TCP(7P)TCP(7P)NAMETCP - Internet Transmission Control Protocol
SYNOPSISDESCRIPTION
The TCP protocol provides reliable, flow-controlled, two-way transmis‐
sion of data. It is a byte-stream protocol used to support the socket
type. TCP constructs virtual circuits between peer entities. A vir‐
tual 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 sock‐
ets, 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 con‐
nection 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 seg‐
ment size will be returned in optval.
(boolean option) causes small amounts of output to be sent imme‐
diately.
(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 termi‐
nates 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 con‐
nection
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 thresholds. If it has
waited longer than the second threshold, TCP ter‐
minates the connection. The default value for
this option is the current value of the ndd tun‐
able 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 considered
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 control 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 speci‐
fied 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 con‐
nect attempt. The default value for this initial
connection establishment 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 config‐
ured 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 synchronized 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 automat‐
ically 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 seg‐
ments that have actually been lost. This option
is useful in cases where multiple segments 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 outstand‐
ing 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, how‐
ever, 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 connec‐
tion 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 sys‐
tem call (for example, returns an error, and is set to See getsock‐
opt(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 Cali‐
fornia, Berkeley.
SEE ALSOndd(1M), getsockopt(2), recv(2), send(2), socket(2), t_open(3), t_opt‐
mgmt(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)
