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High-level vs low-level API¶
The following explains the differences of high- and low-level networking in Godot as well as some fundamentals. If you want to jump in head-first and add networking to your first nodes, skip to Initializing the network below. But make sure to read the rest later on!
Godot always supported standard low-level networking via UDP, TCP and some higher-level protocols such as HTTP and SSL. These protocols are flexible and can be used for almost anything. However, using them to synchronize game state manually can be a large amount of work. Sometimes that work can't be avoided or is worth it, for example when working with a custom server implementation on the backend. But in most cases, it's worthwhile to consider Godot's high-level networking API, which sacrifices some of the fine-grained control of low-level networking for greater ease of use.
This is due to the inherent limitations of the low-level protocols:
TCP ensures packets will always arrive reliably and in order, but latency is generally higher due to error correction. It's also quite a complex protocol because it understands what a "connection" is, and optimizes for goals that often don't suit applications like multiplayer games. Packets are buffered to be sent in larger batches, trading less per-packet overhead for higher latency. This can be useful for things like HTTP, but generally not for games. Some of this can be configured and disabled (e.g. by disabling "Nagle's algorithm" for the TCP connection).
UDP is a simpler protocol, which only sends packets (and has no concept of a "connection"). No error correction makes it pretty quick (low latency), but packets may be lost along the way or received in the wrong order. Added to that, the MTU (maximum packet size) for UDP is generally low (only a few hundred bytes), so transmitting larger packets means splitting them, reorganizing them and retrying if a part fails.
In general, TCP can be thought of as reliable, ordered, and slow; UDP as unreliable, unordered and fast. Because of the large difference in performance, it often makes sense to re-build the parts of TCP wanted for games (optional reliability and packet order), while avoiding the unwanted parts (congestion/traffic control features, Nagle's algorithm, etc). Due to this, most game engines come with such an implementation, and Godot is no exception.
In summary, you can use the low-level networking API for maximum control and implement everything on top of bare network protocols or use the high-level API based on SceneTree that does most of the heavy lifting behind the scenes in a generally optimized way.
Most of Godot's supported platforms offer all or most of the mentioned high- and low-level networking features. As networking is always largely hardware and operating system dependent, however, some features may change or not be available on some target platforms. Most notably, the HTML5 platform currently offers WebSockets and WebRTC support but lacks some of the higher-level features, as well as raw access to low-level protocols like TCP and UDP.
More about TCP/IP, UDP, and networking: https://gafferongames.com/post/udp_vs_tcp/
If you want to use your low-level networking library of choice instead of Godot's built-in networking, see here for an example: https://github.com/PerduGames/gdnet3
Adding networking to your game comes with some responsibility. It can make your application vulnerable if done wrong and may lead to cheats or exploits. It may even allow an attacker to compromise the machines your application runs on and use your servers to send spam, attack others or steal your users' data if they play your game.
This is always the case when networking is involved and has nothing to do with Godot. You can of course experiment, but when you release a networked application, always take care of any possible security concerns.
Before going into how we would like to synchronize a game across the network, it can be hel