FOSS gamedev and creative worlds

Does the client download mods from servers if it has the same version?

I want to know how much needless traffic is generated. Has MT a mechanism to reduce the download sizes?

There are some servers where you wait 10min just for the assets to download.

If not what would the best way to deal with it? Create hashes, transfer them, match then and then copy/link the local copy in the cache for the server?

The client doesnt download the mods, it just downloads the media (textures, sounds, models) and the node and item definitions.
Media is cached, so you dont download it twice (except models).