rsync is a replacement for rcp that has many more features.

rsync uses the "rsync algorithm" which provides a very fast method for
bringing remote files into sync. It does this by sending just the
differences in the files across the link, without requiring that both
sets of files are present at one of the ends of the link beforehand.
This makes rsync a good remote file distribution/synchronisation utility
in a dialup PPP/SLIP environment.

Note: it requires rsync on the destination machine.

There is a Computer Science Technical Report on the rsync algorithm
included in the distribution, and is also available as
	ftp://samba.anu.edu.au/pub/rsync/tech_report.ps