Abstract: A communication system, method and client program for communicating via a first network. The system comprises a first network and a router coupling the second network to the first network, the router comprising an address translator configured to map second-network addresses to a lesser number of first-network addresses identifying the second user nodes collectively to the first network. Each of the second user nodes is installed with a communication client configured so as when executed on the second user nodes to: compare the first-network addresses of two of the user nodes, and, if their first-network addresses match such that both are nodes of the second network, communicate an availability status of one of the two user nodes between the two user nodes directly over the second network without routing via the first network.

Abstract: A method, program and user node for use in a communication system implemented over a network comprising a plurality of user nodes, each being associated with a respective presence status indicating an availability of the user node for communication within the communication system. The method comprises, at each of a first one or more of the user nodes: maintaining a contact list specifying a selection of contacts from the plurality of user nodes; associating a presence update priority level with each of the contacts, the presence update priority level relating to an estimated likelihood of communication between the first user node and the respective contact; and transmitting a presence message to each of a plurality of the contacts in dependence on the respective presence update priority level, each of the presence messages comprising at least one of: a request for the presence status of the contact, and a notification of the presence status of the first user node.

Abstract: A method, program and user node for use in a communication system implemented over a network comprising a plurality of user nodes, each being associated with a respective presence status indicating an availability of the user node for communication within the communication system. The method comprises, at each of a first one or more of the user nodes: transmitting repeated presence messages to each of a plurality of others of the user nodes, each of the presence messages comprising at least one of: a request for the presence status of the other user node, and a notification of the presence status of the first user node; determining a measure of resource usage in relation to at least one of the first user node and the other user nodes; and controlling a regularity of the repeated transmission of presence messages in dependence on the determined measure of resource usage measure.

Abstract: A method, program and user node for use in a communication system implemented over a network comprising a plurality of user nodes, each being associated with a respective presence status indicating an availability of the user node for communication within the communication system. The method comprises, at each of a first one or more of the user nodes: transmitting repeated presence messages to each of a plurality of others of the user nodes, each of the presence messages comprising at least one of: a request for the presence status of the other user node, and a notification of the presence status of the first user node; determining a measure of resource usage in relation to at least one of the first user node and the other user nodes; and controlling a regularity of the repeated transmission of presence messages in dependence on the determined measure of resource usage measure.

Abstract: A method, program and user node for use in a communication system implemented over a network comprising a plurality of user nodes, each being associated with a respective presence status indicating an availability of the user node for communication within the communication system. The method comprises, at each of a first one or more of the user nodes: maintaining a contact list specifying a selection of contacts from the plurality of user nodes; associating a presence update priority level with each of the contacts, the presence update priority level relating to an estimated likelihood of communication between the first user node and the respective contact; and transmitting a presence message to each of a plurality of the contacts in dependence on the respective presence update priority level, each of the presence messages comprising at least one of: a request for the presence status of the contact, and a notification of the presence status of the first user node.