Message Categories

The Microsoft?DirectPlay?protocol is designed to handle the following two basic types of network messaging.

• Reliable versus unreliable messaging determines whether messages are guaranteed to be delivered to the target application.
• Non-sequential versus sequential messaging determines whether messages are received by the target application in the same order they are sent.

Games use messaging for a variety of purposes, each with different demands. To support this range of messaging needs, the DirectPlay protocol enables you to designate a message as belonging to one of four categories:

• Reliable and sequential
• Unreliable and sequential
• Reliable and non-sequential
• Unreliable and non-sequential

The DirectPlay protocol enables you to optimize your messaging strategy by assigning categories on a message-by-message basis.

Reliable and Unreliable Messaging

Messages are sometimes lost in transit. Reliable messaging provides a guarantee that the target will receive every message. This type of messaging is required when data loss cannot be tolerated. Most reliable messaging schemes require the target to acknowledge receipt of each message. If the sender does not receive an acknowledgment within a specified timeout period, it resends the message. This process typically continues until the sender receives an acknowledgment, confirming that the message has arrived.

The DirectPlay protocol imposes a limit on the number of resend attempts. If no acknowledgment is received after a reasonable number of attempts, DirectPlay assumes that the connection has been lost, and closes it.

Unreliable messaging is the simplest form of network communication. It might be faster than reliable messaging because there is no guarantee that the message will be delivered to the target. The sender transmits the message. If the target does not receive the message, the sender will not transmit the message again, and the packet is lost.

Unreliable messaging is used primarily when speed or bandwidth is more important than an occasional lost message. For example, high-bandwidth streaming media applications often use unreliable messaging. They cannot afford to take up bandwidth with acknowledgments and retransmissions, nor can they wait for a lost message to be retransmitted. An occasional lost message normally has only a minor impact on quality, so it can be ignored.

Sequential and Non-Sequential Messaging

Messages leave the sender in a particular sequence. However, there is no guarantee that messages will arrive at the target's computer in the same order that they are sent. For example, if a message is lost and must be retransmitted, that message will typically arrive later than messages that followed it in the original sequence.

Sequential messaging uses sequencing information embedded in the message to ensure that the messages are presented to the target application in the correct order. This type of messaging is required when the target application must receive messages in the correct order. Out-of-order messages are buffered until the missing messages arrive.

Non-sequential messaging presents the received messages to the target as soon as they arrive at the target computer, regardless of the order in which they were sent. Because there is no need to wait for a missing packet, applications often use non-sequential messaging when speed is more important than an occasional out-of-order message. The out-of-order message is ignored.

Choosing the Best Message Category

Choosing the best category for messages is a core issue for multiplayer game developers. While DirectPlay provides the tools to manage your messaging, the choice of a message category ultimately depends on the semantic content of the message and the nature of the game.

The following are general guidelines for choosing the best message category.

• Use non-guaranteed messaging whenever the content permits. For example, your game might send frequent player-location updates. Each update is independent, and it supersedes any previous updates. If an update is lost, the next update is sufficient to maintain the player's game state. A lost and retransmitted message might arrive later than the subsequent update message.
• Use guaranteed messaging when data loss cannot be tolerated. For example, a text-based chat feature depends on every character being delivered to its target.
• Use sequential messaging when the order of the messages is important. For example, streaming media typically uses sequential-unreliable messaging. An occasional dropped message can be tolerated, but an out-of-order message would cause problems.