NAME
tcp —
Internet Transmission Control
Protocol
SYNOPSIS
#include
<sys/socket.h>
#include <netinet/in.h>
int
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 theSOCK_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(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.
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
HISTORY
The tcp protocol stack appeared in
4.2BSD.