Internet Engineering Task Force (IETF)                       I. Nishioka
Request for Comments: 6007                                     NEC Corp.
Category: Informational                                          D. King
ISSN: 2070-1721                                       Old Dog Consulting
                                                          September 2010

Use of the Synchronization VECtor (SVEC) List for
Synchronized Dependent Path Computations

Abstract

   A Path Computation Element (PCE) may be required to perform dependent
   path computations.  Dependent path computations are requests that
   need to be synchronized in order to meet specific objectives.  An
   example of a dependent request would be a PCE computing a set of
   services that are required to be diverse (disjointed) from each
   other.  When a PCE computes sets of dependent path computation
   requests concurrently, use of the Synchronization VECtor (SVEC) list
   is required for association among the sets of dependent path
   computation requests.  The SVEC object is optional and carried within
   the Path Computation Element Communication Protocol (PCEP) PCRequest
   (PCReq) message.

   This document does not specify the PCEP SVEC object or procedure.
   This informational document clarifies the use of the SVEC list for
   synchronized path computations when computing dependent requests.
   The document also describes a number of usage scenarios for SVEC
   lists within single-domain and multi-domain environments.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6007.

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   1. Introduction ....................................................3
      1.1. SVEC Object ................................................4
      1.2. Application of SVEC Lists ..................................5
   2. Terminology .....................................................6
   3. SVEC Association Scenarios ......................................7
      3.1. Synchronized Computation for Diverse Path Requests .........7
      3.2. Synchronized Computation for Point-to-Multipoint
           Path Requests ..............................................8
   4. SVEC Association ................................................9
      4.1. SVEC List ..................................................9
      4.2. Associated SVECs ...........................................9
      4.3. Non-Associated SVECs ......................................10
   5. Processing of SVEC List ........................................10
      5.1. Single-PCE, Single-Domain Environments ....................10
      5.2. Multi-PCE, Single-Domain Environments .....................11
      5.3. Multi-PCE, Multi-Domain Environments ......................11
   6. End-to-End Diverse Path Computation ............................13
      6.1. Disjoint VSPT .............................................13
      6.2. Disjoint VSPT Encoding ....................................14
      6.3. Path Computation Procedure ................................15
   7. Manageability Considerations ...................................15
      7.1. Control of Function and Policy ............................15
      7.2. Information and Data Models (MIB Modules) .................15
      7.3. Liveness Detection and Monitoring .........................15
      7.4. Verifying Correct Operation ...............................15
      7.5. Requirements on Other Protocols and Functional
           Components ................................................16
      7.6. Impact on Network Operation ...............................16
   8. Security Considerations ........................................16
   9. References .....................................................16
      9.1. Normative References ......................................16
      9.2. Informative References ....................................17
   10. Acknowledgements ..............................................18

1. Introduction

   [RFC5440] describes the specifications for the Path Computation
   Element Communication Protocol (PCEP).  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.

   The PCE may be required to compute independent and dependent path
   requests.  Path computation requests are said to be independent if
   they are not related to each other and therefore not required to be

   synchronized.  Conversely, a set of dependent path computation
   requests is such that their computations cannot be performed
   independently of each other, and the requests must be synchronized.
   The Synchronization VECtor (SVEC) with a list of the path computation
   request identifiers carried within the request message allows the PCC
   or PCE to specify a list of multiple path computation requests that
   must be synchronized.  Section 1.1 ("SVEC Object") describes the SVEC
   object.  Section 1.2 ("Application of SVEC Lists") describes the
   application of SVEC lists in certain scenarios.

   This informational document clarifies the handling of dependent and
   synchronized path computation requests, using the SVEC list, based on
   the PCE architecture [RFC4655] and PCEP [RFC5440].  The document also
   describes a number of usage scenarios for SVEC lists within single-
   domain and multi-domain environments.  This document is not intended
   to specify the procedure when using SVEC lists for dependent and
   synchronized path computation requests.

1.1. SVEC Object

   When a PCC or PCE sends path computation requests to a PCE, a PCEP
   Path Computation Request (PCReq) message may carry multiple requests,
   each of which has a unique path computation request identifier.  The
   SVEC with a list of the path computation request identifiers carried
   within the request message allows the PCC or PCE to specify a list of
   multiple path computation requests that must be synchronized, and
   also allows the specification of any dependency relationships between
   the paths.  The path computation requests listed in the SVEC must be
   handled in a specific relation to each other (i.e., synchronized).

   [RFC5440] defines two synchronous path computation modes for
   dependent or independent path computation requests specified by the
   dependency flags (i.e., Node, Link, or Shared Risk Link Group (SRLG)
   diverse flags) in the SVEC:

   o  A set of independent and synchronized path computation requests.

   o  A set of dependent and synchronized path computation requests.

   See [RFC5440] for more details on dependent, independent, and
   synchronous path computation.

   These computation modes are exclusive to each other in a single SVEC.
   If one of the dependency flags in a SVEC is set, it indicates that a
   set of synchronous path computation requests has a dependency and
   does not allow any other path computation requests.  In order to be
   synchronized with other path computation requests with a dependency,
   it is necessary to associate them.

   The aim of the SVEC object carried within a PCReq message is to
   request the synchronization of M path computation requests.  Each
   path computation request is uniquely identified by the Request-ID-
   number carried within the respective Request Parameters (RP) object.
   The SVEC object also contains a set of flags that specify the
   synchronization type.  The SVEC object is defined in Section 7.13
   ("SVEC Object") of [RFC5440].

1.2. Application of SVEC Lists

   It is important for the PCE, when performing path computations, to
   synchronize any path computation requests with a dependency.  For
   example, consider two protected end-to-end services:

   o  It would be beneficial for each back-up path to be disjointed so
      they do not share the same links and nodes as the working path.

   o  Two diverse path computation requests would be needed to compute
      the working and disjointed 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 either providing sub-optimal path disjoint results for the
   protected path or failing to meet the end-to-end 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 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 the synchronous
   approach described in this document.

   The following scenarios are specifically described within this
   document:

   o  Single-domain, single-PCE, dependent and synchronized path
      computation request.

   o  Single-domain, multi-PCE, dependent and synchronized path 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 computations is also described in this
   document, as well as the 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 clarifications and use cases in this document are applicable to
   the Global Concurrent Optimization (GCO) path computation mechanism
   specified in [RFC5557].  The GCO application provides the capability
   to optimize a set of services within the network, in order to
   maximize efficient use of network resources.  A single objective
   function (OF) or a set of 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
   required in [RFC4657].

   The SVEC association and the disjoint VSPT described in this document
   do not require any extension to PCEP messages 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.

2. Terminology

   This document uses PCE terminology defined in [RFC4655], [RFC4875],
   and [RFC5440].

   Associated SVECs: A group of multiple SVECs (Synchronization
      VECtors), defined in this document, to indicate a set of
      synchronized or concurrent path computations.

   Disjoint VSPT: A set of VSPTs, defined in this document, to indicate
      a set of virtual diverse path trees.

   GCO (Global Concurrent Optimization): A concurrent path computation
      application, defined in [RFC5557], where a set of traffic
      engineered (TE) paths is computed concurrently in order to
      efficiently utilize network resources.

   Synchronized: Describes a set of path computation requests that the
      PCE associates and that the PCE does not compute independently of
      each other.

   VSPT: Virtual Shortest Path Tree, defined in [RFC5441].

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) objectives and network
   resource optimization objectives.

   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 tertiary (and beyond) path requests, in which
   all path requests are grouped with one SVEC association.

   Consider two end-to-end services that are to be kept separate by
   using diverse paths.  The path computation requests would need to be
   associated so that diversity could be assured.  Consider further that
   each of these services requires a backup path that can protect
   against any failure in the primary path.  These backup paths must be
   computed using requests that are associated with the primary paths,
   giving rise to a set of four associated requests.

   o  End-to-end primary path and its segmented secondary paths

   When concurrent path computation for segment recovery paths, as shown
   in Figure 1, is requested, SVEC association is needed between a
   primary path and several segmented secondary paths.

                   <------------ primary ----------->

                    A------B------C---D------E------F

                      \          /     \           /

                        P---Q---R        X---Y---Z

                   <--secondary1-->   <--secondary2-->

                     Figure 1.  Segment Recovery Paths

   In this scenario, we assume that the primary path may be pre-computed
   and used for specifying the segment for secondary paths.  Otherwise,
   the segment for secondary path requests is 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 path computation request is represented
      as a set of point-to-point paths [RFC6006], 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 the "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. SVEC List

   PCEP provides the capability to carry one or more SVEC objects in a
   PCReq message, and this set of SVEC objects within the PCReq message
   is termed a SVEC list.  Each SVEC object in the SVEC list contains a
   distinct group of path computation requests.  When requesting
   association among such distinct groups, associated SVECs described in
   this document are used.

4.2. Associated SVECs

   "Associated SVECs" means that there are relationships among multiple
   SVECs in a SVEC list.  Note that there is no automatic association in
   [RFC5440] between the members of one SVEC and the members of another
   SVEC in the same SVEC list.  The associated SVEC is introduced to
   associate these SVECs, especially for correlating among SVECs with
   dependency flags.

   Request identifiers in the SVEC objects are used to indicate the
   association among SVEC objects.  If the same request-IDs exist in
   SVEC objects, this indicates these SVEC objects are associated.  When
   associating among SVEC objects, at least one request identifier must
   be shared between associated SVECs.  The SVEC objects can be
   associated regardless of the dependency flags in each SVEC object,
   but it is recommended to use a single SVEC if the dependency flags
   are not set in all SVEC objects.  Similarly, when associating among
   SVEC objects with dependency flags, it is recommended to construct
   them using a minimum set of associated SVECs, thus avoiding complex
   relational associations.

   Below is an example of associated SVECs.  In this example, the first
   SVEC is associated with the other SVECs, and all of the path
   computation requests contained in the associated SVECs (i.e.,
   Request-IDs #1, #2, #3, #4, #X, #Y, and #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

4.3. Non-Associated SVECs

   "Non-associated SVECs" means that there are no relationships among
   SVECs.  If none of the SVEC objects in the SVEC list on a PCReq
   message contains a common request-ID, there is no association between
   the SVECs and so no association between the requests in one SVEC and
   the requests in another SVEC.

   Below is an example of non-associated SVECs that do not contain any
   common 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

   In this environment, there is a single PCE within the domain.

   When a PCE receives PCReq messages with more than one SVEC object in
   the SVEC list, PCEP has to first check the request-IDs in all SVEC
   objects in order to identify any associations among them.

   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 a PCE that is unable to handle the associated SVEC finds the
   common request-IDs in multiple SVEC objects, the PCE should cancel
   the path computation request and respond to the PCC with the PCErr
   message Error-Type="Capability not supported".

   In the case that M path computation requests are sent across multiple
   PCReq messages, the PCE may start a SyncTimer as recommended in
   Section 7.13.3 ("Handling of the SVEC Object") of [RFC5440].  In this
   case, the associated SVECs should also be handled as described in
   [RFC5440], i.e., after receiving the entire set of M path computation
   requests associated by SVECs, the computation should start at one.
   If the SyncTimer has expired or the subsequent PCReq messages are
   malformed, the PCE should cancel the path computation request and
   respond to the PCC with the relevant PCErr message.

5.2. Multi-PCE, Single-Domain Environments

   There are multiple PCEs in a domain, to which PCCs can communicate
   directly, and PCCs can choose an appropriate PCE for load-balanced
   path computation requests.  In this environment, it is possible that
   dependent path computation requests are sent to different PCEs.

   However, if a PCC sends path computation requests to a PCE, and then
   sends a further path computation request to a different PCE using the
   SVEC list to show that the further request is dependent on the first
   requests, there is no method for the PCE to correlate the dependent
   requests sent to different PCEs.  No SVEC object correlation function
   between the PCEs is specified in [RFC5440].  No mechanism exists to
   resolve this problem, and the issue is open for future study.
   Therefore, a PCC must not send dependent path computation requests
   associated by SVECs to different PCEs.

5.3. Multi-PCE, Multi-Domain Environments

   In this environment, there are multiple domains in which PCEs are
   located in each domain, and end-to-end dependent paths (i.e., diverse
   paths) are computed using multiple PCEs.  Note that we assume a chain
   of PCEs is predetermined and the Backward-Recursive PCE-Based
   Computation (BRPC) procedure [RFC5441] is in use.

   The SVECs 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

   end-to-end diverse path computation across a chain of domains.  In
   the 2-step approaches described in [RFC5521], it is not necessary to
   use the associated SVECs if any of the dependency flags in a SVEC
   object are not set.  On the other hand, the simultaneous approach may
   require the associated SVEC because at least one of the dependency
   flags is required to be set in a SVEC object.  Thus, a use case of
   the simultaneous approach is described in this environment.

   When a chain of PCEs located in separate domains is used for
   simultaneous path computations, additional path computation
   processing is required, as described in Section 6 of this document.

   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
   procedures are applied.

   When a chain of PCEs is a unique sequence for all of the path
   computation requests in a PCReq message, it is not necessary to
   reconstruct associations among SVEC objects.  Thus, the PCReq message
   is passed to the tail-end PCE.  When a PCReq message contains more
   than one SVEC object with the dependency flag set, the contained
   SVECs may then be associated.  PCEs receiving the associated SVECs
   must maintain their association and must consider their relationship
   when performing path computations after receiving a corresponding
   PCReply (PCRep) message.

   When a chain of PCEs is different, it is required that intermediate
   PCEs receiving such PCReq messages may reconstruct associations among
   SVEC objects, and then send PCReq messages to corresponding PCEs
   located in neighboring domains.  If the associated SVECs are
   reconstructed at the intermediate PCE, the PCE must not start its
   path computation until all PCRep messages have been received from all
   neighbor PCEs.  However, a complex PCE implementation is required for
   SVEC reconstruction, and waiting mechanisms must be implemented.
   Therefore, it is not recommended to associate path computation
   requests with different PCE chains.  This is an open issue and is
   currently being discussed in [H-PCE], which proposes a hierarchical
   PCE architecture.

6. End-to-End Diverse Path Computation

In this section, the synchronous approach is provided to compute
primary and secondary paths simultaneously.

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 an end-to-end disjoint path.  In order to
   preserve diversity (or disjointness) information, disjoint VSPTs are
   sent in the PCEP PCRep message.  The PCReq containing a SVEC object
   with the appropriate diverse flag set would signal that the PCE
   should compute a disjoint VSPT.

   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 2 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 3, when the primary path and secondary
   path are requested from S1 to D1.  These VSPTs consist of a disjoint
   VSPT, which is indicated in a PCRep 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].
   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 2.  Example Network for Diverse Path Computation

               VSPT1:            VSPT2:              VSPT3:

                 D1                D1                  D1

                / \               / \                 / \

             BN4   BN5         BN4   BN6           BN5   BN6

                Figure 3.  Disjoint VSPTs from PCE2 to PCE1

6.2. Disjoint VSPT Encoding

   Encoding for the disjoint VSPT follows the definition of PCEP message
   encoding in [RFC5440].

   The 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 Explicit Route
   Objects (EROs) and secondary EROs.

   A PCE sending a 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, the path key
   mechanism defined in [RFC5520] is used for a disjoint VSPT.

   Below are the details of the disjoint VSPT encoding (in Figure 3),
   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]

6.3. Path Computation Procedure

   For end-to-end diverse path computation, the same mode of operation
   as that of the BRPC procedure can be applied (i.e., Step 1 to Step n
   in Section 4.2 of [RFC5441]).  A question that must be considered is
   how to recognize disjoint VSPTs.

   The recognition of disjoint VSPTs is achieved by the PCE sending a
   PCReq to its neighbor PCE, which maintains the path computation
   request (PCReq) information.  If the PCReq has one or more SVEC
   object(s) with the appropriate dependency 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
   requests with disjoint VSPTs.  The selection and application of the
   appropriate algorithm is out of scope in this document.

7. Manageability Considerations

This section describes manageability considerations specified in
[PCE-MNG-REQS].

7.1. Control of Function and Policy

   In addition to [RFC5440], PCEP implementations should allow the PCC
   to be responsible for mapping the requested paths to computation
   requests.  The PCC should construct the SVECs to identify and
   associate SVEC relationships.

7.2. Information and Data Models (MIB Modules)

   There are currently no additional parameters for MIB modules.  There
   would be value in a MIB module that details the SVEC association.
   This work is currently out of scope of this document.

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 the PCE monitoring mechanism specified in [RFC5886].

7.4. Verifying Correct Operation

[RFC5440] provides a sufficient description for this document. 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 descriptions for the mechanisms discussed in this
   document.  There is value in considering that large associated SVECs
   will require greater PCE resources, compared to non-associated SVECs.
   Additionally, the sending of large associated SVECs within multiple
   PCReq messages will require more network resources.  Solving these
   specific issues is out of scope of this document.

8. Security Considerations

   This document describes the usage of the SVEC list, and does not have
   any extensions for PCEP.  The security of the procedures described in
   this document depends on PCEP [RFC5440].  However, a PCE that
   supports associated SVECs may be open to Denial-of-Service (DoS)
   attacks from a rogue PCC.  A PCE may be made to queue large numbers
   of requests waiting for other requests that will never arrive.
   Additionally, a PCE might be made to compute exceedingly complex
   associated SVEC computations.  These DoS attacks may be mitigated
   with the use of practical SVEC list limits, as well as:

   o  Applying provisioning to PCEs, e.g., for a given number of
      simultaneous services (recommended).

   o  Using a priority-based multi-queuing mechanism in which path
      computation requests with a smaller SVEC list are prioritized for
      path computation processing.

   o  Specifying which PCCs may request large SVEC associations through
      PCE access policy control.

9. References

9.1. Normative References

   [RFC4655]      Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
                  Computation Element (PCE)-Based Architecture",
                  RFC 4655, August 2006.

   [RFC4657]      Ash, J., Ed., and J. Le Roux, Ed., "Path Computation
                  Element (PCE) Communication Protocol Generic
                  Requirements", RFC 4657, September 2006.

   [RFC4875]      Aggarwal, R., Ed., Papadimitriou, D., Ed., and S.
                  Yasukawa, Ed., "Extensions to Resource Reservation
                  Protocol - Traffic Engineering (RSVP-TE) for Point-to-
                  Multipoint TE Label Switched Paths (LSPs)", RFC 4875,
                  May 2007.

   [RFC5440]      Vasseur, JP., Ed., and JL. Le Roux, Ed., "Path
                  Computation Element (PCE) Communication Protocol
                  (PCEP)", RFC 5440, March 2009.

   [RFC5441]      Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le
                  Roux, "A Backward-Recursive PCE-Based Computation
                  (BRPC) Procedure to Compute Shortest Constrained
                  Inter-Domain Traffic Engineering Label Switched
                  Paths", RFC 5441, April 2009.

   [RFC5520]      Bradford, R., Ed., Vasseur, JP., and A. Farrel,
                  "Preserving Topology Confidentiality in Inter-Domain
                  Path Computation Using a Path-Key-Based Mechanism",
                  RFC 5520, April 2009.

   [RFC5521]      Oki, E., Takeda, T., and A. Farrel, "Extensions to the
                  Path Computation Element Communication Protocol (PCEP)
                  for Route Exclusions", RFC 5521, April 2009.

   [RFC5557]      Lee, Y., Le Roux, JL., King, D., and E. Oki, "Path
                  Computation Element Communication Protocol (PCEP)
                  Requirements and Protocol Extensions in Support of
                  Global Concurrent Optimization", RFC 5557, July 2009.

9.2. Informative References

   [H-PCE]        King, D., Ed., and A. Farrel, Ed., "The Application of
                  the Path Computation Element Architecture to the
                  Determination of a Sequence of Domains in MPLS &
                  GMPLS", Work in Progress, December 2009.

   [PCE-MNG-REQS] Farrel, A., "Inclusion of Manageability Sections in
                  PCE Working Group Drafts", Work in Progress, July
                  2009.

   [RFC5886]      Vasseur, JP., Ed., Le Roux, JL., and Y. Ikejiri, "A
                  Set of Monitoring Tools for Path Computation Element
                  (PCE)-Based Architecture", RFC 5886, June 2010.

   [RFC6006]      Zhao, Q., Ed., King, D., Ed., Verhaeghe, F., Takeda,
                  T., Ali, Z., and J. Meuric, "Extensions to the Path
                  Computation Element Communication Protocol (PCEP) for
                  Point-to-Multipoint Traffic Engineering Label Switched
                  Paths", RFC 6006, September 2010.

10. Acknowledgements

The authors would like to thank Adrian Farrel, Julien Meuric, and
Filippo Cugini for their valuable comments.

Authors' Addresses

   Itaru Nishioka
   NEC Corp.
   1753 Shimonumabe,
   Kawasaki, 211-8666,
   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