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rfc:rfc4657

Network Working Group J. Ash, Ed. Request for Comments: 4657 AT&T Category: Informational J.L. Le Roux, Ed.

                                                        France Telecom
                                                        September 2006
       Path Computation Element (PCE) Communication Protocol
                        Generic Requirements

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

 The PCE model is described in the "PCE Architecture" document and
 facilitates path computation requests from Path Computation Clients
 (PCCs) to Path Computation Elements (PCEs).  This document specifies
 generic requirements for a communication protocol between PCCs and
 PCEs, and also between PCEs where cooperation between PCEs is
 desirable.  Subsequent documents will specify application-specific
 requirements for the PCE communication protocol.

Table of Contents

 1. Introduction ....................................................2
 2. Conventions Used in This Document ...............................3
 3. Terminology .....................................................3
 4. Overview of PCE Communication Protocol (PCECP) ..................4
 5. PCE Communication Protocol Generic Requirements .................5
    5.1. Basic Protocol Requirements ................................5
         5.1.1. Commonality of PCC-PCE and PCE-PCE Communication ....5
         5.1.2. Client-Server Communication .........................5
         5.1.3. Transport ...........................................5
         5.1.4. Path Computation Requests ...........................5
         5.1.5. Path Computation Responses ..........................7
         5.1.6. Cancellation of Pending Requests ....................7
         5.1.7. Multiple Requests and Responses .....................8
         5.1.8. Reliable Message Exchange ...........................8
         5.1.9. Secure Message Exchange .............................9

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         5.1.10. Request Prioritization ............................10
         5.1.11. Unsolicited Notifications .........................10
         5.1.12. Asynchronous Communication ........................10
         5.1.13. Communication Overhead Minimization ...............10
         5.1.14. Extensibility .....................................11
         5.1.15. Scalability .......................................11
         5.1.16. Constraints .......................................12
         5.1.17. Objective Functions Supported .....................13
    5.2. Deployment Support Requirements ...........................13
         5.2.1. Support for Different Service Provider
                Environments .......................................13
         5.2.2. Policy Support .....................................14
    5.3. Aliveness Detection & Recovery Requirements ...............14
         5.3.1. Aliveness Detection ................................14
         5.3.2. Protocol Recovery ..................................14
         5.3.3. LSP Rerouting & Reoptimization .....................14
 6. Security Considerations ........................................15
 7. Manageability Considerations ...................................16
 8. Contributors ...................................................17
 9. Acknowledgements ...............................................18
 10. References ....................................................19
    10.1. Normative References .....................................19
    10.2. Informative References ...................................19

1. Introduction

 A Path Computation Element (PCE) [RFC4655] supports requests for path
 computation issued by a Path Computation Client (PCC), which may be
 'composite' (co-located) or 'external' (remote) from a PCE.  When the
 PCC is external from the PCE, a request/response communication
 protocol is required to carry the path computation request and return
 the response.  In order for the PCC and PCE to communicate, the PCC
 must know the location of the PCE; PCE discovery is described in
 [PCE-DISC-REQ].
 The PCE operates on a network graph in order to compute paths based
 on the path computation request(s) issued by the PCC(s).  The path
 computation request will include the source and destination of the
 paths to be computed and a set of constraints to be applied during
 the computation, and it may also include an objective function.  The
 PCE response includes the computed paths or the reason for a failed
 computation.

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 This document lists a set of generic requirements for the PCE
 Communication Protocol (PCECP).  Application-specific requirements
 are beyond the scope of this document, and will be addressed in
 separate documents.  For example, application-specific communication
 protocol requirements are given in [PCECP-INTER-AREA] and
 [PCECP-INTER-LAYER] for inter-area and inter-layer PCE applications,
 respectively.

2. Conventions Used in This Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", "MAY NOT", and
 "OPTIONAL" in this document are to be interpreted as described in RFC
 2119 [RFC2119].

3. Terminology

 Domain: Any collection of network elements within a common sphere of
 address management or path computational responsibility.  Examples of
 domains include Interior Gateway Protocol (IGP) areas, Autonomous
 Systems (ASs), multiple ASs within a service provider network, or
 multiple ASs across multiple service provider networks.
 GMPLS: Generalized Multi-Protocol Label Switching
 LSP: MPLS/GMPLS Label Switched Path
 LSR: Label Switch Router
 MPLS: Multi-Protocol Label Switching
 PCC: Path Computation Client: Any client application requesting a
 path computation to be performed by the PCE.
 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 (see
 further description in [RFC4655]).
 TED: Traffic Engineering Database, which contains the topology and
 resource information of the network or network segment used by a PCE.
 TE LSP: Traffic Engineering (G)MPLS Label Switched Path.
 See [RFC4655] for further definitions of terms.

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4. Overview of PCE Communication Protocol (PCECP)

 In the PCE model, path computation requests are issued by a PCC to a
 PCE that may be composite (co-located) or external (remote).  If the
 PCC and PCE are not co-located, a request/response communication
 protocol is required to carry the request and return the response.
 If the PCC and PCE are co-located, a communication protocol is not
 required, but implementations may choose to utilize a protocol for
 exchanges between the components.
 In order for a PCC and PCE to communicate, the PCC must know the
 location of the PCE.  This can be configured or discovered.  The PCE
 discovery mechanism is out of scope of this document, but
 requirements are documented in [PCE-DISC-REQ].
 The PCE operates on a network graph built from the TED in order to
 compute paths.  The mechanism by which the TED is populated is out of
 scope for the PCECP.
 A path computation request issued by the PCC includes a specification
 of the path(s) needed.  The information supplied includes, at a
 minimum, the source and destination for the paths, but may also
 include a set of further requirements (known as constraints) as
 described in Section 5.
 The response from the PCE may be positive in which case it will
 include the paths that have been computed.  If the computation fails
 or cannot be performed, a negative response is required with an
 indication of the type of failure.
 A request/response protocol is also required for a PCE to communicate
 path computation requests to another PCE and for that PCE to return
 the path computation response.  As described in [RFC4655], there is
 no reason to assume that two different protocols are needed, and this
 document assumes that a single protocol will satisfy all requirements
 for PCC-PCE and PCE-PCE communication.
 [RFC4655] describes four models of PCE: composite, external, multiple
 PCE path computation, and multiple PCE path computation with inter-
 PCE communication.  In all cases except the composite PCE model, a
 PCECP is required.  The requirements defined in this document are
 applicable to all models described in [RFC4655].

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5. PCE Communication Protocol Generic Requirements

5.1. Basic Protocol Requirements

5.1.1. Commonality of PCC-PCE and PCE-PCE Communication

 A single protocol MUST be defined for PCC-PCE and PCE-PCE
 communication.  A PCE requesting a path from another PCE can be
 considered a PCC, and in the remainder of this document we refer to
 all communications as PCC-PCE regardless of whether they are PCC-PCE
 or PCE-PCE.

5.1.2. Client-Server Communication

 PCC-PCE communication is by nature client-server based.  The PCECP
 MUST allow a PCC to send a request message to a PCE to request path
 computation, and for a PCE to reply with a response message to the
 requesting PCC once the path has been computed.
 In addition to this request-response mode, there are cases where
 there is unsolicited communication from the PCE to the PCC (see
 Section 5.1.11).

5.1.3. Transport

 The PCECP SHOULD utilize an existing transport protocol that supports
 congestion control.  This transport protocol may also be used to
 satisfy some requirements in other sections of this document, such as
 reliability.  The PCECP SHOULD be defined for one transport protocol
 only in order to ensure interoperability.  The transport protocol
 MUST NOT limit the size of the message used by the PCECP.

5.1.4. Path Computation Requests

 The path computation request message MUST include at least the source
 and destination.  Note that the path computation request is for an
 LSP or LSP segment, and the source and destination supplied are the
 start and end of the computation being requested (i.e., of the LSP
 segment).

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 The path computation request message MUST support the inclusion of a
 set of one or more path constraints, including but not limited to the
 requested bandwidth or resources (hops, affinities, etc.) to
 include/exclude.  For example, a PCC may request the PCE to exclude
 points of failure in the computation of a new path if an LSP setup
 fails.  The actual inclusion of constraints is a choice for the PCC
 issuing the request.  A list of core constraints that must be
 supported by the PCECP is supplied in Section 5.1.16.  Specification
 of constraints MUST be future-proofed as described in Section 5.1.14.
 The requester MUST be allowed to select from or prefer an advertised
 list or minimal subset of standard objective functions and functional
 options.  An objective function is used by the PCE to process
 constraints to a path computation request when it computes a path in
 order to select the "best" candidate paths (e.g., minimum hop path),
 and corresponds to the optimization criteria used for the computation
 of one path, or the synchronized computation of a set of paths.  In
 the case of unsynchronized path computation, this can be, for
 example, the path cost or the residual bandwidth on the most loaded
 path link.  In the case of synchronized path computation, this can
 be, for example, the global bandwidth consumption or the residual
 bandwidth on the most loaded network link.
 A list of core objective functions that MUST be supported by the
 PCECP is supplied in Section 5.1.17.  Specification of objective
 functions MUST be future-proofed as described in Section 5.1.14.
 The requester SHOULD also be able to select a vendor-specific or
 experimental objective function or functional option.  Furthermore,
 the requester MUST be allowed to customize the function/options in
 use.  That is, individual objective functions will often have
 parameters to be set in the request from PCC to PCE.  Support for the
 specification of objective functions and objective parameters is
 required in the protocol extensibility specified in Section 5.1.14.
 A request message MAY include TE parameters carried by the MPLS/GMPLS
 LSP setup signaling protocol.  Also, it MUST be possible for the PCE
 to apply additional objective functions.  This might include policy-
 based routing path computation for load balancing instructed by the
 management plane.
 Shortest path selection may rely either on the TE metric or on the
 IGP metric [METRIC].  Hence the PCECP request message MUST allow the
 PCC to indicate the metric type (IGP or TE) to be used for shortest
 path selection.  Note that other metric types may be specified in the
 future.

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 There may be cases where a single path cannot fit a given bandwidth
 request, while a set of paths could be combined to fit the request.
 Such path combination to serve a given request is called load-
 balancing.  The request message MUST allow the PCC to indicate if
 load-balancing is allowed.  It MUST also include the maximum number
 of paths in a load-balancing path group, and the minimum path
 bandwidth in a load-balancing path group.  The request message MUST
 allow specification of the degree of disjointness of the members of
 the load-balancing group.

5.1.5. Path Computation Responses

 The path computation response message MUST allow the PCE to return
 various elements including, at least, the computed path(s).
 The protocol MUST be capable of returning any explicit path that
 would be acceptable for use for MPLS and GMPLS LSPs once converted to
 an Explicit Route Object for use in RSVP-TE signaling.  In addition,
 anything that can be expressed in an Explicit Route Object MUST be
 capable of being returned in the computed path.  Note that the
 resultant path(s) may be made up of a set of strict or loose hops, or
 any combination of strict and loose hops.  Moreover, a hop may have
 the form of a non-simple abstract node.  See [RFC3209] for the
 definition of strict hop, loose hop, and abstract node.
 A positive response from the PCE MUST include the paths that have
 been computed.  A positive PCECP computation response MUST support
 the inclusion of a set of attributes of the computed path, such as
 the path costs (e.g., cumulative link TE metrics and cumulative link
 IGP metrics) and the computed bandwidth.  The latter is useful when a
 single path cannot serve the requested bandwidth and load balancing
 is applied.
 When a path satisfying the constraints cannot be found, or if the
 computation fails or cannot be performed, a negative response MUST be
 sent.  This response MAY include further details of the reason(s) for
 the failure and MAY include advice about which constraints might be
 relaxed to be more likely to achieve a positive result.
 The PCECP response message MUST support the inclusion of the set of
 computed paths of a load-balancing path group, as well as their
 respective bandwidths.

5.1.6. Cancellation of Pending Requests

 A PCC MUST be able to cancel a pending request using an appropriate
 message.  A PCC that has sent a request to a PCE and no longer needs
 a response, for instance, because it no longer wants to set up the

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 associated service, MUST be able to notify the PCE that it can clear
 the request (i.e., stop the computation if already started, and clear
 the context).  The PCE may also wish to cancel a pending request
 because of some congested state.

5.1.7. Multiple Requests and Responses

 It MUST be possible to send multiple path computation requests within
 the same request message.  Such requests may be correlated (e.g.,
 requesting disjoint paths) or uncorrelated (requesting paths for
 unrelated services).  It MUST be possible to limit by configuration
 of both PCCs and PCEs the number of requests that can be carried
 within a single message.
 Similarly, it MUST be possible to return multiple computed paths
 within the same response message, corresponding either to the same
 request (e.g., multiple suited paths, paths of a load-balancing path
 group) or to distinct requests, correlated or not, of the same
 request message or distinct request messages.
 It MUST be possible to provide "continuation correlation" where all
 related requests or computed paths cannot fit within one message and
 are carried in a sequence of correlated messages.
 The PCE MUST inform the PCC of its capabilities.  Maximum acceptable
 message sizes and the maximum number of requests per message
 supported by a PCE MAY form part of PCE capabilities advertisement
 [PCE-DISC-REQ] or MAY be exchanged through information messages from
 the PCE as part of the protocol described here.
 It MUST be possible for a PCC to specify, in the request message, the
 maximum acceptable response message sizes and the maximum number of
 computed paths per response message it can support.
 It MUST be possible to limit the message size by configuration on
 PCCs and PCEs.

5.1.8. Reliable Message Exchange

 The PCECP MUST support reliable transmission of PCECP packets.  This
 may form part of the protocol itself or may be achieved by the
 selection of a suitable transport protocol (see Section 5.1.3).
 In particular, it MUST allow for the detection and recovery of lost
 messages to occur quickly and not impede the operation of the PCECP.
 In some cases (e.g., after link failure), a large number of PCCs may
 simultaneously send requests to a PCE, leading to a potential

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 saturation of the PCEs.  The PCECP MUST support indication of
 congestion state and rate limitation state.  This should enable, for
 example, a PCE to limit the rate of incoming request messages if the
 request rate is too high.
 The PCECP or its transport protocol MUST provide the following:
  1. Detection and report of lost or corrupted messages
  2. Automatic attempts to retransmit lost messages without reference to

the application

  1. Handling of out-of-order messages
  2. Handling of duplicate messages
  3. Flow control and back-pressure to enable throttling of requests and

responses

  1. Rapid PCECP communication failure detection
  2. Distinction between partner failure and communication channel

failure after the PCECP communication is recovered

 If it is necessary to add functions to PCECP to overcome shortcomings
 in the chosen transport mechanisms, these functions SHOULD be based
 on and re-use where possible techniques developed in other protocols
 to overcome the same shortcomings.  Functionality MUST NOT be added
 to the PCECP where the chosen transport protocol already provides it.

5.1.9. Secure Message Exchange

 The PCC-PCE communication protocol MUST include provisions to ensure
 the security of the exchanges between the entities.  In particular,
 it MUST support mechanisms to prevent spoofing (e.g.,
 authentication), snooping (e.g., preservation of confidentiality of
 information through techniques such as encryption), and Denial of
 Service (DoS) attacks (e.g., packet filtering, rate limiting, no
 promiscuous listening).  Once a PCC is identified and authenticated,
 it has the same privileges as all other PCCs.
 To ensure confidentiality, the PCECP SHOULD allow local policy to be
 configured on the PCE to not provide explicit path(s).  If a PCC
 requests an explicit path when this is not allowed, the PCE MUST
 return an error message to the requesting PCC and the pending path
 computation request MUST be discarded.
 Authorization requirements [RFC3127] include reject capability,
 reauthorization on demand, support for access rules and filters, and
 unsolicited disconnect.

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 IP addresses are used to identify PCCs and PCEs.  Where the PCC-PCE
 communication takes place entirely within one limited domain, the use
 of a private address space that is not available to customer systems
 MAY be used to help protect the information exchange, but other
 mechanisms MUST also be available.
 These functions may be provided by the transport protocol or directly
 by the PCECP.  See Section 6 for further discussion of security
 considerations.

5.1.10. Request Prioritization

 The PCECP MUST allow a PCC to specify the priority of a computation
 request.
 Implementation of priority-based activity within a PCE is subject to
 implementation and local policy.  This application processing is out
 of scope of the PCECP.

5.1.11. Unsolicited Notifications

 The normal operational mode is for the PCC to make path computation
 requests to the PCE and for the PCE to respond.
 The PCECP MUST support unsolicited notifications from PCE to PCC, or
 PCC to PCE.  This requirement facilitates the unsolicited
 communication of information and alerts between PCCs and PCEs.  As
 specified in Section 5.1.8, these notification messages must be
 supported by a reliable transmission protocol.  The PCECP MAY also
 support response messages to the unsolicited notification messages.

5.1.12. Asynchronous Communication

 The PCC-PCE protocol MUST allow for asynchronous communication.  A
 PCC MUST NOT have to wait for a response to one request before it can
 make another request.
 It MUST also be possible to have the order of responses differ from
 the order of the corresponding requests.  This may occur, for
 instance, when path request messages have different priorities (see
 Requirement 5.1.10).  A consequent requirement is that path
 computation responses MUST include a direct correlation to the
 associated request.

5.1.13. Communication Overhead Minimization

 The request and response messages SHOULD be designed so that the
 communication overhead is minimized.  In particular, the overhead per

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 message SHOULD be minimized, and the number of bytes exchanged to
 arrive at a computation answer SHOULD be minimized.  Other
 considerations in overhead minimization include the following:
  1. the number of background messages used by the protocol or its

transport protocol to keep alive any session or association

   between the PCE and PCC
 - the processing cost at the PCE (or PCC) associated with
   request/response messages (as distinct from processing the
   computation requests themselves)

5.1.14. Extensibility

 The PCECP MUST provide a way for the introduction of new path
 computation constraints, diversity types, objective functions,
 optimization methods and parameters, and so on, without requiring
 major modifications in the protocol.
 For example, the PCECP MUST be extensible to support various PCE-
 based applications, such as the following:
  1. intra-area path computation
  2. inter-area path computation [PCECP-INTER-AREA]
  3. inter-AS intra provider and inter-AS inter-provider path

computation [PCECP-INTER-AS]

  1. inter-layer path computation [PCECP-INTER-LAYER]
 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.
 The PCECP SHOULD also be extensible to support future applications
 not currently in the scope of the PCE working group, such as, for
 instance, point-to-multipoint path computations, multi-hop pseudowire
 path computation, etc.
 Note that application specific requirements are out of the scope of
 this document and will be addressed in separate requirements
 documents.

5.1.15. Scalability

 The PCECP MUST scale well, at least as good as linearly, with an
 increase of any of the following parameters.  Minimum order of
 magnitude estimates of what the PCECP should support are given in
 parenthesis (note: these are requirements on the PCECP, not on the
 PCE):

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  1. number of PCCs (1000/domain)
  2. number of PCEs (100/domain)
  3. number of PCCs communicating with a single PCE (1000)
  4. number of PCEs communicated to by a single PCC (100)
  5. number of domains (20)
  6. number of path request messages (average of 10/second/PCE)
  7. handling bursts of requests (burst of 100/second/PCE within a 10-

second interval).

 Note that path requests can be bundled in path request messages, for
 example, 10 PCECP request messages/second may correspond to 100 path
 requests/second.
 Bursts of requests may arise, for example, after a network outage
 when multiple recomputations are requested.  The PCECP MUST handle
 the congestion in a graceful way so that it does not unduly impact
 the rest of the network, and so that it does not gate the ability of
 the PCE to perform computation.

5.1.16. Constraints

 This section provides a list of generic constraints that MUST be
 supported by the PCECP.  Other constraints may be added to service
 specific applications as identified by separate application-specific
 requirements documents.  Note that the provisions of Section 5.1.14
 mean that new constraints can be added to this list without impacting
 the protocol to a level that requires major protocol changes.
 The set of supported generic constraints MUST include at least the
 following:
 o MPLS-TE and GMPLS generic constraints:
   - Bandwidth
   - Affinities inclusion/exclusion
   - Link, Node, Shared Risk Link Group (SRLG) inclusion/exclusion
   - Maximum end-to-end IGP metric
   - Maximum hop count
   - Maximum end-to-end TE metric
   - Degree of paths disjointness (Link, Node, SRLG)
 o MPLS-TE specific constraints
   - Class-type
   - Local protection
   - Node protection
   - Bandwidth protection

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 o GMPLS specific constraints
   - Switching type, encoding type
   - Link protection type

5.1.17. Objective Functions Supported

 This section provides a list of generic objective functions that MUST
 be supported by the PCECP.  Other objective functions MAY be added to
 service specific applications as identified by separate application-
 specific requirements documents.  Note that the provisions of Section
 5.1.14 mean that new objective functions MAY be added to this list
 without impacting the protocol.
 The PCECP MUST support at least the following "unsynchronized"
 functions:
  1. Minimum cost path with respect to a specified metric

(shortest path)

  1. Least loaded path
  2. Maximum available bandwidth path
 Also, the PCECP MUST support at least the following "synchronized"
 objective functions:
  1. Minimize aggregate bandwidth consumption on all links
  2. Maximize the residual bandwidth on the most loaded link
  3. Minimize the cumulative cost of a set of diverse paths

5.2. Deployment Support Requirements

5.2.1. Support for Different Service Provider Environments

 The PCECP must at least support the following environments:
  1. MPLS-TE and GMPLS networks
  2. Packet and non-packet networks
  3. Centralized and distributed PCE path computation
  4. Single and multiple PCE path computation
 For example, PCECP is possibly applicable to packet networks (e.g.,
 IP networks), non-packet networks (e.g., time-division multiplexed
 (TDM) transport), and perhaps to multi-layer GMPLS control plane
 environments.  Definitions of centralized, distributed, single, and
 multiple PCE path computation can be found in [RFC4655].

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5.2.2. Policy Support

 The PCECP MUST allow for the use of policies to accept/reject
 requests.  It MUST include the ability for a PCE to supply sufficient
 detail when it rejects a request for policy reasons to allow the PCC
 to determine the reason for rejection or failure.  For example,
 filtering could be required for a PCE that serves one domain (perhaps
 an AS) such that all requests that come from another domain (AS) are
 rejected.  However, specific policy details are left to application-
 specific PCECP requirements.  Actual policies, configuration of
 policies, and applicability of policies are out of scope.
 Note that work on supported policy models and the corresponding
 requirements/implications is being undertaken as a separate work item
 in the PCE working group.
 PCECP messages MUST be able to carry transparent policy information.

5.3. Aliveness Detection & Recovery Requirements

5.3.1. Aliveness Detection

 The PCECP MUST allow a PCC/PCE to
  1. check the liveliness of the PCC-PCE communication,
  2. rapidly detect PCC-PCE communication failure (indifferently to

partner failure or connectivity failure), and

  1. distinguish PCC/PCE node failures from PCC-PCE connectivity

failures, after the PCC-PCE communication is recovered.

 The aliveness detection mechanism MUST ensure reciprocal knowledge of
 PCE and PCC liveness.

5.3.2. Protocol Recovery

 In the event of the failure of a sender or of the communication
 channel, the PCECP, upon recovery, MUST support resynchronization of
 information (e.g., PCE congestion status) and requests between the
 sender and the receiver; this SHOULD be arranged so as to minimize
 repeat data transfer.

5.3.3. LSP Rerouting & Reoptimization

 If an LSP fails owing to the failure of a link or node that it
 traverses, a new computation request may be made to a PCE in order to
 repair the LSP.  Since the PCC cannot know that the PCE's TED has
 been updated to reflect the failure network information, it is useful
 to include this information in the new path computation request.

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 Also, in order to re-use the resources used by the old LSP, it may be
 advantageous to indicate the route of the old LSP as part of the new
 path computation request.
 Hence the path computation request message MUST allow an indication
 of whether the computation is for LSP restoration, and it MUST
 support the inclusion of the previously computed path as well as the
 identity of the failed element.  Note that the old path might only be
 useful if the old LSP has not yet been torn down.  The PCE MAY choose
 to take failure indication information carried in a given request
 into account when handling subsequent requests.  This should be
 driven by local policy decision.
 Note that a network failure may impact a large number of LSPs.  In
 this case, a potentially large number of PCCs will simultaneously
 send requests to the PCE.  The PCECP MUST properly handle such
 overload situations, such as, for instance, through throttling of
 requests as set forth in Section 5.1.8.
 The path computation request message MUST support TE LSP path
 reoptimization and the inclusion of a previously computed path.  This
 will help ensure optimal routing of a reoptimized path, since it will
 allow the PCE to avoid double bandwidth accounting and help reduce
 blocking issues.

6. Security Considerations

 Key management MUST be provided by the PCECP to provide for the
 authenticity and integrity of PCECP messages.  This will allow
 protecting against PCE or PCC impersonation and also against message
 content falsification.
 The impact of the use of a PCECP MUST be considered in light of the
 impact that it has on the security of the existing routing and
 signaling protocols and techniques in use within the network.
 Intra-domain security is impacted since there is a new interface,
 protocol, and element in the network.  Any host in the network could
 impersonate a PCC and receive detailed information on network paths.
 Any host could also impersonate a PCE, both gathering information
 about the network before passing the request on to a real PCE and
 spoofing responses.  Some protection here depends on the security of
 the PCE discovery process (see [PCE-DISC-REQ]).  An increase in
 inter-domain information flows may increase the vulnerability to
 security attacks, and the facilitation of inter-domain paths may
 increase the impact of these security attacks.
 Of particular relevance are the implications for confidentiality
 inherent in a PCECP for multi-domain networks.  It is not necessarily

Ash & Le Roux Informational [Page 15] RFC 4657 PCE Communication Protocol Generic Reqmnts September 2006

 the case that a multi-domain PCE solution will compromise security,
 but solutions MUST examine their impacts in this area.
 Applicability statements for particular combinations of signaling,
 routing, and path computation techniques are expected to contain
 detailed security sections.
 It should be observed that the use of an external PCE introduces
 additional security issues.  Most notable among these are the
 following:
  1. Interception of PCE requests or responses
  2. Impersonation of PCE or PCC
  3. DoS attacks on PCEs or PCCs
 The PCECP MUST address these issues in detail using authentication,
 encryption, and DoS protection techniques.  See also Section 5.1.9.
 There are security implications of allowing arbitrary objective
 functions, as discussed in Section 5.1.17, and the PCECP MUST allow
 mitigating the risk of, for example, a PCC using complex objectives
 to intentionally drive a PCE into resource exhaustion.

7. Manageability Considerations

 Manageability of the PCECP MUST address the following considerations:
  1. The need for a MIB module for control and monitoring of PCECP
  2. The need for built-in diagnostic tools to test the operation of the

protocol (e.g., partner failure detection, Operations

   Administration and Maintenance (OAM), etc.)
 - Configuration implications for the protocol
 PCECP operations MUST be modeled and controlled through appropriate
 MIB modules.  There are enough specific differences between PCCs and
 PCEs to lead to the need of defining separate MIB modules.
 Statistics gathering will form an important part of the operation of
 the PCECP.  The MIB modules MUST provide information that will allow
 an operator to determine PCECP historical interactions and the
 success rate of requests.  Similarly, it is important for an operator
 to be able to determine PCECP and PCE load and whether an individual
 PCC is responsible for a disproportionate amount of the load.  It
 MUST be possible, through use of MIB modules, to record and inspect
 statistics about the PCECP communications, including issues such as
 malformed messages, unauthorized messages, and messages discarded
 owing to congestion.

Ash & Le Roux Informational [Page 16] RFC 4657 PCE Communication Protocol Generic Reqmnts September 2006

 The new MIB modules should also be used to provide notifications
 (traps) when thresholds are crossed or when important events occur.
 For example, the MIB module may support indication of exceeding the
 congestion state threshold or rate limitation state.
 PCECP techniques must enable a PCC to determine the liveness of a PCE
 both before it sends a request and in the period between sending a
 request and receiving a response.
 It is also important for a PCE to know about the liveness of PCCs to
 gain a predictive view of the likely loading of a PCE in the future
 and to allow a PCE to abandon processing of a received request.
 The PCECP MUST support indication of congestion state and rate
 limitation state, and MAY allow the operator to control such a
 function.

8. Contributors

 This document is the result of the PCE Working Group PCECP
 requirements design team joint effort.  In addition to the
 authors/editors listed in the "Authors' Addresses" section, the
 following are the design team members who contributed to the
 document:
 Alia K.  Atlas
 Google Inc.
 1600 Amphitheatre Parkway
 Mountain View, CA  94043 USA
 EMail: akatlas@alum.mit.edu
 Arthi Ayyangar
 Nuova Systems,
 2600 San Tomas Expressway
 Santa Clara, CA 95051
 EMail: arthi@nuovasystems.com
 Nabil Bitar
 Verizon
 40 Sylvan Road
 Waltham, MA 02145 USA
 EMail: nabil.bitar@verizon.com
 Igor Bryskin
 Independent Consultant
 EMail: i_bryskin@yahoo.com

Ash & Le Roux Informational [Page 17] RFC 4657 PCE Communication Protocol Generic Reqmnts September 2006

 Dean Cheng
 Cisco Systems, Inc.
 3700 Cisco Way
 San Jose CA 95134 USA
 Phone:  408 527 0677
 EMail: dcheng@cisco.com
 Durga Gangisetti
 MCI
 EMail: durga.gangisetti@mci.com
 Kenji Kumaki
 KDDI Corporation
 Garden Air Tower
 Iidabashi, Chiyoda-ku,
 Tokyo 102-8460, JAPAN
 Phone: 3-6678-3103
 EMail: ke-kumaki@kddi.com
 Eiji Oki
 NTT
 Midori-cho 3-9-11
 Musashino-shi, Tokyo 180-8585, JAPAN
 EMail: oki.eiji@lab.ntt.co.jp
 Raymond Zhang
 BT INFONET Services Corporation
 2160 E. Grand Ave.
 El Segundo, CA 90245 USA
 EMail: Raymond_zhang@bt.infonet.com

9. Acknowledgements

 The authors would like to extend their warmest thanks to (in
 alphabetical order) Lou Berger, Ross Callon, Adrian Farrel, Thomas
 Morin, Dimitri Papadimitriou, Robert Sparks, and J.P. Vasseur for
 their review and suggestions.

Ash & Le Roux Informational [Page 18] RFC 4657 PCE Communication Protocol Generic Reqmnts September 2006

10. References

10.1. Normative References

 [RFC2119]           Bradner, S., "Key words for use in RFCs to
                     Indicate Requirement Levels", BCP 14, RFC 2119,
                     March 1997.
 [RFC4655]           Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
                     Computation Element (PCE)-Based Architecture",
                     RFC 4655, August 2006.

10.2. Informative References

 [METRIC]            Le Faucheur, F., Uppili, R., Vedrenne, A.,
                     Merckx, P., and T. Telkamp, "Use of Interior
                     Gateway Protocol (IGP) Metric as a second MPLS
                     Traffic Engineering (TE) Metric", BCP 87, RFC
                     3785, May 2004.
 [PCE-DISC-REQ]      Le Roux, J.L., et al., "Requirements for Path
                     Computation Element (PCE) Discovery", Work in
                     Progress.
 [PCECP-INTER-AREA]  Le Roux, J.L., et al., "PCE Communication
                     Protocol (PCECP) specific requirements for
                     Inter-Area (G)MPLS Traffic Engineering", Work in
                     Progress.
 [PCECP-INTER-LAYER] Oki, E., et al., "PCC-PCE Communication
                     Requirements for Inter-Layer Traffic
                     Engineering", Work in Progress.
 [PCECP-INTER-AS]    Bitar, N., Zhang, R., Kumaki, K., "Inter-AS
                     Requirements for the Path Computation Element
                     Communication Protocol (PCECP)", Work in
                     Progress.
 [RFC3209]           Awduche, D., Berger, L., Gan, D., Li, T.,
                     Srinivasan, V., and G. Swallow, "RSVP-TE:
                     Extensions to RSVP for LSP Tunnels", RFC 3209,
                     December 2001.
 [RFC3127]           Mitton, D., St.Johns, M., Barkley, S., Nelson,
                     D., Patil, B., Stevens, M., and B. Wolff,
                     "Authentication, Authorization, and Accounting:
                     Protocol Evaluation", RFC 3127, June 2001.

Ash & Le Roux Informational [Page 19] RFC 4657 PCE Communication Protocol Generic Reqmnts September 2006

Authors' Addresses

 Jerry Ash (Editor)
 AT&T
 Room MT D5-2A01
 200 Laurel Avenue
 Middletown, NJ 07748, USA
 Phone: (732)-420-4578
 EMail: gash@att.com
 Jean-Louis Le Roux (Editor)
 France Telecom
 2, avenue Pierre-Marzin
 22307 Lannion Cedex, FRANCE
 EMail: jeanlouis.leroux@orange-ft.com

Ash & Le Roux Informational [Page 20] RFC 4657 PCE Communication Protocol Generic Reqmnts September 2006

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Ash & Le Roux Informational [Page 21]

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