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

                                                          Yuichi Ikejiri
                                                      NTT Communications

                                                           Raymond Zhang
                                                              BT Infonet

                                                                May 2007




  OSPF protocol extensions for Path Computation Element (PCE) Discovery

               draft-ietf-pce-disco-proto-ospf-04.txt


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   Copyright (C) The IETF Trust (2007).  All rights reserved.


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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 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
   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 Open Shortest Path First (OSPF) routing
   protocol for the advertisement of PCE Discovery information within an
   OSPF area or within the entire OSPF 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 [RFC2119].

Table of Contents

   1.      Terminology.................................................3
   2.      Introduction................................................4
   3.      Overview....................................................5
   3.1.    PCE Information.............................................5
   3.2.    PCE Discovery Information...................................5
   3.2.1.  PCE Congestion Information..................................6
   3.3.    Flooding Scope..............................................6
   4.      OSPF Extensions.............................................6
   4.1.    The OSPF PCED TLV...........................................6
   4.1.1.  PCE-ADDRESS Sub-TLV.........................................8
   4.1.2.  PATH-SCOPE Sub-TLV..........................................8
   4.1.3.  PCE-DOMAIN Sub-TLV.........................................10
   4.1.4.  NEIG-PCE-DOMAIN Sub-TLV....................................11
   4.1.5.  PCE-CAP-FLAGS Sub-TLV......................................12
   4.1.6.  The CONGESTION Sub-TLV.....................................14
   5.      Elements of Procedure......................................14
   5.1.    CONGESTION sub-TLV Specific Procedures.....................15
   6.      Backward Compatibility.....................................16
   7.      IANA Considerations........................................16
   7.1.    OSPF TLV...................................................16
   7.2.    PCED Sub-TLVs Registry.....................................16
   7.3.    PCE Capability Flags registry..............................17
   8.      Security Considerations....................................18
   9.      Manageability Considerations...............................18
   9.1.    Control of Policy and Functions............................18
   9.2.    Information and Data Model.................................18
   9.3.    Liveness Detection and Monitoring..........................18
   9.4.    Verify Correct Operations..................................18

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   9.5.    Requirements on Other Protocols and Functional
             Components...............................................19
   9.6.    Impact on network operations...............................19
   10.     Acknowledgments............................................19
   11.     References.................................................19
   11.1.   Normative references.......................................19
   11.2.   Informative references.....................................20
   12.     Editor's Addresses:........................................20
   13.     Contributors' Addresses:...................................21
   14.     Intellectual Property Statement............................21


1. Terminology

   Terminology used in this document

      ABR: OSPF Area Border Router.

      AS: Autonomous System.

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

      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.

      LSA: Link State Advertisement

      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.

       PCE-Domain: In a PCE context this refers to any collection of
       network elements within a common sphere of address management or
       path computational responsibility (referred to as "domain" in

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       [RFC4655]). Examples of PCE-Domains include IGP areas and
       Autonomous Systems. This should be distinguished from an OSPF
       routing domain.

      PCEP: Path Computation Element Protocol.

      TE LSP: Traffic Engineered Label Switched Path.

2. Introduction

   [RFC4655] describes the motivations and architecture for a 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 Path
   Computation Client (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. to 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 or 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 OSPF extensions to allow a PCE in an OSPF
   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 OSPF routing domain to
   advertise its processing congestion state.


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

   This document defines a new TLV (named the PCE Discovery (PCED) TLV)
   to be carried within the OSPF Router Information LSA ([OSPF-CAP]).

   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 OSPF elements of procedure. The
   procedures defined in [OSPF-CAP] MUST be used.

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

   In this document, we call TLV any TLV that is carried within an OSPF
   LSA. Any TLV that is itself carried within another TLV is referred to
   as either a TLV or a sub-TLV.

3. Overview

3.1. PCE Information

   The PCE information advertised via OSPF falls into two categories:
   PCE Discovery information and PCE Congestion information.

3.2. 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 path computation scope (i.e. inter-area, inter-AS, inter-
     layer);

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

   - The set of one or more neighbor PCE-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).


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

   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.

3.2.1. PCE Congestion Information

   The PCE Congestion information is optional information and can be
   used to report a PCE's processing congestion state along with an
   estimated congestion duration. This is 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 MUST be given to
   procedures to avoid state oscillations.

3.3. Flooding Scope

   The flooding scope for PCE information advertised through OSPF can be
   limited to one or more OSPF areas the PCE belongs to, or can be
   extended across the entire OSPF 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
   an ABR for instance advertising its PCE information within the
   backbone area and/or a subset of its attached IGP area(s).

4. OSPF Extensions

4.1. The OSPF PCED TLV

   The OSPF PCE Discovery TLV (PCED TLV) is made of a set of non-ordered
   sub-TLVs.

   The format of the OSPF PCED TLV and its sub-TLVs is identical to the
   TLV format used by the Traffic Engineering Extensions to OSPF
   [RFC3630]. That is, the TLV is composed of 2 octets for the type, 2
   octets specifying the TLV length, and a value field. The Length field
   defines the length of the value portion in octets.

   The TLV is padded to four-octet alignment; padding is not included in
   the Length field (so a three octet value would have a length of
   three, but the total size of the TLV would be eight octets). Nested
   TLVs are also four-octet aligned. Unrecognized types are ignored.
   All Type values between 32768 and 65535 are reserved for vendor-


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   specific extensions.  All other undefined Type codes are reserved for
   future assignment by IANA.

   The OSPF PCED TLV has the following format:

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   //                            sub-TLVs                          //
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


         Type     To be defined by IANA (suggested value=5)
         Length   Variable
         Value    This comprises one or more sub-TLVs

   Sub-TLVs types are under IANA control.

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

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

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

   The PCE-CAP-FLAGS sub-TLV is 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 information.

   The PCED TLV is carried within an OSPF Router Information LSA
   defined in [OSPF-CAP].


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   The following sub-sections describe the sub-TLVs which may be carried
   within the PCED sub-TLV.


4.1.1. PCE-ADDRESS Sub-TLV

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

   The format of the PCE-ADDRESS sub-TLV is as follows:

                            1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Type             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     address-type              |          Reserved             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       //                       PCE IP Address                        //
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            PCE-ADDRESS sub-TLV format

         Type     To be assigned by IANA (suggested value =1)
         Length   8 (IPv4) or 20 (IPv6)

         Address-type:
                       1   IPv4
                       2   IPv6

         PCE IP Address: The IP address to be used to reach the PCE.


4.1.2. 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.


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   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:

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|1|2|3|4|5|   Reserved        |PrefL|PrefR|PrefS|PrefY| Res   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


         Type     To be defined by IANA (suggested value =2)
         Length   4
         Value    This comprises a 2 byte flag field where each bit
                  represents a supported path scope, as well as four
                  preference fields used to specify PCE preferences.

         The following bits are defined:

         Bit      Path Scope

          0      L bit:  Can compute intra-area paths
          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 LSP
                         computation
          5      Y bit:  Can compute or take part into the computation
                         of paths across layers.

   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 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 LSPs computation (that is the PCE can compute a
   path towards any neighbor area). Similarly, when set, the Sd bit

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   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 and Sd bit are set the PCED TLV MUST NOT contain any
   NEIG-PCE-DOMAIN sub-TLV (see 4.1.4).

   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 load balance its path computation requests according to
   such PCE preference is 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 scope. For instance an operator that
   wants a 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-DOMAIN Sub-TLV

   The PCE-DOMAIN sub-TLV specifies a PCE-Domain (areas and/or ASes)
   where the PCE has topology visibility and through which the PCE can
   compute paths.

   The PCE-DOMAIN 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).

   A PCED TLV MAY include multiple PCE-DOMAIN sub-TLVs when the PCE has
   visibility in multiple PCE-Domains.








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

                            1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Type             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Domain-type               |          Reserved             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       //                       Domain ID                             //
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            PCE-DOMAIN sub-TLV format

         Type     To be assigned by IANA (suggested value =3)
         Length   Variable

         3 domain-type values are defined:
                       1   IPv4 Area Address
                       2   IPv6 Area Address
                       3   AS Number

         Domain ID: With the address type 1/2 this indicates the IPv4/v6
         address of an area where the PCE has visibility. With address-
         type 3 this indicates an AS number where the PCE has
         visibility. 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. NEIG-PCE-DOMAIN Sub-TLV

   The NEIG-PCE-DOMAIN sub-TLV specifies a neighbour PCE-domain (area,
   AS) toward which a PCE can compute paths. It means that the PCE can
   take part in the computation of inter-domain TE LSPs whose path
   transits this neighbour PCE-domain.

   A PCED sub-TLV MAY include several NEIG-PCE-DOMAIN sub-TLVs when the
   PCE can compute paths towards several neighbour PCE-domains.

   The NEIG-PCE-DOMAIN sub-TLV has the same format as the PCE-DOMAIN
   sub-TLV:




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                            1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Type             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Domain-type               |          Reserved             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       //                       Domain ID                             //
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            NEIG-PCE-DOMAIN sub-TLV format

         Type     To be assigned by IANA (suggested value =3)
         Length   Variable

         3 domain-type values are defined:
                       1   IPv4 Area Address
                       2   IPv6 Area Address
                       3   AS Number

         Domain ID: With the address type 1/2 this indicates the
         IPv4/v6 address of a neighbour area towards which the PCE can
         compute paths. With address-type 3 this indicates the AS number
         of a neighbour AS towards which the PCE can compute paths. 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.

   The NEIG-PCE-DOMAIN 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.

4.1.5. PCE-CAP-FLAGS Sub-TLV

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

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







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   The format of the PCE-CAP-FLAGS sub-TLV is as follows:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Type             |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
     //                 PCE Capability Flags                          //
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


         Type     To be assigned by IANA (suggested value =5)
         Length   Multiple of 4 bytes
         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        Path computation with GMPLS link constraints
      1        Bidirectional path computation
      2        Diverse path computation
      3        Load-balanced path computation
      4        Synchronized paths computation
      5        Support for multiple objective functions
      6        Support for additive path constraints
               (max hop count, etc.)
      7        Support for request prioritization
      8        Support for multiple requests per message

     9-31    Reserved for future assignments by IANA.

   These capabilities are defined in [RFC4657].

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







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4.1.6. The CONGESTION Sub-TLV

   The CONGESTION sub-TLV is used to indicate that a PCE is experiencing
   a processing congestion state and may optionally include the 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.

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Type             |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |C|       Reserved              |      Congestion Duration      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


         Type     To be assigned by IANA (suggested value =6)
         Length   4
         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 OSPFv2 Router Information LSAs
   (Opaque type of 4 and Opaque ID of 0) or OSPFv3 Router information
   LSAs (function code of 12) which are defined in [OSPF-CAP].  As such,
   elements of procedure are inherited from those defined in [OSPF-CAP].

   In OSPFv2 the flooding scope is controlled by the opaque LSA type (as
   defined in [RFC2370]) and in OSPFv3 by the S1/S2 bits (as defined in
   [RFC2740]). If the flooding scope is local to an area then the PCED
   TLV MUST be carried within an OSPFv2 type 10 router information LSA
   or an OSPFV3 Router Information LSA with the S1 bit set and the S2
   bit cleared. If the flooding scope is the entire domain then the PCED
   TLV MUST be carried within an OSPFv2 type 11 Router Information LSA
   or OSPFv3 Router Information LSA with the S1 bit cleared and the S2


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   bit set. When only the L bit of the PATH-SCOPE sub-TLV is set, the
   flooding scope MUST be area local.

   An OSPF router MUST originate a new Router Information LSA whenever
   there is a change in a PCED TLV associated with a PCE it advertises.

   When a PCE is deactivated, the OSPF router advertising this PCE MUST
   originate a new Router Information LSA that does no longer include
   the corresponding PCED TLV.

   The PCE address, i.e. the address indicated within the PCE ADDRESS
   TLV, MUST be reachable via some prefixes advertised by OSPF; 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 OSPF LSA 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 on the PCE
   (e.g., address, preferences, scope) and other information may be
   automatically determined by the PCE (e.g., areas of visibility).

5.1. CONGESTION sub-TLV Specific Procedures

   When a PCE enters into a processing congestion state, the conditions
   of which are implementation dependent, a Router Information LSA with
   a CONGESTION sub-TLV with the C bit set, and optionally a non-null
   expected congestion duration MAY be generated.

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

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

   An OSPF implementation SHOULD support an appropriate dampening
   algorithm so as to dampen OSPF flooding of PCE Congestion information

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   in order to not impact the OSPF scalability. It is RECOMMENDED to
   introduce some hysteresis for congestion state transition, so as to
   avoid state oscillations that may impact OSPF 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.

   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.

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


7. IANA Considerations

7.1. OSPF TLV

   Once the OSPF RI TLVs registry defined in [OSPF-CAP] will have been
   assigned, IANA will assign a new TLV code-point for the PCED TLV
   carried within the Router Information LSA.

   Value      TLV Name                      Reference
   -----     --------                       ----------
     5         PCED                       (this document)


7.2. PCED Sub-TLVs Registry

   The PCED TLV referenced above is constructed from sub-TLVs. Each sub-
   TLV includes a 16-bit type identifier.

   The IANA is requested to create a sub-registry of the OSPF RI TLVs
   registry defined in [OSPF-CAP], named the "OSPF PCED sub-TLV"
   registry, and manage sub-TLV type identifiers as follows:

   - sub-TLV Type
   - sub-TLV Name
   - Reference

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

   Value      TLV name                   References

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   -----     --------                   ----------
    1       PCE-ADDRESS               This document
    2       PATH-SCOPE                This document
    3       PCE-DOMAIN                This document
    4       NEIG-PCE-DOMAIN           This document
    5       PCE-CAP-FLAGS             This document
    6       CONGESTION                This document

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

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 top-level OSPF registry, the
   "PCE Capability Flags" 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.

   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        Path computation with GMPLS link constraints
      1        Bidirectional path computation
      2        Diverse path computation
      3        Load-balanced path computation
      4        Synchronized paths computation
      5        Support for multiple objective functions
      6        Support for additive path constraints
               (max hop count, etc.)
      7        Support for request prioritization
      8        Support for multiple requests per message








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8. Security Considerations

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

   Mechanisms defined to ensure authenticity and integrity of OSPF LSAs
   [RFC2154], and their TLVs, can be used to secure the PCE Discovery
   information as well.

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


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 neighbour PCE 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. OSPF already includes a liveness detection mechanism
   (Hello protocol), and PCE discovery does not require additional
   capabilities.

   Procedures defined in section 5.1 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

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   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 OSPF extensions defined in this documents do not imply any
   requirement on other protocols.

9.6. Impact on network operations

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

   As discussed in section 5.1, a PCE implementation SHOULD support an
   appropriate dampening algorithm so as to dampen OSPF flooding in
   order to not impact the OSPF 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, Adrian Farrel, Les Ginsberg, Mike
   Shand and Lou Berger 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.

   [RFC2740] Coltun, R., Ferguson, D., and J. Moy, "OSPF for IPv6",
             RFC 2740, December 1999.

   [RFC2370] Coltun, R., "The OSPF Opaque LSA Option", RFC 2370, July
   1998.

   [RFC3630] Katz, D., Yeung, D., Kompella, K., "Traffic Engineering
   Extensions to OSPF Version 2", RFC 3630, September 2003.

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   [OSPF-CAP] Lindem, A., Shen, N., Aggarwal, R., Shaffer, S., Vasseur,
   J.P., "Extensions to OSPF for advertising Optional Router
   Capabilities", draft-ietf-ospf-cap, 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.

   [RFC2154] Murphy, S., Badger, M., and B. Wellington, "OSPF with
   Digital Signatures", RFC 2154, June 1997.

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, work in
   progress.

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


12. Editor's 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






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13. Contributors' Addresses:

   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
   Email: raymond_zhang@bt.infonet.com


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   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
   rights, licenses and restrictions contained in BCP 78, and except as
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