We investigate the preservation of quality of service guarantees to a flow of packets in the presence of flow aggregation. In flow aggregation, multiple flows, known as the constituent flows, are merged together resulting in a single aggregate flow. Packet schedulers located after the network point where the aggregation occurred are aware of the aggregate flow, but are unaware of its constituent flows. In spite of this, we show that quality of service may still be guaranteed to the constituent flows if the aggregation is performed fairly. Furthermore, contrary to intuition, the quality of service guaranteed to a flow may be greater under flow aggregation than in the case where no aggregation is performed.

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We develop a simple theory of flows to study the flow of data in real-time computing networks. Flow Theory is based on discrete and nondeterministic mathematics, rather than the customary continuous or probabilistic mathematics. The theory features two types of flows: smooth and uniform, and eight types of flow operators. We prove that, if the input flow to any of these operators is smooth or uniform, then both the internal buffer and delay of that operator are bounded. Linear networks of flow operators are introduced, and their internal buffers and delays are derived from the internal buffers and delays of their constituent operators. We extend Flow Theory so that it can be used in analyzing cyclic networks and networks of multiflows. Since many rate-reservation protocols can be represented as linear networks of flow operators, we use Flow Theory to prove that a number of these protocols (Stop-and-Go, Hierarchical RoundRobin, Weighted Fair Queueing, Self-Clocking Fair Queueing, and Virtual Clock) require bounded buffering and introduce bounded delay.

We present a scheduling protocol, called Time-Shift scheduling, to forward data packets from multiple input flows to a single output channel. Each input flow is guaranteed a predetermined forwarding rate and an upper bound on packet delay. The protocol is an improvement over existing protocols because it satisfies the properties of low delay, fairness, and efficiency, while existing protocols fail to satisfy at least one of these properties. In TimeShift scheduling, each flow is assigned an increasing timestamp, and the packet chosen for transmission is taken from the flow with the least timestamp. The protocol features the novel technique of time shifting, in which the scheduler's real-time clock is adjusted to prevent flow timestamps from increasing faster than the real-time clock. This bounds the difference between any pair of flow timestamps, thus ensuring the fair scheduling of flows.