TCP(7) UNIX System V(Internet Utilities) 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 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)
Postel, Jon, Transmission Control Protocol - DARPA Internet Program
Protocol Specification, RFC 793, Network Information Center, SRI
International, Menlo Park, Calif., September 1981
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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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