Internet Engineering Task Force I. Nishioka
Internet-Draft NEC
Intended Status: Informational Daniel King
Created: March 9, 2009 Old Dog Consulting
Expires: September 9, 2009
The use of SVEC (Synchronization VECtor) list for Synchronized
dependent path computations
draft-ietf-pce-pcep-svec-list-01.txt
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Abstract
A Path Computation Element (PCE) performing dependent path
computations, for instance calculating a diverse working and
protected path do not share common network points, would need to
synchronize the computations in order to increase the probability of
meeting the working and protected path disjoint 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.
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Contents
1. Terminology....................................................
2. Introduction...................................................
3. SVEC association scenarios.....................................
3.1. Synchronized computation for diverse path requests.........
3.2. Synchronized computation for point-to-multipoint
path requests..............................................
4. SVEC association...............................................
4.1. Associated SVECs...........................................
4.2. Non-associated SVECs.......................................
5. Processing of SVEC list........................................
5.1. Single PCE, single domain environments.....................
5.2. Multi-PCE, single domain environments......................
5.3. Single PCE, Multi-domain environments......................
6. End-to-end diverse path computation............................
6.1. Disjoint VSPTs.............................................
6.2. Disjoint VSPTs Encoding....................................
6.3. Path commutation in PCE....................................
7. Manageability considerations...................................
7.1. Control of Function and Police.............................
7.2. Information and Data Models, e.g. MIB modules..............
7.3. Liveness Detection and Monitoring..........................
7.4. Verifying Correct Operation................................
7.5. Requirements on Other Protocols and Functional Components..
7.6. Impact on Network Operation................................
8. Security Considerations........................................
9. IANA Considerations............................................
10. References....................................................
10.1. Normative References......................................
10.2. Informative References....................................
11. Authors Addresses.............................................
12. Intellectual Property Consideration...........................
12. Disclaimer of Validity........................................
13. Full Copyright Statement......................................
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1. Terminology
This document uses PCE terminology defined in [RFC4655],
[RFC4875], and [RFC5440].
GCO (Global Concurrent Optimization): A
concurrent path computation application 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)to indicate a set of synchronized or concurrent path
computations.
VSPT: Virtual Shortest Path Tree
2. Introduction
[RFC5440] describes the specifications for PCEP (Path Computation
Element communication Protocol). PCEP facilitates the communication
between a Path Computation Client(PCC) and a Path Computation Element
(PCE), or between two PCEs based on PCE architecture [RFC4655]. PCEP
interactions include path computation requests and path computation
replies.
[ID.pce-gco] specifies a global concurrent path computation
application for the efficient use of network resources, called GCO,
based on required objective functions (OFs). To compute a set of
traffic-engineered paths for the GCO application, PCEP supports the
synchronous and dependent path computation requests 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 of a set of independent and synchronized path computation
requests.
o Bundle of a set of dependent and synchronized path computation
requests.
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These are exclusive modes. 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 fail to meet the disjoint requirement
altogether.
Additionally SVEC can be applied to an end-to-end diverse path
computation that traverse multiple domains. [ID.pce-brpc] 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 [ID.xro], 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. The following scenarios are specifically described:
o Single domain, single PCE, dependent and synchronized path
computation request.
o Single domain, multiple-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 and the processing rules to
support multiple sets of synchronized dependent path computation
requests is also described in this document. Path computation
algorithms for the associated path computation requests are out of
scope in this document.
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The SVEC association and its processing rule do not require any
extension to PCEP message and object formats, when computing a GCO
for multiple 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 is available to both multiple PCE path
computations as well as a single 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
When computing two or more point-to-point diverse paths, a PCE may
compute these diverse paths concurrently, in order to increase the
probability of meeting primary and secondary path disjoint 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
requests. Note here that each diverse path request consists of
primary, secondary, and etc., in which are grouped with one SVEC.
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.
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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 XRO and/or IOR 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 its 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
[ID.p2mp-te-bypass] 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
4. SVEC association
This section describes the associations among SVECs in a SVEC list.
4.1. Associated SVECs
Associated SVECs mean 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 may 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 of the path
computation requests are synchronized.
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Below is an example of associated SVECs. In this example, the first
SVEC is associating 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.
<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 PCEP 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 later case, the PCE may start a SyncTimer as recommended in
[RFC5440]. After receiving the whole path computation requests,
the analysis for associated SVECs has to be started.
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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".
5.2. Multi-PCE, single domain environments
Currently no mechanisms exist to manage co-ordination of dependent
SVEC requests between multiple PCE`s 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. Single 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.
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 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.
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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, BRPC procedure can be used.
[ID.pce-brpc] 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 is
described in [ID.xro]. This section provides how to compute end-to
-end diverse path in the synchronous approach.
6.1. Disjoint VSPTs
BRPC procedure constructs a VSPT (virtual shortest path tree) to
inform the enquiring PCE of potential paths to the destination node.
In the end-to-end diverse path computation, disjoint information
among the potential paths must be preserved in the VSPT to ensure end
-to-end disjoint path. In order to preserve disjoint information,
disjoint VSPTs are sent in the PCEP PCRep message.
A definition of the disjoint VSPTs is a collection of VSPTs, in which
each VSPT contains a potential set of primary and secondary paths.
Figure 1 is an example network and its disjoint VSPTs when primary
path and secondary path are requested. Here, transit nodes within
domains are not depicted, and PCE1 and PCE2 may be embedded in
boarder nodes.
Example network:
Domain1 Domain2
+----------+ +----------+
| PCE1 | | PCE2 | S1: Source node
| BN1---BN4 | D1: Destination node
| S1 BN2---BR5 D1 | BN1-BN6: Border nodes
| BN3---BN6 |
| | | |
+----------+ +----------+
VSPTs from PCE2:
VSPT1; VSPT2; VSPT3;
D1 D1 D1
/ \ / \ / \
BR4 BR5 BR4 BR6 BR5 BR6
Figure 1: An example of diverse path computation
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6.2. Disjoint VSPT Encoding
Encoding for disjoint VSPTs follows the definition of PCEP message
encoding in [RFC5440].
PCEP PCRep message returns disjoint VSPTs as <path list> for each RP
object. The order of <path> in <path list> among <responses> implies
a set of primary EROs and secondary EROs.
Below shows simple example in Figure 1 if a primary path and a
secondary path are requested.
o Request ID #1 (Primary)
- ERO1 BR4(TE route ID)- ...-D1(TE-Router ID) [for VSPT1]
- ERO2 BR4(TE route ID)- ...-D1(TE-Router ID) [for VSPT2]
- ERO3 BR5(TE route ID)- ...-D1(TE-Router ID) [for VSPT3]
O Request ID #2 (Secondary)
- ERO4 BR5(TE route ID)- ...-D1(TE-Router ID) [for VSPT1]
- ERO5 BR6(TE route ID)- ...-D1(TE-Router ID) [for VSPT2]
- ERO6 BR6(TE route ID)- ...-D1(TE-Router ID) [for VSPT2]
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 [ID.pce-brpc]). During this procedure, a questions 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 VSTP is a normal VSPT based on
the shortest path computation.
Note that the PCE can apply a suitable algorithm for computing
disjoint VSPT. 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].
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7.1. Control of Function and Policy
In addition to section 8.1 to [RFC5440], PCEP implementation
should allow the configuration of association among SVECs on PCCs.
o the capability to configure SVEC association.
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 path. 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].
7.4. Verifying Correct Operation
Section 8.4 in [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 anything on other protocol and
functional components.
7.6. Impact on Network Operation
Section 8.6 in [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 this document
depends on that of 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
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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)," RFCCC4875, May, 2007.
10.2. Informative References
[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)", RFC 5440, March 2009.
[ID.pce-gco] 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," draft-ietf-pce-global-
concurrent-optimization-08 Work in progress, Jan. 2009.
[ID.pce-brpc] 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,"
draft-ietf-pce-brpc-09, Work in progress, April 2008.
[ID.xro] E. Oki, T. Takeda and A. Farrel, "Extensions to the
Path Computation Element Communication Protocol (PCEP)
for Route Exclusions," draft-ietf-pce-pcep-xro-06,
Work in progress, July 2008.
[ID-pce-p2mp-ext] Takeda, T., Chaitou M., Le Roux, J.L., Ali Z.,Zhao,
Q., King, D.,"draft-ietf-pce-pcep-p2mp-extensions-
01,work in progress, October , 2008.
[ID.p2mp-te-bypass] JL. Le Roux, R. Aggarwal, J.P. Vasseur,
and M. Vigoureux, "P2MP MPLS-TE Fast
Reroute with P2MP Bypass Tunnels," draft-
ietf-mpls-p2mp-te-bypass-02," Work in progress, Mar.
2008.
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[ID.pce-mngabl-reqs] A. Farrel, "Inclusion of Manageability
Sections in PCE Working
Group Drafts," draft-ietf-pce-manageability-
requirements-06 Work in progress, Jan. 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-04 Work in
progress, Jan. 2009.
11. 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
UK
Phone: +44 7790 775187
Email: daniel@olddog.co.uk
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Copies of Intellectual Property disclosures made to the IETF
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the result of an attempt made to obtain a general license or
permission for the use of such proprietary rights by implementers or
users of this specification can be obtained from the IETF on-line IPR
repository at http://www.ietf.org/ipr
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