TCP(7P) TCP(7P)
NAME
TCP - Internet Transmission Control Protocol
SYNOPSIS
DESCRIPTION
The TCP protocol provides reliable, flow-controlled, two-way transmission of data. It is a byte-stream protocol used to support the socket
type. TCP constructs virtual circuits between peer entities. A virtual circuit consists of remote Internet addresses, remote ports, local
Internet addresses and local ports. IP uses the Internet addresses to direct messages between hosts, and the port numbers to identify a
TCP entity at a particular host.
Sockets using TCP are either active or passive. creates active sockets, which initiate connections to passive sockets (see connect(2)).
To create a passive socket, use the system call after binding the socket with the system call (see listen(2) and bind(2)). Only passive
sockets can use the call to accept incoming connections (see accept(2)).
Passive sockets can underspecify their location to match incoming connection requests from multiple networks. This technique, called
allows a single server to provide service to clients on multiple networks. To create a socket that listens on all networks, the Internet
address must be bound for AF_INET family and for AF_INET6 family. The TCP port can still be specified even if wildcard addressing is being
used. If the port is specified as zero, the system assigns a port.
Once has a rendezvous with a connect request, a virtual circuit is established between peer entities. supplies the local port and local
Internet address and gathers the remote port and remote Internet address from the peer requesting the connection.
Options
The system supports the following socket options: and (defined in the include file
The option can only be used with while and can be set with and tested with (see getsockopt(2)). These options require level to be set to
in the call.
(non-boolean option) lets an application to receive the current segment size
of the TCP SOCK_STREAM socket. The current segment size will be returned in optval.
(boolean option) causes small amounts of output to be sent immediately.
(non-boolean option) sets the second threshold timer for the connections
that are in ESTABLISHED state. The option value is the threshold time in milliseconds.
When it must retransmit packets because a timer has expired, TCP first compares the total time it has waited against
the two thresholds, as described in RFC 1122, 4.2.3.5. If it has waited longer than the second threshold (R2), TCP
terminates the connection. The default value for this option is the current value of the ndd tunable parameter Refer
to ndd(1M) online help for details on the default value.
(non-boolean option) sets the second threshold timer during connection
establishment. The option value is the threshold time in milliseconds.
This option is the same as except that this value is used during connection establishment. When it must retransmit
the SYN packet because a timer has expired, TCP first compares the total time it has waited against the two thresh-
olds. If it has waited longer than the second threshold, TCP terminates the connection. The default value for this
option is the current value of the ndd tunable See ndd(1M) online help for details on the default value.
(non-boolean option) When the
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 for a certain number of times before a connection is consid-
ered to be broken. Use the option with the option to affect this value for a given socket. If the remote system
does not respond to a keepalive probe, TCP retransmits the probe after some amount of time. The time interval
between each keepalive probe is decided by the TCP's normal retransmission algorithm. Users do not have direct con-
trol of the algorithm. For a given socket, the expression determines when the retransmission attempts will time out
and the connection is broken. If both and are not specified or if either one of them is not specified, then the
retransmission algorithm will run up to This option takes an value, with a range of 1 to 32767.
(non-boolean option) When the
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 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 value, with a range of 1 to 32767.
(non-boolean option) 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 establish-
ment timeout is 75 seconds. The 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 option
value is inherited from the listening socket. This option takes an value, with a range of 1 to 32767.
(non-boolean option) When the
option is enabled, TCP probes a connection that has been idle for some amount of time. This option is used with the
option. Refer to the description of option above. This option takes an value, with a range of 1 to 32767.
(boolean option) 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, (from the
header file) to enable or disable this option. This option takes an value. When this option is enabled, the option
is also enabled.
(boolean option) 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 syn-
chronized TCP connections only. TCP provides a Boolean option, (from the header file) to enable or disable this
option. This option takes an value. This option automatically turns the option on.
(boolean 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 seg-
ments are dropped. TCP provides a Boolean option, (from the header file) to enable or disable this option. This
option takes an value.
If is set, the system sends small amounts of output immediately rather than gathering them into a single packet after an acknowledgement is
received. If is not set, the system sends data when it is presented, if there is no outstanding unacknowledged data. If there is out-
standing unacknowledged data, the system gathers small amounts of data to be sent in a single packet once an acknowledgement is received.
For clients such as window managers that send a stream of mouse events which receive no replies, this packetization may cause significant
delays. The option can be used to avoid this situation. Note, however, that setting the option may result in a large number of small
packets being sent over the network.
By default, is not set when a socket is created.
The option level to use for accessing the TCP option with the or calls is the protocol number for TCP which is available from (see getpro-
toent(3N)).
If the socket option is enabled on an established TCP connection and the connection has been idle for two hours, TCP sends a packet to the
remote socket, expecting the remote TCP to acknowledge that the connection is still active. If the remote TCP does not respond in a timely
manner, TCP continues to send keepalive packets according to its normal retransmission algorithm. If the remote TCP does not respond
within a particular time limit, TCP drops the connection. The next socket system call (for example, returns an error, and is set to See
getsockopt(2) for details on enabling
The default send and receives buffer size is 32768 bytes (see below). The send and receive buffer sizes for TCP stream sockets can be
altered by using the and options of the system call or the and options of the system call. Refer to getsockopt(2) or t_optmgmt(3) for
details.
The maximum transmit buffer size for a TCP stream socket is 2147483647 bytes. The maximum receive buffer size for a TCP stream socket is
1073725440 bytes. These maximum values can be lowered using the ndd variables and
ERRORS
One of the following errors may be returned in if a socket operation fails. For a more detailed list of errors, see the man pages for spe-
cific system calls.
The socket is already connected.
No buffer space is available for an internal data structure.
Connection dropped due to excessive retransmissions.
The connection was forcibly closed by the peer socket.
Remote peer actively refuses connection establishment
(usually because no process is listening to the port).
The specified address is already in use.
The specified address is not available on this machine.
WARNINGS
The default socket buffer size might increase without notice in a future release or patch. Therefore, if an application calls with it
should do so before calling or it should first call with and ensure that the intended new receive buffer size is not less than the current
buffer size. These programming conventions are consistent with TCP protocol restrictions against reducing the TCP receive window after a
connection has been established.
AUTHOR
The socket interfaces to TCP were developed by the University of California, Berkeley.
SEE ALSO
ndd(1M), getsockopt(2), recv(2), send(2), socket(2), t_open(3), t_optmgmt(3), socket(7), inet(7F).
RFC 793 Transmission Control Protocol
RFC 1122 Requirements for Internet hosts
RFC 1323 TCP Extensions for High Performance
RFC 1878 Variable Length Subnet Table for IPv4
RFC 2018 TCP Selective Acknowledgement Options
RFC 2414 Increasing TCP's Initial Window
RFC 2582 NewReno Modifications to TCP's Fast Recovery Algorithm
TCP(7P)