Cisco Systems, Inc.

Canada rgandhi@cisco.com

Juniper Networks

cbarth@juniper.net

Comcast

Bin_Wen@cable.comcast.com

PCE Working Group 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 .

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Table of Contents

• .  Introduction
• .  Conventions Used in This Document
  • .  Key Word Definitions
  • .  Terminology
• .  Overview
  • .  Single-Sided Initiation
    • .  PCE-Initiated Single-Sided Bidirectional LSP
    • .  PCC-Initiated Single-Sided Bidirectional LSP
  • .  Double-Sided Initiation
    • .  PCE-Initiated Double-Sided Bidirectional LSP
    • .  PCC-Initiated Double-Sided Bidirectional LSP
  • .  Co-routed Associated Bidirectional LSP
  • .  Summary of PCEP Extensions
  • .  Operational Considerations
• .  Protocol Extensions
  • .  ASSOCIATION Object
  • .  Bidirectional LSP Association Group TLV
• .  PCEP Procedure
  • .  PCE-Initiated LSPs
  • .  PCC-Initiated LSPs
  • .  Stateless PCE
  • .  Bidirectional (B) Flag
  • .  PLSP-ID Usage
  • .  State Synchronization
  • .  Error Handling
• .  Security Considerations
• .  Manageability Considerations
  • .  Control of Function and Policy
  • .  Information and Data Models
  • .  Liveness Detection and Monitoring
  • .  Verify Correct Operations
  • .  Requirements on Other Protocols
  • .  Impact on Network Operations
• .  IANA Considerations
  • .  Association Types
  • .  Bidirectional LSP Association Group TLV
    • .  Flag Field in Bidirectional LSP Association Group TLV
  • .  PCEP Errors
• .  References
  • .  Normative References
  • .  Informative References
• Acknowledgments
• Authors' Addresses

Introduction 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). 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 , the focus is on active stateful PCE where LSPs are provisioned on the PCC and control over them is delegated to a PCE. Further, describes the setup, maintenance, and teardown of PCE-initiated LSPs for the stateful PCE model. 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 specifies that "MPLS-TP MUST support unidirectional, co-routed bidirectional, and associated bidirectional point-to-point transport paths". 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 . 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 . 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 and is outside the scope of this document.

Conventions Used in This Document

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 when, and only when, they appear in all capitals, as shown here.

Terminology The reader is assumed to be familiar with the terminology defined in , , , and .

Overview As shown in , 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 and described in the following sections.

Example of Associated Bidirectional LSP LSP1 --> LSP1 --> LSP1 --> +-----+ +-----+ +-----+ +-----+ | A +-----------+ B +-----------+ C +-----------+ D | +-----+ +--+--+ +--+--+ +-----+ <-- LSP2 | | <-- LSP2 | | | | +--+--+ +--+--+ | E +-----------+ F | +-----+ +-----+ <-- LSP2

Single-Sided Initiation As specified in , 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 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 . As specified in , 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.

PCE-Initiated Single-Sided Bidirectional LSP

Example of 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

Using partial topology from , as shown in , 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.

PCC-Initiated Single-Sided Bidirectional LSP

Example of 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

Using partial topology from , as shown in , 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.

Double-Sided Initiation As specified in , 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 , for fast reroute bypass tunnel assignment, the LSP with the higher source address is identified as the forward LSP of the double-sided initiated bidirectional LSP.

PCE-Initiated Double-Sided Bidirectional LSP

Example of 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

Using partial topology from , as shown in , 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.

PCC-Initiated Double-Sided Bidirectional LSP

Example of 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

Using partial topology from , as shown in , 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.

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 , where both forward and reverse LSPs of a bidirectional LSP follow the same congruent path in the forward and reverse directions, respectively.

Example of Co-routed Associated Bidirectional LSP LSP3 --> LSP3 --> LSP3 --> +-----+ +-----+ +-----+ +-----+ | A +-----------+ B +-----------+ C +-----------+ D | +-----+ +-----+ +-----+ +-----+ <-- LSP4 <-- LSP4 <-- LSP4

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

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 .
• 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 .
• 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 .
• 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 .

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.

Protocol Extensions

ASSOCIATION Object As per , 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 ) 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 , 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 ). 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 and . 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.

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 .
• 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 .

Bidirectional LSP Association Group TLV Format 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| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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.

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.

PCE-Initiated LSPs As specified in , 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 .

PCC-Initiated LSPs As specified in , 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 .

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

Bidirectional (B) Flag As defined in , 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 to indicate "bidirectional co-routed LSP" is used for GMPLS-signaled bidirectional LSPs and is not applicable to the associated bidirectional LSPs.

PLSP-ID Usage As defined in , 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 and ). 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 MUST be included in all forward and reverse LSPs.

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

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 ). 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) . 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 continue to apply to the Association Types defined in this document.

Security Considerations The security considerations described in , , and 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 and apply to these Association Types. Hence, securing the PCEP session using Transport Layer Security (TLS) is RECOMMENDED.

Manageability Considerations

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 , , and .

Information and Data Models describes the PCEP MIB; there are no new MIB objects defined for LSP associations. The PCEP YANG module defines a data model for LSP associations.

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 , , and .

Verify Correct Operations The mechanisms defined in this document do not imply any new operation verification requirements in addition to those already listed in , , and .

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

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 , , and .

IANA Considerations

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

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

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: Addition to PCEP TLV Type Indicators Subregistry

Value	Meaning	Reference
54	Bidirectional LSP Association Group TLV	RFC 9059

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 . 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: New Bidirectional LSP Association Group TLV Flag Field Subregistry

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

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

Error-Type	Meaning	Error-value	Reference
26	Association Error	14: Association group mismatch	RFC 9059
		15: Tunnel mismatch in the association group	RFC 9059
		16: Path Setup Type not supported	RFC 9059
		17: Bidirectional LSP direction mismatch	RFC 9059
		18: Bidirectional LSP co-routed mismatch	RFC 9059
		19: Endpoint mismatch in the association group	RFC 9059

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document describes the use of RSVP (Resource Reservation Protocol), including all the necessary extensions, to establish label-switched paths (LSPs) in MPLS (Multi-Protocol Label Switching). Since the flow along an LSP is completely identified by the label applied at the ingress node of the path, these paths may be treated as tunnels. A key application of LSP tunnels is traffic engineering with MPLS as specified in RFC 2702. [STANDARDS-TRACK] This document specifies the Path Computation Element (PCE) Communication Protocol (PCEP) for communications between a Path Computation Client (PCC) and a PCE, or between two PCEs. Such interactions include path computation requests and path computation replies as well as notifications of specific states related to the use of a PCE in the context of Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS) Traffic Engineering. PCEP is designed to be flexible and extensible so as to easily allow for the addition of further messages and objects, should further requirements be expressed in the future. [STANDARDS-TRACK] This document describes Resource Reservation Protocol (RSVP) extensions to bind two point-to-point unidirectional Label Switched Paths (LSPs) into an associated bidirectional LSP. The association is achieved by defining new Association Types for use in ASSOCIATION and in Extended ASSOCIATION Objects. One of these types enables independent provisioning of the associated bidirectional LSPs on both sides, while the other enables single-sided provisioning. The REVERSE_LSP Object is also defined to enable a single endpoint to trigger creation of the reverse LSP and to specify parameters of the reverse LSP in the single-sided provisioning case. Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. The Path Computation Element Communication Protocol (PCEP) provides mechanisms for Path Computation Elements (PCEs) to perform path computations in response to Path Computation Client (PCC) requests. Although PCEP explicitly makes no assumptions regarding the information available to the PCE, it also makes no provisions for PCE control of timing and sequence of path computations within and across PCEP sessions. This document describes a set of extensions to PCEP to enable stateful control of MPLS-TE and GMPLS Label Switched Paths (LSPs) via PCEP. The Path Computation Element Communication Protocol (PCEP) defines the mechanisms for the communication between a Path Computation Client (PCC) and a Path Computation Element (PCE), or among PCEs. This document describes PCEPS -- the usage of Transport Layer Security (TLS) to provide a secure transport for PCEP. The additional security mechanisms are provided by the transport protocol supporting PCEP; therefore, they do not affect the flexibility and extensibility of PCEP. This document updates RFC 5440 in regards to the PCEP initialization phase procedures. The Path Computation Element Communication Protocol (PCEP) provides mechanisms for Path Computation Elements (PCEs) to perform path computations in response to Path Computation Client (PCC) requests. The extensions for stateful PCE provide active control of Multiprotocol Label Switching (MPLS) Traffic Engineering Label Switched Paths (TE LSPs) via PCEP, for a model where the PCC delegates control over one or more locally configured LSPs to the PCE. This document describes the creation and deletion of PCE-initiated LSPs under the stateful PCE model. Resource Reservation Protocol (RSVP) association signaling can be used to bind two unidirectional Label Switched Paths (LSPs) into an associated bidirectional LSP. When an associated bidirectional LSP is co-routed, the reverse LSP follows the same path as its forward LSP. This document updates the fast reroute procedures defined in RFC 4090 to support both single-sided and double-sided provisioned associated bidirectional LSPs. This document also updates the procedure for associating two reverse LSPs defined in RFC 7551 to support co-routed bidirectional LSPs. The fast reroute procedures can ensure that, for the co-routed LSPs, traffic flows on co-routed paths in the forward and reverse directions after a failure event. This document introduces a generic mechanism to create a grouping of Label Switched Paths (LSPs) in the context of a Path Computation Element (PCE). This grouping can then be used to define associations between sets of LSPs or between a set of LSPs and a set of attributes (such as configuration parameters or behaviors), and it is equally applicable to the stateful PCE (active and passive modes) and the stateless PCE. Informative References Huawei Technologies Huawei Technologies China Mobile Cisco Systems, Inc. ZTE Corporation 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 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. Work in Progress Work in Progress This document specifies the requirements of an MPLS Transport Profile (MPLS-TP). This document is a product of a joint effort of the International Telecommunications Union (ITU) and IETF to include an MPLS Transport Profile within the IETF MPLS and PWE3 architectures to support the capabilities and functionalities of a packet transport network as defined by International Telecommunications Union - Telecommunications Standardization Sector (ITU-T). This work is based on two sources of requirements: MPLS and PWE3 architectures as defined by IETF, and packet transport networks as defined by ITU-T. The requirements expressed in this document are for the behavior of the protocol mechanisms and procedures that constitute building blocks out of which the MPLS Transport Profile is constructed. The requirements are not implementation requirements. [STANDARDS-TRACK] This memo defines a portion of the Management Information Base (MIB) for use with network management protocols in the Internet community. In particular, it describes managed objects for modeling of the Path Computation Element Communication Protocol (PCEP) for communications between a Path Computation Client (PCC) and a Path Computation Element (PCE), or between two PCEs. A stateful Path Computation Element (PCE) maintains information about Label Switched Path (LSP) characteristics and resource usage within a network in order to provide traffic-engineering calculations for its associated Path Computation Clients (PCCs). This document describes general considerations for a stateful PCE deployment and examines its applicability and benefits, as well as its challenges and limitations, through a number of use cases. PCE Communication Protocol (PCEP) extensions required for stateful PCE usage are covered in separate documents. A Path Computation Element (PCE) can compute Traffic Engineering (TE) paths through a network; these paths are subject to various constraints. Currently, TE paths are Label Switched Paths (LSPs) that are set up using the RSVP-TE signaling protocol. However, other TE path setup methods are possible within the PCE architecture. This document proposes an extension to the PCE Communication Protocol (PCEP) to allow support for different path setup methods over a given PCEP session. Work in Progress

Acknowledgments The authors would like to thank for various discussions on association groups and inputs to this document. The authors would also like to thank , , , and for reviewing this document and providing valuable comments. The authors would like to thank the following IESG members for their review comments and suggestions: , , , , , and .

Authors' Addresses Cisco Systems, Inc.

Canada rgandhi@cisco.com

Juniper Networks

cbarth@juniper.net

Comcast

Bin_Wen@cable.comcast.com