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