Internet Engineering Task Force (IETF)                    R. Gandhi, Ed.
Request for Comments: 9059                           Cisco Systems, Inc.
Category: Standards Track                                       C. Barth
ISSN: 2070-1721                                         Juniper Networks
                                                                  B. Wen
                                                                 Comcast
                                                               June 2021


 Path Computation Element Communication Protocol (PCEP) Extensions for
          Associated Bidirectional Label Switched Paths (LSPs)

Abstract

   This document defines Path Computation Element Communication Protocol
   (PCEP) extensions for grouping two unidirectional MPLS-TE Label
   Switched Paths (LSPs), one in each direction in the network, into an
   associated bidirectional LSP.  These PCEP extensions can be applied
   either using a stateful PCE for both PCE-initiated and PCC-initiated
   LSPs or using a stateless PCE.  The PCEP procedures defined are
   applicable to the LSPs using RSVP-TE for signaling.

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/rfc9059.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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.  Key Word Definitions
     2.2.  Terminology
   3.  Overview
     3.1.  Single-Sided Initiation
       3.1.1.  PCE-Initiated Single-Sided Bidirectional LSP
       3.1.2.  PCC-Initiated Single-Sided Bidirectional LSP
     3.2.  Double-Sided Initiation
       3.2.1.  PCE-Initiated Double-Sided Bidirectional LSP
       3.2.2.  PCC-Initiated Double-Sided Bidirectional LSP
     3.3.  Co-routed Associated Bidirectional LSP
     3.4.  Summary of PCEP Extensions
     3.5.  Operational Considerations
   4.  Protocol Extensions
     4.1.  ASSOCIATION Object
     4.2.  Bidirectional LSP Association Group TLV
   5.  PCEP Procedure
     5.1.  PCE-Initiated LSPs
     5.2.  PCC-Initiated LSPs
     5.3.  Stateless PCE
     5.4.  Bidirectional (B) Flag
     5.5.  PLSP-ID Usage
     5.6.  State Synchronization
     5.7.  Error Handling
   6.  Security Considerations
   7.  Manageability Considerations
     7.1.  Control of Function and Policy
     7.2.  Information and Data Models
     7.3.  Liveness Detection and Monitoring
     7.4.  Verify Correct Operations
     7.5.  Requirements on Other Protocols
     7.6.  Impact on Network Operations
   8.  IANA Considerations
     8.1.  Association Types
     8.2.  Bidirectional LSP Association Group TLV
       8.2.1.  Flag Field in Bidirectional LSP Association Group TLV
     8.3.  PCEP Errors
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   [RFC5440] describes the Path Computation Element Communication
   Protocol (PCEP) as a communication mechanism between a Path
   Computation Client (PCC) and a Path Computation Element (PCE), or
   between PCE and PCC, that enables computation of Multiprotocol Label
   Switching (MPLS) - Traffic Engineering (TE) Label Switched Paths
   (LSPs).

   [RFC8231] specifies extensions to PCEP to enable stateful control of
   MPLS-TE LSPs.  It describes two modes of operation: passive stateful
   PCE and active stateful PCE.  In [RFC8231], the focus is on active
   stateful PCE where LSPs are provisioned on the PCC and control over
   them is delegated to a PCE.  Further, [RFC8281] describes the setup,
   maintenance, and teardown of PCE-initiated LSPs for the stateful PCE
   model.

   [RFC8697] introduces a generic mechanism for creating a grouping of
   LSPs.  This grouping can then be used to define associations between
   sets of LSPs or between a set of LSPs and a set of attributes, and it
   is equally applicable to the stateful PCE (active and passive modes)
   and the stateless PCE.

   The MPLS Transport Profile (MPLS-TP) requirements document [RFC5654]
   specifies that "MPLS-TP MUST support unidirectional, co-routed
   bidirectional, and associated bidirectional point-to-point transport
   paths".  [RFC7551] defines RSVP signaling extensions for binding
   forward and reverse unidirectional LSPs into an associated
   bidirectional LSP.  The fast reroute (FRR) procedures for associated
   bidirectional LSPs are described in [RFC8537].

   This document defines PCEP extensions for grouping two unidirectional
   MPLS-TE LSPs into an associated bidirectional LSP for both single-
   sided and double-sided initiation cases either when using a stateful
   PCE for both PCE-initiated and PCC-initiated LSPs or when using a
   stateless PCE.  The procedures defined are applicable to the LSPs
   using Resource Reservation Protocol - Traffic Engineering (RSVP-TE)
   for signaling [RFC3209].  Specifically, this document defines two new
   Association Types, Single-Sided Bidirectional LSP Association and
   Double-Sided Bidirectional LSP Association, as well as the
   Bidirectional LSP Association Group TLV, to carry additional
   information for the association.

   The procedure for associating two unidirectional Segment Routing (SR)
   paths to form an associated bidirectional SR path is defined in
   [BIDIR-PATH] and is outside the scope of this document.

2.  Conventions Used in This Document

2.1.  Key Word Definitions

   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.

2.2.  Terminology

   The reader is assumed to be familiar with the terminology defined in
   [RFC5440], [RFC7551], [RFC8231], and [RFC8697].

3.  Overview

   As shown in Figure 1, forward and reverse unidirectional LSPs can be
   grouped to form an associated bidirectional LSP.  Node A is the
   ingress node for LSP1 and egress node for LSP2, whereas node D is the
   ingress node for LSP2 and egress node for LSP1.  There are two
   methods of initiating the Bidirectional LSP Association, single-sided
   and double-sided, as defined in [RFC7551] and described in the
   following sections.

               LSP1 -->          LSP1 -->          LSP1 -->
      +-----+           +-----+           +-----+           +-----+
      |  A  +-----------+  B  +-----------+  C  +-----------+  D  |
      +-----+           +--+--+           +--+--+           +-----+
               <-- LSP2    |                 |     <-- LSP2
                           |                 |
                           |                 |
                        +--+--+           +--+--+
                        |  E  +-----------+  F  |
                        +-----+           +-----+
                                <-- LSP2

             Figure 1: Example of Associated Bidirectional LSP

3.1.  Single-Sided Initiation

   As specified in [RFC7551], in the single-sided case, the
   bidirectional tunnel is provisioned only on one endpoint node (PCC)
   of the tunnel.  Both endpoint nodes act as PCCs.  Both forward and
   reverse LSPs of this tunnel are initiated with the Association Type
   set to "Single-Sided Bidirectional LSP Association" on the
   originating endpoint node.  The forward and reverse LSPs are
   identified in the Bidirectional LSP Association Group TLV of their
   PCEP ASSOCIATION objects.

   The originating endpoint node signals the properties for the reverse
   LSP in the RSVP REVERSE_LSP object [RFC7551] of the forward LSP Path
   message.  The remote endpoint node then creates the corresponding
   reverse tunnel and reverse LSP, and it then signals the reverse LSP
   in response to the received RSVP-TE Path message.  Similarly, the
   remote endpoint node deletes the reverse LSP when it receives the
   RSVP-TE message to delete the forward LSP [RFC3209].

   As specified in [RFC8537], for fast reroute bypass tunnel assignment,
   the LSP starting from the originating endpoint node is identified as
   the forward LSP of the single-sided initiated bidirectional LSP.

3.1.1.  PCE-Initiated Single-Sided Bidirectional LSP

                                   +-----+
                                   | PCE |
                                   +-----+
       Initiates:                   |    \
       Tunnel 1 (F)                 |     \
       (LSP1 (F, 0), LSP2 (R, 0))   |      \
       Association #1               v       \
                                 +-----+    +-----+
                                 |  A  |    |  D  |
                                 +-----+    +-----+


                                   +-----+
                                   | PCE |
                                   +-----+
       Reports:                     ^    ^      Reports:
       Tunnel 1 (F)                 |     \     Tunnel 2 (F)
       (LSP1 (F, P1), LSP2 (R, P2)) |      \    (LSP2 (F, P3))
       Association #1               |       \   Association #1
                                 +-----+    +-----+
                                 |  A  |    |  D  |
                                 +-----+    +-----+

     Legend: F = Forward LSP, R = Reverse LSP, (0,P1,P2,P3) = PLSP-IDs

     Figure 2: Example of PCE-Initiated Single-Sided Bidirectional LSP

   Using partial topology from Figure 1, as shown in Figure 2, the
   forward Tunnel 1 and both forward LSP1 and reverse LSP2 are initiated
   on the originating endpoint node A by the PCE.  The PCEP-specific LSP
   identifiers (PLSP-IDs) used are P1 and P2 on the originating endpoint
   node A and P3 on the remote endpoint node D.  The originating
   endpoint node A reports Tunnel 1 and forward LSP1 and reverse LSP2 to
   the PCE.  The endpoint (PCC) node D reports Tunnel 2 and LSP2 to the
   PCE.

3.1.2.  PCC-Initiated Single-Sided Bidirectional LSP

                                   +-----+
                                   | PCE |
                                   +-----+
       Reports/Delegates:           ^    ^      Reports:
       Tunnel 1 (F)                 |     \     Tunnel 2 (F)
       (LSP1 (F, P1), LSP2 (R, P2)) |      \    (LSP2 (F, P3))
       Association #2               |       \   Association #2
                                 +-----+    +-----+
                                 |  A  |    |  D  |
                                 +-----+    +-----+

     Legend: F = Forward LSP, R = Reverse LSP, (P1,P2,P3) = PLSP-IDs

     Figure 3: Example of PCC-Initiated Single-Sided Bidirectional LSP

   Using partial topology from Figure 1, as shown in Figure 3, the
   forward Tunnel 1 and both forward LSP1 and reverse LSP2 are initiated
   on the originating endpoint node A (the originating PCC).  The PLSP-
   IDs used are P1 and P2 on the originating endpoint node A and P3 on
   the remote endpoint node D.  The originating endpoint (PCC) node A
   may delegate the forward LSP1 and reverse LSP2 to the PCE.  The
   originating endpoint node A reports Tunnel 1 and forward LSP1 and
   reverse LSP2 to the PCE.  The endpoint (PCC) node D reports Tunnel 2
   and LSP2 to the PCE.

3.2.  Double-Sided Initiation

   As specified in [RFC7551], in the double-sided case, the
   bidirectional tunnel is provisioned on both endpoint nodes (PCCs) of
   the tunnel.  The forward and reverse LSPs of this tunnel are
   initiated with the Association Type set to "Double-Sided
   Bidirectional LSP Association" on both endpoint nodes.  The forward
   and reverse LSPs are identified in the Bidirectional LSP Association
   Group TLV of their ASSOCIATION objects.

   As specified in [RFC8537], for fast reroute bypass tunnel assignment,
   the LSP with the higher source address [RFC3209] is identified as the
   forward LSP of the double-sided initiated bidirectional LSP.

3.2.1.  PCE-Initiated Double-Sided Bidirectional LSP

                               +-----+
                               | PCE |
                               +-----+
             Initiates:         |    \      Initiates:
             Tunnel 1 (F)       |     \     Tunnel 2 (F)
             (LSP1 (F, 0))      |      \    (LSP2 (F, 0))
             Association #3     v       v   Association #3
                             +-----+    +-----+
                             |  A  |    |  D  |
                             +-----+    +-----+


                               +-----+
                               | PCE |
                               +-----+
             Reports:           ^    ^      Reports:
             Tunnel 1 (F)       |     \     Tunnel 2 (F)
             (LSP1 (F, P4))     |      \    (LSP2 (F, P5))
             Association #3     |       \   Association #3
                             +-----+    +-----+
                             |  A  |    |  D  |
                             +-----+    +-----+

     Legend: F = Forward LSP, (0,P4,P5) = PLSP-IDs

     Figure 4: Example of PCE-Initiated Double-Sided Bidirectional LSP

   Using partial topology from Figure 1, as shown in Figure 4, the
   forward Tunnel 1 and forward LSP1 are initiated on the endpoint node
   A, and the reverse Tunnel 2 and reverse LSP2 are initiated on the
   endpoint node D by the PCE.  The PLSP-IDs used are P4 on the endpoint
   node A and P5 on the endpoint node D.  The endpoint node A (PCC)
   reports the forward LSP1, and endpoint node D reports the forward
   LSP2 to the PCE.

3.2.2.  PCC-Initiated Double-Sided Bidirectional LSP

                               +-----+
                               | PCE |
                               +-----+
           Reports/Delegates:   ^    ^      Reports/Delegates:
           Tunnel 1 (F)         |     \     Tunnel 2 (F)
           (LSP1 (F, P4))       |      \    (LSP2 (F, P5))
           Association #4       |       \   Association #4
                             +-----+    +-----+
                             |  A  |    |  D  |
                             +-----+    +-----+

     Legend: F = Forward LSP, (P4,P5) = PLSP-IDs

     Figure 5: Example of PCC-Initiated Double-Sided Bidirectional LSP

   Using partial topology from Figure 1, as shown in Figure 5, the
   forward Tunnel 1 and forward LSP1 are initiated on the endpoint node
   A, and the reverse Tunnel 2 and reverse LSP2 are initiated on the
   endpoint node D (the PCCs).  The PLSP-IDs used are P4 on the endpoint
   node A and P5 on the endpoint node D.  Both endpoint (PCC) nodes may
   delegate the forward LSP1 and LSP2 to the PCE.  The endpoint node A
   (PCC) reports the forward LSP1, and endpoint node D reports the
   forward LSP2 to the PCE.

3.3.  Co-routed Associated Bidirectional LSP

   In both single-sided and double-sided initiation cases, forward and
   reverse LSPs can be co-routed as shown in Figure 6, where both
   forward and reverse LSPs of a bidirectional LSP follow the same
   congruent path in the forward and reverse directions, respectively.

               LSP3 -->          LSP3 -->          LSP3 -->
      +-----+           +-----+           +-----+           +-----+
      |  A  +-----------+  B  +-----------+  C  +-----------+  D  |
      +-----+           +-----+           +-----+           +-----+
              <-- LSP4          <-- LSP4          <-- LSP4

        Figure 6: Example of Co-routed Associated Bidirectional LSP

   The procedure specified in [RFC8537] for fast reroute bypass tunnel
   assignment is also applicable to the co-routed associated
   bidirectional LSPs.

3.4.  Summary of PCEP Extensions

   The PCEP extensions defined in this document cover the following
   modes of operation under the stateful PCE model:

   *  A PCC initiates the forward and reverse LSP of a single-sided
      bidirectional LSP and retains control of the LSPs.  Similarly,
      both PCCs initiate the forward LSPs of a double-sided
      bidirectional LSP and retain control of the LSPs.  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 bidirectional LSP.  This is the passive stateful
      mode defined in [RFC8051].

   *  A PCC initiates the forward and reverse LSP of a single-sided
      bidirectional LSP and delegates control of the LSPs to a stateful
      PCE.  Similarly, both PCCs initiate the forward LSPs of a double-
      sided bidirectional LSP and delegate control of the LSPs to a
      stateful PCE.  During delegation, the association group
      identifying the bidirectional LSP is included.  The PCE computes
      the path of the LSP and updates the PCC with the information about
      the path as long as it controls the LSP.  This is the active
      stateful mode defined in [RFC8051].

   *  A PCE initiates the forward and reverse LSP of a single-sided
      bidirectional LSP on a PCC and retains control of the LSP.
      Similarly, a PCE initiates the forward LSPs of a double-sided
      bidirectional LSP on both PCCs and retains control of the LSPs.
      The PCE is responsible for computing the path of the LSP and
      updating the PCC with the information about the path as well as
      the association group identifying the bidirectional LSP.  This is
      the PCE-initiated mode defined in [RFC8281].

   *  A PCC requests co-routed or non-co-routed paths for forward and
      reverse LSPs of a bidirectional LSP, including when using a
      stateless PCE [RFC5440].

3.5.  Operational Considerations

   The double-sided case has an advantage when compared to the single-
   sided case, summarized as follows:

   *  In the double-sided case, two existing unidirectional LSPs in
      reverse directions in the network can be associated to form a
      bidirectional LSP without significantly increasing the operational
      complexity.

   The single-sided case has some advantages when compared to the
   double-sided case, summarized as follows:

   *  Some Operations, Administration, and Maintenance (OAM) use cases
      may require an endpoint node to know both forward and reverse
      paths for monitoring the bidirectional LSP.  For such use cases,
      the single-sided case may be preferred.

   *  For co-routed associated bidirectional LSPs in PCC-initiated mode,
      the single-sided case allows the originating PCC to dynamically
      compute co-routed forward and reverse paths.  This may not be
      possible with the double-sided case where the forward and reverse
      paths are computed separately as triggered by two different PCCs.

   *  The associated bidirectional LSPs in the single-sided case can be
      deployed in a network where PCEP is only enabled on the
      originating endpoint nodes as remote endpoint nodes create the
      reverse tunnels using RSVP-TE Path messages.

4.  Protocol Extensions

4.1.  ASSOCIATION Object

   As per [RFC8697], LSPs are associated by adding them to a common
   association group.  This document defines two new Association Types,
   called "Single-Sided Bidirectional LSP Association" (4) and "Double-
   Sided Bidirectional LSP Association" (5), using the generic
   ASSOCIATION object (Object-Class value 40).  A member of the
   Bidirectional LSP Association can take the role of a forward or
   reverse LSP and follows the following rules:

   *  An LSP (forward or reverse) MUST NOT be part of more than one
      Bidirectional LSP Association.

   *  The LSPs in a Bidirectional LSP Association MUST have matching
      endpoint nodes in the reverse directions.

   *  The same tunnel (as defined in Section 2.1 of [RFC3209]) MUST
      contain the forward and reverse LSPs of the Single-Sided
      Bidirectional LSP Association on the originating node, albeit both
      LSPs have reversed endpoint nodes.

   The Bidirectional LSP Association Types are considered to be both
   dynamic and operator configured in nature.  As per [RFC8697], the
   association group could be manually created by the operator on the
   PCEP peers, and the LSPs belonging to this association are conveyed
   via PCEP messages to the PCEP peer; alternately, the association
   group could be created dynamically by the PCEP speaker, and both the
   association group information and the LSPs belonging to the
   association group are conveyed to the PCEP peer.  The operator-
   configured Association Range MUST be set for this Association Type to
   mark a range of Association Identifiers that are used for operator-
   configured associations to avoid any Association Identifier clash
   within the scope of the Association Source (refer to [RFC8697]).

   Specifically, for the PCE-initiated bidirectional LSPs, these
   associations are dynamically created by the PCE on the PCE peers.
   Similarly, for both the PCE-initiated and the PCC-initiated single-
   sided cases, these associations are also dynamically created on the
   remote endpoint node using the information received from the RSVP
   message from the originating node.

   The Association ID, Association Source, optional Global Association
   Source TLV, and optional Extended Association ID TLV in the
   Bidirectional LSP ASSOCIATION object are initialized using the
   procedures defined in [RFC8697] and [RFC7551].

   [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 Bidirectional LSP Association Types MUST
   be done before using the Bidirectional LSP Association.  Thus, the
   PCEP speaker MUST include the Bidirectional LSP Association Types in
   the ASSOC-Type-List TLV and MUST receive the same from the PCEP peer
   before using the Bidirectional LSP Association in PCEP messages.

4.2.  Bidirectional LSP Association Group TLV

   The Bidirectional LSP Association Group TLV is an OPTIONAL TLV for
   use with Bidirectional LSP Associations (ASSOCIATION object with
   Association Type 4 for Single-Sided Bidirectional LSP Association or
   5 for Double-Sided Bidirectional LSP Association).

   *  The Bidirectional LSP Association Group TLV follows the PCEP TLV
      format from [RFC5440].

   *  The Type (16 bits) of the TLV is 54.

   *  The Length is 4 bytes.

   *  The value comprises of a single field, the Flags field (32 bits),
      where each bit represents a flag option.

   *  If the Bidirectional LSP Association Group TLV is missing, it
      means the LSP is the forward LSP, and it is not a co-routed LSP.

   *  When the Bidirectional LSP Association Group TLV is present, the R
      flag MUST be reset for the forward LSP for both co-routed and non-
      co-routed LSPs.

   *  For co-routed LSPs, this TLV MUST be present and the C flag set.

   *  For reverse LSPs, this TLV MUST be present and the R flag set.

   *  The Bidirectional LSP Association Group 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 format of the Bidirectional LSP Association Group TLV is shown in
   Figure 7.

     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 = 54             |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Flags                               |C|R|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 7: Bidirectional LSP Association Group TLV Format

   Flags for the Bidirectional LSP Association Group TLV are defined as
   follows.

   R (Reverse LSP, 1 bit, bit number 31):  Indicates whether the LSP
      associated is the reverse LSP of the bidirectional LSP.  If this
      flag is set, the LSP is a reverse LSP.  If this flag is not set,
      the LSP is a forward LSP.

   C (Co-routed Path, 1 bit, bit number 30):  Indicates whether the
      bidirectional LSP is co-routed.  This flag MUST be set for both
      the forward and reverse LSPs of a co-routed bidirectional LSP.

   The C flag is used by the PCE (both stateful and stateless) to
   compute bidirectional paths of the forward and reverse LSPs of a co-
   routed bidirectional LSP.

   The unassigned flags (bit numbers 0-29) MUST be set to 0 when sent
   and MUST be ignored when received.

5.  PCEP Procedure

   The PCEP procedure defined in this document is applicable to the
   following three scenarios:

   *  Neither unidirectional LSP exists, and both must be established.

   *  Both unidirectional LSPs exist, but the association must be
      established.

   *  One LSP exists, but the reverse associated LSP must be
      established.

5.1.  PCE-Initiated LSPs

   As specified in [RFC8697], Bidirectional LSP Associations can be
   created and updated by a stateful PCE.

   *  For a Single-Sided Bidirectional LSP Association initiated by the
      PCE, the PCE MUST send a PCInitiate message to the originating
      endpoint node with both forward and reverse LSPs.  For a Double-
      Sided Bidirectional LSP Association initiated by the PCE, it MUST
      send a PCInitiate message to both endpoint nodes with forward
      LSPs.

   *  Both PCCs MUST report the forward and reverse LSPs in the
      Bidirectional LSP Association to the PCE.  A PCC reports via a
      PCRpt message.

   *  Stateful PCEs MAY create and update the forward and reverse LSPs
      independently for the Single-Sided Bidirectional LSP Association
      on the originating endpoint node.

   *  Stateful PCEs MAY create and update the forward LSP independently
      for the Double-Sided Bidirectional LSP Association on the endpoint
      nodes.

   *  Stateful PCEs establish and remove the association relationship on
      a per-LSP basis.

   *  Stateful PCEs create and update the LSP and the association on
      PCCs via PCInitiate and PCUpd messages, respectively, using the
      procedures described in [RFC8697].

5.2.  PCC-Initiated LSPs

   As specified in [RFC8697], Bidirectional LSP Associations can also be
   created and updated by a PCC.

   *  For a Single-Sided Bidirectional LSP Association initiated at a
      PCC, the PCC MUST send a PCRpt message to the PCE with both
      forward and reverse LSPs.  For a Double-Sided Bidirectional LSP
      Association initiated at the PCCs, both PCCs MUST send a PCRpt
      message to the PCE with forward LSPs.

   *  PCCs on the originating endpoint node MAY create and update the
      forward and reverse LSPs independently for the Single-Sided
      Bidirectional LSP Association.

   *  PCCs on the endpoint nodes MAY create and update the forward LSP
      independently for the Double-Sided Bidirectional LSP Association.

   *  PCCs establish and remove the association group on a per-LSP
      basis.  PCCs MUST report the change in the association group of an
      LSP to PCE(s) via a PCRpt message.

   *  PCCs report the forward and reverse LSPs in the Bidirectional LSP
      Association independently to PCE(s) via a PCRpt message.

   *  PCCs for the single-sided case MAY delegate the forward and
      reverse LSPs independently to a stateful PCE, where the PCE would
      control the LSPs.  In this case, the originating (PCC) endpoint
      node SHOULD delegate both forward and reverse LSPs of a tunnel
      together to a stateful PCE in order to avoid any race condition.

   *  PCCs for the double-sided case MAY delegate the forward LSPs to a
      stateful PCE, where the PCE would control the LSPs.

   *  A stateful PCE updates the LSPs in the Bidirectional LSP
      Association via a PCUpd message, using the procedures described in
      [RFC8697].

5.3.  Stateless PCE

   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 [RFC8697].  A PCC can request co-routed or non-co-routed
   forward and reverse paths from a stateless PCE for a Bidirectional
   LSP Association.

5.4.  Bidirectional (B) Flag

   As defined in [RFC5440], the Bidirectional (B) flag in the Request
   Parameters (RP) object is set when the PCC specifies that the path
   computation request is for a bidirectional TE LSP with the same TE
   requirements in each direction.  For an associated bidirectional LSP,
   the B flag is also set when the PCC makes the path computation
   request for the same TE requirements for the forward and reverse
   LSPs.

   Note that the B flag defined in a Stateful PCE Request Parameter
   (SRP) object [STATEFUL-PCE-GMPLS] to indicate "bidirectional co-
   routed LSP" is used for GMPLS-signaled bidirectional LSPs and is not
   applicable to the associated bidirectional LSPs.

5.5.  PLSP-ID Usage

   As defined in [RFC8231], a PCEP-specific LSP Identifier (PLSP-ID) is
   created by a PCC to uniquely identify an LSP, and it remains the same
   for the lifetime of a PCEP session.

   In the case of a Single-Sided Bidirectional LSP Association, the
   reverse LSP of a bidirectional LSP created on the originating
   endpoint node is identified by the PCE using two different PLSP-IDs,
   based on the PCEP session on the ingress or egress node PCCs for the
   LSP.  In other words, the LSP will have a PLSP-ID P2 allocated at the
   ingress node PCC, while it will have a PLSP-ID P3 allocated at the
   egress node PCC (as shown in Figures 2 and 3).  There is no change in
   the PLSP-ID allocation procedure for the forward LSP of a single-
   sided bidirectional LSP created on the originating endpoint node.

   In the case of a Double-Sided Bidirectional LSP Association, there is
   no change in the PLSP-ID allocation procedure for the forward LSPs on
   either PCC.

   For an associated bidirectional LSP, the LSP-IDENTIFIERS TLV
   [RFC8231] MUST be included in all forward and reverse LSPs.

5.6.  State Synchronization

   During state synchronization, a PCC MUST report all the existing
   Bidirectional LSP Associations to the stateful PCE, as per [RFC8697].
   After the state synchronization, the PCE MUST remove all previous
   Bidirectional LSP Associations absent in the report.

5.7.  Error Handling

   If a PCE speaker receives an LSP with a Bidirectional LSP Association
   Type that it does not support, the PCE speaker MUST send PCErr with
   Error-Type = 26 (Association Error) and Error-value = 1 (Association
   Type is not supported).

   An LSP (forward or reverse) cannot be part of more than one
   Bidirectional LSP Association.  If a PCE speaker receives an LSP not
   complying to this rule, the PCE speaker MUST send PCErr with Error-
   Type = 26 (Association Error) and Error-value = 14 (Association group
   mismatch).

   The LSPs (forward or reverse) in a Single-Sided Bidirectional
   Association MUST belong to the same TE tunnel (as defined in
   [RFC3209]).  If a PCE speaker attempts to add an LSP in a Single-
   Sided Bidirectional LSP Association for a different tunnel, the PCE
   speaker MUST send PCErr with Error-Type = 26 (Association Error) and
   Error-value = 15 (Tunnel mismatch in the association group).

   The PCEP Path Setup Type (PST) for RSVP-TE is set to "Path is set up
   using the RSVP-TE signaling protocol" (Value 0) [RFC8408].  If a PCEP
   speaker receives a different PST value for the Bidirectional LSP
   Associations defined in this document, the PCE speaker MUST return a
   PCErr message with Error-Type = 26 (Association Error) and Error-
   value = 16 (Path Setup Type not supported).

   A Bidirectional LSP Association cannot have both unidirectional LSPs
   identified as reverse LSPs or both LSPs identified as forward LSPs.
   If a PCE speaker receives an LSP not complying to this rule, the PCE
   speaker MUST send PCErr with Error-Type = 26 (Association Error) and
   Error-value = 17 (Bidirectional LSP direction mismatch).

   A Bidirectional LSP Association cannot have one unidirectional LSP
   identified as co-routed and the other identified as non-co-routed.
   If a PCE speaker receives an LSP not complying to this rule, the PCE
   speaker MUST send PCErr with Error-Type = 26 (Association Error) and
   Error-value = 18 (Bidirectional LSP co-routed mismatch).

   The unidirectional LSPs forming the Bidirectional LSP Association
   MUST have matching endpoint nodes in the reverse directions.  If a
   PCE speaker receives an LSP not complying to this rule, the PCE
   speaker MUST send PCErr with Error-Type = 26 (Association Error) and
   Error-value = 19 (Endpoint mismatch in the association group).

   The processing rules as specified in Section 6.4 of [RFC8697]
   continue to apply to the Association Types defined in this document.

6.  Security Considerations

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

   Two new Association Types for the ASSOCIATION object, Single-Sided
   Bidirectional LSP Association and Double-Sided Bidirectional LSP
   Association, are introduced in this document.  Additional security
   considerations related to LSP associations due to a malicious PCEP
   speaker are described in [RFC8697] and apply to these Association
   Types.  Hence, securing the PCEP session using Transport Layer
   Security (TLS) [RFC8253] is RECOMMENDED.

7.  Manageability Considerations

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 LSP associations.

   The PCEP YANG module [PCE-PCEP-YANG] defines a data model for LSP
   associations.

7.3.  Liveness Detection and Monitoring

   The 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

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

7.5.  Requirements on Other Protocols

   The mechanisms defined in this document do not add any new
   requirements on other protocols.

7.6.  Impact on Network Operations

   The mechanisms defined in this document do not have any impact on
   network operations in addition to those already listed in [RFC5440],
   [RFC8231], and [RFC8281].

8.  IANA Considerations

8.1.  Association Types

   This document defines two new Association Types [RFC8697].  IANA has
   assigned the following new values in the "ASSOCIATION Type Field"
   subregistry [RFC8697] within the "Path Computation Element Protocol
   (PCEP) Numbers" registry:

     +======+============================================+===========+
     | Type | Name                                       | Reference |
     +======+============================================+===========+
     | 4    | Single-Sided Bidirectional LSP Association | RFC 9059  |
     +------+--------------------------------------------+-----------+
     | 5    | Double-Sided Bidirectional LSP Association | RFC 9059  |
     +------+--------------------------------------------+-----------+

          Table 1: Additions to ASSOCIATION Type Field Subregistry

8.2.  Bidirectional LSP Association Group TLV

   This document defines a new TLV for carrying additional information
   about LSPs within a Bidirectional LSP Association.  IANA has assigned
   the following value in the "PCEP TLV Type Indicators" subregistry
   within the "Path Computation Element Protocol (PCEP) Numbers"
   registry:

      +=======+=========================================+===========+
      | Value | Meaning                                 | Reference |
      +=======+=========================================+===========+
      | 54    | Bidirectional LSP Association Group TLV | RFC 9059  |
      +-------+-----------------------------------------+-----------+

         Table 2: Addition to PCEP TLV Type Indicators Subregistry

8.2.1.  Flag Field in Bidirectional LSP Association Group TLV

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

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

   *  Description

   *  Reference

   The initial contents of this registry are as follows:

                 +======+====================+===========+
                 | Bit  | Description        | Reference |
                 +======+====================+===========+
                 | 0-29 | Unassigned         |           |
                 +------+--------------------+-----------+
                 | 30   | C - Co-routed Path | RFC 9059  |
                 +------+--------------------+-----------+
                 | 31   | R - Reverse LSP    | RFC 9059  |
                 +------+--------------------+-----------+

                       Table 3: New Bidirectional LSP
                      Association Group TLV Flag Field
                                Subregistry

8.3.  PCEP Errors

   This document defines new Error-values for Error-Type 26 (Association
   Error).  IANA has allocated the following new 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              | Reference |
    +============+=============+==========================+===========+
    | 26         | Association | 14: Association group    | RFC 9059  |
    |            | Error       | mismatch                 |           |
    |            |             +--------------------------+-----------+
    |            |             | 15: Tunnel mismatch in   | RFC 9059  |
    |            |             | the association group    |           |
    |            |             +--------------------------+-----------+
    |            |             | 16: Path Setup Type not  | RFC 9059  |
    |            |             | supported                |           |
    |            |             +--------------------------+-----------+
    |            |             | 17: Bidirectional LSP    | RFC 9059  |
    |            |             | direction mismatch       |           |
    |            |             +--------------------------+-----------+
    |            |             | 18: Bidirectional LSP    | RFC 9059  |
    |            |             | co-routed mismatch       |           |
    |            |             +--------------------------+-----------+
    |            |             | 19: Endpoint mismatch in | RFC 9059  |
    |            |             | the association group    |           |
    +------------+-------------+--------------------------+-----------+

       Table 4: Additions to PCEP-ERROR Object Error Types and Values
                                Subregistry

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>.

   [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>.

   [RFC7551]  Zhang, F., Ed., Jing, R., and R. Gandhi, Ed., "RSVP-TE
              Extensions for Associated Bidirectional Label Switched
              Paths (LSPs)", RFC 7551, DOI 10.17487/RFC7551, May 2015,
              <https://www.rfc-editor.org/info/rfc7551>.

   [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>.

   [RFC8537]  Gandhi, R., Ed., Shah, H., and J. Whittaker, "Updates to
              the Fast Reroute Procedures for Co-routed Associated
              Bidirectional Label Switched Paths (LSPs)", RFC 8537,
              DOI 10.17487/RFC8537, February 2019,
              <https://www.rfc-editor.org/info/rfc8537>.

   [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

   [BIDIR-PATH]
              Li, C., Chen, M., Cheng, W., Gandhi, R., and Q. Xiong,
              "Path Computation Element Communication Protocol (PCEP)
              Extensions for Associated Bidirectional Segment Routing
              (SR) Paths", Work in Progress, Internet-Draft, draft-ietf-
              pce-sr-bidir-path-05, 26 January 2021,
              <https://tools.ietf.org/html/draft-ietf-pce-sr-bidir-path-
              05>.

   [PCE-PCEP-YANG]
              Dhody, D., Ed., 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-16, 22 February
              2021,
              <https://tools.ietf.org/html/draft-ietf-pce-pcep-yang-16>.

   [RFC5654]  Niven-Jenkins, B., Ed., Brungard, D., Ed., Betts, M., Ed.,
              Sprecher, N., and S. Ueno, "Requirements of an MPLS
              Transport Profile", RFC 5654, DOI 10.17487/RFC5654,
              September 2009, <https://www.rfc-editor.org/info/rfc5654>.

   [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>.

   [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>.

   [STATEFUL-PCE-GMPLS]
              Lee, Y., Ed., Zheng, H., Ed., de Dios, O., Lopez, V., and
              Z. Ali, "Path Computation Element (PCE) Protocol
              Extensions for Stateful PCE Usage in GMPLS-controlled
              Networks", Work in Progress, Internet-Draft, draft-ietf-
              pce-pcep-stateful-pce-gmpls-14, 28 December 2020,
              <https://tools.ietf.org/html/draft-ietf-pce-pcep-stateful-
              pce-gmpls-14>.

Acknowledgments

   The authors would like to thank Dhruv Dhody for various discussions
   on association groups and inputs to this document.  The authors would
   also like to thank Mike Taillon, Harish Sitaraman, Al Morton, and
   Marina Fizgeer for reviewing this document and providing valuable
   comments.  The authors would like to thank the following IESG members
   for their review comments and suggestions: Barry Leiba, √Čric Vyncke,
   Benjamin Kaduk, Murray Kucherawy, Martin Duke, and Alvaro Retana.

Authors' Addresses

   Rakesh Gandhi (editor)
   Cisco Systems, Inc.
   Canada

   Email: rgandhi@cisco.com


   Colby Barth
   Juniper Networks

   Email: cbarth@juniper.net


   Bin Wen
   Comcast

   Email: Bin_Wen@cable.comcast.com