TCP(4)                  NetBSD 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 com-
     posed of an Internet address specifying the host and network, with a spe-
     cific 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 cre-
     ated 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 pre-
                    sented; when outstanding data has not yet been acknowl-
                    edged, 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 negotiation, 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 cur-
                    rent 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 adver-
                    tising 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 supported.

                    In order for this option to function correctly, it is nec-
                    essary for the administrator to add a tcp-md5 key entry to
                    the system's security associations 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 sec-
                    onds 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 con-
                    nection 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 con-
                    nection 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 connection establish-
                    ment 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 listening 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 connection 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 behavior; 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 Appropriate 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 retrans-
                      missions;

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

     [ECONNREFUSED]   when the remote peer actively refuses connection estab-
                      lishment (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 net-
                      work address for which no network interface exists.

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

     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.

NetBSD 5.0_STABLE                June 19, 2007               NetBSD 5.0_STABLE
