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TCP(4)		       FreeBSD Kernel Interfaces Manual			TCP(4)

     tcp -- Internet Transmission Control Protocol

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

     socket(AF_INET, SOCK_STREAM, 0);

     socket(AF_INET6, SOCK_STREAM, 0);

     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

     Sockets utilizing the TCP protocol	are either "active" or "passive".  Ac-
     tive 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

     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 as-
     signed to the socket is the address associated with the network interface
     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).

     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.

     By	convention, the	TCP sender will	set the	"push" bit and begin transmis-
     sion 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 de-
     lay sending any data at all until either the socket is closed, the	inter-
     nal send buffer is	filled,	or this	option is set to a zero	value.

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

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

     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.

     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

     [ETIMEDOUT]      when a connection	was dropped due	to excessive retrans-

     [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

     [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.

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

     The tcp protocol stack appeared in	4.2BSD.

FreeBSD	13.0			 May 11, 2018			  FreeBSD 13.0


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