1.\" Copyright (c) 1983, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. All advertising materials mentioning features or use of this software 13.\" must display the following acknowledgement: 14.\" This product includes software developed by the University of 15.\" California, Berkeley and its contributors. 16.\" 4. 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.\" $FreeBSD$ 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 a number of 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.It TCPCTL_RTTDFLT 211.Pq tcp.rttdflt 212The value of the default maximum TCP Round Trip Time. 213.It TCPCTL_SENDSPACE 214.Pq tcp.sendspace 215Maximum TCP send window. 216.It TCPCTL_RECVSPACE 217.Pq tcp.recvspace 218Maximum TCP receive window. 219.It tcp.log_in_vain 220Log any connection attempts to ports where there is not a socket 221accepting connections. 222.It tcp.blackhole 223If enabled, disable sending of RST when a connection is attempted 224to a port where there is not a socket accepting connections. 225See 226.Xr blackhole 4 . 227.It tcp.delayed_ack 228Delay ACK to try and piggyback it onto a data packet. 229.It tcp.newreno 230Enable TCP NewReno Fast Recovery algorithm, 231as described in RFC 2582. 232.It tcp.path_mtu_discovery 233Enable Path MTU Discovery 234.El 235.Sh DIAGNOSTICS 236A socket operation may fail with one of the following errors returned: 237.Bl -tag -width [EADDRNOTAVAIL] 238.It Bq Er EISCONN 239when trying to establish a connection on a socket which 240already has one; 241.It Bq Er ENOBUFS 242when the system runs out of memory for 243an internal data structure; 244.It Bq Er ETIMEDOUT 245when a connection was dropped 246due to excessive retransmissions; 247.It Bq Er ECONNRESET 248when the remote peer 249forces the connection to be closed; 250.It Bq Er ECONNREFUSED 251when the remote 252peer actively refuses connection establishment (usually because 253no process is listening to the port); 254.It Bq Er EADDRINUSE 255when an attempt 256is made to create a socket with a port which has already been 257allocated; 258.It Bq Er EADDRNOTAVAIL 259when an attempt is made to create a 260socket with a network address for which no network interface 261exists. 262.It Bq Er EAFNOSUPPORT 263when an attempt is made to bind or connect a socket to a multicast 264address. 265.El 266.Sh SEE ALSO 267.Xr getsockopt 2 , 268.Xr socket 2 , 269.Xr sysctl 3 , 270.Xr inet 4 , 271.Xr intro 4 , 272.Xr ip 4 , 273.Xr ttcp 4 274.Xr blackhole 4 275.Rs 276.%A V. Jacobson 277.%A R. Braden 278.%A D. Borman 279.%T "TCP Extensions for High Performance" 280.%O RFC 1323 281.Re 282.Rs 283.%A R. Braden 284.%T "T/TCP \- TCP Extensions for Transactions" 285.%O RFC 1644 286.Re 287.Sh HISTORY 288The 289.Nm 290protocol appeared in 291.Bx 4.2 . 292The RFC 1323 extensions for window scaling and timestamps were added 293in 294.Bx 4.4 . 295