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TCP(4)                    NetBSD Programmer's Manual                    TCP(4)


NAME

     tcp - Internet Transmission Control Protocol


SYNOPSIS

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

     int
     socket(AF_INET, SOCK_STREAM, 0);


DESCRIPTION

     The TCP 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 com-
     posed of an Internet address specifying the host and network, with a spe-
     cific TCP port on the host identifying the peer entity.

     Sockets utilizing TCP are either ``active'' or ``passive''.  Active sock-
     ets 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 pas-
     sive 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 sock-
     et's address is fixed by the peer entity's location.  The address as-
     signed 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 gath-
     ers small amounts of output to be sent in a single packet once an ac-
     knowledgement 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 algo-
     rithm.  The option level for the setsockopt(2) call is the protocol num-
     ber for TCP, available from getprotobyname(3).

     Options at the IP network level may be used with TCP; see ip(4).  Incom-
     ing 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

     getsockopt(2), socket(2), intro(4), inet(4), ip(4)

     Transmission Control Protocol, RFC, 793, September 1981.

     Requirements for Internet Hosts -- Communication Layers, RFC, 1122,
     October 1989.


HISTORY

     The tcp protocol stack appeared in 4.2BSD.

NetBSD 1.4                       June 5, 1993                                2