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Versions: (draft-ietf-pce-disco-proto-igp) 00 01 02 03 04 05 06 07 08 RFC 5089

Network Working Group                              J.L. Le Roux (Editor)
Internet Draft                                            France Telecom
Category: Standard Track
Expires: August 2007                               J.P. Vasseur (Editor)
                                                       Cisco System Inc.

                                                          Yuichi Ikejiri
                                                      NTT Communications

                                                           Raymond Zhang
                                                              BT Infonet

                                                           February 2007


  IS-IS protocol extensions for Path Computation Element (PCE) Discovery

                  draft-ietf-pce-disco-proto-isis-02.txt


Status of this Memo

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Abstract

   There are various circumstances where it is highly desirable for a
   Path Computation Client (PCC) to be able to dynamically and
   automatically discover a set of Path Computation Elements (PCE),
   along with some of information that can be used for PCE selection.
   When the PCE is a Label Switching Router (LSR) participating in the
   Interior Gateway Protocol (IGP), or even a server participating

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   passively in the IGP, a simple and efficient way to discover PCEs
   consists of using IGP flooding. For that purpose this document
   defines extensions to the Intermediate System to Intermediate System
   (IS-IS) routing protocol for the advertisement of PCE Discovery
   information within an IS-IS area or within the entire IS-IS routing
   domain.


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.      Terminology.................................................3
   2.      Introduction................................................4
   3.      Overview....................................................5
   3.1.    PCE Information.............................................5
   3.1.1.  PCE Discovery Information...................................5
   3.1.2.  PCE Status Information......................................6
   3.2.    Flooding scope..............................................6
   4.      IS-IS extensions............................................6
   4.1.    The IS-IS PCED TLV..........................................6
   4.1.1.  PCE-ADDRESS sub-TLV.........................................7
   4.1.2.  The PATH-SCOPE sub-TLV......................................8
   4.1.3.  PCE-DOMAINS sub-TLV........................................10
   4.1.3.1.  Area ID DOMAIN sub-TLV...................................10
   4.1.3.2.  AS Number DOMAIN sub-TLV.................................11
   4.1.4.  PCE-NEIG-DOMAINS sub-TLV...................................11
   4.1.5.  PCE-CAP-FLAGS sub-TLV......................................11
   4.1.6.  The CONGESTION sub-TLV.....................................12
   5.      Elements of Procedure......................................13
   5.1.1.  CONGESTION sub-TLV specific procedures.....................14
   6.      Backward compatibility.....................................15
   7.      IANA considerations........................................15
   7.1.    IS-IS sub-TLV..............................................15
   7.2.    PCED sub-TLVs registry.....................................15
   7.3.    PCE Capability Flags registry..............................16
   8.      Security Considerations....................................16
   9.      Manageability Considerations...............................17
   9.1.    Control of Policy and Functions............................17
   9.2.    Information and Data Model.................................17
   9.3.    Liveness Detection and Monitoring..........................17
   9.4.    Verify Correct Operations..................................17
   9.5.    Requirements on Other Protocols and Functional
             Components...............................................17
   9.6.    Impact on network operations...............................18
   10.     Acknowledgments............................................18
   11.     References.................................................18
   11.1.   Normative references.......................................18

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   11.2.   Informative references.....................................19
   12.     Editors' Addresses:........................................19
   13.     Contributors' Adresses:....................................19
   14.     Intellectual Property Statement............................20


1. Terminology

   Terminology used in this document

      ABR: IGP Area Border Router (L1L2 router).

      AS: Autonomous System.

      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.

      IGP: Interior Gateway Protocol. Either of the two routing
      protocols Open Shortest Path First (OSPF) or Intermediate System
      to Intermediate system (IS-IS).

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

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

      Inter-area TE LSP: A TE LSP whose path transits two or
      more IGP areas. That is a TE-LSP that crosses at least one IGP
      area boundary.

      Inter-AS TE LSP: A TE LSP whose path transits two or more
      ASes or sub-ASes (BGP confederations). That is a TE-LSP that
      crosses at least one AS boundary.

      IS-IS LSP: Link State PDU

      LSR: Label Switching Router.

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

      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.

      PCEP: Path Computation Element communication Protocol.

      TE LSP: Traffic Engineered Label Switched Path.



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2. Introduction

   [RFC4655] describes the motivations and architecture for a Path
   Computation Element (PCE)-based path computation model for Multi
   Protocol Label Switching (MPLS) and Generalized MPLS (GMPLS) Traffic
   Engineered Label Switched Paths (TE-LSPs). The model allows for the
   separation of the PCE from a PCC (also referred to as a non co-
   located PCE) and allows for cooperation between PCEs. This relies on
   a communication protocol between PCC and PCE, and between PCEs. The
   requirements for such a communication protocol can be found in
   [RFC4657] and the communication protocol is defined in [PCEP].

   The PCE 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 TE LSP computation.

   A network may contain a large number of PCEs with potentially
   distinct capabilities. In such a context it is highly desirable to
   have a mechanism for automatic and dynamic PCE discovery, which
   allows PCCs to automatically discover a set of PCEs, along with
   additional information about each PCE that may be required for the
   PCC to perform PCE selection. Additionally, it is valuable for a PCC
   to dynamically detect new PCEs or any modification of the PCE
   information. Detailed requirements for such a PCE discovery mechanism
   are provided in [RFC4674].

   Moreover, it may also be useful to discover when a PCE experiences
   processing congestion and when it exits such a state, in order for
   the PCCs to take some appropriate actions (e.g. redirect their
   requests to another PCE). Note that the PCE selection algorithm
   applied by a PCC is out of the scope of this document.

   When PCCs are LSRs participating in the IGP (OSPF, IS-IS), and PCEs
   are either LSRs or servers also participating in the IGP, an
   effective mechanism for PCE discovery within an IGP routing domain
   consists of utilizing IGP advertisements.

   This document defines IS-IS extensions to allow a PCE in an IS-IS
   routing domain to advertise its location along with some information
   useful to a PCC for PCE selection, so as to satisfy dynamic PCE
   discovery requirements set forth in [RFC4674]. This document also
   defines extensions allowing a PCE in an IS-IS routing domain to
   advertise its processing congestion state.

   Generic capability advertisement mechanisms for IS-IS are defined in
   [IS-IS-CAP]. These allow a router to advertise its capabilities
   within an IS-IS area or an entire IS-IS routing domain. This document
   leverages this generic capability advertisement mechanism to fully
   satisfy the aforementioned dynamic PCE discovery requirements.

   This document defines a new sub-TLV (named the PCE Discovery (PCED)
   to be carried within the IS-IS Router Capability TLV ([IS-IS-CAP]).

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   The PCE information advertised is detailed in section 3. Protocol
   extensions and procedures are defined in section 4 and 5.

   This document does not define any new IS-IS elements of procedure.
   The procedures defined in [IS-IS-CAP] should be used.

   The IS-IS extensions defined in this document allow for PCE discovery
   within an IS-IS Routing domain. Solutions for PCE discovery across AS
   boundaries are beyond the scope of this document, and for further
   study.

   This document defines a set of sub-TLVs that are nested within each
   other. When the degree of nesting TLVs is 2 (a TLV is carried within
   another TLV) the TLV carried within a TLV is called a sub-TLV.
   Strictly speaking, when the degree of nesting is 3, a subsub-TLV is
   carried within a sub-TLV that is itself carried within a TLV. For the
   sake of terminology simplicity, we refer to sub-TLV, a TLV carried
   within a TLV regardless of the degree of nesting.

3. Overview

3.1. PCE Information

   The PCE information advertised via IS-IS falls into two categories:
   PCE Discovery information and PCE Status information.

3.1.1. PCE Discovery Information

   The PCE Discovery information is comprised of:

   - The PCE location: an IPv4 and/or IPv6 address that is used to reach
     the PCE. It is RECOMMENDED to use an address that is always
     reachable;

   - The PCE inter-domain functions: PCE path computation scope (i.e.
     inter-area, inter-AS, inter-layer…);

   - The PCE domain(s): set of one or more domain(s) into which the PCE
     has visibility and can compute paths;

   - The PCE neighbor domain(s): set of one or more neighbor domain(s)
      towards which a PCE can compute paths;

   -  A set of communication capabilities (e.g. support for
      request prioritization) and path computation specific capabilities
      (e.g. supported constraints).

   Optional elements to describe more complex capabilities may also be
   advertised.



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   PCE Discovery information is by nature fairly static and does not
   change with PCE activity. Changes in PCE Discovery information may
   occur as a result of PCE configuration updates, PCE
   deployment/activation, PCE deactivation/suppression, or PCE failure.
   Hence, this information is not expected to change frequently.

3.1.2. PCE Status Information

   The PCE Status is optional and can be used to report a PCE's
   processing congestion state along with an estimated congestion
   duration. This is a dynamic information, which may change with PCE
   activity.

   Procedures for a PCE to move from a processing congestion state to a
   non-congestion state are beyond the scope of this document, but the
   rate at which a PCE Status change is advertised MUST NOT impact by
   any means the IGP scalability. Particular attention should be given
   on procedures to avoid state oscillations.

3.2. Flooding scope

   The flooding scope for PCE information advertised through IS-IS can
   be limited to one or more IS-IS areas the PCE belongs to, or can be
   extended across the entire IS-IS routing domain.
   Note that some PCEs may belong to multiple areas, in which case the
   flooding scope may comprise these areas. This could be the case for a
   L1L2 router for instance advertising its PCE information within the
   L2 area and/or a subset of its attached L1 area(s).

4. IS-IS extensions

4.1. The IS-IS PCED TLV

   The IS-IS PCED TLV is made of a set of non ordered sub-TLVs.

   The format of the IS-IS PCED TLV and its sub-TLVs is the identical to
   the TLV format used by the Traffic Engineering Extensions to IS-IS
   [RFC3784]. That is, the TLV is composed of 1 octet for the type, 1
   octet specifying the TLV length, and a value field. The Length field
   defines the length of the value portion in octets.

   The IS-IS PCED TLV has the following format:

      TYPE: To be assigned by IANA  (suggested value = 5)
      LENGTH: Variable
      VALUE: set of sub-TLVs







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   Sub-TLVs types are under IANA control.

   Currently six sub-TLVs are defined (suggested type values to be
   assigned by IANA):
            Sub-TLV type  Length               Name
                1      variable     PCE-ADDRESS sub-TLV
                2         3         PATH-SCOPE sub-TLV
                3      variable     PCE-DOMAINS sub-TLV
                4      variable     PCE-NEIG-DOMAINS sub-TLV
                5      variable     PCE-CA-FLAGS sub-TLV
                6         1         CONGESTION sub-TLV


   The PCE-ADDRESS and PATH-SCOPE sub-TLVs MUST always be present within
   the PCED TLV.

   The PCE-DOMAINS and PCE-NEIG-DOMAINS sub-TLVs are optional. They may
   be present in the PCED TLV to facilitate selection of inter-domain
   PCEs.

   The PCE-CAP-FLAGS sub-TLVs are optional and MAY be present in the
   PCED TLV to facilitate the PCE selection process.

   The CONGESTION sub-TLV is optional and MAY be present in the PCED
   TLV, to indicate a PCE's processing congestion state.

   Any non recognized sub-TLV MUST be silently ignored.

   Additional sub-TLVs could be added in the future to advertise
   additional PCE information.

   The PCED TLV is carried within an IS-IS CAPABILITY TLV defined in
   [IS-IS-CAP].


4.1.1. PCE-ADDRESS sub-TLV

   The PCE-ADDRESS sub-TLV specifies the IP address that can be
   used to reach the PCE. It is RECOMMENDED to make use of an address
   that is always reachable, provided the PCE is alive.

   The PCE-ADDRESS sub-TLV is mandatory; it MUST be present within the
   PCED TLV. It MAY appear twice, when the PCE has both an IPv4 and
   IPv6 address. It MUST NOT appear more than once for the same address
   type.








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   The PCE-ADDRESS sub-TLV has the following format:

      TYPE: To be assigned by IANA (Suggested value =1)
      LENGTH: 5 for IPv4 address and 17 for IPv6 address
      VALUE: This comprises one octet indicating the address-type and 4
             or 16 octets encoding the IPv4 or IPv6 address to be used
             to reach the PCE

   Address-type:
                  1   IPv4
                  2   IPv6


4.1.2. The PATH-SCOPE sub-TLV

   The PATH-SCOPE sub-TLV indicates the PCE path computation scope,
   which refers to the PCE's ability to compute or take part in the
   computation of intra-area, inter-area, inter-AS, or inter-layer_TE
   LSP(s).

   The PATH-SCOPE sub-TLV is mandatory; it MUST be present within the
   PCED TLV. There MUST be exactly one instance of the PATH-SCOPE sub-
   TLV within each PCED TLV.

   The PATH-SCOPE sub-TLV contains a set of bit flags indicating the
   supported path scopes, and four fields indicating PCE preferences.

   The PATH-SCOPE sub-TLV has the following format:

   TYPE: To be assigned by IANA (Suggested value =2)
   LENGTH: 3
   VALUE: This comprises a one-byte flags field where flag
          represents a supported path scope, followed by a 2-bytes
          preferences field indicating PCE preferences.

   Here is the structure of the bits flag:

      +-+-+-+-+-+-+-+-+
      |0|1|2|3|4|5|Res|
      +-+-+-+-+-+-+-+-+


   Bit      Path Scope

   0      L bit:  Can compute intra-area path
   1      R bit:  Can act as PCE for inter-area TE LSP computation
   2      Rd bit: Can act as a default PCE for inter-area TE LSP
                  computation
   3      S bit:  Can act as PCE for inter-AS TE LSP computation
   4      Sd bit: Can act as a default PCE for inter-AS TE LSPs
                  computation

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   5      Y bit:  Can compute or take part into the computation of
                  paths across layers
   6-7   Reserved for future usage.


   Here is the structure of the preferences field

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |PrefL|PrefR|PrefS|PrefY| Res   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Res: Reserved for future usage.

   Pref-L field: PCE's preference for intra-area TE LSPs computation.

   Pref-R field: PCE's preference for inter-area TE LSPs computation.

   Pref-S field: PCE's preference for inter-AS TE LSPs computation.

   Pref-Y field: PCE's preference for inter-layer TE LSPs computation.

   Res: Reserved for future usage.

   The bits L, R, S, and Y bits are set when the PCE can act as a PCE
   for intra-area, inter-area, inter-AS or inter-layer TE LSPs
   computation respectively. These bits are non-exclusive.

   When set the Rd bit indicates that the PCE can act as a default PCE
   for inter-area TE LSP computation (that is the PCE can compute a path
   towards any neighbor area). Similarly, when set, the Sd bit indicates
   that the PCE can act as a default PCE for inter-AS TE LSP computation
   (the PCE can compute a path towards any neighbor AS).

   When the Rd bit is set, the PCE-NEIG-DOMAIN TLV (see 5.1.4) MUST NOT
   contain any Area ID DOMAIN sub-TLVs.

   Similarly, when the Sd bit is set, the PCE-NEIG-DOMAIN TLV MUST NOT
   contain any AS-DOMAIN sub-TLVs.

   When the R/S bit is cleared, the RD/Sd bit SHOULD be cleared and MUST
   be ignored.

   The PrefL, PrefR, PrefS and PrefY fields are each three bits long and
   allow the PCE to specify a preference for each computation scope,
   where 7 reflects the highest preference. Such preference can be used
   for weighted load balancing of requests. An operator may decide to
   configure a preference for each computation scope to each PCE so as
   to balance the path computation load among them. The algorithms used
   by a PCC to balance its path computation requests according to such
   PCE preference are out of the scope of this document and is a matter
   for local or network wide policy. The same or distinct preferences
   may be used for each scopes. For instance an operator that wants a

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   PCE capable of both inter-area and inter-AS computation to be used
   preferably for inter-AS computation may configure a PrefS higher than
   the PrefR.

   When the L bit, R bit, S bit or Y bit are cleared the PrefL, PrefR,
   PrefS, PrefY fields SHOULD respectively be set to 0 and MUST be
   ignored.

   Both reserved fields SHOULD be set to zero on transmission and MUST
   be ignored on receipt.


4.1.3. PCE-DOMAINS sub-TLV

   The PCE-DOMAINS sub-TLV specifies the set of domains (areas and/or
   ASes) where the PCE has topology visibility and through which the PCE
   can compute paths. It contains a set of one or more sub-TLVs where
   each sub-TLV identifies a domain.


   The PCE-DOMAINS sub-TLV MAY be present when PCE domains cannot be
   inferred by other IGP information, for instance when the PCE is
   inter-domain capable (i.e. when the R bit or S bit is set) and the
   flooding scope is the entire routing domain (see section 5 for a
   discussion of how the flooding scope is set and interpreted).

   The PCE-DOMAINS sub-TLV has the following format:

   TYPE: To be assigned by IANA (Suggested value =3)
   LENGTH: Variable
   VALUE: This comprises a set of one or more DOMAIN sub-TLVs where
          each DOMAIN sub-TLV identifies a domain where the PCE has
          topology visibility and can compute paths.

   Two DOMAIN sub-TLVs are defined

            Sub-TLV type   Length        Name
                 1         Variable     Area ID sub-TLV
                 2           4          AS number sub-TLV

   At least one DOMAIN sub-TLV MUST be present in the PCE-DOMAINS sub-
   TLV. Note than when the PCE visibility is an entire AS, the PCE-
   DOMAINS sub-TLV MUST include exactly one AS number sub-TLV, and MUST
   not contain an area-ID sub-TLV.


4.1.3.1. Area ID DOMAIN sub-TLV

   This sub-TLV carries an IS-IS area ID. It has the following format

   TYPE: 1
   LENGTH: Variable

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   VALUE: This comprises a variable length IS-IS area ID. This is the
          combination of an Initial Domain Part (IDP) and High Order
          part of the Domain Specific part (HO-DSP)

4.1.3.2. AS Number DOMAIN sub-TLV

   The AS Number sub-TLV carries an AS number. It has the following
   format:

   TYPE: 2
   LENGTH: 4
   VALUE: AS number identifying an AS. When coded in two
          bytes (which is the current defined format as the
          time of writing this document), the AS Number field
          MUST have its left two bytes set to 0.


4.1.4. PCE-NEIG-DOMAINS sub-TLV

   The PCE-NEIG-DOMAINS sub-TLV specifies the set of neighbour domains
   (areas, ASes) toward which a PCE can compute paths. It means that the
   PCE can compute or take part in the computation of inter-domain TE
   LSPs whose path transits one of these domains. It contains a set of
   one or more DOMAIN sub-TLVs where each DOMAIN sub-TLV identifies a
   domain.

   The PCE-NEIG-DOMAINS sub-TLV has the following format:

   TYPE: To be assigned by IANA (Suggested value =4)
   LENGTH: Variable
   VALUE: This comprises a set of one or more area or/and AS DOMAIN sub-
          TLVs where each sub-TLV identifies a neighbour domain toward
          which a PCE can compute path.

   The PCE-NEIG-DOMAINS sub-TLV MUST be present if the R bit is set and
   the Rd bit is cleared, and/or, if the S bit is set and the Sd bit is
   cleared.

   The PCE-NEIG-DOMAINS sub-TLV MUST include at least one DOMAIN sub-
   TLV. It MUST include at least one Area ID sub-TLV, if the R bit of
   the PATH-SCOPE TLV is set and the Rd bit of the PATH-SCOPE TLV is
   cleared. Similarly, it MUST include at least one AS number sub-TLV if
   the S bit of the PATH-SCOPE TLV is set and the Sd bit of the PATH-
   SCOPE TLV is cleared.

4.1.5. PCE-CAP-FLAGS sub-TLV

   The PCE-CAP-FLAGs sub-TLV is an optional TLV used to indicate PCEP
   related capabilities. It MAY be present within the PCED TLV. It MUST
   NOT be present more than once.



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   The value field of the PCE-CAP-FLAGS sub-TLV is made up of an array
   of units of 32 bit flags numbered from the most significant as bit
   zero, where each bit represents one PCE capability.

   The GENERAL-CAP sub-TLV has the following format:

      TYPE: To be assigned by IANA (Suggested value =4)
      LENGTH: Multiple of 4
      VALUE: This contains an array of units of 32 bit flags numbered
             from the most significant as bit zero, where each bit
             represents one PCE capability.

   IANA is requested to manage the space of the PCE Capability Flags

   The following bits are to be assigned by IANA:


     Bit       Capabilities

      0        Capability to handle GMPLS link constraints
      1        Capability to compute bidirectional paths
      2        Capability to compute PSC path
      3        Capability to compute a TDM path
      4        Capability to compute a LSC path
      5        Capability to compute a FSC path

      6        Capability to compute link/node/SRLG diverse paths
      7        Capability to compute load-balanced paths
      8        Capability to compute a set of paths in a
               synchronized Manner
      9        Support for multiple objective functions
      10       Capability to handle path constraints (e.g. max hop count,
               max path metric)
      11       Support for Request prioritization.
      12       Support for multiple requests within the same
               request message.

     13-31    Reserved for future assignments by IANA.

   Reserved bits SHOULD be set to zero on transmission and MUST be
   ignored on receipt.


4.1.6. The CONGESTION sub-TLV

   The CONGESTION sub-TLV is used to indicate a PCE's experiences a
   processing congestion state and may optionally include expected PCE
   congestion duration.
   The CONGESTION sub-TLV is optional, it MAY be carried within the PCED
   TLV. It MUST NOT be present more than once.



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   The format of the CONGESTION sub-TLV is as follows:

   TYPE: To be assigned by IANA (Suggested value =6)
   LENGTH: 3
   VALUE: This comprises a one-byte bit flags indicating the
          congestion status, followed by a 2-bytes field indicating the
          congestion duration.


   Here is the TLV structure

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |C|     Reserved|      Congestion Duration      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Value
           -C bit: When set this indicates that the PCE is experiencing
                   congestion and cannot accept any new request. When
                   cleared this indicates that the PCE is not
                   experiencing congestion and can accept new requests.

           -Congestion Duration: 2-bytes, the estimated PCE congestion
                                 duration in seconds.

   When C is set and the Congestion Duration field is equal to 0, this
   means that the Congestion Duration is unknown.
   When C is cleared the Congestion Duration SHOULD be set to 0 and MUST
   be ignored.


5. Elements of Procedure

   The PCED TLV is advertised within an IS-IS Router Capability TLV
   defined in [IS-IS-CAP]. A such, elements of procedures are inherited
   from those defined in [IS-IS-CAP].

   The flooding scope is controlled by the S flag in the IS-IS Router
   Capability TLV (see [IS-IS-CAP]). When the scope of the PCED TLV is
   area local it MUST be carried within an IS-IS CAPABILITY TLV having
   the S bit cleared. When the scope of the PCED TLV is the entire IGP
   domain, itMUST be carried within an IS-IS CAPABILITY TLV having the S
   bit set. When only the L bit of the PATH-SCOPE sub-TLV is set, the
   flooding scope MUST be local.

   A PCE MUST originate a new IS-IS LSP whenever the content
   of any of the PCED TLV changes or whenever required by the regular
   IS-IS procedure.

   When the PCE function is deactivated on a node, the node MUST
   originate a new IS-IS LSP with no longer any PCED TLV. A PCC MUST be
   able to detect that the PCED TLV has been removed from an IS-IS LSP.

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   The PCE address, i.e. the address indicated within the PCE ADDRESS
   sub-TLV, MUST be distributed as part of IS-IS routing; this allows
   speeding up the detection of a PCE failure. Note that when the PCE
   address is no longer reachable, this means that the PCE node has
   failed or has been torn down, or that there is no longer IP
   connectivity to the PCE node.

   The PCED TLV is OPTIONAL. When an IS-IS LSP does not contain any PCED
   TLV, this means that the PCE information of that node is unknown.

   A change in PCED information MUST not trigger any SPF computation at
   a receiving router.

   The way PCEs determine the information they advertise is out of the
   scope of this document. Some information may be configured (e.g.,
   address, preferences, scope) and other information may be
   automatically determined by the PCE (e.g. areas of visibility).

5.1.1. CONGESTION sub-TLV specific procedures

   When a PCE enters into a processing congestion state, the conditions
   of which are implementation dependent, it MAY originate a new IS-IS
   LSP with a CONGESTION sub-TLV with the C bit set and optionally a
   non-null expected congestion duration.

   When a PCE exists from the processing congestion state, the
   conditions of which are implementation dependent, two cases are
   considered:
        - If the congestion duration in the previously originated
   CONGESTION sub-TLV was null, it SHOULD originate a CONGESTION sub-TLV
   with the C bit cleared and a null congestion duration;
        - If the congestion duration in the previously originated
   CONGESTION sub-TLV was non null, it MAY originate a CONGESTION sub-
   TLV with the C bit cleared. Note that in some particular cases it may
   be desired to originate a PCES TLV with the C bit cleared if the
   congestion duration was over estimated.

   The congestion duration allows a reduction in the amount of IS-IS
   flooding, as only uncongested-to-congested state transitions need
   advertised.

   A PCE implementation SHOULD support an appropriate dampening
   algorithm so as to dampen IS-IS flooding in order to not impact the
   IS-IS scalability. It is RECOMMENDED to introduce some hysteresis for
   congestion state transition, so as to avoid state oscillations that
   may impact IS-IS performance. For instance two thresholds MAY be
   configured: a resource congestion upper-threshold and a resource
   congestion lower-threshold. An LSR enters the congested state when
   the CPU load reaches the upper threshold and leaves the congested
   state when the CPU load goes under the lower threshold.


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   Upon receipt of an updated CONGESTION sub-TLV a PCC should take
   appropriate actions. In particular, the PCC SHOULD stop sending
   requests to a congested PCE, and SHOULD gradually start sending again
   requests to a PCE that is no longer congested

6. Backward compatibility

   The PCED TLV defined in this document does not introduce any
   interoperability issues.

   An IS-IS router not supporting the PCED TLV will just silently ignore
   the TLV as specified in [IS-IS-CAP].

7. IANA considerations

7.1. IS-IS sub-TLV

   Once a registry for the IS-IS Router Capability TLV defined in
   [IS-IS-CAP] will have been assigned, IANA will assign a new
   TLV code-point for the PCED TLV carried within the Router Capability
   TLV.

   Value      Sub-TLV                   References
   -----     --------                   ----------
     5    PCED TLV                      (this document)

     7.2.                         PCED sub-TLVs registry

   The PCED TLV referenced above is constructed from sub-TLVs. Each sub-
   TLV includes a 8-bit type identifier.

   The IANA is requested to create a new registry and manage TLV type
   identifiers as follows:

   - TLV Type
   - TLV Name
   - Reference

   This document defines five TLVs as follows (suggested values):

   Value      TLV name                   References
   -----     --------                   ----------
    1       PCE-ADDRESS               This document
    2       PATH-SCOPE                This document
    3       PCE-DOMAINS               This document
    4       PCE-NEIG-DOMAINS          This document
    5       PCE-CAP-FLAGS             This document
    6       CONGESTION                This document

   New TLV type values may be allocated only by an IETF Consensus
   action.


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7.3. PCE Capability Flags registry

   This document provides new capability bit flags, which are present
   in the PCE-CAP-FLAGS TLV referenced in section 4.1.5.

   The IANA is requested to create a new registry and to manage the
   space of PCE capability bit flags numbering them in the usual IETF
   notation starting at zero, and continuing at least through 31, with
   the most significant bit as bit zero.

   The same registry is defined for OSPF based PCE discovery [PCED-OSPF].
   A single registry must be defined for both protocols.

   New bit numbers may be allocated only by an IETF Consensus action.

   Each bit should be tracked with the following qualities:

   - Bit number
   - Defining RFC
   - Capability Description

   Several bits are defined in this document. Here are the suggested
   values:

     Bit       Capability Description

      0        GMPLS link constraints
      1        Bidirectional paths
      2        PSC paths
      3        TDM paths
      4        LSC paths
      5        FSC paths
      6        Diverse paths
      7        Load-balanced paths
      8        Synchronized computation
      9        Multiple objective functions
      10       Additive path constraints (e.g. max hop count)
      11       Request prioritization
      12       Multiple requests per message


8. Security Considerations

   This document defines IS-IS extensions for PCE discovery within an
   administrative domain. Hence the security of the PCE discovery relies
   on the security of IS-IS.

   Mechanisms defined to ensure authenticity and integrity of IS-IS LSPs
   [RFC3567], and their TLVs, can be used to secure the PCED TLV as well.

   IS-IS provides no mechanism for protecting the privacy of LSAs, and
   in particular the privacy PCE discovery information.

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9. Manageability Considerations

   Manageability considerations for PCE Discovery are addressed in
   section 4.10 of [RFC4674].

9.1. Control of Policy and Functions

   Requirements on the configuration of PCE discovery parameters on PCCs
   and PCEs are discussed in section 4.10.1 of [RFC4674].

   Particularly, a PCE implementation SHOULD allow configuring the
   following parameters on the PCE:
        -The PCE IPv4/IPv6 address(es) (see section 4.1.1)
        -The PCE Scope, including the inter-domain functions (inter-
         area, inter-AS, inter-layer), the preferences, and whether the
         PCE can act as default PCE (see section 4.1.2)
        -The PCE domains (see section 4.1.3)
        -The PCE neighbour domains (see section 4.1.4)
        -The PCE capabilities (see section 4.1.5)

9.2. Information and Data Model

   A MIB module for PCE Discovery is defined in [PCED-MIB].

9.3. Liveness Detection and Monitoring

   PCE Discovery Protocol liveness detection relies upon OSPF liveness
   detection. IS-IS already includes a liveness detection mechanism
   (Hello PDUs), and PCE discovery does not require additional
   capabilities.

   Procedures defined in section 5 allow a PCC detecting when a PCE has
   been deactivated, or is no longer reachable.

9.4. Verify Correct Operations

   The correlation of information advertised against information
   received can be achieved by comparing the PCED information in the PCC
   and in the PCE, which is stored in the PCED MIB [PCED-MIB].  The
   number of dropped, corrupt, and rejected information elements are
   stored in the PCED MIB.

9.5. Requirements on Other Protocols and Functional Components

   The IS-IS extensions defined in this documents does not imply any
   requirement on other protocols.





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9.6. Impact on network operations

   Frequent changes in PCE information, and particularly in PCE
   congestion information, may have a significant impact on IS-IS and
   might destabilize the operation of the network by causing the PCCs to
   swap between PCEs.

   As discussed in section 5, a PCE implementation SHOULD support an
   appropriate dampening algorithm so as to dampen IS-IS flooding in
   order to not impact the IS-IS scalability.

   Also, as discussed in section 4.10.4 of [RFC4674], 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.


10. Acknowledgments

We would like to thank Lucy Wong and Adrian Farrel for their useful
comments and suggestions.

11. References

11.1. Normative references

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

   [RFC3784] Li, T., Smit, H., "IS-IS extensions for Traffic
   Engineering", RFC 3784, June 2004.

   [IS-IS-CAP] Vasseur, J.P. et al., "IS-IS extensions for advertising
   router information", draft-ietf-isis-caps, work in progress.

   [RFC4655] Farrel, A., Vasseur, J.P., Ash, J., "Path Computation
   Element (PCE)-based Architecture", RFC4655, august 2006.

   [RFC4674] Le Roux, J.L., et al. "Requirements for PCE discovery",
   RFC4674, October 2006.

   [RFC4205] Kompella, Rekhter, " IS-IS Extensions in Support of
   Generalized Multi-Protocol Label Switching (GMPLS)", RFC4205, October
   2005.

   [RFC3567] Li, T. and R. Atkinson, "Intermediate System to


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   Intermediate System (IS-IS) Cryptographic Authentication", RFC 3567,
   July 2003.

11.2. Informative references

   [RFC4657] Ash, J., Le Roux, J.L., " PCE Communication Protocol
   Generic Requirements", RFC4657, September 2006.

   [PCEP] Vasseur, Le Roux, et al., “Path Computation Element (PCE)
   communication Protocol (PCEP) - Version 1”, draft-ietf-pce-pcep, work
   in progress.

   [PCED-MIB] Stephan, E., "Definitions of Managed Objects for Path
   Computation Element Discovery", draft-ietf-pce-disc-mib-00, work in
   progress.

   [PCED-OSPF] Le Roux, Vasseur, "OSPF protocol extensions for Path
   Computation Element (PCE) Discovery", draft-ietf-pce-disco-proto-
   ospf, work in progress.



12. Editors' Addresses:

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

   Jean-Philippe Vasseur (Editor)
   Cisco Systems, Inc.
   1414 Massachusetts avenue
   Boxborough , MA - 01719
   USA
   Email: jpv@cisco.com

13. Contributors' Adresses:

   Yuichi Ikejiri
   NTT Communications Corporation
   1-1-6, Uchisaiwai-cho, Chiyoda-ku
   Tokyo 100-8019
   JAPAN
   Email: y.ikejiri@ntt.com

   Raymond Zhang
   BT Infonet
   2160 E. Grand Ave.
   El Segundo, CA 90025
   USA

Le Roux, Vasseur et al. IS-IS extensions for PCE discovery   [Page 19]

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   Email: raymond_zhang@bt-infonet.com


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   Disclaimer of Validity

   This document and the information contained herein are provided
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   Copyright Statement

   Copyright (C) The IETF Trust (2007). This document is subject to the
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