You may know that muticast is based on UDP. so IP packets send out may get dropped or are duplicated before they are received. Since there is no error correction in UDP itself, the receiver may get incorrect data. Two month ago I found MAD/FLUTE (http://www.atm.tut.fi/mad/home.html) wich implements a protocol for reliable unidirectional transport, such as multicast. Yes, you can use it to share your files over multicast to many users over a limited ADSL upstream... ;-) It adds FEC and congestion control to UDP, to compensate the transmission errors. At this time I have seen an sdr-announcement (since there are no good other tools to send out valid SDP-records , sdr or minisapserver/vlc) from an other flute user at the same SixxS-pop (deham01) and we tried it out. The main problem we first had to find, was that the default packetsize of 1428 bytes is to big. So I only got the FDT and the transmission hang at under 0.5% of the file. After we used -l:1192, which is the maximum for the default MTU of 1280 and a german DSL line with PPPoE, it works fine. IIRC we used the standards for FEC and the transmission of the flute source (135 kb) over 8 1kb channels took around 58 seconds. I hope that multicast on IPv6 does not end up as with IPv4, where no one outside the MBone are using it, because it must turned on on the routers by hand... :-( The idea behind it is to nice, but leaving it turned off, the carriers can earn much money for filling up the lines with thousends copies of the same data. :-((