librsync is a library for building rsync workalikes. It is not compatible with rsync itself.

librsync and the rdiff binary that wraps it can create a signature from a destination file, create a patch from a signature and a source file, and can apply a patch to a destination file. And that's about it. librsync doesn't concern itself with the networking. That's up to you.

rdiff is a thin wrapper around librsync. librsync can easily do anything rdiff can do, without having to fork a new process. You might wish the rsync executable were built this way, but it is not.