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(4).
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(4), inet(4), ip(4)