Congestion Control

In an ideal world, your game can send messages as often as it needs to. They arrive at the target immediately and are processed instantaneously. If all of the computers in your game have ample processing power and are connected by a lightly used high-bandwidth network link, you might approach this ideal situation. You can then send messages as often as you like. However, a number of factors can create congestion and cause messaging to work more slowly than this ideal:

• Network latency. Even under ideal conditions, messages take a finite time to traverse the network from sender to target, especially over the Internet. There might be further delays for acknowledgments, retransmission of lost packets, or reassembly of out-of-order packets.
• Network bandwidth. The network bandwidth controls the rate at which a message can be sent or received by a computer. Network links have a wide range of bandwidths, and even high-bandwidth networks might be slowed by high traffic levels. If one or more of your players has a low-bandwidth connection, they will be able to send and receive messages only at a limited rate.
• Processing speed. Even if network bandwidth is high and latency low, the target application still needs some time to process a received message. If one or more of the players in a session is using a relatively slow computer, the rate at which they can process received messages might be below the rate at which messages can be sent.

Message Throttling

If there is no control over the rate at which messages are sent, a target can be flooded by more messages than it can handle. To prevent this situation, the Microsoft® DirectPlay® protocol throttles the rate at which messages are sent. The net effect of throttling is that the rate at which messages are sent is controlled by the rate at which the target can handle them.

Throttling is implemented with a sliding window mechanism. The sliding window is basically a queue with a limited number of slots that holds messages that have been sent but not yet received. All outgoing messages are placed in this queue, regardless of their category. When the sent-message queue is full, it accepts no more outgoing messages until one of the messages in the queue has been received.

For optimal performance, the size of the sliding window must be matched to current network conditions. The DirectPlay protocol automatically monitors such factors as the number of messages and the total number of bytes in the sent-message queue. This information is then used to dynamically adjust the size of the sliding window to optimize messaging for the current network conditions.

Connection Checking

If there is no activity on a link, the DirectPlay protocol periodically tests the connection by sending an empty reliable packet. If no acknowledgment is received from the target after a reasonable number of attempts, DirectPlay concludes that the link has been disconnected.