tcp - Internet Transmission Control Protocol
socket(AF_INET, SOCK_STREAM, 0);
socket(AF_INET6, SOCK_STREAM, 0);
The TCP protocol provides a reliable, flow-controlled, twoway transmission
of data. It is a byte-stream protocol used to support
SOCK_STREAM abstraction. TCP uses the standard Internet address format
and, in addition, provides a per-host collection of ``port
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
Sockets utilizing the TCP protocol are either ``active'' or
Active sockets initiate connections to passive sockets. By
sockets are created active; to create a passive socket the
call must be used after binding the socket with the
call. Only passive sockets may use the accept(2) call to
connections. Only active sockets may use the connect(2)
call to initiate
Passive sockets may ``underspecify'' their location to match
connection requests from multiple networks. This technique,
``wildcard addressing'', allows a single server to provide
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
system will assign one. Once a connection has been established the socket's
address is fixed by the peer entity's location. The
to the socket is the address associated with the network interface
through which packets are being transmitted and received.
address corresponds to the peer entity's network.
TCP supports several socket options which are set with setsockopt(2) and
tested with getsockopt(2).
TCP_NODELAY [Toc] [Back]
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
For a small number of clients, such as window systems that
send a stream
of mouse events which receive no replies, this packetization
significant delays. Therefore, TCP provides a boolean option,
TCP_NODELAY (from <netinet/tcp.h>), to defeat this algorithm.
TCP_MAXSEG [Toc] [Back]
Set the maximum segment size for this connection. The maximum segment
size can only be lowered.
TCP_SACK_ENABLE [Toc] [Back]
Use selective acknowledgements for this connection. See options(4).
TCP_MD5SIG [Toc] [Back]
Use TCP MD5 signatures per RFC 2385. This requires Security
to be set up, which can be done using ipsecadm(8). When a
socket has TCP_MD5SIG set, it accepts connections with MD5
from sources for which a Security Association is set up.
without MD5 signatures are only accepted from sources for
Security Association is set up. The connected socket only
set if the connection is protected with MD5 signatures.
The option level for the setsockopt(2) call is the protocol
TCP, available from getprotobyname(3).
Options at the IP transport level may be used with TCP; see
ip6(4). Incoming connection requests that are source-routed
and the reverse source route is used in responding.
A socket operation may fail with one of the following errors
[EISCONN] when trying to establish a connection on a
already has one;
[ENOBUFS] when the system runs out of memory for an
[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
[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
getsockopt(2), socket(2), inet(4), inet6(4), ip(4), ip6(4),
The tcp protocol stack appeared in 4.2BSD.
OpenBSD 3.6 June 5, 1993
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