Path Computation Element Communication Protocol (PCEP) Extensions for Signaling Multipath Information
draft-ietf-pce-multipath-27
| Document | Type | Active Internet-Draft (pce WG) | |
|---|---|---|---|
| Authors | Mike Koldychev , Samuel Sidor | ||
| Last updated | 2026-06-15 (Latest revision 2026-06-09) | ||
| Replaces | draft-koldychev-pce-multipath | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Associated WG milestone |
|
||
| Document shepherd | Adrian Farrel | ||
| Shepherd write-up | Show Last changed 2026-04-07 | ||
| IESG | IESG state | IESG Evaluation | |
| Action Holder | |||
| Consensus boilerplate | Yes | ||
| Telechat date |
On agenda of 2026-06-18 IESG telechat
Has a DISCUSS. Needs 6 more YES or NO OBJECTION positions to pass. |
||
| Responsible AD | Ketan Talaulikar | ||
| Send notices to | adrian@olddog.co.uk | ||
| IANA | IANA review state | IANA OK - Actions Needed |
draft-ietf-pce-multipath-27
PCE Working Group M. Koldychev, Ed.
Internet-Draft Ciena Corporation
Intended status: Standards Track S. Sidor, Ed.
Expires: 11 December 2026 Cisco Systems.
9 June 2026
Path Computation Element Communication Protocol (PCEP) Extensions for
Signaling Multipath Information
draft-ietf-pce-multipath-27
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 11 December 2026.
Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
Koldychev & Sidor Expires 11 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 . . . . . . . . . . . . . . . . . . . . . . . 18
6.3. Huawei Technologies . . . . . . . . . . . . . . . . . . . 18
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7.1. PCEP Object . . . . . . . . . . . . . . . . . . . . . . . 18
7.2. PCEP TLV . . . . . . . . . . . . . . . . . . . . . . . . 19
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 11 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 11 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 11 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
For example, 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 11 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 <intended-path>/<actual-path> 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 11 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 11 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 <intended-path>/<actual-
path>, 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 11 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 11 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 11 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 11 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 11 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"). When a PCE
receives a PCRpt message containing more paths than its own
computation capability, it MUST NOT treat this as an error condition;
PCRpt conveys PCC-reported operational state and is not subject to
the PCE's computation limits.
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.
Koldychev & Sidor Expires 11 December 2026 [Page 13]
Internet-Draft PCEP Extensions for Multipath June 2026
* 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).
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 11 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 11 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 <intended-path> and/or <actual-path>. 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 <intended-path> or <actual-path>
constructs.
As specified in Section 6.1 of [RFC8231], within the context of
messages that use these constructs, <intended-path> is represented by
the ERO and <actual-path> is represented by the RRO:
<intended-path> ::= <ERO>
<actual-path> ::= <RRO>
This document extends [RFC8231] by allowing multiple EROs/RROs to be
present in the <intended-path>/<actual-path>:
<intended-path> ::= <ERO> |
<PATH-ATTRIB><ERO>[<intended-path-multipath>]
<intended-path-multipath> ::= <PATH-ATTRIB><ERO>
[<intended-path-multipath>]
<actual-path> ::= <RRO> |
<PATH-ATTRIB><RRO>[<actual-path-multipath>]
<actual-path-multipath> ::= <PATH-ATTRIB><RRO>
[<actual-path-multipath>]
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 11 December 2026 [Page 16]
Internet-Draft PCEP Extensions for Multipath June 2026
<PCE-initiated-lsp-instantiation> ::= <SRP>
<LSP>
[<END-POINTS>]
<intended-path>
[<attribute-list>]
where <intended-path> 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.
The RBNF extensions defined in this section are additive and apply
only to sessions where the capabilities from the MULTIPATH-CAP TLV
have been successfully negotiated (Section 4). Legacy PCEP peers
that do not advertise the MULTIPATH-CAP TLV will not receive PATH-
ATTRIB objects introduced by the updated RBNF.
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
Koldychev & Sidor Expires 11 December 2026 [Page 17]
Internet-Draft PCEP Extensions for Multipath June 2026
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
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].
Koldychev & Sidor Expires 11 December 2026 [Page 18]
Internet-Draft PCEP Extensions for Multipath June 2026
7.2. PCEP TLV
IANA is requested to confirm the following allocations in the "PCEP
TLV Type Indicators" registry:
+------------+-----------------------------------+-----------------+
| 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 11 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 11 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 11 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 11 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 11 December 2026 [Page 23]
Internet-Draft PCEP Extensions for Multipath June 2026
PCE implementations should minimize unnecessary churn in multipath
state. With multiple paths per LSP, frequent re-computation or
weight changes cause flow redistribution across Segment Lists, which
may perturb congestion control state and trigger PMTUD re-discovery
on a broader scale than with single-path LSPs.
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 11 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 11 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, <https://datatracker.ietf.org/doc/html/
draft-ietf-pce-pceps-tls13-04>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
DOI 10.17487/RFC5440, March 2009,
<https://www.rfc-editor.org/rfc/rfc5440>.
[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, <https://www.rfc-editor.org/rfc/rfc5511>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8231] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
Computation Element Communication Protocol (PCEP)
Extensions for Stateful PCE", RFC 8231,
DOI 10.17487/RFC8231, September 2017,
<https://www.rfc-editor.org/rfc/rfc8231>.
[RFC8253] Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
"PCEPS: Usage of TLS to Provide a Secure Transport for the
Path Computation Element Communication Protocol (PCEP)",
RFC 8253, DOI 10.17487/RFC8253, October 2017,
<https://www.rfc-editor.org/rfc/rfc8253>.
[RFC8281] Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path
Computation Element Communication Protocol (PCEP)
Extensions for PCE-Initiated LSP Setup in a Stateful PCE
Model", RFC 8281, DOI 10.17487/RFC8281, December 2017,
<https://www.rfc-editor.org/rfc/rfc8281>.
Koldychev & Sidor Expires 11 December 2026 [Page 26]
Internet-Draft PCEP Extensions for Multipath June 2026
[RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "Path Computation Element Communication
Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
DOI 10.17487/RFC8664, December 2019,
<https://www.rfc-editor.org/rfc/rfc8664>.
[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,
<https://www.rfc-editor.org/rfc/rfc9256>.
[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, <https://www.rfc-editor.org/rfc/rfc9325>.
[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,
<https://www.rfc-editor.org/rfc/rfc9603>.
[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,
<https://www.rfc-editor.org/rfc/rfc9862>.
[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, <https://www.rfc-editor.org/rfc/rfc9863>.
12.2. Informative References
[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,
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-sr-
bidir-path-25>.
Koldychev & Sidor Expires 11 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, <https://datatracker.ietf.org/doc/html/
draft-ietf-spring-cs-sr-policy-17>.
[RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture", RFC 4655,
DOI 10.17487/RFC4655, August 2006,
<https://www.rfc-editor.org/rfc/rfc4655>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/rfc/rfc7942>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/rfc/rfc8126>.
[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,
<https://www.rfc-editor.org/rfc/rfc9059>.
[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,
<https://www.rfc-editor.org/rfc/rfc9826>.
Appendix A. Examples
A.1. SR Policy Candidate Path with Multiple Segment Lists
Consider the following sample SR Policy.
Koldychev & Sidor Expires 11 December 2026 [Page 28]
Internet-Draft PCEP Extensions for Multipath June 2026
SR policy POL1 <headend, color, endpoint>
Candidate Path CP1 <protocol-origin = 20, originator-asn = 100,
originator-address = 192.0.2.1,
discriminator = 1>
Preference 200
Weight W1, SID-List1 <SID11...SID1i>
Weight W2, SID-List2 <SID21...SID2j>
Candidate Path CP2 <protocol-origin = 20, originator-asn = 100,
originator-address = 198.51.100.1,
discriminator = 2>
Preference 100
Weight W3, SID-List3 <SID31...SID3i>
Weight W4, SID-List4 <SID41...SID4j>
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:
<state-report> =
<LSP PLSP-ID=100>
<ASSOCIATION>
<PATH-ATTRIB Path ID=1 <WEIGHT-TLV Weight=W1>>
<ERO SID-List1>
<PATH-ATTRIB Path ID=2 <WEIGHT-TLV Weight=W2>>
<ERO SID-List2>
The state-report for CP2 can be encoded as:
<state-report> =
<LSP PLSP-ID=200>
<ASSOCIATION>
<PATH-ATTRIB Path ID=1 <WEIGHT-TLV Weight=W3>>
<ERO SID-List3>
<PATH-ATTRIB Path ID=2 <WEIGHT-TLV Weight=W4>>
<ERO SID-List4>
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
<PATH-ATTRIB Path ID=1 <WEIGHT-TLV Weight=W1>>
Koldychev & Sidor Expires 11 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 <headend = H1, color = 100, endpoint = E1>
Candidate Path CP1 <protocol-origin = 20, originator-asn = 100,
originator-address = 192.0.2.1,
discriminator = 1>
Preference 200
Weight W1, SR policy <color = 1>
Weight W2, SR policy <color = 2>
This is signaled in PCEP as:
<LSP PLSP-ID=100>
<ASSOCIATION>
<PATH-ATTRIB Path ID=1
<WEIGHT-TLV Weight=W1>
<COLOR-TLV Color=1>>
<ERO (empty)>
<PATH-ATTRIB Path ID=2
<WEIGHT-TLV Weight=W2>
<COLOR-TLV Color=2>>
<ERO (empty)>
A.3. Opposite Direction Tunnels
Consider the two opposite-direction SR Policies between endpoints H1
and E1.
Koldychev & Sidor Expires 11 December 2026 [Page 30]
Internet-Draft PCEP Extensions for Multipath June 2026
SR policy POL1 <headend = H1, color, endpoint = E1>
Candidate Path CP1
Preference 200
Bidirectional Association = A1
SID-List = <H1,M1,M2,E1>
SID-List = <H1,M3,M4,E1>
Candidate Path CP2
Preference 100
Bidirectional Association = A2
SID-List = <H1,M5,M6,E1>
SID-List = <H1,M7,M8,E1>
SR policy POL2 <headend = E1, color, endpoint = H1>
Candidate Path CP1
Preference 200
Bidirectional Association = A1
SID-List = <E1,M2,M1,H1>
SID-List = <E1,M4,M3,H1>
Candidate Path CP2
Preference 100
Bidirectional Association = A2
SID-List = <E1,M6,M5,H1>
The state-report for POL1, CP1 can be encoded as:
<state-report> =
<LSP PLSP-ID=100>
<BIDIRECTIONAL ASSOCIATION = A1>
<PATH-ATTRIB Path ID=1 R-flag=0
<OPPDIR-PATH-TLV OppositePath ID=3>>
<ERO <H1,M1,M2,E1>>
<PATH-ATTRIB Path ID=2 R-flag=0
<OPPDIR-PATH-TLV OppositePath ID=4>>
<ERO <H1,M3,M4,E1>>
<PATH-ATTRIB Path ID=3 R-flag=1
<OPPDIR-PATH-TLV OppositePath ID=1>>
<ERO <E1,M2,M1,H1>>
<PATH-ATTRIB Path ID=4 R-flag=1
<OPPDIR-PATH-TLV OppositePath ID=2>>
<ERO <E1,M4,M3,H1>>
The state-report for POL1, CP2 can be encoded as:
Koldychev & Sidor Expires 11 December 2026 [Page 31]
Internet-Draft PCEP Extensions for Multipath June 2026
<state-report> =
<LSP PLSP-ID=200>
<BIDIRECTIONAL ASSOCIATION = A2>
<PATH-ATTRIB Path ID=1 R-flag=0
<OPPDIR-PATH-TLV OppositePath ID=3>>
<ERO <H1,M5,M6,E1>>
<PATH-ATTRIB Path ID=2 R-flag=0>
<ERO <H1,M7,M8,E1>>
<PATH-ATTRIB Path ID=3 R-flag=1
<OPPDIR-PATH-TLV OppositePath ID=1>>
<ERO <E1,M6,M5,H1>>
The state-report for POL2, CP1 can be encoded as:
<state-report> =
<LSP PLSP-ID=100>
<BIDIRECTIONAL ASSOCIATION = A1>
<PATH-ATTRIB Path ID=1 R-flag=0
<OPPDIR-PATH-TLV OppositePath ID=3>>
<ERO <E1,M2,M1,H1>>
<PATH-ATTRIB Path ID=2 R-flag=0
<OPPDIR-PATH-TLV OppositePath ID=4>>
<ERO <E1,M4,M3,H1>>
<PATH-ATTRIB Path ID=3 R-flag=1
<OPPDIR-PATH-TLV OppositePath ID=1>>
<ERO <H1,M1,M2,E1>>
<PATH-ATTRIB Path ID=4 R-flag=1
<OPPDIR-PATH-TLV OppositePath ID=2>>
<ERO <H1,M3,M4,E1>>
The state-report for POL2, CP2 can be encoded as:
<state-report> =
<LSP PLSP-ID=200>
<BIDIRECTIONAL ASSOCIATION = A2>
<PATH-ATTRIB Path ID=1 R-flag=0
<OPPDIR-PATH-TLV OppositePath ID=3>>
<ERO <E1,M6,M5,H1>>
<PATH-ATTRIB Path ID=2 R-flag=1>
<ERO <H1,M7,M8,E1>>
<PATH-ATTRIB Path ID=3 R-flag=1
<OPPDIR-PATH-TLV OppositePath ID=1>>
<ERO <H1,M5,M6,E1>>
Authors' Addresses
Mike Koldychev (editor)
Ciena Corporation
Koldychev & Sidor Expires 11 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 11 December 2026 [Page 33]