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NetBSD 6.1.5 - man page for tcp (netbsd section 4)

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TCP(4)				   BSD Kernel Interfaces Manual 			   TCP(4)

NAME
     tcp -- Internet Transmission Control Protocol

SYNOPSIS
     #include <sys/socket.h>
     #include <netinet/in.h>

     int
     socket(AF_INET, SOCK_STREAM, 0);

     int
     socket(AF_INET6, SOCK_STREAM, 0);

DESCRIPTION
     The TCP provides reliable, flow-controlled, two-way transmission of data.	It is a byte-
     stream protocol used to support the SOCK_STREAM abstraction.  TCP 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 using 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(2) system call must be used after binding the socket with the bind(2) system
     call.  Only passive sockets may use the accept(2) call to accept incoming connections.  Only
     active sockets may use the connect(2) call to initiate connections.

     Passive sockets may ``underspecify'' their location to match incoming connection requests
     from multiple networks.  This technique, termed ``wildcard addressing'', allows a single
     server to provide service to clients on multiple networks.  To create a socket which listens
     on all networks, the Internet address INADDR_ANY 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 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 a number of socket options which can be set with setsockopt(2) and tested with
     getsockopt(2):

     TCP_NODELAY    Under most circumstances, TCP sends data when it is presented; when outstand-
		    ing 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
		    number of clients, such as window systems that send a stream of mouse events
		    which receive no replies, this packetization may cause significant delays.
		    Therefore, TCP provides a boolean option, TCP_NODELAY (from <netinet/tcp.h>,
		    to defeat this algorithm.

     TCP_MAXSEG     By default, a sender- and receiver-TCP will negotiate among themselves to
		    determine the maximum segment size to be used for each connection.	The
		    TCP_MAXSEG option allows the user to determine the result of this negotia-
		    tion, and to reduce it if desired.

     TCP_MD5SIG     This option enables the use of MD5 digests (also known as TCP-MD5) on writes
		    to the specified socket.  In the current release, only outgoing traffic is
		    digested; digests on incoming traffic are not verified.  The current default
		    behavior for the system is to respond to a system advertising this option
		    with TCP-MD5; this may change.

		    One common use for this in a NetBSD router deployment is to enable based
		    routers to interwork with Cisco equipment at peering points.  Support for
		    this feature conforms to RFC 2385.	Only IPv4 (AF_INET) sessions are sup-
		    ported.

		    In order for this option to function correctly, it is necessary for the
		    administrator to add a tcp-md5 key entry to the system's security associa-
		    tions database (SADB) using the setkey(8) utility.	This entry must have an
		    SPI of 0x1000 and can therefore only be specified on a per-host basis at this
		    time.

		    If an SADB entry cannot be found for the destination, the outgoing traffic
		    will have an invalid digest option prepended, and the following error message
		    will be visible on the system console: tcp_signature_compute: SADB lookup
		    failed for %d.%d.%d.%d.

     TCP_KEEPIDLE   TCP probes a connection that has been idle for some amount of time.  The
		    default value for this idle period is 4 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 unsigned
		    int value, with a value greater than 0.

     TCP_KEEPINTVL  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 150 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 retransmitting a keepalive probe.	This
		    option takes an unsigned int value, with a value greater than 0.

     TCP_KEEPCNT    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_KEEPCNT 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 unsigned int value, with a
		    value greater than 0.

     TCP_KEEPINIT   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 con-
		    nection establishment timeout is 150 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 lis-
		    tening socket.  This option takes an unsigned int value, with a value greater
		    than 0.

     The option level for the setsockopt(2) call is the protocol number for TCP, available from
     getprotobyname(3).

     In the historical BSD TCP implementation, if the TCP_NODELAY option was set on a passive
     socket, the sockets returned by accept(2) erroneously did not have the TCP_NODELAY option
     set; the behavior was corrected to inherit TCP_NODELAY in NetBSD 1.6.

     Options at the IP network level may be used with TCP; see ip(4) or ip6(4).  Incoming connec-
     tion requests that are source-routed are noted, and the reverse source route is used in
     responding.

     There are many adjustable parameters that control various aspects of the NetBSD TCP behav-
     ior; these parameters are documented in sysctl(7), and they include:
     o	 RFC 1323 extensions for high performance
     o	 Send/receive buffer sizes
     o	 Default maximum segment size (MSS)
     o	 SYN cache parameters
     o	 Initial window size
     o	 Hughes/Touch/Heidemann Congestion Window Monitoring algorithm
     o	 Keepalive parameters
     o	 newReno algorithm for congestion control
     o	 Logging of connection refusals
     o	 RST packet rate limits
     o	 SACK (Selective Acknowledgment)
     o	 ECN (Explicit Congestion Notification)
     o	 Congestion window increase methods; the traditional packet counting or RFC 3465 Appro-
	 priate Byte Counting

DIAGNOSTICS
     A socket operation may fail with one of the following errors returned:

     [EISCONN]	      when trying to establish a connection on a socket which already has one;

     [ENOBUFS]	      when the system runs out of memory for an internal data structure;

     [ETIMEDOUT]      when a connection was dropped due to excessive retransmissions;

     [ECONNRESET]     when the remote peer forces the connection to be closed;

     [ECONNREFUSED]   when the remote peer actively refuses connection establishment (usually
		      because no process is listening to the port);

     [EADDRINUSE]     when an attempt is made to create a socket with a port which has already
		      been allocated;

     [EADDRNOTAVAIL]  when an attempt is made to create a socket with a network address for which
		      no network interface exists.

SEE ALSO
     getsockopt(2), socket(2), inet(4), inet6(4), intro(4), ip(4), ip6(4), sysctl(7)

     Transmission Control Protocol, RFC, 793, September 1981.

     Requirements for Internet Hosts -- Communication Layers, RFC, 1122, October 1989.

HISTORY
     The tcp protocol stack appeared in 4.2BSD.

BSD					  June 19, 2007 				      BSD
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