.\"	$OpenBSD: tcp.4,v 1.27 2022/08/11 14:04:08 jmc Exp $
.\"	$NetBSD: tcp.4,v 1.3 1994/11/30 16:22:35 jtc Exp $
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.\"     @(#)tcp.4	8.1 (Berkeley) 6/5/93
.\"
.Dd $Mdocdate: August 11 2022 $
.Dt TCP 4
.Os
.Sh NAME
.Nm tcp
.Nd Internet Transmission Control Protocol
.Sh SYNOPSIS
.In sys/socket.h
.In netinet/in.h
.In netinet/tcp.h
.Ft int
.Fn socket AF_INET SOCK_STREAM 0
.Ft int
.Fn socket AF_INET6 SOCK_STREAM 0
.Sh DESCRIPTION
The
.Tn TCP
protocol provides a reliable, flow-controlled, two-way
transmission of data.
It is a byte-stream protocol used to support the
.Dv SOCK_STREAM
abstraction.
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 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 several socket options which are set with
.Xr setsockopt 2
and tested with
.Xr getsockopt 2 .
.Bl -ohang
.It Cd TCP_INFO
Retrieve information about a socket's underlying TCP session.
.Dv TCP_INFO
is only used with
.Fn getsockopt .
The argument is a pointer to an instance of
.Vt "struct tcp_info"
(from
.In netinet/tcp.h ) .
.It Cd 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.
Therefore,
.Tn TCP
provides a boolean option,
.Dv TCP_NODELAY
(from
.In netinet/tcp.h ) ,
to defeat this algorithm.
.It Cd TCP_NOPUSH
By convention, the
.Tn TCP
sender 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, the internal send buffer is filled, or this option
is set to a zero value.
.It Cd TCP_MAXSEG
Set the maximum segment size for this connection.
The maximum segment size can only be lowered.
.It Cd TCP_SACK_ENABLE
Use selective acknowledgements for this connection.
See
.Xr options 4 .
.It Cd TCP_MD5SIG
Use TCP MD5 signatures per RFC 2385.
This requires
.Em Security Associations
to be set up, which can be done using
.Xr ipsecctl 8 .
When a listening socket has
.Em TCP_MD5SIG
set, it accepts connections with MD5 signatures only from sources for which a
.Em Security Association
is set up.
Connections without MD5 signatures are only accepted from sources for which no
.Em Security Association
is set up.
The connected socket only has
.Em TCP_MD5SIG
set if the connection is protected with MD5 signatures.
.El
.Pp
The option level for the
.Xr setsockopt 2
call is the protocol number for
.Tn TCP ,
available from
.Xr getprotobyname 3 .
.Pp
Options at the
.Tn IP
transport 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.
.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 tcpbench 1 ,
.Xr getsockopt 2 ,
.Xr socket 2 ,
.Xr inet 4 ,
.Xr inet6 4 ,
.Xr ip 4 ,
.Xr ip6 4 ,
.Xr netintro 4 ,
.Xr ipsecctl 8 ,
.Xr tcpdrop 8
.Sh HISTORY
The
.Nm
protocol stack appeared in
.Bx 4.2 .