The IPv6 flow mechanism assumes that the state associated with a given flow label is stored in routers, so they know how to handle packets that carry that flow label.

A design requirement is to flush flow labels that are no longer being used (stale flow label) from routers.

a. Assume that a source always send a control message to all affected routers deleting a flow label when the source finishes with that flow. In that case, how could a stale flow label persist?

b. Suggest router and source mechanisms to overcome the problem of stale flow labels.