



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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   This document is subject to BCP 78 and the IETF Trust's Legal
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   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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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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