SR Policy Selector

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Table of Contents

1. Introduction

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.¶

The [I-D.ietf-idr-performance-routing] specification defines a mechanism for disseminating path delay information across multiple Autonomous Systems (ASes). This information is used for BGP route computation.¶

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 Policies. For convenience, the composite candidate path formed by the combination of SR Policies is called parent SR Policy in [I-D.ietf-spring-sr-policy-group].¶

Different enterprise applications have varying network performance requirements. For instance, conference is highly sensitive to packet loss and jitter, while CRM applications are not highly demanding in terms of latency and packet loss.¶

This document describes a policy selection mechanism among the candidate SR Policies based on network quality in IPv6 environments.¶

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

The definitions of the basic terms are identical to those found in Segment Routing Policy Architecture [RFC9256].¶

CRM: Customer Relationship Management is a critical application that requires low bandwidth and low latency network connection.¶

Parent SR Policy: Refer to [I-D.ietf-spring-sr-policy-group]. A Parent SR Policy is the composite candidate path that acts as a container for grouping SR Policies which meet different service optimization objectives and constraints and have the same destination endpoint.¶

4. Problem and Requirements

Take the network 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. CRM and conference traffic have different SLA requirement, and expected be carried by different SR Policies. Generally, from CE1 to CE2, conference services with low latency requirements are forwarded along SR Policy PE1->P1->P2->PE2 and PE1->P3->P4->PE2. The CRM traffic is forwarded along the other SR Policy PE1->P5->P6->PE2. When failure or degradation happened in CRM SR Policy, it should be possible to switchover CRM traffic to conference SR Policy.¶

                     +---------------+
                     |   Controller  |
                     +---------------+

                    +------+    +------+
              +-----+  P1  +----+  P2  +-------+
              |     +------+    +------+       |
              |                                |
              |     +------+    +------+       |
              +-----+  P3  +----+  P4  +-------+
              |     +------+    +------+       |
              |                                |
+-----+   +---+--+                         +---+--+   +-----+
| CE1 +---+  PE1 |                         |  PE2 +---+ CE2 |
+-----+   +---+--+                         +---+--+   +-----+
              |                                |
              |     +------+    +------+       |
              +-----+  P5  +----+  P6  +-------+
                    +------+    +------+

Based on such scenario, the following requirements should be met:¶

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.¶

Refer to [I-D.ietf-spring-sr-policy-group], the services with different forwarding quality requirements to the same destination endpoint can be implemented through parent SR Policy.¶

This document proposes an SR Policy selector for parent SR Policy based on network quality requirement. The head end node of parent SR Policy selects the best constituent SR Policy for the application according to the quality of the constituent SR Policy.¶

Take Figure 1 as an example, there is a parent SR Policy between PE1 to PE2, which has multiple constituent SR Policies. An SR Policy selection mechanism is needed, which should select best constituent SR Policy in the parent SR Policy. When the head node detects the quality degradation of the active constituent SR Policy, it will select another one in the parent SR Policy.¶

5. SR Policy Selector

                       +----------------------+
+----------------+     |  Parent SR Policy    |    +------------+
|      Flow      +---->|                      +--->|    Flow    |
| Classification |     | SR Policy Selector   |    | Forwarding |
+----------------+     +----------------------+    +------------+
                                    ^
                                    |
                          +---------+-------+
                          | Network Quality |
                          |   Measurement   |
                          +-----------------+

               +--------------------------------------------------+
               |                 Parent SR Policy                 |
               |                                  +-------------+ |
               | +------------------------------+ |+-----------+| |
               | |     SR Policy Selector       | || SR Policy || |
+-----------+  | |                            +-|-|+ (Color X) || |
| Classified|  | | +-----------+  +------+   /  | |+-----------+| |
|           +--|-|-+ Threshold +--+ Prio +--+   | |             | |
|  Traffic  |  | | +-----+-----+  +--+---+      | |+-----------+| |
+-----------+  | |       ^           ^        +-|-|+ SR Policy || |
               | |       |           |          | || (Color Y) || |
               | +-------|-----------|----------+ |+------+----+| |
               |         |           |            +-------|-----+ |
               |         |           | Prio Configuration |       |
               |         |           \--------------------+       |
               +---------|--------------------------------|-------+
                         |                                |
               +---------+--------------+   Quality data  |
               |    Network  Quality    +<----------------/
               |  Measurement Function  |
               +------------------------+

6. Examples of SR Policy Selector

The application of SR Policy Selector is described in detail in L3VPN over TE scenario. Take the example shown in Figure 1.¶

There are two services between CE1 and CE2: conference and CRM. The traffic from CE1 to CE2 can be forwarded through two paths: Path1 (PE1->P1->P2->PE2 and PE1->P3->P4->PE2) and Path2 (PE1->P5->P6->PE2).¶

The conference service traffic will be forwarded through Path1 first. The CRM 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 conference service to Path2.¶

When the remaining bandwidth of Path2 is less than the bandwidth guarantee threshold, if Path1 still has enough remaining bandwidth, the CRM traffic exceeding the bandwidth will be directed to Path1.¶

The configuration on the head node PE1 includes the following three parts. These configurations can be directly configured on the node or distributed through the controller.¶

1. Configure the parent SR Policy.¶

   parent-sr-policy sr-policy-1(color 10, PE2_SID)
     service conference use routing-policy-selector irp1
     service crm use routing-policy-selector irp2
   
   

   ¶

2. Configure constituent SR Policy.¶

   sr-policy path1 (color 100, PE2_SID)
     segment-list <SID_P1, SID_P2, SID_PE2>
     segment-list <SID_P3, SID_P4, SID_PE2>
   sr-policy path2 (color 200, PE2_SID)
     segment-list <SID_P5, SID_P6, SID_PE2>
   
   

   ¶

3. Define three SR Policy Selector policies, and specify the threshold of network quality, priority.¶

   routing-policy-selector irp1
     traffic-delay threshold 1000ms
     priority 1 mapping-to color 100
     priority default mapping-to color 200
   routing-policy-selector irp2
     remaining-bandwidth threshold 50M
     priority 1 mapping-to color 200
     priority default mapping-to color 100
   
   

   ¶

7. IANA Considerations

This memo includes no request to IANA.¶

8. Security Considerations

This document does not introduce any security considerations.¶

9. References

Acknowledgements

The authors would like to thank the following for their valuable contributions of this document.¶

TBD.¶

Authors' Addresses