TCP(7) (TCP/IP) TCP(7)
NAME
TCP - Internet Transmission Control Protocol
SYNOPSIS
#include <sys/socket.h>
#include <netinet/in.h>
s = socket(AFINET, SOCKSTREAM, 0);
t = topen("/dev/tcp", ORDWR);
DESCRIPTION
TCP is the virtual circuit protocol of the Internet protocol family.
It provides reliable, flow-controlled, in order, two-way transmission
of data. It is a byte-stream protocol layered above the Internet
Protocol (IP), the Internet protocol family's internetwork datagram
delivery protocol.
Programs can access TCP using the socket interface as a SOCKSTREAM
socket type, or using the Transport Level Interface (TLI) where it
supports the connection-oriented (TCOTSORD) service type.
TCP uses IP's host-level addressing and adds its own per-host
collection of port addresses. The endpoints of a TCP connection are
identified by the combination of an IP address and a TCP port number.
Although other protocols, such as the User Datagram Protocol (UDP),
may use the same host and port address format, the port space of
these protocols is distinct. See inet(7) for details on the common
aspects of addressing in the Internet protocol family.
Sockets utilizing TCP are either active or passive. Active sockets
initiate connections to passive sockets. Both types of sockets must
have their local IP address and TCP port number bound with the
bind(2) system call after the socket is created. By default, TCP
sockets are active. A passive socket is created by calling the
listen(2) system call after binding the socket with bind(). This
establishes a queueing parameter for the passive socket. After this,
connections to the passive socket can be received with the accept(2)
system call. Active sockets use the connect(2) call after binding to
initiate connections.
By using the special value INADDRANY, the local IP address can be
left unspecified in the bind() call by either active or passive TCP
sockets. This feature is usually used if the local address is either
unknown or irrelevant. If left unspecified, the local IP address
will be bound at connection time to the address of the network
interface used to service the connection.
Once a connection has been established, data can be exchanged using
the read(2) and write(2) system calls.
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TCP(7) (TCP/IP) TCP(7)
TCP supports one socket option which is set with setsockopt() and
tested with getsockopt(2). Under most circumstances, 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, TCP provides a boolean option,
TCPNODELAY (defined in /usr/include/netinet/tcp.h), to defeat this
algorithm. The option level for the setsockopt() call is the
protocol number for TCP, available from getprotobyname() [see
getprotoent(3N)].
Options at the IP level may be used with TCP; See ip(7).
TCP provides an urgent data mechanism, which may be invoked using the
out-of-band provisions of send(2). The caller may mark one byte as
urgent with the MSGOOB flag to send(2). This sets an urgent pointer
pointing to this byte in the TCP stream. The receiver on the other
side of the stream is notified of the urgent data by a SIGURG signal.
The SIOCATMARK ioctl() request returns a value indicating whether the
stream is at the urgent mark. Because the system never returns data
across the urgent mark in a single read(2) call, it is possible to
advance to the urgent data in a simple loop which reads data, testing
the socket with the SIOCATMARK ioctl() request, until it reaches the
mark.
Incoming connection requests that include an IP source route option
are noted, and the reverse source route is used in responding.
A checksum over all data helps TCP implement reliability. Using a
window-based flow control mechanism that makes use of positive
acknowledgements, sequence numbers, and a retransmission strategy,
TCP can usually recover when datagrams are damaged, delayed,
duplicated or delivered out of order by the underlying communication
medium.
If the local TCP receives no acknowledgements from its peer for a
period of time, as would be the case if the remote machine crashed,
the connection is closed and an error is returned to the user. If
the remote machine reboots or otherwise loses state information about
a TCP connection, the connection is aborted and an error is returned
to the user.
SEE ALSO
read(2), write(2), accept(3N), bind(3N), connect(3N),
getprotoent(3N), getsockopt(3N), listen(3N), send(3N), inet(7),
ip(7).
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TCP(7) (TCP/IP) TCP(7)
Postel, Jon, Transmission Control Protocol - DARPA Internet Program
Protocol Specification, RFC 793, Network Information Center, SRI
International, Menlo Park, Calif., September 1981.
DIAGNOSTICS
A socket operation may fail if:
EISCONN A connect() operation was attempted on a socket
on which a connect() operation had already been
performed.
ETIMEDOUT A connection was dropped due to excessive
retransmissions.
ECONNRESET The remote peer forced the connection to be
closed (usually because the remote machine has
lost state information about the connection due
to a crash).
ECONNREFUSED The remote peer actively refused connection
establishment (usually because no process is
listening to the port).
EADDRINUSE A bind() operation was attempted on a socket with
a network address/port pair that has already been
bound to another socket.
EADDRNOTAVAIL A bind() operation was attempted on a socket with
a network address for which no network interface
exists.
EACCES A bind() operation was attempted with a reserved
port number and the effective user ID of the
process was not the privileged user.
ENOBUFS The system ran out of memory for internal data
structures.
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