srv6ops F. Yang
Internet-Draft China Mobile
Intended status: Informational C. Lin
Expires: 1 January 2026 New H3C Technologies
30 June 2025
Intelligent Routing Method of SR Policy
draft-yang-srv6ops-intelligent-routing-00
Abstract
Segment routing (SR) [RFC8402] is a source routing paradigm that
explicitly indicates the forwarding path for packets at the ingress
node. An SR Policy is associated with one or more candidate paths,
and each candidate path is either dynamic, explicit or composite.
This document describes an intelligent routing method for SR Policy
based on network quality in MPLS and IPv6 environments.
Status of This Memo
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This Internet-Draft will expire on 1 January 2026.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Problem and Requriements . . . . . . . . . . . . . . . . . . 3
4. Intelligent Routing Method for SR Policy . . . . . . . . . . 5
4.1. Processing Model . . . . . . . . . . . . . . . . . . . . 5
4.2. Flow Classification . . . . . . . . . . . . . . . . . . . 6
4.3. Flow Steering . . . . . . . . . . . . . . . . . . . . . . 6
4.4. Intelligent Routing . . . . . . . . . . . . . . . . . . . 7
4.5. Network Quality Measurement . . . . . . . . . . . . . . . 8
4.6. Flow Forwarding . . . . . . . . . . . . . . . . . . . . . 9
5. Examples of intelligent routing . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . 11
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Segment routing (SR) [RFC8402] is a source routing paradigm that
explicitly indicates the forwarding path for packets at the ingress
node. The ingress node steers packets into a specific path according
to the Segment Routing Policy (SR Policy) as defined in [RFC9256].
In order to distribute SR Policies to the headend,
[I-D.ietf-idr-segment-routing-te-policy] specifies a mechanism by
using BGP.
An SR Policy is associated with one or more candidate paths. A
composite candidate path acts as a container for grouping SR
Policies. As described in section 2.2 in [RFC9256], the composite
candidate path construct enables combination of SR Policies, each
with explicit candidate paths and/or dynamic candidate paths with
potentially different optimization objectives and constraints, for
load-balanced steering of packet flows over its constituent SR
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Policies. For convenience, the composite candidate path formed by
the combination of SR Policies is called parent SR Policy in
[I-D.cheng-spring-sr-policy-group].
This document describes an intelligent routing method for SR Policy
based on network quality in MPLS and IPv6 environments.
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.
2. Terminology
The definitions of the basic terms are identical to those found in
Segment Routing Policy Architecture [RFC9256].
3. Problem and Requriements
Take the networking shown in Figure 1 below as an example to
illustrate the current problems.
CE1 and CE2 are the two access endpoints of the IP telecom network.
There are many service flows between CE1 and CE2 that have different
requirements for forwarding quality. E.g. OA and voice traffic have
different SLA requirement, and were carried by different SR Policies.
Generally, from CE1 to CE2, voice services with low latency
requirements are forwarded along the highly reliable path
PE1->PE2->CE2. The OA traffic is forwarded along the high bandwidth
path PE3->P5->P6->PE2->CE2. When failure or degradation happened in
OA traffic SR Policy, there should be possible to assure basic
communication for OA traffic by using voice bandwidth.
In single SR Policy, there are many mechanism provide failure/degrade
protection, such as TILFA, VPN FRR. However, it is not clear how to
handle failure or degradation between multiple SR Policies.
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+---------------+
| Controller |
+---------------+
+------+ +------+
+-----+ P1 +-----+ P2 +------+
| +---+--+ +---+--+ |
+---+--+ | \ / | |
+----+ PE1 | | \/ | |
| +---+--+ | /\ | |
+--+--+ | +---+--+/ \ +---+--+ +---+--+ +-----+
| CE1 | +-----+ P3 +-----+ P4 +--+ PE2 +--+ CE2 |
+--+--+ +------+ +------+ +---+--+ +-----+
| |
| +------+ +------+ +------+ |
+----+ PE3 +--+ P5 +-----+ P6 +------+
+------+ +------+ +------+
Figure 1
Based on such scenarios, the following requirements are proposed:
1. Maximize failure/degradation protection
In case of failure or degradation detected on one SR policy, it
should be possible to do inter-policy protection.
2. Minimal impact after taking repairing action
Repair action can be done on flow level to minimize the ripple
effect cause by forwarding path switchover.
3. Maximize bandwidth efficiency
For some critical applications, it should be possible to forward
the traffic over lower class policy in case of higher class SR
Policy degradation.
In order to better meet these requirements, this document proposes an
intelligent routing method for SR policy based on network quality
requirement. The head end node selects the optimal path according to
the current network quality to improve the path switching speed and
forwarding performance.
Refer to [I-D.cheng-spring-sr-policy-group], the services with
different forwarding quality requirements to the same destination
endpoint can be implemented through parent SR Policy group.
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Define a parent SR Policy group for the above CE1 to CE2 services.
Specify the steering policies of services in the parent SR Policy
group. Different services can select different SR Policy paths in
the parent SR Policy group according to different quality
requirements. When the head node perceives that the quality of the
path of a service is deteriorating (such as bandwidth or delay
degradation), it searches for other path in the group that is
suboptimal and also meets its quality requirements.
4. Intelligent Routing Method for SR Policy
4.1. Processing Model
The path priority is assigned to the SR policy forwarding path
manually by the controller. Each path with quality requirement will
be assigned with a priority value. The lower the value, the higher
the priority. That is, when there is a group of qualified paths,
best path will be selected with higher priority.
Configure multiple SR policy paths for the service flows with
specified characteristics in the parent SR Policy group. Assign the
corresponding path priority to each path according to the priority
order of the path. If there is a backup path for the SR policies,
lower priority value should be used according to the quality
requirements.
After receiving the service packet with the specified
characteristics, when the network quality is good, the traffic is
forwarded through the path with high priority. When the network
quality degradation is happened on the high priority path, such as
the packet loss rate exceeds the acceptable range, switch to the next
high priority path of the service.
If the quality of the high priority forwarding path is restored and
the specified quality requirements are met, the traffic is switched
from the low priority forwarding path to the high priority forwarding
path after a period of wait-to-restore time.
According to the processing logic, the SR policy intelligent routing
model can be divided into five units, including Flow Classification,
Flow Steering, Intelligent Routing, Flow Forwarding, and Network
Quality Measurement, as shown in Figure 2 below.
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+----------------+ +----------+ +-------------+ +------------+
| | | | | | | |
| Flow +->| Flow +->| Intelligent +->| Flow |
| Classification | | Steering | | Routing | | Forwarding |
| | | | | | | |
+----------------+ +----------+ +-------------+ +------------+
^
|
+---------+-------+
| |
| Network Quality |
| Measurement |
| |
+-----------------+
Figure 2
The functions of each unit are described below.
4.2. Flow Classification
After receiving the traffic, the head node first needs to label the
traffic with forwarding class according to classification
configuration.
The head node can match flow characteristics in its ingress
interfaces (upon any field such as Ethernet destination/source/VLAN/
TOS or IP destination/source/DSCP or transport ports or application
attribute etc.) and color them with an internal per-packet
forwarding-class variable.
4.3. Flow Steering
According to the forwarding class variables determined by the Flow
classification, the header node selects the matching forwarding path,
that is, selects the SR policy or the parent SR policy representing a
group of policies.
If multiple SR policy forwarding paths are configured for the traffic
flow with the specified characteristics, all valid SR policies will
be retrieved and handed over to the Intelligent Routing unit to
select the optimal forwarding path.
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4.4. Intelligent Routing
According to the SR policy(policies) provided by Flow Steering, the
Intelligent Routing unit obtains the current quality of each SR
policy path from the Network Quality Measurement unit. Based on the
mapping between the quality and the priority of intelligent routing,
it selects the forwarding path with the highest priority and the
quality measurement of the SR Policy. Only those qulified SR
Policies which can reach the threshold are considered as candidate SR
Policies.
+---------------------------------------------------+
| Parent SR Policy |
| +-----------------+ |
| | SR Policy | |
| | (high priority) | |
| | +-------------+ | |
| | | Active Path | | |
| | +-------------+ | |
| +----------+ | +-------------+ | |
| | | | | Standby Path| | |
| | +->| +-------------+ | |
| +------------+ | | +-------------+---+ |
| | Classified | | Policy | / |
| | +->| Decision |<-Measurement-+ |
| | Traffic | | | \ |
| +------------+ | | + ------------+---+ |
| | +->| +-------------+ | |
| | | | Active Path | | |
| +----------+ | +-------------+ | |
| | +-------------+ | |
| | | Standby Path| | |
| | +-------------+ | |
| | SR Policy | |
| | (lower priority)| |
| + ----------------+ |
+---------------------------------------------------+
Figure 3
When the network quality is better than the threshold, the traffic is
forwarded by the policy with high priority. When the network quality
of the high priority SR Policy degrades, such as the loss rate
increasion, the Policy Decision module will switch the traffic to the
next high priority one in the candidate SR Policies. Similarly, when
the next higher priority SR Policy forwarding path cannot meet the
forwarding quality requirements, switch to the lower priority SR
Policy path.
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If the quality of the high priority forwarding path gets better and
meets the specified quality requirements, the traffic can be
recovered from the low priority forwarding path to the high priority
SR Policy forwarding path after a period of wait-to-restore time.
The purpose of wait-to-restore time is specified in order to prevent
flapping between SR Policies.
To avoid frequent path switching when the network quality is
unstable, if the current path can meet the forwarding quality
requirements, the head node can choose not to automatically switch
back to the higher priority path in case of the quality of the higher
priority path is restored. The device can provide a configuration
for automatic failback, and add a wait-to-restore timer. Only after
automatic restore is allowed and the wait-to-restore timer is
timeout, the forwarding path switch from the current path that meets
the quality requirements to the path with higher priority.
4.5. Network Quality Measurement
The Network Quality Measurement unit regularly monitors the quality
of all effective forwarding paths according to the measurement cycle,
records the current performance measurement data of the path, and
reports it to the Intelligent Routing unit, which decides whether to
switch paths.
The following network quality parameters of forwarding path can be
used for path scheduling:
* Jitter
* Latency
* Packet loss
* Available bandwidth
* Bandwidth utilization
* Current traffic statistics
* Other forwarding performance parameters
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The quality parameters of network forwarding path can be obtained
through active or passive performance measurement methods, such as
iOAM, STAMP, TWAMP, etc. The network quality parameters can be
calculated by the controller and distributed to the head end node, or
calculated by the head end node according to the network measurement
data. The measurement method and quality parameter acquisition
method are beyond the scope of this document.
4.6. Flow Forwarding
The service flow is forwarded according to the path determined by the
Intelligent Routing unit.
When there are multiple paths with the same priority, the traffic
will share the load among these SR Policy paths with the same
priority according to the weight value.
5. Examples of intelligent routing
The application of intelligent routing is described in detail in
L3VPN over TE scenario. The networking is shown in Figure 4 below.
CE1 and CE2 belong to the same L3VPN and access the public network
through PE1, PE2 and PE3 respectively.
There are two services between CE1 and CE2: voice and OA. The
traffic from CE1 to CE2 can be forwarded through two paths: Path1
(PE1->PE2->CE2) and Path2 (PE3->P5->P6->PE2->CE2). Among them, the
reliability of path 1 is high and the transmission delay is low.
Path 2 has a large bandwidth.
The voice service traffic will be forwarded through Path1 first. The
OA service traffic will be forwarded through Path2 first. When the
transmission delay of Path1 exceeds the threshold value and Path2 can
meet the delay requirements, switch the voice service to Path2.
When the remaining bandwidth of Path2 is less than the bandwidth
guarantee threshold, if Path1 still has enough remaining bandwidth,
the OA traffic exceeding the bandwidth will be directed to Path1.
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+------+ +------+
+------+ P1 +-----+ P2 +-------+
| +---+--+ +---+--+ |
+---+--+ | \ / | |
+-----+ PE1 | | \/ | |
| +---+--+ | /\ | |
+--+--+ | +---+--+/ \ +---+--+ +---+--+ +-----+
| CE1 | +------+ P3 +-----+ P4 +---+ PE2 +---+ CE2 |
+--+--+ +------+ +------+ +---+--+ +-----+
| |
| +------+ +------+ +------+ |
+-----+ PE3 +---+ P5 +-----+ P6 +-------+
+------+ +------+ +------+
Figure 4
The configuration on the head node CE1 includes the following three
parts. These configurations can be directly configured on the node
or distributed through the controller.
1. Define three intelligent routing policies, and specify the
threshold of network quality, path priority and the corresponding
path color value for routing.
intelligent-routing-policy irp1
traffic-delay threshold 1000ms
priority 1 mapping-to color 100
priority default mapping-to color 200
intelligent-routing-policy irp2
remaining-bandwidth threshold 50M
priority 1 mapping-to color 200
priority default mapping-to color 100
2. Configure forwarding paths.
sr-policy policy-A (color 100, CE2_SID)
segment-list <SID_PE1, SID_PE2, SID_CE2>
sr-policy policy-B (color 200, CE2_SID)
segment-list <SID_PE3, SID_P5, SID_P6, SID_PE2, SID_CE2>
3. Configure corresponding intelligent routing policies for services
with specified characteristics in the parent SR Policy group.
parent-sr-policy sr-policy-1(color 10, CE2_SID)
service voice use intelligent-routing-policy irp1
service oa use intelligent-routing-policy irp2
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6. IANA Considerations
This memo includes no request to IANA.
7. Security Considerations
This document does not introduce any security considerations.
8. References
8.1. Normative References
[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/rfc/rfc8402>.
[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>.
[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>.
[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>.
8.2. Informative References
[I-D.ietf-idr-segment-routing-te-policy]
Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., and
D. Jain, "Advertising Segment Routing Policies in BGP",
Work in Progress, Internet-Draft, draft-ietf-idr-segment-
routing-te-policy-26, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-
segment-routing-te-policy-26>.
[I-D.cheng-spring-sr-policy-group]
Cheng, W., Wenying, J., Lin, C., Chen, R., Zhang, Y., and
Y. Liang, "SR Policy Group", Work in Progress, Internet-
Draft, draft-cheng-spring-sr-policy-group-08, 17 June
2025, <https://datatracker.ietf.org/doc/html/draft-cheng-
spring-sr-policy-group-08>.
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Acknowledgements
The authors would like to thank the following for their valuable
contributions of this document.
TBD.
Authors' Addresses
Feng Yang
China Mobile
Beijing
China
Email: yangfeng@chinamobile.com
Changwang Lin
New H3C Technologies
Beijing
China
Email: linchangwang.04414@h3c.com
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