1.\" $OpenBSD: tcp.4,v 1.11 2000/12/21 21:01:20 aaron Exp $ 2.\" $NetBSD: tcp.4,v 1.3 1994/11/30 16:22:35 jtc Exp $ 3.\" 4.\" Copyright (c) 1983, 1991, 1993 5.\" The Regents of the University of California. All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. All advertising materials mentioning features or use of this software 16.\" must display the following acknowledgement: 17.\" This product includes software developed by the University of 18.\" California, Berkeley and its contributors. 19.\" 4. Neither the name of the University nor the names of its contributors 20.\" may be used to endorse or promote products derived from this software 21.\" without specific prior written permission. 22.\" 23.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 24.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 25.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 26.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 27.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 28.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 29.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 30.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 31.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 32.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 33.\" SUCH DAMAGE. 34.\" 35.\" @(#)tcp.4 8.1 (Berkeley) 6/5/93 36.\" 37.Dd June 5, 1993 38.Dt TCP 4 39.Os 40.Sh NAME 41.Nm tcp 42.Nd Internet Transmission Control Protocol 43.Sh SYNOPSIS 44.Fd #include <sys/socket.h> 45.Fd #include <netinet/in.h> 46.Ft int 47.Fn socket AF_INET SOCK_STREAM 0 48.Ft int 49.Fn socket AF_INET6 SOCK_STREAM 0 50.Sh DESCRIPTION 51The 52.Tn TCP 53protocol provides a reliable, flow-controlled, two-way 54transmission of data. It is a byte-stream protocol used to 55support the 56.Dv SOCK_STREAM 57abstraction. TCP uses the standard 58Internet address format and, in addition, provides a per-host 59collection of 60.Dq port addresses . 61Thus, each address is composed 62of an Internet address specifying the host and network, with 63a specific 64.Tn TCP 65port on the host identifying the peer entity. 66.Pp 67Sockets utilizing the TCP protocol are either 68.Dq active 69or 70.Dq passive . 71Active sockets initiate connections to passive 72sockets. By default 73.Tn TCP 74sockets are created active; to create a 75passive socket the 76.Xr listen 2 77system call must be used 78after binding the socket with the 79.Xr bind 2 80system call. Only 81passive sockets may use the 82.Xr accept 2 83call to accept incoming connections. Only active sockets may 84use the 85.Xr connect 2 86call to initiate connections. 87.Pp 88Passive sockets may 89.Dq underspecify 90their location to match 91incoming connection requests from multiple networks. This 92technique, termed 93.Dq wildcard addressing , 94allows a single 95server to provide service to clients on multiple networks. 96To create a socket which listens on all networks, the Internet 97address 98.Dv INADDR_ANY 99must be bound. The 100.Tn TCP 101port may still be specified 102at this time; if the port is not specified the system will assign one. 103Once a connection has been established the socket's address is 104fixed by the peer entity's location. The address assigned to the 105socket is the address associated with the network interface 106through which packets are being transmitted and received. Normally 107this address corresponds to the peer entity's network. 108.Pp 109.Tn TCP 110supports several socket options which are set with 111.Xr setsockopt 2 112and tested with 113.Xr getsockopt 2 . 114.Bl -ohang 115.It Cd TCP_NODELAY 116Under most circumstances, 117.Tn TCP 118sends data when it is presented; 119when outstanding data has not yet been acknowledged, it gathers 120small amounts of output to be sent in a single packet once 121an acknowledgement is received. 122For a small number of clients, such as window systems 123that send a stream of mouse events which receive no replies, 124this packetization may cause significant delays. 125Therefore, 126.Tn TCP 127provides a boolean option, 128.Dv TCP_NODELAY 129(from 130.Aq Pa netinet/tcp.h , 131to defeat this algorithm. 132.It Cd TCP_MAXSEG 133Set the maximum segment size for this connection. The maximum segment 134size can only be lowered. 135.It Cd TCP_SACK_DISABLE 136Do not use selective acknowledgements for this connection. See 137.Xr options 4 . 138.El 139.Pp 140The option level for the 141.Xr setsockopt 2 142call is the protocol number for 143.Tn TCP , 144available from 145.Xr getprotobyname 3 . 146.Pp 147Options at the 148.Tn IP 149transport level may be used with 150.Tn TCP ; 151see 152.Xr ip 4 153or 154.Xr ip6 4 . 155Incoming connection requests that are source-routed are noted, 156and the reverse source route is used in responding. 157.Sh DIAGNOSTICS 158A socket operation may fail with one of the following errors returned: 159.Bl -tag -width [EADDRNOTAVAIL] 160.It Bq Er EISCONN 161when trying to establish a connection on a socket which 162already has one; 163.It Bq Er ENOBUFS 164when the system runs out of memory for 165an internal data structure; 166.It Bq Er ETIMEDOUT 167when a connection was dropped 168due to excessive retransmissions; 169.It Bq Er ECONNRESET 170when the remote peer 171forces the connection to be closed; 172.It Bq Er ECONNREFUSED 173when the remote 174peer actively refuses connection establishment (usually because 175no process is listening to the port); 176.It Bq Er EADDRINUSE 177when an attempt 178is made to create a socket with a port which has already been 179allocated; 180.It Bq Er EADDRNOTAVAIL 181when an attempt is made to create a 182socket with a network address for which no network interface 183exists. 184.El 185.Sh SEE ALSO 186.Xr getsockopt 2 , 187.Xr socket 2 , 188.Xr inet 4 , 189.Xr inet6 4 , 190.Xr ip 4 , 191.Xr ip6 4 , 192.Xr netintro 4 193.Sh HISTORY 194The 195.Nm 196protocol stack appeared in 197.Bx 4.2 . 198