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tcp (Internet Transmission Control Protocol)
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#include <sys/types.h>
#include  <sys/socket.h>
#include <netinet/in.h>

socket(AF_INET, SOCK_STREAM, 0);

TCP (Internet Transmission Control 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 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, 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 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 a number of socket options which can be set with setsockopt and tested with getsockopt:

The option level for the getsockopt call is the protocol number for TCP, available from getprotobyname(), or IPPROTO_TCP. All options are declared in <netinet/tcp.h>.

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

MIB variables

The TCP protocol implements a number of variables in the net.inet.tcp branch of the sysctl MIB.


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
EAFNOSUPPORT When an attempt is made to bind or connect a socket to a multicast address


The TCP protocol appeared in 4.2BSD. The RFC 1323 extensions for window scaling and timestamps were added in 4.4BSD. The TCP_INFO option was introduced in Linux 2.6 and is subject to change.


Versions Link to
INtime 4.0 netlib.lib

See Also

getsockopt, socket, sysctl, blackhole, inet, IP, syncache, syncookies

V. Jacobson, R. Braden, and D. Borman, TCP Extensions for High Performance, RFC 1323.

A. Heffernan, Protection of BGP Sessions via the TCP MD5 Signature Option, RFC 2385.