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TCP(4)                   BSD Kernel Interfaces Manual                   TCP(4)

NAME
     tcp -- Internet Transmission Control Protocol

SYNOPSIS
     #include <&lt;sys/socket.h>&gt;
     #include <&lt;netinet/in.h>&gt;

     int
     socket(AF_INET, SOCK_STREAM, 0);

     int
     socket(AF_INET6, SOCK_STREAM, 0);

DESCRIPTION
     The TCP protocol provides a reliable, flow-controlled, two-way transmis-
     sion of data.  It is a byte-stream protocol used to support the
     SOCK_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) sys-
     tem 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 several socket options which are set with setsockopt(2) and
     tested with getsockopt(2).

     TCP_NODELAY
     Under most circumstances, TCP sends data when it is presented; when out-
     standing 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.

     TCP_NOPUSH
     By convention, the TCP sender will set the ``push'' bit and begin trans-
     mission 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, the
     internal send buffer is filled, or this option is set to a zero value.

     TCP_MAXSEG
     Set the maximum segment size for this connection.  The maximum segment
     size can only be lowered.

     TCP_SACK_ENABLE
     Use selective acknowledgements for this connection.  See options(4).

     TCP_MD5SIG
     Use TCP MD5 signatures per RFC 2385.  This requires Security Associations
     to be set up, which can be done using ipsecctl(8).  When a listening
     socket has TCP_MD5SIG set, it accepts connections with MD5 signatures
     only from sources for which a Security Association is set up.  Connec-
     tions without MD5 signatures are only accepted from sources for which no
     Security Association is set up.  The connected socket only has TCP_MD5SIG
     set if the connection is protected with MD5 signatures.

     The option level for the setsockopt(2) 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) or
     ip6(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 retrans-
                      missions;

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

     [ECONNREFUSED]   when the remote peer actively refuses connection estab-
                      lishment (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.

SEE ALSO
     tcpbench(1), getsockopt(2), socket(2), inet(4), inet6(4), ip(4), ip6(4),
     netintro(4), ipsecctl(8), tcpdrop(8)

HISTORY
     The tcp protocol stack appeared in 4.2BSD.

BSD                             March 28, 2017                             BSD