Java Network Programming, Second Edition

Chapter 10. Sockets for Clients

Data is transmitted across the Internet in packets of finite size called datagrams. Each datagram contains a header and a payload. The header contains the address and port to which the packet is going, the address and port from which the packet came, and various other housekeeping information used to ensure reliable transmission. The payload contains the data itself. However, since datagrams have finite length, it’s often necessary to split the payload across multiple packets and reassemble it at the destination. It’s also possible that one or more packets may be lost or corrupted in transit and need to be retransmitted or that packets will arrive out of order. Keeping track of this—splitting the payload into packets, generating headers, parsing the headers of incoming packets, keeping track of what packets have and haven’t been received, etc.—is a lot of work and requires a lot of intricate code.

Fortunately, you don’t have to do the work yourself. Sockets are an innovation of Berkeley Unix that allow the programmer to treat a network connection as just another stream onto which bytes can be written and from which bytes can be read. Historically, sockets are an extension of one of Unix’s most important ideas: that all I/O should look like file I/O to the programmer, whether you’re working with a keyboard, a graphics display, a regular file, or a network connection. Sockets shield the programmer from low-level details of the network, such as media ...