Thanks to Max Bowsher on the rsync list, this is it.

$ rsync --ignore-times rsync://ftp.gwdg.de/pub/linux/knoppix/KNOPPIX_V3.1-2003-01-20-EN.iso KNOPPIX_V3.1-2003-01-20-EN.iso

That short form of the option is -I, I for India, not l for lollypop.

Bob