tcp(7)tcp(7)NAMEtcp - Internet Transmission Control Protocol (TCP)
SYNOPSIS
#include <sys/socket.h>
#include <netinet/in.h>
The following is the socket call for AF_INET sockets: s =
socket(AF_INET, SOCK_STREAM, 0);
The following is the socket call for AF_INET6 sockets: s =
socket(AF_INET6, SOCK_STREAM, 0);
DESCRIPTION
The TCP protocol provides reliable, flow-controlled, two-way transmis‐
sion of data. It is a byte-stream protocol used to support the
SOCK_STREAM abstraction. The TCP protocol 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 spec‐
ifying the host and network, with a specific TCP port on the host iden‐
tifying the peer entity.
Sockets utilizing the TCP 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() function must
be used after binding the socket with the bind() function. Only pas‐
sive sockets may use the accept() function to accept incoming connec‐
tions. Only active sockets may use the connect() function to initiate
connections.
Passive sockets may underspecify their location to match incoming con‐
nection requests from multiple networks. This technique, termed wild‐
card addressing, allows a single server to provide service to clients
on multiple networks. To create a socket that listens on all networks,
the Internet address INADDR_ANY (for AF_INET sockets) or in6addr_any
(for AF_INET6 sockets) must be bound. The TCP port may still be speci‐
fied 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 the following socket options: When the SO_KEEPALIVE option
is enabled, TCP probes a connection that has been idle for some amount
of time. If the remote system does not respond to a keepalive probe,
TCP retransmits the probe a certain number of times before a connection
is considered to be broken. The default value for this keepalive probe
retransmit limit is 8. The TCP_KEEPCNT option can be used to affect
this value for a given socket, and specifies the maximum number of
keepalive probes to be sent. This option takes an int value, with a
range of 1 to tcp_keepcnt. For more information on tcp_keepcnt see the
information on the inet subsystem configuration attributes in the Sys‐
tem Configuration and Tuning guide. When the SO_KEEPALIVE option is
enabled, TCP probes a connection that has been idle for some amount of
time. The default value for this idle period is 2 hours. The
TCP_KEEPIDLE option can be used to affect this value for a given
socket, and specifies the number of seconds of idle time between
keepalive probes. This option takes an int value, with a range of 1 to
N (where N is tcp_keepidle divided by PR_SLOWHZ). For more information
on tcp_keepidle see the information on the inet subsystem configuration
attributes in the System Configuration and Tuning guide. PR_SLOWHZ is
defined in the <sys/protosw.h> header file. If a TCP connection cannot
be established within some amount of time, TCP will time out the con‐
nect attempt. The default value for this initial connection establish‐
ment timeout is 75 seconds. The TCP_KEEPINIT option can be used to
affect this initial timeout period for a given socket, and specifies
the number of seconds to wait before the connect attempt is timed out.
For passive connections, the TCP_KEEPINIT option value is inherited
from the listening socket. This option takes an int value, with a
range of 1 to N (where N is tcp_keepinit divided by PR_SLOWHZ). For
more information on tcp_keepinit see the information on the inet sub‐
system configuration attributes in the System Configuration and Tuning
guide. PR_SLOWHZ is defined in the <sys/protosw.h> header file. When
the SO_KEEPALIVE option is enabled, TCP probes a connection that has
been idle for some amount of time. If the remote system does not
respond to a keepalive probe, TCP retransmits the probe after some
amount of time. The default value for this retransmit interval is 75
seconds. The TCP_KEEPINTVL option can be used to affect this value for
a given socket, and specifies the number of seconds to wait before
retransmitting a keepalive probe. This option takes an int value, with
a range of 1 to N (where N is tcp_keepintvl divided by PR_SLOWHZ). For
more information on tcp_keepintvl see the information on the inet sub‐
system configuration attributes in the System Configuration and Tuning
guide. PR_SLOWHZ is defined in the <sys/protosw.h> header file. The
maximum size of a TCP segment is based on the network MTU for destina‐
tions on local networks or on a default MTU of 576 bytes for destina‐
tions on nonlocal networks. The default behavior can be altered by
setting the TCP_MAXSEG option to an integer value from 1 to 65,535.
However, TCP will not use a maximum segment size smaller than 32 or
larger than the local network MTU. Setting the TCP_MAXSEG option to a
value of zero results in default behavior. The TCP_MAXSEG option can
only be set prior to calling listen or connect on the socket. For pas‐
sive connections, the TCP_MAXSEG option value is inherited from the
listening socket. This option takes an int value, with a range of 0 to
65535. 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 gath‐
ering of output can cause significant delays. Therefore, TCP provides
a Boolean option, TCP_NODELAY (from the netinet/tcp.h header file), to
defeat this algorithm. This option takes an int value. When this
option is enabled, the sender places a timestamp in each data segment.
The receiver, if configured to accept them, sends these timestamps back
in ACK segments. This provides the sender with a mechanism with which
to measure round-trip time. TCP provides a Boolean option, TCP_TSOPTENA
(from the netinet/tcp.h header file) to enable or disable this option.
This option takes an int value. When the PAWS (Protect Against Wrapped
Sequence numbers) option is enabled, the receiver rejects any old
duplicate segments that are received. This option is used on synchro‐
nized TCP connections only. TCP provides a Boolean option, TCP_PAWS
(from the netinet/tcp.h header file) to enable or disable this option.
This option takes an int value. In order to use this option, you must
also enable the TCP_TSOPTENA option. When the Selective Acknowledgment
(SACK) option is enabled, the data receiver can inform the sender about
all segments that have arrived successfully. In this way, the sender
need retransmit only those segments that have actually been lost. This
option is useful in cases where multiple segments are dropped. TCP
provides a Boolean option, TCP_SACKENA (from the netinet/tcp.h header
file) to enable or disable this option. This option takes an int value.
These options can be retrieved with the getsockopt() function and set
with the setsockopt() function.
The option level for the setsockopt() and getsockopt() functions is the
protocol number for TCP, available from the getprotobyname() function.
Options at the IP transport level may be used with TCP; see ip(7).
Incoming connection requests that are source-routed are noted, and the
reverse source route is used in responding.
ERRORS
If a socket operation fails, errno may be set to one of the following
values: The option_len parameter provided to the setsockopt() or get‐
sockopt() function is less than sizeof(int) or the option_value parame‐
ter provided to the setsockopt() function is outside the valid range
for the option. The socket to be connected already has a connection.
The system ran out of memory for an internal data structure. Setting
the TCP_MAXSEG option is not supported after connect() or listen(). A
connection was dropped due to excessive retransmissions. The remote
peer forced the connection to be closed. The remote peer actively
refuses connection establishment (usually because no process is listen‐
ing to the port). An attempt is made to create a socket with a port
which has already been allocated. An attempt is made to create a
socket with a network address for which no network interface exists.
SEE ALSO
Functions: getsockopt(2), socket(2)
Network Information: netintro(7), inet(7), ip(7)tcp(7)