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.\"     @(#)tcp.4	8.1 (Berkeley) 6/5/93
.\"
.Dd November 23, 2003
.Dt TCP 4
.Os
.Sh NAME
.Nm tcp
.Nd Internet Transmission Control Protocol
.Sh SYNOPSIS
.In sys/socket.h
.In netinet/in.h
.Ft int
.Fn socket AF_INET SOCK_STREAM 0
.Ft int
.Fn socket AF_INET6 SOCK_STREAM 0
.Sh DESCRIPTION
The
.Tn TCP
provides reliable, flow-controlled, two-way transmission of data.
It is a byte-stream protocol used to support the
.Dv SOCK_STREAM
abstraction.
.Tn TCP
uses the standard Internet address format and, in addition, provides
a per-host collection of
.Dq port addresses .
Thus, each address is composed of an Internet address specifying
the host and network, with a specific
.Tn TCP
port on the host identifying the peer entity.
.Pp
Sockets using
.Tn TCP
are either
.Dq active
or
.Dq passive .
Active sockets initiate connections to passive
sockets.
By default
.Tn TCP
sockets are created active; to create a passive socket the
.Xr listen 2
system call must be used
after binding the socket with the
.Xr bind 2
system call.
Only passive sockets may use the
.Xr accept 2
call to accept incoming connections.
Only active sockets may use the
.Xr connect 2
call to initiate connections.
.Pp
Passive sockets may
.Dq underspecify
their location to match incoming connection requests from multiple networks.
This technique, termed
.Dq 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
.Dv INADDR_ANY
must be bound.
The
.Tn 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.
.Pp
.Tn TCP
supports one socket option which is set with
.Xr setsockopt 2
and tested with
.Xr getsockopt 2 .
Under most circumstances,
.Tn 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,
.Tn TCP
provides a boolean option,
.Dv TCP_NODELAY
(from
.Aq Pa netinet/tcp.h ,
to defeat this algorithm.
The option level for the
.Xr setsockopt 2
call is the protocol number for
.Tn TCP ,
available from
.Xr getprotobyname 3 .
In the historical
.Bx
.Tn TCP
implementation, if the
.Dv TCP_NODELAY
option was set on a passive socket, the sockets returned by
.Xr accept 2
erroneously did not have the
.Dv TCP_NODELAY
option set; the behavior was corrected to inherit
.Dv TCP_NODELAY
in
.Nx 1.6 .
.Pp
Options at the
.Tn IP
network level may be used with
.Tn TCP ;
see
.Xr ip 4
or
.Xr ip6 4 .
Incoming connection requests that are source-routed are noted,
and the reverse source route is used in responding.
.Pp
There are many adjustable parameters that control various aspects
of the
.Nx
TCP behavior; these parameters are documented in
.Xr sysctl 3 ,
and they include:
.Bl -bullet -compact
.It
RFC 1323 extensions for high performance
.It
Send/receive buffer sizes
.It
Default maximum segment size (MSS)
.It
SYN cache parameters
.It
Initial window size
.It
Hughes/Touch/Heidemann Congestion Window Monitoring algorithm
.It
Keepalive parameters
.It
newReno algorithm for congestion control
.It
Logging of connection refusals
.It
RST packet rate limits
.El
.Sh DIAGNOSTICS
A socket operation may fail with one of the following errors returned:
.Bl -tag -width [EADDRNOTAVAIL]
.It Bq Er EISCONN
when trying to establish a connection on a socket which
already has one;
.It Bq Er ENOBUFS
when the system runs out of memory for
an internal data structure;
.It Bq Er ETIMEDOUT
when a connection was dropped
due to excessive retransmissions;
.It Bq Er ECONNRESET
when the remote peer
forces the connection to be closed;
.It Bq Er ECONNREFUSED
when the remote
peer actively refuses connection establishment (usually because
no process is listening to the port);
.It Bq Er EADDRINUSE
when an attempt
is made to create a socket with a port which has already been
allocated;
.It Bq Er EADDRNOTAVAIL
when an attempt is made to create a
socket with a network address for which no network interface
exists.
.El
.Sh SEE ALSO
.Xr getsockopt 2 ,
.Xr socket 2 ,
.Xr sysctl 3 ,
.Xr inet 4 ,
.Xr inet6 4 ,
.Xr intro 4 ,
.Xr ip 4 ,
.Xr ip6 4
.Rs
.%R RFC
.%N 793
.%D September 1981
.%T "Transmission Control Protocol"
.Re
.Rs
.%R RFC
.%N 1122
.%D October 1989
.%T "Requirements for Internet Hosts -- Communication Layers"
.Re
.Sh HISTORY
The
.Nm
protocol stack appeared in
.Bx 4.2 .