CSM
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CitiesSkylinesMultiplayer
Use relay server so no port forwarding is necessary
To simplify hosting a server, we should provide a possibility where port forwarding is not necessary. Different levels of relaying could be possible:
- Use a technique called UDP hole punching to create the connection, then send messages directly (https://en.wikipedia.org/wiki/UDP_hole_punching). LiteNetLib has support for this. An example can be found here: https://github.com/RevenantX/LiteNetLib/issues/51#issuecomment-292790482
- Relay all messages (needs much more performance on the relay)
Things to discuss:
- What if the relay is not reachable? Fallback to the current solution?
- Hardcode the relay IP/domain? Allow the user to enter a custom IP?
I have a webserver that we can use for the first method as it doesn't have much performance or bandwidth.
It's not quite as reliable as using a 3rd-party server to holepunch (since some routers don't support the feature or have it disabled), but I would also look into UPnP Port Mapping.
It's not quite as reliable as using a 3rd-party server to holepunch (since some routers don't support the feature or have it disabled), but I would also look into UPnP Port Mapping.
UPnP Port Mapping is in theory implemented, but I have no idea if it works at all (the async work around might be broken, Cities: Skylines uses an old version of .NET that does not support async/await)
https://github.com/CitiesSkylinesMultiplayer/CSM/blob/c6205aa8c06f8ea00852b15915bb187b537f7de6/src/Networking/Server.cs#L93-L94
It does work, I guess partly. For me I was able to launch and play without doing manual port-forwarding. Although I know many do have to do it manually.
Good to know, thanks. I'll take a look today at producing a simple server + client + relay example (for UDP hole punching) (make sure it works), and then attempt to integrate that into the mod (I've got a VPS I can use).
Last time I checked on Windows, UPnP was working, although it doesn't work on my current Linux system (But I also didn't bother looking into it, so it may be my fault). I guess many people just have UPnP deactivated on their routers, which is a good decision from a security perspective for most people.
About UDP hole punching: The LunaMultiplayer mod for Kerbal Space Program has an implementation for that. A very elaborate description can be found here: https://github.com/LunaMultiplayer/LunaMultiplayer/wiki/Nat-punchthrough and the source code might also be useful.
Another option for hosting a relay server could be the oracle cloud. you can get a free vps there.
Current testing code is here:
https://github.com/CitiesSkylinesMultiplayer/HolePunchTest
Managed to get it working one way (with no port forwarding), but not the other way. More testing required. (example client code is super messy).