RIAS FAIRNESS REFERENCE MODEL

Overview

Weighted Fairness in the presence of spatial reuse is an important goal of the RPR protocol. The definition of fairness in 802.17 has two goals: (1) to have a precise and rigorous definition of the Fairness Reference Model under an idealized scenario; (2) to have a set of compliance criteria (including both mandatory and optional) against which practical implementations of the reference model can be measured against.

Toward the first goal, RIAS (Ring Ingress-Aggregated with Spatial Reuse), of which the exact mathematical definition is presented in Appendix X, is defined as the Fairness Reference Model. Toward the second goal, four scenarios are specified. For each scenario, the steady state RIAS fairness rates are defined.

Properties

RIAS Fairness has two key components. The first component defines the level of traffic granularity for fairness determination at a link as an ingress-aggregated (IA) flow, i.e., the aggregate of all flows originating from a given ingress node. The second component of RIAS fairness ensures maximal spatial reuse subject to this first constraint. That is, bandwidth can be reclaimed by IA flows when it is unused either due to lack of demand or in cases of sufficient demand in which flows are bottlenecked elsewhere.

Scenarios

The following scenarios demonstrate the RIAS rates, expressed as a share of the total capacity. In each case, a tandem segment of a ring is depicted. The input traffic is assumed to be constant rate with an input rate equal to the link capacity for each depicted flow. The RIAS rate refers to the ideal end-to-end throughput of the flow, and hence the ideal ingress throttling rate.

Scenario I (Parking Lot)

Scenario II (Parallel Parking Lot)

Scenario III (Upstream Parallel Parking Lot)

Scenario IV (Two-Exit Parking Lot)

A fairness algorithm is said to be RPR compliant if for Scenario I, the algorithm achieves the defined RIAS end-to-end throughput shares in steady state via throttling traffic to the RIAS rates at the ingress points. Satisfying the additional three scenarios is optional.

Mathematical Definition

See the appendix

Available Downloads

RIAS matlab code

RIAS definition (this document)