Resource retrieval over a data network

A method of downloading resources to a client (1) from a content server (3) over a data network. A resource request message is sent from the client (1) and is intercepted at a proxy (2) located in the data network between the client (1) and the content server (3). A header request is sent from the proxy (2) to the content server (3), requesting the content server (3) to transmit a header, associated with the requested resource, to the proxy (2). The header is received at the proxy (2) which determines whether or not the header contains billing and/or access restrictions. In the event that the header does contain billing and/or access restrictions, the client's right to receive the requested resource is authenticated and, providing the client is authenticated, the resource request message is delivered from the proxy (2) to the content server (3) and subsequently the resource is downloaded from the content server (3) to the client (1).