You might want to have a look at zsync (http://zsync.moria.org.uk/ http://zsync.moria.org.uk/), which is exactly a reversed rsync: the seed server calculates a

You might want to have a look at zsync (http://zsync.moria.org.uk/ http://zsync.moria.org.uk/), which is exactly a reversed rsync: the seed server calculates a signatures file over a file, the clients download that signatures file (through any means, but usually HTTP) and calculate a diff on their side; once they have instructions they can download only the parts of what they need through HTTP Ranges. Which means that any dumb HTTP storage that respects Ranges will work. The big advantage over rsync, on top of being available over HTTP, is that the server only calculates the signatures file once for the life of the content, instead of calculating it for each and every client. I know some distributions use that, but it's not very well known... yet ?