The Internet protocol family is a collection of protocols layered on the Internet Protocol (IP) transport layer, and using the Internet address format. This section describes the Transmission Control Protocol and User Datagram Protocol.
The Transmission Control Protocol (TCP) provides a reliable, flow- controlled, two-way transmission of data. It is a byte-stream protocol used to support the SOCK_STREAM abstraction. TCP uses the standard Internet address format and, in addition, provides a per host collection of port addresses. Thus, each address is composed of an Internet address specifying the host and network with a specific TCP port on the host identifying the peer entity.
Sockets utilizing the TCP protocol are either active or passive. Active sockets initiate connections to passive sockets. By default, TCP sockets are created active; to create a passive socket the listen system call must be used after binding the socket with the bind system call. Only passive sockets may use the accept call to accept incoming connections. Only active sockets may use the connect call to initiate connections.
Passive sockets may underspecify their location to match incoming connection requests from multiple networks. This technique, called wildcard addressing, allows a single server to provide service to clients on multiple networks. To create a socket that listens to all hosts on any network, the Internet address INADDR_ANY must be bound. The TCP port must be specified at this time. If the Internet address is not INADDR and the port is not specified, the system will assign a port. Once a connection has been established, the socket's address is fixed by the peer entity's location. The address assigned to the socket is the address associated with the network interface through which packets are being transmitted and received. Normally this address corresponds to the peer entity's network.
TCP supports one socket option that is set with setsockopt and tested with getsockopt. Under most circumstances, TCP sends data when it is presented; when outstanding data has not yet been acknowledged, it gathers small amounts of output to be sent in a single packet once an acnowledgement is received. For a small number of clients, such as window systems that send a stream of mouse events that receive no replies, this packetization may cause significant delays. Therefore, TCP provides a Boolean option, TCP_ NODELAY (from <netinet/tcp.h>), to defeat this algorithm. The option level for the setsockopt call is the protocol number for TCP, which is available from getprotobyname.
User Datagram Protocol (UDP) is a simple, unreliable datagram protocol used to support the SOCK_DGRAM abstraction for the Internet protocol family. UDP sockets are connectionless and are normally used with the sendto and recvfrom calls, though the connect call may also be used to fix the destination for future packets (in which case the recv or read or write system calls may be used).
UDP address formats are identical to those used by TCP. In particular, UDP provides a port identifier in addition to the normal Internet address format. Note that the UDP port space is separate from the TCP port space (for example, a UDP port may not be connected to a TCP port). Also, broadcast packets may be sent (assuming the underlying network supports this) by using a reserved broadcast address; this address is network interface dependent. The SO_BROADCAST option must be set on the socket and the process must have the SYSPRV or BYPASS privilege for broadcasting to succeed.