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Let's get started

What DirecPlay is About

DirectPlay is a layer above your normal network protocols (IPX, TCP/IP etc). Once the connection is made, you can send messages without needing to know what the user is connecting with. This is one of the best features (IMO) and though some of you may come up with more efficient messaging protocols with WinSock, I'd rather use DirectPlay and save me hordes of trouble. BTW, DirectPlay uses Winsock too.

Sessions in DirectPlay

A DirectPlay session is a communication channel between several computers. An application must be in a session before it can communicate with other machines. An application can either join an existing session or create a new session and wait for people to join it. Each session has one and only one host, which is the application that creates it. Only the host can change the session properties (we will get to that detail later).

The default mode in a session is peer-to-peer, meaning that the session complete state is replicated on all the machines. When one computer changes something, other computers are notified.

The other mode is client/server, which means everything is routed through a server. You manage the data in this mode, which is probably the best in a chat program. However, I used sort of a hybrid in this tutorial.

Players in DirectPlay

An application must create a player to send and receive message. Messages are always directed to a player and not the computer. At this point, your application should not even know about the computer's location. Every message you sent is directed at a specific player and every received message is directed at a specific local player (except for system messages; more on that later). Players are identified only as local (exist on your computer) or remote (exist on another computer). Though you can create more than one local player on your machine, I find it not very useful. DirectPlay offers additional methods to store application specific data so you don't have to implement a list of players but I'd rather do my own list.

You can also group players in the same session together so any message sent to you will get directed to the group. This is great for games where you can ally with other people and you want to send messages to your allies only. Unfortunately, you have to explore that area on your own.

If at this point you find all this very troublesome, try writing your own network API. You will be glad what DirectPlay does for you.

Messages in DirectPlay

Now we have a session and a player, we can start sending messages. As I said, each message sent is marked by a specific local player and can be sent to any player. Each message received is placed in a queue for that local player. Since I create only one player, all the messages belong to that player. You don't need to bother about this queue; all you need is to extract the messages from it and act on them. The application can poll the receive queue for messages or use a separate thread and events. I tried the thread method and it works great, except for when I use MessageBox to notify the user (in case you don't know, MessageBox displays a message box in Windows). If you want to use threads, you need to pause the thread when the application displays a message box and somehow it gets very messy with synchronization. So I opted for the poll method. Feel free to use threads if you think you can handle it.

There are two types of messages: player and system. Player messages have a sender and receiver. System messages are sent to every player and are marked sent from the system (DPID_SYSMSG). DP stands for DirectPlay, ID for identification. If you cannot understand messages, go and learn more about DirectDraw first. System messages are generated when the session state is changed, i.e. when a new player joins the session.

Note: There are also security features using the Security Support Provider Interface (SSPI) on windows. These messages are encrypted and such. I don't think this is of much use in gaming.

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