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(2) system call must be used after binding the socket with the bind(2) system call. Only passive sockets may use the accept(2) call to accept incoming connections. Only active sockets may use the connect(2) call to initiate connections.
Passive sockets may ``underspecify'' their location to match incoming connection requests from multiple networks. This technique, termed ``wildcard addressing'' allows a single server to provide service to clients on multiple networks. To create a socket which listens on all networks, the Internet address INADDR_ANY must be bound. The TCP port may still be specified at this time; if the port is not specified the system will assign one. Once a connection has been established the socket's address is fixed by the peer entity's location. The address assigned 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 which is set with setsockopt(2) and tested with getsockopt(2). 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 acknowledgement is received. For a small number of clients, such as window systems that send a stream of mouse events which receive no replies, this packetization may cause significant delays. Therefore, TCP provides a boolean option, TCP_NODELAY (from Aq Pa netinet/tcp.h , to defeat this algorithm. The option level for the setsockopt call is the protocol number for TCP available from getprotobyname(3).
Options at the IP transport level may be used with TCP see ip(4). Incoming connection requests that are source-routed are noted, and the reverse source route is used in responding.