MSD Consideration in Path Computation Element Communication Protocol (PCEP)
draft-ali-pce-sr-policy-msd-consideration-00
This document is an Internet-Draft (I-D).
Anyone may submit an I-D to the IETF.
This I-D is not endorsed by the IETF and has no formal standing in the
IETF standards process.
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Zafar Ali , Andrew Stone , Diego Achaval , Samuel Sidor | ||
| Last updated | 2025-07-07 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ali-pce-sr-policy-msd-consideration-00
PCE Working Group Z. Ali
Internet-Draft Cisco Systems, Inc.
Intended status: Standards Track A. Stone
Expires: 8 January 2026 D. Achaval
Nokia
S. Sidor
Cisco Systems, Inc.
7 July 2025
MSD Consideration in Path Computation Element Communication Protocol
(PCEP)
draft-ali-pce-sr-policy-msd-consideration-00
Abstract
Segment Routing (SR) allows a node to steer a packet flow along any
path. SR Policy is an ordered list of segments (i.e., instructions)
that represent a source-routed policy. The packets steered into an
SR Policy carry an ordered list of segments associated with that SR
Policy. An SR Policy can be instantiated SR-MPLS and SRv6 data
planes.
Maximum SID Depth (MSD) specifies the maximum number of SIDs that a
Path Computation Client (PCC) is capable of imposing on a packet.
The number of SIDs in an SR-TE path computed by the PCE on behalf of
a PCC is dictated by the MSD value at the PCC. This draft specifies
some MSD considerations PCE needs to take into account when computing
the number of SIDs in an SR-TE path.
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.
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/.
Ali, et al. Expires 8 January 2026 [Page 1]
Internet-Draft PCE MSD consideration July 2025
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 8 January 2026.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include 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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview of PCEP Extensions . . . . . . . . . . . . . . . . . 3
3.1. New flag in SR-PCE-CAPABILITY sub-TLV . . . . . . . . . . 3
3.2. New flag in SRv6-PCE-CAPABILITY sub-TLV . . . . . . . . . 4
4. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Backward compatibility . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 4
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
8.1. Normative References . . . . . . . . . . . . . . . . . . 4
8.2. Informative References . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Terminology
This document uses the following terms defined in [RFC5440]: PCC,
PCE, PCEP.
SR: Segment Routing.
SID: Segment Identifier.
SRv6: Segment Routing over IPv6 data plane.
Ali, et al. Expires 8 January 2026 [Page 2]
Internet-Draft PCE MSD consideration July 2025
2. Introduction
Segment Routing (SR) [RFC8402] allows a node to steer a packet flow
along any path. A Segment Routing Policy (SR Policy) [RFC8402] is an
ordered list of segments that represent a source-routed policy. The
headend node is said to steer a flow into an SR Policy. The packets
steered into an SR Policy have an ordered list of segments associated
with that SR Policy written into them. Segment Routing Policy
Architecture [RFC9256] updates [RFC8402] as it details the concepts
of SR Policy and steering into an SR Policy. An SR Policy can be
instantiated SR-MPLS and SRv6 data planes.
Maximum SID Depth (MSD) specifies the maximum number of SIDs that a
Path Computation Client (PCC) is capable of imposing on a packet.
The number of SIDs in an SR-TE path computed by the PCE on behalf of
a PCC is dictated by the MSD value at the PCC.
[RFC8664] defines the SR-PCE-CAPABILITY sub-TLV. PCEP speakers use
this sub-TLV to exchange information about their SR capability,
including MSD, which indicates that a PCC is capable of imposing on a
packet. [RFC8664] also specifies MSD considerations PCE needs to
take into account when computing the number of SIDs in an SR-TE path.
Specifically, it mandates that once an SR-capable PCEP session is
established with a non-zero MSD value, the corresponding PCE MUST NOT
send SR-TE paths with a number of SIDs exceeding that MSD value.
However, when an adjacency SID is the first SID in an SR Policy SID
list, the top adjacency SID is not imposed on the packet.
This draft specifies a procedure for optimizing the number of SIDs in
an SR-TE path that PCE can compute when the first SID in the SR
Policy SID list is an adjacency SID. The procedure applies to the
SR-MPLS data plane and SRv6 data plane. The procedure is backward
compatible with [RFC8664].
3. Overview of PCEP Extensions
3.1. New flag in SR-PCE-CAPABILITY sub-TLV
A-flag (Adjacency SID exclusion for MSD consideration flag) is
proposed in the SR-PCE-CAPABILITY sub-TLV defined in [RFC8664]. The
bit position for the flag in the SR PCE Capability Flag Field
registry is to be defined by IANA.
A (Adjacency SID exclusion for MSD consideration flag) - 1 bit (Bit
Position TBD1):
* If set to 1, it indicates support for the A-flag by the PCEP peer.
Ali, et al. Expires 8 January 2026 [Page 3]
Internet-Draft PCE MSD consideration July 2025
3.2. New flag in SRv6-PCE-CAPABILITY sub-TLV
A-flag (Adjacency SID exclusion for MSD consideration flag) is
proposed in the SRv6-PCE-CAPABILITY sub-TLV defined in [RFC9603].
The bit position for the flag in the SRv6 Capability Flag Field
registry is to be defined by IANA.
A (Adjacency SID exclusion for MSD consideration flag) - 1 bit (Bit
Position TBD1):
* If set to 1, it indicates support for the A-flag by the PCEP peer.
4. Operation
[RFC8664] mandates that once an SR-capable PCEP session is
established with a non-zero MSD value, the corresponding PCE MUST NOT
send SR-TE paths with a number of SIDs exceeding that MSD value.
This procedure MUST only be applied if both the PCE and PCC have
advertised support for the capability by setting the A-flag in their
respective SR-PCE-CAPABILITY sub-TLV [[RFC8664]] or SRv6-PCE-
CAPABILITY sub-TLVs [[RFC9603]]. Under these conditions, if the
first SID in an SR-TE path is an adjacency SID, the PCE MUST NOT send
SR-TE paths with a number of SIDs exceeding that (MSD+1) value.
5. Backward compatibility
The proposed procedure is backward compatible with [RFC8664] as it
requires both PCE and PCC to support the optimization capabilities
during the PCEP initialization phase by setting the A-flag in the SR-
PCE-CAPABILITY and SRv6-PCE-CAPABILITY sub-TLV in the Open message.
Specifically, if at least one PCEP peer is not capable of supporting
the A-flag, the PCE MUST NOT send SR-TE paths with a number of SIDs
exceeding that MSD value.
6. Security Considerations
TBA
7. IANA Considerations
TBA
8. References
8.1. Normative References
Ali, et al. Expires 8 January 2026 [Page 4]
Internet-Draft PCE MSD consideration July 2025
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
DOI 10.17487/RFC5440, March 2009,
<https://www.rfc-editor.org/info/rfc5440>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[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/info/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/info/rfc9256>.
[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/info/rfc9603>.
8.2. Informative References
Authors' Addresses
Zafar Ali
Cisco Systems, Inc.
Email: zali@cisco.com
Andrew Stone
Nokia
Email: andrew.stone@nokia.com
Ali, et al. Expires 8 January 2026 [Page 5]
Internet-Draft PCE MSD consideration July 2025
Diego Achaval
Nokia
Email: diego.achaval@nokia.com
Samuel Sidor
Cisco Systems, Inc.
Email: ssidor@cisco.com
Ali, et al. Expires 8 January 2026 [Page 6]