To be able to use the TCP/IP protocol suite, application programmers need an interface to the transport layer. However, such interfaces are operating system-dependent, and so there is no standard for them.
This section will briefly introduce the socket interface, which is the one used by 4.3BSD Unix OS. Its many useful and efficient features have been widely copied, but it still is not a standard. But first we will study the client-server model of interaction, which provides the basis for most network communications.