A communications interface between an end-user and an edge node of a communication network for establishing, prior to data transmission, the differentiated service code points supported by the network. The end-user first issues a service request message (42) to an edge node of the communication network. The edge node responds with an acknowledgment message (44) carrying information as to which DSCP's the network is able to support. Finally, the application at the end-user can decide how to mark the relevant packets during data transmission depending on the levels of differentiated service supported by the network.