Network Working Group                                       S. Sivabalan
Internet-Draft                                                 J. Medved
Intended status: Standards Track                             C. Filsfils
Expires: January 13, 2014                            Cisco Systems, Inc.
                                                               E. Crabbe
                                                            Google, Inc.
                                                               R. Raszuk
                                                                  NTT I3
                                                           July 12, 2013


                  PCEP Extensions for Segment Routing
               draft-sivabalan-pce-segment-routing-01.txt

Abstract

   Segment Routing (SR) enables any head-end node to select any path
   without relying on a hop-by-hop signaling technique (e.g., LDP or
   RSVP-TE).  It depends only on "segments" that are advertised by Link-
   State Interior Gateway Protocols (IGPs).  A Segment Routed Path can
   be derived from a variety of mechanisms, including an IGP Shortest
   Path Tree (SPT), explicit configuration, or a Path Computation
   Element (PCE).  This document specifies extensions to the Path
   Computation Element Protocol (PCEP) that allow a stateful PCE to
   compute and instantiate Traffic Engineering paths, as well as a PCC
   to request a path subject to certain constraint(s) and optimization
   criteria in SR networks.

Requirements Language

   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].

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."



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   This Internet-Draft will expire on January 13, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Overview of PCEP Operation in SR Networks  . . . . . . . . . .  6
   4.  SR-Specific PCEP Message Extensions  . . . . . . . . . . . . .  7
     4.1.  The PCReq Message  . . . . . . . . . . . . . . . . . . . .  7
     4.2.  The PCRep Message  . . . . . . . . . . . . . . . . . . . .  7
     4.3.  The PCInitiate Message . . . . . . . . . . . . . . . . . .  8
     4.4.  The PCRpt Message  . . . . . . . . . . . . . . . . . . . .  8
     4.5.  The PCUpd Message  . . . . . . . . . . . . . . . . . . . .  9
   5.  Object Formats . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.1.  The OPEN Object  . . . . . . . . . . . . . . . . . . . . .  9
       5.1.1.  The SR PCE Capability TLV  . . . . . . . . . . . . . .  9
     5.2.  The RP Object  . . . . . . . . . . . . . . . . . . . . . . 11
       5.2.1.  The LSP-PATH-TYPE TLV  . . . . . . . . . . . . . . . . 11
     5.3.  The SR-ERO Object  . . . . . . . . . . . . . . . . . . . . 12
       5.3.1.  The SR-ERO Subobject . . . . . . . . . . . . . . . . . 12
       5.3.2.  NAI Associated with SID  . . . . . . . . . . . . . . . 14
       5.3.3.  SR-ERO Processing  . . . . . . . . . . . . . . . . . . 15
   6.  Backward Compatibility . . . . . . . . . . . . . . . . . . . . 15
   7.  Management Considerations  . . . . . . . . . . . . . . . . . . 16
     7.1.  Policy . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     7.2.  The PCEP Data Model  . . . . . . . . . . . . . . . . . . . 16
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
     9.1.  PCEP Objects . . . . . . . . . . . . . . . . . . . . . . . 16
       9.1.1.  LSP-SIG-TYPE field in the LSP object . . . . . . . . . 16
     9.2.  PCEP-Error Object  . . . . . . . . . . . . . . . . . . . . 17
     9.3.  PCEP TLV Type Indicators . . . . . . . . . . . . . . . . . 17
       9.3.1.  LSP-PATH-TYPE Indicators . . . . . . . . . . . . . . . 17
   10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 17
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 18
     12.2. Informative References . . . . . . . . . . . . . . . . . . 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19














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1.  Introduction

   SR technology leverages the source routing and tunneling paradigms.
   A source node can choose a path without relying on hop-by-hop
   signaling protocols such as LDP or RSVP-TE.  Each path is specified
   as a set of "segments" advertised by link-state routing protocols
   (IS-IS or OSPF).  [I-D.filsfils-rtgwg-segment-routing] provides an
   introduction to SR technology.  The corresponding IS-IS and OSPF
   extensions are specified in
   [I-D.previdi-isis-segment-routing-extensions] and
   [I-D.psenak-ospf-segment-routing-extensions], respectively.  Two
   types of segments have been defined; nodal and adjacency segments.  A
   nodal segment represents a path to a node, whereas an adjacency
   segment represents a specific adjacency to a node.  The SR
   architecture can be applied to MPLS forwarding plane without any
   change as well as IPv6 forwarding plane with a new type of routing
   extension header.  A Segment Identifier (SID) is a 32-bit value.  In
   the case of the MPLS data plane, an SR path corresponds to an MPLS
   Label Switching Path (LSP)

   A Segment Routed path (SR path) can be derived from an IGP Shortest
   Path Tree (SPT).  Segment Routed Traffic Engineering paths (SR-TE
   paths) may not follow IGP SPT.  Such paths may be chosen by a
   suitable network planning tool and provisioned on the source node of
   the SR-TE path.

   [RFC5440] describes Path Computation Element Protocol (PCEP) for
   communication between a Path Computation Client (PCC) and a Path
   Control Element (PCE) or between one a pair of PCEs.  A PCE computes
   paths for MPLS Traffic Engineering LSPs (MPLS-TE LSPs) based on
   various constraints and optimization criteria.
   [I-D.ietf-pce-stateful-pce] specifies extensions to PCEP that allow a
   stateful PCE to compute and recommend network paths in compliance
   with [RFC4657] and defines objects and TLVs for MPLS-TE LSPs.
   Stateful PCEP extensions provide synchronization of LSP state between
   a PCC and a PCE or between a pair of PCEs, delegation of LSP control,
   reporting of LSP state from a PCC to a PCE, controlling the setup and
   path routing of an LSP from a PCE to a PCC.  Stateful PCEP extensions
   are intended for an operational model in which LSPs are configured on
   the PCC, and control over them is delegated to the PCE.

   A mechanism to dynamically instantiate LSPs on a PCC based on the
   requests from a stateful PCE or a controller using stateful PCE is
   specified in [I-D.crabbe-pce-pce-initiated-lsp].  Such mechanism is
   useful in Software Driven Networks (SDN) applications, such as demand
   engineering, or bandwidth calendaring.

   It is possible to use a stateful PCE for computing one or more SR-TE



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   paths taking into account various constraints and objective
   functions.  Once a path is chosen, the stateful PCE can instantiate
   an SR-TE path on the PCC using PCEP extensions specified in
   [I-D.crabbe-pce-pce-initiated-lsp] along with the SR specific PCEP
   extensions provided in this document.  Similarly, a PCC can request
   an SR path from either stateful or a stateless PCE.


2.  Terminology

   The following terminologies are used in this document:

   ERO:  Explicit Route Object

   IGP:  Interior Gateway Protocol

   IS-IS:  Intermediate System to Intermediate System

   LSR:  Label Switching Router

   MSD:  Maximum SID Depth

   NAI:  Node or Adjacency Identifier

   OSPF:  Open Shortest Path First

   PCC:  Path Computation Client

   PCE:  Path Computation Element

   PCEP:  Path Computation Element Protocol

   RRO:  Record Route Object

   SID:  Segment Identifier

   SR:  Segment Routing

   SR-ERO:  Segment Routed Explicit Route Object

   SR Path:  Segment Routed Path

   SR-RRO:  Segment Routed Record Route Object

   SR-TE Path:  Segment Routed Traffic Engineering Path






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3.  Overview of PCEP Operation in SR Networks

   In SR networks, an ingress node of an SR path appends all outgoing
   packets with an SR header consisting of a list of Segment IDs (SIDs).
   The header has all necessary information to guide the packets from
   the ingress node to the egress node of the path, and hence there is
   no need for any signaling protocol.  A SID can represent a nodal
   segment representing a path to a node or adjacency segment
   representing path over a specific adjacency.

   In a PCEP session, path information is carried in the Explicit Route
   Object (ERO), which consists of a sequence of subobjects.  Various
   types of ERO subobjects have been specified in [RFC3209], [RFC3473],
   and [RFC3477].  In SR networks, a PCE needs to specify ERO containing
   SIDs, and a PCC should be capable of processing such ERO.  An ERO
   containing SIDs can be included in the Path Computation LSP Initiate
   Request message (PCInitiate) defined in
   [I-D.crabbe-pce-pce-initiated-lsp], as well as in the Path
   Computation LSP Update Request (PCUpd) and Path Computation LSP State
   Report (PCRpt) messages defined in Report (PCRpt) messages defined in
   [I-D.ietf-pce-stateful-pce].

   When a PCEP session between a PCC and a PCE is established, both PCEP
   Speakers exchange information to indicate their ability to support
   SR-specific functionality.  A PCEP session can carry EROs of
   different types.  However, an ERO carrying SIDs MUST NOT include any
   other form of EROs, i.e., all subobjects within an ERO MUST represent
   SID.  Furthermore, if an SR path is established using SR-ERO,
   subsequent PCEP Update and Report messages for that path MUST NOT
   contain other ERO types.  This document specifies new error codes to
   handle these errors.  Should the need to change the ERO type arise,
   the SR path must be deleted and re-created using a new ERO type.

   A PCC MAY include an ERO object in a PCRpt message.  In SR networks,
   a PCC MAY learn the SR actual path actually taken by data packets and
   report that path to a PCE.  Methods used by a PCC to learn SR-TE
   paths are outside the scope of this document.

   In summary, this document:

   o  Defines a new PCEP capability, new subobjects, a new TLV, and new
      PCEP error codes

   o  Specifies how two PCEP Speakers can establish a PCEP session that
      can cary segment routing paths

   o  Defines the formats of SR-specific PCEP messages in Backus-Naur
      Format (BNF).



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   This document specifies SR extensions for the stateless PCE model
   defined in [RFC5440], as well as for the active stateful and passive
   stateful PCE models defined in [I-D.ietf-pce-stateful-pce].


4.  SR-Specific PCEP Message Extensions

   As defined in [RFC5440], a PCEP message consists of a common header
   followed by a variable length body made up of mandatory and/or
   optional objects.  PCEP messages an their formats for stateless PCE
   are defined in [RFC5440].  PCEP messages and their formats for
   stateful PCE are defined in [I-D.ietf-pce-stateful-pce].  Finally,
   PCEP messages and their formats for PCE-initiated LSP instantiation
   are defined in [I-D.crabbe-pce-pce-initiated-lsp].

   This document defines changes to PCEP messages and their formats
   required to carry SR-specific information.

4.1.  The PCReq Message

   This document does not specify any changes to the PCReq message
   format.  This document proposes a new optional TLV carried in the RP
   Object (Section 5.2.1), which can be used by a PCC to request path
   computation for one or more SR TE Paths.

4.2.  The PCRep Message

   This document defines the format of the PCRep message carrying SR TE
   Paths.  The message is sent by a PCE to a PCC in response to a
   previously received PCReq message, where the PCC requested
   computation of SR TE Paths.  The format of the SR-specific PCRep
   message is as follows:

      <PCRep Message> ::= <Common Header>
                          <response-list>
   Where:

      <response-list>::=<response>[<response-list>]

      <response> ::= <RP>
                     [<NO-PATH>]
                     [<path-list>]

   Where:
       <path-list>::=<SR-ERO>[<path-list>]

   The RP and NO-PATH Objects are defined in [RFC5440].  The <SR-ERO>
   object contains the SR TE path and is defined in Section 5.3.



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4.3.  The PCInitiate Message

   The format of the PCInitiate message is as follows:

      <PCInitiate Message> ::= <Common Header>
                               <PCE-initiated-lsp-list>
   Where:

      <PCE-initiated-lsp-list> ::=
                 <PCE-initiated-lsp-request>[<PCE-initiated-lsp-list>]

      <PCE-initiated-lsp-request> ::= <SRP>
                                      <LSP>
                                      <SR-ERO>

   The <SR-ERO> object contains the SR TE path and is defined in
   Section 5.3.  The <LSP> object in the Common Header MUST include the
   SYMBOLIC-PATH-NAME TLV.

4.4.  The PCRpt Message

   An SR-specific PCRpt message is sent by a PCC to a PCE to report the
   current state of an SR TE Path.  A PCRpt message can carry more than
   one LSP State Report, but all LSP State reports in the SR-Specific
   PCRpt message MUST be for SR TE Paths.  A PCC can send an LSP State
   Report either in response to an LSP Update Request from a PCE, or
   asynchronously when the state of an SR TE Path changes.

   The format of the SR-specific PCRpt message is as follows:

      <PCRpt Message> ::= <Common Header>
                          <state-report-list>
   Where:

      <state-report-list> ::= <state-report>[<state-report-list>]

      <state-report> ::= <SRP>
                         <LSP>
                         <sr-te-path>

   Where:
       <sr-te-path> ::= <SR-ERO>

   The <SR-ERO> object contains the actual SR TE path used by the PCC
   and is defined in Section 5.3.  The actual SR TE Path may be
   different from the programmed SR TE Path, for example, when the
   programmed SR TE Path contains loose hops and the PCC must compute
   the path between loose hops locally.



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   The <SRP> and <LSP> objects are defined in
   [I-D.ietf-pce-stateful-pce].  The <LSP> object MUST include the
   SYMBOLIC-PATH-NAME TLV.  The LSP-sig-type filed in the LSP object
   MUST be set to TBD (Segment Routing).

4.5.  The PCUpd Message

   An SR-Specific PCUpd message is sent by a PCE to a PCC to update an
   SR TE Path.  A PCUpd message can carry more than one LSP Update
   Request.

   The format of the SR-specific PCUpd message is as follows:

      <PCUpd Message> ::= <Common Header>
                          <udpate-request-list>
   Where:

      <update-request-list> ::= <update-request>[<update-request-list>]

      <update-request> ::= <SRP>
                           <LSP>
                           <sr-te-path>
   Where:
      <sr-te-path>::= <SR-ERO>

   The <SR-ERO> object contains the SR TE path computed by the PCE, and
   is defined in Section 5.3.  The <SRP> and <LSP> objects are defined
   in [I-D.ietf-pce-stateful-pce].  The LSP object MUST include the
   SYMBOLIC-PATH-NAME TLV.  The LSP-sig-type filed in the LSP object
   MUST be set to TBD (Segment Routing).  Note that compared to the
   RSVP-TE-specific PCUpd message defined in
   [I-D.ietf-pce-stateful-pce], the path in the SR-specific PCUpd
   message does not have attributes, only hops specified in the <SR-ERO>
   object.


5.  Object Formats

5.1.  The OPEN Object

   This document defines a new optional TLV for use in the OPEN Object.

5.1.1.  The SR PCE Capability TLV

   The SR-PCE-CAPABILITY TLV is an optional TLV for use in the OPEN
   Object to negotiate Segment Routing capability on the PCEP session.
   The format of the SR-PCE-CAPABILITY TLV is shown in the following
   figure:



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       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=TBD           |            Length=4           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Reserved              |     Flags     |      MSD      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                  Figure 1: SR-PCE-CAPABILITY TLV format

   The code point for the TLV type is to be defined by IANA.  The TLV
   length is 4 octets.

   The 32-bit value is formatted as follows.  The "Maximum SID Depth" (1
   octet) field (MSD) specifies the maximum number of SIDs that a PCC is
   capable of imposing on a packet.  The "Flags" (1 octet) and
   "Reserved" (2 octets) fields are currently unused, and MUST be set to
   zero and ignored on receipt.

5.1.1.1.  Negotiating SR Capability

   The SR capability TLV is contained in the OPEN object.  By including
   the TLV in the OPEN message to a PCE, a PCC indicates its support for
   SR-TE Paths.  By including the TLV in the OPEN message to a PCC, a
   PCE indicates that it is capable of computing SR-TE paths.

   The number of SIDs that can be imposed on a packet depends on PCC's
   data plane's capability.  The default value of MSD is 0 meaning that
   a PCC does not impose any limitation on the number of SIDs included
   in any SR-TE path coming from PCE.  Once an SR-capable PCEP session
   is established with a non-default MSD value, the corresponding PCE
   cannot send SR-TE paths with SIDs exceeding the MSD value.  If a PCC
   needs to modify the MSD value, the PCEP session must be closed and
   re-established with the new MSD value.  If a PCEP session is
   established with a non-default MSD value, and the PCC receives an
   SR-TE path containing more SIDs than specified in the MSD value, the
   PCC MUST send out a PCErr message with Error-Type 10 (Reception of an
   invalid object) and Error-value 3 (Unsupported number of Segment
   ERO).

   The SR Capability TLV is meaningful only in the OPEN message sent
   from a PCC to a PCE.  As such, a PCE does not need to set MSD value
   in outbound message to a PCC.  Similarly, an MSD value received by a
   PCC is ignored.  If there are multiple SR capability TLVs, only the
   first TLV is processed.

   All bits in the Reserved and Flags fields SHOULD be set to 0 on



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   outbound OPEN messages, and MUST be ignored on inbound OPEN messages.

5.2.  The RP Object

   This document defines a new optional TLV for use in the RP Object.

5.2.1.  The LSP-PATH-TYPE TLV

   A PCC can simultaneously support both RSVP-TE signaled MPLS LSPs as
   well as SR-TE paths.  In this case, the PCC needs to query and
   receive paths specified with RSVP-TE EROs (defined in [RFC5440]) and
   paths specified with SR-EROs (defined in this document).  Thus, there
   is a need for a PCC to identify the ERO type that it wishes to
   receive from a PCE.

   This document defines a new optional TLV called "LSP-PATH-TYPE" that
   MAY be included in the RP object (defined in [RFC5440]) on a PCReq
   message from a PCC to a PCE.  The format of this TLV is shown in the
   following figure:

       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= TBD           |            Length=1           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | LSP-Path-Type |
      +-+-+-+-+-+-+-+-+


                    Figure 2: LSP-PATH-TYPE TLV format

   The type of the TLV is to be defined by IANA.  The TLV length is 1
   octets.

   The 8-bit value contains the Path-Type (PT).  The following values
   for Path Type are defined:

   o  PT = 0: Requested path is to be used with RSVP-TE signaling
      (default).

   o  PT = 1: SR-TE path is requested.

   If this TLV is not included in the PCReq message, the default Path
   Type of 0 is assumed, otherwise the path type specified in the TLV is
   used.  An RP object SHOULD carry no more than one LSP-PATH-TYPE TLV,
   only the first is used if several are present, the others are
   ignored.




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5.3.  The SR-ERO Object

   An SR-TE path consists of one or more SID(s) where each SID is
   associated with the identifier that represents the node or adjacency
   corresponding to the SID.  This identifier is referred to as the
   'Node or Adjacency Identifier' (NAI).  As described later, a NAI can
   be represented in various formats (e.g., IPv4 address, IPv6 address,
   etc).  Furthermore, a NAI is used only for troubleshooting purposes,
   and MUST not be used to replace or modify any fields in a data packet
   header.  An SR-ERO object consists of one or more ERO subobjects
   described in the following section.

5.3.1.  The SR-ERO Subobject

   An SR-ERO subobject consists of a 32-bit header followed by the SID
   and the NAI associated with the SID.  The SID is a 32-bit number.
   The size of the NAI depends on its respective type, as described in
   the following sections.

      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    |  ST   |     Flags     |F|S|C|M|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              SID                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                        NAI (variable)                       //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 3: SR-ERO Subobject format

   The fields in the ERO Subobject are as follows:

   The 'L' Flag  indicates whether the subobject represents a loose-hop
      in the explicit route [RFC3209].  If this flag is unset, a PCC
      MUST not overwrite the SID value present in the SR-ERO subobject.
      Otherwise, a PCC MAY expand or replace one or more SID value(s) in
      the received SR-ERO based on its local policy.


   Type  is the type of the SR-ERO Subobject.  This document defines the
      SR-ERO Subobject type.  A new code point will be requested for the
      SR-ERO Subobject from IANA.








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   Length  contains the total length of the subobject in octets,
      including the L, Type and Length fields.  Length MUST be at least
      4, and MUST be a multiple of 4.


   SID Type (ST)  indicates the type of information associated with the
      SID contained in the object body.  The SID-Type values are
      described later in this document.


   Flags  is used to carry any additional information pertaining to SID.
      Currently, the following flag bits are defined:



      *  M: When this bit is set, the SID value represents an MPLS label
         stack entry as specified in [RFC5462] where only the label
         value is specified by the PCE.  Other fields (TC, S, and TTL)
         fields MUST be considered invalid, and PCC MUST set these
         fields according to its local policy and MPLS forwarding rules.


      *  C: When this bit as well as the M bit are set, then the SID
         value represents an MPLS label stack entry as specified in
         [RFC5462], where all the entry's fields (Label, TC, S, and TTL)
         are specified by the PCE.  However, a PCC MAY choose to
         override TC, S, and TTL values according its local policy and
         MPLS forwarding rules.


      *  S: When this bit is set, the SID value in the subobject body is
         null.  In this case, the PCC is responsible for choosing the
         SID value, e.g., by looking up its Traffic Engineering Database
         (TED) using node/adjacency identifier in the subobject body.


      *  F: When this bit is set, the NAI value in the subobject body is
         null.


   SID  is the Segment Identifier.


   NAI  contains the NAI associated with the SID.  Depending on the
      value of ST, the NAI can have different format as described in the
      following section.





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5.3.2.  NAI Associated with SID

   This document defines the following NAIs:

   'IPv4 Node ID'  is specified as an IPv4 address.  In this case, ST
      and Length are 1 and 12 respectively.


   'IPv6 Node ID'  is specified as an IPv6 address.  In this case, ST
      and Length are 2 and 24 respectively.


   'IPv4 Adjacency'  is specified as a pair of IPv4 addresses.  In this
      case, ST and Length are 3 and 16, respectively, and the format of
      the NAI is shown in the following figure:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Local IPv4 address                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Remote IPv4 address                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 4: NAI for IPv4 Adjacency


   'IPv6 Adjacency'  is specified as a pair of IPv6 addresses.  In this
      case, ST and Length are 4 and 40 respectively, and the format of
      the NAI is shown in the following figure:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //               Local IPv6 address (16 bytes)                 //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //               Remote IPv6 address (16 bytes)                //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 5: NAI for IPv6 adjacency


   'Unnumbered Adjacency with IPv4 NodeIDs'  is specified as a pair of
      Node ID / Interface ID tuples.  In this case, ST and Length are 5
      and 24 respectively, and the format of the NAI is shown in the
      following figure:





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      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Local Node-ID                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Local Interface ID                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Remote Node-ID                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Remote Interface ID                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 6: NAI for Unnumbered adjacency with IPv4 Node IDs

   We are yet to decide if another SID subobject is required for
   unnumbered adjacency with 128 bit node ID.

5.3.3.  SR-ERO Processing

   A PCEP Speaker that does not recognize the new ERO subobject in the
   PCCreate, PCUpd or PCRpt message MUST reject the entire PCEP message
   and MUST send a PCE error message with Error-Type=3 ("Unknown
   Object") and Error-Value=2 ("Unrecognized object Type") or Error-
   Type=4 ("Not supported object") and Error-Value=2 ("Not supported
   object Type"), defined in [RFC5440].

   When the SID represents an MPLS label (i.e. the M bit is set), its
   value (20 most significant bits) MUST be larger than 15, unless it is
   special purpose label, such as an Entropy Label Indicator (ELI) or an
   Entropy Label (EL).  If a PCEP Speaker receives a label ERO subobject
   with an invalid value, it MUST send the PCE error message with Error-
   Type = "Reception of an invalid object" and Error-Value = "Bad label
   value".  If both M and C bits of an ERO subobject are set, and if a
   PCEP Speaker finds erroneous setting in one or more of TC, S, and TTL
   fields, it MUST send a PCE error with Error-Type = "Reception of an
   invalid object" and Error-Value = "Bad label format".

   If a PCC receives a stack of SR-ERO subobjects, and the number of
   stack exceeds the maximum number of SIDs that the PCC can impose on
   the packet, it MAY send a PCE error with Error-Type = "Reception of
   an invalid object" and Error-Value = "Unsupported number of Segment
   ERO subobjects".


6.  Backward Compatibility

   An LSR that does not support the SR PCEP capability negotiation
   cannot recognize the SR-ERO subobjects.  As such, it shall send a



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   PCEP error with Error-Type = 4 (Not supported object) and Error-Value
   = 2 (Not supported object Type) as per [RFC5440].


7.  Management Considerations

7.1.  Policy

   PCEP implementation:

   o  Can enable SR-PCEP capability either by default or via explicit
      configuration.

   o  May generate PCEP error due to unsupported number of SR-ERO
      subobjects either by default or via explicit configuration.

7.2.  The PCEP Data Model

   A PCEP MIB module is defined in [I-D.ietf-pce-pcep-mib] needs be
   extended to cover additional functionality provided by [RFC5440] and
   [I-D.crabbe-pce-pce-initiated-lsp].  Such extension will cover the
   new functionality specified in this document.


8.  Security Considerations

   The security considerations described in [RFC5440] and
   [I-D.crabbe-pce-pce-initiated-lsp] are applicable to this
   specification.  No additional security measure is required.


9.  IANA Considerations

9.1.  PCEP Objects

   IANA is requested to allocate a ERO subobject type (recommended value
   = 5) for the SR-ERO subobject.

9.1.1.  LSP-SIG-TYPE field in the LSP object

   This document requests that a new value is allocated for Segment
   Routing LSP Type in the LSP-SIG_TYPE registry.









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                     Value     Meaning
                       1        Segment Routing

9.2.  PCEP-Error Object

   This document defines new Error-Type and Error-Value for the
   following new conditions:

    Error-Type  Meaning
       10       Reception of an invalid object
                 Error-value=2:  Bad label value
                 Error-value=3:  Unsupported number of Segment ERO
                                 subobjects

9.3.  PCEP TLV Type Indicators

   This document defines the following new PCEP TLVs:

      Value   Meaning                              Reference
    --------  ------------------------------------ -----------------
       26      SR-PCE-CAPABILITY                    This document
       27      LSP-PATH-TYPE                        This document

9.3.1.  LSP-PATH-TYPE Indicators

   This document requests that a registry is created to manage the value
   of the LSP-Path-Type field in the LSP object, which defines the
   technology of the LSP path.

                     Value     Meaning
                       0        RSVP
                       1        Segment Routing


10.  Contributors

   The following people contributed to this document:
      - Lakshmi Sharma (Cisco Systems)


11.  Acknowledgements

   We'd like to thank Ina Minei and George Swallow for valuable
   comments.


12.  References




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12.1.  Normative References

   [I-D.crabbe-pce-pce-initiated-lsp]
              Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "PCEP
              Extensions for PCE-initiated LSP Setup in a Stateful PCE
              Model", draft-crabbe-pce-pce-initiated-lsp-01 (work in
              progress), April 2013.

   [I-D.filsfils-rtgwg-segment-routing]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
              Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
              Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe,
              "Segment Routing Architecture",
              draft-filsfils-rtgwg-segment-routing-00 (work in
              progress), June 2013.

   [I-D.ietf-pce-pcep-mib]
              Koushik, K., Stephan, E., Zhao, Q., King, D., and J.
              Hardwick, "PCE communication protocol (PCEP) Management
              Information Base", draft-ietf-pce-pcep-mib-04 (work in
              progress), February 2013.

   [I-D.ietf-pce-stateful-pce]
              Crabbe, E., Medved, J., Minei, I., and R. Varga, "PCEP
              Extensions for Stateful PCE",
              draft-ietf-pce-stateful-pce-05 (work in progress),
              July 2013.

   [I-D.previdi-isis-segment-routing-extensions]
              Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
              Decraene, B., Litkowski, S., Geib, R., Milojevic, I.,
              Shakir, R., Ytti, S., Henderickx, W., and J. Tantsura,
              "IS-IS Extensions for Segment Routing",
              draft-previdi-isis-segment-routing-extensions-01 (work in
              progress), July 2013.

   [I-D.psenak-ospf-segment-routing-extensions]
              Psenak, P., Previdi, S., Filsfils, C., Gredler, H., and R.
              Shakir, "OSPF Extensions for Segment Routing",
              draft-psenak-ospf-segment-routing-extensions-01 (work in
              progress), July 2013.

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

   [RFC5440]  Vasseur, JP. and JL. Le Roux, "Path Computation Element
              (PCE) Communication Protocol (PCEP)", RFC 5440,
              March 2009.



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   [RFC5462]  Andersson, L. and R. Asati, "Multiprotocol Label Switching
              (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
              Class" Field", RFC 5462, February 2009.

12.2.  Informative References

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, December 2001.

   [RFC3473]  Berger, L., "Generalized Multi-Protocol Label Switching
              (GMPLS) Signaling Resource ReserVation Protocol-Traffic
              Engineering (RSVP-TE) Extensions", RFC 3473, January 2003.

   [RFC3477]  Kompella, K. and Y. Rekhter, "Signalling Unnumbered Links
              in Resource ReSerVation Protocol - Traffic Engineering
              (RSVP-TE)", RFC 3477, January 2003.

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


Authors' Addresses

   Siva Sivabalan
   Cisco Systems, Inc.
   2000 Innovation Drive
   Kanata, Ontario  K2K 3E8
   Canada

   Email: msiva@cisco.com


   Jan Medved
   Cisco Systems, Inc.
   170 West Tasman Dr.
   San Jose, CA  95134
   US

   Email: jmedved@cisco.com










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   Clarence Filsfils
   Cisco Systems, Inc.
   Pegasus Parc
   De kleetlaan 6a, DIEGEM  BRABANT 1831
   BELGIUM

   Email: cfilsfil@cisco.com


   Edward Crabbe
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   US

   Email: edward.crabbe@gmail.com


   Robert Raszuk
   NTT I3
   101 S. Ellsworth Ave
   San Mateo, CA  94401
   US

   Email: robert@raszuk.net


























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