Socket types

Sockets have types that reflect the communication properties visible to a user. Processes are presumed to communicate only between sockets of the same type, although there is nothing that prevents communication between sockets of different types should the underlying communication protocols support this.

There are several types of sockets currently available:

• A stream socket provides for the bi-directional, reliable, sequenced, and unduplicated flow of data without record boundaries. A pair of connected stream sockets provides an interface nearly identical to that of pipes.

• A datagram socket supports bi-directional flow of data that is not promised to be sequenced, reliable, or unduplicated. That is, a process receiving messages on a datagram socket may find messages duplicated and possibly in an order different from the order in which they were sent. An important characteristic of a datagram socket is that record boundaries in the data are preserved. Datagram sockets closely model the facilities found in many contemporary packet switched networks such as the Ethernet.

• A raw socket provides access to the underlying communication protocols that support socket abstractions. These sockets are normally datagram oriented, although their exact characteristics are dependent on the interface provided by the protocol. Raw sockets are not intended for the general user; they have been provided mainly for users interested in developing new communication protocols or gaining access to some of the more esoteric facilities of an existing protocol. The use of raw sockets is considered under ``Advanced topics'' below.