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Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" From: @(#)tcp.4 8.1 (Berkeley) 6/5/93 33.\" $Id: tcp.4,v 1.8 1997/02/22 13:24:45 peter Exp $ 34.\" 35.Dd February 14, 1995 36.Dt TCP 4 37.Os BSD 4.2 38.Sh NAME 39.Nm tcp 40.Nd Internet Transmission Control Protocol 41.Sh SYNOPSIS 42.Fd #include <sys/types.h> 43.Fd #include <sys/socket.h> 44.Fd #include <netinet/in.h> 45.Ft int 46.Fn socket AF_INET SOCK_STREAM 0 47.Sh DESCRIPTION 48The 49.Tn TCP 50protocol provides reliable, flow-controlled, two-way 51transmission of data. It is a byte-stream protocol used to 52support the 53.Dv SOCK_STREAM 54abstraction. TCP uses the standard 55Internet address format and, in addition, provides a per-host 56collection of 57.Dq port addresses . 58Thus, each address is composed 59of an Internet address specifying the host and network, with 60a specific 61.Tn TCP 62port on the host identifying the peer entity. 63.Pp 64Sockets utilizing the tcp protocol are either 65.Dq active 66or 67.Dq passive . 68Active sockets initiate connections to passive 69sockets. By default 70.Tn TCP 71sockets are created active; to create a 72passive socket the 73.Xr listen 2 74system call must be used 75after binding the socket with the 76.Xr bind 2 77system call. Only 78passive sockets may use the 79.Xr accept 2 80call to accept incoming connections. Only active sockets may 81use the 82.Xr connect 2 83call to initiate connections. 84.Tn TCP 85also supports a more datagram-like mode, called Transaction 86.Tn TCP , 87which is described in 88.Xr ttcp 4 . 89.Pp 90Passive sockets may 91.Dq underspecify 92their location to match 93incoming connection requests from multiple networks. This 94technique, termed 95.Dq wildcard addressing , 96allows a single 97server to provide service to clients on multiple networks. 98To create a socket which listens on all networks, the Internet 99address 100.Dv INADDR_ANY 101must be bound. The 102.Tn TCP 103port may still be specified 104at this time; if the port is not specified the system will assign one. 105Once a connection has been established the socket's address is 106fixed by the peer entity's location. The address assigned the 107socket is the address associated with the network interface 108through which packets are being transmitted and received. Normally 109this address corresponds to the peer entity's network. 110.Pp 111.Tn TCP 112supports a number of socket options which can be set with 113.Xr setsockopt 2 114and tested with 115.Xr getsockopt 2 : 116.Bl -tag -width TCP_NODELAYx 117.It Dv TCP_NODELAY 118Under most circumstances, 119.Tn TCP 120sends data when it is presented; 121when outstanding data has not yet been acknowledged, it gathers 122small amounts of output to be sent in a single packet once 123an acknowledgement is received. 124For a small number of clients, such as window systems 125that send a stream of mouse events which receive no replies, 126this packetization may cause significant delays. 127The boolean option 128.Dv TCP_NODELAY 129defeats this algorithm. 130.It Dv TCP_MAXSEG 131By default, a sender\- and receiver-TCP 132will negotiate among themselves to determine the maximum segment size 133to be used for each connection. The 134.Dv TCP_MAXSEG 135option allows the user to determine the result of this negotiation, 136and to reduce it if desired. 137.It Dv TCP_NOOPT 138.Tn TCP 139usually sends a number of options in each packet, corresponding to 140various 141.Tn TCP 142extensions which are provided in this implementation. The boolean 143option 144.Dv TCP_NOOPT 145is provided to disable 146.Tn TCP 147option use on a per-connection basis. 148.It Dv TCP_NOPUSH 149By convention, the sender-TCP 150will set the 151.Dq push 152bit and begin transmission immediately (if permitted) at the end of 153every user call to 154.Xr write 2 155or 156.Xr writev 2 . 157The 158.Dv TCP_NOPUSH 159option is provided to allow servers to easily make use of Transaction 160TCP (see 161.Xr ttcp 4 ). 162When the option is set to a non-zero value, 163.Tn TCP 164will delay sending any data at all until either the socket is closed, 165or the internal send buffer is filled. 166.El 167.Pp 168The option level for the 169.Xr setsockopt 2 170call is the protocol number for 171.Tn TCP , 172available from 173.Xr getprotobyname 3 , 174or 175.Dv IPPROTO_TCP . 176All options are declared in 177.Aq Pa netinet/tcp.h . 178.Pp 179Options at the 180.Tn IP 181transport level may be used with 182.Tn TCP ; 183see 184.Xr ip 4 . 185Incoming connection requests that are source-routed are noted, 186and the reverse source route is used in responding. 187.Sh MIB VARIABLES 188The 189.Nm 190protocol implements three variables in the 191.Li net.inet 192branch of the 193.Xr sysctl 3 194MIB. 195.Bl -tag -width TCPCTL_DO_RFC1644 196.It Dv TCPCTL_DO_RFC1323 197.Pq tcp.rfc1323 198Implement the window scaling and timestamp options of RFC 1323 199(default true). 200.It Dv TCPCTL_DO_RFC1644 201.Pq tcp.rfc1644 202Implement Transaction 203.Tn TCP , 204as described in RFC 1644. 205.It Dv TCPCTL_MSSDFLT 206.Pq tcp.mssdflt 207The default value used for the maximum segment size 208.Pq Dq MSS 209when no advice to the contrary is received from MSS negotiation. 210.El 211.Sh DIAGNOSTICS 212A socket operation may fail with one of the following errors returned: 213.Bl -tag -width [EADDRNOTAVAIL] 214.It Bq Er EISCONN 215when trying to establish a connection on a socket which 216already has one; 217.It Bq Er ENOBUFS 218when the system runs out of memory for 219an internal data structure; 220.It Bq Er ETIMEDOUT 221when a connection was dropped 222due to excessive retransmissions; 223.It Bq Er ECONNRESET 224when the remote peer 225forces the connection to be closed; 226.It Bq Er ECONNREFUSED 227when the remote 228peer actively refuses connection establishment (usually because 229no process is listening to the port); 230.It Bq Er EADDRINUSE 231when an attempt 232is made to create a socket with a port which has already been 233allocated; 234.It Bq Er EADDRNOTAVAIL 235when an attempt is made to create a 236socket with a network address for which no network interface 237exists. 238.It Bq Er EAFNOSUPPORT 239when an attempt is made to bind or connect a socket to a multicast 240address. 241.El 242.Sh SEE ALSO 243.Xr getsockopt 2 , 244.Xr socket 2 , 245.Xr sysctl 3 , 246.Xr inet 4 , 247.Xr intro 4 , 248.Xr ip 4 , 249.Xr ttcp 4 250.Rs 251.%A V. Jacobson, R. Braden, and D. Borman 252.%T "TCP Extensions for High Performance" 253.%O RFC 1323 254.Re 255.Rs 256.%A R. Braden 257.%T "T/TCP \- TCP Extensions for Transactions" 258.%O RFC 1644 259.Re 260.Sh HISTORY 261The 262.Nm 263protocol appeared in 264.Bx 4.2 . 265The RFC 1323 extensions for window scaling and timestamps were added 266in 267.Bx 4.4 . 268