.Dd June 3, 2017
.Dt TCP 4
.Os
.Sh NAME
.Nm tcp
.Nd 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 IPPROTO_TCP
.Sh DESCRIPTION
The Transmission Control Protocol (TCP) is a connection-oriented transport layer
for the Internet Protocol
.Xr ip 4
that provides a reliable byte stream connection between two hosts.
It is designed for packet-switched networks and provides sequenced data,
retransmissions on packet loss, handling of duplicated packets, flow control,
basic data integrity checks, multiplexing with a 16-bit port number, support for
out-of-band urgent data, and detection of lost connection.
TCP provides the
.Dv SOCK_STREAM
abstraction for the
.Xr inet 4
protocol family.
.Pp
TCP sockets are made with
.Xr socket 2
by passing an appropriate
.Fa domain
.Dv ( AF_INET ) ,
.Dv SOCK_STREAM
as the
.Fa type ,
and 0 or
.Dv IPPROTO_TCP
as the
.Fa protocol .
Newly created TCP sockets are not bound to a local address nor connected to a
remote socket.
They can be bound to a local address with
.Xr bind 2 ,
or a local address will be assigned on
.Xr connect 2
or
.Xr listen 2 .
.Pp
A connection to a remote TCP socket can be established with
.Xr connect 2 .
Connections can be established when both sides calls
.Xr connect 2
on each other.
.Pp
Incoming connections can be listened for using
.Xr listen 2
and accepted with
.Xr accept 2 .
.Pp
Bytes can be received form the remote TCP socket with
.Xr recv 2 ,
.Xr recvmsg 2 ,
.Xr recvfrom 2 ,
.Xr read 2 ,
or
.Xr readv 2 .
Bytes can be transmitted to the remote TCP socket with
.Xr send 2 ,
.Xr sendmsg 2 ,
.Xr sendto 2 ,
.Xr write 2 ,
or
.Xr writev 2 .
Transmitting when the connection has broken will result in the process being
sent the
.Dv SIGPIPE
signal and fail with
.Er EPIPE .
.Pp
The receiving socket will acknowledge any received data.
If no acknowledgement is received in a timely manner, the transmitting socket
will transmit the data again.
If a acknowledgement still isn't received after a while, the connection is
considered to be broken and no further receipt or transmission is possible.
.Pp
The connection can be shut down with
.Xr shutdown 2
in either the reading direction (discarding further received data) or the
writing direction (sending the finish control flag).
The connection is closed when both sockets have sent and acknowledged the finish
control flag.
Upon the
.Xr close 2
of the last file descriptor for a connected socket, the socket is shut down in
both directions.
.Sh SEE ALSO
.Xr accept 2 ,
.Xr bind 2 ,
.Xr connect 2 ,
.Xr getsockopt 2 ,
.Xr recv 2 ,
.Xr send 2 ,
.Xr setsockopt 2 ,
.Xr shutdown 2 ,
.Xr socket 2 ,
.Xr inet 4 ,
.Xr ip 4 ,
.Xr kernel 7
.Sh STANDARDS
.Rs
.%A J. Postel (ed.)
.%D September 1981
.%R STD 7
.%R RFC 793
.%T Transmission Control Protocol
.%Q USC/Information Sciences Institute
.Re
.Pp
.Rs
.%A Internet Engineering Task Force
.%A R. Braden (ed.)
.%D October 1989
.%R STD 3
.%R RFC 1122
.%T Requirements for Internet Hosts -- Communication Layers
.%Q USC/Information Sciences Institute
.Re
.Pp
.St -p1003.1-2008 specifies the TCP socket programming interface.
.Sh BUGS
The implementation is incomplete and has known bugs.
.Pp
Out-of-band data is not yet supported and is ignored on receipt.
.Pp
The round trip time is not estimated which prevents efficient retransmission
when data is lost
Retransmissions happen after a second, which means unnecessary retransmissions
happen if the round trip time is more than a second.
.Pp
Options are not supported and are ignored on receipt.
.Pp
No extensions are implemented yet that improve efficiency for long fat networks
with large bandwidth * delay products.
.Pp
There is not yet any support for sending keep-alive packets.
.Pp
There is not yet any support for respecting
.Xr icmp 4
condition such as destination unreachable or source quench.
.Pp
Half-open connections use memory, but until the handshake is complete, it is not
confirmed whether the remote is actually able to transmit from the source
qaddress.
An attacker may be able to transmit many packets from forged addresses,
exhausting the available memory for TCP sockets and thus deny service to further
legitimate connections.
A SYN queue or SYN cookies would mitigate this problem, but neither is yet
implemented.