So UDP is more or less the best choice for high action client-server games like Quake from what I've seen. This can also apply to other similar games and I think also applies to the RTS we are making using client-server architecture rather than lockstep peer to peer.

The problem with UDP is it has no flow control and that has to be implemented on top of it, or it can just be forgotten and not implemented as long as there is some limit to packet send rate. DCCP looks like a version of UDP but with flow control already implemented for us.

My main concern is whether or not Steam supports games that use DCCP. I heard from people who sell their games on Steam that you have to integrate with their API and replace some netcode. I don't know the full details of that because they don't release any of that info to people who aren't signed on with them yet, and the only way to get signed on with them is to have a complete enough demo of a game which we don't have either.

Anyone out there know how flexible the Steamworks API is for supporting DCCP rather than the most common UDP and TCP/IP protocols?

Try out ENet http://enet.bespin.org/index.html it is simple and gives you what you need. The great thing about ENet is that the library is not bloated and the licensing is great.

But the problem with libraries is I have no idea how easy it will be to integrate with Steam. I can't find the info on their site because they don't reveal enough info to people unless they are already signed on as developers. I have a feeling TCP/IP and UDP will work just fine. I want to make sure that before I commit to using a library whether or not I'm wasting time and will have to reimplement the whole thing with pure UDP later myself.