287 lines
8.1 KiB
Groff
287 lines
8.1 KiB
Groff
.\" $NetBSD: tcp.4,v 1.21 2006/10/20 09:39:55 yamt Exp $
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.\" $FreeBSD: tcp.4,v 1.11.2.16 2004/02/16 22:21:47 bms Exp $
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.\"
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.\" Copyright (c) 1983, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)tcp.4 8.1 (Berkeley) 6/5/93
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.\"
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.Dd October 20, 2006
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.Dt TCP 4
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.Os
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.Sh NAME
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.Nm tcp
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.Nd Internet Transmission Control Protocol
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.Sh SYNOPSIS
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.In sys/socket.h
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.In netinet/in.h
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.Ft int
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.Fn socket AF_INET SOCK_STREAM 0
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.Ft int
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.Fn socket AF_INET6 SOCK_STREAM 0
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.Sh DESCRIPTION
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The
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.Tn TCP
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provides reliable, flow-controlled, two-way transmission of data.
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It is a byte-stream protocol used to support the
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.Dv SOCK_STREAM
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abstraction.
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.Tn TCP
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uses the standard Internet address format and, in addition, provides
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a per-host collection of
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.Dq port addresses .
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Thus, each address is composed of an Internet address specifying
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the host and network, with a specific
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.Tn TCP
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port on the host identifying the peer entity.
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.Pp
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Sockets using
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.Tn TCP
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are either
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.Dq active
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or
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.Dq passive .
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Active sockets initiate connections to passive
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sockets.
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By default
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.Tn TCP
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sockets are created active; to create a passive socket the
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.Xr listen 2
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system call must be used
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after binding the socket with the
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.Xr bind 2
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system call.
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Only passive sockets may use the
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.Xr accept 2
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call to accept incoming connections.
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Only active sockets may use the
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.Xr connect 2
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call to initiate connections.
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.Pp
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Passive sockets may
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.Dq underspecify
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their location to match incoming connection requests from multiple networks.
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This technique, termed
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.Dq wildcard addressing ,
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allows a single
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server to provide service to clients on multiple networks.
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To create a socket which listens on all networks, the Internet
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address
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.Dv INADDR_ANY
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must be bound.
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The
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.Tn TCP
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port may still be specified at this time; if the port is not
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specified the system will assign one.
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Once a connection has been established the socket's address is
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fixed by the peer entity's location.
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The address assigned the socket is the address associated with the
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network interface through which packets are being transmitted and received.
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Normally this address corresponds to the peer entity's network.
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.Pp
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.Tn TCP
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supports a number of socket options which can be set with
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.Xr setsockopt 2
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and tested with
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.Xr getsockopt 2 :
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.Bl -tag -width TCP_MD5SIGx
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.It Dv TCP_NODELAY
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Under most circumstances,
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.Tn TCP
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sends data when it is presented;
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when outstanding data has not yet been acknowledged, it gathers
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small amounts of output to be sent in a single packet once
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an acknowledgement is received.
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For a small number of clients, such as window systems
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that send a stream of mouse events which receive no replies,
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this packetization may cause significant delays.
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Therefore,
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.Tn TCP
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provides a boolean option,
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.Dv TCP_NODELAY
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(from
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.Aq Pa netinet/tcp.h ,
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to defeat this algorithm.
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.It Dv TCP_MAXSEG
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By default, a sender- and receiver-TCP
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will negotiate among themselves to determine the maximum segment size
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to be used for each connection.
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The
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.Dv TCP_MAXSEG
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option allows the user to determine the result of this negotiation,
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and to reduce it if desired.
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.It Dv TCP_MD5SIG
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This option enables the use of MD5 digests (also known as TCP-MD5)
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on writes to the specified socket.
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In the current release, only outgoing traffic is digested;
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digests on incoming traffic are not verified.
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The current default behavior for the system is to respond to a system
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advertising this option with TCP-MD5; this may change.
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.Pp
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One common use for this in a
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.Nx
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router deployment is to enable
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based routers to interwork with Cisco equipment at peering points.
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Support for this feature conforms to RFC 2385.
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Only IPv4 (AF_INET) sessions are supported.
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.Pp
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In order for this option to function correctly, it is necessary for the
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administrator to add a tcp-md5 key entry to the system's security
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associations database (SADB) using the
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.Xr setkey 8
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utility.
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This entry must have an SPI of 0x1000 and can therefore only be specified
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on a per-host basis at this time.
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.Pp
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If an SADB entry cannot be found for the destination, the outgoing traffic
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will have an invalid digest option prepended, and the following error message
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will be visible on the system console:
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.Em "tcp_signature_compute: SADB lookup failed for %d.%d.%d.%d" .
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.El
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.Pp
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The option level for the
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.Xr setsockopt 2
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call is the protocol number for
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.Tn TCP ,
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available from
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.Xr getprotobyname 3 .
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.Pp
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In the historical
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.Bx
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.Tn TCP
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implementation, if the
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.Dv TCP_NODELAY
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option was set on a passive socket, the sockets returned by
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.Xr accept 2
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erroneously did not have the
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.Dv TCP_NODELAY
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option set; the behavior was corrected to inherit
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.Dv TCP_NODELAY
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in
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.Nx 1.6 .
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.Pp
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Options at the
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.Tn IP
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network level may be used with
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.Tn TCP ;
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see
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.Xr ip 4
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or
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.Xr ip6 4 .
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Incoming connection requests that are source-routed are noted,
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and the reverse source route is used in responding.
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.Pp
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There are many adjustable parameters that control various aspects
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of the
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.Nx
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TCP behavior; these parameters are documented in
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.Xr sysctl 3 ,
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and they include:
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.Bl -bullet -compact
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.It
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RFC 1323 extensions for high performance
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.It
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Send/receive buffer sizes
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.It
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Default maximum segment size (MSS)
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.It
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SYN cache parameters
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.It
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Initial window size
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.It
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Hughes/Touch/Heidemann Congestion Window Monitoring algorithm
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.It
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Keepalive parameters
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.It
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newReno algorithm for congestion control
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.It
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Logging of connection refusals
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.It
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RST packet rate limits
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.It
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SACK (Selective Acknowledgment)
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.It
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ECN (Explicit Congestion Notification)
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.It
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Congestion window increase methods; the traditional packet counting or
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RFC 3465 Appropriate Byte Counting
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.El
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.Sh DIAGNOSTICS
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A socket operation may fail with one of the following errors returned:
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.Bl -tag -width [EADDRNOTAVAIL]
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.It Bq Er EISCONN
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when trying to establish a connection on a socket which
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already has one;
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.It Bq Er ENOBUFS
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when the system runs out of memory for
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an internal data structure;
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.It Bq Er ETIMEDOUT
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when a connection was dropped
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due to excessive retransmissions;
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.It Bq Er ECONNRESET
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when the remote peer
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forces the connection to be closed;
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.It Bq Er ECONNREFUSED
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when the remote
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peer actively refuses connection establishment (usually because
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no process is listening to the port);
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.It Bq Er EADDRINUSE
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when an attempt
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is made to create a socket with a port which has already been
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allocated;
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.It Bq Er EADDRNOTAVAIL
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when an attempt is made to create a
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socket with a network address for which no network interface
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exists.
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.El
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.Sh SEE ALSO
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.Xr getsockopt 2 ,
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.Xr socket 2 ,
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.Xr sysctl 3 ,
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.Xr inet 4 ,
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.Xr inet6 4 ,
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.Xr intro 4 ,
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.Xr ip 4 ,
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.Xr ip6 4
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.Rs
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.%R RFC
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.%N 793
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.%D September 1981
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.%T "Transmission Control Protocol"
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.Re
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.Rs
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.%R RFC
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.%N 1122
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.%D October 1989
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.%T "Requirements for Internet Hosts -- Communication Layers"
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.Re
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.Sh HISTORY
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The
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.Nm
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protocol stack appeared in
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.Bx 4.2 .
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