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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
     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
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