Path MTU (PMTU) for Segment Routing (SR) Policy
draft-ietf-spring-pmtu-sr-policy-03
| Document | Type | Active Internet-Draft (spring WG) | |
|---|---|---|---|
| Authors | Shuping Peng , Dhruv Dhody , Ketan Talaulikar , Gyan Mishra | ||
| Last updated | 2025-09-05 | ||
| Replaces | draft-peng-spring-pmtu-sr-policy | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-spring-pmtu-sr-policy-03
SPRING Working Group S. Peng
Internet-Draft D. Dhody
Intended status: Standards Track Huawei
Expires: 9 March 2026 K. Talaulikar
Cisco Systems
G. Mishra
Verizon Inc.
5 September 2025
Path MTU (PMTU) for Segment Routing (SR) Policy
draft-ietf-spring-pmtu-sr-policy-03
Abstract
This document defines the Path MTU (PMTU) for Segment Routing (SR)
Policy (called SR-PMTU). It applies to both Segment Routing over
IPv6 (SRv6) and SR-MPLS. This document specifies the framework of
SR-PMTU for SR Policy including the link MTU collection, the SR-PMTU
computation, the SR-PMTU enforcement, and the handling behaviours on
the headend.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 9 March 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
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and restrictions with respect to this document. Code Components
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. SR-PMTU Definition for SR Policy . . . . . . . . . . . . . . 4
4.1. SR-PMTU of a Segment List . . . . . . . . . . . . . . . . 4
4.2. SR-PMTU of a Candidate Path . . . . . . . . . . . . . . . 4
4.3. SR-PMTU of an SR Policy . . . . . . . . . . . . . . . . . 5
5. The Framework of SR-PMTU for SR Policy . . . . . . . . . . . 5
5.1. Link MTU Collection . . . . . . . . . . . . . . . . . . . 5
5.2. SR-PMTU Computation . . . . . . . . . . . . . . . . . . . 6
5.2.1. Loose Path . . . . . . . . . . . . . . . . . . . . . 6
5.2.2. Strict TE Path . . . . . . . . . . . . . . . . . . . 6
5.2.3. Mixed Path . . . . . . . . . . . . . . . . . . . . . 6
5.2.4. Binding Path . . . . . . . . . . . . . . . . . . . . 6
5.2.5. TI-LFA . . . . . . . . . . . . . . . . . . . . . . . 7
5.3. SR-PMTU Enforcement . . . . . . . . . . . . . . . . . . . 7
5.4. Handling behaviors on the headend . . . . . . . . . . . . 8
5.4.1. SR-PMTU Constraints and Optimization . . . . . . . . 8
5.4.2. Fragmentation processing . . . . . . . . . . . . . . 8
5.5. Link MTU Change . . . . . . . . . . . . . . . . . . . . . 9
6. SRv6-Specific Handling . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Segment Routing (SR) [RFC8402] allows a node to steer a packet flow
along any given path. The headend is a node where the instructions
for source routing (i.e., segments) are encoded in the packet and
hence becomes the starting node for a specific segment routing path.
Intermediate per-path states are eliminated thanks to source routing.
A Segment Routing Policy (SR Policy) [RFC9256] is an ordered list of
segments (i.e., instructions) that represent a source-routed policy.
The headend node is said to steer a flow into a SR Policy. The
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packets steered into an SR Policy have an ordered list of segments
associated with that SR Policy written into them. [RFC8660]
describes the representation and processing of this ordered list of
segments as an MPLS label stack for SR-MPLS, while [RFC8754] and
[RFC8986] describe the same for Segment Routing over IPv6 (SRv6) with
the use of the Segment Routing Header (SRH).
[RFC8402] introduces the SR Policy construct and provides an overview
of how it is leveraged for Segment Routing use-cases. [RFC9256]
updates [RFC8402] to specify detailed concepts of SR Policy and
steering packets into an SR Policy.
This document extends the SR Policy to also include the Path MTU
information to SR Policy and applies to both SRv6 and SR-MPLS. The
SRv6-specific handling is specified in Section 6.
1.1. Motivation
The motivation for handling SR-PMTU for the SR paths includes (but is
not limited to):
* Being able to avoid fragmentation by being aware of the SR-PMTU
associated with the SR paths and policies at the headend.
* Being able to generate ICMP messages at the headend.
* When fragmentation is unavoidable, the ability to do it correctly
at the headend.
* Ability to use SR-PMTU as path computation constraint and
optimization criteria at the headend or controller/PCE.
2. 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.
3. Terminology
Link MTU: As per [RFC8899], this could more properly be called the
IP MTU. It is the size in bytes of the largest IP packet,
including the IP header and payload, that can be transmitted over
a link. In case of MPLS, it also includes the label stack, and in
case of IPv6, it includes IPv6 extension headers (including SRH).
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Path MTU (PMTU): See [RFC8899]. In the scope of this document,
this is also called SR-PMTU for the SR paths and policies.
SR overhead: The SR-MPLS label stack or SRH. The link MTU takes
the SR overhead into consideration.
4. SR-PMTU Definition for SR Policy
The Segment Routing policy architecture is specified in [RFC9256].
An SR Policy is associated with one or more candidate paths. A
candidate path is either dynamic, explicit, or composite. The
related concepts with the SR-PMTU definition in this document are
listed as follows.
An explicit/dynamic candidate path is expressed as a Segment-List or
a set of Segment-Lists directly or by computation. If a candidate
path is associated with a set of Segment-Lists, each Segment-List is
associated with weight for weighted load balancing. The default
weight is 1.
A composite candidate path is defined in [RFC9256].
4.1. SR-PMTU of a Segment List
A Segment-List represents a specific source-routed path to send
traffic from the headend to the endpoint of the corresponding SR
policy [RFC9256]. The SR-PMTU of a segment list is defined as the
minimum Link MTU of all the links in a path between a source node and
a destination node. Refer to Section 5.2 for specific handling for
Node, Adjacency and Binding SID (as well as their combinations).
4.2. SR-PMTU of a Candidate Path
In the case of an explicit/dynamic candidate path, if it is expressed
as a single Segment-List, then the SR-PMTU of the candidate path is
the same as that of the SR-PMTU of the segment list as described in
Section 4.1.
In the case of an explicit/dynamic candidate path, if it is expressed
as a set of Segment-Lists (for load-balancing), then the SR-PMTU of
the candidate path is defined as the minimum SR-PMTU of all the
Segment-Lists in the set.
In the case of a composite candidate path, the SR-PMTU is defined as
the minimum SR-PMTU of all the constituent SR Policies.
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4.3. SR-PMTU of an SR Policy
According to [RFC9256], an SR Policy is associated with one or more
candidate paths. A candidate path is selected when it is valid and
it is determined to be the best path of the SR Policy. The selected
path is referred to as the "active path" of the SR policy. Then the
SR-PMTU for an SR Policy is defined as the SR-PMTU of the selected/
active candidate path of this SR policy.
In the case of an explicit/dynamic candidate path, the SR-PMTU
definition can be referred to in Section 4.2.
In the case of a composite candidate path, the SR-PMTU is defined as
the minimum SR-PMTU of all the constituent SR policies. Since the
constituent SR Policies of a composite candidate path can only be
explicit/dynamic candidate paths, then the SR-PMTU definition of
explicit/dynamic candidate path is as per Section 4.2.
5. The Framework of SR-PMTU for SR Policy
The framework of SR-PMTU for SR Policy includes link MTU collection,
SR-PMTU computation, SR-PMTU enforcement, and handling behaviors on
the headend.
+------------------+
+--------|Network Controller| SR-PMTU computation
| +--------/|\-------+
| |
SR-PMTU enforcement Link MTU Collection
| |
+-\|/-+ +-----------|-----------+ +-----+
Handling |Head |---| Segment Routing |---|End |
behaviors |end | | Network Domain | |Point|
+-----+ +-----------------------+ +-----+
<---------Link MTU collection---------|
Figure 1. The Framework of SR-PMTU for SR Policy
5.1. Link MTU Collection
The SR-PMTU of a segment list is defined as the minimum Link MTU of
all the links in a path, see Section 4.1. The Link MTU can be
collected in network through various mechanisms such as the ones
defined in [I-D.hu-lsr-igp-path-mtu] and
[I-D.ietf-idr-bgp-ls-link-mtu] without the knowledge of the services.
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5.2. SR-PMTU Computation
The collected link MTU of all the related links are sent to the
network controller or the headend where the SR-PMTU is computed.
Depending upon the path type, the computation methods are different,
which are described in the following subsections.
5.2.1. Loose Path
In a loose path [RFC7855], only Node SIDs are used along the path.
Between two adjacent Node-SIDs, generally, there are equal-cost
multipaths (ECMP). The SR-PMTU of the loose path is computed by
finding the minimum SR-PMTU of all the ECMPs between two adjacent
Node SIDs along the loose path.
5.2.2. Strict TE Path
In a strict TE path [RFC7855], only Adj-SIDs are used along the path.
Since the link MTU of all the links being indicated by the Adj-SIDs
of the strict TE path are known to the network controller, the SR-
PMTU of the strict SR-TE path is computed by finding out the minimum
link MTU of all the links in the strict SR-TE path between its source
node and destination node.
5.2.3. Mixed Path
In a mixed path, both Node SIDs-and Adj-SIDs are used along the path.
The PMTU of the mixed TE path is computed by finding the minimum SR-
PMTU of all the ECMPs between two adjacent Node SIDs and the link MTU
of all the links indicated by the Adj SIDs.
5.2.4. Binding Path
The Binding SID (BSID) [RFC8402] is bound to an SR Policy,
instantiation of which may involve a list of SIDs. The SR-PMTU of
the binding path is the same as that of an SR Policy as specified in
the above section modulo that it also includes the encapsulation
overhead associated with it (i.e. the additional label stack pushed
in case of SR-MPLS and the outer IPv6 header with its own SRH in case
of SRv6). This is done to make sure the headend of the SR path that
includes a BSID is able to compute the SR-PMTU correctly by taking
the correct SR-PMTU of the binding path into consideration along with
other SIDs in the SR path.
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5.2.5. TI-LFA
Topology Independent Loop-free Alternate Fast Re-route (TI-LFA)
[I-D.ietf-rtgwg-segment-routing-ti-lfa], aims to provide protection
for node and adjacency segments within the SR framework. The PMTU of
the repair path might be different from the original path's, which
could lead to fragmentation while the repair path is in use.
To avoid fragmentation, it is possible for the headend (or
controller) to consider the FRR overhead when computing the SR-PMTU
of the original path.
5.3. SR-PMTU Enforcement
SR Policy as per [RFC9256] does not include SR-PMTU in the SR Policy
encoding structure. As specified in
[I-D.ietf-idr-sr-policy-path-mtu], the SR-PMTU is encoded in the SR
policy structure as shown in Figure 2. After the SR-PMTU
computation, the SR-PMTU is enforced along with the SR Policy to the
headend of the corresponding path.
SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
Attributes:
Tunnel Encaps Attribute (23)
Tunnel Type: SR Policy
Binding SID
Preference
Priority
Policy Name
Explicit NULL Label Policy (ENLP)
Segment List
Weight
----> Path MTU (SR-PMTU)
Segment
Segment
...
...
Figure 2. The SR Policy encoding structure with SR-PMTU
When there are multiple paths that can be selected, the one with the
highest SR-PMTU will be used in order to avoid fragmentation on the
headend.
The PCEP extension to handle PMTU is specified in
[I-D.ietf-pce-pcep-pmtu].
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5.4. Handling behaviors on the headend
After the SR-PMTU is computed, the headend performs the handling
behaviors such as encapsulation and fragmentation, if needed. Note
that this behavior is similar to the existing behaviors of MPLS and
IPv6 dataplane.
5.4.1. SR-PMTU Constraints and Optimization
Generally, considering the services being carried, the operators set
an SR-PMTU constraint aiming for a proper path selection that
fulfills packet size requirements hence avoiding fragmentation.
Furthermore, the encapsulation on the headend will introduce the
overhead on top of the packet to be encapsulated. Generally, the
encapsulation overhead has to be estimated according to the possible
path hops and sometimes the repair paths. Therefore, the SR-PMTU
constraint is set considering both the carried services and the
encapsulation overhead.
When SR-PMTU-based path optimization is done, PCE will select the
path with the highest SR-PMTU among all the possible paths.
Even if the SR-PMTU is not considered by the PCE at the time of path
computation, the computed SR-PMTU is useful at the headend for the
reasons already stated in Section 1.1.
Once the SR-PMTU constraint is set on the headend, it is supposed to
be the lowest bound of the SR-PMTUs of all the paths being computed
locally or enforced by the controller in order to avoid
fragmentation.
5.4.2. Fragmentation processing
If the SR-PMTU of all the paths being computed locally or enforced by
the controller is smaller than the SR-PMTU constraint set on the
headend, the fragmentation will have to be handled. If fragmentation
is not possible, the headend could generate the ICMP messages
[RFC4443] to notify the traffic source.
Over this selected path, on the headend, the packets are fragmented
in order to guarantee the size of the encapsulated packets is smaller
than the PMTU of the selected path.
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5.5. Link MTU Change
The Link MTU collected as described in Section 5.1 may change over
time due to factors such as device configuration updates or
topological modifications, such as the addition of a new link with a
lower MTU. These changes can impact the SR-PMTU of the data path,
and the computed SR-PMTU value may remain outdated until the control
plane converges. This behavior is similar to changes in other link
metrics.
6. SRv6-Specific Handling
In the case of SRv6, the SRH is included in the calculation of the
Link MTU and thus in the SR-PMTU. Note that the PMTU considerations
for IPv6 [RFC8201] apply for the SRv6. [RFC8754] also specify the
MTU considerations related to encapsulation with an outer IPv6 header
with SRH.
7. Security Considerations
[RFC9256] specifies in detail the SR Policy construct (introduced in
[RFC8402]) and its security considerations. The additional SR-MTU
attribute information can be sensitive in some deployments and could
be used to influence SR path setup and selection with adverse effect.
The protocol extensions that include SR-PMTU need to take this into
consideration. This document does not define any new protocol
extensions and thus does not introduce any further security
considerations.
8. IANA Considerations
This document does not include any IANA requests.
9. Acknowledgement
Thanks to xx for useful discussions and comments.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[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>.
[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>.
[RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing with the MPLS Data Plane", RFC 8660,
DOI 10.17487/RFC8660, December 2019,
<https://www.rfc-editor.org/info/rfc8660>.
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/info/rfc8986>.
[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>.
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.
[RFC8899] Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
Völker, "Packetization Layer Path MTU Discovery for
Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
September 2020, <https://www.rfc-editor.org/info/rfc8899>.
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[I-D.ietf-rtgwg-segment-routing-ti-lfa]
Bashandy, A., Litkowski, S., Filsfils, C., Francois, P.,
Decraene, B., and D. Voyer, "Topology Independent Fast
Reroute using Segment Routing", Work in Progress,
Internet-Draft, draft-ietf-rtgwg-segment-routing-ti-lfa-
21, 12 February 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-rtgwg-
segment-routing-ti-lfa-21>.
10.2. Informative References
[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<https://www.rfc-editor.org/info/rfc4443>.
[RFC7855] Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
Litkowski, S., Horneffer, M., and R. Shakir, "Source
Packet Routing in Networking (SPRING) Problem Statement
and Requirements", RFC 7855, DOI 10.17487/RFC7855, May
2016, <https://www.rfc-editor.org/info/rfc7855>.
[I-D.ietf-idr-bgp-ls-link-mtu]
Zhu, Y., Hu, Z., Peng, S., and R. Mwehair, "Signaling
Maximum Transmission Unit (MTU) using BGP-LS", Work in
Progress, Internet-Draft, draft-ietf-idr-bgp-ls-link-mtu-
09, 21 March 2025, <https://datatracker.ietf.org/doc/html/
draft-ietf-idr-bgp-ls-link-mtu-09>.
[I-D.hu-lsr-igp-path-mtu]
Hu, Z., Peng, S., and X. Xi, "IGP Extensions for Path
MTU", Work in Progress, Internet-Draft, draft-hu-lsr-igp-
path-mtu-00, 19 October 2021,
<https://datatracker.ietf.org/doc/html/draft-hu-lsr-igp-
path-mtu-00>.
[I-D.ietf-idr-sr-policy-path-mtu]
Li, C., Zhu, Y., El Sawaf, A., and Z. Li, "Segment Routing
Path MTU in BGP", Work in Progress, Internet-Draft, draft-
ietf-idr-sr-policy-path-mtu-11, 3 April 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-sr-
policy-path-mtu-11>.
[I-D.ietf-pce-pcep-pmtu]
Peng, S., Li, C., Han, L., Ndifor, L., and S. Sidor,
"Support for Path MTU (PMTU) in the Path Computation
Element (PCE) Communication Protocol (PCEP)", Work in
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Progress, Internet-Draft, draft-ietf-pce-pcep-pmtu-08, 17
August 2025, <https://datatracker.ietf.org/doc/html/draft-
ietf-pce-pcep-pmtu-08>.
Authors' Addresses
Shuping Peng
Huawei
Huawei Campus, No. 156 Beiqing Rd.
Beijing
100095
China
Email: pengshuping@huawei.com
Dhruv Dhody
Huawei
India
Email: dhruv.ietf@gmail.com
Ketan Talaulikar
Cisco Systems
India
Email: ketant.ietf@gmail.com
Gyan Mishra
Verizon Inc.
United States of America
Email: gyan.s.mishra@verizon.com
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