I'm not entirely sure I understand what you're asking for, but it sounds like zsync (http://zsync.moria.org.uk/ http://zsync.moria.org.uk/) is what you're looki

I'm not entirely sure I understand what you're asking for, but it sounds like zsync (http://zsync.moria.org.uk/ http://zsync.moria.org.uk/) is what you're looking for.