Overlay network options

When creating an overlay network for a swarm there are a number of options I can pass in as detailed here https://docs.docker.com/engine/reference/commandline/network_create/#options

However one that I am interested in seems to be missing some documentation.

–opt, -o map[] Set driver specific options

I can’t find any reference to what options I can pass in for a network with an overlay driver, I’m really interested specifically in the secure / encryption options but a full list would be nice to have!

From some research I have found that I am able to pass in encrypted as well as secure and maybe securenet flags, but I’m also able to validly pass in fubar so I don’t know which flags are valid or what they do.

Any help very much appreciated.