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Versions: (draft-leroux-pce-discovery-reqs) 00 01 02 03 04 05 RFC 4674

Network Working Group                              J.L. Le Roux (Editor)
Internet Draft                                            France Telecom
Category: Informational
Expires: December 2006



                                                               June 2006


        Requirements for Path Computation Element (PCE) Discovery

               draft-ietf-pce-discovery-reqs-05.txt


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Abstract

   This document presents a set of requirements for a Path Computation
   Element (PCE) discovery mechanism that would allow a Path Computation
   Client (PCC) to discover dynamically and automatically a set of PCEs
   along with certain information relevant for PCE selection. It is
   intended that solutions that specify procedures and protocols or
   extensions to existing protocols for such PCE discovery satisfy these
   requirements.





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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.

Table of Contents

   1.      Contributors................................................3
   2.      Terminology.................................................3
   3.      Introduction................................................4
   4.      Problem Statement and Requirements Overview.................5
   4.1.    Problem Statement...........................................5
   4.2.    Requirements overview.......................................6
   5.      Example of application scenario.............................6
   6.      Detailed Requirements.......................................7
   6.1.    PCE Information to be disclosed.............................7
   6.1.1.  General PCE Information (Mandatory support).................8
   6.1.1.1.  Discovery of PCE Location.................................8
   6.1.1.2.  Discovery of PCE Domains and Inter-domain Functions.......8
   6.1.2.  Detailed PCE Information (Optional support).................9
   6.1.2.1.  Discovery of PCE Capabilities.............................9
   6.1.2.2.  Discovery of Alternate PCEs...............................9
   6.2.    Scope of PCE Discovery.....................................10
   6.2.1.  Inter-AS specific requirements.............................10
   6.3.    PCE Information Synchronization............................11
   6.4.    Discovery of PCE deactivation..............................11
   6.5.    Policy Support.............................................11
   6.6.    Security Requirements......................................12
   6.7.    Extensibility..............................................12
   6.8.    Scalability................................................12
   6.9.    Operational orders of magnitudes...........................13
   6.10.   Manageability considerations...............................13
   6.10.1.  Configuration of PCE Discovery parameters.................13
   6.10.2.  PCE Discovery MIB modules.................................14
   6.10.2.1.  PCC MIB module..........................................14
   6.10.2.2.  PCE MIB module..........................................14
   6.10.3.  Monitoring Protocol Operations............................15
   6.10.4.  Impact on network operations..............................15
   7.      Security Considerations....................................15
   8.      Acknowledgments............................................16
   9.      References.................................................16
   9.1.    Normative references.......................................16
   9.2.    Informative references.....................................16
   10.     Editor Address.............................................16
   11.     Contributors' Addresses....................................16
   12.     Intellectual Property Statement............................17






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1. Contributors

   The following are the authors that contributed to the present
   document:

   Jean-Louis Le Roux (France Telecom)
   Paul Mabey (Qwest Communications)
   Eiji Oki (NTT)
   Richard Rabbat (Fujitsu)
   Ting Wo Chung (Bell Canada)
   Raymond Zhang (BT Infonet)

2. Terminology

   Terminology used in this document

   LSR: Label Switch Router

   TE-LSP: Traffic Engineered Label Switched Path

   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.

   PCC: Path Computation Client: any client application requesting a
   path computation to be performed by a Path Computation Element.

   IGP Area: OSPF Area or ISIS level/area

   ABR: IGP Area Border Router (OSPF ABR or ISIS L1L2 router)

   AS: Autonomous System

   ASBR: AS Border Router

   Intra-area TE LSP: A TE LSP whose path does not cross IGP area
   boundaries.

   Inter-area TE LSP: A TE LSP whose path transits through two or more
   IGP areas.

   Inter-AS MPLS TE LSP: A TE LSP whose path transits through two or
   more ASs or sub-ASs (BGP confederations).

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





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3. Introduction

   The PCE-based network Architecture [PCE-ARCH] defines a Path
   Computation Element (PCE) as an entity capable of computing TE-LSP
   paths based on a network graph, and applying computational
   constraints. A PCE serves path computation requests sent by Path
   Computation Clients (PCC).
   A PCC is a client application requesting a path computation to be
   performed by a PCE. This can be, for instance, an LSR requesting a
   path for a TE-LSP for which it is the head-end, or a PCE requesting a
   path computation of another PCE (inter-PCE communication). The
   communication between a PCC and a PCE requires a client-server
   protocol whose generic requirements are listed in [PCE-COM-REQ].

   The PCE based architecture requires, that a PCC be aware of the
   location of one or more PCEs in its domain, and also potentially of
   some PCEs in other domains, e.g. in case of inter-domain path
   computation.

   In that context it would be highly desirable to define a mechanism
   for automatic and dynamic PCE discovery, which would allow PCCs to
   automatically discover a set of PCEs, to determine additional
   information required for PCE selection, and to dynamically detect new
   PCEs or any modification of the PCEs' information. This includes the
   discovery by a PCC of a set of one or more PCEs in its domain, and
   potentially in some other domains. The latter is a desirable function
   in the case of inter-domain path computation, for example.

   This document lists a set of functional requirements for such an
   automatic and dynamic PCE discovery mechanism. Section 4 points out
   the problem statement. Section 5 illustrates an application scenario.
   Finally, section 6 addresses detailed requirements.

   It is intended that solutions that specify procedures and protocols
   or protocol extensions for PCE discovery satisfy these requirements.
   There is no intent either to specify solution-specific requirements
   or to make any assumption on the protocols that could be used for the
   discovery.

   Note that requirements listed in this document apply equally to PCEs
   that are capable of computing paths in MPLS-TE-enabled networks and
   PCEs that are capable of computing paths in GMPLS-enabled networks
   (and PCEs capable of both).

   It is also important to note that the notion of a PCC encompasses a
   PCE acting as PCC when requesting a path computation of another PCE
   (inter-PCE communication). Hence, this document does not make the
   distinction between PCE discovery by PCCs and PCE discovery by PCEs.




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4. Problem Statement and Requirements Overview

4.1. Problem Statement

   A routing domain may, in practice, contain multiple PCEs:
   - The path computation load may be balanced among a set of PCEs
     to improve scalability;
   - For the purpose of redundancy, primary and backup PCEs may be
     used;
   - PCEs may have distinct path computation capabilities (multi-
     constrained path computation, backup path computation, etc.);
   - In an inter-domain context there can be several PCEs with
     distinct inter-domain functions (inter-area, inter-AS, inter-
     layer), each PCE being responsible for path computation in one or
     more domains.

   In order to allow for effective PCE selection by PCCs, that is to
   select the appropriate PCE based on its capabilities and perform
   efficient load balancing of requests, a PCC needs to know the
   location of PCEs in its domain, along with some information relevant
   to PCE selection, and also potentially needs to know the location of
   some PCEs in other domains, for inter-domain path computation
   purpose.
   Such PCE information could be learnt through manual configuration, on
   each PCC, of the set of PCEs along with their capabilities. Such a
   manual configuration approach may be sufficient, and even desired in
   some particular situations, (e.g. inter-AS PCE discovery, where
   manual configuration of neighbor PCEs may be preferred for security
   reasons), but it obviously faces several limitations:
   - This may imply a substantial configuration overhead;
   - This would not allow a PCC to dynamically detect that a new PCE is
     available, that an existing PCE is no longer available, or that
     there is a change in the PCE's information.

   Furthermore, as with any manual configuration approach, there is a
   risk of configuration errors.

   As an example, in a multi-area network made up of one backbone area
   and N peripheral areas, and where inter-area MPLS-TE path computation
   relies on multiple-PCE path computation with ABRs acting as PCEs, the
   backbone area would comprise at least N PCEs, and the configuration
   of PCC would be too cumbersome (e.g. in existing multi-area networks,
   N can be beyond fifty).

   Hence, an automated PCE discovery mechanism allowing a PCC to
   dynamically discover a set of PCEs is highly desirable.







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4.2. Requirements overview

   A PCE discovery mechanism that satisfies the requirements set forth
   in this document MUST allow a PCC to automatically discover the
   location of one or more of the PCEs in its domain.
   Where inter-domain path computation is required and policy permits,
   the PCE discovery method MUST allow a PCC to automatically discover
   the location of PCEs in other domains that can assist with inter-
   domain path computation.

   A PCE discovery mechanism MUST allow a PCC to discover the set of one
   or more domains where a PCE has TE topology visibility and can
   compute paths. It MUST also allow the discovery of the potential
   inter-domain path computation functions of a PCE (inter-area, inter-
   AS, inter-layer, etc.).

   A PCE discovery mechanism MUST allow the control of the discovery
   scope, that is the set of one or more domains (areas, ASs) where
   information related to a given PCE has to be disclosed.

   A PCE discovery mechanism MUST allow PCCs in a given discovery scope
   to dynamically discover that a new PCE has appeared or that there is
   a change in PCE's information.

   A PCE discovery mechanism MUST allow PCCs to dynamically discover
   that a PCE is no longer available.

   A PCE discovery MUST support security procedures. In particular, key
   consideration MUST be given in terms of how to establish a trust
   model for PCE discovery.

   OPTIONALLY a PCE discovery mechanism MAY be used so as to disclose a
   set of detailed PCE capabilities so that the PCC may make advanced
   and informed choices about which PCE to use.

5. Example of application scenario

   <----------------AS1-------------------->           <----AS2---
    Area 1           Area 0        Area 2
  R1---------R3-----R5-------R6-----------R9----------R11----R13
  |          |               |             |           |
  |          |               |             |           |
  R2---------R4-----R7-------R8-----------R10---------R12----R14
       |
       |
       --
      |S1|
       --

                          Figure 1



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   Figure 1 illustrates a multi-area/AS network with several PCEs:
   - The ABR R3 is a PCE that can take part in inter area path
     computation. It can compute paths in area 1 and area 0;
   - The ABR R6 is a PCE that can take part in inter-area path
     computation. It can compute paths in area 0 and area2;
   - The ASBR R9 is a PCE that can take part in inter-AS path
     computation. It is responsible for path computation in AS1 towards
     AS2;
   - The ASBR R12 is a PCE that can take part in inter-AS path
     computation. It is responsible for path computation in AS2 towards
     AS1;
   - The server S1 is a PCE that can be used to compute diverse paths
     and backup paths in area 1.

   By meeting the requirements set out in this document, the PCE
   discovery mechanism will allow:
   - each PCC in areas 1 and 0 to dynamically discover R3, as a PCE for
     inter-area path computation, and that R3 can compute paths in area0
     and area1;
   - each PCC in areas 0 and 2 to dynamically discover R6, as a PCE for
     inter-area path computation, and that R6 can compute paths in area2
     and area0;
   - each PCC in AS1 and one or more PCCs in AS2 to dynamically discover
     R9 as a PCE for inter-AS path computation in AS1 towards AS2;
   - each PCC in AS2 and one or more PCCs in AS1 to dynamically discover
     R12 as a PCE for inter-AS path computation in AS2 towards AS1;
   - each PCC in area 1 to dynamically discover S1, as a PCE for intra-
     area path computation in area1, and optionally to discover its path
     computation capabilities (diverse path computation and backup path
     computation).

6. Detailed Requirements

6.1. PCE Information to be disclosed

   We distinguish two levels of PCE information to be disclosed by a PCE
   discovery mechanism:
   - General information. Disclosure MUST be supported by the
     PCE discovery mechanism.
   - Detailed information. Disclosure MAY be supported by the
     PCE discovery mechanism.

   The PCE discovery mechanism MUST allow disclosure of general PCE
   information that will allow PCCs to select appropriate PCEs. This
   comprises discovery of PCE location, PCE domains supported by the
   PCEs, and PCE inter-domain functions.

   The PCE discovery mechanism MAY also allow disclosure of detailed PCE
   information. This comprises any or all information about PCE path
   computation capabilities and alternate PCEs. This information is not
   part of PCE discovery; this is additional information that can

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   facilitate the selection of a PCE by a PCC. Support of the exchange
   of this information is optional in the context of the PCE discovery
   mechanism itself. This does not mean that the availability of this
   information is optional in the PCE-based architecture, but such
   information could also be obtained by other mechanisms, such as the
   PCC-PCE communication protocol.

6.1.1. General PCE Information (Mandatory support)

6.1.1.1. Discovery of PCE Location

   The PCE discovery mechanism MUST allow the discovery, for a given
   PCE, of the IPv4 and/or IPv6 address to be used to reach the PCE.
   This address will typically be an address that is always reachable,
   if there is any connectivity to the PCE.

   This address will be used by PCCs to communicate with a PCE, through
   a PCC-PCE communication protocol.

6.1.1.2. Discovery of PCE Domains and Inter-domain Functions

   Inter-domain path computation is a key application of the PCE
   architecture. This can rely on a multiple-PCE path computation, where
   PCEs in each domain compute a part of the end-to-end path and
   collaborate with each other to find the end-to-end-path. Inter-domain
   path computation can also rely on a single-PCE path computation where
   a PCE has visibility inside multiple domains and can compute an
   entire end-to-end inter-domain path (that is a path from the inter-
   domain TE-LSP head-end to the inter-domain TE-LSP tail end).

   Hence the PCE discovery mechanism MUST allow the discovery of the set
   of one or more domains where a PCE has visibility and can compute
   paths. These domains could be identified using a domain identifier:
   For instance, an IGP area can be identified by the Area ID (OSPF or
   ISIS), and an AS can be identified by the AS number.

   Also the PCE discovery mechanism MUST allow discovery of the inter-
   domain functions of a PCE, i.e. whether a PCE can be used to compute
   or to take part in the computation of end-to-end paths across domain
   borders. The inter-domain functions include non exhaustively: inter-
   area, inter-AS and inter-layer path computation. Note that these
   functions are not mutually exclusive.

   Note that the inter-domain functions are not necessarily inferred
   from the set of domains where a PCE has visibility. For instance a
   PCE may have visibility limited to a single domain, but may be able
   to take part into the computation of inter-domain paths, by
   collaborating with PCEs in other domains. Conversely, a PCE may have
   visibility in multiple domains but the operator may not want that the
   PCE be used for inter-domain path computations.



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   The PCE discovery mechanisms MUST also allow discovery of the set of
   one or more domains toward which a PCE can compute paths. For
   instance in an inter-AS path computation context, there may be
   several PCEs in an AS, each one responsible for taking part in the
   computation of inter-AS paths toward a set of one or more destination
   ASs, and a PCC may have to discover the destination ASs each PCE is
   responsible for.

6.1.2. Detailed PCE Information (Optional support)

6.1.2.1. Discovery of PCE Capabilities

   In the case where there are several PCEs with distinct capabilities
   available, a PCC has to select one or more appropriate PCEs.

   For that purpose the PCE discovery mechanism MAY support the
   disclosure of some detailed PCE capabilities.

   For the sake of illustration this could include the following path
   computation related PCE capabilities:
   - The link constraints supported: e.g. bandwidth, affinities.
   - The path constraints supported: maximum IGP/TE cost, maximum hop
     count;
   - The objective functions supported: e.g. shortest path, widest path;
   - The capability to compute multiple correlated paths: e.g. diverse
     paths, load balanced paths;
   - The capability to compute bidirectional paths;
   - The GMPLS technology specific constraints supported: e.g. the
     supported interface switching capabilities, encoding types.

   And this could also include some specific PCE capabilities:
   - The capability to handle request prioritization;
   - The maximum size of a request message;
   - The maximum number of path requests in a request message;
   - The PCE computation power (static parameters to be used for
     weighted load balancing of requests).

   Such information regarding PCE capabilities could then be used by a
   PCC to select an appropriate PCE from a list of candidate PCEs.

   Note that the exact definition and description of PCE capabilities is
   out of the scope of this document. It is expected that this will be
   described in one or more separate documents which may be application
   specific.

6.1.2.2. Discovery of Alternate PCEs

   In the case of a PCE failure, a PCC has to select another PCE, if one
   is available. It could be useful in various situations, for a PCE to
   indicate a set of one or more alternate PCEs that can be selected in
   case the given PCE fails.


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   Hence the PCE Discovery mechanism MAY allow the discovery, for a
   given PCE, of the location of one or more assigned alternate PCEs.

   The PCE Discovery mechanism MAY also allow the discovery, for a given
   PCE, of the set of one or more PCEs for which it acts as alternate
   PCE.

6.2. Scope of PCE Discovery

   The PCE Discovery mechanism MUST allow control of the scope of the
   PCE information disclosure on a per PCE basis. In other words it MUST
   allow control of to which PCC or group of PCCs the information
   related to a PCE may be disclosed.

   The choice for the discovery scope of a given PCE MUST include at
   least the followings settings:

   - All PCCs in a single IGP area

   - All PCCs in a set of adjacent IGP areas

   - All PCCs in a single AS

   - All PCCs in a set of ASs

   - A set of one or more PCCs in a set of one or more ASs

   In particular, this also implies that the PCE Discovery mechanism
   MUST allow for the discovery of PCE information across IGP areas and
   across AS boundaries.

   The discovery scope MUST be configurable on a per PCE basis.

   It MUST be possible to deactivate PCE discovery on a per PCE basis.

6.2.1. Inter-AS specific requirements

   When using a PCE-based approach for inter-AS path computation, a PCC
   in one AS may need to learn information related to inter-AS capable
   PCEs located in other ASs. For that purpose, and as pointed out in
   the previous section, the PCE discovery mechanism MUST allow
   disclosure of information related to inter-AS capable PCEs across AS
   boundaries.

   Such inter-AS PCE discovery must be carefully controlled. For
   security and confidentiality reasons, particularly in an inter-
   provider context, the discovery mechanism MUST allow the discovery
   scope to be limited to a set of ASs and MUST also provide control of
   the PCE information to be disclosed across ASs. This is achieved by
   applying policies (See also section 6.4). This implies the capability
   to contain a PCE advertisement to a restricted set of one or more
   ASs, and to filter and translate any PCE parameter (PCE domains, PCE

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   inter-domain functions, PCE capabilities, etc.) in disclosures that
   cross AS borders. For the sake of illustration, it may be useful to
   disclose detailed PCE information (such as detailed capabilities)
   locally in the PCE's AS but only general information (such as
   location and supported domains) in other ASs.

6.3. PCE Information Synchronization

   The PCE discovery mechanism MUST allow a PCC to discover any change
   in the information related to a PCE that it has previously
   discovered. This includes changes to both general information (e.g.
   a change in the PCE domains supported), and detailed information if
   supported (e.g. a modification of the PCE's capabilities).

   In addition, the PCE discovery mechanism MUST allow to dynamically
   discover new PCEs in a given discovery scope.

   Note that there is no requirement for real-time detection of these
   changes, the PCE Discovery Mechanism SHOULD rather allow discovery of
   these changes in an order of magnitude of 60 seconds, and the
   operator should have the ability to configure the Discovery delay.

   Note that PCE information is relatively static, and is expected to be
   fairly stable and to not change frequently.

6.4. Discovery of PCE deactivation

   The PCE discovery mechanism MUST allow a PCC to discover when a PCE
   that it has previously discovered is no longer alive or is
   deactivated. This may help reducing or avoiding path computation
   service disruption.

   Note that there is no requirement for real-time detection of PCE
   failure/deactivation, the PCE Discovery Mechanism SHOULD rather allow
   such discovery in an order of magnitude of 60 seconds, and the
   operator should have the ability to configure the Discovery delay.

6.5. Policy Support

   The PCE Discovery mechanism MUST allow for policies to restrict the
   discovery scope to a set of authorized domains, to control and
   restrict the type and nature of the information to be disclosed, and
   also to filter and translate some information at domains borders. It
   MUST be possible to apply these policies on a per PCE basis.

   Note that the Discovery mechanisms MUST allow disclosing policy
   information so as to control the disclosure policies at domain
   boundaries.

   Also, it MUST be possible to apply different policies when disclosing
   PCE information to different domains.


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6.6. Security Requirements

   The five major threats related to PCE discovery mechanisms are:
   - Impersonation of PCE;
   - Interception of PCE discovery information (sniffing);
   - Falsification of PCE discovery information;
   - Information disclosure to non-authorized PCCs (PCC spoofing).
   - DoS Attacks

   Note that security of the PCE Discovery procedures is of particular
   importance in an inter-AS context, where PCE discovery may increase
   the vulnerability to attacks and the consequences of these attacks.

   Hence mechanisms MUST be defined to ensure authenticity, integrity,
   confidentiality, and containment of PCE discovery information:
   - There MUST be a mechanism to authenticate discovery information;
   - There MUST be a mechanism to verify discovery information
     integrity;
   - There MUST be a mechanism to encrypt discovery information;
   - There MUST be a mechanism to restrict the scope of discovery to a
     set of authorized PCCs and to filter PCE information disclosed
     at domain boundaries (as per defined in 6.5).

   A PCE and PCC MUST be identified by a globally unique ID, which may
   be for instance a combination of AS number an IP address.

   Mechanisms MUST be defined in order to limit the impact of a
   DoS attack on the PCE discovery procedure (e.g. filter out excessive
   PCE information change and flapping PCEs). Note also that DOS
   attacks may be either accidental (caused by a mis-behaving
   PCE system) or intentional. As discussed in [PCE-COM-REQ] such
   mechanisms may include packet filtering, rate limiting, no
   promiscuous listening, and where applicable use of private addresses
   spaces.

   Also, key consideration MUST be given in terms of how to establish a
   trust model for PCE discovery. The PCE discovery mechanism MUST
   explicitly support a specific set of one or more trust models.

6.7. Extensibility

   The PCE discovery mechanism MUST be flexible and extensible so as to
   easily allow for the inclusion of additional PCE information that
   could be defined in the future.

6.8. Scalability

   The PCE discovery mechanism MUST be designed to scale well with an
   increase of any of the following parameters:
   - Number of PCCs discovering a given PCE;
   - Number of PCEs to be discovered by a given PCC;
   - Number of domains in the discovery scope.

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   The PCE discovery mechanism MUST NOT have an adverse effect in the
   performance of other protocols (especially routing and signaling)
   already operating in the network.

   Note that there is no scalability requirement with regards to the
   amount of information to be exchanged.
   Information disclosed in the PCE discovery mechanism is relatively
   static. Changes in PCE information may occur as result of PCE
   configuration updates, PCE deployment/activation or PCE
   deactivation/suppression, and should not occur as a result of the PCE
   activity itself. Hence, this information is quite stable and will not
   change frequently.

6.9. Operational orders of magnitudes

   This section gives minimum order of magnitude estimates of what the
   PCE discovery mechanism should support.

   - Number of PCCs discovering a given PCE: 1000
   - Number of PCEs to be discovered by a given PCC: 100

6.10. Manageability considerations

   Mechanisms are REQUIRED to manage PCE discovery operations. This
   includes the configuration of PCE Discovery functions and policies,
   as well as, the monitoring of the discovery protocol activity.

6.10.1. Configuration of PCE Discovery parameters

   It MUST be possible to enable and disable the PCE discovery function
   at a PCC and at a PCE.

   On the PCC it MUST be possible for an operator to activate/deactivate
   automatic PCE discovery. The activation of automatic discovery MUST
   not prevent static configuration of PCE information that may
   supplement discovered information.

   On the PCE it MUST be possible for an operator to control the
   application of discovery policies by which the specific PCE is
   discovered. As described in Section 6.5, this control MUST include
   the ability to:
   - restrict the discovery scope to a set of authorized domains;
   - define the type and nature of the information disclosed;
   - specify the filtering and translation to be applied to the PCE
     information disclosed at domain borders.

   These configuration options MAY be supported through an
   implementation-specific local configuration interface, or MAY be
   supported via a standardised interface (such as a MIB module, as
   below).

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6.10.2. PCE Discovery MIB modules

   PCE Discovery MIB modules MUST be specified for the control of the
   function on PCCs and PCEs.

6.10.2.1. PCC MIB module

   The MIB module that will run on PCCs MUST include at least:
   - A control to disable automatic discovery by the PCC;
   - The set of known PCEs;
   - The number of known PCEs, and the number of discovered PCEs.

   For each PCE reported in the MIB module, the following information
   MUST be available:
   - Information advertised by the PCE (i.e., discovered information);
   - Information locally configured about the PCE;
   - The time since the PCE was discovered;
   - The time since any change to the discovered information for the PCE;

   Note that when a PCE is no longer alive (see section 6.4), it SHOULD
   no longer be reported in the PCC MIB module.

   The MIB module SHOULD also provide the average and maximum rates of
   arrival, departure and modification of PCE discovery to enable
   effective analysis of the operation of the protocols. Further, the
   MIB module SHOULD report on the operation of the discovery protocol
   by counting the number of unacceptable and incomprehensible
   information exchanges.

   The PCC MIB module SHOULD also be used to provide notifications
   when thresholds (e.g. on the maximum rate of change, on the number of
   unacceptable messages) are crossed, or when important events occur
   (e.g. the number of discovered PCEs decreases to zero).

6.10.2.2. PCE MIB module

   The MIB module that will run on PCEs MUST include at least:
   - A control to disable automatic discovery announcements by the PCE;
   - Information to be advertised by the PCE, although this information
     MAY be present as read-only;
   - The discovery policies active on the PCE, although this information
     MAY be present as read-only.

   The MIB module SHOULD also include:
   - The time since the last change to the advertised PCE information;
   - The time since the last change to the advertisement policies;
   - Control of on which interfaces the PCE issues advertisements where
     this is applicable to the protocol solution selected.



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   Note that a PCE MAY also be configured to discover other PCEs. In
   this case it SHOULD operate the MIB module described in section
   6.10.2.1 as well as the module described here.

6.10.3. Monitoring Protocol Operations

   It MUST be possible to monitor the operation of any PCE discovery
   protocol. Where an existing protocol is used to support the PCE
   discovery function, this monitoring SHOULD be achieved using the
   techniques already defined for that protocol, enhanced by the MIB
   modules described above. Where, those techniques are inadequate, new
   techniques MUST be developed.

   Monitoring of the protocol operation demands support for at least the
   following functions:
   - Correlation of information advertised against information received;
   - Counts of dropped, corrupt, and rejected information elements;
   - Detection of 'segmented' networks. That is, the ability to detect
     and diagnose the failure of a PCE advertisement to reach a PCC.

6.10.4. Impact on network operations

   Frequent changes in PCE information may have a significant impact on
   PCCs that receive the advertisements, might destabilise the operation
   of the network by causing the PCCs to swap between PCEs, and might
   harm the network through excessive advertisement traffic. Hence it
   MUST be possible to apply at least the following controls:

   - Configurable limit on the rate of announcement of changed
     parameters at a PCE;
   - Control of the impact on PCCs such as through discovery messages
     rate-limiting;
   - Configurable control of triggers that cause a PCC to swap to
     another PCE.


7. Security Considerations

   This document is a requirement document and hence does not raise by
   itself any particular security issue.

   A set of security requirements that MUST be addressed when
   considering the design and deployment of a PCE Discovery mechanism
   have been identified in section 6.6.








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8. Acknowledgments

   We would like to thank, in chronological order, Benoit Fondeviole,
   Thomas Morin, Emile Stephan, Jean-Philippe Vasseur, Dean Cheng,
   Adrian Farrel, Renhai Zhang, Mohamed Boucadair, Eric Gray, Igor
   Bryskin, Dimitri Papadimitriou, Arthi Ayyangar, Andrew Dolganow, Lou
   Berger, Nabil Bitar, and Kenji Kumaki.

   Thanks also to Ross Callon, Ted Hardie, Dan Romascanu, Russ Housley
   and Sam Hartman for their review and constructive discussions during
   the final stages of publication.


9. References

9.1. Normative references

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

   [PCE-ARCH] Farrel, A., Vasseur, J.P., Ash, J., "Path Computation
   Element (PCE) Architecture", draft-ietf-pce-architecture, work in
   progress.

9.2. Informative references

   [PCE-COM-REQ] Ash, J., Le Roux, J.L., "PCE Communication Protocol
   Generic Requirements", draft-ietf-pce-comm-protocol-gen-reqs, work in
   progress.

10. Editor Address

   Jean-Louis Le Roux (Editor)
   France Telecom
   2, avenue Pierre-Marzin
   22307 Lannion Cedex
   FRANCE
   Email: jeanlouis.leroux@francetelecom.com

11. Contributors' Addresses

   Paul Mabey
   Qwest Communications
   950 17th Street,
   Denver, CO 80202,
   USA
   Email: pmabey@qwest.com

   Eiji Oki
   NTT
   Midori-cho 3-9-11
   Musashino-shi, Tokyo 180-8585,

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   JAPAN
   Email: oki.eiji@lab.ntt.co.jp

   Richard Rabbat
   Fujitsu Laboratories of America
   1240 East Arques Ave, MS 345
   Sunnyvale, CA 94085
   USA
   Email: richard@us.fujitsu.com

   Ting Wo Chung
   Bell Canada
   181 Bay Street, Suite 350
   Toronto, Ontario, M5J 2T3
   CANADA,
   Email: ting_wo.chung@bell.ca

   Raymond Zhang
   BT Infonet
   2160 E. Grand Ave.
   El Segundo, CA 90025
   USA
   Email: raymond_zhang@infonet.com

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   Disclaimer of Validity

   This document and the information contained herein are provided on an
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   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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