GameNetworkingSockets VS comms_champion

Steam does have a NAT traversal/punchthrough service too. It's apparently usable without Steam according to their README on https://github.com/ValveSoftware/GameNetworkingSockets but honestly the only easy to use implementation I know is in Facepunch.Steamworks which requires a SteamID to initialize

Halo was mostly all about single player and early multiplayer/local multiplayer but their online netcode has sucked since Blood Gulch. Lots of games do networking horribly, I have been in gamedev making networking and I hate most of what people do. The ones that have a clean natting, based on enet style reliable UDP channels, RakNet style punch are better (RakNet was good until Facebook bought it). It has come a long way but also fallen back. Valve source netcode (on github) is probably the best and you can check it out here. They started with the best in Quake networking, then to Source.

There are some general APIs that are tied to Steam, like some networking stuff (even then there's an open source library), but the devs are not required to use any of those. If the devs chose to develop exclusively for steam and use their libraries, then that's their own decision.

Keep in mind that if you do go the GodotSteam route, there's GameNetworkingSockets from valve that doesn't have the requirement of needing Steam that you can use for releasing on other platforms. It has the same interface as the Steamworks SDK without requiring the Steamworks SDK.

Their UDP-but-connection-based game networking protocol can be used completely separetely from Steam (it also supports encryption). I think that does not include their relay networking solution, but still.. maybe it's helpful. They also have a TCP-like interface to make it easy to plug into an existing solution.

You can also use the non-Steam parts of that library in any game in case you don't just release on Steam (have a look at https://github.com/ValveSoftware/GameNetworkingSockets )

If you want to go the route of using a raw protocol and packing your messages yourself, I suggest to at least have a look at Valve's GameNetworkingSockets either for inspiration of what you might need or even to just use the library: https://github.com/ValveSoftware/GameNetworkingSockets

There is early NAT piercing support in Valve's GameNetworkingSockets. Check out:

You can also try Valve's GameNetworkingSockets with a serialization of your choice.

I've been developing CommsChampion Ecosystem for about 9 years as my pet project. It's about easy and compile-time configurable implementation of binary communication protocols using C++11 programming language, with main focus on embedded systems (including bare-metal ones).

If this is the case, for the third stage of the application specific protocol handling I recommend CommsChampion Ecosystem.

I suppose if you develop something that is going to be used in a single project / product without much of a customization, then meta-programming is not really justified. The meta-programming is justified in many cases when you implement some kind of a library, which can be used in multiple independent products, and these uses may require some product specific customizations. For example, I'm developing a solution for implementing binary communication protocols for embedded systems in C++, called CommsChampion Ecosystem. The core component of which is the COMMS library. Every single use of this library requires different customization. Every application may require different polymorphic interface to handle its message objects. I use template meta-programming there to define virtual functions only needed by the application and not adding unnecessary ones. I also use template meta programming to allow customization of the storage data structures and may use different, more optimized code for some.

I use commschamp. You describe the protocol in an XML format, it only supports C++ though