Some years ago, I had heard of a hashing algorithm that allows one to transmit
only diffs (or, rather, hashes) that are used to reconstruct the source file at
the destination. The name escapes me at this moment but I believe it started
with 'r'.

AFAIK, CVS doesn't use this algorithm but will transmit only diff's when it can
(eg if the client is holding a copy of the original base version, it can diff
that against the working copy). My news is at least a couple of years old,
though, so things may have changed. You might want to check with the CVS people.

I know nothing about how Perforce deals with file transmissions so I'll let
those more knowledgable answer.

HTH, Noel