Nowadays Peer-to-Peer (P2P) systems became an important
part of Internet, millions of users have been
attracted to use their structures and services. The popularity
of Peer-to-Peer systems speed up academic research
joining researchers from systems, networking
and theory. The most popular P2P applications support
file-sharing and content distribution, new applications
are emerging in different fields, Internet telephony
is an example.
This paper discusses the issues of P2P systems such
as characteristics, structures, protocols, drawbacks,
open problems and futures fields of development.