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Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)tcp.4 8.1 (Berkeley) 6/5/93 .\" .Dd February 28, 2007 .Dt TCP 4 .Os BSD 4.2 .Sh NAME .Nm tcp .Nd Internet Transmission Control Protocol .Sh SYNOPSIS .In sys/types.h .In sys/socket.h .In netinet/in.h .Ft int .Fn socket AF_INET SOCK_STREAM 0 .Sh DESCRIPTION The .Tn TCP protocol provides reliable, flow-controlled, two-way transmission of data. It is a byte-stream protocol used to support the .Dv SOCK_STREAM abstraction. .Tn TCP uses the standard Internet address format and, in addition, provides a per-host collection of .Dq "port addresses" . Thus, each address is composed of an Internet address specifying the host and network, with a specific .Tn TCP port on the host identifying the peer entity. .Pp Sockets utilizing the .Tn TCP protocol are either .Dq active or .Dq passive . Active sockets initiate connections to passive sockets. By default, .Tn TCP sockets are created active; to create a passive socket, the .Xr listen 2 system call must be used after binding the socket with the .Xr bind 2 system call. Only passive sockets may use the .Xr accept 2 call to accept incoming connections. Only active sockets may use the .Xr connect 2 call to initiate connections. .Pp Passive sockets may .Dq underspecify their location to match incoming connection requests from multiple networks. This technique, termed .Dq "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 .Dv INADDR_ANY must be bound. The .Tn 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 to 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. .Pp .Tn TCP supports a number of socket options which can be set with .Xr setsockopt 2 and tested with .Xr getsockopt 2 : .Bl -tag -width ".Dv TCP_CONNECTIONTIMEOUT" .It Dv TCP_NODELAY Under most circumstances, .Tn TCP sends data when it is presented; when outstanding 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. The boolean option .Dv TCP_NODELAY defeats this algorithm. .It Dv TCP_MAXSEG By default, a sender- and .No receiver- Ns Tn TCP will negotiate among themselves to determine the maximum segment size to be used for each connection. The .Dv TCP_MAXSEG option allows the user to determine the result of this negotiation, and to reduce it if desired. .It Dv TCP_NOOPT .Tn TCP usually sends a number of options in each packet, corresponding to various .Tn TCP extensions which are provided in this implementation. The boolean option .Dv TCP_NOOPT is provided to disable .Tn TCP option use on a per-connection basis. .It Dv TCP_NOPUSH By convention, the .No sender- Ns Tn TCP will set the .Dq push bit, and begin transmission immediately (if permitted) at the end of every user call to .Xr write 2 or .Xr writev 2 . When this option is set to a non-zero value, .Tn TCP will delay sending any data at all until either the socket is closed, or the internal send buffer is filled. .It Dv TCP_KEEPALIVE .Tn The .Dv TCP_KEEPALIVE options enable to specify the amount of time, in seconds, that the connection must be idle before keepalive probes (if enabled) are sent. The default value is specified by the .Tn MIB variable .Va net.inet.tcp.keepidle . .It Dv TCP_CONNECTIONTIMEOUT .Tn The .Dv TCP_CONNECTIONTIMEOUT option allows to specify the timeout, in seconds, for new, non established .Tn TCP connections. This option can be useful for both active and passive .Tn TCP connections. The default value is specified by the .Tn MIB variable .Va net.inet.tcp.keepinit . .El .Pp The option level for the .Xr setsockopt 2 call is the protocol number for .Tn TCP , available from .Xr getprotobyname 3 , or .Dv IPPROTO_TCP . All options are declared in .In netinet/tcp.h . .Pp Options at the .Tn IP transport level may be used with .Tn TCP ; see .Xr ip 4 . Incoming connection requests that are source-routed are noted, and the reverse source route is used in responding. .Ss "Non-blocking connect" .Pp When a .Tn TCP socket is set non-blocking, and the connection cannot be established immediatly, .Xr connect 2 returns with the error .Dv EINPROGRESS , and the connection is established asynchronously. .Pp When the asynchronous connection completes successfully, .Xr select 2 or .Xr poll 2 or .Xr kqueue 2 will indicate the file descriptor is ready for writing. If the connection encounters an error, the file descriptor is marked ready for both reading and writing, and the pending error can be retrieved via the socket option .Dv SO_ERROR . .Pp Note that even if the socket is non-blocking, it is possible for the connection to be established immediatly. In that case .Xr connect 2 does not return with .Dv EINPROGRESS . .Sh DIAGNOSTICS A socket operation may fail with one of the following errors returned: .Bl -tag -width Er .It Bq Er EISCONN when trying to establish a connection on a socket which already has one; .It Bq Er ENOBUFS when the system runs out of memory for an internal data structure; .It Bq Er ETIMEDOUT when a connection was dropped due to excessive retransmissions; .It Bq Er ECONNRESET when the remote peer forces the connection to be closed; .It Bq Er ECONNREFUSED when the remote peer actively refuses connection establishment (usually because no process is listening to the port); .It Bq Er EADDRINUSE when an attempt is made to create a socket with a port which has already been allocated; .It Bq Er EADDRNOTAVAIL when an attempt is made to create a socket with a network address for which no network interface exists; .It Bq Er EAFNOSUPPORT when an attempt is made to bind or connect a socket to a multicast address; .It Bq Er EINPROGRESS returned by .Xr connect 2 when the socket is set nonblocking, and the connection cannot be immediately established; .It Bq Er EALREADY returned by .Xr connect 2 when connection request is already in progress for the specified socket. . .El .Sh SEE ALSO .Xr connect 2 , .Xr getsockopt 2 , .Xr kqueue 2 , .Xr poll 2 , .Xr select 2 , .Xr socket 2 , .Xr sysctl 3 , .Xr inet 4 , .Xr inet6 4 , .Xr ip 4 , .Xr ip6 4 , .Xr netintro 4 , .Xr setkey 8 .Sh HISTORY The .Tn TCP protocol appeared in .Bx 4.2 . .Pp The socket option .Dv TCP_CONNECTIONTIMEOUT first appeared in Mac OS X 10.6.