tcp(5P)                                                   tcp(5P)



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(2N) system call must be
     used after binding the socket with the bind(2N) system call.
     Only passive sockets may use the accept(2N) call to accept
     incoming connections.  Only active sockets may use the
     connect(2N) 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 that 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.

STATUS MESSAGES AND VALUES
     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



January 1992                                                    1

tcp(5P)                                                   tcp(5P)



          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
          that has already been allocated;

     EADDRNOTAVAIL
          when an attempt is made to create a socket with a
          network address for which no network interface exists.

LIMITATIONS
     It should be possible to send and receive TCP options.  The
     system always tries to negotiate the maximum TCP segment
     size to be 1024 bytes.  This can result in poor performance
     if an intervening network performs excessive fragmentation.

SEE ALSO
     intro(5), inet(5F)
























2                                                    January 1992