PCE Working Group M. Koldychev, Ed. Internet-Draft Ciena Corporation Intended status: Standards Track S. Sidor, Ed. Expires: 7 December 2026 Cisco Systems. 5 June 2026 Path Computation Element Communication Protocol (PCEP) Extensions for Signaling Multipath Information draft-ietf-pce-multipath-26 Abstract A Segment Routing (SR) Policy Candidate Path can contain multiple Segment Lists, allowing for load-balancing and redundancy across diverse paths. However, current PCEP extensions for SR Policy only allow signaling of a single Segment List per Candidate Path. This document defines PCEP extensions to encode multiple Segment Lists within an SR Policy Candidate Path, enabling multipath capabilities such as weighted or equal-cost load-balancing across Segment Lists. These extensions are designed to be generic and reusable for future path types beyond SR Policy, and are applicable to both stateless and stateful PCEP. 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 7 December 2026. Copyright Notice Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved. Koldychev & Sidor Expires 7 December 2026 [Page 1] Internet-Draft PCEP Extensions for Multipath June 2026 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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Signaling Multiple Segment Lists of an SR Candidate Path . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2. Splitting of Requested Bandwidth . . . . . . . . . . . . 5 2.3. Reverse Path Information . . . . . . . . . . . . . . . . 5 3. Protocol Extensions . . . . . . . . . . . . . . . . . . . . . 6 3.1. PATH-ATTRIB Object . . . . . . . . . . . . . . . . . . . 6 3.2. METRIC Object . . . . . . . . . . . . . . . . . . . . . . 7 3.3. MULTIPATH-WEIGHT TLV . . . . . . . . . . . . . . . . . . 7 3.4. MULTIPATH-OPPDIR-PATH TLV . . . . . . . . . . . . . . . . 8 3.5. Composite Candidate Path . . . . . . . . . . . . . . . . 9 3.5.1. Per-Flow Candidate Path . . . . . . . . . . . . . . . 10 4. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.1. Capability Negotiation . . . . . . . . . . . . . . . . . 11 4.1.1. Multipath Capability TLV . . . . . . . . . . . . . . 11 4.2. Path ID . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.3. Signaling Multiple Paths for Load-Balancing . . . . . . . 14 4.4. Signaling Opposite-Direction Path Information . . . . . . 15 5. PCEP Message Extensions . . . . . . . . . . . . . . . . . . . 16 6. Implementation Status . . . . . . . . . . . . . . . . . . . . 17 6.1. Cisco Systems . . . . . . . . . . . . . . . . . . . . . . 17 6.2. Ciena Corp . . . . . . . . . . . . . . . . . . . . . . . 17 6.3. Huawei Technologies . . . . . . . . . . . . . . . . . . . 18 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 7.1. PCEP Object . . . . . . . . . . . . . . . . . . . . . . . 18 7.2. PCEP TLV . . . . . . . . . . . . . . . . . . . . . . . . 18 7.3. PCEP-Error Object . . . . . . . . . . . . . . . . . . . . 19 7.4. Flags in the MULTIPATH-CAP TLV . . . . . . . . . . . . . 20 7.5. Flags in the PATH-ATTRIB Object . . . . . . . . . . . . . 20 7.6. Flags in the MULTIPATH-OPPDIR-PATH TLV . . . . . . . . . 21 7.7. Flags in the MULTIPATH-FORWARD-CLASS TLV . . . . . . . . 21 8. Security Considerations . . . . . . . . . . . . . . . . . . . 22 9. Operational Considerations . . . . . . . . . . . . . . . . . 22 9.1. Control of Function and Policy . . . . . . . . . . . . . 22 Koldychev & Sidor Expires 7 December 2026 [Page 2] Internet-Draft PCEP Extensions for Multipath June 2026 9.2. Information and Data Models . . . . . . . . . . . . . . . 22 9.3. Liveness Detection and Monitoring . . . . . . . . . . . . 22 9.4. Verify Correct Operations . . . . . . . . . . . . . . . . 23 9.5. Requirements On Other Protocols . . . . . . . . . . . . . 23 9.6. Impact On Network Operations . . . . . . . . . . . . . . 23 10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 24 11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 24 11.1. Original Authors . . . . . . . . . . . . . . . . . . . . 24 11.2. Additional Contributors . . . . . . . . . . . . . . . . 25 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 26 12.1. Normative References . . . . . . . . . . . . . . . . . . 26 12.2. Informative References . . . . . . . . . . . . . . . . . 27 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 28 A.1. SR Policy Candidate Path with Multiple Segment Lists . . 28 A.2. Composite Candidate Path . . . . . . . . . . . . . . . . 30 A.3. Opposite Direction Tunnels . . . . . . . . . . . . . . . 30 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32 1. Introduction Segment Routing Policy for Traffic Engineering [RFC9256] details the concepts of Segment Routing (SR) Policy and approaches to steering traffic into an SR Policy. In particular, it describes the SR Candidate Path as a collection of one or more Segment Lists. The current PCEP specifications only allow for signaling of one Segment List per Candidate Path. The PCEP extension to support Segment Routing Policy Candidate Paths [RFC9862] specifically kept the signaling of multiple Segment Lists outside its scope. This document defines the required extensions that allow the signaling of multipath information via PCEP. Although these extensions are motivated by the SR Policy use case, they are also applicable to other technologies. For SR Policy, support for [RFC9862] is a prerequisite for using the multipath extensions defined in this document with SR Policy Candidate Paths. 1.1. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 1.2. Terminology The following terms are used in this document: Koldychev & Sidor Expires 7 December 2026 [Page 3] Internet-Draft PCEP Extensions for Multipath June 2026 ECMP: Equal-Cost Multipath, equally distributing traffic among multiple paths/links, where each path/link gets the same share of traffic as others. W-ECMP: Weighted Equal-Cost Multipath, unequally distributing traffic among multiple paths/links, where some paths/links get more traffic than others. PLSP: PCE Label Switched Path, a path or set of paths computed or controlled by the PCE. In the context of SR Policy, a PLSP corresponds to a Candidate Path. Path: In the context of this document, a path refers to a single forwarding path encoded in an ERO or RRO. For SR Policy, a path corresponds to a Segment List. The mechanisms defined in this document use the generic term "path" to allow applicability beyond SR Policy. LSP: Label Switched Path. The base PCEP specification [RFC4655] originally defined the use of the PCE architecture for MPLS and GMPLS networks with LSPs instantiated using the RSVP-TE signaling protocol. Over time, support for additional path setup types such as SRv6 has been introduced [RFC9603]. The term "LSP" is used extensively in PCEP specifications and, while the multipath extensions defined in this document are applicable beyond SR Policy, in the context of PCEP for SR Policy [RFC9862], an LSP object represents an SR Policy Candidate Path, which may be an SRv6 path (still represented using the LSP object as specified in [RFC8231]). A single LSP may contain multiple paths (Segment Lists). Segment List: An ordered list of segments that defines a forwarding path in Segment Routing, as defined in [RFC9256]. In PCEP for SR Policy, each Segment List is encoded as an ERO or RRO. ERO: Koldychev & Sidor Expires 7 December 2026 [Page 4] Internet-Draft PCEP Extensions for Multipath June 2026 Explicit Route Object, defined in [RFC5440], encodes an explicit path. In the context of SR Policy, an ERO encodes a Segment List. RRO: Record Route Object, defined in [RFC5440], encodes the actual signaled path. In the context of SR Policy, an RRO reports a Segment List. 2. Motivation This extension is motivated by the use-cases described below. 2.1. Signaling Multiple Segment Lists of an SR Candidate Path The Candidate Path of an SR Policy is the unit of signaling in PCEP [RFC9862]. A single Candidate Path can consist of multiple Segment Lists. Each Segment List is represented by an Explicit Route Object (ERO). In existing PCEP specifications, a PCEP Label Switched Path (LSP) object is associated with exactly one ERO. This restriction prevents the encoding of multiple Segment Lists (i.e., multiple EROs) within the single LSP. 2.2. Splitting of Requested Bandwidth A Path Computation Client (PCC) may request a path with 80 Gbps of bandwidth, but all links in the network have only 60 Gbps of capacity each. The Path Computation Element (PCE) can return two paths that can together carry 80 Gbps. The PCC can then equally or unequally split the incoming 80 Gbps of traffic among the two paths. Section 3.3 introduces a new TLV that carries the path weight that facilitates control of load-balancing of traffic among the multiple paths. 2.3. Reverse Path Information Path Computation Element Communication Protocol (PCEP) Extensions for Associated Bidirectional LSPs [RFC9059] defines a mechanism in PCEP to associate two opposite direction SR Policy Candidate Paths. However, within each Candidate Path there can be multiple Segment Lists, and [RFC9059] does not define a mechanism to specify mapping between Segment Lists of the forward and reverse Candidate Paths. Certain applications such as Circuit Style SR Policy [I-D.ietf-spring-cs-sr-policy], require the knowledge of reverse paths per Segment List, not just per Candidate Path. For example, when the headend knows the reverse Segment List for each forward Segment List, then Performance Measurement (PM)/Bidirectional Koldychev & Sidor Expires 7 December 2026 [Page 5] Internet-Draft PCEP Extensions for Multipath June 2026 Forwarding Detection (BFD) can run a separate session on every Segment List, by imposing a double stack (forward stack followed by reverse stack) onto the packet. If the reverse Segment List is co- routed with the forward Segment List, then the PM/BFD session would traverse the same links in the forward and reverse directions, thus allowing detection of link/node failures in both directions. 3. Protocol Extensions 3.1. PATH-ATTRIB Object This document defines the PATH-ATTRIB object that is used to carry per-path information and to act as a separator between EROs/RROs in the / Routing Backus-Naur Form (RBNF) [RFC5511] element. The PATH-ATTRIB object always precedes the ERO or RRO that it applies to. If multiple EROs or RROs are present, then each ERO or RRO MUST be preceded by a PATH-ATTRIB object that describes it. The PATH-ATTRIB Object-Class value is 45. The PATH-ATTRIB Object-Type value is 1. The format of the PATH-ATTRIB object is shown in Figure 1. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags |R| O | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Path ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ Optional TLVs ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: PATH-ATTRIB object format Flags (32 bits): * O (Operational - 3 bits): operational state of the path, same values as the identically named field in the LSP object [RFC8231]. The relationship between the per-path Operational state and the LSP-level Operational state in the LSP object is outside the scope of this document; for SR Policy, the Candidate Path validity criterion is defined in Section 2.8 of [RFC9256]. Koldychev & Sidor Expires 7 December 2026 [Page 6] Internet-Draft PCEP Extensions for Multipath June 2026 * R (Reverse - 1 bit): Indicates this path is reverse, i.e., it originates on the LSP destination and terminates on the LSP source (usually the PCC headend itself). Paths with this flag set are not installed in forwarding for load-balancing purposes, but MAY be used by the PCC for operations such as Performance Measurement (PM) or Bidirectional Forwarding Detection (BFD). * Unassigned bits MUST be set to 0 on transmission and MUST be ignored on receipt. Path ID (32 bits): 4-octet identifier that identifies a path (encoded in the ERO/RRO) within the set of multiple paths under the PCEP LSP. See Section 4.2 for details. Optional TLVs: Variable length field that can contain one or more TLVs that carry additional per-path information. The specific TLVs that can be included are defined in subsequent sections of this document. 3.2. METRIC Object The PCEP METRIC object can continue to be used at the LSP level to describe properties of the overall LSP. Mechanisms for encoding per- path metrics (e.g., a separate METRIC for each path) are outside the scope of this document and would require further extensions. 3.3. MULTIPATH-WEIGHT TLV A new MULTIPATH-WEIGHT TLV is optional in the PATH-ATTRIB object. 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 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Weight | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: MULTIPATH-WEIGHT TLV format Type (16 bits): 61 for "MULTIPATH-WEIGHT" TLV. Length (16 bits): 4 octets. Weight (32 bits): unsigned integer weight of this path within the multipath, if W-ECMP is desired. The fraction of flows that a specific ERO/RRO carries is derived from the ratio of its weight to the sum of the weights of all paths in the multipath (including this Koldychev & Sidor Expires 7 December 2026 [Page 7] Internet-Draft PCEP Extensions for Multipath June 2026 path): see Section 4.3 for details. For SR Policy, if the Weight value is 0, the corresponding Segment List is declared invalid per Section 5.1 of [RFC9256] and carries no traffic. If all paths within an LSP have Weight 0, the sum of weights is zero, making the candidate path invalid per Section 2.11 of [RFC9256]. Signaling a zero-weight path alongside paths with non-zero weights can be used to drain traffic from a path while retaining its forwarding instructions. When the MULTIPATH-WEIGHT TLV is absent from the PATH-ATTRIB object, or the PATH-ATTRIB object is absent from the /, then the Weight of the corresponding path is taken to be 1. 3.4. MULTIPATH-OPPDIR-PATH TLV A new MULTIPATH-OPPDIR-PATH TLV is optional in the PATH-ATTRIB object. Multiple instances of the TLV are allowed in the same PATH- ATTRIB object. Each TLV instance identifies one opposite-direction path for the path described by this PATH-ATTRIB object. This provides per-path level opposite-direction mapping within an LSP. In the context of SR Policy, this corresponds to per-Segment List mapping within a Candidate Path, complementing the Candidate Path level bidirectional association defined in [I-D.ietf-pce-sr-bidir-path], which also describes the usage of this TLV in the context of associated bidirectional SR Paths. See Section 4.4 for operational details. 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 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Flags |L|N| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opposite Direction Path ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: MULTIPATH-OPPDIR-PATH TLV format Type (16 bits): 63 for "MULTIPATH-OPPDIR-PATH" TLV Length (16 bits): 8 octets. Reserved: This field MUST be set to zero on transmission and MUST be ignored on receipt. Flags (16 bits): Koldychev & Sidor Expires 7 December 2026 [Page 8] Internet-Draft PCEP Extensions for Multipath June 2026 * N (Node co-routed): If set, indicates this path is node co-routed with its opposite direction path specified in this TLV. Two opposite direction paths are node co-routed if they traverse the same nodes, but MAY traverse different links. If not set, the paths are not guaranteed to be node co-routed (they may or may not traverse the same set of nodes). * L (Link co-routed): If set, indicates this path is link co-routed with its opposite direction path, specified in this TLV. Two opposite direction paths are link co-routed if they traverse the same links (but in opposite directions). Link co-routing implies node co-routing; therefore, it is not necessary to set the N flag when the L flag is set. * Unassigned bits MUST be set to 0 on transmission and MUST be ignored on receipt. Opposite Direction Path ID (32 bits): References the Path ID field (see Section 4.2) of a PATH-ATTRIB object that identifies a path going in the opposite direction to this path. The value 0 is reserved and MUST NOT be used in this field. If no opposite- direction path exists, the MULTIPATH-OPPDIR-PATH TLV MUST NOT be included in the PATH-ATTRIB object (see Section 4.4). If a PCEP speaker receives a MULTIPATH-OPPDIR-PATH TLV with Opposite Direction Path ID set to 0, it MUST send a PCError message with Error-Type = 19 ("Invalid Operation") and Error-Value = TBD4 ("Invalid opposite- direction path mapping"). 3.5. Composite Candidate Path SR Policy Architecture [RFC9256] defines the concept of a Composite Candidate Path. A regular SR Policy Candidate Path outputs traffic to a set of Segment Lists, while an SR Policy Composite Candidate Path outputs traffic recursively to a set of SR Policies on the same headend. In PCEP, the Composite Candidate Path still consists of PATH-ATTRIB objects, but ERO is replaced by Color of the recursively used SR Policy. To signal the Composite Candidate Path, this document makes use of the COLOR TLV, defined in [RFC9863]. For a Composite Candidate Path, the COLOR TLV is included in the PATH-ATTRIB Object, thus allowing each Composite Candidate Path to do ECMP/W-ECMP among SR Policies identified by its constituent Colors. To achieve W-ECMP, the MULTIPATH-WEIGHT TLV (Section 3.3) is included alongside the COLOR TLV in each PATH-ATTRIB object. If multiple COLOR TLVs are contained in the PATH-ATTRIB object, the first one is processed and the others MUST be ignored. Koldychev & Sidor Expires 7 December 2026 [Page 9] Internet-Draft PCEP Extensions for Multipath June 2026 An ERO MUST be included as per the existing RBNF; this ERO MUST contain no sub-objects. This empty ERO serves as a placeholder to maintain compatibility with existing implementations based on the RBNF defined in [RFC8231]. If the head-end receives a non-empty ERO for a Composite Candidate Path, it MUST send a PCError message with Error-Type = 19 ("Invalid Operation") and Error-Value = 21 ("Non- empty path"). See Appendix A.2 for an example of the encoding. 3.5.1. Per-Flow Candidate Path Per-Flow Candidate Path builds on the concept of the Composite Candidate Path. Each Path in a Per-Flow Candidate Path is assigned a 3-bit forwarding class value, which allows Quality of Service (QoS) classified traffic to be steered depending on the forwarding class. A new MULTIPATH-FORWARD-CLASS TLV is optional in the PATH-ATTRIB object. 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 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags |T| FC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: MULTIPATH-FORWARD-CLASS TLV format Type (16 bits): TBD1 for "MULTIPATH-FORWARD-CLASS" TLV. Length (16 bits): 4 octets. Flags (29 bits): Unassigned bits MUST be set to 0 on transmission and MUST be ignored on receipt. T (1 bit): MPLS TC type. When set, indicates that the FC value is derived from the MPLS Traffic Class (TC) bits as described in Section 8.6 of [RFC9256]. When not set, the interpretation of the FC value is reserved for future use. FC (3 bits): Forwarding class value. When the T flag is set, this carries the MPLS TC-based forwarding class value as defined in Section 8.6 of [RFC9256]. This value is given by the QoS classifier to traffic entering the given Candidate Path. Different classes of traffic that enter the given Candidate Path can be differentially steered into different Colors. The FC field allows up to 8 different Koldychev & Sidor Expires 7 December 2026 [Page 10] Internet-Draft PCEP Extensions for Multipath June 2026 forwarding classes (values 0-7). The semantics of specific FC values are significant at the headend node (PCC) that implements the SR Policy and are determined by that node's local QoS policy or configuration. Coordination of FC value meanings between PCEP peers (e.g., through out-of-band configuration management or operational procedures) is outside the scope of this document. 4. Operation 4.1. Capability Negotiation 4.1.1. Multipath Capability TLV A new MULTIPATH-CAP TLV is defined. This TLV MAY be present in the OPEN object during PCEP session establishment. It MAY also be present in the LSP object for each individual LSP from the PCC. 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 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Number of Multipaths | Flags |C|F|O| |W| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: MULTIPATH-CAP TLV format Type (16 bits): 60 for "MULTIPATH-CAP" TLV. Length (16 bits): 4 octets. Number of Multipaths (16 bits): When sent from a PCC, it indicates how many forward primary multipaths the PCC can install in forwarding. From a PCE, it indicates how many forward primary multipaths the PCE can compute. This value is per-LSP when carried in the LSP object; the effective value governing a given LSP is the LSP object value if present, otherwise the OPEN object value (see below). This count does not include reverse paths (R-flag=1), which are not installed in forwarding for load-balancing purposes. Therefore, the total number of PATH-ATTRIB objects in an LSP may exceed this value when reverse paths are also signaled. The value 0 indicates an unlimited number. Flags (16 bits): * W-flag: whether MULTIPATH-WEIGHT TLV is supported. This flag covers the use of MULTIPATH-WEIGHT for both regular and Composite Candidate Paths. Koldychev & Sidor Expires 7 December 2026 [Page 11] Internet-Draft PCEP Extensions for Multipath June 2026 * O-flag: In the OPEN object, this flag indicates whether the MULTIPATH-OPPDIR-PATH TLV is supported. In the LSP object, this flag indicates that opposite-direction path information is requested or provided for that specific LSP. When set by the PCC (in PCRpt/PCReq), it requests the PCE to provide reverse path information. When set by the PCE (in PCInit/PCUpd/PCRep), it indicates the PCE is providing or will provide reverse path information. In both cases, the PCE SHOULD provide the reverse path information, if it is able to. For PCC-Initiated LSPs, if the PCC has not set the O-flag in the MULTIPATH-CAP TLV of the LSP object, the PCE SHOULD NOT include reverse path information in the corresponding response. If the PCC receives unsolicited reverse path information, it MAY ignore it. * F-flag: whether MULTIPATH-FORWARD-CLASS TLV is supported. * C-flag: whether Composite Candidate Path (Section 3.5) is supported, including the use of the COLOR TLV in the PATH-ATTRIB object. * Unassigned bits MUST be set to 0 on transmission and MUST be ignored on receipt. Note that F-flag and C-flag can be set independently for capability negotiation purposes. While Per-Flow Candidate Path (Section 3.5.1) builds on top of Composite Candidate Path, the F-flag reflects whether the MULTIPATH-FORWARD-CLASS TLV is supported, and the C-flag reflects whether Composite Candidate Path signaling is supported. A peer that supports Per-Flow Candidate Path MUST set both C-flag and F-flag. Note that the F-flag is defined independently of the C-flag to allow for future use cases that may use the MULTIPATH-FORWARD- CLASS TLV for purposes other than Per-Flow Candidate Path; in such cases, the F-flag MAY be set without the C-flag. When a PCE computes an LSP path, it MUST NOT return more forward multipaths than the minimum of the effective "Number of Multipaths" values of both the PCE and PCC. The effective value for a given LSP is determined by the per-LSP MULTIPATH-CAP TLV in the LSP object if present; otherwise, it defaults to the value from the MULTIPATH-CAP TLV in the OPEN object. This ensures the PCE does not exceed either its own computation capability or the PCC's installation capability. If this TLV is absent from both OPEN and LSP objects, the PCEP speaker does not support multipath and the behavior is consistent with existing PCEP specifications, where a single path is associated with each LSP. Koldychev & Sidor Expires 7 December 2026 [Page 12] Internet-Draft PCEP Extensions for Multipath June 2026 If a PCC receives more paths than it advertised support for, it MUST send a PCError message with Error-Type = 19 ("Invalid Operation") and Error-Value = TBD3 ("Unsupported multipath capability"). The PCC MAY also include the MULTIPATH-CAP TLV in the LSP object for each individual LSP, to specify per-LSP values. The PCC MUST NOT include this TLV in the LSP object if the TLV was not present in the OPEN objects of both PCEP peers. TLV values in the LSP object override the session default values in the OPEN object. If a PCEP speaker receives a PATH-ATTRIB object but the multipath capability was not successfully negotiated during session establishment, it MUST treat this as an error. The PCEP speaker MUST send a PCError message with Error-Type = 10 ("Reception of an invalid object") and Error- Value = TBD2 ("Unexpected PATH-ATTRIB object"). For example, the PCC includes this TLV in the OPEN object at session establishment, setting "Number of Multipaths" to 4 and "O-flag" to 0. The PCC also includes this TLV in the LSP object for a particular LSP, setting "Number of Multipaths" to 16 and "O-flag" to 1. This indicates that the PCC only wants to receive the reverse path information for that particular LSP and that this LSP can have up to 16 multipaths, while other LSPs can only have up to 4 multipaths. Additionally, if a PCEP speaker receives a TLV within the PATH-ATTRIB object (such as MULTIPATH-WEIGHT, MULTIPATH-OPPDIR-PATH, or MULTIPATH-FORWARD-CLASS) but the corresponding capability flag was not set in the negotiated MULTIPATH-CAP TLV, it MUST treat this as an error. The PCEP speaker MUST send a PCError message with Error-Type = 19 ("Invalid Operation") and Error-Value = TBD3 ("Unsupported multipath capability"). 4.2. Path ID The Path ID uniquely identifies a Path within the context of an LSP. A single Path ID space is shared among all paths within the LSP, including forward paths and reverse paths (R-flag=1). Path IDs MUST be unique across forward and reverse paths within the same LSP. The meaning of "Path" depends on the type of LSP: * For a regular SR Policy Candidate Path, the Paths within that LSP are the Segment Lists. * For a Composite Candidate Path (Section 3.5), the Paths within that LSP are the constituent SR Policies, each of which is identified by its Color (carried in the COLOR TLV within the corresponding PATH-ATTRIB object). Koldychev & Sidor Expires 7 December 2026 [Page 13] Internet-Draft PCEP Extensions for Multipath June 2026 Value 0 indicates an unallocated Path ID. The value of 0 MAY be used when this Path is not referenced and the allocation of a Path ID is not necessary. Path IDs are allocated by the PCEP peer that owns the LSP. If the LSP is delegated to the PCE, then the PCE allocates the Path IDs and sends them in the PCReply/PCUpd/PCInitiate messages. If the LSP is locally computed on the PCC, then the PCC allocates the Path IDs and sends them in the PCReq/PCRpt messages. When LSP delegation changes (e.g., the PCC revokes delegation from the PCE), the new path owner SHOULD retain the existing Path IDs to simplify path correlation in the event of re-delegation. If the new owner cannot retain them, it MAY allocate new Path IDs. If a PCEP speaker detects that there are two Paths with the same non- zero Path ID, then the PCEP speaker MUST send a PCError message with Error-Type = 10 ("Reception of an invalid object") and Error-Value = 38 ("Conflicting Path ID"). Multiple paths MAY have Path ID set to 0, as this value indicates those paths are not referenced and do not require unique identification. 4.3. Signaling Multiple Paths for Load-Balancing The PATH-ATTRIB object can be used to signal multiple paths and indicate equal or unequal load-balancing amongst the set of multipaths. In this case, the PATH-ATTRIB is populated for each ERO as follows: 1. The PCE MAY assign a unique Path ID to each ERO path and populate it inside the PATH-ATTRIB object. The Path ID is unique within the context of a PLSP (PCE Label Switched Path) (when non-zero). 2. The PCE MAY include the MULTIPATH-WEIGHT TLV inside the PATH- ATTRIB object, populating a weight value to reflect the relative share of traffic to be carried by the path. If the MULTIPATH- WEIGHT is not carried inside a PATH-ATTRIB object, the PCC MUST assume the default weight of 1 when computing the traffic share. 3. The PCC derives the fraction of flows carried by a specific primary path from the ratio of its weight to the sum of the weights of all paths in the multipath. For SR Policy, the use of weights for load-balancing between Segment Lists of a Candidate Path is described in Section 2.11 of [RFC9256]. Koldychev & Sidor Expires 7 December 2026 [Page 14] Internet-Draft PCEP Extensions for Multipath June 2026 4.4. Signaling Opposite-Direction Path Information The PATH-ATTRIB object can be used to signal opposite-direction path associations within a PCEP LSP. This capability is used to establish bidirectional path relationships where forward and reverse paths can be explicitly mapped to each other. In this case, the PATH-ATTRIB is populated for each ERO as follows: 1. The PCEP peer (PCC or PCE) allocates a unique Path ID to each path and populates it inside the PATH-ATTRIB object. The Path ID is unique within the context of a PLSP (PCE Label Switched Path). 2. For paths that have opposite-direction counterparts, the MULTIPATH-OPPDIR-PATH TLV is added to the PATH-ATTRIB object. The Opposite Direction Path ID field is set to reference the Path ID of the corresponding opposite-direction path. 3. Multiple instances of the MULTIPATH-OPPDIR-PATH TLV MAY be present in the same PATH-ATTRIB object to support many-to-many mappings between forward and reverse paths. This allows a single forward path to map to multiple reverse paths and vice versa. Many-to-many mapping can occur when a Segment List contains Node Segment(s) that traverse parallel links at a midpoint. The reverse of this Segment List may require multiple Reverse Segment Lists to cover all the parallel links at the midpoint. 4. The N-flag and L-flag in the MULTIPATH-OPPDIR-PATH TLV MAY be set to indicate node co-routing or link co-routing respectively. These flags inform the receiver about the relationship between the forward and reverse paths. 5. For paths that have no opposite-direction counterpart, the MULTIPATH-OPPDIR-PATH TLV is omitted from the PATH-ATTRIB object. Forward paths (R-flag=0) and reverse paths (R-flag=1) are included in the same PCEP LSP, allowing bidirectional relationships to be established in a single message exchange. The opposite-direction path associations MUST be symmetric within the same LSP: for each (A -> B) reference expressed via a MULTIPATH-OPPDIR-PATH TLV instance in path A's PATH-ATTRIB object, the corresponding (B -> A) reference MUST also be present as a MULTIPATH-OPPDIR-PATH TLV instance in path B's PATH-ATTRIB object. Multiple MULTIPATH-OPPDIR-PATH TLV instances per PATH-ATTRIB object are permitted, supporting many-to-many mappings. Additionally, the R-flags of opposite-direction paths MUST have opposite values (one set to 0, the other to 1). Koldychev & Sidor Expires 7 December 2026 [Page 15] Internet-Draft PCEP Extensions for Multipath June 2026 If a PCEP speaker receives an opposite-direction path mapping that is asymmetric or where the R-flags are inconsistent, it MUST send a PCError message with Error-Type = 19 ("Invalid Operation") and Error- Value = TBD4 ("Invalid opposite-direction path mapping"). See Appendix A.3 for an example of usage. 5. PCEP Message Extensions The RBNF of PCRpt and PCUpd messages, as defined in [RFC8231], uses a combination of and/or . PCReq and PCRep messages, as defined in [RFC5440] and extended by [RFC8231], directly include ERO and RRO within their respective message structures rather than encapsulating them within or constructs. As specified in Section 6.1 of [RFC8231], within the context of messages that use these constructs, is represented by the ERO and is represented by the RRO: ::= ::= This document extends [RFC8231] by allowing multiple EROs/RROs to be present in the /: ::= | [] ::= [] ::= | [] ::= [] Similarly, this document extends [RFC8281] by allowing multiple paths in the PCInitiate message by allowing multiple EROs with their associated path attributes. The PCE-initiated LSP instantiation format is updated to: Koldychev & Sidor Expires 7 December 2026 [Page 16] Internet-Draft PCEP Extensions for Multipath June 2026 ::= [] [] where follows the recursive definition above, allowing multiple paths to be signaled in a single PCInitiate message. When multiple paths are present, each ERO MUST be preceded by a PATH-ATTRIB object that describes it. A single path MAY be sent as a bare ERO without PATH-ATTRIB for backward compatibility. 6. 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 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". 6.1. Cisco Systems Organization: Cisco Systems Implementation: IOS-XR PCC and PCE Description: Circuit-Style SR Policies Maturity Level: Supported feature Coverage: Multiple Segment Lists and reverse paths in SR Policy Contact: mkoldych@cisco.com 6.2. Ciena Corp Koldychev & Sidor Expires 7 December 2026 [Page 17] Internet-Draft PCEP Extensions for Multipath June 2026 Organization: Ciena Corp Implementation: Head-end and controller Maturity Level: Proof of concept Coverage: Partial Contact: byadav@ciena.com 6.3. Huawei Technologies Organization: Huawei Technologies Co.,Ltd. Implementation: Huawei's Router and Controller Maturity Level: Proof of concept Coverage: Partial Contact: tanren@huawei.com 7. IANA Considerations All IANA actions in this section pertain to the "Path Computation Element Protocol (PCEP) Numbers" registry group. 7.1. PCEP Object IANA is requested to confirm the following allocation in the "PCEP Objects" registry: +--------------+-------------+-------------------+-----------------+ | Object-Class | Name | Object-Type | Reference | | Value | | Value | | +--------------+-------------+-------------------+-----------------+ | 45 | PATH-ATTRIB | 0: Reserved | | | | | 1: PATH-ATTRIB | This document | | | | 2-15: Unassigned | | +--------------+-------------+-------------------+-----------------+ Object-Type values are managed via the IETF Review policy as per [RFC8126]. 7.2. PCEP TLV IANA is requested to confirm the following allocations in the "PCEP TLV Type Indicators" registry: Koldychev & Sidor Expires 7 December 2026 [Page 18] Internet-Draft PCEP Extensions for Multipath June 2026 +------------+-----------------------------------+-----------------+ | TLV Type | TLV Name | Reference | | Value | | | +------------+-----------------------------------+-----------------+ | 60 | MULTIPATH-CAP | This document | +------------+-----------------------------------+-----------------+ | 61 | MULTIPATH-WEIGHT | This document | +------------+-----------------------------------+-----------------+ | 63 | MULTIPATH-OPPDIR-PATH | This document | +------------+-----------------------------------+-----------------+ IANA is requested to make new allocations in the "PCEP TLV Type Indicators" registry: +------------+-----------------------------------+-----------------+ | TLV Type | TLV Name | Reference | | Value | | | +------------+-----------------------------------+-----------------+ | TBD1 | MULTIPATH-FORWARD-CLASS | This document | +------------+-----------------------------------+-----------------+ 7.3. PCEP-Error Object IANA is requested to confirm the following allocations in the "PCEP- ERROR Object Error Types and Values" registry: +------------+-----------------------------------+-----------------+ | Error-Type | Error-Value | Reference | +------------+-----------------------------------+-----------------+ | 10 | 38 - Conflicting Path ID | This document | +------------+-----------------------------------+-----------------+ | 19 | 21 - Non-empty path | This document | +------------+-----------------------------------+-----------------+ IANA is requested to make new allocations in the "PCEP-ERROR Object Error Types and Values" registry: Koldychev & Sidor Expires 7 December 2026 [Page 19] Internet-Draft PCEP Extensions for Multipath June 2026 +------------+-----------------------------------+-----------------+ | Error-Type | Error-Value | Reference | +------------+-----------------------------------+-----------------+ | 10 | TBD2 - Unexpected PATH-ATTRIB | This document | | | Object | | +------------+-----------------------------------+-----------------+ | 19 | TBD3 - Unsupported multipath | This document | | | capability | | +------------+-----------------------------------+-----------------+ | 19 | TBD4 - Invalid opposite-direction | This document | | | path mapping | | +------------+-----------------------------------+-----------------+ 7.4. Flags in the MULTIPATH-CAP TLV IANA is requested to create a new registry called "Flags in MULTIPATH-CAP TLV" to manage the Flag field of the MULTIPATH-CAP TLV. New values are to be assigned by "IETF review" [RFC8126] +------------+-----------------------------------+-----------------+ | Bit | Description | Reference | +------------+-----------------------------------+-----------------+ | 0-10 | Unassigned | This document | +------------+-----------------------------------+-----------------+ | 11 | C-flag: Composite Candidate | This document | | | Path support | | +------------+-----------------------------------+-----------------+ | 12 | F-flag: MULTIPATH-FORWARD-CLASS | This document | | | TLV support | | +------------+-----------------------------------+-----------------+ | 13 | O-flag: MULTIPATH-OPPDIR-PATH | This document | | | TLV support | | +------------+-----------------------------------+-----------------+ | 14 | Unassigned | This document | +------------+-----------------------------------+-----------------+ | 15 | W-flag: MULTIPATH-WEIGHT TLV | This document | | | support | | +------------+-----------------------------------+-----------------+ 7.5. Flags in the PATH-ATTRIB Object IANA is requested to create a new registry called "Flags in PATH- ATTRIB Object" to manage the Flag field of the PATH-ATTRIB object. New values are to be assigned by "IETF review" [RFC8126] Koldychev & Sidor Expires 7 December 2026 [Page 20] Internet-Draft PCEP Extensions for Multipath June 2026 +------------+-----------------------------------+-----------------+ | Bit | Description | Reference | +------------+-----------------------------------+-----------------+ | 0-27 | Unassigned | This document | +------------+-----------------------------------+-----------------+ | 28 | R-flag: Reverse path | This document | +------------+-----------------------------------+-----------------+ | 29-31 | O-flag: Operational state | This document | +------------+-----------------------------------+-----------------+ 7.6. Flags in the MULTIPATH-OPPDIR-PATH TLV IANA is requested to create a new registry called "Flags in MULTIPATH-OPPDIR-PATH TLV" to manage the Flag field of the MULTIPATH- OPPDIR-PATH TLV. New values are to be assigned by "IETF review" [RFC8126] +------------+-----------------------------------+-----------------+ | Bit | Description | Reference | +------------+-----------------------------------+-----------------+ | 0-13 | Unassigned | This document | +------------+-----------------------------------+-----------------+ | 14 | L-flag: Link co-routed | This document | +------------+-----------------------------------+-----------------+ | 15 | N-flag: Node co-routed | This document | +------------+-----------------------------------+-----------------+ 7.7. Flags in the MULTIPATH-FORWARD-CLASS TLV IANA is requested to create a new registry called "Flags in MULTIPATH-FORWARD-CLASS TLV" to manage the Flag field of the MULTIPATH-FORWARD-CLASS TLV. New values are to be assigned by "IETF review" [RFC8126] +------------+-----------------------------------+-----------------+ | Bit | Description | Reference | +------------+-----------------------------------+-----------------+ | 0-27 | Unassigned | This document | +------------+-----------------------------------+-----------------+ | 28 | T-flag: MPLS TC type | This document | +------------+-----------------------------------+-----------------+ | 29-31 | FC: Forwarding class | This document | +------------+-----------------------------------+-----------------+ Koldychev & Sidor Expires 7 December 2026 [Page 21] Internet-Draft PCEP Extensions for Multipath June 2026 8. Security Considerations The security considerations described in [RFC5440], [RFC8231], [RFC8281], [RFC8664], [RFC9256], [RFC9862] and [RFC9863] are applicable to this specification. As per [RFC8231], it is RECOMMENDED that these PCEP extensions can only be activated on authenticated and encrypted sessions across PCEs and PCCs belonging to the same administrative authority, using Transport Layer Security (TLS) [RFC8253] [I-D.ietf-pce-pceps-tls13] as per the recommendations and best current practices in [RFC9325]. The multipath extensions defined in this document allow a PCE to signal multiple paths per LSP, which increases the per-LSP state maintained by the PCC. A misbehaving or compromised PCE could exploit this to amplify state at the PCC by maximizing multipath fan- out. The "Number of Multipaths" field in the MULTIPATH-CAP TLV provides an upper bound on the number of paths a PCC will accept per LSP, and operators SHOULD configure a non-zero value to limit exposure. The existing PCEP authentication and encryption recommendations (TLS per [RFC8253]) mitigate the risk of unauthorized PCE access. 9. Operational Considerations All manageability requirements and considerations listed in [RFC5440], [RFC8231], [RFC8664], and [RFC9256] apply to the PCEP protocol extensions defined in this document. In addition, the requirements and considerations listed in this section apply. 9.1. Control of Function and Policy A PCEP speaker (PCC or PCE) implementation SHOULD allow an operator to enable or disable the multipath capabilities advertised in the MULTIPATH-CAP TLV (see Section 4). 9.2. Information and Data Models It is expected that a future version of the PCEP YANG module [RFC9826] will be extended to include the PCEP extensions defined in this document. 9.3. Liveness Detection and Monitoring The mechanisms defined in this document do not introduce any new liveness detection or monitoring requirements in addition to those already defined in [RFC5440] and [RFC8231]. Koldychev & Sidor Expires 7 December 2026 [Page 22] Internet-Draft PCEP Extensions for Multipath June 2026 9.4. Verify Correct Operations In addition to the verification requirements in [RFC5440] and [RFC8231], the following considerations apply: * An implementation SHOULD allow an operator to view the capabilities advertised in the MULTIPATH-CAP TLV by each PCEP peer for a session and for individual LSPs. * An implementation SHOULD allow an operator to view the PATH-ATTRIB object and all its associated TLVs for each path within an LSP. This includes the Path ID, weight, and opposite-direction path associations. * An implementation SHOULD provide a mechanism to log and display the new PCEP errors defined in this document. 9.5. Requirements On Other Protocols The PCEP extensions defined in this document do not impose any new requirements on other protocols. 9.6. Impact On Network Operations The mechanisms in this document allow for more complex LSP structures with multiple paths. Network operators should be aware of the potential increase in PCEP message sizes and the additional state that must be maintained by PCEP speakers. The "Number of Multipaths" field in the MULTIPATH-CAP TLV can be used to control the scale of multipath computations and state. Paths within a single LSP may have significantly different properties, such as MTU, latency, and available bandwidth. When flow-based load-balancing is used across such paths, individual flows are hashed to a single path, so per-path MTU or latency divergence is a per-flow concern rather than a per-packet one. However, operators should be aware that advertising weighted load-balancing across paths with very different latencies can degrade application performance. Similarly, if paths have different MTUs, the effective MTU for flows on each path will differ; operators should ensure that PMTUD or consistent MTU configuration is in place. For bidirectional applications relying on Performance Measurement (PM) or BFD, note that forward and reverse paths may have asymmetric properties, and implementations should account for this. Koldychev & Sidor Expires 7 December 2026 [Page 23] Internet-Draft PCEP Extensions for Multipath June 2026 For Composite Candidate Paths, recursive policy resolution is therefore bounded, as Section 2.2 of [RFC9256] prohibits constituent SR Policies of a composite Candidate Path from themselves using composite Candidate Paths. 10. Acknowledgement Thanks to Adrian Farrel for shepherding this document, Ketan Talaulikar for his thorough AD review, Dhruv Dhody for ideas and discussion, and Diego Achaval, Quan Xiong, Giuseppe Fioccola, Italo Busi, Yuan Yaping, and Cheng Li for their reviews. 11. Contributors 11.1. Original Authors The following individuals are the original authors who initiated and developed the core work of this document. Mike Koldychev is also listed as editor in the Authors' Addresses section. The remaining individuals appear here rather than in the Authors' Addresses section due to the IETF guidelines on the maximum number of listed authors, but should be considered co-authors of this document. Samuel Sidor joined the effort at a later stage as an additional editor. Koldychev & Sidor Expires 7 December 2026 [Page 24] Internet-Draft PCEP Extensions for Multipath June 2026 Mike Koldychev (also listed as editor) Ciena Corporation Email: mkoldych@ciena.com Siva Sivabalan Ciena Corporation Email: ssivabal@ciena.com Tarek Saad Cisco Systems Email: tsaad@cisco.com Vishnu Pavan Beeram Juniper Networks, Inc. Email: vbeeram@juniper.net Hooman Bidgoli Nokia Email: hooman.bidgoli@nokia.com Shuping Peng Huawei Technologies Email: pengshuping@huawei.com Bhupendra Yadav Ciena Email: byadav@ciena.com Gyan Mishra Verizon Inc. Email: hayabusagsm@gmail.com 11.2. Additional Contributors The following individuals made contributions to this document: Zafar Ali Cisco Systems Email: zali@cisco.com Andrew Stone Nokia Email: andrew.stone@nokia.com Chen Ran ZTE Email: chen.ran@zte.com.cn Koldychev & Sidor Expires 7 December 2026 [Page 25] Internet-Draft PCEP Extensions for Multipath June 2026 12. References 12.1. Normative References [I-D.ietf-pce-pceps-tls13] Dhody, D., Turner, S., and R. Housley, "Updates for PCEPS: TLS Connection Establishment Restrictions", Work in Progress, Internet-Draft, draft-ietf-pce-pceps-tls13-04, 9 January 2024, . [I-D.ietf-pce-sr-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) LSPs", Work in Progress, Internet-Draft, draft-ietf- pce-sr-bidir-path-25, 6 March 2026, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation Element (PCE) Communication Protocol (PCEP)", RFC 5440, DOI 10.17487/RFC5440, March 2009, . [RFC5511] Farrel, A., "Routing Backus-Naur Form (RBNF): A Syntax Used to Form Encoding Rules in Various Routing Protocol Specifications", RFC 5511, DOI 10.17487/RFC5511, April 2009, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [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, . Koldychev & Sidor Expires 7 December 2026 [Page 26] Internet-Draft PCEP Extensions for Multipath June 2026 [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, . [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, . [RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W., and J. Hardwick, "Path Computation Element Communication Protocol (PCEP) Extensions for Segment Routing", RFC 8664, DOI 10.17487/RFC8664, December 2019, . [RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov, A., and P. Mattes, "Segment Routing Policy Architecture", RFC 9256, DOI 10.17487/RFC9256, July 2022, . [RFC9325] Sheffer, Y., Saint-Andre, P., and T. Fossati, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November 2022, . [RFC9603] Li, C., Ed., Kaladharan, P., Sivabalan, S., Koldychev, M., and Y. Zhu, "Path Computation Element Communication Protocol (PCEP) Extensions for IPv6 Segment Routing", RFC 9603, DOI 10.17487/RFC9603, July 2024, . [RFC9862] Koldychev, M., Sivabalan, S., Sidor, S., Barth, C., Peng, S., and H. Bidgoli, "Path Computation Element Communication Protocol (PCEP) Extensions for Segment Routing (SR) Policy Candidate Paths", RFC 9862, DOI 10.17487/RFC9862, October 2025, . [RFC9863] Rajagopalan, B., Beeram, V., Peng, S., Koldychev, M., and G. Mishra, "Path Computation Element Protocol (PCEP) Extension for Color", RFC 9863, DOI 10.17487/RFC9863, October 2025, . 12.2. Informative References Koldychev & Sidor Expires 7 December 2026 [Page 27] Internet-Draft PCEP Extensions for Multipath June 2026 [I-D.ietf-spring-cs-sr-policy] Schmutzer, C., Ali, Z., Maheshwari, P., Rokui, R., and A. Stone, "Circuit Style Segment Routing Policy", Work in Progress, Internet-Draft, draft-ietf-spring-cs-sr-policy- 17, 12 March 2026, . [RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path Computation Element (PCE)-Based Architecture", RFC 4655, DOI 10.17487/RFC4655, August 2006, . [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, . [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, . [RFC9059] Gandhi, R., Ed., Barth, C., and B. Wen, "Path Computation Element Communication Protocol (PCEP) Extensions for Associated Bidirectional Label Switched Paths (LSPs)", RFC 9059, DOI 10.17487/RFC9059, June 2021, . [RFC9826] Dhody, D., Ed., Beeram, V., Hardwick, J., and J. Tantsura, "A YANG Data Model for the Path Computation Element Communication Protocol (PCEP)", RFC 9826, DOI 10.17487/RFC9826, September 2025, . Appendix A. Examples A.1. SR Policy Candidate Path with Multiple Segment Lists Consider the following sample SR Policy. Koldychev & Sidor Expires 7 December 2026 [Page 28] Internet-Draft PCEP Extensions for Multipath June 2026 SR policy POL1 Candidate Path CP1 Preference 200 Weight W1, SID-List1 Weight W2, SID-List2 Candidate Path CP2 Preference 100 Weight W3, SID-List3 Weight W4, SID-List4 As specified in [RFC9862], CP1 and CP2 are signaled as separate state-report elements and each has a unique PLSP-ID, assigned by the PCC. For this example, PLSP-ID 100 is assigned to CP1 and PLSP-ID 200 to CP2. The state-report (as defined in [RFC8231]) for CP1 can be encoded as: = > > The state-report for CP2 can be encoded as: = > > The above sample state-report elements only specify the minimum mandatory objects, of course other objects like SRP, LSPA, METRIC, etc., are allowed to be inserted. Note that the syntax > Koldychev & Sidor Expires 7 December 2026 [Page 29] Internet-Draft PCEP Extensions for Multipath June 2026 means that this is PATH-ATTRIB object with Path ID field set to 1 and with a MULTIPATH-WEIGHT TLV carrying weight of "W1". A.2. Composite Candidate Path Consider the following Composite Candidate Path. SR policy POL100 Candidate Path CP1 Preference 200 Weight W1, SR policy Weight W2, SR policy This is signaled in PCEP as: > > A.3. Opposite Direction Tunnels Consider the two opposite-direction SR Policies between endpoints H1 and E1. Koldychev & Sidor Expires 7 December 2026 [Page 30] Internet-Draft PCEP Extensions for Multipath June 2026 SR policy POL1 Candidate Path CP1 Preference 200 Bidirectional Association = A1 SID-List = SID-List = Candidate Path CP2 Preference 100 Bidirectional Association = A2 SID-List = SID-List = SR policy POL2 Candidate Path CP1 Preference 200 Bidirectional Association = A1 SID-List = SID-List = Candidate Path CP2 Preference 100 Bidirectional Association = A2 SID-List = The state-report for POL1, CP1 can be encoded as: = > > > > > > > > The state-report for POL1, CP2 can be encoded as: Koldychev & Sidor Expires 7 December 2026 [Page 31] Internet-Draft PCEP Extensions for Multipath June 2026 = > > > > > The state-report for POL2, CP1 can be encoded as: = > > > > > > > > The state-report for POL2, CP2 can be encoded as: = > > > > > Authors' Addresses Mike Koldychev (editor) Ciena Corporation Koldychev & Sidor Expires 7 December 2026 [Page 32] Internet-Draft PCEP Extensions for Multipath June 2026 Email: mkoldych@ciena.com Samuel Sidor (editor) Cisco Systems. Email: ssidor@cisco.com Koldychev & Sidor Expires 7 December 2026 [Page 33]