TCP(7)                               SysV                               TCP(7)




NAME
     tcp - Internet Transmission Control Protocol

SYNOPSIS
     #include <sys/socket.h>
     #include <netinet/in.h>

     s = socket(AF_INET, SOCK_STREAM, 0);

DESCRIPTION
     The TCP protocol provides 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(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 <netinet/tcp.h>, to defeat
     this algorithm.  The option level for the setsockopt call is the protocol
     number for TCP, available from getprotobyname(3N).

     Options at the IP transport level may be used with TCP; see ip(4P).
     Incoming connection requests that are source-routed are noted, and the
     reverse source route is used in responding.

DIAGNOSTICS
     A socket operation may fail with one of the following errors returned:

     [EISCONN]           when trying to establish a connection on a socket
                         which already has one;

     [ENOBUFS]           when the system runs out of memory for an internal

                         data structure;

     [ETIMEDOUT]         when a connection was dropped due to excessive
                         retransmissions;

     [ECONNRESET]        when the remote peer forces the connection to be
                         closed;

     [ECONNREFUSED]      when the remote peer actively refuses connection
                         establishment (usually because no process is
                         listening to the port);

     [EADDRINUSE]        when an attempt is made to create a socket with a
                         port which has already been allocated;

     [EADDRNOTAVAIL]     when an attempt is made to create a socket with a
                         network address for which no network interface
                         exists.

SEE ALSO
     getsockopt(2), socket(2), intro(4N), inet(4F), ip(4P).