TCP(4) Device Drivers Manual TCP(4)
NAME
tcp – Internet Transmission Control Protocol
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
int
socket(AF_INET, SOCK_STREAM, 0);
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) or connectx(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 to 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.
TCP supports a number of socket options which can be set with
setsockopt(2) and tested with getsockopt(2):
TCP_NODELAY 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. The boolean
option TCP_NODELAY defeats this algorithm.
TCP_MAXSEG By default, a sender- and receiver-TCP will
negotiate among themselves to determine the
maximum segment size to be used for each
connection. The TCP_MAXSEG option allows the
user to determine the result of this
negotiation, and to reduce it if desired.
TCP_NOOPT TCP usually sends a number of options in each
packet, corresponding to various TCP
extensions which are provided in this
implementation. The boolean option TCP_NOOPT
is provided to disable TCP option use on a
per-connection basis.
TCP_NOPUSH By convention, the sender-TCP will set the
“push” bit, and begin transmission immediately
(if permitted) at the end of every user call
to write(2) or writev(2). When this option is
set to a non-zero value, TCP will delay
sending any data at all until either the
socket is closed, or the internal send buffer
is filled.
TCP_KEEPALIVE The TCP_KEEPALIVE options enable to specify
the amount of time, in seconds, that the
connection must be idle before keepalive
probes (if enabled) are sent. The default
value is specified by the MIB variable
net.inet.tcp.keepidle.
TCP_CONNECTIONTIMEOUT The TCP_CONNECTIONTIMEOUT option allows to
specify the timeout, in seconds, for new, non
established TCP connections. This option can
be useful for both active and passive TCP
connections. The default value is specified by
the MIB variable net.inet.tcp.keepinit.
TCP_KEEPINTVL When keepalive probes are enabled, this option
will set the amount of time in seconds between
successive keepalives sent to probe an
unresponsive peer.
TCP_KEEPCNT When keepalive probes are enabled, this option
will set the number of times a keepalive probe
should be repeated if the peer is not
responding. After this many probes, the
connection will be closed.
TCP_SENDMOREACKS When a stream of TCP data packets are
received, OS X uses an algorithm to reduce the
number of acknowlegements by generating a TCP
acknowlegement for 8 data packets instead of
acknowledging every other data packet. When
this socket option is enabled, the connection
will always send a TCP acknowledgement for
every other data packet.
TCP_ENABLE_ECN Using Explicit Congestion Notification (ECN)
on TCP allows bi-directional end-to-end
notification of congestion without dropping
packets. Conventionally TCP/IP networks signal
congestion by dropping packets. When ECN is
successfully negotiated, an ECN-aware router
may set a mark in the IP header instead of
dropping a packet in order to signal impending
congestion. The TCP receiver of the packet
echoes congestion indication to the TCP
sender, which reduces it's transmission rate
as if it detected a dropped packet. This will
avoid unnecessary retransmissions and will
improve latency by saving the time required
for recovering a lost packet.
TCP_NOTSENT_LOWAT The send socket buffer of a TCP sender has
unsent and unacknowledged data. This option
allows a TCP sender to control the amount of
unsent data kept in the send socket buffer.
The value of the option should be the maximum
amount of unsent data in bytes. Kevent, poll
and select will generate a write notification
when the unsent data falls below the amount
given by this option. This will allow an
application to generate just-in-time fresh
updates for real-time communication.
TCP_FASTOPEN The TCP listener can set this option to use
TCP Fast Open feature. After setting this
option, an accept(2) may return a socket that
is in SYN_RECEIVED state but is readable and
writable.
TCP_CONNECTION_INFO This socket option can be used to obtain TCP
connection level statistics. The "struct
tcp_connection_info" defined in
<netinet/tcp_var.h> is copied to the user
buffer.
The option level for the setsockopt(2) call is the protocol number for
TCP, available from getprotobyname(3), or IPPROTO_TCP. All options are
declared in <netinet/tcp.h>.
Options at the IP transport level may be used with TCP; see ip(4).
Incoming connection requests that are source-routed are noted, and the
reverse source route is used in responding.
Non-blocking connect
When a TCP socket is set non-blocking, and the connection cannot be
established immediately, connect(2) or connectx(2) returns with the error
EINPROGRESS, and the connection is established asynchronously.
When the asynchronous connection completes successfully, select(2) or
poll(2) or kqueue(2) will indicate the file descriptor is ready for
writing. If the connection encounters an error, the file descriptor is
marked ready for both reading and writing, and the pending error can be
retrieved via the socket option SO_ERROR.
Note that even if the socket is non-blocking, it is possible for the
connection to be established immediately. In that case connect(2) or
connectx(2) does not return with EINPROGRESS.
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;
[EAFNOSUPPORT] when an attempt is made to bind or connect a socket to
a multicast address;
[EINPROGRESS] returned by connect(2) or connectx(2) when the socket
is set nonblocking, and the connection cannot be
immediately established;
[EALREADY] returned by connect(2) or connectx(2) when connection
request is already in progress for the specified
socket.
[ENODATA] returned by recv(2) or send(2) in case a connection is
experiencing a data-stall (probably due to a middlebox
issue). It is advised that the current connection
gets closed by the application and a new attempt is
being made.
SEE ALSO
connect(2), connectx(2), getsockopt(2), kqueue(2), poll(2), select(2),
socket(2), sysctl(3), inet(4), inet6(4), ip(4), ip6(4), netintro(4),
setkey(8)
HISTORY
The TCP protocol appeared in 4.2BSD.
The socket option TCP_CONNECTIONTIMEOUT first appeared in Mac OS X 10.6.
BSD 4.2 March 18, 2015 BSD 4.2