Network Working Group                                             E. Oki
Internet Draft                                                       NTT
Category: Standards Track                                      A. Farrel
Expires: June 2007                                    Old Dog Consulting

                                                            January 2007

    Extensions to the Path Computation Element Communication Protocol
                       (PCEP) for Route Exclusions

                      draft-oki-pce-pcep-xro-00.txt

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Abstract

   The Path Computation Element (PCE) provides functions of path
   computation in support of traffic engineering in Multi-Protocol
   Label Switching (MPLS) and Generalized MPLS (GMPLS) networks.

   When a Path Computation Client (PCC) requests a PCE for a route, it
   may be useful for the PCC to specify as constraints to the path
   computation abstract nodes, resources, and Shared Risk Link Groups
   (SRLGs) that are to be explicitly excluded from routes. Such
   constraints are termed route exclusions.

   The PCE Communication Protocol (PCEP) is designed as a communication
   protocol between PCCs and PCEs. This document presents PCEP
   extensions for route exclusions.


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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
   [RFC2119].


Table of Contents

   1. Introduction...................................................2
   2. Protocol Procedures and Extensions.............................3
   2.1.  Exclude Route Object (XRO)..................................4
   2.2.1.  Processing Rules..........................................7
   2.2.  Explicit Route Exclusion....................................8
   2.2.1. Processing Rules...........................................9
   3. IANA Considerations...........................................10
   3.1.  PCEP Objects...............................................10
   3.2.  Error Object Field Values..................................10
   4. Manageability considerations..................................10
   5. Security Considerations.......................................11
   6. References....................................................11
   6.1.  Normative Reference........................................11
   6.2.  Informative Reference......................................11
   7. AuthorsEAddresses............................................11


1. Introduction

   The Path Computation Element (PCE) defined in [RFC4655] is an entity
   that is capable of computing a network path or route based on a
   network graph, and applying computational constraints. A Path
   Computation Client (PCC) may make requests to a PCE for paths to be
   computed.

   When a PCC requests a PCE for a route, it may be useful for the PCC
   to specify abstract nodes, resources, and Shared Risk Link Groups
   (SRLGs) that are to be explicitly excluded from the route.

   For example, disjoint paths for inter-domain LSPs may be computed by
   cooperation between PCEs, each of which computes segments of the
   paths across one domain. In order to achieve path computation for a
   secondary (backup) path, a PCE may act as a PCC to request another
   PCE for a route that must be a node/link/SRLG disjoint from the
   primary (working) path. Another example is where a network operator
   wants path to avoid specified nodes for administrative reasons
   perhaps because the specified nodes will be out-of-services in near
   future.


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   [RFC4657] specifies generic requirements for a communication
   protocol between PCCs and PCEs. Generic constraints described in
   [RFC4657] include route exclusions for links, nodes, and SRLGs. That
   is, the requirement for support of route exclusions within the PCC-
   PCE communication protocol is already established.

   The PCE communication protocol (PCEP) is designed as a communication
   protocol between PCCs and PCEs and is defined in [PCEP]. This
   document presents PCEP extensions to satisfy the requirements for
   route exclusions as described in Sections 5.1.4 and 5.1.16 of
   [RFC4657].

   Note that MPLS-TE and GMPLS signaling extensions for communicating
   route exclusions between network nodes for specific Label Switched
   Paths (LSPs) are described in [XRO]. Route exclusions may be
   specified during provisioning requests for specific LSPs using the
   mplsTunnelHopInclude object of MPLS-TE-STD-MIB defined in [RFC3812].

2. Protocol Procedures and Extensions

   This section describes the procedures adopted by a PCE handling a
   request for path computation with route exclusions received from a
   PCC, and defines how those exclusions are encoded.

   There are two types of route exclusion described in [XRO].

   1. Exclusion of certain abstract nodes or resources on the whole
      path. This set of abstract nodes is referred to as the Exclude
      Route List.

   2. Exclusion of certain abstract nodes or resources between a
      specific pair of abstract nodes present in an explicit path. Such
      specific exclusions are referred to as an Explicit Route
      Exclusion.

   This document defines protocol extensions to allow a PCC to specify
   both types of route exclusions to a PCE on a path computation
   request.

   A new PCEP object is defined as the Exclude Route Object (XRO) to
   convey the Exclude Route List. The existing Include Route Object
   (IRO) in PCEP [PCEP] is modified by introducing a new IRO subobject,
   the Explicit Exclusion Route Subobject (EXRS), to convey Explicit
   Route Exclusions.







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2.1. Exclude Route Object (XRO)

   The XRO is OPTIONAL and MAY be carried within PCReq and PCRep
   messages.

   When present in a PCReq message, the XRO provides a list of network
   resources that the PCE is requested to exclude from the path that it
   computes. Flags associated with each list member instruct the PCE as
   to whether the network resources must be excluded from the computed
   path or whether the PCE should make best efforts to exclude the
   resources from the computed path.

   The XRO MAY be used on PCRep message with the NO-PATH object to
   indicate the set of elements of the original XRO that prevented  the
   PCE from finding a path. The XRO MAY also be used on a PCRep message
   for a successful path computation when the PCE wishes to provide a
   set of exclusions to be signaled during LSP setup.

   XRO Object-Class is to be assigned by IANA (recommended value=17)

   XRO Object-Type is to be assigned by IANA (recommended value=1)

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   //                        (Subobjects)                         //
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 1: XRO body format

   Subobjects. The XRO is up made of one or more subobject(s). An XRO
   with no subobjects MUST not be sent and SHOULD be ignored on receipt.

   In the following subobject definitions a set of fields have
   consistent meaning as follows:

      X
        The X-bit indicates whether the exclusion is mandatory or
        desired. 0 indicates that the resource specified MUST be
        excluded from the path computed by the PCE 1 indicates that the
        resource specified SHOULD be excluded from the path computed by
        the PCE, but MAY be included subject to PCE policy and the
        absence of a viable path that meets the other other constraints
        and excludes the resource.





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      Type
        The type of the subobject. The following subobject types are
        defined.

        Type           Subobject
        -------------+-------------------------------
        1              IPv4 prefix
        2              IPv6 prefix
        3              Unnumbered Interface ID
        4              Autonomous system number
        5              SRLG

      Length
        The length of the subobject including the Type and Length
        fields.

      Prefix Length
        Where present, this field can be used to indicate a set of
        addresses matching a prefix. If the subobject indicates a
        single address, the prefix length MUST be set to the full
        length of the address.

      Attribute
        The Attribute field indicates how the exclusion subobject is to
        be interpreted.

        0 Interface
          The subobject is to be interpreted as an interface or set of
          interfaces. All interfaces identified by the subobject are to
          be excluded from the computed path according to the setting
          of the X-bit. This value is valid only for subobject types 1,
          2, and 3.

        1 Node
          The subobject is to be interpreted as a node or set of nodes.
          All nodes identified by the subobject are to be excluded from
          the computed path according to the setting of the X-bit. This
          value is valid only for subobject types 1, 2, 3, and 4.

        2 SRLG
          The subobject identifies an SRLG explicitly or indicates all
          of the SRLGs associated with the resource or resources
          identified by the subobject. Resources that share any SRLG
          with those identified are to be excluded from the computed
          path according to the setting of the X-bit. This value is
          valid for all subobjects.

      Reserved
        Reserved fields MUST be transmitted as zero and SHOULD be
        ignored on receipt.

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   The subobjects are encoded as follows:

    IPv4 prefix Subobject

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |X|  Type = 1   |     Length    | IPv4 address (4 bytes)        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | IPv4 address (continued)      | Prefix Length |   Attribute   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    IPv6 prefix Subobject

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |X|  Type = 2   |     Length    | IPv6 address (16 bytes)       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | IPv6 address (continued)                                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | IPv6 address (continued)                                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | IPv6 address (continued)                                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | IPv6 address (continued)      | Prefix Length |   Attribute   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Unnumbered Interface ID Subobject

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |X|  Type = 3   |     Length    |    Reserved   |  Attribute    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        TE Router ID                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Interface ID                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       The TE Router ID and Interface ID fields are as defined in
       [RFC3477].









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    Autonomous System Number Subobject

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |X|  Type = 4   |     Length    |    Reserved   |  Attribute    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Optional AS Number High Octets|      2-Octet AS Number        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      If a two-octet AS number is used, the optional AS Number High
      Octets MUST be set to zero.

    SRLG Subobject

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |X|  Type = 5   |     Length    |       SRLG Id (4 bytes)       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      SRLG Id (continued)      |    Reserved   |  Attribute    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The Attribute SHOULD be set to two (2) and SHOULD be ignored on
      receipt.

2.2.1. Processing Rules

   A PCC builds an XRO to encode all of the resources that it wishes
   the PCE to exclude from the path that it is requested to compute.
   For each exclusion, the PCC clears the X-bit to indicate that the
   PCE is required to exclude the resources, or sets the X-bit to
   indicate that the PCC simply desires that the resources are excluded.
   For each exclusion, the PCC also sets the Attribute field to
   indicate how the PCE should interpret the contents of the exclusion
   subobject.

   When a PCE receives a PCReq message it looks for an XRO to see if
   exclusions are required. If the PCE finds more than one XRO it MUST
   use the first one in the message and MUST ignore subsequent
   instances.

   If the PCE does not recognize the XRO it MUST return a PCErr message
   with Error-Type "Unknown Object" as described in [PCEP].

   If the PCE is unwilling on unable to process the XRO it MUST return
   a PCErr message with the Error-Type "Not supported object" and
   follow the relevant procedures described in [PCEP].



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   If the PCE processes the XRO and attempts to compute a path, it MUST
   adhere to the requested exclusions as expressed in the XRO. That is,
   the returned path MUST NOT include any resources encoded with the X-
   bit clear, and SHOULD NOT include any with the X-bit set unless
   alternate paths that match the other constraints expressed in the
   PCReq are unavailable.

   When a PCE returns a path in a PCRep it MAY also supply an XRO. In
   this case, the PCC SHOULD apply the contents using the same rules as
   in [XRO] and SHOULD signal the an RSVP-TE XRO to indicate the
   exclusions that downstream LSRs should apply. This may be
   particularly useful in per-domain path computation scenarios. [Note
   that this does not match the behavior for an explicit path where an
   IRO is used to force inclusions and an ERO is used to report a
   computed path. We could consider using a separate object to report
   the XRO that should be signaled.]

   In the event that no suitable path can be computed and the PCE
   returns a PCRep message containing a NO-PATH object, the PCE MAY
   also include an XRO that lists one or more subobjects from the
   original XRO that have contributed to the PCE's inability to select
   a path.

2.2. Explicit Route Exclusion

   Explicit Route Exclusion defines network elements that must not or
   should not be used on the path between two abstract nodes or
   resources explicitly indicated in the Include Route Object (IRO)
   [PCEP]. This information is encoded by defining a new subobject for
   the IRO .

   The new IRO subobject, the Explicit Exclusion Route Subobject (EXRS),
   has type defined by IANA (see Section 3.). The EXRS contains one or
   more subobjects in its own right. An EXRS MUST NOT be sent with no
   subobjects, and if received with no subobjects MUST be ignored.

   The format of the EXRS 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |L|    Type     |     Length    |           Reserved            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     //                One or more EXRS subobjects                  //
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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       L
         MUST be set to zero on transmission and MUST be ignored on
         receipt.

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

   The EXRS subobject may carry any of the subobjects defined for
   inclusion in the XRO by this document or by future documents. The
   meanings of the fields of the XRO subobjects are unchanged when the
   subobjects are included in an EXRS, except that scope of the
   exclusion is limited to the single hop between the previous and
   subsequent elements in the IRO.

2.2.1. Processing Rules

   A PCC that supplies a partial explicit route to a PCE in an IRO MAY
   also specify explicit exclusions by including one or more EXRSes in
   the IRO.

   If a PCE parses an IRO in a received PCReq message and encounters an
   EXRS and does not recognize the subobject it MUST respond with a
   PCErr message using the Error-Type "Unrecognized IRO subobject" and
   set the Error-Value to the subobject type code of the EXRS (see
   Section 3).

   If a PCE parses an IRO and encounters an EXRS that it recognizes,
   but detects an EXRS subobject that it does not recognize it MUST act
   according to the setting of the X-bit in the subobject. If the X-bit
   is clear, the PCE MUST respond with a PCErr with Error-Type
   "Unrecognized EXRS subobject" and set the Error-Value to the EXRS
   subobject type code (see Section 3). If the X-bit is set, the PCE
   MAY respond with a PCErr as already stated or MAY ignore the EXRS
   subobject: this choice is a local policy decision.

   If a PCE parses an IRO and encounters an EXRS subobject that it
   recognizes, it MUST act according to the requirements expressed in
   the subobject. That is, if the X-bit is clear, the PCE MUST NOT
   produce a path that includes any resource identified by the EXRS
   subobject in the path between the previous abstract node in the IRO
   and the next abstract node in the IRO. If the X-bit is set, the PCE
   SHOULD NOT produce a path that includes any resource identified by
   the EXRS subobject in the path between the previous abstract node in
   the IRO and the next abstract node in the IRO unless it is not
   possible to construct a path that avoids that resource while still
   complying with the other constraints expressed in the PCReq message.

   A successful path computation reported in a PCRep message MUST
   include an ERO to specify the path that has been computed. That ERO

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   MAY contain specific route exclusions using the EXRS as specified in
   [XRO].

   If the path computation fails and a PCErr is returned with a NO-PATH
   object, the PCE MAY include an IRO to report the hops that could not
   be complied with, and that IRO MAY include EXRSes.

3. IANA Considerations

3.1. PCEP Objects

   The "PCEP Parameters" registry contains a subregistry "PCEP Objects".
   IANA is requested to make the following allocations from this
   registry.

   Object  Name          Object  Name
   Class                 Type
     17    XRO             1     Route exclusion

3.2. Error Object Field Values.

   The "PCEP Parameters" registry contains a subregistry "PCEP Errors".
   IANA is requested to make the following allocations from this
   registry.

   Values in this section are recommended and to be confirmed by IANA.

      Error   Meaning and Error-Values
      Type

      11      Unrecognized IRO subobject

   Note that this Error-Type has been omitted from [PCEP] where it is
   required. It is expected that it will be added to a later version of
   [PCEP] and removed from this document.

      12      Unrecognized EXRS subobject

4. Manageability Considerations

   A MIB module for management of the PCEP is specified in a separate
   document. This MIB module allows examination of individual PCEP
   messages, in particular requests, responses and errors.

   The MIB module MUST be extended to include the ability to view the
   route exclusion extensions defined in this document.





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5. Security Considerations

   The new exclude route mechanisms defined in this document allow
   finer and more specific control of the path computed by a PCE. Such
   control increases the risk if a PCEP message is intercepted,
   modified, or spoofed. Therefore, the security techniques described
   in [PCEP] are considered more important.

6. References

6.1. Normative Reference

   [RFC2119] Bradner, S., "Key words for use in RFCs to indicate
   requirements levels", RFC 2119, March 1997.

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

6.2. Informative Reference

   [RFC3812] Srinivasan, C., Viswanathan, A., and T. Nadeau,
   "Multiprotocol Label Switching (MPLS) Traffic Engineering (TE)
   Management Information Base (MIB)", RFC 3812, June 2004.

   [RFC4655] A. Farrel, JP. Vasseur and J. Ash, "A Path Computation
   Element (PCE)-Based Architecture", RFC 4655, September 2006.

   [RFC4657] J. Ash and J.L. Le Roux, "Path Computation Element (PCE)
   Communication Protocol Generic Requirements", RFC 4657, September
   2006.

   [XRO] Lee et al, "Exclude Routes - Extension to RSVP-TE", draft-
   ietf-ccamp-rsvp-te-exclude-route, (work in progress).

7. Authors' Addresses

   Eiji Oki
   NTT
   3-9-11 Midori-cho,
   Musashino-shi, Tokyo 180-8585, Japan
   Email: oki.eiji@lab.ntt.co.jp

   Adrian Farrel
   Old Dog Consulting
   Email: adrian@olddog.co.uk





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