PCE Communication Protocol (PCEP) Extensions for Label Switched Path (LSP) Scheduling with Stateful PCE
RFC 8934

Document Type RFC - Proposed Standard (October 2020; No errata)
Authors Huaimo Chen  , Zhuangyan  , Qin Wu  , Daniele Ceccarelli 
Last updated 2020-10-31
Replaces draft-zhuang-pce-stateful-pce-lsp-scheduling
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Internet Engineering Task Force (IETF)                      H. Chen, Ed.
Request for Comments: 8934                                     Futurewei
Category: Standards Track                                 Y. Zhuang, Ed.
ISSN: 2070-1721                                                    Q. Wu
                                                                  Huawei
                                                           D. Ceccarelli
                                                                Ericsson
                                                            October 2020

  PCE Communication Protocol (PCEP) Extensions for Label Switched Path
                   (LSP) Scheduling with Stateful PCE

Abstract

   This document defines a set of extensions to the stateful PCE
   Communication Protocol (PCEP) to enable Label Switched Path (LSP)
   path computation, activation, setup, and deletion based on scheduled
   time intervals for the LSP and the actual network resource usage in a
   centralized network environment, as stated in RFC 8413.

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/rfc8934.

Copyright Notice

   Copyright (c) 2020 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.

Table of Contents

   1.  Introduction
   2.  Conventions Used in This Document
     2.1.  Terminology
   3.  Motivation and Objectives
   4.  Procedures and Mechanisms
     4.1.  LSP Scheduling Overview
     4.2.  Support of LSP Scheduling
       4.2.1.  LSP Scheduling
       4.2.2.  Periodical LSP Scheduling
     4.3.  Scheduled LSP Creation
     4.4.  Scheduled LSP Modifications
     4.5.  Scheduled LSP Activation and Deletion
   5.  PCEP Objects and TLVs
     5.1.  Stateful PCE Capability TLV
     5.2.  LSP Object
       5.2.1.  SCHED-LSP-ATTRIBUTE TLV
       5.2.2.  SCHED-PD-LSP-ATTRIBUTE TLV
   6.  The PCEP Messages
     6.1.  The PCRpt Message
     6.2.  The PCUpd Message
     6.3.  The PCInitiate Message
     6.4.  The PCReq message
     6.5.  The PCRep Message
     6.6.  The PCErr Message
   7.  Security Considerations
   8.  Manageability Consideration
     8.1.  Control of Function and Policy
     8.2.  Information and Data Models
     8.3.  Liveness Detection and Monitoring
     8.4.  Verify Correct Operations
     8.5.  Requirements on Other Protocols
     8.6.  Impact on Network Operations
   9.  IANA Considerations
     9.1.  PCEP TLV Type Indicators
       9.1.1.  SCHED-PD-LSP-ATTRIBUTE TLV Opt Field
       9.1.2.  Schedule TLVs Flag Field
     9.2.  STATEFUL-PCE-CAPABILITY TLV Flag Field
     9.3.  PCEP-ERROR Object Error Types and Values
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Acknowledgments
   Contributors
   Authors' Addresses

1.  Introduction

   The PCE Communication Protocol (PCEP) defined in [RFC5440] is used
   between a Path Computation Element (PCE) and a Path Computation
   Client (PCC) (or other PCE) to enable path computation of
   Multiprotocol Label Switching (MPLS) Traffic Engineering Label
   Switched Paths (TE LSPs).

   [RFC8231] describes a set of extensions to PCEP to provide stateful
   control.  A stateful PCE has access to not only the information
   carried by the network's Interior Gateway Protocol (IGP) but also the
   set of active paths and their reserved resources for its
   computations.  The additional state allows the PCE to compute
   constrained paths while considering individual LSPs and their
   interactions.

   Traditionally, the usage and allocation of network resources,
   especially bandwidth, can be supported by a Network Management System
   (NMS) operation such as path pre-establishment.  However, this does
   not provide efficient usage of network resources.  The established
   paths reserve the resources forever, so they cannot be used by other
   services even when they are not used for transporting any service.
   [RFC8413] then provides a framework that describes and discusses the
   problem and defines an appropriate architecture for the scheduled
   reservation of TE resources.

   The scheduled reservation of TE resources allows network operators to
   reserve resources in advance according to the agreements with their
   customers and allows them to transmit data about scheduling, such as
   a specified start time and duration (for example, for a scheduled
   bulk data replication between data centers).  It enables the
   activation of bandwidth usage at the time the service is really being
   used while letting other services use the bandwidth when it is not
   being used by this service.  The requirement of scheduled LSP
   provisioning is mentioned in [RFC8231] and [RFC7399].  Also, for
   deterministic networks [RFC8655], the scheduled LSP or temporal LSP
   can provide better network resource usage for guaranteed links.  This
   idea can also be applied in segment routing [RFC8402] to schedule the
   network resources over the whole network in a centralized manner.

   With this in mind, this document defines a set of needed extensions
   to PCEP used for stateful PCEs so as to enable LSP scheduling for
   path computation and LSP setup/deletion based on the actual network
   resource usage duration of a traffic service.  A scheduled LSP is
   characterized by a start time and a duration.  When the end of the
   LSP life is reached, it is deleted to free up the resources for other
   LSPs (scheduled or not).

2.  Conventions Used in This Document

   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.

2.1.  Terminology

   The following terminology is reused from existing PCE documents.

   *  Active Stateful PCE [RFC8051]

   *  Delegation [RFC8051]

   *  PCE-initiated LSP [RFC8281]

   *  PCC [RFC5440]

   *  PCE [RFC5440]

   *  TE LSP [RFC5440]

   *  TED (Traffic Engineering Database) [RFC5440]

   *  LSP-DB (LSP State Database) [RFC8051]

   In addition, this document defines the following terminologies.

   Scheduled TE LSP (or Scheduled LSP for short):  An LSP with
      scheduling attributes that carries traffic flow demand at a start
      time and lasts for a certain duration (or from a start time to an
      end time, where the end time is the start time plus the duration).
      A scheduled LSP is also called a "temporal LSP".  The PCE operates
      path computation per LSP availability for the required time and
      duration.

   Scheduled LSP-DB (SLSP-DB):  A database of scheduled LSPs.

   Scheduled TED:  Traffic engineering database with the awareness of
      scheduled resources for TE.  This database is generated by the PCE
      from the information in the TED and scheduled LSP-DB; it allows
      knowing, at any time, the expected amount of available resources
      (discounting the possibility of failures in the future).

   Start time (Start-Time):  This value indicates when the scheduled LSP
      is used and the corresponding LSP must be set up and active.  At
      other times (i.e., before the start time or after the start time
      plus duration), the LSP can be inactive to include the possibility
      of the resources being used by other services.

   Duration:  This value indicates the length of time that the LSP
      carries a traffic flow and the corresponding LSP must be set up
      and active.  At the end of the duration, the LSP is torn down and
      removed from the database.

3.  Motivation and Objectives

   A stateful PCE [RFC8231] can support better efficiency by using LSP
   scheduling described in the use case of [RFC8051].  This requires the
   PCE to maintain the scheduled LSPs and their associated resource
   usage (e.g., bandwidth for packet-switched network) as well as have
   the ability to trigger signaling for the LSP setup/tear-down at the
   correct time.

   Note that existing configuration tools can be used for LSP
   scheduling, but as highlighted in Section 3.1.3 of [RFC8231] as well
   as discussions in [RFC8413], doing this as a part of PCEP in a
   centralized manner has obvious advantages.

   This document provides a set of extensions to PCEP to enable LSP
   scheduling for LSP creation/deletion under the stateful control of a
   PCE and according to traffic service requests from customers, so as
   to improve the usage of network resources.

4.  Procedures and Mechanisms

4.1.  LSP Scheduling Overview

   LSP scheduling allows PCEs and PCCs to provide scheduled LSP for
   customers' traffic services at its actual usage time, so as to
   improve the network resource utilization efficiency.

   For stateful PCE supporting LSP scheduling, there are two types of
   LSP databases used in this document.  One is the LSP-DB defined in
   PCEP [RFC8231], while the other is the scheduled LSP database (SLSP-
   DB).  The SLSP-DB records scheduled LSPs and is used in conjunction
   with the TED and LSP-DB.  Note that the two types of LSP databases
   can be implemented in one physical database or two different
   databases.  This is an implementation matter, and this document does
   not state any preference.

   Furthermore, a scheduled TED can be generated from the scheduled LSP-
   DB, LSP-DB, and TED to indicate the network links and nodes with
   resource availability information for now and the future.  The
   scheduled TED MUST be maintained by all PCEs within the network
   environment.

   In the case of implementing PCC-initiated scheduled LSPs, when
   delegating a scheduled LSP, a PCC MUST include that LSP's scheduling
   parameters (see Section 5.2.1), including the start time and
   duration, using a Path Computation State Report (PCRpt) message.
   Since the LSP is not yet signaled, at the time of delegation, the LSP
   would be in down state.  Upon receiving the delegation of the
   scheduled LSP, a stateful PCE MUST check whether the parameters are
   valid.  If they are valid, it SHALL check the scheduled TED for the
   network resource availability on network nodes, compute a path for
   the LSP with the scheduling information, and update to the PCC as per
   the active stateful PCE techniques [RFC8231].

   Note that the active stateful PCE can update to the PCC with the path
   for the scheduled LSP at any time.  However, the PCC should not
   signal the LSP over the path after receiving these messages since the
   path is not active yet; the PCC signals the LSP at the start time.

   In the case of multiple PCEs within a single domain, the PCE would
   need to synchronize their scheduling information with other PCEs
   within the domain.  This could be achieved by proprietary database-
   synchronization techniques or via a possible PCEP extension (see
   [PCE-STATE-SYNC]).  The technique used to synchronize an SLSP-DB is
   out of scope for this document.  When the scheduling information is
   out of synchronization among some PCEs, some scheduled LSPs may not
   be set up successfully.

   The scheduled LSP can also be initiated by a PCE itself.  In the case
   of implementing a PCE-initiated scheduled LSP, the stateful PCE SHALL
   check the network resource availability for the traffic, compute a
   path for the scheduled LSP, and initiate a scheduled LSP at the PCC
   and synchronize the scheduled LSP to other PCEs.  Note that the PCC
   could be notified immediately or at the start time of the scheduled
   LSP, based on the local policy.  In the former case, the SCHED-LSP-
   ATTRIBUTE TLV (see Section 5.2.1) MUST be included in the message,
   whereas for the latter, the SCHED-LSP-ATTRIBUTE TLV SHOULD NOT be
   included.  Either way, the synchronization to other PCEs MUST be done
   when the scheduled LSP is created.

   In both modes, for activation of scheduled LSPs, the PCC MUST
   initiate the setup of the scheduled LSP at the start time.
   Similarly, on the scheduled usage expiry, the PCC MUST initiate the
   removal of the LSP based on the flag set in the SCHED-LSP-ATTRIBUTE
   TLV.

4.2.  Support of LSP Scheduling

4.2.1.  LSP Scheduling

   For a scheduled LSP, a user configures it with an arbitrary
   scheduling period from time Ta to time Tb, which may be represented
   as [Ta, Tb].

   When an LSP is configured with arbitrary scheduling period [Ta, Tb],
   a path satisfying the constraints for the LSP in the scheduling
   period is computed, and the LSP along the path is set up to carry
   traffic from time Ta to time Tb.

4.2.2.  Periodical LSP Scheduling

   In addition to LSP scheduling at an arbitrary time period, there is
   also periodical LSP scheduling.

   Periodical LSP scheduling means an LSP has multiple time intervals
   and the LSP is set up to carry traffic in every time interval.  It
   has a scheduling period such as [Ta, Tb], a number of repeats such as
   10 (repeats 10 times), and a repeat cycle/time interval such as a
   week (repeats every week).  The scheduling interval "[Ta, Tb] repeats
   n times with repeat cycle C" represents n+1 scheduling intervals as
   follows:

      [Ta, Tb], [Ta+C, Tb+C], [Ta+2C, Tb+2C], ..., [Ta+nC, Tb+nC]

   When an LSP is configured with a scheduling interval such as "[Ta,
   Tb] repeats 10 times with a repeat cycle of a week" (representing 11
   scheduling intervals), a path satisfying the constraints for the LSP
   in every interval represented by the periodical scheduling interval
   is computed once.  Note that the path computed for one recurrence may
   be different from the path for another recurrence.  And then the LSP
   along the path is set up to carry traffic in each of the scheduling
   intervals.  If there is no path satisfying the constraints for some
   of the intervals, the LSP MUST NOT be set up at all.  It MUST
   generate a PCEP Error (PCErr) with Error-Type = 29 (Path computation
   failure) and Error-value = 5 (Constraints could not be met for some
   intervals).

4.2.2.1.  Elastic Time LSP Scheduling

   In addition to the basic LSP scheduling at an arbitrary time period,
   another option is elastic time intervals, which is represented as
   within -P and Q, where P and Q are amounts of time such as 300
   seconds.  P is called the elastic range lower bound, and Q is called
   the elastic range upper bound.

   For a simple time interval such as [Ta, Tb] with an elastic range,
   elastic time interval "[Ta, Tb] within -P and Q" means a time period
   from (Ta+X) to (Tb+X), where -P <= X <= Q.  Note that both Ta and Tb
   are shifted by the same X.  This elastic time interval is suitable
   for the case where a user wants to have a scheduled LSP up to carry
   the traffic in time interval [Ta, Tb] and has some flexibility on
   shifting the time interval a little bit, such as up to P seconds
   earlier/left or some time such as up to Q seconds later/right.

   When an LSP is configured with elastic time interval "[Ta, Tb] within
   -P and Q", a path is computed such that the path satisfies the
   constraints for the LSP in the time period from (Ta+Xv) to (Tb+Xv),
   and an optimization is performed on Xv from -P to Q.  The
   optimization makes [Ta+Xv, Tb+Xv] the time interval closest to time
   interval [Ta, Tb] within the elastic range.  The LSP along the path
   is set up to carry traffic in the time period from (Ta+Xv) to
   (Tb+Xv).

   Similarly, for a recurrent time interval with an elastic range,
   elastic time interval "[Ta, Tb] repeats n times with repeat cycle C
   within -P and Q" represents n+1 simple elastic time intervals as
   follows:

      [Ta+X0, Tb+X0], [Ta+C+X1, Tb+C+X1], ..., [Ta+nC+Xn, Tb+nC+Xn],
      where -P <= Xi <= Q, i = 0, 1, 2, ..., n.

   If a user wants to keep the same repeat cycle between any two
   adjacent time intervals, elastic time interval "[Ta, Tb] repeats n
   times with repeat cycle C within -P and Q SYNC" may be used, which
   represents n+1 simple elastic time intervals as follows:

      [Ta+X, Tb+X], [Ta+C+X, Tb+C+X], ..., [Ta+nC+X, Tb+nC+X], where -P
      <= X <= Q.

4.2.2.2.  Grace Periods

   Besides the stated time scheduling, a user may want to have some
   grace periods (short for "graceful time periods") for each or some of
   the time intervals for the LSP.  Two grace periods may be configured
   for a time interval.  One is the grace period before the time
   interval, called "Grace-Before", which extends the lifetime of the
   LSP by an amount of time (such as 30 seconds) before the time
   interval.  The other grace period is after the time interval and is
   called "Grace-After"; it extends the lifetime of the LSP by an amount
   of time (such as 60 seconds) after the time interval.  Note that no
   network resources such as link bandwidth will be reserved for the LSP
   during the grace periods.

   When an LSP is configured with a simple time interval such as [Ta,
   Tb] with grace periods such as Grace-Before GrB and Grace-After GrA,
   a path is computed such that the path satisfies the constraints for
   the LSP in the time period from Ta to Tb.  The LSP along the path is
   set up to carry traffic in the time period from (Ta-GrB) to (Tb+GrA).
   During grace periods from (Ta-GrB) to Ta and from Tb to (Tb+GrA), the
   LSP is up to carry traffic in best effort.

4.3.  Scheduled LSP Creation

   In order to realize PCC-initiated scheduled LSPs in a centralized
   network environment, a PCC MUST separate the setup of an LSP into two
   steps.  The first step is to request/delegate and get an LSP but not
   signal it over the network.  The second step is to signal the
   scheduled LSP over the Label Switching Routers (LSRs) at its start
   time.

   For PCC-initiated scheduled LSPs, a PCC MUST delegate the scheduled
   LSP by sending a PCRpt message by including its demanded resources
   with the scheduling information to a stateful PCE.  Note that the PCC
   MAY use Path Computation Request (PCReq) and Path Computation Reply
   (PCRep) messages with scheduling information before delegating.

   Upon receiving the delegation via PCRpt message, the stateful PCE
   MUST compute a path for the scheduled LSP per its start time and
   duration based on the network resource availability stored in the
   scheduled TED (see Section 4.1).

   The stateful PCE will send a Path Computation Update Request (PCUpd)
   message with the scheduled path information and the scheduled
   resource information for the scheduled LSP to the PCC.  The stateful
   PCE MUST update its local scheduled LSP-DB and scheduled TED with the
   scheduled LSP and would need to synchronize the scheduling
   information with other PCEs in the domain.

   For a PCE-initiated scheduled LSP, the stateful PCE MUST
   automatically compute a path for the scheduled LSP per requests from
   network management systems, based on the network resource
   availability in the scheduled TED, and send an LSP Initiate Request
   (PCInitiate) message with the path information to the PCC.  Based on
   the local policy, the PCInitiate message could be sent immediately to
   ask the PCC to create a scheduled LSP (as per this document), or the
   PCInitiate message could be sent at the start time to the PCC to
   create a normal LSP (as per [RFC8281]).

   For both PCC-initiated and PCE-initiated Scheduled LSPs:

   *  The stateful PCE MUST update its local scheduled LSP-DB and
      scheduled TED with the scheduled LSP.

   *  Upon receiving the PCUpd message or PCInitiate message for the
      scheduled LSP from PCEs with a found path, the PCC determines that
      it is a scheduled path for the LSP by the SCHED-LSP-ATTRIBUTE TLV
      (see Section 5.2.1) or SCHED-PD-LSP-ATTRIBUTE TLV (see
      Section 5.2.2) in the message and does not trigger signaling for
      the LSP setup on LSRs immediately.

   *  The stateful PCE MUST update the scheduled LSP parameters on any
      network events using the PCUpd message to the PCC.  These changes
      are also synchronized to other PCEs.

   *  When it is time for the LSP to be set up (i.e., at the start
      time), based on the value of the C flag for the scheduled TLV,
      either the PCC MUST trigger the LSP to be signaled or the
      delegated PCE MUST send a PCUpd message to the head-end LSR
      providing the updated path to be signaled (with the A flag set to
      indicate LSP activation).

4.4.  Scheduled LSP Modifications

   After a scheduled LSP is configured, a user may change its
   parameters, including the requested time and the bandwidth.  For a
   periodic-scheduled LSP, its unused recurrences can be modified or
   canceled.  For a scheduled LSP that is currently active, its duration
   (the lifetime) can be reduced.

   In the PCC-initiated case, the PCC MUST send the PCE a PCRpt message
   for the scheduled LSP with updated parameters, as well as scheduled
   information included in the SCHED-LSP-ATTRIBUTE TLV (see
   Section 5.2.1) or SCHED-PD-LSP-ATTRIBUTE TLV (see Section 5.2.2)
   carried in the LSP object.  The PCE SHOULD take the updated resources
   and schedule into consideration, and update the new path for the
   scheduled LSP to the PCC, and synchronize to other PCEs in the
   network.  If the path cannot be set based on new requirements, the
   previous LSP will not be impacted, and this MUST be conveyed by the
   use of an empty Explicit Route Object (ERO) in the PCEP messages.

   In the PCE-initiated case, the stateful PCE would recompute the path
   based on updated parameters and scheduled information.  If it has
   already conveyed this information to the PCC by sending a PCInitiate
   message, it SHOULD update the path and other scheduling and resource
   information by sending a PCUpd message.

4.5.  Scheduled LSP Activation and Deletion

   In the PCC-initiated case, when it is time for the LSP to be set up
   (i.e., at the start time), based on the value of the C flag for the
   scheduled TLV, either the PCC MUST trigger the LSP to be signaled, or
   the delegated PCE MUST send a PCUpd message to the head-end LSR
   providing the updated path to be signaled (with the A flag set to
   indicate LSP activation).  The PCC MUST report the status of the
   active LSP as per the procedures in [RFC8231], and at this time, the
   LSP MUST be considered part of the LSP-DB.  The A flag MUST be set in
   the scheduled TLV to indicate that the LSP is active now.  After the
   scheduled duration expires, based on the C flag, the PCC MUST trigger
   the LSP deletion on itself, or the delegated PCE MUST send a PCUpd
   message to the PCC to delete the LSP as per the procedures in
   [RFC8231].

   In the PCE-initiated case, based on the local policy, if the
   scheduled LSP is already conveyed to the PCC at the time of creation,
   the handling of LSP activation and deletion is handled in the same
   way as the PCC-initiated case, as per the setting of the C flag.
   Otherwise, the PCE MUST send the PCInitiate message to the PCC at the
   start time to create a normal LSP without the scheduled TLV and
   remove the LSP after the duration expires, as per [RFC8281].

5.  PCEP Objects and TLVs

5.1.  Stateful PCE Capability TLV

   A PCC and a PCE indicate their ability to support LSP scheduling
   during their PCEP session establishment phase.  For an environment
   with multiple PCEs, the PCEs SHOULD also establish a PCEP session and
   indicate its ability to support LSP scheduling among PCEP peers.  The
   OPEN object in the Open message contains the STATEFUL-PCE-CAPABILITY
   TLV.  Note that the STATEFUL-PCE-CAPABILITY TLV is defined in
   [RFC8231] and updated in [RFC8281] and [RFC8232].  In this document,
   we define a new flag bit B (LSP-SCHEDULING-CAPABILITY) in the Flags
   field of the STATEFUL-PCE-CAPABILITY TLV to indicate the support of
   LSP scheduling.  We also define another flag bit PD (PD-LSP-
   CAPABILITY) to indicate the support of LSP periodical scheduling.

   B (LSP-SCHEDULING-CAPABILITY) - 1 bit (Bit Position 22):  If set to 1
      by a PCC, the B flag indicates that the PCC allows LSP scheduling;
      if set to 1 by a PCE, the B flag indicates that the PCE is capable
      of LSP scheduling.  The B bit MUST be set by both PCEP peers in
      order to support LSP scheduling for path computation.

   PD (PD-LSP-CAPABILITY) - 1 bit (Bit Position - 21):  If set to 1 by a
      PCC, the PD flag indicates that the PCC allows LSP scheduling
      periodically; if set to 1 by a PCE, the PD flag indicates that the
      PCE is capable of periodical LSP scheduling.  Both the PD bit and
      the B bit MUST be set to 1 by both PCEP peers in order to support
      periodical LSP scheduling for path computation.  If the PD bit or
      B bit is 0, then the periodical LSP scheduling capability MUST be
      ignored.

5.2.  LSP Object

   The LSP object is defined in [RFC8231].  This document adds an
   optional SCHED-LSP-ATTRIBUTE TLV for normal LSP scheduling and an
   optional SCHED-PD-LSP-ATTRIBUTE TLV for periodical LSP scheduling.
   The LSP object for a scheduled LSP MUST NOT include these two TLVs.
   Only one scheduling, either normal or periodical, is allowed for a
   scheduled LSP.

   The presence of the SCHED-LSP-ATTRIBUTE TLV in the LSP object
   indicates that this LSP is normal scheduling while the SCHED-PD-LSP-
   ATTRIBUTE TLV indicates that this scheduled LSP is periodical.  The
   SCHED-LSP-ATTRIBUTE TLV MUST be present in the LSP object for each
   normal-scheduled LSP carried in the PCEP messages.  The SCHED-PD-LSP-
   ATTRIBUTE TLV MUST be used in the LSP object for each periodic-
   scheduled LSP carried in the PCEP messages.

   Only one SCHED-LSP-ATTRIBUTE TLV SHOULD be present in the LSP object.
   If more than one SCHED-LSP-ATTRIBUTE TLV is found, the first instance
   is processed and others ignored.  The SCHED-PD-LSP-ATTRIBUTE TLV is
   the same as the SCHED-LSP-ATTRIBUTE TLV with regard to its presence
   in the LSP object.

5.2.1.  SCHED-LSP-ATTRIBUTE TLV

   The SCHED-LSP-ATTRIBUTE TLV MAY be included as an optional TLV within
   the LSP object for LSP scheduling for the requesting traffic service.

   This TLV MUST NOT be included unless both PCEP peers have set the B
   (LSP-SCHEDULING-CAPABILITY) bit in the STATEFUL-PCE-CAPABILITY TLV
   carried in the Open message to one.  If the TLV is received by a peer
   when both peers didn't set the B bit to one, the peer MUST generate a
   PCEP Error (PCErr) with a PCEP-ERROR object having Error-Type = 19
   (Invalid Operation) and Error-value = 15 (Attempted LSP scheduling
   while the scheduling capability was not advertised).

   The format of the SCHED-LSP-ATTRIBUTE TLV is shown in Figure 1.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type (49)         |          Length               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Flags |R|C|A|G|               Reserved (0)                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Start-Time                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Duration                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   GrB / Elastic-Lower-Bound   |   GrA / Elastic-Upper-Bound   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 1: SCHED-LSP-ATTRIBUTE TLV

   The type of the TLV is 49, and the TLV has a fixed length of 16
   octets.

   The fields in the format are:

   Flags (8 bits):  The following flags are defined in this document.

      R (1 bit):  Set to 1 to indicate that the Start-Time is a relative
         time, which is the number of seconds from the current time.
         The PCEs and PCCs MUST synchronize their clocks when relative
         time is used.  It is RECOMMENDED that the Network Time Protocol
         [RFC5905] be used to synchronize clocks among them.  When the
         transmission delay from a PCE or PCC to another PCE or PCC is
         too big (such as greater than 1 second), the scheduling
         interval represented is not accurate if the delay is not
         considered.  Set to 0 to indicate that the 32-bit Start-Time is
         an absolute time, which is the number of seconds since the
         epoch.  The epoch is 1 January 1970 at 00:00 UTC.  It wraps
         around every 2^32 seconds, which is roughly 136 years.  The
         next wraparound will occur in the year 2106.  The received
         Start-Time is considered after the wraparound if the resulting
         value is less than the current time.  In that case, the value
         of the 32-bit Start-Time is considered to be the number of
         seconds from the time of wraparound (because the Start-Time is
         always a future time).

      C (1 bit):  Set to 1 to indicate that the PCC is responsible to
         set up and remove the scheduled LSP based on the Start-Time and
         Duration.  The PCE holds these responsibilities when the bit is
         set to zero.

      A (1 bit):  Set to 1 to indicate that the scheduled LSP has been
         activated.

      G (1 bit):  Set to 1 to indicate that the grace period is included
         in the fields GrB/Elastic-Lower-Bound and GrA/Elastic-Upper-
         Bound; set to 0 to indicate that the elastic range is included
         in the fields.

   Reserved (24 bits):  This field MUST be set to zero on transmission
      and MUST be ignored on receipt.

   Start-Time (32 bits):  This value, in seconds, indicates when the
      scheduled LSP is used to carry traffic and the corresponding LSP
      MUST be set up and activated.  Note that the transmission delay
      SHOULD be considered when R=1 and the value of Start-Time is
      small.

   Duration (32 bits):  This value, in seconds, indicates the duration
      that the LSP carries a traffic flow and the corresponding LSP MUST
      be up to carry traffic.  At the expiry of this duration, the LSP
      MUST be torn down and deleted.  A value of 0 MUST NOT be used in
      Duration since it does not make any sense.  The value of Duration
      SHOULD be greater than a constant MINIMUM-DURATION seconds, where
      MINIMUM-DURATION is 5.

   Start-Time indicates a time at or before which the scheduled LSP MUST
   be set up.  When the R bit is set to 0, the value of Start-Time
   represents the number of seconds since the epoch.  When the R bit is
   set to 1, the value of Start-Time represents the number of seconds
   from the current time.

   In addition, the SCHED-LSP-ATTRIBUTE TLV contains the G flag set to 1
   and a nonzero Grace-Before and Grace-After in the fields GrB/Elastic-
   Lower-Bound and GrA/Elastic-Upper-Bound if grace periods are
   configured.  It includes the G flag set to 0 and a nonzero elastic
   range lower bound and upper bound in the fields if there is an
   elastic range configured.  A TLV can configure a nonzero grace period
   or elastic range, but it MUST NOT provide both for an LSP.

   GrB (Grace-Before, 16 bits):  The grace period time length, in
      seconds, before the start time.

   GrA (Grace-After, 16 bits):  The grace period time length, in
      seconds, after time interval [start time, start time + duration].

   Elastic-Lower-Bound (16 bits):  The maximum amount of time, in
      seconds, that the time interval can shift lower/left.

   Elastic-Upper-Bound (16 bits):  The maximum amount of time, in
      seconds, that the time interval can shift higher/right.

5.2.2.  SCHED-PD-LSP-ATTRIBUTE TLV

   The periodical LSP is a special case of LSP scheduling.  The traffic
   service happens in a series of repeated time intervals.  The SCHED-
   PD-LSP-ATTRIBUTE TLV can be included as an optional TLV within the
   LSP object for this periodical LSP scheduling.

   This TLV MUST NOT be included unless both PCEP peers have set the B
   (LSP-SCHEDULING-CAPABILITY) bit and PD (PD-LSP-CAPABILITY) bit in
   STATEFUL-PCE-CAPABILITY TLV carried in Open message to one.  If the
   TLV is received by a peer when either bit is zero (or both bits are
   zero), the peer MUST generate a PCEP Error (PCErr) with a PCEP-ERROR
   object having Error-Type = 19 (Invalid Operation) and Error-value =
   15 (Attempted LSP scheduling while the scheduling capability was not
   advertised).

   The format of the SCHED-PD-LSP-ATTRIBUTE TLV is shown in Figure 2.

    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 (50)          |         Length                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Flags|R|C|A|G| Opt   |           NR          |  Reserved (0) |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Start-Time                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Duration                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Repeat-time-length                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   GrB / Elastic-Lower-Bound   |   GrA / Elastic-Upper-Bound   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 2: SCHED-PD-LSP-ATTRIBUTE TLV

   The type of the TLV is 50, and the TLV has a fixed length of 20
   octets.  The description, format, and meaning of the flags (R, C, A,
   and G bits), Start-Time, Duration, GrB, GrA, Elastic-Lower-Bound, and
   Elastic-Upper-Bound fields remain the same as in the SCHED-LSP-
   ATTRIBUTE TLV.

   The following fields are new:

   Opt (4 bits):  Indicates options to repeat.  When a PCE receives a
      TLV with an unknown Opt value, it does not compute any path for
      the LSP.  It MUST generate a PCEP Error (PCErr) with a PCEP-ERROR
      object having Error-Type = 4 (Not supported object) and Error-
      value = 4 (Unsupported parameter).

         Opt = 1: repeat every month

         Opt = 2: repeat every year

         Opt = 3: repeat every Repeat-time-length

      A user may configure a Repeat-time-length in time units weeks,
      days, hours, minutes, and/or seconds.  The value represented by
      these units is converted to the number of seconds in the TLV.  For
      example, repeat every 2 weeks is equivalent to repeat every
      Repeat-time-length = 2*7*86,400 (seconds), where 86,400 is the
      number of seconds per day.

   NR (12 bits):  The number of repeats.  During each repetition, LSP
      carries traffic.

   Reserved (8 bits):  This field MUST be set to zero on transmission
      and MUST be ignored on receipt.

   Repeat-time-length (32 bits):  The time in seconds between the Start-
      Time of one repetition and the Start-Time of the next repetition.

6.  The PCEP Messages

6.1.  The PCRpt Message

   The Path Computation State Report (PCRpt) message is a PCEP message
   sent by a PCC to a PCE to report the status of one or more LSPs, as
   per [RFC8231].  Each LSP State Report in a PCRpt message contains the
   actual LSP's path, bandwidth, operational and administrative status,
   etc.  An LSP Status Report carried in a PCRpt message is also used in
   delegation or revocation of control of an LSP to/from a PCE.  In the
   case of a scheduled LSP, a scheduled TLV MUST be carried in the LSP
   object, and the ERO conveys the intended path for the scheduled LSP.
   The scheduled LSP MUST be delegated to a PCE.

6.2.  The PCUpd Message

   The Path Computation Update Request (PCUpd) message is a PCEP message
   sent by a PCE to a PCC to update LSP parameters on one or more LSPs,
   as per [RFC8231].  Each LSP Update Request in a PCUpd message
   contains all LSP parameters that a PCE wishes to be set for a given
   LSP.  In the case of a scheduled LSP, a scheduled TLV MUST be carried
   in the LSP object, and the ERO conveys the intended path for the
   scheduled LSP.  If no path can be found, an empty ERO is used.  The A
   bit is used in the PCUpd message to indicate the activation of the
   scheduled LSP if the PCE is responsible for the activation (as per
   the C bit).

6.3.  The PCInitiate Message

   The LSP Initiate Request (PCInitiate) message is a PCEP message sent
   by a PCE to a PCC to trigger LSP instantiation or deletion, as per
   [RFC8281].  In the case of a scheduled LSP, based on the local
   policy, the PCE MAY convey the scheduled LSP to the PCC by including
   a scheduled TLV in the LSP object.  Alternatively, the PCE would
   initiate the LSP only at the start time of the scheduled LSP, as per
   [RFC8281], without the use of scheduled TLVs.

6.4.  The PCReq message

   The Path Computation Request (PCReq) message is a PCEP message sent
   by a PCC to a PCE to request a path computation [RFC5440], and it may
   contain the LSP object [RFC8231] to identify the LSP for which the
   path computation is requested.  In the case of a scheduled LSP, a
   scheduled TLV MUST be carried in the LSP object in the PCReq message
   to request the path computation based on the scheduled TED and LSP-
   DB.  A PCC MAY use the PCReq message to obtain the scheduled path
   before delegating the LSP.  The parameters of the LSP may be changed
   (refer to Section 4.4).

6.5.  The PCRep Message

   The Path Computation Reply (PCRep) message is a PCEP message sent by
   a PCE to a PCC in reply to a path computation request [RFC5440], and
   it may contain the LSP object [RFC8231] to identify the LSP for which
   the path is computed.  A PCRep message can contain either a set of
   computed paths if the request can be satisfied or a negative reply if
   not.  A negative reply may indicate the reason why no path could be
   found.  In the case of a scheduled LSP, a scheduled TLV MUST be
   carried in the LSP object in PCRep message to indicate the path
   computation based on the scheduled TED and LSP-DB.  A PCC and PCE MAY
   use PCReq and PCRep messages to obtain the scheduled path before
   delegating the LSP.

6.6.  The PCErr Message

   The PCEP Error (PCErr) message is a PCEP message, as described in
   [RFC5440], for error reporting.  This document defines new error
   values for several error types to cover failures specific to
   scheduling and reuses the applicable error types and error values of
   [RFC5440] and [RFC8231] wherever appropriate.

   The PCEP extensions for scheduling MUST NOT be used if one or both of
   the PCEP speakers have not set the corresponding bits in the
   STATEFUL-PCE-CAPABILITY TLV in their respective Open messages to one.
   If the PCEP speaker supports the extensions of this specification but
   did not advertise this capability, then upon receipt of LSP object
   with the scheduled TLV, it MUST generate a PCEP Error (PCErr) with
   Error-Type = 19 (Invalid Operation) and Error-value = 15 (Attempted
   LSP scheduling while the scheduling capability was not advertised),
   and it SHOULD ignore the TLV.  As per Section 7.1 of [RFC5440], a
   legacy PCEP implementation that does not understand this
   specification would consider a scheduled TLV unknown and ignore it.

   If the PCC decides that the scheduling parameters proposed in the
   PCUpd/PCInitiate message are unacceptable, it MUST report this error
   by including the LSP-ERROR-CODE TLV (Section 7.3.3 of [RFC8231]) with
   LSP Error-value = 4 (Unacceptable parameters) in the LSP object (with
   the scheduled TLV) in the PCRpt message to the PCE.

   The scheduled TLV MUST be included in the LSP object for the
   scheduled LSPs.  If the TLV is missing, the receiving PCEP speaker
   MUST send a PCErr message with Error-Type = 6 (Mandatory Object
   missing) and Error-value = 16 (Scheduled TLV missing).

7.  Security Considerations

   This document defines the LSP-SCHEDULING-CAPABILITY TLV and SCHED-
   LSP-ATTRIBUTE TLV; the security considerations discussed in
   [RFC5440], [RFC8231], and [RFC8281] continue to apply.  In some
   deployments, the scheduling information could provide details about
   the network operations that could be deemed extra sensitive.
   Additionally, snooping of PCEP messages with such data or using PCEP
   messages for network reconnaissance may give an attacker sensitive
   information about the operations of the network.  A single PCEP
   message can now instruct a PCC to set up and tear down an LSP every
   second for a number of times.  That single message could have a
   significant effect on the network.  Thus, such deployments SHOULD
   employ suitable PCEP security mechanisms like TCP Authentication
   Option (TCP-AO), which is discussed in [RFC5925] and [RFC8253].  Note
   that [RFC8253] is considered a security enhancement and thus is much
   better suited for sensitive information.  PCCs may also need to apply
   some form of rate limit to the processing of scheduled LSPs.

8.  Manageability Consideration

8.1.  Control of Function and Policy

   The LSP scheduling feature MUST be controlled per tunnel by the
   active stateful PCE.  The values for parameters like start time and
   duration SHOULD be configurable by customer applications and based on
   the local policy at PCE.  The suggested default values for start time
   and duration are one day (in seconds) from the current time and one
   year (in seconds), respectively.  One day has 86,400 seconds.  One
   year has 31,536,000 seconds.

   When configuring the parameters for time, a user SHOULD consider leap
   years and leap seconds.  If a scheduled LSP has a time interval
   containing a leap year, the duration of the LSP is 366 days plus the
   rest of the interval.

8.2.  Information and Data Models

   An implementation SHOULD allow the operator to view the information
   about each scheduled LSP defined in this document.  To serve this
   purpose, the PCEP YANG module [PCE-PCEP-YANG] could be extended.

8.3.  Liveness Detection and Monitoring

   Mechanisms defined in this document do not imply any new liveness
   detection and monitoring requirements in addition to those already
   listed in [RFC5440].

8.4.  Verify Correct Operations

   Mechanisms defined in this document do not imply any new operation
   verification requirements in addition to those already listed in
   [RFC5440].  An implementation SHOULD allow a user to view
   information, including the status of a scheduled LSP, through a
   Command Line Interface (CLI) tool.  In addition, it SHOULD check and
   handle the cases where there is a significant time correction or a
   clock skew between PCC and PCE.

8.5.  Requirements on Other Protocols

   Mechanisms defined in this document do not imply any new requirements
   on other protocols.

8.6.  Impact on Network Operations

   Mechanisms defined in this document do not have any impact on network
   operations in addition to those already listed in [RFC5440].

9.  IANA Considerations

9.1.  PCEP TLV Type Indicators

   IANA maintains the "PCEP TLV Type Indicators" subregistry within the
   "Path Computation Element Protocol (PCEP) Numbers" registry.  IANA
   has made the following allocations in this subregistry for the new
   PCEP TLVs defined in this document.

            +=======+========================+===============+
            | Value | Description            | Reference     |
            +=======+========================+===============+
            | 49    | SCHED-LSP-ATTRIBUTE    | This document |
            +-------+------------------------+---------------+
            | 50    | SCHED-PD-LSP-ATTRIBUTE | This document |
            +-------+------------------------+---------------+

              Table 1: Additions to PCEP TLV Type Indicators
                               Subregistry

9.1.1.  SCHED-PD-LSP-ATTRIBUTE TLV Opt Field

   IANA has created and will maintain a new subregistry named "SCHED-PD-
   LSP-ATTRIBUTE TLV Opt Field" within the "Path Computation Element
   Protocol (PCEP) Numbers" registry.  Initial values for the
   subregistry are given below.  New values are assigned by Standards
   Action [RFC8126].

        +=======+=================================+===============+
        | Value | Description                     | Reference     |
        +=======+=================================+===============+
        | 0     | Reserved                        |               |
        +-------+---------------------------------+---------------+
        | 1     | REPEAT-EVERY-MONTH              | This document |
        +-------+---------------------------------+---------------+
        | 2     | REPEAT-EVERY-YEAR               | This document |
        +-------+---------------------------------+---------------+
        | 3     | REPEAT-EVERY-REPEAT-TIME-LENGTH | This document |
        +-------+---------------------------------+---------------+
        | 4-14  | Unassigned                      |               |
        +-------+---------------------------------+---------------+
        | 15    | Reserved                        |               |
        +-------+---------------------------------+---------------+

             Table 2: New SCHED-PD-LSP-ATTRIBUTE TLV Opt Field
                                Subregistry

9.1.2.  Schedule TLVs Flag Field

   IANA has created a new subregistry named "Schedule TLVs Flag Field"
   within the "Path Computation Element Protocol (PCEP) Numbers"
   registry.  New values are assigned by Standards Action [RFC8126].
   Each bit should be tracked with the following qualities:

   *  Bit number (counting from bit 0 as the most significant bit)

   *  Capability description

   *  Defining RFC

   The following values are defined in this document:

          +=====+===============================+===============+
          | Bit | Description                   | Reference     |
          +=====+===============================+===============+
          | 0-3 | Unassigned                    |               |
          +-----+-------------------------------+---------------+
          | 4   | Relative Time (R-bit)         | This document |
          +-----+-------------------------------+---------------+
          | 5   | PCC Responsible (C-bit)       | This document |
          +-----+-------------------------------+---------------+
          | 6   | LSP Activated (A-bit)         | This document |
          +-----+-------------------------------+---------------+
          | 7   | Grace Period Included (G-bit) | This document |
          +-----+-------------------------------+---------------+

             Table 3: New Schedule TLVs Flag Field Subregistry

9.2.  STATEFUL-PCE-CAPABILITY TLV Flag Field

   This document defines new bits in the Flags field in the STATEFUL-
   PCE-CAPABILITY TLV in the OPEN object.  IANA maintains the "STATEFUL-
   PCE-CAPABILITY TLV Flag Field" subregistry within the "Path
   Computation Element Protocol (PCEP) Numbers" registry.  IANA has made
   the following allocations in this subregistry.

        +=====+===================================+===============+
        | Bit | Description                       | Reference     |
        +=====+===================================+===============+
        | 22  | LSP-SCHEDULING-CAPABILITY (B-bit) | This document |
        +-----+-----------------------------------+---------------+
        | 21  | PD-LSP-CAPABILITY (PD-bit)        | This document |
        +-----+-----------------------------------+---------------+

           Table 4: Additions to STATEFUL-PCE-CAPABILITY TLV Flag
                             Field Subregistry

9.3.  PCEP-ERROR Object Error Types and Values

   IANA has allocated the following new error types to the existing
   error values within the "PCEP-ERROR Object Error Types and Values"
   subregistry of the "Path Computation Element Protocol (PCEP) Numbers"
   registry:

      +============+================+===============================+
      | Error-Type | Meaning        | Error-value                   |
      +============+================+===============================+
      | 6          | Mandatory      | 16: Scheduled TLV missing     |
      |            | Object missing |                               |
      +------------+----------------+-------------------------------+
      | 19         | Invalid        | 15: Attempted LSP scheduling  |
      |            | Operation      | while the scheduling          |
      |            |                | capability was not advertised |
      +------------+----------------+-------------------------------+
      | 29         | Path           | 5: Constraints could not be   |
      |            | computation    | met for some intervals        |
      |            | failure        |                               |
      +------------+----------------+-------------------------------+

          Table 5: Additions to PCEP-ERROR Object Error Types and
                             Values Subregistry

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>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <https://www.rfc-editor.org/info/rfc5905>.

   [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>.

   [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>.

   [RFC8281]  Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for PCE-Initiated LSP Setup in a Stateful PCE
              Model", RFC 8281, DOI 10.17487/RFC8281, December 2017,
              <https://www.rfc-editor.org/info/rfc8281>.

   [RFC8413]  Zhuang, Y., Wu, Q., Chen, H., and A. Farrel, "Framework
              for Scheduled Use of Resources", RFC 8413,
              DOI 10.17487/RFC8413, July 2018,
              <https://www.rfc-editor.org/info/rfc8413>.

10.2.  Informative References

   [PCE-PCEP-YANG]
              Dhody, D., Hardwick, J., Beeram, V. P., and J. Tantsura,
              "A YANG Data Model for Path Computation Element
              Communications Protocol (PCEP)", Work in Progress,
              Internet-Draft, draft-ietf-pce-pcep-yang-15, 31 October
              2020,
              <https://tools.ietf.org/html/draft-ietf-pce-pcep-yang-15>.

   [PCE-STATE-SYNC]
              Litkowski, S., Sivabalan, S., Li, C., and H. Zheng, "Inter
              Stateful Path Computation Element (PCE) Communication
              Procedures.", Work in Progress, Internet-Draft, draft-
              litkowski-pce-state-sync-08, 12 July 2020,
              <https://tools.ietf.org/html/draft-litkowski-pce-state-
              sync-08>.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <https://www.rfc-editor.org/info/rfc5925>.

   [RFC7399]  Farrel, A. and D. King, "Unanswered Questions in the Path
              Computation Element Architecture", RFC 7399,
              DOI 10.17487/RFC7399, October 2014,
              <https://www.rfc-editor.org/info/rfc7399>.

   [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>.

   [RFC8253]  Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
              "PCEPS: Usage of TLS to Provide a Secure Transport for the
              Path Computation Element Communication Protocol (PCEP)",
              RFC 8253, DOI 10.17487/RFC8253, October 2017,
              <https://www.rfc-editor.org/info/rfc8253>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8655]  Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", RFC 8655,
              DOI 10.17487/RFC8655, October 2019,
              <https://www.rfc-editor.org/info/rfc8655>.

Acknowledgments

   The authors of this document would also like to thank Rafal Szarecki,
   Adrian Farrel, and Cyril Margaria for the review and comments.

Contributors

   Dhruv Dhody
   Huawei
   Divyashree Techno Park, Whitefield
   Bangalore 560066
   Karnataka
   India

   Email: dhruv.ietf@gmail.com

   Xufeng Liu
   Ericsson
   United States of America

   Email: xliu@kuatrotech.com

   Mehmet Toy
   Verizon
   United States of America

   Email: mehmet.toy@verizon.com

   Vic Liu
   China Mobile
   No.32 Xuanwumen West Street, Xicheng District
   Beijing
   100053
   China

   Email: liu.cmri@gmail.com

   Lei Liu
   Fujitsu
   United States of America

   Email: lliu@us.fujitsu.com

   Khuzema Pithewan
   Infinera

   Email: kpithewan@infinera.com

   Zitao Wang
   Huawei
   101 Software Avenue, Yuhua District
   Nanjing
   Jiangsu, 210012
   China

   Email: wangzitao@huawei.com

   Xian Zhang
   Huawei Technologies
   Research Area F3-1B
   Huawei Industrial Base
   Shenzhen
   518129
   China

   Email: zhang.xian@huawei.com

Authors' Addresses

   Huaimo Chen (editor)
   Futurewei
   Boston, MA
   United States of America

   Email: huaimo.chen@futurewei.com

   Yan Zhuang (editor)
   Huawei
   Yuhua District
   101 Software Avenue
   Nanjing
   Jiangsu, 210012
   China

   Email: zhuangyan.zhuang@huawei.com

   Qin Wu
   Huawei
   Yuhua District
   101 Software Avenue
   Nanjing
   Jiangsu, 210012
   China

   Email: bill.wu@huawei.com

   Daniele Ceccarelli
   Ericsson
   Via A. Negrone 1/A
   Genova - Sestri Ponente
   Italy

   Email: daniele.ceccarelli@ericsson.com