tcp(4) BSD Kernel Interfaces Manual tcp(4)
NAME
tcp -- Internet Transmission Control Protocol
SYNOPSIS
#include <sys/types.h>
#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 the SOCK_STREAM
abstraction. TCP uses the standard Internet address format and, in addi-
tion, provides a per-host collection of ``port addresses''. Thus, each
address is composed of an Internet address specifying the host and net-
work, 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 to the socket is the address associated with the network inter-
face 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(2) and tested with getsockopt(2):
TCP_NODELAY 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.
The boolean option TCP_NODELAY defeats this algo-
rithm.
TCP_MAXSEG By default, a sender- and receiver-TCP will nego-
tiate among themselves to determine the maximum
segment size to be used for each connection. The
TCP_MAXSEG option allows the user to determine the
result of this negotiation, and to reduce it if
desired.
TCP_NOOPT TCP usually sends a number of options in each
packet, corresponding to various TCP extensions
which are provided in this implementation. The
boolean option TCP_NOOPT is provided to disable
TCP option use on a per-connection basis.
TCP_NOPUSH By convention, the sender-TCP will set the
``push'' bit, and begin transmission immediately
(if permitted) at the end of every user call to
write(2) or writev(2). When this option is set to
a non-zero value, TCP will delay sending any data
at all until either the socket is closed, or the
internal send buffer is filled.
TCP_KEEPALIVE The TCP_KEEPALIVE options enable to specify the
amount of time, in seconds, that the connection
must be idle before keepalive probes (if enabled)
are sent. The default value is specified by the
MIB variable net.inet.tcp.keepidle.
TCP_CONNECTIONTIMEOUT The TCP_CONNECTIONTIMEOUT option allows to specify
the timeout, in seconds, for new, non established
TCP connections. This option can be useful for
both active and passive TCP connections. The
default value is specified by the MIB variable
net.inet.tcp.keepinit.
The option level for the setsockopt(2) call is the protocol number for
TCP, available from getprotobyname(3), or IPPROTO_TCP. All options are
declared in <netinet/tcp.h>.
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.
Non-blocking connect
When a TCP socket is set non-blocking, and the connection cannot be
established immediately, connect(2) returns with the error EINPROGRESS,
and the connection is established asynchronously.
When the asynchronous connection completes successfully, select(2) or
poll(2) or kqueue(2) will indicate the file descriptor is ready for writ-
ing. If the connection encounters an error, the file descriptor is
marked ready for both reading and writing, and the pending error can be
retrieved via the socket option SO_ERROR.
Note that even if the socket is non-blocking, it is possible for the con-
nection to be established immediately. In that case connect(2) does not
return with EINPROGRESS.
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 net-
work address for which no network interface exists;
[EAFNOSUPPORT] when an attempt is made to bind or connect a socket to
a multicast address;
[EINPROGRESS] returned by connect(2) when the socket is set non-
blocking, and the connection cannot be immediately
established;
[EALREADY] returned by connect(2) when connection request is
already in progress for the specified socket.
SEE ALSO
connect(2), getsockopt(2), kqueue(2), poll(2), select(2), socket(2),
sysctl(3), inet(4), inet6(4), ip(4), ip6(4), netintro(4), setkey(8)
HISTORY
The TCP protocol appeared in 4.2BSD.
The socket option TCP_CONNECTIONTIMEOUT first appeared in Mac OS X 10.6.
4.2 Berkeley Distribution February 28, 2007 4.2 Berkeley Distribution
Mac OS X 10.9 - Generated Wed Oct 16 18:09:26 CDT 2013