Connectionless Sockets

Summary

This chapter described the functions necessary to send UDP messages across the network to a remote host. To use UDP, the Socket object must use the SocketType.Dgram and ProtocolType.Udp parameters. Once the UDP Socket object is created, you can use the SendTo() and ReceiveFrom()methods to send and receive UDP packets.

Because UDP is a connectionless protocol, you must specify the destination address for each packet transmitted. Alternatively, you can use the standard Connect() method to specify the destination address one time, and then use the Receive() and Send() methods.

An important benefit of UDP communication is that it preserves message boundaries. All data sent in a SendTo() method is received in a single ReceiveFrom() method on the destination host. On the other hand, a drawback of UDP is that the data packet is not guaranteed to arrive at the destination. One way to detect dropped packets is to implement a retransmission system. Every packet sent to a destination should get a packet in return. If that doesn’t happen within a set time-out period, the original packet can be retransmitted. If, after several retransmissions (retries) you do not get a response, your program should inform the customer of the network problem. The ReceiveTimeout socket option can be used to set the time-out value of the socket.

Another issue for UDP is data buffer size. When the ReceiveFrom() method reads the incoming packet, it must have a data buffer big enough to accept all of the data. Any data not accepted will be lost. You can prevent this by geometrically increasing the buffer size each time an Exception is thrown on a received packet.

You can monitor the ReceiveFrom() method for Exceptions using a try-catch block, and you can test the Exception for which socket error triggered it. The ErrorCode property of the SocketException is used to determine the underlying socket error code. If the error code is 10040, the received message was too long for the receive buffer, and the message must be resent. Error code 10054 means the remote client has disconnected from the server and the connection session can be terminated.

The next chapter describes how to use the asynchronous socket methods in network programs. This enables event-driven Windows programs to utilize network programming techniques without having to worry about blocking network function calls