10.2;tcp, revision 1.0, 86/5/16
tcp - Internet Transmission Control Protocol
DESCRIPTION
The TCP protocol provides reliable, flow-controlled, two-way transmission
of data. It is a byte-stream protocol used to support the SOCK_STREAM
abstraction. TCP uses the standard Internet address format and, in
addition, provides a per-host collection of "port addresses". Thus, each
address is composed of an Internet address specifying the host and
network, with a specific TCP port on the host identifying the peer
entity.
Sockets utilizing the tcp protocol are either "active" or "passive".
Active sockets initiate connections to passive sockets. By default TCP
sockets are created active; to create a passive socket the listen(2)
system call must be used after binding the socket with the bind(2) system
call. Only passive sockets may use the accept(2) call to accept incoming
connections. Only active sockets may use the connect(2) call to initiate
connections.
Passive sockets may "underspecify" their location to match incoming
connection requests from multiple networks. This technique, termed
"wildcard addressing", allows a single server to provide service to
clients on multiple networks. To create a socket which listens on all
networks, the Internet address INADDR_ANY must be bound. The TCP port
may still be specified at this time; if the port is not specified the
system will assign one. Once a connection has been established the
socket's address is fixed by the peer entity's location. The address
assigned the socket is the address associated with the network interface
through which packets are being transmitted and received. Normally this
address corresponds to the peer entity's network.
Options at the IP transport level may be used with TCP; see ip(4P).
Incoming connection requests that are source-routed are noted, and the
reverse source route is used in responding.
DIAGNOSTICS
A socket operation may fail with one of the following errors returned:
[EISCONN] when trying to establish a connection on a socket
which already has one;
[ENOBUFS] when the system runs out of memory for an internal
data structure;
[ETIMEDOUT] when a connection was dropped due to excessive
retransmissions;
[ECONNRESET] when the remote peer forces the connection to be
closed;
[ECONNREFUSED] when the remote peer actively refuses connection
establishment (usually because no process is
listening to the port);
[EADDRINUSE] when an attempt is made to create a socket with a
port which has already been allocated;
[EADDRNOTAVAIL] when an attempt is made to create a socket with a
network address for which no network interface
exists.
SEE ALSO
getsockopt(2), socket(2), intro(4N), inet(4F), ip(4P).