tcp, TCP - Internet Transmission Control Protocol
s = socket(AF_INET, SOCK_STREAM, 0);
s = socket(AF_INET6, SOCK_STREAM, 0);
t = t_open("/dev/tcp", O_RDWR);
t = t_open("/dev/tcp6", O_RDWR);
TCP is the virtual circuit protocol of the Internet protocol family. It
provides reliable, flow-controlled, in order, two-way transmission of
data. It is a byte-stream protocol layered above the Internet Protocol
(IP), or the Internet Protocol Version 6 (IPv6), the Internet protocol
family's internetwork datagram delivery protocol.
Programs can access TCP using the socket interface as a SOCK_STREAM
socket type, or using the Transport Level Interface (TLI) where it sup‐
ports the connection-oriented (T_COTS_ORD) service type.
TCP uses IP's host-level addressing and adds its own per-host collec‐
tion of "port addresses." The endpoints of a TCP connection are identi‐
fied by the combination of an IP or IPv6 address and a TCP port number.
Although other protocols, such as the User Datagram Protocol (UDP), may
use the same host and port address format, the port space of these pro‐
tocols is distinct. See inet(7P) and inet6(7P) for details on the com‐
mon aspects of addressing in the Internet protocol family.
Sockets utilizing TCP are either "active" or "passive." Active sockets
initiate connections to passive sockets. Both types of sockets must
have their local IP or IPv6 address and TCP port number bound with the
bind(3SOCKET) system call after the socket is created. By default, TCP
sockets are active. A passive socket is created by calling the lis‐
ten(3SOCKET) system call after binding the socket with bind(). This
establishes a queueing parameter for the passive socket. After this,
connections to the passive socket can be received with the
accept(3SOCKET) system call. Active sockets use the connect(3SOCKET)
call after binding to initiate connections.
By using the special value INADDR_ANY with IP, or the unspecified
address (all zeroes) with IPv6, the local IP address can be left
unspecified in the bind() call by either active or passive TCP sockets.
This feature is usually used if the local address is either unknown or
irrelevant. If left unspecified, the local IP or IPv6 address will be
bound at connection time to the address of the network interface used
to service the connection.
Note that no two TCP sockets can be bound to the same port unless the
bound IP addresses are different. IPv4 INADDR_ANY and IPv6 unspecified
addresses compare as equal to any IPv4 or IPv6 address. For example, if
a socket is bound to INADDR_ANY or unspecified address and port X, no
other socket can bind to port X, regardless of the binding address.
This special consideration of INADDR_ANY and unspecified address can be
changed using the socket option SO_REUSEADDR. If SO_REUSEADDR is set on
a socket doing a bind, IPv4 INADDR_ANY and IPv6 unspecified address do
not compare as equal to any IP address. This means that as long as the
two sockets are not both bound to INADDR_ANY/unspecified address or the
same IP address, the two sockets can be bound to the same port.
If an application does not want to allow another socket using the
SO_REUSEADDR option to bind to a port its socket is bound to, the
application can set the socket level option SO_EXCLBIND on a socket.
The option values of 0 and 1 mean enabling and disabling the option
respectively. Once this option is enabled on a socket, no other socket
can be bound to the same port.
Once a connection has been established, data can be exchanged using the
read(2) and write(2) system calls.
Under most circumstances, TCP sends data when it is presented. When
outstanding data has not yet been acknowledged, TCP gathers small
amounts of output to be sent in a single packet once an acknowledgement
has been received. For a small number of clients, such as window sys‐
tems that send a stream of mouse events which receive no replies, this
packetization may cause significant delays. To circumvent this problem,
TCP provides a socket-level boolean option, TCP_NODELAY. TCP_NODELAY is
defined in <netinet/tcp.h>, and is set with setsockopt(3SOCKET) and
tested with getsockopt(3SOCKET). The option level for the setsockopt()
call is the protocol number for TCP, available from getprotoby‐
For some applications, it may be desirable for TCP not to send out data
unless a full TCP segment can be sent. To enable this behavior, an
application can use the TCP_CORK socket option. When TCP_CORK is set
with a non-zero value, TCP sends out a full TCP segment only. When
TCP_CORK is set to zero after it has been enabled, all buffered data is
sent out (as permitted by the peer's receive window and the current
congestion window). TCP_CORK is defined in <netinet/tcp.h>, and is set
with setsockopt(3SOCKET) and tested with getsockopt(3SOCKET). The
option level for the setsockopt() call is the protocol number for
TCP, available from getprotobyname(3SOCKET).
The SO_RCVBUF socket level option can be used to control the window
that TCP advertises to the peer. IP level options may also be used with
TCP. See ip(7P) and ip6(7P).
Another socket level option, SO_RCVBUF, can be used to control the win‐
dow that TCP advertises to the peer. IP level options may also be used
with TCP. See ip(7P) and ip6(7P).
TCP provides an urgent data mechanism, which may be invoked using the
out-of-band provisions of send(3SOCKET). The caller may mark one byte
as "urgent" with the MSG_OOB flag to send(3SOCKET). This sets an
"urgent pointer" pointing to this byte in the TCP stream. The receiver
on the other side of the stream is notified of the urgent data by a
SIGURG signal. The SIOCATMARK ioctl(2) request returns a value indicat‐
ing whether the stream is at the urgent mark. Because the system never
returns data across the urgent mark in a single read(2) call, it is
possible to advance to the urgent data in a simple loop which reads
data, testing the socket with the SIOCATMARK ioctl() request, until it
reaches the mark.
Incoming connection requests that include an IP source route option are
noted, and the reverse source route is used in responding.
A checksum over all data helps TCP implement reliability. Using a win‐
dow-based flow control mechanism that makes use of positive acknowl‐
edgements, sequence numbers, and a retransmission strategy, TCP can
usually recover when datagrams are damaged, delayed, duplicated or
delivered out of order by the underlying communication medium.
If the local TCP receives no acknowledgements from its peer for a
period of time, (for example, if the remote machine crashes), the con‐
nection is closed and an error is returned.
TCP follows the congestion control algorithm described in RFC 2581, and
also supports the initial congestion window (cwnd) changes in RFC 3390.
The initial cwnd calculation can be overridden by the socket option
TCP_INIT_CWND. An application can use this option to set the initial
cwnd to a specified number of TCP segments. This applies to the cases
when the connection first starts and restarts after an idle period.
The process must have the PRIV_SYS_NET_CONFIG privilege if it wants to
specify a number greater than that calculated by RFC 3390.
SunOS supports TCP Extensions for High Performance (RFC 1323) which
includes the window scale and time stamp options, and Protection
Against Wrap Around Sequence Numbers (PAWS). SunOS also supports Selec‐
tive Acknowledgment (SACK) capabilities (RFC 2018) and Explicit Conges‐
tion Notification (ECN) mechanism (RFC 3168).
Turn on the window scale option in one of the following ways:
o An application can set SO_SNDBUF or SO_RCVBUF size in the
setsockopt() option to be larger than 64K. This must be done
before the program calls listen() or connect(), because the
window scale option is negotiated when the connection is
established. Once the connection has been made, it is too
late to increase the send or receive window beyond the
default TCP limit of 64K.
o For all applications, use ndd(1M) to modify the configura‐
tion parameter tcp_wscale_always. If tcp_wscale_always is
set to 1, the window scale option will always be set when
connecting to a remote system. If tcp_wscale_always is 0,
the window scale option will be set only if the user has
requested a send or receive window larger than 64K. The
default value of tcp_wscale_always is 1.
o Regardless of the value of tcp_wscale_always, the window
scale option will always be included in a connect acknowl‐
edgement if the connecting system has used the option.
Turn on SACK capabilities in the following way:
o Use ndd to modify the configuration parameter tcp_sack_per‐
mitted. If tcp_sack_permitted is set to 0, TCP will not
accept SACK or send out SACK information. If tcp_sack_per‐
mitted is set to 1, TCP will not initiate a connection with
SACK permitted option in the SYN segment, but will respond
with SACK permitted option in the SYN|ACK segment if an
incoming connection request has the SACK permitted option.
This means that TCP will only accept SACK information if the
other side of the connection also accepts SACK information.
If tcp_sack_permitted is set to 2, it will both initiate and
accept connections with SACK information. The default for
tcp_sack_permitted is 2 (active enabled).
Turn on TCP ECN mechanism in the following way:
o Use ndd to modify the configuration parameter tcp_ecn_per‐
mitted. If tcp_ecn_permitted is set to 0, TCP will not nego‐
tiate with a peer that supports ECN mechanism. If
tcp_ecn_permitted is set to 1 when initiating a connection,
TCP will not tell a peer that it supports ECN mechanism.
However, it will tell a peer that it supports ECN mechanism
when accepting a new incoming connection request if the peer
indicates that it supports ECN mechanism in the SYN segment.
If tcp_ecn_permitted is set to 2, in addition to negotiating
with a peer on ECN mechanism when accepting connections, TCP
will indicate in the outgoing SYN segment that it supports
ECN mechanism when TCP makes active outgoing connections.
The default for tcp_ecn_permitted is 1.
Turn on the time stamp option in the following way:
o Use ndd to modify the configuration parameter
tcp_tstamp_always. If tcp_tstamp_always is 1, the time stamp
option will always be set when connecting to a remote
machine. If tcp_tstamp_always is 0, the timestamp option
will not be set when connecting to a remote system. The
default for tcp_tstamp_always is 0.
o Regardless of the value of tcp_tstamp_always, the time stamp
option will always be included in a connect acknowledgement
(and all succeeding packets) if the connecting system has
used the time stamp option.
Use the following procedure to turn on the time stamp option only when
the window scale option is in effect:
o Use ndd to modify the configuration parameter
tcp_tstamp_if_wscale. Setting tcp_tstamp_if_wscale to 1
will cause the time stamp option to be set when connecting
to a remote system, if the window scale option has been set.
If tcp_tstamp_if_wscale is 0, the time stamp option will not
be set when connecting to a remote system. The default for
tcp_tstamp_if_wscale is 1.
Protection Against Wrap Around Sequence Numbers (PAWS) is always used
when the time stamp option is set.
SunOS also supports multiple methods of generating initial sequence
numbers. One of these methods is the improved technique suggested in
RFC 1948. We HIGHLY recommend that you set sequence number generation
parameters as close to boot time as possible. This prevents sequence
number problems on connections that use the same connection-ID as ones
that used a different sequence number generation. The svc:/network/ini‐
tial:default service configures the initial sequence number generation.
The service reads the value contained in the configuration file
/etc/default/inetinit to determine which method to use.
The /etc/default/inetinit file is an unstable interface, and may change
in future releases.
TCP may be configured to report some information on connections that
terminate by means of an RST packet. By default, no logging is done. If
the ndd(1M) parameter tcp_trace is set to 1, then trace data is col‐
lected for all new connections established after that time.
The trace data consists of the TCP headers and IP source and destina‐
tion addresses of the last few packets sent in each direction before
RST occurred. Those packets are logged in a series of strlog(9F) calls.
This trace facility has a very low overhead, and so is superior to such
utilities as snoop(1M) for non-intrusive debugging for connections ter‐
minating by means of an RST.
SunOS supports the keep-alive mechanism described in RFC 1122. It is
enabled using the socket option SO_KEEPALIVE. When enabled, the first
keep-alive probe is sent out after a TCP is idle for two hours If the
peer does not respond to the probe within eight minutes, the TCP con‐
nection is aborted. You can alter the interval for sending out the
first probe using the socket option TCP_KEEPALIVE_THRESHOLD. The option
value is an unsigned integer in milliseconds. The system default is
controlled by the TCP ndd parameter tcp_keepalive_interval. The minimum
value is ten seconds. The maximum is ten days, while the default is two
hours. If you receive no response to the probe, you can use the
TCP_KEEPALIVE_ABORT_THRESHOLD socket option to change the time thresh‐
old for aborting a TCP connection. The option value is an unsigned
integer in milliseconds. The value zero indicates that TCP should never
time out and abort the connection when probing. The system default is
controlled by the TCP ndd parameter tcp_keepalive_abort_interval. The
default is eight minutes.
socket options TCP_KEEPIDLE, TCP_KEEPCNT and TCP_KEEPINTVL are also
supported for compatibility with other Unix Flavors. TCP_KEEPIDLE
option specifies the interval in seconds for sending out the first
keep-alive probe. TCP_KEEPCNT specifies the number of keep-alive probes
to be sent before aborting the connection in the event of no response
from peer. TCP_KEEPINTVL specifies the interval in seconds between suc‐
cessive keep-alive probes.
SEE ALSOsvcs(1), ndd(1M), ioctl(2), read(2), svcadm(1M), write(2),
accept(3SOCKET), bind(3SOCKET), connect(3SOCKET), getprotoby‐
name(3SOCKET), getsockopt(3SOCKET), listen(3SOCKET), send(3SOCKET),
smf(5), inet(7P), inet6(7P), ip(7P), ip6(7P)
Ramakrishnan, K., Floyd, S., Black, D., RFC 3168, The Addition of
Explicit Congestion Notification (ECN) to IP, September 2001.
Mathias, M. and Hahdavi, J. Pittsburgh Supercomputing Center; Ford, S.
Lawrence Berkeley National Laboratory; Romanow, A. Sun Microsystems,
Inc. RFC 2018, TCP Selective Acknowledgement Options, October 1996.
Bellovin, S., RFC 1948, Defending Against Sequence Number Attacks, May
Jacobson, V., Braden, R., and Borman, D., RFC 1323, TCP Extensions for
High Performance, May 1992.
Postel, Jon, RFC 793, Transmission Control Protocol - DARPA Internet
Program Protocol Specification, Network Information Center, SRI Inter‐
national, Menlo Park, CA., September 1981.
A socket operation may fail if:
A connect() operation was attempted on a socket on
which a connect() operation had already been per‐
A connection was dropped due to excessive retransmis‐
The remote peer forced the connection to be closed
(usually because the remote machine has lost state
information about the connection due to a crash).
The remote peer actively refused connection establish‐
ment (usually because no process is listening to the
A bind() operation was attempted on a socket with a
network address/port pair that has already been bound
to another socket.
A bind() operation was attempted on a socket with a
network address for which no network interface exists.
A bind() operation was attempted with a "reserved"
port number and the effective user ID of the process
was not the privileged user.
The system ran out of memory for internal data struc‐
The tcp service is managed by the service management facility, smf(5),
under the service identifier:
Administrative actions on this service, such as enabling, disabling, or
requesting restart, can be performed using svcadm(1M). The service's
status can be queried using the svcs(1) command.
Jun 30, 2006 TCP(7P)