Peer-to-peer systems for video delivery

In the very recent years, peer-to-peer for streaming videoapplications has gained considerable attention: commercialsystems have appeared, raising interest among a communityof potential users that have quickly increased and reachedan impressive size. The pioneering experience of theCoolstreaming prototype has been soon paired by popularsystems such as PPlive, Sopcast and PPStream, platformscapable of attracting thousands of users to watch videoprograms over the Internet.Among academia too, peer-to-peer streaming architectureshave spurred curiosity: a significant body of papershas been produced, bringing forth studies on heterogeneousaspects related to these systems.Nevertheless, much work remains to be done. The fieldof modeling is only partly explored; the understanding ofthe basilar laws that shape the behaviour of these systems isfar from being satisfying; the mutual relation betweensystem performance and the different algorithms that can beemployed for scheduling and peer selection still awaits for asound answer. How to provide differentiated service levelsto different users is a further open question, and so appearsthe validation of peer-to-peer approaches for video applicationswith tight reliability constraints.In this special issue we make an attempt to focus on afew specific issues of this realm: the contributionspresented below shed some light on the examined problems,and let the reader appreciate the strength and noveltyof their reach.