How does the Half-Life 2 multiplayer protocol work?

Question

I was wondering how the Half-Life 2 multiplayer protocol works in mods like Counter-Strike: Source or Day Of Defeat: Source. I believe that they use some kind of obfuscation and proprietary compression algorithm. I would like to know how different kinds of messages are encoded in a packet.

Answer 1

This is a really complicated question, my suggestion would be to look at some of the open source network game engines:

• http://www.hawksoft.com/hawknl/
• http://www.zoidcom.com/
• http://sourceforge.net/projects/opentnl
• http://www.gillius.org/gne/

You could also look at the source code for the quake series upon which the original half life engine is based.

Answer 2

Half-Life 2, Counter-Strike:Source etc all use Valves Source engine. Valve has a developer wiki which covers a lot of stuff (its pretty cool check it out!)...

These articles might interest you:

Latency Compensating Methods in Client/Server In-game Protocol, Design and Optimization

Source Multiplayer Networking

Answer 3

Though details might differ, the general framework is pretty old. Here's a quick overview:

In early fps games such as doom and Quake the player's position was updated only on the server's response to your move command. That is, you pressed the move-forward button and the client communicated that to the server, the server udpated your position on its memory and then relayed a new game-state to your client with your new position. This led to very laggy play: shooting, even moving in narrow corridors was a game of predicting lag.

Newer games let the client handle the player's shooting and movement by themselves. Though this led to lag-less movement and fire it opened more possibilities of cheating by hacking the client code. Now every player moves and fires independently on their own computer and communicates to the server what they have done. This only breaks down when two players bump into one another or try to catch a power up at the same time.

Now the server has this stream of client state coming from each player and has to sync them and make a coherent game out of them. The trick is to measure each player's latency. The ultimate goal is to be able to fire a very low latency weapon (such as sniper rifle or railgun) on an enemy moving sideways and have it hit correctly. If the latency from each player is know, suppose player A (latency 50ms) fires a gun on B (latency 60ms). To make a hit, the shot has to hit B where B was 60ms ago, from where A was 50ms ago.

That's a very rough overview but should give you the general idea.

Answer 4

You should check out Luigi Auriemmas papers on Half-Life. You'll find a packet decoder and some disassembled algorithms there, too.

Reverse engineering information on Half-Life 2 may be hard to come by, because of its relevance for cheating. I'd guess boards like mpcforum are your best bet.

Answer 5

I suggest that you look into Quake 1-3 engines. They are available with source code. Half-life's protocol might be a bit different but most likely close enough.