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Versions: (draft-ash-pce-comm-protocol-gen-reqs) 00 01 02 03 04 05 06 07 RFC 4657

IETF Internet Draft PCE Working Group                 Jerry Ash (AT&T)
Proposed Status: Informational                                  Editor
Expires: August 2006                     J.L. Le Roux (France Telecom)
                                                                Editor

                                                         February 2006


           draft-ietf-pce-comm-protocol-gen-reqs-04.txt

         PCE Communication Protocol Generic Requirements


Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on August 1, 2006.

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.

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Table of Contents

1. Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions used in this document . . . . . . . . . . . . . . . . 3
3. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Overview of PCE Communication Protocol (PCECP)  . . . . . . . . . 3
6. PCE Communication Protocol Generic Requirements . . . . . . . . . 4
   6.1 Basic Protocol Requirements . . . . . . . . . . . . . . . . . 5
       6.1.1 Commonality of PCC-PCE and PCE-PCE Communication  . . . 5
       6.1.2 Client-Server Communication . . . . . . . . . . . . . . 5
       6.1.3 Transport . . . . . . . . . . . . . . . . . . . . . . . 5
       6.1.4 Path Computation Requests . . . . . . . . . . . . . . . 6
       6.1.5 Path Computation Responses  . . . . . . . . . . . . . . 7
       6.1.6 Cancellation of Pending Requests  . . . . . . . . . . . 8
       6.1.7 Multiple Requests and Responses . . . . . . . . . . . . 8
       6.1.8 Reliable Message Exchange . . . . . . . . . . . . . . . 8
       6.1.9 Secure Message Exchange . . . . . . . . . . . . . . . . 9
       6.1.10 Request Prioritization . . . . . . . . . . . . . . . . 9
       6.1.11 Unsolicited Notifications  . . . . . . . . . . . . . . 10
       6.1.12 Asynchronous Communication . . . . . . . . . . . . . . 10
       6.1.13 Communication Overhead Minimization  . . . . . . . . . 10
       6.1.14 Extensibility  . . . . . . . . . . . . . . . . . . . . 10
       6.1.15 Scalability  . . . . . . . . . . . . . . . . . . . . . 11
       6.1.16 Constraints  . . . . . . . . . . . . . . . . . . . . . 11
       6.1.17 Objective Functions Supported  . . . . . . . . . . . . 12
   6.2 Deployment Support Requirements . . . . . . . . . . . . . . . 13
       6.2.1 Support for Different Service Provider Environments . . 13
       6.2.2 Policy Support  . . . . . . . . . . . . . . . . . . . . 13
   6.3 Aliveness Detection & Recovery Requirements . . . . . . . . . 13
       6.3.1 Aliveness Detection . . . . . . . . . . . . . . . . . . 13
       6.3.2 Protocol Recovery . . . . . . . . . . . . . . . . . . . 14
       6.3.3 LSP Rerouting & Reoptimization  . . . . . . . . . . . . 14
   6.4 Requirements Summary  . . . . . . . . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . . . 17
8. Manageability Considerations  . . . . . . . . . . . . . . . . . . 17
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . . 18
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 18
11. Normative References . . . . . . . . . . . . . . . . . . . . . . 18
12. Informational References . . . . . . . . . . . . . . . . . . . . 18
13. Authors' & Contributors' Addresses . . . . . . . . . . . . . . . 19
Intellectual Property Statement  . . . . . . . . . . . . . . . . . . 20
Disclaimer of Validity . . . . . . . . . . . . . . . . . . . . . . . 21
Copyright Statement  . . . . . . . . . . . . . . . . . . . . . . . . 21

1. Contributors

   This document is the result of the PCE Working Group PCE
   Communication Protocol (PCECP) requirements design team joint effort.
   The following are the design team member authors that contributed to

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   the present document:

   Jerry Ash (AT&T)
   Alia Atlas (Google, Inc.)
   Arthi Ayyangar (Juniper)
   Nabil Bitar (Verizon)
   Igor Bryskin (Independent Consultant)
   Dean Cheng (Cisco)
   Durga Gangisetti (MCI)
   Kenji Kumaki (KDDI)
   Jean-Louis Le Roux (France Telecom)
   Eiji Oki (NTT)
   Raymond Zhang (BT Infonet)

2. Conventions used in this document

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

3. Introduction

   A Path Computation Element (PCE) [PCE-ARCH] 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, a set of constraints to be applied during the
   computation, and may also include an objective function.  The PCE
   response includes the computed paths or the reason for a failed
   computation.

   This document lists a set of generic requirements for the 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.

4. Terminology

   Domain: any collection of network elements within a common sphere of
   address management or path computational responsibility.  Examples of
   domains include IGP areas, Autonomous Systems (ASs), multiple ASs

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   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 [PCE-ARCH]).

   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 [PCE-ARCH] for further definitions of terms.

5. 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 that a PCC and PCE can 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 6.


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   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 [PCE-ARCH], 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.

   [PCE-ARCH] 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 the [PCE-ARCH].

6. PCE Communication Protocol Generic Requirements

   Section 6.4 contains a summary of the requirements in this section.

6.1 Basic Protocol Requirements

6.1.1 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 as 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.

6.1.2 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 6.1.11).

6.1.3 Transport

   The PCECP may utilize an existing transport protocol or operate
   directly over IP.

   If a transport protocol is used, it MAY be used to satisfy some
   requirements stated in other sections of this document (for example,
   reliability and security). Where requirements expressed in this

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   document match the function of existing transport protocols,
   consideration MUST be given to the use of those protocols.

   If a transport protocol is used, it MUST NOT limit the size of the
   message used by the PCECP.

6.1.4 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).

   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 6.1.16. Specification
   of constraints MUST be future-proofed as described in Section 6.1.14.

   The requester MUST be allowed to select or prefer from 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 6.1.17. Specification of objective
   functions MUST be future-proofed as described in Section 6.1.14.

   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 6.1.14.

   A request message MAY include TE parameters carried by the MPLS/GMPLS
   LSP setup signaling protocol.  Also, it MUST be possible for the PCE

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   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.

   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 or not.  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.

6.1.5 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 [RFC 3209] 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

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   respective bandwidths.

6.1.6 Cancellation of Pending Requests

   A PCC MUST be able to cancel a pending request using a notification
   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
   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.

6.1.7 Multiple Requests and Responses

   It MUST be possible to send multiple path computation requests
   within the same request message. Such requests may be correlated (for
   example, 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.

6.1.8 Reliable Message Exchange

   The PCECP MUST include reliability. This may form part of the
   protocol itself or may be achieved by the selection of a suitable
   transport protocol (see Section 6.1.3).

   In particular, it MUST allow for the detection and recovery of lost

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   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
   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 MUST provide:

   - Detection and report of lost or corrupted messages
   - Automatic attempts to retransmit lost messages without reference to
     the application
   - Handling of out-of-order messages
   - Handling of duplicate messages
   - Flow control and back-pressure to enable throttling of requests and
     responses
   - Rapid PCECP communication failure detection
   - 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.

6.1.9 Secure Message Exchange

   The PCC-PCE communication protocol MUST include provisions to insure
   the security of the exchanges between the entities.  In particular,
   it MUST support mechanisms to prevent spoofing (e.g.,
   authentication), snooping (e.g., encryption) and DOS attacks (e.g.,
   rate limiting, no promiscuous listening).

   This function may be provided by the transport protocol or directly
   by the PCECP.

   See Section 7 for further discussion of security considerations.

6.1.10 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.


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6.1.11 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.

6.1.12 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 6.1.10). A consequent requirement is that path
   computation responses MUST include a direct correlation to the
   associated request.

6.1.13 Communication Overhead Minimization

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

   - 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).

6.1.14 Extensibility

   The PCECP MUST provide a way for the introduction of new path
   computation constraints, diversity types, objective functions,
   optimization methods and parameters, etc., without requiring
   major modifications in the protocol.

   The PCECP MUST be easily extensible to support various PCE based
   applications that have been currently identified including:

   - intra-area path computation [PCECP-INTER-AREA]
   - inter-area path computation
   - inter-AS intra provider and inter-AS inter-provider path
     computation

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   - inter-layer path computation [PCECP-MULTI-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.

6.1.15 Scalability

   The PCECP MUST scale well, at least as good as linearly, with an
   increase of any of the following parameters (note, minimum order of
   magnitude estimates of what the PCECP should support are given in
   parenthesis):

   - number of PCCs (1000/domain)
   - number of PCEs (100/domain)
   - number of PCCs communicating with a single PCE (1000)
   - number of PCEs communicated to by a single PCC (100)
   - number of domains (20)
   - number of path request messages (average of 10/second/PCE)
   - 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.

6.1.16 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 absence of a constraint in this list does not mean that
   the constraint must not be supported.  Note also that the provisions

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   of Section 6.1.14 mean that new constraints can be added to this list
   without impacting the protocol to a level that requires major
   protocol changes.

   Here is the list of generic constraints that MUST be supported:

   o MPLS-TE and GMPLS generic constraints:
     - Bandwidth
     - Affinities inclusion/exclusion
     - Link, Node, SRLG inclusion/exclusion
     - Maximum end-to-end IGP metric
     - Maximum Hop Count
     - Maximum end-to-end TE metric
     - Degree of paths disjointess (Link, Node, SRLG)

   o MPLS-TE specific constraints
     - Class-type
     - Local protection
     - Node protection
     - Bandwidth protection

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

6.1.17 Objective Functions Supported

   This section provides a list of generic objective functions that MUST
   be supported by the PCECP.  Other objectives functions MAY be added
   to service specific applications as identified by separate
   application-specific requirements documents.

   Note that the absence of an objective function in this list does not
   mean that the objective function may not be supported.  Note also
   that the provisions of Section 6.1.14 mean that new objective
   functions MAY be added to this list without impacting the protocol.

   The PCECP MUST support the following "unsynchronized" objective
   functions:

   - Minimum cost path with respect to a specified metric(shortest path)
   - Least loaded path
   - Maximum available bandwidth path

   Also the PCECP MUST support the following "synchronized" objective
   functions:

   - Minimize aggregate bandwidth consumption on all links
   - Maximize the residual bandwidth on the most loaded link
   - Minimize the cumulative cost of a set of diverse paths.


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6.2 Deployment Support Requirements

6.2.1 Support for Different Service Provider Environments

   The PCECP MUST operate in various different service provider network
   environments that utilize an IP-based control plane, including

   - MPLS-TE and GMPLS networks
   - packet and non-packet networks
   - centralized and distributed PCE path computation
   - single and multiple PCE path computation

   Definitions of centralized, distributed, single, and multiple PCE
   path computation can be found in [PCE-ARCH].

6.2.2 Policy Support

   The PCECP MUST allow for the use of policies to accept/reject
   requests, and 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.

6.3 Aliveness Detection & Recovery Requirements

6.3.1 Aliveness Detection

   The PCECP MUST allow a PCC to

   - check the liveliness of the PCC-PCE communication
   - rapidly detect PCC-PCE communication failure (indifferently to
     partner failure or connectivity failure),
   - 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.


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6.3.2 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 and requests between the sender and the receiver, and
   this SHOULD be arranged so as to minimize repeat data transfer.

6.3.3 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. 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 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.

   Note that a network failure may impact a large number of LSPs. In
   this case, a potentially large number of PCCs is going to
   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 6.1.8.

   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.

6.4 Requirements Summary

   The following is a summary of the requirements in Section 6:

   Requirement                                       Necessity  Ref.
   ------------------------------------------------------------------
   Commonality of PCC-PCE and PCE-PCE communication  MUST       6.1.1
   Client-server communication                       MUST       6.1.2
   Support PCC/PCE request message to request path
     computation                                     MUST       6.1.2
   Support PCE response message with computed path   MUST       6.1.2
   Support unsolicited communication PCE-PCC         SHOULD     6.1.2
   Maintain PCC-PCE session                          NON-RQMT   6.1.2
   Use of existing transport protocol                MAY        6.1.3

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   Transport protocol satisfy reliability & security
     requirements                                    MAY        6.1.3
   Transport protocol limits size of message         MUST NOT   6.1.3
   Support path computation requests                 MUST       6.1.4
   Path computation request includes source &
     destination                                     MUST       6.1.4
   Support path constraints (e.g., bandwidth, hops,
     affinities) to include/exclude                  MUST       6.1.4
   Allow to select/prefer from advertised list of
     standard objective functions/options            MUST       6.1.4
   Allow to customize objective function/options     MUST       6.1.4
   Allow indicating the metric type (IGP or TE) to
     be used for shortest path selection             MUST       6.1.4
   Allow indicating the set of path attributes
     required in response message                    MUST       6.1.4
   Allow indicating if load-balancing is allowed     MUST       6.1.4
   Support path computation responses                MUST       6.1.5
   Negative response support reasons for failure,
     constraints to relax to achieve positive result SHOULD     6.1.5
   Support inclusion of set of path attributes       MUST       6.1.5
   Support inclusion of set of computed paths of a
     load-balancing path group, as well as their
     respective bandwidth                            MUST       6.1.5
   Cancellation of pending requests                  MUST       6.1.6
   Multiple requests and responses                   MUST       6.1.7
   Limit by configuration number of requests within
     a message                                       MUST       6.1.7
   Support multiple computed paths in response       MUST       6.1.7
   Support "continuation correlation" where related
     requests or computed paths cannot fit within
     one message                                     MUST       6.1.7
   Maximum message size & maximum number of requests
     per message exchanged through PCE messages to
     PCC, or indicated in request message            MAY        6.1.7
   Reliable message exchange (achieved by PCECP
     itself or transport protocol)                   MUST       6.1.8
   Allow detection & recovery of lost messages to
     occur quickly & not impede operation of PCECP   MUST       6.1.8
   Handle overload situations without significant
     decrease in performance, e.g., through
     throttling of requests                          MUST       6.1.8
   Detect/report lost/corrupted messages, retransmit
     lost messages, handle out-of-order messages &
     duplicate messages, provide flow control/
     back-pressure to throttle messages, detect
     PCECP communication failure detection           MUST       6.1.8
   Functionality added to PCECP if transport
     protocol provides it                            SHOULD NOT 6.1.8
   Secure message exchange (provided by PCECP or
     transport protocol                              MUST       6.1.9

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   Support mechanisms to prevent spoofing (e.g.,
     authentication), snooping (e.g., encryption),
     DOS attacks                                     MUST       6.1.9
   Request prioritization                            MUST       6.1.10
   Unsolicited notifications                         SHOULD     6.1.11
   Allow asynchronous communication                  MUST       6.1.12
   PCC has to wait for response before making
     another request                                 MUST NOT   6.1.12
   Allow order of responses differ from order of
     requests                                        MUST       6.1.12
   Communication overhead minimization               SHOULD     6.1.13
   Give particular attention to message size         SHOULD     6.1.13
   Extensibility without requiring modifications to
     protocol                                        MUST       6.1.14
   Easily extensible to support intra-area,
     inter-area, inter-AS intra provider, inter-AS
     inter-provider, multi-layer path & virtual
     network topology path computation               MUST       6.1.14
   Easily extensible to support future applications
     not in scope (e.g., point-to-multipoint path
     computations)                                   SHOULD     6.1.14
   Scale at least linearly with number of PCCs,
     PCEs, PCCs communicating with single PCE, PCEs
     communicated to by single PCC, domains, path
     requests, handling bursts of requests           MUST       6.1.15
   Support path computation constraints              MUST       6.1.16
   Support "unsynchronized" & "synchronized"
     objective functions                             MUST       6.1.17
   Support different service provider environments
     (e.g., MPLS-TE and GMPLS networks, centralized
     & distributed PCE path computation, single &
     multiple PCE path computation)                  MUST       6.2.1
   Policy support for policies to accept/reject
     requests, PCC to determine reason for
     rejection, notification of policy violation     MUST       6.2.2
   Aliveness detection of PCCs/PCEs, PCECP failure
     detection                                       MUST       6.3.1
   Protocol recovery support resynchronization of
     information & requests between sender &
     receiver                                        MUST       6.3.2
   Minimize repeat data transfer, allow PCE to
     respond to computation requests issued before
     failure without requests being re-issued        SHOULD     6.3.2
   Stateful PCE able to resynchronize/recover
     states (e.g., LSP status, paths) after restart  SHOULD     6.3.2
   Allow indicating if computation is for LSP
     restoration (support inclusion of previously
     computed path & failed element)                 MUST       6.3.3
   Support path reoptimization & inclusion of a
     previously computed path                        MUST       6.3.3


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7. Security Considerations

   The impact of the use of a PCECP MUST be considered in the 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
   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 amongst these are:

   - interception of PCE requests or responses
   - impersonation of PCE or PCC
   - DoS attacks on PCEs or PCCs

   The PCECP MUST address these issues in detail using authentication,
   encryption and DoS protection techniques.  See also Section 6.1.9.

8. Manageability Considerations

   Manageability of the PCECP MUST address the following considerations:

   - the need for a MIB module for control and monitoring of PCECP
   - the need for built-in diagnostic tools to test the operation of the
     protocol (e.g., partner failure detection, OAM, etc.)
   - configuration implications for the protocol

   PCECP operations MUST be modeled and controlled through appropriate
   MIB modules.  Statistics gathering will form an important part of the
   operation of the PCECP.  The operator MUST be able to determine PCECP
   historical interactions and the success rate of requests using data
   from MIB modules.  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

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   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.

   The new MIB modules should also be used to provide notifications
   (traps) when thresholds are crossed or when important events occur.

   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.

9. IANA Considerations

   This document makes no requests for IANA action.

10. Acknowledgements

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

11. Normative References

   [PCE-ARCH] Farrel, A., Vasseur, JP, Ash, J., "Path Computation
   Element (PCE) Architecture", work in progress.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Levels", BCP 14, RFC 2119, March 1997.

12. Informational References

   [METRIC] Le Faucheur, F., et. al., "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, JL, et. al., "Requirements for Path
   Computation Element (PCE) Discovery," work in progress.

   [RFC3209] Awduche, D., et. al., "RSVP-TE: Extensions to RSVP for LSP
   Tunnels," RFC 3209, December 2001.


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   [PCE-INTER-AREA]  Le Roux, JL, et. al., "PCE Communication Protocol
   (PCECP) specific requirements for Inter-Area (G)MPLS Traffic
   Engineering," work in progress.

   [PCE-INTER-LAYER] Oki, E., et. al., "PCC-PCE Communication
   Requirements for Inter-Layer Traffic Engineering," work in progress.

13. Authors' & Contributors' 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@francetelecom.com

   Alia K. Atlas
   Google Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   Email: akatlas@alum.mit.edu

   Arthi Ayyangar
   Juniper Networks, Inc.
   1194 N.Mathilda Ave
   Sunnyvale, CA 94089 USA
   Email: arthi@juniper.net

   Nabil Bitar
   Verizon
   40 Sylvan Road
   Waltham, MA 02145
   Email: nabil.bitar@verizon.com

   Igor Bryskin
   Independent Consultant
   Email: i_bryskin@yahoo.com

   Dean Cheng
   Cisco Systems Inc.
   3700 Cisco Way
   San Jose CA 95134 USA
   Phone:  408 527 0677
   Email: dcheng@cisco.com

PCE Design Team <draft-ietf-pce-comm-protocol-gen-reqs-04.txt> [Page 19]

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   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

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