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tcp(4)                   BSD Kernel Interfaces Manual                   tcp(4)


     tcp -- Internet Transmission Control Protocol


     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netinet/in.h>

     socket(AF_INET, SOCK_STREAM, 0);


     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) or connectx(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-

     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

     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.

                            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

     TCP_KEEPINTVL          When keepalive probes are enabled, this option
                            will set the amount of time in seconds between
                            successive keepalives sent to probe an unrespon-
                            sive peer.

     TCP_KEEPCNT            When keepalive probes are enabled, this option
                            will set the number of times a keepalive probe
                            should be repeated if the peer is not responding.
                            After this many probes, the connection will be

     TCP_SENDMOREACKS       When a stream of TCP data packets are received, OS
                            X uses an algorithm to reduce the number of
                            acknowlegements by generating a TCP acknowlegement
                            for 8 data packets instead of acknowledging every
                            other data packet. When this socket option is
                            enabled, the connection will always send a TCP
                            acknowledgement for every other data packet.

     TCP_ENABLE_ECN         Using Explicit Congestion Notification (ECN) on
                            TCP allows bi-directional end-to-end notification
                            of congestion without dropping packets. Conven-
                            tionally TCP/IP networks signal congestion by
                            dropping packets. When ECN is successfully negoti-
                            ated, an ECN-aware router may set a mark in the IP
                            header instead of dropping a packet in order to
                            signal impending congestion. The TCP receiver of
                            the packet echoes congestion indication to the TCP
                            sender, which reduces it's transmission rate as if
                            it detected a dropped packet. This will avoid
                            unnecessary retransmissions and will improve
                            latency by saving the time required for recovering
                            a lost packet.

     TCP_NOTSENT_LOWAT      The send socket buffer of a TCP sender has unsent
                            and unacknowledged data. This option allows a TCP
                            sender to control the amount of unsent data kept
                            in the send socket buffer. The value of the option
                            should be the maximum amount of unsent data in
                            bytes. Kevent, poll and select will generate a
                            write notification when the unsent data falls
                            below the amount given by this option. This will
                            allow an application to generate just-in-time
                            fresh updates for real-time communication.

     TCP_FASTOPEN           The TCP listener can set this option to use TCP
                            Fast Open feature. After setting this option, an
                            accept(2) may return a socket that is in
                            SYN_RECEIVED state but is readable and writable.

     TCP_CONNECTION_INFO    This socket option can be used to obtain TCP con-
                            nection level statistics. The "struct tcp_connec-
                            tion_info" defined in <netinet/tcp_var.h> is
                            copied to the user buffer.

     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) or connectx(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) or
     connectx(2) does not return with EINPROGRESS.


     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

     [ECONNRESET]       when the remote peer forces the connection to be

     [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) or connectx(2) when the socket
                        is set nonblocking, and the connection cannot be imme-
                        diately established;

     [EALREADY]         returned by connect(2) or connectx(2) when connection
                        request is already in progress for the specified

     [ENODATA]          returned by recv(2) or send(2) in case a connection is
                        experiencing a data-stall (probably due to a middlebox
                        issue).  It is advised that the current connection
                        gets closed by the application and a new attempt is
                        being made.


     connect(2), connectx(2), getsockopt(2), kqueue(2), poll(2), select(2),
     socket(2), sysctl(3), inet(4), inet6(4), ip(4), ip6(4), netintro(4),


     The TCP protocol appeared in 4.2BSD.

     The socket option TCP_CONNECTIONTIMEOUT first appeared in Mac OS X 10.6.

4.2 Berkeley Distribution       March 18, 2015       4.2 Berkeley Distribution

Mac OS X 10.11.6 - Generated Sat Feb 4 07:47:59 CST 2017
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