Validity of SR Policy Candidate Path

Abstract

An SR Policy comprises one or more candidate paths (CP) of which at a given time one and only one may be active (i.e., installed in forwarding and usable for steering of traffic). Each CP in turn may have one or more SID-List of which one or more may be active; when multiple SID-List are active then traffic is load balanced over them. However, a candidate path is valid when at least one SID-List is active. This candidate path validity criterion cannot meet the needs of some scenarios.¶

This document defines the new candidate path validity criterion.¶

1. Introduction

SR Policy architecture are specified in [RFC9256]. An SR Policy comprises one or more candidate paths (CP) of which at a given time one and only one may be active (i.e., installed in forwarding and usable for steering of traffic). Each CP in turn may have one or more SID-List of which one or more may be active; when multiple SID-List are active then traffic is load balanced over them. However, a candidate path is valid when at least one SID-List is active. This candidate path validity criterion cannot meet the needs of some scenarios.¶

This document defines the new candidate path validity criterions based on [RFC9256]. For the segment list invalidation rules, refer to [RFC9256] and [I-D.liu-spring-sr-policy-flexible-path-selection]. This document does not change the segment list invalidation rules.¶

2. Motivation

The candidate path validity criterion defined in [RFC9256] can't meet the needs of the following scenarios:¶

                                          +----------------------+
                                +---------| SL1(Weight 1, 100MB) |
+----------------+      |         +----------------------+
|  CP1 (200MB)   |------+
+----------------+      |         +----------------------+
                        +---------| SL2(Weight 1, 100MB) |
                                  +----------------------+

The SR Policy POL1 has two candidate paths: CP1 and CP2, and CP1 is the active candidate path (it is valid and has the highest Preference). The two segment lists (SL1 and SL2) of CP1 are installed as the forwarding instantiation of the SR Policy POL1. Each segment list is assumed to have a maximum capacity of 100Mbps. CP1 carries a total of 200Mbps of traffic. Within POL1, flow-based hashing is performed across each SL based on its relative weight. With an equal weight assigned to each SL, the fraction of flows steered into each SL is 50%, meaning each SL carries 100 Mbps of traffic.¶

At this time, if one of the segment lists is determined to be invalid by the rule defined in [RFC9256], the remaining Segment List cannot carry the full 200Mbps of traffic due to its capacity limit. However, the CP1 remains the active candidate path according to [RFC9256], as a candidate path is valid as long as it has at least one valid Segment List.¶

3. Validity of a Candidate Path

A headend MAY be informed about the validity control parameters of a candidate path for an SR Policy <Color, Endpoint> by various means including: via configuration, PCEP, or BGP. The detailed protocol extension will be described in a separate document.¶

This document defines the following validity control parameters under candidate Path to control the validity judgment of candidate Path:¶

• Minimum valid SL count: 8-bit value, The value is 1-0xff.¶

  Indicates the minimum number of valid segment Lists under the active candidate path. When the number of valid segment Lists under candidate path is greater than or equal to this field, the candidate path is considered valid.¶

  0xff indicates that the candidate path is considered valid only if all the segment Lists are valid.¶

• Minimum Cumulative SL Weight: 32-bit value, The value is 0-0xffffffff.¶

  Indicates the minimum value of the sum of the weights of the valid segment List under the active candidate Path.¶

  When the sum of the weights of the valid segment Lists under the candidate path is greater than or equal to this field, the candidate Path is considered valid.¶

  0 indicates no requirement for weight.¶

  0xffffffff indicates that the candidate path is considered valid only if all the segment Lists are valid.¶

Candidate path is considered valid only if both validity control parameters are satisfied.¶

4. Use Cases for Candidate Path Validity

The following scenarios illustrate how the validity control parameters of a candidate path defined in Section 3 address the capacity and validity issues described in Section 2.¶

• Minimum valid SL count: Following the scenario in Section 2, where the aggregate traffic load is 200 Mbps and each SL has a capacity of 100 Mbps, an operator can configure a "Minimum valid SL count" of 2. In this case, the candidate path (CP) is rendered invalid as soon as any single Segment List (SL) becomes invalid. This prevents the CP from remaining active when its capacity is insufficient to carry the full traffic load.¶

• Minimum Cumulative SL Weight: Alternatively, an operator can assign a weight of 1 to both SL1 and SL2 to reflect their identical 100 Mbps capacity. By setting the "Minimum Cumulative SL Weight" to 2, the operator ensures the candidate path remains active only when the aggregate capacity meets the 200 Mbps demand. If one SL becomes invalid, the sum of the weights of the remaining valid SLs becomes 1, falling below the threshold. Consequently, CP1 is declared invalid, thus preventing the steering of 200 Mbps of traffic into a single 100 Mbps link.¶

8. References