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A flow is a stream of packets that is transmitted between a source and a destination. Flows generally follow the same route through a network, although that route may change at any time to bypass downed links and other problems. A voice or video session consists of long flows that usually require a certain amount of guaranteed bandwidth. An IP over ATM technique that provides QoS is to detect flows and set up switched paths across the ATM network that can satisfy the QoS requirements of the flows.

Flows may be implicit or explicit. An implicit flow is one in which the router detects a flow by inspecting header information in packets and then manages the flow as necessary. An explicit flow is a flow that is predefined, in other words, an end device tells the network that a flow is about to begin and the network sets itself up to handle the flow. In both cases, the network manages the flow in order to allocate resources (e.g., bandwidth and buffers) for the flow. When flows are recognized, congestion problems can be avoided.

Cut-through routing is a technique of detecting flows and switching the flows at high speed, using switching techniques rather than routing techniques. This case points out that flows are not related to routing because the routing functions are removed.

Flows are allocated some bandwidth, which means that some flows may have more bandwidth than others. This gets into issues of priority and/or fair sharing of bandwidth. See "Bandwidth Management" and "QoS (Quality of Service)."

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