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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 con‐
     nection 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 per‐
			    mitted) 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
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