A media session between an initiating endpoint and a responding endpoint is established via a communication network. A set of candidate pairs is generated, each comprising a respective network address available to the initiating endpoint and a respective network address available to the responding endpoint by exchanging network addresses between the initiating endpoint and the responding endpoint. The media session is established using a candidate pair of the set determined to be valid. Connectivity checks are performed for at least one candidate pair of the set to determine whether or not the candidate pair is valid. The at least one candidate pair is selected in dependence on selection data pertaining to at least one of the network addresses and indicative of the quality of a path through the network that would be traversed were that network address to be used for the media session.