The librsync library does network delta-compression of
streams and files. The algorithm is similar to that used in the rsync and xdelta programs. Unlike
diff and xdelta, librsync does not require access to both of
the files on the same machine, but rather only a short
signature of the old file and the complete contents of the new
librsync ships with a tool called rdiff which does delta compression from the command line or from scripts.
librsync does not speak the application-specific protocol used by rsync. They only share an algorithm, not any code.
The current release is librsync-0.9.6, which was released in 2004 and seems stable.
From April 2014, the code has been moved to https://github.com/librsync/librsync.