Have a look around the IETF site for RFCs on Reliable Multicast. There is an entire working group on this. Several protocols have been developed for different purposes. Also have a look around Oracle/Sun for the Java Reliable Multicast Service project (JRMS). It was a research project of Sun, never supported, but it did contain Java bindings for the TRAM and LRMS protocols.

Reliable udp broadcast libraries?
I want get running squid3 and obligate users to authenticate again a local and simple database users-pass file. Can be user-pass file of the system or a simple .txt file, whatever is more easy. 
