that would involve describing it

Describing just one part of it could be sufficient to sue, but not sufficient to develop a client.

Software patents are intentionally vague, so I doubt anybody could write a working client based on a patent alone (even if whole protocol was claimed to be described).

You are probably correct, but this is an abuse of the patent system, since the quid pro quo for patent protection is the disclosure of the invention to advance the state of the art.