Flow labeling

Outside of basic prioritization of traffic, IPv6 defines a mechanism for specifying a particular flow of packets. In IPv6 terms, a flow is defined as 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.

This flow identification can be used for priority control, but might also be used for any number of other controls.

The flow label is chosen randomly, and does not identify any characteristic of the traffic other than the flow to which it belongs. This means that a router cannot determine that a packet is a particular type by examining the flow label. It can, however, determine that it is part of the same sequence of packets as the last packet containing that label.