The application relates to an internet payment system based on return traffic. In regard of payment for content delivered across the internet an aspect of the invention provides for initiation of an explicit return flow of packets. Reception by the server of these return packets entails reception of payment tokens of the client by the server to continue with the delivery of the content. Maskerading by the client can be prevented by sending challenges along with the data packets and by having the client to send responses to the same along with the return packets.