How Halo 2's matchmaking works may seems like quite a mystery at first, but it's actually pretty easy to understand. Let's dive right in and I'll explain.

You're probably familiar with the traditional matchmaking model. You either host a game or you join someone else's. If you're hosting, you rule the day, controlling every aspect of the game, who's allowed in it, the map, etc. If you're joining you specify criteria for the type of game you'd like to play, things like map, number of players, game rules, etc. The game then returns a list of games most similar to these criteria, sorted by how similar they are, or sometimes by connection speed. You choose one of these games to join, hope that it hasn't filled up by the time you do, and if you're lucky grab a spot in a game with a good connection, and if you're even luckier with settings somewhat matching what you're looking for. If not you back up, choose another game, and try again. This model works. Sort of.

Behind the scenes Halo 2's matchmaking works in a similar manner. You specify match criteria by choosing Optimatch, by choosing Quickmatch to quickly recall your last used criteria, or by changing your match settings once in a Pregame Lobby. You start the game from the lobby. Halo 2 then takes over, requesting a list of games that match your criteria from the Xbox Live servers, or sometimes deciding to create a game that fits your search criteria and advertising it to others. If you're not creating then Halo 2 evaluates the list of available games, determines the game with the best connection that best matches your match criteria, and tries to join it. If the join fails another game is chosen automatically and you again try to join it. Wash , rinse, repeat, all completely automated until a match is finally made.

So what are the advantages of automating this process? There are actually quite a few, which we'll cover in detail. First, if you've read my other article about the stats system you might recall the very last paragraph, where I expound on the goals of skill levels in Halo 2. I won't repeat all of it, but the gist of it is this: our ranking system is not about bragging rights, it's about helping people find challenging but fair games, the types of games we consider the most enjoyable. The same can be said about our matchmaking system. Halo 2 matchmaking is not intended to give you complete control over every aspect of who and what you play, its goal is to find you challenging opponents that you can square off against on a level playing field. The level system is key for finding opponents of comparable skill, and matchmaking makes good use of the level data when searching for games. But keep the "level playing field" portion of the above statement in mind for a moment and read on, there's more to matchmaking than just skill level.

Some of the worst abuses, from games that do not use third party controlled dedicated servers, involve taking advantage of being the host. If you want a solid game, want to be sure that that host doesn't end it just as you're about to score and win, you need to host yourself. Of course not everyone's bandwidth supports this, and of course if everyone could host then no one would join anyone else's game, and so most players are left at the host's mercy. This is one of the biggest hurdles many games face, but Halo 2 dispenses with these problems. First, the matchmaking system determines the host automatically and invisibly (this is true for all Halo 2 games, not just matchmade). Second, the host of a matchmade game has no option to end the game, all he can do is leave the game. Finally, if the host leaves, powers off, loses his connection, whatever, a new host is automatically chosen and the game continues without him (we call this host migration).

Of course, pairing you up with others is not just about bandwidth and connection quality, your skill level, the size of your party, your geographic region, and other factors are all taken into account to ensure a quality match. This is another benefit of this automated system, these evaluations happen quickly and efficiently, completely behind the scenes.

Alright, let's review what we know so far. Matchmaking in Halo 2 does the following:

You choose a matchmaking playlist and start the game from the Pregame Lobby...

	We find a large set of other players that want to play using the same playlist
	We use a number of criteria to group you with others that want to play in the same playlist

So now we've managed to find a very solid group of similarly skilled opponents with good connectivity. The hard part is done. However, there's another problem we must deal with. In the traditional hosting model there's actually more advantage to being the host than just the ability to end the game when you're losing. Even if the host plays the game fair, he still has total control over what games you play. Sure, if the host insists on playing games that you hate, or that just aren't fairly balanced, you can leave and try to find another host. But there's no guarantee you'll find a better game, and certainly no guarantee you'll find as good a host. This situation is exacerbated if you have friends in the game with you, as leaving the bad host also means leaving your friends. Thus not only does Halo 2 matchmaking choose the host for you automatically, it also chooses the map and game rules automatically.

This inability to control exactly what you play and which map you play it on can be a little frustrating at times, but there's reason behind it. The choice of map and game is not random, they are chosen based on criteria provided by Bungie, designed to be fun for the size and composition of the group of players in the game. Of course you do have a little bit of say in what you play through your choice of matchmaking playlists. Want to play 1x1 games? Sure thing. FFA games? No problem. Small team games? Large team games? You got it.

The games in each playlist and the very playlists themselves are all part of a dynamic system. The set of playlists available at launch is what we consider the mainstay of Halo 2 online, matching the sizes and types of groups that we've designed the multiplayer game for. That said, we actively manage this system and almost certainly will add playlists as the game evolves and as we get a better feel for what people find most enjoyable. Some of these will probably be short-lived, a break from the ordinary, but if something is popular we'll definitely consider keeping it around. The possibilities are endless, from super-specialized one game on one map playlists to wildly varying playlists that incorporate just about every game on every map. You can have a say in this process by voicing your thoughts on the playlists and playlist games in the Bungie.net forum dedicated to just this topic, the Halo 2 Matchmaking Forum (post Nov. 9th ).

At this point you might be wondering why, since we allow you to specify certain criteria like size of game, we don't just allow all other sorts of other criteria, or make a billion different playlists to cater to every whim. This is simply an issue of scale. Despite Halo 2's popularity there's actually a pretty small set of quality matches for any given player. Remember, we're filtering and sorting the list based on a large number of restrictive factors, not the least of which are skill level and connection quality. To be blunt, we give you as much choice as we feel comfortably able to give without over-fragmenting the players using matchmaking. No one wants to have to wait all evening to find a match, and keeping the number of playlists to a minimum ensures that matches are made quickly. Again, keep in mind that, although the current set of playlists is admittedly small, we have the ability to add new playlists and we almost certainly will once the dust settles after launch.

Alright, where was I. Ah...now we're just about done. We've found you a match. If you're playing a team game we organize you into as fair teams as possible, respecting party groupings so that party members play on the same team. This not only guarantees that you get to play with your friends instead of against them, it also ensures that no one on your team is working for the enemy. For details on the team balancing see the Stats Overview article.

Once teams are determined you're given 10 seconds to see which game we've chosen, to meet your opponents in FFA games, or to meet your teammates in team games, and then the matchmade game starts, again automatically. The final matchmaking process breaks down as follows, where these steps roughly match the steps shown on the matchmaking progress screen:

(you choose a matchmaking playlist and start the game from the Pregame Lobby...)

	We find a large set of other players that want to play using the same playlist
	We use a number of criteria to group you with others that want to play in the same playlist
	We configure the game settings and organize you into teams
	We give you a 10 second count and start the game

Whew, that's it! If we've done our jobs then you've found a great game and had a blast playing it. Win or lose, it has been a fun challenge. You're now at the Postgame Carnage Report, where you're free to chat with the entire group of players, not just your teammates. You're also able to choose other players in the game and send Party invites, Friend requests, and Clan invites. If you like some of the people you've been matched with try forming a party with them and jumping back into matchmaking together or playing a custom game together. There's no permanent commitment, but if things go well you might make some new friends. Once you're done playing be sure to return to Bungie.net to discuss the matchmaking playlists and to help us make the system better. Until then, see you online!