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RFC 8281

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Path Computation Element Communication Protocol (PCEP) Extensions for PCE-Initiated LSP Setup in a Stateful PCE Model

 


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Internet Engineering Task Force (IETF)                         E. Crabbe
Request for Comments: 8281                        Individual Contributor
Category: Standards Track                                       I. Minei
ISSN: 2070-1721                                             Google, Inc.
                                                            S. Sivabalan
                                                     Cisco Systems, Inc.
                                                                R. Varga
                                               Pantheon Technologies SRO
                                                           December 2017


 Path Computation Element Communication Protocol (PCEP) Extensions for
            PCE-Initiated LSP Setup in a Stateful PCE Model

Abstract

   The Path Computation Element Communication Protocol (PCEP) provides
   mechanisms for Path Computation Elements (PCEs) to perform path
   computations in response to Path Computation Client (PCC) requests.

   The extensions for stateful PCE provide active control of
   Multiprotocol Label Switching (MPLS) Traffic Engineering Label
   Switched Paths (TE LSPs) via PCEP, for a model where the PCC
   delegates control over one or more locally configured LSPs to the
   PCE.  This document describes the creation and deletion of
   PCE-initiated LSPs under the stateful PCE model.

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/rfc8281.

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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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   3.  Architectural Overview  . . . . . . . . . . . . . . . . . . .   5
     3.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Operation Overview  . . . . . . . . . . . . . . . . . . .   6
   4.  Support of PCE-Initiated LSPs . . . . . . . . . . . . . . . .   7
     4.1.  STATEFUL-PCE-CAPABILITY TLV . . . . . . . . . . . . . . .   8
   5.  PCE-Initiated LSP Instantiation and Deletion  . . . . . . . .   8
     5.1.  The LSP Initiate Request  . . . . . . . . . . . . . . . .   8
     5.2.  The R Flag in the SRP Object  . . . . . . . . . . . . . .  10
     5.3.  LSP Instantiation . . . . . . . . . . . . . . . . . . . .  10
       5.3.1.  The Create Flag . . . . . . . . . . . . . . . . . . .  12
       5.3.2.  The SPEAKER-ENTITY-ID TLV . . . . . . . . . . . . . .  13
     5.4.  LSP Deletion  . . . . . . . . . . . . . . . . . . . . . .  13
   6.  LSP Delegation and Cleanup  . . . . . . . . . . . . . . . . .  14
   7.  LSP State Synchronization . . . . . . . . . . . . . . . . . .  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     8.1.  PCEP Messages . . . . . . . . . . . . . . . . . . . . . .  15
     8.2.  LSP Object  . . . . . . . . . . . . . . . . . . . . . . .  15
     8.3.  SRP object  . . . . . . . . . . . . . . . . . . . . . . .  16
     8.4.  STATEFUL-PCE-CAPABILITY TLV . . . . . . . . . . . . . . .  16
     8.5.  PCEP-Error Object . . . . . . . . . . . . . . . . . . . .  17
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
     9.1.  Malicious PCE . . . . . . . . . . . . . . . . . . . . . .  18
     9.2.  Malicious PCC . . . . . . . . . . . . . . . . . . . . . .  18
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     10.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

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1.  Introduction

   [RFC5440] describes the Path Computation Element Communication
   Protocol (PCEP).  PCEP defines the communication between a Path
   Computation Client (PCC) and a Path Computation Element (PCE), or
   between PCE and PCE, enabling computation of Multiprotocol Label
   Switching (MPLS) for Traffic Engineering Label Switched Path (TE LSP)
   characteristics.

   [RFC8231] specifies a set of extensions to PCEP to enable stateful
   control of TE LSPs between and across PCEP sessions in compliance
   with [RFC4657].  It includes:

   o  mechanisms to effect LSP State Synchronization between PCCs and
      PCEs

   o  delegation of control of LSPs to PCEs

   o  PCE control of timing and sequence of path computations within and
      across PCEP sessions

   It focuses on a model where LSPs are configured on the PCC, and
   control over them is delegated to the PCE.

   This document describes the setup, maintenance, and teardown of
   PCE-initiated LSPs under the stateful PCE model, without the need for
   local configuration on the PCC, thus allowing for a dynamic network
   that is centrally controlled and deployed.

2.  Terminology

   This document uses the following terms defined in [RFC5440]: PCC,
   PCE, and PCEP Peer.

   This document uses the following terms defined in [RFC8051]: Stateful
   PCE and Delegation.

   This document uses the following terms defined in [RFC8231]:
   Redelegation Timeout Interval, State Timeout Interval, LSP State
   Report, and LSP Update Request.

   The following terms are defined in this document:

   PCE-initiated LSP:  LSP that is instantiated as a result of a request
      from the PCE.

   The message formats in this document are specified using Routing
   Backus-Naur Form (RBNF) encoding as specified in [RFC5511].

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2.1.  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.

3.  Architectural Overview

3.1.  Motivation

   [RFC8231] provides active control over LSPs that are locally
   configured on the PCC.  This model relies on the Label Edge Router
   (LER) taking an active role in delegating locally configured LSPs to
   the PCE and is well suited in environments where the LSP placement is
   fairly static.  However, in environments where the LSP placement
   needs to change in response to application demands, it is useful to
   support dynamic creation and teardown of LSPs.  The ability for a PCE
   to trigger the creation of LSPs on demand can be seamlessly
   integrated into a controller-based network architecture, where
   intelligence in the controller can determine when and where to set up
   paths.

   A possible use case is a software-defined network, where applications
   request network resources and paths from the network infrastructure.
   For example, an application can request a path with certain
   constraints between two Label Switching Routers (LSRs) by contacting
   the PCE.  The PCE can compute a path satisfying the constraints, and
   instruct the head end LSR to instantiate and signal it.  When the
   path is no longer required by the application, the PCE can request
   its teardown.

   Another use case is dynamically adjusting aggregate bandwidth between
   two points in the network using multiple LSPs.  This functionality is
   very similar to auto-bandwidth, but it allows for providing the
   desired capacity through multiple LSPs.  This approach overcomes two
   of the limitations auto-bandwidth can experience: 1) growing the
   capacity between the endpoints beyond the capacity of individual
   links in the path and 2) achieving good bin packing through use of
   several small LSPs instead of a single large one.  The number of LSPs
   varies based on the demand, and LSPs are created and deleted
   dynamically to satisfy the bandwidth requirements.

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   Another use case is demand engineering, where a PCE with visibility
   into both the network state and the demand matrix can anticipate and
   optimize how traffic is distributed across the infrastructure.  Such
   optimizations may require creating new paths across the
   infrastructure.

3.2.  Operation Overview

   This document defines the new I flag in the STATEFUL-PCE-CAPABILITY
   TLV to indicate that the sender supports PCE-initiated LSPs (see
   details in Section 4.1).  A PCC or PCE sets this flag in the Open
   message during the PCEP initialization phase to indicate that it
   supports the procedures of this document.

   This document defines a new PCEP message, the LSP Initiate Request
   (PCInitiate) message, which a PCE can send to a PCC to request the
   initiation or deletion of an LSP.  The decision when to instantiate
   or delete a PCE-initiated LSP is out of the scope of this document.

   The PCE sends a PCInitiate message to the PCC to request the
   initiation of an LSP.  The PCC creates the LSP using the attributes
   communicated by the PCE and local values for any unspecified
   parameters.  The PCC generates a Path Computation State Report
   (PCRpt) for the LSP, carrying a newly assigned PLSP-ID for the LSP
   and delegating the LSP to the PCE via the Delegate flag in the LSP
   object.

   The PCE can update the attributes of the LSP by sending subsequent
   Path Computation Update Request (PCUpd) messages.  Subsequent PCRpt
   and PCUpd messages that the PCC and PCE, respectively, send for the
   LSP will carry the PCC-assigned PLSP-ID, which uniquely identifies
   the LSP.  See details in Section 5.3.

   The PCE sends a PCInitiate message to the PCC to request the deletion
   of an LSP.  To indicate a delete operation, this document defines the
   new R flag in the Stateful PCE Request Parameter (SRP) object in the
   PCInitiate message, as described in Section 5.2.  As a result of the
   deletion request, the PCC removes the LSP and sends a PCRpt for the
   removed state.  See details in Section 5.4.

   Figure 1 illustrates these message exchanges.

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         +-+-+                            +-+-+
         |PCC|                            |PCE|
         +-+-+                            +-+-+
           |                                |
           |<--PCInitiate-------------------| (Initiate LSP)
           |                                |
           |---PCRpt, PLSP_ID=1, D=1------->| (Confirm initiation)
           |            .                   |
           |            .                   |
           |                                |
           |<--PCUpd, PLSP_ID=1-------------| (Update LSP)
           |                                |
           |---PCRpt, PLSP_ID=1, D=1------->| (Confirm update)
           |            .                   |
           |            .                   |
           |                                |
           |<--PCInitiate, PLSP_ID=1, R=1---| (Delete LSP)
           |                                |
           |---PCRpt, PLSP_ID=1, R=1------->| (Confirm delete)

                  Figure 1: PCE-Initiated LSP Life Cycle

4.  Support of PCE-Initiated LSPs

   A PCEP speaker indicates its ability to support PCE-initiated LSPs
   during the PCEP initialization phase, as follows.  When the PCEP
   session is created, it sends an Open message with an OPEN object that
   contains the STATEFUL-PCE-CAPABILITY TLV, as defined in [RFC8231].  A
   new flag, the I (LSP-INSTANTIATION-CAPABILITY) flag, is introduced to
   this TLV to indicate support for instantiation of PCE-initiated LSPs.
   A PCE can initiate LSPs only for PCCs that advertised this
   capability.  A PCC will follow the procedures described in this
   document only on sessions where the PCE advertised the I flag.

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4.1.  STATEFUL-PCE-CAPABILITY TLV

   The format of the STATEFUL-PCE-CAPABILITY TLV is defined in [RFC8231]
   and included here for easy reference with the addition of the new I
   flag.

      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=4           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Flags                                      |I|S|U|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+

               Figure 2: STATEFUL-PCE-CAPABILITY TLV Format

   A new flag is defined to indicate the sender's support for LSP
   instantiation by a PCE:

   I (LSP-INSTANTIATION-CAPABILITY -- 1 bit):  If set to 1 by a PCC, the
      I flag indicates that the PCC allows instantiation of an LSP by a
      PCE.  If set to 1 by a PCE, the I flag indicates that the PCE
      supports instantiating LSPs.  The LSP-INSTANTIATION-CAPABILITY
      flag must be set by both the PCC and PCE in order to enable
      PCE-initiated LSP instantiation.

5.  PCE-Initiated LSP Instantiation and Deletion

   To initiate an LSP, a PCE sends a PCInitiate message to a PCC.  The
   message format, objects, and TLVs are discussed separately below for
   the creation and the deletion cases.

5.1.  The LSP Initiate Request

   An LSP Initiate Request (PCInitiate) message is a PCEP message sent
   by a PCE to a PCC to trigger LSP instantiation or deletion.  The
   Message-Type field of the PCEP common header for the PCInitiate
   message is set to 12.  The PCInitiate message MUST include the SRP
   and the LSP objects and MAY contain other objects, as discussed later
   in this section.

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   The format of a PCInitiate message is as follows:

     <PCInitiate Message> ::= <Common Header>
                              <PCE-initiated-lsp-list>
   Where:
     <Common Header> is defined in RFC 5440

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

     <PCE-initiated-lsp-request> ::= (<PCE-initiated-lsp-instantiation>|
                                      <PCE-initiated-lsp-deletion>)

     <PCE-initiated-lsp-instantiation> ::= <SRP>
                                           <LSP>
                                           [<END-POINTS>]
                                           <ERO>
                                           [<attribute-list>]

     <PCE-initiated-lsp-deletion> ::= <SRP>
                                      <LSP>

   Where:
     <attribute-list> is defined in RFC 5440 and extended by
     PCEP extensions.

   The LSP object is defined in [RFC8231].  The END-POINTS and Explicit
   Route Objects (EROs) are defined in [RFC5440].

   The SRP object is defined in [RFC8231].  The SRP object contains an
   SRP-ID-number that is unique within a PCEP session.  The PCE
   increments the last-used SRP-ID-number before it sends each
   PCInitiate message.  The PCC MUST echo the value of the SRP-ID-number
   in PCEP Error (PCErr) and PCRpt messages that it sends as a result of
   the PCInitiate; this allows the PCE to correlate them with the
   corresponding PCInitiate message.

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5.2.  The R Flag in the SRP Object

   The format of the SRP object is defined in [RFC8231] and included
   here for easy reference with the addition of the new R flag.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Flags                              |R|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        SRP-ID-number                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      //                      Optional TLVs                          //
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 3: The SRP Object Format

   A new flag is defined to indicate a delete operation initiated by the
   PCE:

   R (LSP-REMOVE -- 1 bit):  If set to 0, it indicates a request to
      create an LSP.  If set to 1, it indicates a request to remove an
      LSP.

5.3.  LSP Instantiation

   The LSP is instantiated by sending a PCInitiate message.  The LSP is
   set up using RSVP-TE.  Extensions for other setup methods are outside
   the scope of this document.

   The PCInitiate message, when used to instantiate an LSP, MUST contain
   an LSP object with the reserved PLSP-ID 0.  The LSP object MUST
   include the SYMBOLIC-PATH-NAME TLV, which is used to correlate
   between the PCC-assigned PLSP-ID and the LSP.

   The PCInitiate message, when used to instantiate an LSP, MUST contain
   an ERO for the LSP.

   For an instantiation request of an RSVP-signaled LSP, the destination
   address may be needed.  The PCC MAY determine it from a provided
   object (e.g., ERO) or a local decision.  Alternatively, the
   END-POINTS object MAY be included to explicitly convey the
   destination addresses to be used in the RSVP-TE signaling.  The
   source address MUST be either specified or left for the PCC to choose
   by setting it to "0.0.0.0" (if the destination is an IPv4 address) or
   "::" (if the destination is an IPv6 address).

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   The PCE MAY include various attributes as per [RFC5440].  The PCC
   MUST use these values in the LSP instantiation and local values for
   unspecified parameters.  After the LSP setup, the PCC MUST send a
   PCRpt to the PCE, reflecting these values.  The SRP object in the
   PCRpt message MUST echo the value of the PCInitiate message that
   triggered the setup.  LSPs that were instantiated as a result of a
   PCInitiate message MUST have the Create flag (Section 5.3.1) set in
   the LSP object.

   If the PCC receives a PCInitiate message with a non-zero PLSP-ID and
   the R flag in the SRP object set to zero, then it MUST send a PCErr
   message with Error-type=19 (Invalid Operation) and Error-value=8
   (Non-zero PLSP-ID in the LSP Initiate Request).

   If the PCC receives a PCInitiate message without an ERO and the R
   flag in the SRP object set to zero, then it MUST send a PCErr message
   with Error-type=6 (Mandatory Object missing) and Error-value=9 (ERO
   object missing).

   If the PCC receives a PCInitiate message without a SYMBOLIC-PATH-NAME
   TLV, then it MUST send a PCErr message with Error-type=10 (Reception
   of an invalid object) and Error-value=8 (SYMBOLIC-PATH-NAME TLV
   missing).

   The PCE MUST NOT provide a symbolic path name that conflicts with the
   symbolic path name of any existing LSP in the PCC.  (Existing LSPs
   may be either statically configured or initiated by another PCE.)  If
   there is a conflict with the symbolic path name of an existing LSP,
   the PCC MUST send a PCErr message with Error-type=23 (Bad Parameter
   value) and Error-value=1 (SYMBOLIC-PATH-NAME in use).  The only
   exception to this rule is for LSPs for which the State Timeout
   Interval timer is running (see Section 6).

   If the PCC determines that the LSP parameters proposed in the
   PCInitiate message are unacceptable, it MUST send a PCErr message
   with Error-type=24 (PCE instantiation error) and Error-value=1
   (Unacceptable instantiation parameters).  If the PCC encounters an
   internal error during the processing of the PCInitiate message, it
   MUST send a PCErr message with Error-type=24 (PCE instantiation
   error) and Error-value=2 (Internal error).

   A PCC MUST relay errors it encounters in the setup of a PCE-initiated
   LSP to the PCE by sending a PCErr message with Error-type=24 (PCE
   instantiation error) and Error-value=3 (Signaling error).  The PCErr
   message MUST echo the SRP-ID-number of the PCInitiate message.  The
   PCEP-ERROR object SHOULD include the RSVP_ERROR_SPEC TLV (if an RSVP
   ERROR_SPEC object was returned to the PCC by a downstream node).

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   After the LSP is set up, errors in RSVP signaling are reported in
   PCRpt messages, as described in [RFC8231].

   On successful completion of the LSP instantiation, the PCC MUST send
   a PCRpt message.  The LSP object message MUST contain a non-zero
   PLSP-ID that uniquely identifies the LSP within this PCC and MUST
   have the Create flag (Section 5.3.1) and Delegate flag set.  The SRP
   object MUST contain an SRP-ID-number that echoes the value from the
   PCInitiate message that triggered the setup.  The PCRpt MUST include
   the attributes that the PCC used to instantiate the LSP.

   A PCC SHOULD be able to place a limit on either the number of LSPs or
   the percentage of resources that are allocated to honor PCE-initiated
   LSP requests.  As soon as that limit is reached, the PCC MUST send a
   PCErr message with Error-type=19 (Invalid Operation) and
   Error-value=6 (PCE-initiated LSP limit reached) and is free to drop
   any incoming PCInitiate messages without additional processing.

   Similarly, the PCE SHOULD be able to place a limit on either the
   number of PCInitiate messages pending for a particular PCC or the
   time it waits for a response (positive or negative) to a PCInitiate
   message from a PCC, and it MAY take further action (such as closing
   the session or removing all its LSPs) if this limit is reached.

5.3.1.  The Create Flag

   The LSP object is defined in [RFC8231] and included here for easy
   reference with the addition of the new Create (C) flag.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                PLSP-ID                |Flags  |C|  O  |A|R|S|D|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                        TLVs                                 //
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 4: The LSP Object Format

   A new flag, the C flag, is introduced.  On a PCRpt message, the C
   flag set to 1 indicates that this LSP was created via a PCInitiate
   message.  The C flag MUST be set to 1 on each PCRpt message for the
   LSP's duration of existence.  The C flag allows PCEs to be aware of
   which LSPs were PCE initiated (a state that would otherwise only be
   known by the PCC and the PCE that initiated them).

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5.3.2.  The SPEAKER-ENTITY-ID TLV

   The optional SPEAKER-ENTITY-ID TLV defined in [RFC8232] MAY be
   included in the LSP object in a PCRpt message as an optional TLV for
   LSPs for which the C flag is 1.  The SPEAKER-ENTITY-ID TLV identifies
   the PCE that initiated the creation of the LSP on all PCEP sessions,
   a state that would otherwise only be known by the PCC and the PCE
   that initiated the LSP.  If the TLV appears in a PCRpt for an LSP for
   which the C flag is 0, the LSP MUST be ignored, and the PCE MUST send
   a PCErr message with Error-type=23 (Bad parameter value) and
   Error-value=2 (Speaker identity included for an LSP that is not PCE
   initiated).

5.4.  LSP Deletion

   A PCE can initiate the removal of a PCE-initiated LSP by sending a
   PCInitiate message with an LSP object carrying the PLSP-ID of the LSP
   to be removed and an SRP object with the R flag set (see
   Section 5.2).  A PLSP-ID of zero removes all LSPs with the C flag set
   to 1 (in their LSP object) that are delegated to the PCE.

   If the PLSP-ID is unknown, the PCC MUST send a PCErr message with
   Error-type=19 (Invalid Operation) and Error-value=3 (Unknown PLSP-ID)
   [RFC8231].

   If the PLSP-ID specified in the PCInitiate message is not delegated
   to the PCE, the PCC MUST send a PCErr message with Error-type=19
   (Invalid operation) and Error-value=1 (LSP is not delegated)
   [RFC8231].

   If the PLSP-ID specified in the PCInitiate message was not created by
   a PCE, the PCC MUST send a PCErr message with Error-type=19 (Invalid
   operation) and Error-value=9 (LSP is not PCE initiated).

   Following the removal of the LSP, the PCC MUST send a PCRpt as
   described in [RFC8231].  The SRP object in the PCRpt MUST include the
   SRP-ID-number from the PCInitiate message that triggered the removal.
   The R flag in the SRP object MUST be set.

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6.  LSP Delegation and Cleanup

   The PCC MUST delegate PCE-initiated LSPs to the PCE upon
   instantiation.  The PCC MUST set the delegation bit to 1 in the PCRpt
   that includes the assigned PLSP-ID.

   The PCC MUST NOT revoke the delegation for a PCE-initiated LSP on an
   active PCEP session.  Therefore, all PCRpt messages from the PCC to
   the PCE that owns the delegation MUST have the delegation bit set to
   1.  If the PCE that owns the delegation receives a PCRpt message with
   the delegation bit set to 0, then it MUST send a PCErr message with
   Error-type=19 (Invalid Operation) and Error-value=7 (Delegation for
   PCE-initiated LSP cannot be revoked).  The PCE MAY further react by
   closing the session.

   Control over a PCE-initiated LSP can revert to the PCC in two ways.
   A PCE MAY return a delegation to the PCC to allow for LSP transfer
   between PCEs.  Alternatively, the PCC gains control of an LSP if the
   PCEP session that it was delegated on fails and the Redelegation
   Timeout Interval timer expires.  In both cases, the LSP becomes an
   orphan until the expiration of the State Timeout Interval timer
   [RFC8231].

   The PCC MAY attempt to redelegate an orphaned LSP by following the
   procedures of [RFC8231].  Alternatively, if the orphaned LSP was
   PCE-initiated, then a PCE MAY obtain control over it, as follows.

   A PCE (either the original or one of its backups) sends a PCInitiate
   message that includes just the SRP and LSP objects and carries the
   PLSP-ID of the LSP it wants to take control of.  If the PCC receives
   a PCInitiate message with a PLSP-ID pointing to an orphaned
   PCE-initiated LSP, then it MUST redelegate that LSP to the PCE.  Any
   other non-zero PLSP-ID MUST result in the generation of a PCErr
   message using the rules described in Section 5.4.  The State Timeout
   Interval timer for the LSP is stopped upon the redelegation.  After
   obtaining control of the LSP, the PCE may remove it using the
   procedures described in this document.

   The State Timeout Interval timer ensures that a PCE crash does not
   result in automatic and immediate disruption for the services using
   PCE-initiated LSPs.  PCE-initiated LSPs are not removed immediately
   upon PCE failure.  Instead, they are cleaned up on the expiration of
   this timer.  This allows for network cleanup without manual
   intervention.  The PCC MUST support removal of PCE-initiated LSPs as
   one of the behaviors applied on expiration of the State Timeout
   Interval timer.  The behavior MUST be picked based on local policy
   and can result in either LSP removal or reverting to operator-defined
   default parameters.

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7.  LSP State Synchronization

   LSP State Synchronization procedures are described in Section 5.6 of
   [RFC8231].  During State Synchronization, a PCC reports the state of
   its LSPs to the PCE using PCRpt messages, setting the SYNC flag in
   the LSP object.  For PCE-initiated LSPs, the PCC MUST also set the
   Create flag in the LSP object and MAY include the SPEAKER-ENTITY-ID
   TLV identifying the PCE that requested the LSP creation.  At the end
   of State Synchronization, the PCE SHOULD send a PCInitiate message to
   initiate any missing LSPs and/or remove any LSPs that are not wanted.
   Under some circumstances, depending on the deployment, it might be
   preferable for a PCE not to send this PCInitiate immediately, or at
   all.  For example, the PCC may be a slow device, or the operator
   might prefer not to disrupt active flows.

8.  IANA Considerations

   As detailed below, IANA has allocated code points for the protocol
   elements defined in this document.

8.1.  PCEP Messages

   IANA has registered the following message type within the "PCEP
   Messages" subregistry of the PCEP Numbers registry.  (Note that the
   early allocation for this message type was called "Initiate"; it has
   been changed as follows.)

               Value     Meaning                  Reference
               -----     --------------------     -------------
                 12      LSP Initiate Request     RFC 8281

8.2.  LSP Object

   [RFC8231] defines the LSP object; per that RFC, IANA created a
   registry to manage the value of the LSP object's Flag field.  IANA
   has allocated a new bit in the "LSP Object Flag Field" subregistry,
   as follows:

                    Bit     Description       Reference
                    ---     -----------       -------------

                     4      Create            RFC 8281

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8.3.  SRP object

   IANA has created a new subregistry, named "SRP Object Flag Field",
   within the "Path Computation Element Protocol (PCEP) Numbers"
   registry, to manage the Flag field of the SRP object.  New values are
   to be assigned by Standards Action [RFC8126].  Each bit is tracked
   with the following qualities: bit number (counting from bit 0 as the
   most significant bit), description, and defining RFC.

   The following values are defined in this document:

                    Bit     Description       Reference
                    ---     -----------       -------------

                     31     LSP-Remove        RFC 8281

8.4.  STATEFUL-PCE-CAPABILITY TLV

   [RFC8231] defines the STATEFUL-PCE-CAPABILITY TLV; per that RFC, IANA
   created a registry to manage the value of the STATEFUL-PCE-CAPABILITY
   TLV's Flag field.  IANA has allocated a new bit in the STATEFUL-PCE-
   CAPABILITY TLV Flag Field registry, as follows:

            Bit  Description                      Reference
            ---  -------------------------------- -------------

             29  LSP-INSTANTIATION-CAPABILITY (I) RFC 8281

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8.5.  PCEP-Error Object

   IANA has registered the following error types and error values within
   the "PCEP-ERROR Object Error Types and Values" subregistry of the
   PCEP Numbers registry.

   Error-Type  Meaning
   ----------  --------------
      10       Reception of an invalid object

                Error-value=8:  SYMBOLIC-PATH-NAME TLV missing

      19       Invalid Operation

                Error-value=6:  PCE-initiated LSP limit reached
                Error-value=7:  Delegation for PCE-initiated LSP cannot
                                 be revoked
                Error-value=8:  Non-zero PLSP-ID in LSP Initiate Request
                Error-value=9:  LSP is not PCE initiated
                Error-value=10: PCE-initiated operation-frequency limit
                                 reached

      23       Bad parameter value

                Error-value=1:  SYMBOLIC-PATH-NAME in use
                Error-value=2:  Speaker identity included for an LSP
                                 that is not PCE initiated

      24       LSP instantiation error

                Error-value=1:  Unacceptable instantiation parameters
                Error-value=2:  Internal error
                Error-value=3:  Signaling error

Top      ToC       Page 18 
9.  Security Considerations

   The security considerations described in [RFC8231] apply to the
   extensions described in this document.  Additional considerations
   related to a malicious PCE are introduced.

9.1.  Malicious PCE

   The LSP instantiation mechanism described in this document allows a
   PCE to generate state on the PCC and throughout the network.  As a
   result, it introduces a new attack vector: an attacker may flood the
   PCC with LSP instantiation requests and consume network and LSR
   resources by either spoofing messages or compromising the PCE itself.

   A PCC can protect itself from such an attack by imposing a limit on
   either the number of LSPs or the percentage of resources that are
   allocated to honor PCE-initiated LSP requests.  As soon as that limit
   is reached, the PCC MUST send a PCErr message with Error-type=19
   (Invalid Operation) and Error-value=6 (PCE-initiated LSP limit
   reached) and is free to drop any incoming PCInitiate messages for LSP
   initiation without additional processing.

   Rapid flaps triggered by the PCE can also be an attack vector.  A PCC
   can protect itself from such an attack by imposing a limit on the
   number of flaps per unit of time that it allows a PCE to generate.
   As soon as that limit is reached, a PCC MUST send a PCErr message
   with Error-type=19 (Invalid Operation) and Error-value=10
   (PCE-initiated operation-frequency limit reached) and is free to
   treat the session as having reached the limit in terms of resources
   allocated to honor PCE-initiated LSP requests, either permanently or
   for a locally-defined cool-off period.

9.2.  Malicious PCC

   The LSP instantiation mechanism described in this document requires
   the PCE to keep state for LSPs that it instantiates and relies on the
   PCC responding (with either a state report or an error message) to
   requests for LSP instantiation.  A malicious PCC or one that reached
   the limit of the number of PCE-initiated LSPs can ignore PCE requests
   and consume PCE resources.  A PCE can protect itself by imposing a
   limit on the number of requests pending or by setting a timeout, and
   it MAY take further action such as closing the session or removing
   all the LSPs it initiated.

Top      ToC       Page 19 
10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <https://www.rfc-editor.org/info/rfc5440>.

   [RFC5511]  Farrel, A., "Routing Backus-Naur Form (RBNF): A Syntax
              Used to Form Encoding Rules in Various Routing Protocol
              Specifications", RFC 5511, DOI 10.17487/RFC5511, April
              2009, <https://www.rfc-editor.org/info/rfc5511>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8231]  Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for Stateful PCE", RFC 8231,
              DOI 10.17487/RFC8231, September 2017,
              <https://www.rfc-editor.org/info/rfc8231>.

   [RFC8232]  Crabbe, E., Minei, I., Medved, J., Varga, R., Zhang, X.,
              and D. Dhody, "Optimizations of Label Switched Path State
              Synchronization Procedures for a Stateful PCE", RFC 8232,
              DOI 10.17487/RFC8232, September 2017,
              <https://www.rfc-editor.org/info/rfc8232>.

10.2.  Informative References

   [RFC4657]  Ash, J., Ed. and J. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol Generic
              Requirements", RFC 4657, DOI 10.17487/RFC4657, September
              2006, <https://www.rfc-editor.org/info/rfc4657>.

   [RFC8051]  Zhang, X., Ed. and I. Minei, Ed., "Applicability of a
              Stateful Path Computation Element (PCE)", RFC 8051,
              DOI 10.17487/RFC8051, January 2017,
              <https://www.rfc-editor.org/info/rfc8051>.

Top      ToC       Page 20 
   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

Acknowledgments

   We would like to thank Jan Medved, Ambrose Kwong, Ramon Casellas,
   Cyril Margaria, Dhruv Dhody, Raveendra Trovi, and Jon Hardwick for
   their contributions to this document.

Authors' Addresses

   Edward Crabbe
   Individual Contributor

   Email: edward.crabbe@gmail.com


   Ina Minei
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   United States of America

   Email: inaminei@google.com


   Siva Sivabalan
   Cisco Systems, Inc.
   170 West Tasman Dr.
   San Jose, CA  95134
   United States of America

   Email: msiva@cisco.com


   Robert Varga
   Pantheon Technologies SRO
   Mlynske Nivy 56
   Bratislava  821 05
   Slovakia

   Email: robert.varga@pantheon.tech