|The Design and Implementation of the 4.4BSD Operating System|
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Interprocess communication in 4.4BSD is organized in communication domains. Domains currently supported include the local domain, for communication between processes executing on the same machine; the internet domain, for communication between processes using the TCP/IP protocol suite (perhaps within the Internet); the ISO/OSI protocol family for communication between sites required to run them; and the XNS domain, for communication between processes using the XEROX Network Systems (XNS) protocols.
Within a domain, communication takes place between communication endpoints known as sockets. As mentioned in Section 2.6, the socket system call creates a socket and returns a descriptor; other IPC system calls are described in Chapter 11. Each socket has a type that defines its communications semantics; these semantics include properties such as reliability, ordering, and prevention of duplication of messages.
Each socket has associated with it a communication protocol. This protocol provides the semantics required by the socket according to the latter's type. Applications may request a specific protocol when creating a socket, or may allow the system to select a protocol that is appropriate for the type of socket being created.
Sockets may have addresses bound to them. The form and meaning of socket addresses are dependent on the communication domain in which the socket is created. Binding a name to a socket in the local domain causes a file to be created in the filesystem.
Normal data transmitted and received through sockets are untyped. Data-representation issues are the responsibility of libraries built on top of the interprocess-communication facilities. In addition to transporting normal data, communication domains may support the transmission and reception of specially typed data, termed access rights. The local domain, for example, uses this facility to pass descriptors between processes.
Networking implementations on UNIX before 4.2BSD usually worked by overloading the character-device interfaces. One goal of the socket interface was for naive programs to be able to work without change on stream-style connections. Such programs can work only if the read and write systems calls are unchanged. Consequently, the original interfaces were left intact, and were made to work on stream-type sockets. A new interface was added for more complicated sockets, such as those used to send datagrams, with which a destination address must be presented with each send call.
Another benefit is that the new interface is highly portable. Shortly after a test release was available from Berkeley, the socket interface had been ported to System III by a UNIX vendor (although AT&T did not support the socket interface until the release of System V Release 4, deciding instead to use the Eighth Edition stream mechanism). The socket interface was also ported to run in many Ethernet boards by vendors, such as Excelan and Interlan, that were selling into the PC market, where the machines were too small to run networking in the main processor. More recently, the socket interface was used as the basis for Microsoft's Winsock networking interface for Windows.