General :: Upload Only Modified Files To FTP Server

I want to upload files from my computer to an FTP site and I don't want to upload files that are already on the server. So I need a tool that finds out which local files that are different from the ones on the server, or that don't exists on the server.

Some requirements:

I'm using a cheap provider that does not support rsync or ssh, so I can only use FTP. I generate the files before uploading them, so comparing timestamps is meaningless. I've tried lftp with the mirror command. It's slow (I think it uploads all the files). I upload the files from different computers, so I can't use sitecopy, which uses a local database to keep track of which files are on the server. I'd like to be able to upload all changed files with one command. Preferably no GUI application. And it needs to run in Ubuntu.

I was thinking about creating a tool similar to sitecopy, but which stores checksums of all the files on the FTP server on the server itself. But then I thought that there may already be such a tool.