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Versions: (draft-li-pce-sr-bidir-path) 00 01 02

PCE Working Group                                                  C. Li
Internet-Draft                                                   M. Chen
Intended status: Standards Track                     Huawei Technologies
Expires: August 17, 2020                                        W. Cheng
                                                            China Mobile
                                                               R. Gandhi
                                                     Cisco Systems, Inc.
                                                                Q. Xiong
                                                         ZTE Corporation
                                                       February 14, 2020


PCEP Extensions for Associated Bidirectional Segment Routing (SR) Paths
                    draft-ietf-pce-sr-bidir-path-01

Abstract

   The Path Computation Element Communication Protocol (PCEP) provides
   mechanisms for Path Computation Elements (PCEs) to perform path
   computations in response to Path Computation Clients (PCCs) requests.
   Segment routing (SR) leverages the source routing and tunneling
   paradigms.  The Stateful PCEP extensions allow stateful control of
   Segment Routing (SR) Traffic Engineering (TE) Paths.  Furthermore,
   PCEP can be used for computing SR TE paths in the network.

   This document defines PCEP extensions for grouping two unidirectional
   SR Paths (one in each direction in the network) into a single
   Associated Bidirectional SR Path.  The mechanisms defined in this
   document can also be applied using a Stateful PCE for both PCE-
   Initiated and PCC-Initiated LSPs, as well as when using a Stateless
   PCE.

Status of This Memo

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

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

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

   This Internet-Draft will expire on August 17, 2020.



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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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   3.  PCEP Extensions . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Double-sided Bidirectional SR Path Association Group  . .   5
       3.1.1.  Bidirectional LSP Association Group TLV . . . . . . .   5
   4.  PCEP Procedures . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  PCE Initiated Associated Bidirectional SR Paths . . . . .   6
     4.2.  PCC Initiated Associated Bidirectional SR Paths . . . . .   7
     4.3.  Stateless PCE . . . . . . . . . . . . . . . . . . . . . .   9
     4.4.  Bidirectional (B) Flag  . . . . . . . . . . . . . . . . .   9
     4.5.  State Synchronization . . . . . . . . . . . . . . . . . .   9
     4.6.  Error Handling  . . . . . . . . . . . . . . . . . . . . .   9
   5.  Implementation Status . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Huawei's Commercial Delivery  . . . . . . . . . . . . . .  10
     5.2.  ZTE's Commercial Delivery . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  Manageability Considerations  . . . . . . . . . . . . . . . .  11
     7.1.  Control of Function and Policy  . . . . . . . . . . . . .  11
     7.2.  Information and Data Models . . . . . . . . . . . . . . .  11
     7.3.  Liveness Detection and Monitoring . . . . . . . . . . . .  11
     7.4.  Verify Correct Operations . . . . . . . . . . . . . . . .  11
     7.5.  Requirements On Other Protocols . . . . . . . . . . . . .  12
     7.6.  Impact On Network Operations  . . . . . . . . . . . . . .  12
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Association Type  . . . . . . . . . . . . . . . . . . . .  12
     8.2.  PCEP Errors . . . . . . . . . . . . . . . . . . . . . . .  12
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  15



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   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   Segment routing (SR) [RFC8402] leverages the source routing and
   tunneling paradigms.  SR supports steering packets onto an explicit
   forwarding path at the ingress node.  SR is specified for
   unidirectional paths.  However, some applications require
   bidirectional paths in SR networks, for example, in mobile backhaul
   transport networks.  The requirement for bidirectional SR Paths is
   specified in [I-D.ietf-spring-mpls-path-segment].

   [RFC5440] describes the Path Computation Element (PCE) Communication
   Protocol (PCEP).  PCEP enables the communication between a Path
   Computation Client (PCC) and a PCE, or between PCE and PCE, for the
   purpose of computation of Traffic Engineering (TE) Label Switched
   Paths (LSP).  [RFC8231] specifies a set of extensions to PCEP to
   enable stateful control of TE LSPs within and across PCEP sessions.
   The mode of operation where LSPs are initiated from the PCE is
   described in [RFC8281].

   [RFC8408] specifies extensions to the Path Computation Element
   Protocol (PCEP) [RFC5440] for SR networks, that allow a stateful PCE
   to compute and initiate SR TE paths, as well as a PCC to request,
   report or delegate them.

   [RFC8697] introduces a generic mechanism to create a grouping of LSPs
   which can then be used to define associations between a set of LSPs
   and/or a set of attributes, and is equally applicable to the active
   and passive modes of a Stateful PCE [RFC8231] or a stateless PCE
   [RFC5440].

   [I-D.ietf-pce-association-bidir] defines PCEP extensions for grouping
   two unidirectional RSVP-TE LSPs into an Associated Bidirectional LSP
   when using a Stateful PCE for both PCE-Initiated and PCC-Initiated
   LSPs as well as when using a Stateless PCE.  It specifies the
   procedure for Double-sided Bidirectional LSP Association, where the
   PCE creates the association and provisions the forward LSPs at their
   ingress nodes.  The RSVP-TE signals the forward LSPs to the egress
   nodes.  Thus, both endpoints learn the reverse LSPs forming the
   bidirectional LSP association.

   This document extends the bidirectional LSP association to SR by
   specifying PCEP extensions for grouping two unidirectional SR Paths
   into a bidirectional SR Path.  For bidirectional SR, there are use
   cases such as directed BFD [I-D.ietf-mpls-bfd-directed] and SR
   performance measurement [I-D.gandhi-spring-twamp-srpm] those require



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   PCC to be aware of the reverse direction SR path.  For such use-
   cases, the reverse SR paths are also communicated to the ingress
   nodes using the PCEP extensions defined in this document.  This
   allows both endpoints to be aware of SR Paths in both directions,
   including their status and all other path related information.

2.  Terminology

   This document makes use of the terms defined in [RFC8408].  The
   reader is assumed to be familiar with the terminology defined in
   [RFC5440], [RFC8231], [RFC8281], [RFC8697], and
   [I-D.ietf-pce-association-bidir].

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.  PCEP Extensions

   As per [RFC8697], TE LSPs are associated by adding them to a common
   association group by a PCEP peer.  [I-D.ietf-pce-association-bidir]
   uses the the association group object and the procedures as specified
   in [RFC8697] to group two unidirectional RSVP-TE LSPs.  Similarly,
   two SR Paths can also be associated using similar technique.  This
   document extends these association mechanisms for bidirectional SR
   Paths.  Two unidirectional SR Paths (one in each direction in the
   network) can be associated together by using the Bidirectional SR
   Path Association Group defined in this document for PCEP messages.

   Note that the association group defined in this document is specific
   to the bidirectional SR Paths.  The procedure for this association
   group is different than the RSVP-TE bidirectional association groups
   defined in [I-D.ietf-pce-association-bidir].

   [I-D.ietf-pce-sr-path-segment] defines a mechanism for communicating
   Path Segment Identifier (PSID) in PCEP for SR.  The PSID is defined
   for SR-MPLS in [I-D.ietf-spring-mpls-path-segment].  The PSID can be
   used for identifying an SR Path of an associated bidirectional SR
   Path.  The PATH-SEGMENT TLV MAY be included for each SR Path in the
   LSP object to support required use-cases.  The PATH-SEGMENT TLV MUST
   be handled as defined in [I-D.ietf-pce-sr-path-segment] and is not
   modified for associated bidirectional SR Path.





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3.1.  Double-sided Bidirectional SR Path Association Group

   For associating two unidirectional SR paths, this document defines a
   new Association Type called 'Double-sided Bidirectional SR Path
   Association Group' for Association Group Object (Class-Value 40) as
   follows:

   o  Association Type (TBD1 to be assigned by IANA) = Double-sided
      Bidirectional SR Path Association Group

   Similar to RSVP-TE bidirectional LSP associations, this Association
   Type is operator-configured in nature and statically created by the
   operator on the PCEP peers.  The paths belonging to this association
   is conveyed via PCEP messages to the PCEP peer.  Operator-configured
   Association Range TLV [RFC8697] MUST NOT be sent for these
   Association Types, and MUST be ignored, so that the entire range of
   association ID can be used for them.  The handling of the Association
   ID, Association Source, optional Global Association Source and
   optional Extended Association ID in this association are set in the
   same way as [I-D.ietf-pce-association-bidir].

   A member of the 'Double-sided Bidirectional SR Path Association
   Group' can take the role of a forward or reverse SR Path and follow
   the similar rules defined in [I-D.ietf-pce-association-bidir] for
   LSPs.

   o  An SR Path (forward or reverse) cannot be part of more than one
      'Double-sided Bidirectional SR Path Association Group'.

   o  The endpoints of the SR Paths in this associations cannot be
      different.

3.1.1.  Bidirectional LSP Association Group TLV

   In Bidirectional SR Association Group, for properties such as forward
   and reverse direction and co-routed path, it uses the Bidirectional
   LSP Association Group TLV defined in
   [I-D.ietf-pce-association-bidir].  All fields and processing rules
   are as per [I-D.ietf-pce-association-bidir].

4.  PCEP Procedures

   For bidirectional SR path, an ingress PCC is aware of the forward
   direction SR path beginning from itself to the egress PCC using the
   existing PCEP procedures.  For the use-cases which require the
   ingress PCC to be aware of the reverse direction SR path, PCE informs
   the reverse SR Path to the ingress PCC.  To achieve this, a
   PCInitiate message for the reverse SR Path is sent to the ingress PCC



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   and a PCInitiate message for the forward SR Path is sent to the
   egress PCC (with the matching association group).  These PCInitiate
   message MUST NOT trigger initiation of SR Paths on PCCs.

   For a bidirectional LSP computation when using both direction LSPs on
   a node, the same LSP would need to be identified using 2 different
   PLSP-IDs based on the PCEP session to the ingress or the egress node.
   Note that the PLSP-ID space is independent at each PCC, the PLSP-ID
   allocated by the egress PCC cannot be used for the LSP at the ingress
   PCC (PLSP-ID conflict may occur).  As per normal PCInitiate
   operations, PCC assigns the PLSP-IDs for the local LSPs.  Hence, when
   the PCE notifies an ingress PCC of the reverse LSP, it does so by
   using PCInitiate operations and sets PLSP-ID to zero and sets the R
   bit in the Bidirectional LSP Association Group TLV in the association
   object to indicate that this PCInitiate LSP is a reverse LSP.  The
   PCC upon receiving the PCInitiate MUST locally assign a new PLSP-ID
   and it MUST issue a PCRpt to PCE for this LSP containing the new
   PLSP-ID.  This reverse direction LSP MUST NOT be instantiated on the
   PCC.

   In other words, a given LSP will be identified by PLSP-ID A at the
   ingress node while it will be identified by PLSP-ID B at the egress
   node.  The PCE will maintain two PLSP-IDs for the same LSP.  For
   example, ingress PCC1 may report to PCE an LSP1 with PLSP-ID 100.
   Egress PCC2 may report to PCE an LSP2 with PLSP-ID 200.  Both of
   these LSPs are part of a bidirectional association.  When PCE
   notifies PCC1 of the reverse direction LSP2, it does so by sending a
   PCInitiate to PCC1 with PLSP-ID set to zero and R bit set in the
   Bidirectional LSP Association Group TLV.  PCC1 upon reception of this
   generates a new PLSP-ID (example PLSP-ID 300) and issues a PCRpt to
   PCE.  Thus there would two PLSP-ID associated for LSP2 (300 at PCC1
   and 200 at PCC2).

4.1.  PCE Initiated Associated Bidirectional SR Paths

   As specified in [RFC8697], Bidirectional SR Path Association Group
   can be created by a Stateful PCE as shown in Figure 1.

   o  Stateful PCE can create and update the forward and reverse SR
      Paths independently for 'Double-sided Bidirectional SR Path
      Association Group'.

   o  Stateful PCE can establish and remove the association relationship
      on a per SR Path basis.

   o  Stateful PCE can create and update the SR Path and the association
      on a PCC via PCInitiate and PCUpd messages, respectively, using
      the procedures described in [RFC8697].



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   o  The reverse direction SR Path (LSP2(R) at node S, LSP1(R) at node
      D as shown in Figure 1) SHOULD be informed by the PCE via
      PCInitiate message with the matching association group for the
      use-cases which require the PCC to be aware of the reverse
      direction SR path.

                                  +-----+
                                  | PCE |
                                  +-----+
      PCInitiate/PCUpd:           /     \       PCInitiate/PCUpd:
      Tunnel 1 (F)               /       \      Tunnel 2 (F)
      (LSP1 (F), LSP2 (R))      /         \     (LSP2 (F), LSP1 (R))
      Association #1           /           \    Association #1
                              /             \
                             v               v
                        +-----+    LSP1     +-----+
                        |  S  |------------>|  D  |
                        |     |<------------|     |
                        +-----+    LSP2     +-----+
                              <no signaling>

         Figure 1: PCE-Initiated Double-sided Bidirectional SR Path
                   with Forward and Reverse Direction SR Paths


4.2.  PCC Initiated Associated Bidirectional SR Paths

   As specified in [RFC8697], Bidirectional SR Path Association Group
   can also be created by a PCC as shown in Figure 2a and 2b.

   o  PCC can create and update the forward SR Path and update the
      reverse SR Path independently for a 'Double-sided Bidirectional SR
      Path Association Group'.

   o  PCC cannot instantiate a reverse SR Path in a bidirectional SR
      Path.

   o  PCC can establish and remove the association relationship on a per
      SR Path basis.

   o  PCC MUST report the change in the association group of an SR Path
      to PCE(s) via PCRpt message.

   o  PCC can report the forward and reverse SR Paths independently to
      PCE(s) via PCRpt message.

   o  PCC can delegate the forward and reverse SR Paths independently to
      a Stateful PCE, where PCE would control the SR Paths.



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   o  Stateful PCE can update the SR Paths in the 'Double-sided
      Bidirectional SR Path Association Group' via PCUpd message, using
      the procedures described in [RFC8697].

   o  The reverse direction SR Path (LSP2(R) at node S, LSP1(R) at node
      D as shown in Figure 2b) SHOULD be informed by the PCE via
      PCInitiate message with the matching association group for the
      use-cases which require the PCC to be aware of the reverse
      direction SR path.

                                 +-----+
                                 | PCE |
                                 +-----+
      Report/Delegate:           ^     ^        Report/Delegate:
      Tunnel 1 (F)              /       \       Tunnel 2 (F)
      (LSP1 (F))               /         \      (LSP2 (F))
      Association #2          /           \     Association #2
                             /             \
                            /               \
                       +-----+    LSP1     +-----+
                       |  S  |------------>|  D  |
                       |     |<------------|     |
                       +-----+    LSP2     +-----+
                             <no signaling>

    Figure 2a: Step 1: PCC-Initiated Double-sided Bidirectional
                       SR Path with Forward Direction SR Paths


                                 +-----+
                                 | PCE |
                                 +-----+
      PCUpd/PCInitiate:          /     \       PCUpd/PCInitiate:
      Tunnel 1 (F)              /       \      Tunnel 2 (F)
      (LSP1 (F), LSP2 (R))     /         \     (LSP2 (F), LSP1 (R))
      Association #2          /           \    Association #2
                             /             \
                            v               v
                       +-----+    LSP1     +-----+
                       |  S  |------------>|  D  |
                       |     |<------------|     |
                       +-----+    LSP2     +-----+
                             <no signaling>

    Figure 2b: Step 2: PCE-Updated/Initiated Double-sided Bidirectional
                       SR Path with Reverse Direction SR Paths





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4.3.  Stateless PCE

   As defined in [I-D.ietf-pce-association-bidir], for a stateless PCE,
   it might be useful to associate a path computation request to an
   association group, thus enabling it to associate a common set of
   configuration parameters or behaviors with the request.  A PCC can
   request co-routed or non-co-routed forward and reverse direction
   paths from a stateless PCE for a bidirectional SR association group.

4.4.  Bidirectional (B) Flag

   As defined in [RFC5440], the Bidirectional (B) flag in Request
   Parameters (RP) object MUST be set when the PCC specifies that the
   path computation request relates to a bidirectional TE LSP.  The
   B-flag also MUST be set when the PCC specifies that the path
   computation request relates to an associated bidirectional SR Path.

   Note that the B-flag defined in Stateful PCE Request Parameter (SRP)
   object [I-D.ietf-pce-pcep-stateful-pce-gmpls] is not required for
   associated bidirectional SR path as association group is used to
   indicate that the path is bidirectional.

4.5.  State Synchronization

   During state synchronization, a PCC MUST report all the existing
   Bidirectional SR Association Groups to the Stateful PCE as per
   [RFC8697].  After the state synchronization, the PCE MUST remove all
   stale Bidirectional SR Associations.

4.6.  Error Handling

   The error handling as described in section 5.7 of
   [I-D.ietf-pce-association-bidir] continue to apply.

   The PCEP Path Setup Type (PST) for SR is set to 'TE Path is Setup
   using Segment Routing' [RFC8408].  If a PCEP speaker receives a
   different PST value for Bidirectional SR Path association group and
   it does not support; it MUST send a PCErr message with Error-Type =
   26 (Association Error) and Error-Value = TBD2 (Bidirectional LSP
   Association - Path Setup Type Not Supported).

5.  Implementation Status

   [Note to the RFC Editor - remove this section before publication, as
   well as remove the reference to [RFC7942].

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this



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   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   According to [RFC7942], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

5.1.  Huawei's Commercial Delivery

   The feature is developing based on Huawei VRP8.

   o  Organization: Huawei

   o  Implementation: Huawei's Commercial Delivery implementation based
      on VRP8.

   o  Description: The implementation is under development.

   o  Maturity Level: Product

   o  Contact: tanren@huawei.com

5.2.  ZTE's Commercial Delivery

   o  Organization: ZTE

   o  Implementation: ZTE's Commercial Delivery implementation based on
      Rosng v8.

   o  Description: The implementation is under development.

   o  Maturity Level: Product

   o  Contact: zhan.shuangping@zte.com.cn






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6.  Security Considerations

   The security considerations described in [RFC5440], [RFC8231],
   [RFC8281], and [RFC8408] apply to the extensions defined in this
   document as well.

   A new Association Type for the Association Object, 'Double-sided
   Associated Bidirectional SR Path Association Group' is introduced in
   this document.  Additional security considerations related to LSP
   associations due to a malicious PCEP speaker is described in
   [RFC8697] and apply to this Association Type.  Hence, securing the
   PCEP session using Transport Layer Security (TLS) [RFC8253] is
   recommended.

7.  Manageability Considerations

   All manageability requirements and considerations listed in
   [RFC5440], [RFC8231], and [RFC8281] apply to PCEP protocol extensions
   defined in this document.  In addition, requirements and
   considerations listed in this section apply.

7.1.  Control of Function and Policy

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

7.2.  Information and Data Models

   [RFC7420] describes the PCEP MIB, there are no new MIB Objects
   defined for Bidirectional SR Path associations.  The PCEP YANG module
   [I-D.ietf-pce-pcep-yang] defines data model for Bidirectional SR Path
   associations.

7.3.  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].

7.4.  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 [RFC8408].






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7.5.  Requirements On Other Protocols

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

7.6.  Impact On Network Operations

   Mechanisms defined in [RFC5440], [RFC8231], and [RFC8408] also apply
   to PCEP extensions defined in this document.

8.  IANA Considerations

8.1.  Association Type

   This document defines a new Association Type for the Association
   Object (Class Value 40) defined [RFC8697].  IANA is requested to make
   the assignment of a type for the sub-registry "ASSOCIATION Type" as
   follows:

   Type    Name                                          Reference
   -------------------------------------------------------------------
   TBD1    Double-sided Bidirectional SR Path            This document
           Association Group


8.2.  PCEP Errors

   This document defines new Error value for Error Type 26 (Association
   Error).  IANA is requested to allocate new Error value within the
   "PCEP-ERROR Object Error Types and Values" sub-registry of the PCEP
   Numbers registry, as follows:

   Error Type  Description                              Reference
   -------------------------------------------------------------------
    26         Association Error

               Error value: TBD2                        This document
               Bidirectional LSP Association -
               Path Setup Type Not Supported

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>.



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   [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>.

   [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>.

   [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>.

   [I-D.ietf-pce-association-bidir]
              Gandhi, R., Barth, C., and B. Wen, "PCEP Extensions for
              Associated Bidirectional Label Switched Paths (LSPs)",
              draft-ietf-pce-association-bidir-05 (work in progress),
              February 2020.

   [I-D.ietf-pce-sr-path-segment]
              Li, C., Chen, M., Cheng, W., Gandhi, R., and Q. Xiong,
              "Path Computation Element Communication Protocol (PCEP)
              Extension for Path Segment in Segment Routing (SR)",
              draft-ietf-pce-sr-path-segment-00 (work in progress),
              October 2019.

9.2.  Informative References

   [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>.



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   [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>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

   [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>.

   [RFC8408]  Sivabalan, S., Tantsura, J., Minei, I., Varga, R., and J.
              Hardwick, "Conveying Path Setup Type in PCE Communication
              Protocol (PCEP) Messages", RFC 8408, DOI 10.17487/RFC8408,
              July 2018, <https://www.rfc-editor.org/info/rfc8408>.

   [I-D.ietf-mpls-bfd-directed]
              Mirsky, G., Tantsura, J., Varlashkin, I., and M. Chen,
              "Bidirectional Forwarding Detection (BFD) Directed Return
              Path", draft-ietf-mpls-bfd-directed-13 (work in progress),
              December 2019.

   [I-D.gandhi-spring-twamp-srpm]
              Gandhi, R., Filsfils, C., Voyer, D., Chen, M., and B.
              Janssens, "Performance Measurement Using TWAMP Light for
              Segment Routing Networks", draft-gandhi-spring-twamp-
              srpm-05 (work in progress), December 2019.

   [I-D.ietf-spring-mpls-path-segment]
              Cheng, W., Li, H., Chen, M., Gandhi, R., and R. Zigler,
              "Path Segment in MPLS Based Segment Routing Network",
              draft-ietf-spring-mpls-path-segment-01 (work in progress),
              September 2019.

   [I-D.ietf-pce-pcep-yang]
              Dhody, D., Hardwick, J., Beeram, V., and J. Tantsura, "A
              YANG Data Model for Path Computation Element
              Communications Protocol (PCEP)", draft-ietf-pce-pcep-
              yang-13 (work in progress), October 2019.







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   [I-D.ietf-pce-pcep-stateful-pce-gmpls]
              Lee, Y., Zheng, H., Dios, O., Lopezalvarez, V., and Z.
              Ali, "Path Computation Element (PCE) Protocol Extensions
              for Stateful PCE Usage in GMPLS-controlled Networks",
              draft-ietf-pce-pcep-stateful-pce-gmpls-12 (work in
              progress), October 2019.

Acknowledgments

   Many thanks to Marina Fizgeer, Adrian Farrel, and Andrew Stone for
   the detailed review of this document and providing many useful
   comments.

Contributors

   The following people have substantially contributed to this document:

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

    Email: dhruv.ietf@gmail.com

    Zhenbin Li
    Huawei Technologies
    Huawei Campus, No. 156 Beiqing Rd.
    Beijing  100095
    China

    Email: lizhenbin@huawei.com


    Jie Dong
    Huawei Technologies
    Huawei Campus, No. 156 Beiqing Rd.
    Beijing  100095
    China

    Email: jie.dong@huawei.com










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Authors' Addresses

   Cheng Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing  100095
   China

   Email: chengli13@huawei.com


   Mach(Guoyi) Chen
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing  100095
   China

   Email: Mach.chen@huawei.com


   Weiqiang Cheng
   China Mobile
   China

   Email: chengweiqiang@chinamobile.com


   Rakesh Gandhi
   Cisco Systems, Inc.
   Canada

   Email: rgandhi@cisco.com


   Quan Xiong
   ZTE Corporation
   China

   Email: xiong.quan@zte.com.cn












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