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.\"     @(#)tcp.4	8.1 (Berkeley) 6/5/93
.\"
.Dd February 28, 2007
.Dt TCP 4
.Os BSD 4.2
.Sh NAME
.Nm tcp
.Nd Internet Transmission Control Protocol
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.In netinet/in.h
.Ft int
.Fn socket AF_INET SOCK_STREAM 0
.Sh DESCRIPTION
The
.Tn TCP
protocol 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 utilizing the
.Tn TCP
protocol 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 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.
.Pp
.Tn TCP
supports a number of socket options which can be set with
.Xr setsockopt 2
and tested with
.Xr getsockopt 2 :
.Bl -tag -width ".Dv TCP_CONNECTIONTIMEOUT"
.It Dv TCP_NODELAY
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.
The boolean option
.Dv TCP_NODELAY
defeats this algorithm.
.It Dv TCP_MAXSEG
By default, a sender- and
.No receiver- Ns Tn TCP
will negotiate among themselves to determine the maximum segment size
to be used for each connection.
The
.Dv TCP_MAXSEG
option allows the user to determine the result of this negotiation,
and to reduce it if desired.
.It Dv TCP_NOOPT
.Tn TCP
usually sends a number of options in each packet, corresponding to
various
.Tn TCP
extensions which are provided in this implementation.
The boolean option
.Dv TCP_NOOPT
is provided to disable
.Tn TCP
option use on a per-connection basis.
.It Dv TCP_NOPUSH
By convention, the
.No sender- Ns Tn TCP
will set the
.Dq push
bit, and begin transmission immediately (if permitted) at the end of
every user call to
.Xr write 2
or
.Xr writev 2 .
When this option is set to a non-zero value,
.Tn TCP
will delay sending any data at all until either the socket is closed,
or the internal send buffer is filled.
.It Dv TCP_KEEPALIVE
.Tn The
.Dv TCP_KEEPALIVE
options enable to specify the amount of time, in seconds, that the 
connection must be idle before keepalive probes (if enabled) are sent. 
The default value is specified by the 
.Tn MIB
variable
.Va net.inet.tcp.keepidle .  
.It Dv TCP_CONNECTIONTIMEOUT
.Tn The
.Dv TCP_CONNECTIONTIMEOUT
option allows to specify the timeout, in seconds, for new, non established
.Tn TCP
connections. This option can be useful for both active and passive
.Tn TCP
connections. The default value is specified by the 
.Tn MIB
variable
.Va net.inet.tcp.keepinit .
.El
.Pp
The option level for the
.Xr setsockopt 2
call is the protocol number for
.Tn TCP ,
available from
.Xr getprotobyname 3 ,
or
.Dv IPPROTO_TCP .
All options are declared in
.In netinet/tcp.h .
.Pp
Options at the
.Tn IP
transport level may be used with
.Tn TCP ;
see
.Xr ip 4 .
Incoming connection requests that are source-routed are noted,
and the reverse source route is used in responding.
.Ss "Non-blocking connect"
.Pp
When a
.Tn TCP
socket is set non-blocking, and the connection cannot be established immediatly, 
.Xr connect 2
returns with the error
.Dv EINPROGRESS ,
and the connection is established asynchronously.
.Pp
When the asynchronous connection completes successfully,
.Xr select 2
or
.Xr poll 2
or
.Xr kqueue 2
will indicate the file descriptor is ready for writing. 
If the connection encounters an error, the file descriptor 
is marked ready for both reading and writing, and the pending error 
can be retrieved via the socket option 
.Dv SO_ERROR .
.Pp
Note that even if the socket is non-blocking, it is possible for the connection 
to be established immediatly. In that case 
.Xr connect 2
does not return with 
.Dv EINPROGRESS .
.Sh DIAGNOSTICS
A socket operation may fail with one of the following errors returned:
.Bl -tag -width Er
.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;
.It Bq Er EAFNOSUPPORT
when an attempt is made to bind or connect a socket to a multicast
address;
.It Bq Er EINPROGRESS
returned by
.Xr connect 2
when the socket is set nonblocking, and the connection cannot be 
immediately established;
.It Bq Er EALREADY
returned by
.Xr connect 2
when connection request is already in progress for the specified socket.
.
.El
.Sh SEE ALSO
.Xr connect 2 ,
.Xr getsockopt 2 ,
.Xr kqueue 2 ,
.Xr poll 2 ,
.Xr select 2 ,
.Xr socket 2 ,
.Xr sysctl 3 ,
.Xr inet 4 ,
.Xr inet6 4 ,
.Xr ip 4 ,
.Xr ip6 4 ,
.Xr netintro 4 ,
.Xr setkey 8
.Sh HISTORY
The
.Tn TCP
protocol appeared in
.Bx 4.2 .
.Pp
The socket option
.Dv TCP_CONNECTIONTIMEOUT
first appeared in Mac OS X 10.6.