPT, which is replied to PCE1. When receiving the disjoint VSPT,
PCE1 recognizes the disjoint request and disjoint VSPT information.
PCE1 will then continue to process the request and compute the
diverse path using the BRPC procedure [RFC5441]. The detail encoding
for the disjoint VSPT is described in Section 6.2.
Domain1 Domain2
+----------+ +----------+
| PCE1 | | PCE2 | S1: Source node
| BN1---BN4 | D1: Destination node
| S1 BN2---BN5 D1 | BN1-BN6: Border nodes
| BN3---BN6 |
+----------+ +----------+
Figure-1: Example network for diverse path computation
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VSPT1: VSPT2: VSPT3:
D1 D1 D1
/ \ / \ / \
BN4 BN5 BN4 BN6 BN5 BN6
Figure-2: Disjoint VSPT from PCE2 to PCE1
6.2. Disjoint VSPT encoding
Encoding for disjoint VSPT follows the definition of PCEP message
encoding in [RFC5440].
PCEP PCRep message returns a disjoint VSPT as <path list> for each
RP object (Request Parameter object). The order of <path> in <path
list> among <responses> implies a set of primary EROs (Explicit
Route Objects) and secondary EROs.
A PCE sending PCRep with a disjoint VSPT can reply with a partial
disjoint VSPT based on its network operation policy, but the order
of <path> in <path list> must be aligned correctly.
If confidentiality is required between domains, path key mechanism
defined in [RFC5520] is used for a disjoint VSPT.
Detailed disjoint VSPT encoding in Figure-2 is shown below, when a
primary path and a secondary path are requested from S1 to D1.
o Request ID #1 (Primary)
- ERO1 BN4(TE route ID)- ...-D1(TE-Router ID) [for VSPT1]
- ERO2 BN4(TE route ID)- ...-D1(TE-Router ID) [for VSPT2]
- ERO3 BN5(TE route ID)- ...-D1(TE-Router ID) [for VSPT3]
O Request ID #2 (Secondary)
- ERO4 BN5(TE route ID)- ...-D1(TE-Router ID) [for VSPT1]
- ERO5 BN6(TE route ID)- ...-D1(TE-Router ID) [for VSPT2]
- ERO6 BN6(TE route ID)- ...-D1(TE-Router ID) [for VSPT3]
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6.3. Path computation procedure
For end-to-end diverse path computation, the same mode of operation
as BRPC procedure can be applied (i.e. Step 1 to Step n in Section
4.2 [RFC5441]). During this procedure, a question is how to
recognize disjoint VSPTs.
The recognition of disjoint VSPT is achieved by the PCE sending
PCReq to its neighbor PCE which maintains the path computation
request (PCReq) information. If PCReq has one or more SVEC object(s)
with the appropriate diverse flags, the received PCRep will contain
the disjoint VSPT. If not, the received VSPT is a normal VSPT based
on the shortest path computation.
Note that the PCE will apply a suitable algorithm for computing
disjoint VSPT. The selection and application of the appropriate
algorithm is out of scope in this draft.
7. Manageability considerations
This section describes manageability considerations specified in
[ID.pce-mngabl-reqs].
7.1. Control of Function and Policy
In addition to [RFC5440], PCEP implementation should allow the
configuration of association among SVECs on PCCs.
7.2. Information and Data Models, e.g. MIB modules
There are no additional parameters for MIB modules.
7.3. Liveness Detection and Monitoring
The associated SVEC in this document allows PCEs to compute optimal
sets of diverse paths. This type of path computation may require
more time to obtain its results. Therefore, it is recommended for
PCEP to support PCE monitoring mechanism specified in [ID.pce-
monitor].
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7.4. Verifying Correct Operation
[RFC5440] provides the sufficient descriptions for this document. So,
there are no additional considerations.
7.5. Requirements on Other Protocols and Functional Components
This document does not require any other protocol and functional
components.
7.6. Impact on Network Operation
[RFC5440] provides the sufficient descriptions for this document. So,
there are no additional considerations.
8. Security Considerations
This document defines the usage of SVEC list, and does not have any
extensions for PCEP protocol. Therefore, the security of the
procedures described in this document depends on PCEP protocol.
9. IANA Considerations
This document has no specific extension for PCEP messages, objects
and its parameters and does not require any registry assignment.
10. References
10.1. Normative References
[RFC4655] A. Farrel, JP. Vasseur and J. Ash, "A Path Computation
Element (PCE)-Based Architecture," RFC 4655, September
2006.
[RFC4657] J. Ash and J.L. Le Roux, "Path Computation Element (PCE)
Communication Protocol Generic Requirements," RFC 4757,
September 2006.
[RFC4875] R. Aggarwal, D. Papadimitriou, and S. Yasukawa,
"Extensions to Resource Reservation Protocol - Traffic
Engineering (RSVP-TE) for Point-to-Multipoint TE Label
Switched Paths (LSPs)," RFC4875, May 2007.
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[RFC5440] Ayyangar, A., Farrel, A., Oki, E., Atlas, A., Dolganow A.
Ikejiri, Y., Kumaki, K., Vasseur, J., and J. Roux, "Path
Computation Element (PCE) communication Protocol (PCEP),"
RFC5440, March. 2009.
[RFC5441] JP. Vasseur, R. Zhang, N. Bitar and JL. Le Roux, "A
Backward Recursive PCE-based Computation (BRPC) Procedure
to Compute Shortest Constrained Inter-domain Traffic
Engineering Label Switched Paths," RFC5441, April 2009.
[RFC5520] R. Bradford, JP. Vasseur, and A. Farrel, "Preserving
Topology Confidentiality in Inter-Domain Path Computations
Using a Path-Key-Based mechanism," RFC5520, April 2009.
[RFC5521] E. Oki, T. Takeda and A. Farrel, "Extensions to the Path
Computation Element Communication Protocol (PCEP) for
Route Exclusions," RFC5521, April 2009.
[RFC5557] Y. Lee, JL. Le Roux, D. King and E. Oki, "Path Computation
Element Communication Protocol (PCECP) Requirements and
Protocol Extensions In Support of Global Concurrent
Optimization," RFC5557, July 2009.
10.2. Informative References
[ID.pce-p2mp-ext] Takeda, T., Chaitou M., Le Roux, J.L., Ali Z.,Zhao,
Q., King, D., "Extensions to the Path Computation Element
Communication Protocol (PCEP) for Point-to-Multipoint
Traffic Engineering Label Switched Paths," draft-ietf-pce-
pcep-p2mp-extensions, work in progress, August, 2009.
[ID.pce-mngabl-reqs] A. Farrel, "Inclusion of Manageability Sections
in PCE Working Group Drafts," draft-ietf-pce-
manageability-requirements, work in progress, July. 2009.
[ID.pce-monitor] JP. Vasseur, JL. Le Roux and Y. Ikejiri, "A set of
monitoring tools for Path Computation Element based
Architecture," draft-ietf-pce-monitoring, work in
progress.txt, July. 2009.
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11. Acknowledgements
The authors would like to thank Julien Meuric and Filippo Cugini
for their valuable comments
Authors' Addresses
Itaru Nishioka
NEC Corp.
1753 Shimonumabe,
Kawasaki, 211-8555,
Japan
Phone: +81 44 396 3287
Email: i-nishioka@cb.jp.nec.com
Daniel King
Old Dog Consulting
United Kingdom
Phone: +44 7790 775187
Email: daniel@olddog.co.uk
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