TCP(4P) 																   TCP(4P)

NAME

       tcp - Internet Transmission Control Protocol

SYNOPSIS

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

       s = 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  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) 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 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 one socket option which is set with setsockopt(2) and tested with getsockopt(2).  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.   Therefore,  TCP provides a boolean option, TCP_NODELAY (from
       <netinet/tcp.h>, to defeat this algorithm.  The option level for the setsockopt call is the protocol number for TCP, available from getpro-
       tobyname(3N).

       Options at the IP transport level may be used with TCP; see ip(4P).  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 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 network address for which no network interface exists.

SEE ALSO

       getsockopt(2), socket(2), intro(4N), inet(4F), ip(4P)

4.2 Berkeley Distribution					   May 16, 1986 							   TCP(4P)