Gaffer on Games

Hi, I’m Glenn Fiedler, I’m a professional game developer working in Los Angeles.

In this series of articles, I share some of the basic game networking techniques with you. Until now, a lot of this information has been hard to find, now it’s all in one place and written specifically covering techniques for multiplayer action games.

Maybe you are a hobbyist programmer looking to break-in to game programming, or just somebody interested in learning how multiplayer games work. You may even be a professional game programmer searching for hard to find information about how to network your game.

I hope you find what you are looking for here.

If you find these articles useful and you would like to read more, please donate! Donations encourage me to write more (each article usually takes all my spare time for at least a week…), and I’m very busy these days working for Sony Santa Monica, so I don’t get a lot of free time!

Again, let me reiterate that, if you would like me to finish writing this series please donate. If I get enough donations, I’ll publish a new article, it’s as simple as that!

Thanks!

Glenn Fiedler
gaffer@gaffer.org

UDP vs. TCP

What is the best way to send data between machines? Do you use TCP sockets, UDP sockets or a mixture of both? In this article we explore this fundamental choice, then decide on the best option for networking games.

Sending and Receiving Packets

This article shows how to send and receive UDP packets using BSD sockets. We then wrap this code into a platform independent socket class you can use across MacOS X, Windows and Unix.

Virtual Connection over UDP

In this article I show you how to build your own concept of virtual connection between two machines on top of UDP. We cover the basics of what it means to be connected, how to detect connection, how to filter out packets from other machines, and how to detect disconnection with timeouts.

Reliability and Flow Control

We want to use UDP to network our game, but UDP does not provide any reliability or flow control. This article covers the basic concepts you need to implement your own reliability system: sequence numbers and acks. We then implement a simple flow control system on top of our reliability system to avoid flooding the connection with too much data.

Debugging Multiplayer Games

Multiplayer games are much harder to debug than singleplayer ones. This article describes they key techniques professional game developers use when debugging multiplayer games.

What Every Programmer Needs To Know About Game Networking

An overview of common multiplayer programming techniques and the history of their development. Discover how RTS games were able to synchronize thousands of units over a 28k modem, and how first person shooters hide lag with client side prediction and latency compensation.

Floating Point Determinism

For one reason or another it is considered very difficult to get exactly the same result from floating point calculations on two different machines. People even report different results on the same machine from run to run, and between debug and release builds. Other folks say that AMDs give different results to Intel machines, and that SSE results are different from x87. What exactly is going on? Are floating point calculations deterministic or not?

Building a Node Mesh

In this article we discuss client/server topology vs. peer-to-peer, then design a simple node mesh structure that may be used to represent either topology. The concept of delta encoding is also introduced, then applied as a bandwidth optimization for mesh updates.

Basic LAN Matchmaking

Even wondered how classic games like Doom or Quake were able to automatically “see” servers running on your LAN? The answer is UDP broadcast packets, a way to send packets to all computers on the LAN-subnet. In this article I show you how to use broadcast packets to create your own basic LAN matchmaking system.

Real World Networking

Welcome to the real world, where routers and real world difficulties like NAT punch-through get in the way of you and the computers you want to talk to. Instead of writing our own NAT punch-through, we solve this problem by creating an abstract packet transport interface, then create implementations of this transport layer using OpenTNL, RakNet and eNet, as well as our own LAN matchmaking and BSD sockets based comms.

Reading and Writing Packets

In this article I show you how to efficiently read and write packets with a bit-packer. I also show you how to implement checkpoints and journaling to detect when you desync packet read and write, as well as detect invalid packets. Finally, I show you a cool trick that unifies read and write into a single serialize operation.

Synchronizing State

How do you synchronize state between two machines? I show you the basics in this article covering object prioritization, object views, and ack aware state replication. Using this system you can easily synchronize the positions of players, and state of game objects in your world. I also introduce the concept of “soft delta-encoding” and show you how to packet the most important objects in your packet, while maintaining a constant packet size.

Network Events

Not everything can be represented as state, even though it would be nice. Sometimes you have to send events. In this article I show you how to implement a reliable event system based on acks from our packet level reliability system. I’ll show you how to implement multiple streams of events, with different reliability guarantees: unreliable, reliable, reliable-ordered, most-recent and important. The last one the is the key for absolute minimum latency events!