Feedback control in a distributed scheduling architecture

Multiple-module, multistage packet switching systems are gaining popularity as a scalable solution to the ever increasing demand for the aggregate switching capacity. Due to additional contention points between the stages, such systems differ in their behavior from single module output buffered switches, which serve as a model for most of the advanced quality of service (QoS) scheduling algorithms. Recently, a distributed scheduling reference architecture has been proposed to extend the QoS provisioning framework to the practical multistage switches. While keeping all the per-flow state information in the port card, it aggregates the individual traffic flows into a few QoS based fabric channels and relies on the intelligent selective feedback to ensure that the QoS guarantees of the individual flows are met. In this paper we discuss a novel credit-based feedback mechanism that allows to aggregate multiple traffic components with diverse QoS requirements into the same downstream FIFO queue. We specifically apply it to merge the guaranteed-bandwidth (GB) and best-effort (BE) QoS channels of the distributed scheduling architecture into a single non-guaranteed-delay queue in the switch fabric. The mechanism allows to satisfy the bandwidth requirements of the GB traffic while maximizing the throughput of the BE traffic and distributes the available excess bandwidth between different types of traffic fairly. The presentation is supported by fluid stationary analysis and packet-level simulations.