During these last years the Internet Protocol Television (IPTV) service and the different peer-to-peer (P2P) technologies have generated an increasing interest for the developers and the research community that find in them the solution to deal with the scalability problem of media streaming and reducing costs at the same time. However, despite of the benefits obtained in Internet-based applications and the growing deployment of commercial IPTV systems, there has been a little effort in combining them both. With the advent of the next-generation-network platforms such as the IP Multimedia Subsystem (IMS), which advocates for an open and inter-operable service infrastructure, P2P emerges as a possible solution in situations where the traditional streaming mechanisms are not possible or not economically feasible. In this paper, we propose an IPTV service architecture for the IMS that combines a centralized control layer and a distributed. P2P-like, media layer that relies on the IMS devices or peers located in the customers' premises to act as streaming forwarding nodes. We extend the existing IMS IPTV standardization work that has already been done in 3GPP and ETSI TISPAN in order to require a minimum number of architectural changes. The objective is to obtain a system with a similar performance to the one in currently deployed systems and with the flexibility of P2P. One of the main challenges is to achieve comparable response times to user actions such as changing and tuning into channels, as well as providing a fast recovery mechanism when streaming nodes leave. To accomplish this we introduce the idea of foster peers as peers having inactive multimedia sessions and reserved resources. These peers are on stand-by until their functionality is required and at that moment, they are able to accept downstream peers at short notice for events requiring urgent treatment like channel changing and recovery. Copyright (C). 2009 John Wiley & Sons, Ltd.
