383 lines
12 KiB
Groff
383 lines
12 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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.Pp
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Port numbers are 16-bit and range from 1 to 65535.
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Port 0 is not valid.
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Binding to port 0 will assign an available port on the requested address.
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Connecting to port 0 will fail with
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.Er EADDRNOTAVAIL .
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Received packets whose source or destination address is port 0 will be silently
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dropped.
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TCP ports are distinct from ports in other transport layer protocols.
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.Pp
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Packets contain a 16-bit ones' complement checksum.
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Received packets will be silently discarded if their checksum does not match
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the contents.
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.Pp
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Sockets can be bound to a local address and port with
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.Xr bind 2
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(if not already bound),
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or an local address and port will be automatically assigned when connected.
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The local address and port can be read with
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.Xr getsockname 2 .
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If the socket hasn't been bound, the local address and port is reported as the
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any address on port 0.
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Binding to a well-known port (port 1 through port 1023) requires superuser
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privileges.
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.Pp
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Sockets can be bound to the any address, the broadcast address, the address of
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a network interface, or the broadcast address of a network interface.
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Binding to port 0 will automatically assign an available port on the requested
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local address or fail with
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.Er EAGAIN
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if no port is available.
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No two sockets can bind to the same local address and port.
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No two sockets can be bound such that one is bound to the any address and a
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port, and the other socket is bound to another address and the same port; unless
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both sockets had the
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.Dv SO_REUSEADDR
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socket option set when the second socket was bound, and the current user is the
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same that bound the first socket or the current user has superuser privileges.
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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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If the socket is not bound,
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.Xr connect 2
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will determine which network interface will be used to send to the remote
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address, and then bind to the address of that network interface together with an
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available port.
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.Xr connect 2
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will fail if there is no route from the local address to the requested remote
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address.
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.Pp
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Incoming connections can be received by binding to a local address with
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.Xr bind 2
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and listening for connections with
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.Xr listen 2 ,
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after which incoming connections can be retrieved with
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.Xr accept 2 .
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.Pp
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Bytes can be received from 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 broken and no further receipt or transmission is possible.
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.Pp
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The condition of the socket can be tested with
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.Xr poll 2
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where
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.Dv POLLIN
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signifies new data been received or the remote socket has shut down for writing
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or an incoming connection can be retrieved with
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.Xr accept 2 ,
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.Dv POLLOUT
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signifies new data can be sent now (and the socket is not shut down for
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writing),
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.Dv POLLHUP
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signifies the socket is shut down for writing, and
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.Dv POLLERR
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signifies an asynchronous error is pending.
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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 (which sends 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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.Pp
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Socket options can be set with
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.Xr setsockopt 2
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and read with
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.Xr getsockopt 2
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and exist on the
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.Dv IPPROTO_TCP
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level as well as applicable underlying protocol levels.
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.Sh SOCKET OPTIONS
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TCP sockets support these
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.Xr setsockopt 2 /
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.Xr getsockopt 2
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options at level
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.Dv SOL_SOCKET :
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.Bl -tag -width "12345678"
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.It Dv SO_BINDTODEVICE Fa "char[]"
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Bind to a network interface by its name.
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(Described in
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.Xr if 4 )
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.It Dv SO_BINDTOINDEX Fa "unsigned int"
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Bind to a network interface by its index number.
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(Described in
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.Xr if 4 )
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.It Dv SO_DEBUG Fa "int"
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Whether the socket is in debug mode.
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This option is not implemented and is initially 0.
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Attempting to set it to non-zero will fail with
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.Er EPERM .
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(Described in
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.Xr if 4 )
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.It Dv SO_DOMAIN Fa "sa_family_t"
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The socket
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.Fa domain
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(the address family).
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This option can only be read.
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(Described in
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.Xr if 4 )
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.It Dv SO_ERROR Fa "int"
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The asynchronous pending error
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(an
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.Xr errno 3
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value).
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Errors are permanent.
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This option can only be read.
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(Described in
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.Xr if 4 )
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.It Dv SO_PROTOCOL Fa "int"
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The socket protocol
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.Dv ( IPPROTO_TCP ) .
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This option can only be read.
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(Described in
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.Xr if 4 )
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.It Dv SO_RCVBUF Fa "int"
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How many bytes the receive queue can use (default is 64 KiB).
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(Described in
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.Xr if 4 )
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.It Dv SO_REUSEADDR Fa "int"
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Whether binding to the any address on a port doesn't conflict with binding to
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another address and the same port, if both sockets have this option set and the
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user binding the second socket is the same that bound the first socket or the
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user binding the second socket has superuser privileges.
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(Described in
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.Xr if 4 )
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.It Dv SO_SNDBUF Fa "int"
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How many bytes the send queue can use (default is 64 KiB).
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(Described in
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.Xr if 4 )
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.It Dv SO_TYPE Fa "int"
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The socket type
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.Dv ( SOCK_STREAM ) .
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This option can only be read.
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(Described in
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.Xr if 4 )
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.El
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.Pp
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TCP sockets currently implement no
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.Xr setsockopt 2 /
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.Xr getsockopt 2
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options at level
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.Dv IPPROTO_TCP .
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.Sh IMPLEMENTATION NOTES
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Connections time out when a segment has not been acknowledged by the remote
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socket after 6 attempts to deliver the segment.
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Each retransmission happens after 1 second plus 1 second per failed
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transmissions so far.
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Successful delivery of any segment resets the retransmission count to 0.
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.Pp
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The receive and transmission buffers are both 64 KiB by default.
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.Pp
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If no specific port is requested, one is randomly selected in the dynamic port
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range 32768 (inclusive) through 61000 (exclusive).
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.Pp
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The Maximum Segment Lifetime (MSL) is set to 30 seconds and the quiet time of
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two MSLs before reusing sockets is 60 seconds.
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.Sh ERRORS
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Socket operations can fail due to these error conditions, in addition to the
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error conditions of the network and link layer, and the error conditions of the
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invoked function.
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.Bl -tag -width [EADDRNOTAVAIL]
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.It Bq Er EADDRINUSE
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The socket cannot be bound to the requested address and port because another
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socket was already bound to 1) the same address and port 2) the any address
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and the same port (and
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.Dv SO_REUSEADDR
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was not set on both sockets), or 3) some address and the same port but the
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requested address was the any address (and
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.Dv SO_REUSEADDR
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was not set on both sockets).
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.It Bq Er EADDRNOTAVAIL
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The socket cannot be bound to the requested address because no network interface
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had that address or broadcast address.
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.It Bq Er EADDRNOTAVAIL
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The socket was connected to port 0.
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.It Bq Er EAGAIN
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A port could not be assigned because each port in the dynamic port range had
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already been bound to a socket in a conflicting manner.
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.It Bq Er ECONNREFUSED
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The destination host refused the connection.
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.It Bq Er ECONNRESET
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The connection was reset by the remote socket.
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.It Bq Er EHOSTDOWN
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The destination host is not up.
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This error can happen asynchronously.
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.It Bq Er EHOSTUNREACH
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The destination host was unreachable.
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This error can happen asynchronously.
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.It Bq Er ENETDOWN
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The network interface isn't up.
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This error can happen asynchronously.
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.It Bq Er ENETUNREACH
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The destination network was unreachable.
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This error can happen asynchronously.
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.It Bq Er ENETUNREACH
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The remote address could not be connected because there was no route from the
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local address to the remote address.
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.It Bq Er ENOBUFS
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There was not enough memory available for network packets.
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.It Bq Er EPERM
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The unimplemented
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.Dv SO_DEBUG
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socket options was attempted to be set to a non-zero value.
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.It Bq Er EPIPE
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The transmission failed because the connetion is broken.
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The
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.Dv SIGPIPE
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signal is sent as well unless disabled.
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.It Bq Er ETIMEDOUT
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The connection timed out delivering a segment.
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This error can happen asynchronously.
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.El
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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 getpeername 2 ,
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.Xr getsockname 2 ,
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.Xr getsockopt 2 ,
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.Xr poll 2 ,
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.Xr recv 2 ,
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.Xr recvfrom 2 ,
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.Xr recvmsg 2 ,
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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 setsockopt 2 ,
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.Xr shutdown 2 ,
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.Xr socket 2 ,
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.Xr if 4 ,
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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 fast 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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reaching the limit on pending TCP sockets in the listen queue and thus deny
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service to further 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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.Pp
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.Xr bind 2
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does not yet enforce that binding to a well-known port (port 1 through port
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1023) requires superuser privileges.
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.Pp
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The automatic assignment of ports is random, but is statistically biased.
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A random port is picked, and if it is taken, the search sequentially iterates
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ports in ascending order until an available port is found or the search
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terminates.
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