What Is a Flow?

A flow is a sequence of packets sent from a particular source to a particular (unicast or multicast) destination for which the source desires special handling by the intervening routers. The nature of that special handling might be conveyed to the routers by a control protocol, such as a resource reservation protocol, or by information within the flow's packets themselves, for example, in a hop-by-hop option.

Active flows from a source to a destination can be multiple and can contain traffic that is not associated with any flow. The combination of a source address and a non-zero flow label uniquely identifies a flow. Packets that do not belong to a flow carry a flow label of zero.

The flow's source node assigns a flow label to a flow. New flow labels must be chosen (pseudo-)randomly and uniformly from the range 1 to FFFFFF hex. This random allocation makes any set of bits within the flow label field suitable for use as a hash key by routers for looking up the state associated with the flow.