Using CVS_CLIENT_LOG

You can also log exactly what gets sent by the client and by the server.

Set the CVS_CLIENT_LOG environment variable to the base filename you want to use.
Logs will be created as basename.in and basename.out

Example:

$ export CVS_CLIENT_LOG=log20010610
$ cvs -Q update
$ ls log*
log20010610.in  log20010610.out
$

Note that this only works with connections to remote repositories. It will not work if commands occur on same machine.