Path Computation Element Communication Protocol (PCEP) Extensions for Associating Working and Protection Label Switched Paths (LSPs) with Stateful PCE
RFC 8745



Internet Engineering Task Force (IETF)                H. Ananthakrishnan
Request for Comments: 8745                                       Netflix
Category: Standards Track                                   S. Sivabalan
ISSN: 2070-1721                                                    Cisco
                                                                C. Barth
                                                        Juniper Networks
                                                                I. Minei
                                                             Google, Inc
                                                                 M. Negi
                                                     Huawei Technologies
                                                              March 2020

 Path Computation Element Communication Protocol (PCEP) Extensions for
  Associating Working and Protection Label Switched Paths (LSPs) with
                              Stateful PCE

Abstract

   An active stateful Path Computation Element (PCE) is capable of
   computing as well as controlling via Path Computation Element
   Communication Protocol (PCEP) Multiprotocol Label Switching Traffic
   Engineering (MPLS-TE) Label Switched Paths (LSPs).  Furthermore, it
   is also possible for an active stateful PCE to create, maintain, and
   delete LSPs.  This document defines the PCEP extension to associate
   two or more LSPs to provide end-to-end path protection.

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/rfc8745.

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
     1.1.  Requirements Language
   2.  Terminology
   3.  PCEP Extensions
     3.1.  Path Protection Association Type
     3.2.  Path Protection Association TLV
   4.  Operation
     4.1.  State Synchronization
     4.2.  PCC-Initiated LSPs
     4.3.  PCE-Initiated LSPs
     4.4.  Session Termination
     4.5.  Error Handling
   5.  Other Considerations
   6.  IANA Considerations
     6.1.  Association Type
     6.2.  Path Protection Association TLV
     6.3.  PCEP Errors
   7.  Security Considerations
   8.  Manageability Considerations
     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.  References
     9.1.  Normative References
     9.2.  Informative References
   Acknowledgments
   Contributors
   Authors' Addresses

1.  Introduction

   [RFC5440] describes Path Computation Element Communication Protocol
   (PCEP) for communication between a Path Computation Client (PCC) and
   a PCE or between a pair of PCEs as per [RFC4655].  A PCE computes
   paths for MPLS-TE Label Switched Paths (LSPs) based on various
   constraints and optimization criteria.

   Stateful PCE [RFC8231] specifies a set of extensions to PCEP to
   enable stateful control of paths such as MPLS-TE LSPs between and
   across PCEP sessions in compliance with [RFC4657].  It includes
   mechanisms to affect LSP state synchronization between PCCs and PCEs,
   delegation of control of LSPs to PCEs, and PCE control of timing and
   sequence of path computations within and across PCEP sessions.  The
   focus is on a model where LSPs are configured on the PCC, and control
   over them is delegated to the stateful PCE.  Furthermore, [RFC8281]
   specifies a mechanism to dynamically instantiate LSPs on a PCC based
   on the requests from a stateful PCE or a controller using stateful
   PCE.

   Path protection [RFC4427] refers to a paradigm in which the working
   LSP is protected by one or more protection LSP(s).  When the working
   LSP fails, protection LSP(s) is/are activated.  When the working LSPs
   are computed and controlled by the PCE, there is benefit in a mode of
   operation where protection LSPs are also computed and controlled by
   the same PCE.  [RFC8051] describes the applicability of path
   protection in PCE deployments.

   This document specifies a stateful PCEP extension to associate two or
   more LSPs for the purpose of setting up path protection.  The
   extension defined in this document covers the following scenarios:

   *  A PCC initiates a protection LSP and retains the control of the
      LSP.  The PCC computes the path itself or makes a request for path
      computation to a PCE.  After the path setup, it reports the
      information and state of the path to the PCE.  This includes the
      association group identifying the working and protection LSPs.
      This is the passive stateful mode [RFC8051].

   *  A PCC initiates a protection LSP and delegates the control of the
      LSP to a stateful PCE.  During delegation, the association group
      identifying the working and protection LSPs is included.  The PCE
      computes the path for the protection LSP and updates the PCC with
      the information about the path as long as it controls the LSP.
      This is the active stateful mode [RFC8051].

   *  A protection LSP could be initiated by a stateful PCE, which
      retains the control of the LSP.  The PCE is responsible for
      computing the path of the LSP and updating to the PCC with the
      information about the path.  This is the PCE-Initiated mode
      [RFC8281].

   Note that a protection LSP can be established (signaled) before the
   failure (in which case the LSP is said to be either in standby mode
   [RFC4427] or a primary LSP [RFC4872]) or after failure of the
   corresponding working LSP (known as a secondary LSP [RFC4872]).
   Whether to establish it before or after failure is according to
   operator choice or policy.

   [RFC8697] introduces a generic mechanism to create a grouping of
   LSPs, which can then be used to define associations between a set of
   LSPs.  The mechanism is equally applicable to stateful PCE (active
   and passive modes) and stateless PCE.

   This document specifies a PCEP extension to associate one working LSP
   with one or more protection LSPs using the generic association
   mechanism.

   This document describes a PCEP extension to associate protection LSPs
   by creating the Path Protection Association Group (PPAG) and encoding
   this association in PCEP messages for stateful PCEP sessions.

1.1.  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.

2.  Terminology

   The following terms are used in this document:

      ERO:  Explicit Route Object

      LSP:  Label Switched Path

      PCC:  Path Computation Client

      PCE:  Path Computation Element

      PCEP:  Path Computation Element Communication Protocol

      PPAG:  Path Protection Association Group

      TLV:  Type, Length, and Value

3.  PCEP Extensions

3.1.  Path Protection Association Type

   As per [RFC8697], LSPs are not associated by listing the other LSPs
   with which they interact but, rather, by making them belong to an
   association group.  All LSPs join an association group individually.
   The generic ASSOCIATION object is used to associate two or more LSPs
   as specified in [RFC8697].  This document defines a new Association
   type called "Path Protection Association Type" of value 1 and a "Path
   Protection Association Group" (PPAG).  A member LSP of a PPAG can
   take the role of working or protection LSP.  A PPAG can have one
   working LSP and/or one or more protection LSPs.  The source,
   destination, Tunnel ID (as carried in LSP-IDENTIFIERS TLV [RFC8231],
   with description as per [RFC3209]), and Protection Type (PT) (in Path
   Protection Association TLV) of all LSPs within a PPAG MUST be the
   same.  As per [RFC3209], a TE tunnel is used to associate a set of
   LSPs during reroute or to spread a traffic trunk over multiple paths.

   The format of the ASSOCIATION object used for PPAG is specified in
   [RFC8697].

   [RFC8697] specifies the mechanism for the capability advertisement of
   the Association types supported by a PCEP speaker by defining an
   ASSOC-Type-List TLV to be carried within an OPEN object.  This
   capability exchange for the Association type described in this
   document (i.e., Path Protection Association Type) MAY be done before
   using this association, i.e., the PCEP speaker MAY include the Path
   Protection Association Type (1) in the ASSOC-Type-List TLV before
   using the PPAG in the PCEP messages.

   This Association type is dynamic in nature and created by the PCC or
   PCE for the LSPs belonging to the same TE tunnel (as described in
   [RFC3209]) originating at the same head node and terminating at the
   same destination.  These associations are conveyed via PCEP messages
   to the PCEP peer.  As per [RFC8697], the association source is set to
   the local PCEP speaker address that created the association unless
   local policy dictates otherwise.  Operator-configured Association
   Range MUST NOT be set for this Association type and MUST be ignored.

3.2.  Path Protection Association TLV

   The Path Protection Association TLV is an optional TLV for use in the
   ASSOCIATION object with the Path Protection Association Type.  The
   Path Protection Association TLV MUST NOT be present more than once.
   If it appears more than once, only the first occurrence is processed
   and any others MUST be ignored.

   The Path Protection Association TLV follows the PCEP TLV format of
   [RFC5440].

   The Type (16 bits) of the TLV is 38.  The Length field (16 bits) has
   a fixed value of 4.

   The value is comprised of a single field, the Path Protection
   Association Flags (32 bits), where each bit represents a flag option.

   The format of the Path Protection Association TLV (Figure 1) is as
   follows:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Type = 38             |            Length = 4         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   PT      |               Unassigned Flags                |S|P|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 1: Path Protection Association TLV Format

   Path Protection Association Flags (32 bits)

   The following flags are currently defined:

   *  Protecting (P): 1 bit - This bit is as defined in Section 14.1 of
      [RFC4872] to indicate if the LSP is a working (0) or protection
      (1) LSP.

   *  Secondary (S): 1 bit - This bit is as defined in Section 14.1 of
      [RFC4872] to indicate if the LSP is a primary (0) or secondary (1)
      LSP.  The S flag is ignored if the P flag is not set.

   *  Protection Type (PT): 6 bits - This field is as defined in
      Section 14.1 of [RFC4872] (as "LSP (Protection Type) Flags") to
      indicate the LSP protection type in use.  Any type already defined
      or that could be defined in the future for use in the RSVP-TE
      PROTECTION object is acceptable in this TLV unless explicitly
      stated otherwise.

   *  Unassigned bits are considered reserved.  They MUST be set to 0 on
      transmission and MUST be ignored on receipt.

   If the TLV is missing in the PPAG ASSOCIATION object, it is
   considered that the LSP is a working LSP (i.e., as if the P bit is
   unset).

4.  Operation

   An LSP is associated with other LSPs with which it interacts by
   adding them to a common association group via the ASSOCIATION object.
   All procedures and error handling for the ASSOCIATION object is as
   per [RFC8697].

4.1.  State Synchronization

   During state synchronization, a PCC reports all the existing LSP
   states as described in [RFC8231].  The association group membership
   pertaining to an LSP is also reported as per [RFC8697].  This
   includes PPAGs.

4.2.  PCC-Initiated LSPs

   A PCC can associate a set of LSPs under its control for path
   protection purposes.  Similarly, the PCC can remove one or more LSPs
   under its control from the corresponding PPAG.  In both cases, the
   PCC reports the change in association to PCE(s) via a Path
   Computation Report (PCRpt) message.  A PCC can also delegate the
   working and protection LSPs to an active stateful PCE, where the PCE
   would control the LSPs.  The stateful PCE could update the paths and
   attributes of the LSPs in the association group via a Path
   Computation Update (PCUpd) message.  A PCE could also update the
   association to the PCC via a PCUpd message.  These procedures are
   described in [RFC8697].

   It is expected that both working and protection LSPs are delegated
   together (and to the same PCE) to avoid any race conditions.  Refer
   to [STATE-PCE-SYNC] for the problem description.

4.3.  PCE-Initiated LSPs

   A PCE can create/update working and protection LSPs independently.
   As specified in [RFC8697], Association Groups can be created by both
   the PCE and the PCC.  Furthermore, a PCE can remove a protection LSP
   from a PPAG as specified in [RFC8697].  The PCE uses PCUpd or Path
   Computation Initiate (PCInitiate) messages to communicate the
   association information to the PCC.

4.4.  Session Termination

   As per [RFC8697], the association information is cleared along with
   the LSP state information.  When a PCEP session is terminated, after
   expiry of State Timeout Interval at the PCC, the LSP state associated
   with that PCEP session is reverted to operator-defined default
   parameters or behaviors as per [RFC8231].  The same procedure is also
   followed for the association information.  On session termination at
   the PCE, when the LSP state reported by PCC is cleared, the
   association information is also cleared as per [RFC8697].  Where
   there are no LSPs in an association group, the association is
   considered to be deleted.

4.5.  Error Handling

   As per the processing rules specified in Section 6.4 of [RFC8697], if
   a PCEP speaker does not support this Path Protection Association
   Type, it would return a PCErr message with Error-Type 26 "Association
   Error" and Error-Value 1 "Association type is not supported".

   All LSPs (working or protection) within a PPAG MUST belong to the
   same TE tunnel (as described in [RFC3209]) and have the same source
   and destination.  If a PCEP speaker attempts to add or update an LSP
   to a PPAG and the Tunnel ID (as carried in the LSP-IDENTIFIERS TLV
   [RFC8231], with a description as per [RFC3209]) or source or
   destination of the LSP is different from the LSP(s) in the PPAG, the
   PCEP speaker MUST send PCErr with Error-Type 26 (Association Error)
   [RFC8697] and Error-Value 9 (Tunnel ID or endpoints mismatch for Path
   Protection Association).  In case of Path Protection, an LSP-
   IDENTIFIERS TLV SHOULD be included for all LSPs (including Segment
   Routing (SR) [RFC8664]).  If the Protection Type (PT) (in the Path
   Protection Association TLV) is different from the LSPs in the PPAG,
   the PCEP speaker MUST send PCErr with Error-Type 26 (Association
   Error) [RFC8697] and Error-Value 6 (Association information mismatch)
   as per [RFC8697].

   When the PCEP peer does not support the protection type set in PPAG,
   the PCEP peer MUST send PCErr with Error-Type 26 (Association Error)
   [RFC8697] and Error-Value 11 (Protection type is not supported).

   A given LSP MAY belong to more than one PPAG.  If there is a conflict
   between any of the two PPAGs, the PCEP peer MUST send PCErr with
   Error-Type 26 (Association Error) [RFC8697] and Error-Value 6
   (Association information mismatch) as per [RFC8697].

   When the protection type is set to 1+1 (i.e., protection type=0x08 or
   0x10), there MUST be at maximum only one working LSP and one
   protection LSP within a PPAG.  If a PCEP speaker attempts to add
   another working/protection LSP, the PCEP peer MUST send PCErr with
   Error-Type 26 (Association Error) [RFC8697] and Error-Value 10
   (Attempt to add another working/protection LSP for Path Protection
   Association).

   When the protection type is set to 1:N (i.e., protection type=0x04),
   there MUST be at maximum only one protection LSP, and the number of
   working LSPs MUST NOT be more than N within a PPAG.  If a PCEP
   speaker attempts to add another working/protection LSP, the PCEP peer
   MUST send PCErr with Error-Type 26 (Association Error) [RFC8697] and
   Error-Value 10 (Attempt to add another working/protection LSP for
   Path Protection Association).

   During the make-before-break (MBB) procedure, two paths will briefly
   coexist.  The error handling related to the number of LSPs allowed in
   a PPAG MUST NOT be applied during MBB.

   All processing as per [RFC8697] continues to apply.

5.  Other Considerations

   The working and protection LSPs are typically resource disjoint
   (e.g., node, Shared Risk Link Group [SRLG] disjoint).  This ensures
   that a single failure will not affect both the working and protection
   LSPs.  The disjoint requirement for a group of LSPs is handled via
   another Association type called "Disjointness Association" as
   described in [PCEP-LSP-EXT].  The diversity requirements for the
   protection LSP are also handled by including both ASSOCIATION objects
   identifying both the protection association group and the disjoint
   association group for the group of LSPs.  The relationship between
   the Synchronization VECtor (SVEC) object and the Disjointness
   Association is described in Section 5.4 of [PCEP-LSP-EXT].

   [RFC4872] introduces the concept and mechanisms to support the
   association of one LSP to another LSP across different RSVP Traffic
   Engineering (RSVP-TE) sessions using the ASSOCIATION and PROTECTION
   object.  The information in the Path Protection Association TLV in
   PCEP as received from the PCE is used to trigger the signaling of the
   working LSP and protection LSP, with the Path Protection Association
   Flags mapped to the corresponding fields in the PROTECTION object in
   RSVP-TE.

6.  IANA Considerations

6.1.  Association Type

   This document defines a new Association type, originally defined in
   [RFC8697], for path protection.  IANA has assigned new value in the
   "ASSOCIATION Type Field" subregistry (created by [RFC8697]) as
   follows:

            +------+-----------------------------+-----------+
            | Type | Name                        | Reference |
            +======+=============================+===========+
            | 1    | Path Protection Association | RFC 8745  |
            +------+-----------------------------+-----------+

                     Table 1: ASSOCIATION Type Field

6.2.  Path Protection Association TLV

   This document defines a new TLV for carrying the additional
   information of LSPs within a path protection association group.  IANA
   has assigned a new value in the "PCEP TLV Type Indicators"
   subregistry as follows:

       +-------+---------------------------------------+-----------+
       | Value | Description                           | Reference |
       +=======+=======================================+===========+
       | 38    | Path Protection Association Group TLV | RFC 8745  |
       +-------+---------------------------------------+-----------+

                     Table 2: PCEP TLV Type Indicators

   Per this document, a new subregistry named "Path protection
   Association Group TLV Flag Field" has been created within the "Path
   Computation Element Protocol (PCEP) Numbers" registry to manage the
   Flag field in the Path Protection Association Group TLV.  New values
   are to be assigned by Standards Action [RFC8126].  Each bit should be
   tracked with the following qualities:

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

   *  Name of the flag

   *  Reference

               +------+-----------------------+-----------+
               | Bit  |          Name         | Reference |
               +======+=======================+===========+
               |  31  |   P - PROTECTION-LSP  |  RFC 8745 |
               +------+-----------------------+-----------+
               |  30  |   S - SECONDARY-LSP   |  RFC 8745 |
               +------+-----------------------+-----------+
               | 6-29 |       Unassigned      |  RFC 8745 |
               +------+-----------------------+-----------+
               | 0-5  | Protection Type Flags |  RFC 8745 |
               +------+-----------------------+-----------+

                   Table 3: Path Protection Association
                           Group TLV Flag Field

6.3.  PCEP Errors

   This document defines new Error-Values related to path protection
   association for Error-type 26 "Association Error" defined in
   [RFC8697].  IANA has allocated new error values within the "PCEP-
   ERROR Object Error Types and Values" subregistry of the PCEP Numbers
   registry as follows:

   +------------+-------------+---------------------------+-----------+
   | Error-Type | Meaning     | Error-value               | Reference |
   +============+=============+===========================+===========+
   | 26         | Association |                           | [RFC8697] |
   |            | Error       |                           |           |
   +------------+-------------+---------------------------+-----------+
   |            |             | 9: Tunnel ID or endpoints | RFC 8745  |
   |            |             | mismatch for Path         |           |
   |            |             | Protection Association    |           |
   +------------+-------------+---------------------------+-----------+
   |            |             | 10: Attempt to add        | RFC 8745  |
   |            |             | another working/          |           |
   |            |             | protection LSP for Path   |           |
   |            |             | Protection Association    |           |
   +------------+-------------+---------------------------+-----------+
   |            |             | 11: Protection type is    | RFC 8745  |
   |            |             | not supported             |           |
   +------------+-------------+---------------------------+-----------+

            Table 4: PCEP-ERROR Object Error Types and Values

7.  Security Considerations

   The security considerations described in [RFC8231], [RFC8281], and
   [RFC5440] apply to the extensions described in this document as well.
   Additional considerations related to associations where a malicious
   PCEP speaker could be spoofed and could be used as an attack vector
   by creating associations are described in [RFC8697].  Adding a
   spurious protection LSP to the Path Protection Association group
   could give a false sense of network reliability, which leads to
   issues when the working LSP is down and the protection LSP fails as
   well.  Thus, securing the PCEP session using Transport Layer Security
   (TLS) [RFC8253], as per the recommendations and best current
   practices in BCP 195 [RFC7525], is RECOMMENDED.

8.  Manageability Considerations

8.1.  Control of Function and Policy

   Mechanisms defined in this document do not imply any control or
   policy requirements in addition to those already listed in [RFC5440],
   [RFC8231], and [RFC8281].

8.2.  Information and Data Models

   [RFC7420] describes the PCEP MIB; there are no new MIB Objects for
   this document.

   The PCEP YANG module [PCEP-YANG] supports associations.

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], [RFC8231], and [RFC8281].

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], [RFC8231], and [RFC8281].

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],
   [RFC8231], and [RFC8281].

9.  References

9.1.  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>.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <https://www.rfc-editor.org/info/rfc3209>.

   [RFC4872]  Lang, J.P., Ed., Rekhter, Y., Ed., and D. Papadimitriou,
              Ed., "RSVP-TE Extensions in Support of End-to-End
              Generalized Multi-Protocol Label Switching (GMPLS)
              Recovery", RFC 4872, DOI 10.17487/RFC4872, May 2007,
              <https://www.rfc-editor.org/info/rfc4872>.

   [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>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [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>.

   [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>.

   [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>.

   [RFC8697]  Minei, I., Crabbe, E., Sivabalan, S., Ananthakrishnan, H.,
              Dhody, D., and Y. Tanaka, "Path Computation Element
              Communication Protocol (PCEP) Extensions for Establishing
              Relationships between Sets of Label Switched Paths
              (LSPs)", RFC 8697, DOI 10.17487/RFC8697, January 2020,
              <https://www.rfc-editor.org/info/rfc8697>.

9.2.  Informative References

   [PCEP-LSP-EXT]
              Litkowski, S., Sivabalan, S., Barth, C., and M. Negi,
              "Path Computation Element Communication Protocol (PCEP)
              Extension for LSP Diversity Constraint Signaling", Work in
              Progress, Internet-Draft, draft-ietf-pce-association-
              diversity-14, 26 January 2020,
              <https://tools.ietf.org/html/draft-ietf-pce-association-
              diversity-14>.

   [PCEP-YANG]
              Dhody, D., Hardwick, J., Beeram, V., and J. Tantsura, "A
              YANG Data Model for Path Computation Element
              Communications Protocol (PCEP)", Work in Progress,
              Internet-Draft, draft-ietf-pce-pcep-yang-13, 31 October
              2019,
              <https://tools.ietf.org/html/draft-ietf-pce-pcep-yang-13>.

   [RFC4427]  Mannie, E., Ed. and D. Papadimitriou, Ed., "Recovery
              (Protection and Restoration) Terminology for Generalized
              Multi-Protocol Label Switching (GMPLS)", RFC 4427,
              DOI 10.17487/RFC4427, March 2006,
              <https://www.rfc-editor.org/info/rfc4427>.

   [RFC4655]  Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
              Computation Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,
              <https://www.rfc-editor.org/info/rfc4655>.

   [RFC4657]  Ash, J., Ed. and J.L. 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>.

   [RFC7420]  Koushik, A., Stephan, E., Zhao, Q., King, D., and J.
              Hardwick, "Path Computation Element Communication Protocol
              (PCEP) Management Information Base (MIB) Module",
              RFC 7420, DOI 10.17487/RFC7420, December 2014,
              <https://www.rfc-editor.org/info/rfc7420>.

   [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>.

   [RFC8664]  Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "Path Computation Element Communication
              Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
              DOI 10.17487/RFC8664, December 2019,
              <https://www.rfc-editor.org/info/rfc8664>.

   [STATE-PCE-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-07, 11 January 2020,
              <https://tools.ietf.org/html/draft-litkowski-pce-state-
              sync-07>.

Acknowledgments

   We would like to thank Jeff Tantsura, Xian Zhang, and Greg Mirsky for
   their contributions to this document.

   Thanks to Ines Robles for the RTGDIR review.

   Thanks to Pete Resnick for the GENART review.

   Thanks to Donald Eastlake for the SECDIR review.

   Thanks to Barry Leiba, Benjamin Kaduk, Éric Vyncke, and Roman Danyliw
   for the IESG review.

Contributors

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

   Email: dhruv.ietf@gmail.com

   Raveendra Torvi
   Juniper Networks
   1194 N Mathilda Ave
   Sunnyvale, CA 94086
   United States of America

   Email: rtorvi@juniper.net

   Edward Crabbe
   Individual Contributor

   Email: edward.crabbe@gmail.com

Authors' Addresses

   Hariharan Ananthakrishnan
   Netflix
   United States of America

   Email: hari@netflix.com

   Siva Sivabalan
   Cisco
   2000 Innovation Drive
   Kanata Ontario K2K 3E8
   Canada

   Email: msiva@cisco.com

   Colby Barth
   Juniper Networks
   1194 N Mathilda Ave
   Sunnyvale, CA 94086
   United States of America

   Email: cbarth@juniper.net

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

   Email: inaminei@google.com

   Mahendra Singh Negi
   Huawei Technologies
   Divyashree Techno Park, Whitefield
   Bangalore 560066
   Karnataka
   India

   Email: mahend.ietf@gmail.com