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RFC 8306

Proposed STD
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Extensions to the Path Computation Element Communication Protocol (PCEP) for Point-to-Multipoint Traffic Engineering Label Switched Paths

Part 1 of 3, p. 1 to 16
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Obsoletes:    6006


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Internet Engineering Task Force (IETF)                           Q. Zhao
Request for Comments: 8306                                 D. Dhody, Ed.
Obsoletes: 6006                                               R. Palleti
Category: Standards Track                            Huawei Technologies
ISSN: 2070-1721                                                  D. King
                                                      Old Dog Consulting
                                                           November 2017


                             Extensions to
       the Path Computation Element Communication Protocol (PCEP)
    for Point-to-Multipoint Traffic Engineering Label Switched Paths

Abstract

   Point-to-point Multiprotocol Label Switching (MPLS) and Generalized
   MPLS (GMPLS) Traffic Engineering Label Switched Paths (TE LSPs) may
   be established using signaling techniques, but their paths may first
   need to be determined.  The Path Computation Element (PCE) has been
   identified as an appropriate technology for the determination of the
   paths of point-to-multipoint (P2MP) TE LSPs.

   This document describes extensions to the PCE Communication Protocol
   (PCEP) to handle requests and responses for the computation of paths
   for P2MP TE LSPs.

   This document obsoletes RFC 6006.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8306.

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Copyright Notice

   Copyright (c) 2017 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
   (https://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.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1. Introduction ....................................................4
      1.1. Terminology ................................................5
      1.2. Requirements Language ......................................5
   2. PCC-PCE Communication Requirements ..............................5
   3. Protocol Procedures and Extensions ..............................6
      3.1. P2MP Capability Advertisement ..............................7
           3.1.1. IGP Extensions for P2MP Capability Advertisement ....7
           3.1.2. Open Message Extension ..............................7
      3.2. Efficient Presentation of P2MP LSPs ........................8
      3.3. P2MP Path Computation Request/Reply Message Extensions .....9
           3.3.1. The Extension of the RP Object ......................9
           3.3.2. The P2MP END-POINTS Object .........................11
      3.4. Request Message Format ....................................13
      3.5. Reply Message Format ......................................15

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      3.6. P2MP Objective Functions and Metric Types .................16
           3.6.1. Objective Functions ................................16
           3.6.2. METRIC Object-Type Values ..........................17
      3.7. Non-Support of P2MP Path Computation ......................17
      3.8. Non-Support by Back-Level PCE Implementations .............17
      3.9. P2MP TE Path Reoptimization Request .......................17
      3.10. Adding and Pruning Leaves to/from the P2MP Tree ..........18
      3.11. Discovering Branch Nodes .................................22
           3.11.1. Branch Node Object ................................22
      3.12. Synchronization of P2MP TE Path Computation Requests .....22
      3.13. Request and Response Fragmentation .......................23
           3.13.1. Request Fragmentation Procedure ...................24
           3.13.2. Response Fragmentation Procedure ..................24
           3.13.3. Fragmentation Example .............................24
      3.14. UNREACH-DESTINATION Object ...............................25
      3.15. P2MP PCEP-ERROR Objects and Types ........................27
      3.16. PCEP NO-PATH Indicator ...................................28
   4. Manageability Considerations ...................................28
      4.1. Control of Function and Policy ............................28
      4.2. Information and Data Models ...............................28
      4.3. Liveness Detection and Monitoring .........................29
      4.4. Verifying Correct Operation ...............................29
      4.5. Requirements for Other Protocols and Functional
           Components ................................................29
      4.6. Impact on Network Operation ...............................29
   5. Security Considerations ........................................30
   6. IANA Considerations ............................................31
      6.1. PCEP TLV Type Indicators ..................................31
      6.2. Request Parameter Bit Flags ...............................31
      6.3. Objective Functions .......................................31
      6.4. METRIC Object-Type Values .................................32
      6.5. PCEP Objects ..............................................32
      6.6. PCEP-ERROR Objects and Types ..............................34
      6.7. PCEP NO-PATH Indicator ....................................35
      6.8. SVEC Object Flag ..........................................35
      6.9. OSPF PCE Capability Flag ..................................35
   7. References .....................................................36
      7.1. Normative References ......................................36
      7.2. Informative References ....................................37
   Appendix A. Summary of Changes from RFC 6006 ......................39
   Appendix A.1. RBNF Changes from RFC 6006 ..........................39
   Acknowledgements ..................................................41
   Contributors ......................................................42
   Authors' Addresses ................................................43

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1.  Introduction

   The Path Computation Element (PCE) as 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.

   [RFC4875] describes how to set up point-to-multipoint (P2MP) Traffic
   Engineering Label Switched Paths (TE LSPs) for use in Multiprotocol
   Label Switching (MPLS) and Generalized MPLS (GMPLS) networks.

   The PCE has been identified as a suitable application for the
   computation of paths for P2MP TE LSPs [RFC5671].

   The PCE Communication Protocol (PCEP) is designed as a communication
   protocol between PCCs and PCEs for point-to-point (P2P) path
   computations and is defined in [RFC5440].  However, that
   specification does not provide a mechanism to request path
   computation of P2MP TE LSPs.

   A P2MP LSP is comprised of multiple source-to-leaf (S2L) sub-LSPs.
   These S2L sub-LSPs are set up between ingress and egress Label
   Switching Routers (LSRs) and are appropriately overlaid to construct
   a P2MP TE LSP.  During path computation, the P2MP TE LSP may be
   determined as a set of S2L sub-LSPs that are computed separately and
   combined to give the path of the P2MP LSP, or the entire P2MP TE LSP
   may be determined as a P2MP tree in a single computation.

   This document relies on the mechanisms of PCEP to request path
   computation for P2MP TE LSPs.  One Path Computation Request message
   from a PCC may request the computation of the whole P2MP TE LSP, or
   the request may be limited to a subset of the S2L sub-LSPs.  In the
   extreme case, the PCC may request the S2L sub-LSPs to be computed
   individually; the PCC is responsible for deciding whether to signal
   individual S2L sub-LSPs or combine the computation results to signal
   the entire P2MP TE LSP.  Hence, the PCC may use one Path Computation
   Request message or may split the request across multiple path
   computation messages.

   This document obsoletes [RFC6006] and incorporates the following
   errata:

   o  Erratum IDs 3819, 3830, 3836, 4867, and 4868 for [RFC6006]

   o  Erratum ID 4956 for [RFC5440]

   All changes from [RFC6006] are listed in Appendix A.

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1.1.  Terminology

   Terminology used in this document:

   TE LSP: Traffic Engineering Label Switched Path.

   LSR: Label Switching Router.

   OF: Objective Function.  A set of one or more optimization criteria
      used for the computation of a single path (e.g., path cost
      minimization) or for the synchronized computation of a set of
      paths (e.g., aggregate bandwidth consumption minimization).

   P2MP: Point-to-Multipoint.

   P2P: Point-to-Point.

   This document also uses the terminology defined in [RFC4655],
   [RFC4875], and [RFC5440].

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  PCC-PCE Communication Requirements

   This section summarizes the PCC-PCE communication requirements as met
   by the protocol extension specified in this document for P2MP MPLS-TE
   LSPs.  The numbering system in the list below corresponds to the
   requirement numbers (e.g., R1, R2) used in [RFC5862].

   1.  The PCC MUST be able to specify that the request is a P2MP path
       computation request.

   2.  The PCC MUST be able to specify that objective functions are to
       be applied to the P2MP path computation request.

   3.  The PCE MUST have the capability to reject a P2MP path
       computation request and indicate non-support of P2MP path
       computation.

   4.  The PCE MUST provide an indication of non-support of P2MP path
       computation by back-level PCE implementations.

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   5.  A P2MP path computation request MUST be able to list multiple
       destinations.

   6.  A P2MP path computation response MUST be able to carry the path
       of a P2MP LSP.

   7.  By default, the path returned by the PCE SHOULD use the
       compressed format.

   8.  It MUST be possible for a single P2MP path computation request or
       response to be conveyed by a sequence of messages.

   9.  It MUST NOT be possible for a single P2MP path computation
       request to specify a set of different constraints, traffic
       parameters, or quality-of-service requirements for different
       destinations of a P2MP LSP.

   10. P2MP path modification and P2MP path diversity MUST be supported.

   11. It MUST be possible to reoptimize existing P2MP TE LSPs.

   12. It MUST be possible to add and remove P2MP destinations from
       existing paths.

   13. It MUST be possible to specify a list of applicable branch nodes
       to use when computing the P2MP path.

   14. It MUST be possible for a PCC to discover P2MP path computation
       capability.

   15. The PCC MUST be able to request diverse paths when requesting a
       P2MP path.

3.  Protocol Procedures and Extensions

   The following section describes the protocol extensions required to
   satisfy the requirements specified in Section 2 ("PCC-PCE
   Communication Requirements") of this document.

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3.1.  P2MP Capability Advertisement

3.1.1.  IGP Extensions for P2MP Capability Advertisement

   [RFC5088] defines a PCE Discovery (PCED) TLV carried in an OSPF
   Router Information Link State Advertisement (LSA) as defined in
   [RFC7770] to facilitate PCE discovery using OSPF.  [RFC5088]
   specifies that no new sub-TLVs may be added to the PCED TLV.  This
   document defines a flag in the OSPF PCE Capability Flags to indicate
   the capability of P2MP computation.

   Similarly, [RFC5089] defines the PCED sub-TLV for use in PCE
   discovery using IS-IS.  This document will use the same flag for the
   OSPF PCE Capability Flags sub-TLV to allow IS-IS to indicate the
   capability of P2MP computation.

   The IANA assignment for a shared OSPF and IS-IS P2MP Capability Flag
   is documented in Section 6.9 ("OSPF PCE Capability Flag") of this
   document.

   PCEs wishing to advertise that they support P2MP path computation
   would set the bit (10) accordingly.  PCCs that do not understand this
   bit will ignore it (per [RFC5088] and [RFC5089]).  PCEs that do not
   support P2MP will leave the bit clear (per the default behavior
   defined in [RFC5088] and [RFC5089]).

   PCEs that set the bit to indicate support of P2MP path computation
   MUST follow the procedures in Section 3.3.2 ("The P2MP END-POINTS
   Object") to further qualify the level of support.

3.1.2.  Open Message Extension

   Based on the Capabilities Exchange requirement described in
   [RFC5862], if a PCE does not advertise its P2MP capability during
   discovery, PCEP should be used to allow a PCC to discover, during the
   Open Message Exchange, which PCEs are capable of supporting P2MP path
   computation.

   To satisfy this requirement, we extend the PCEP OPEN object by
   defining an optional TLV to indicate the PCE's capability to perform
   P2MP path computations.

   IANA has allocated value 6 from the "PCEP TLV Type Indicators"
   subregistry, as documented in Section 6.1 ("PCEP TLV Type
   Indicators").  The description is "P2MP capable", and the length
   value is 2 bytes.  The value field is set to default value 0.

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   The inclusion of this TLV in an OPEN object indicates that the sender
   can perform P2MP path computations.

   The capability TLV is meaningful only for a PCE, so it will typically
   appear only in one of the two Open messages during PCE session
   establishment.  However, in the case of PCE cooperation (e.g.,
   inter-domain), when a PCE behaving as a PCC initiates a PCE session
   it SHOULD also indicate its path computation capabilities.

3.2.  Efficient Presentation of P2MP LSPs

   When specifying additional leaves or when optimizing existing P2MP TE
   LSPs as specified in [RFC5862], it may be necessary to pass existing
   P2MP LSP route information between the PCC and PCE in the request and
   reply messages.  In each of these scenarios, we need path objects for
   efficiently passing the existing P2MP LSP between the PCE and PCC.

   We specify the use of the Resource Reservation Protocol Traffic
   Engineering (RSVP-TE) extensions Explicit Route Object (ERO) to
   encode the explicit route of a TE LSP through the network.  PCEP ERO
   sub-object types correspond to RSVP-TE ERO sub-object types.  The
   format and content of the ERO are defined in [RFC3209] and [RFC3473].

   The Secondary Explicit Route Object (SERO) is used to specify the
   explicit route of an S2L sub-LSP.  The path of each subsequent S2L
   sub-LSP is encoded in a P2MP_SECONDARY_EXPLICIT_ROUTE object SERO.
   The format of the SERO is the same as the format of an ERO as defined
   in [RFC3209] and [RFC3473].

   The Secondary Record Route Object (SRRO) is used to record the
   explicit route of the S2L sub-LSP.  The class of the P2MP SRRO is the
   same as the class of the SRRO as defined in [RFC4873].

   The SERO and SRRO are used to report the route of an existing TE LSP
   for which a reoptimization is desired.  The format and content of the
   SERO and SRRO are defined in [RFC4875].

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   PCEP Object-Class and Object-Type values for the SERO and SRRO have
   been assigned:

      Object-Class Value    29
      Name                  SERO
      Object-Type           0: Reserved
                            1: SERO
                            2-15: Unassigned
      Reference             RFC 8306

      Object-Class Value    30
      Name                  SRRO
      Object-Type           0: Reserved
                            1: SRRO
                            2-15: Unassigned
      Reference             RFC 8306

   The IANA assignments are documented in Section 6.5 ("PCEP Objects").

   Since the explicit path is available for immediate signaling by the
   MPLS or GMPLS control plane, the meanings of all of the sub-objects
   and fields in this object are identical to those defined for the ERO.

3.3.  P2MP Path Computation Request/Reply Message Extensions

   This document extends the existing P2P RP (Request Parameters) object
   so that a PCC can signal a P2MP path computation request to the PCE
   receiving the PCEP request.  The END-POINTS object is also extended
   to improve the efficiency of the message exchange between the PCC and
   PCE in the case of P2MP path computation.

3.3.1.  The Extension of the RP Object

   The PCE path computation request and reply messages will need the
   following additional parameters to indicate to the receiving PCE
   (1) that the request and reply messages have been fragmented across
   multiple messages, (2) that they have been requested for a P2MP path,
   and (3) whether the route is represented in the compressed or
   uncompressed format.

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   This document adds the following flags to the RP object:

   The F-bit is added to the flag bits of the RP object to indicate to
   the receiver that the request is part of a fragmented request or
   is not a fragmented request.

   o  F (RP fragmentation bit - 1 bit):

      0: This indicates that the RP is not fragmented or it is the last
         piece of the fragmented RP.

      1: This indicates that the RP is fragmented and this is not the
         last piece of the fragmented RP.  The receiver needs to wait
         for additional fragments until it receives an RP with the same
         RP-ID and with the F-bit set to 0.

   The N-bit is added in the flag bits field of the RP object to signal
   the receiver of the message that the request/reply is for P2MP or
   is not for P2MP.

   o  N (P2MP bit - 1 bit):

      0: This indicates that this is not a Path Computation Request
         (PCReq) or Path Computation Reply (PCRep) message for P2MP.

      1: This indicates that this is a PCReq or PCRep message for P2MP.

   The E-bit is added in the flag bits field of the RP object to signal
   the receiver of the message that the route is in the compressed
   format or is not in the compressed format.  By default, the path
   returned by the PCE SHOULD use the compressed format.

   o  E (ERO-compression bit - 1 bit):

      0: This indicates that the route is not in the compressed format.

      1: This indicates that the route is in the compressed format.

   The IANA assignments are documented in Section 6.2 ("Request
   Parameter Bit Flags") of this document.

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3.3.2.  The P2MP END-POINTS Object

   The END-POINTS object is used in a PCReq message to specify the
   source IP address and the destination IP address of the path for
   which a path computation is requested.  To represent the end points
   for a P2MP path efficiently, we define two types of END-POINTS
   objects for the P2MP path:

   o  Old leaves whose path can be modified/reoptimized.

   o  Old leaves whose path must be left unchanged.

   With the P2MP END-POINTS object, the PCE Path Computation Request
   message is expanded in a way that allows a single request message to
   list multiple destinations.

   In total, there are now four possible types of leaves in a
   P2MP request:

   o  New leaves to add (leaf type = 1)

   o  Old leaves to remove (leaf type = 2)

   o  Old leaves whose path can be modified/reoptimized (leaf type = 3)

   o  Old leaves whose path must be left unchanged (leaf type = 4)

   A given END-POINTS object gathers the leaves of a given type.  The
   type of leaf in a given END-POINTS object is identified by the
   END-POINTS object leaf type field.

   Using the P2MP END-POINTS object, the END-POINTS portion of a request
   message for the multiple destinations can be reduced by up to 50% for
   a P2MP path where a single source address has a very large number of
   destinations.

   Note that a P2MP path computation request can mix the different types
   of leaves by including several END-POINTS objects per RP object as
   shown in the PCReq Routing Backus-Naur Form (RBNF) [RFC5511] format
   in Section 3.4 ("Request Message Format").

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   The format of the P2MP END-POINTS object body for IPv4
   (Object-Type 3) 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Leaf type                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Source IPv4 address                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  Destination IPv4 address                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                           ...                                 ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  Destination IPv4 address                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 1: The P2MP END-POINTS Object Body Format for IPv4

   The format of the END-POINTS object body for IPv6 (Object-Type 4) 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Leaf type                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                Source IPv6 address (16 bytes)                 |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |              Destination IPv6 address (16 bytes)              |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                           ...                                 ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |              Destination IPv6 address (16 bytes)              |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 2: The P2MP END-POINTS Object Body Format for IPv6

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   The END-POINTS object body has a variable length.  These are

   o  multiples of 4 bytes for IPv4

   o  multiples of 16 bytes, plus 4 bytes, for IPv6

3.4.  Request Message Format

   As per [RFC5440], a Path Computation Request message (also referred
   to as a PCReq message) is a PCEP message sent by a PCC to a PCE to
   request a path computation.  A PCReq message may carry more than one
   path computation request.

   As per [RFC5541], the OF object MAY be carried within a PCReq
   message.  If an objective function is to be applied to a set of
   synchronized path computation requests, the OF object MUST be carried
   just after the corresponding SVEC (Synchronization Vector) object and
   MUST NOT be repeated for each elementary request.

   The PCReq message is encoded as follows using RBNF as defined in
   [RFC5511].

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   Below is the message format for the request message:

        <PCReq Message> ::= <Common Header>
                           [<svec-list>]
                           <request-list>

        where:

             <svec-list> ::= <SVEC>
                           [<OF>]
                           [<metric-list>]
                           [<svec-list>]

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

             <request> ::= <RP>
                          <end-point-rro-pair-list>
                          [<OF>]
                          [<LSPA>]
                          [<BANDWIDTH>]
                          [<metric-list>]
                          [<IRO>|<BNC>]
                          [<LOAD-BALANCING>]

        where:

             <end-point-rro-pair-list> ::=
                                <END-POINTS>[<RRO-List>[<BANDWIDTH>]]
                                [<end-point-rro-pair-list>]

             <RRO-List> ::= (<RRO>|<SRRO>)[<RRO-List>]
             <metric-list> ::= <METRIC>[<metric-list>]

           Figure 3: The Message Format for the Request Message

   Note that we preserve compatibility with the definition of <request>
   provided in [RFC5440].  At least one instance of <END-POINTS> MUST be
   present in this message.

   We have documented the IANA assignment of additional END-POINTS
   Object-Type values in Section 6.5 ("PCEP Objects") of this document.

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3.5.  Reply Message Format

   The PCEP Path Computation Reply message (also referred to as a
   PCRep message) is a PCEP message sent by a PCE to a requesting PCC in
   response to a previously received PCReq message.  PCEP supports the
   bundling of multiple replies to a set of path computation requests
   within a single PCRep message.

   The PCRep message is encoded as follows using RBNF as defined in
   [RFC5511].

   Below is the message format for the reply message:

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

        where:

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

            <response> ::= <RP>
                   [<end-point-path-pair-list>]
                   [<NO-PATH>]
                   [<UNREACH-DESTINATION>]
                   [<attribute-list>]

            <end-point-path-pair-list> ::=
                    [<END-POINTS>]<path>
                    [<end-point-path-pair-list>]

            <path> ::= (<ERO>|<SERO>) [<path>]

        where:

            <attribute-list> ::= [<OF>]
                               [<LSPA>]
                               [<BANDWIDTH>]
                               [<metric-list>]
                               [<IRO>]

            Figure 4: The Message Format for the Reply Message

   The optional END-POINTS object in the reply message is used to
   specify which paths are removed, changed, not changed, or added for
   the request.  The path is only needed for the end points that are
   added or changed.

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   If the E-bit (ERO-Compress bit) was set to 1 in the request, then the
   path will be formed by an ERO followed by a list of SEROs.

   Note that we preserve compatibility with the definition of <response>
   provided in [RFC5440] and with the optional
   <end-point-path-pair-list> and <path>.



(page 16 continued on part 2)

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