1. A method comprising: receiving a keepalive message from a client station, and sending to the client station an acknowledgement of the received keepalive message; determining a measure of network load; based on the measure of network load, selecting a keepalive period; reporting the selected keepalive period to the client station in the acknowledgement of the received keepalive message; and the client station responsively sending a next keepalive message to a presence server at a time determined based on the selected keepalive period reported in the acknowledgement.