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)