tcp(7) Miscellaneous Information Manual tcp(7)
NAME
tcp - 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 transmission 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 specifying the host and network, with a specific TCP port on the
host identifying 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 connections. Only active sockets may use the connect function to initiate con-
nections.
Passive sockets may underspecify their location to match incoming connection 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 net-
works, 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 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 the following socket options: 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 num-
ber of clients, such as window systems that send a stream of mouse events which receive no replies, this gathering of output can cause sig-
nificant 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. The maximum size of a TCP segment is based on the network MTU for destinations on local networks or on a
default MTU of 576 bytes for destinations 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 lis-
ten or connect on the socket. For passive 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. 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 con-
figuration 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 retrans-
mitting 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 subsystem 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 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_KEEP-
CNT 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 System Configuration and Tuning guide. If a TCP connection cannot be established within some amount of
time, TCP will time out the connect attempt. The default value for this initial connection establishment 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 subsystem configuration attributes in the System Configuration and Tuning guide. PR_SLOWHZ is
defined in the <sys/protosw.h> header file. These options can be retrieved with the getsockopt function and set with the setsockopt func-
tion.
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 parameter 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.
RELATED INFORMATION
Functions: getsockopt(2), socket(2)
Network Information: netintro(7), inet(7), ip(7) delim off
tcp(7)