428 lines
14 KiB
Groff
428 lines
14 KiB
Groff
.\" 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. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. 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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.\" From: @(#)tcp.4 8.1 (Berkeley) 6/5/93
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.\" $FreeBSD$
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.\"
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.Dd February 14, 1995
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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/types.h
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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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.Sh DESCRIPTION
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The
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.Tn TCP
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protocol provides reliable, flow-controlled, two-way
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transmission of data. It is a byte-stream protocol used to
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support the
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.Dv SOCK_STREAM
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abstraction. TCP uses the standard
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Internet address format and, in addition, provides a per-host
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collection of
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.Dq port addresses .
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Thus, each address is composed
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of an Internet address specifying the host and network, with
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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 utilizing the tcp protocol 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. By default
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.Tn TCP
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sockets are created active; to create a
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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. Only
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passive sockets may use the
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.Xr accept 2
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call to accept incoming connections. Only active sockets may
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use the
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.Xr connect 2
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call to initiate connections.
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.Tn TCP
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also supports a more datagram-like mode, called Transaction
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.Tn TCP ,
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which is described in
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.Xr ttcp 4 .
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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
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incoming connection requests from multiple networks. This
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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. The
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.Tn TCP
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port may still be specified
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at this time; if the port is not 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. The address assigned the
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socket is the address associated with the network interface
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through which packets are being transmitted and received. Normally
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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_NODELAYx
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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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The boolean option
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.Dv TCP_NODELAY
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defeats 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. 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_NOOPT
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.Tn TCP
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usually sends a number of options in each packet, corresponding to
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various
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.Tn TCP
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extensions which are provided in this implementation. The boolean
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option
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.Dv TCP_NOOPT
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is provided to disable
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.Tn TCP
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option use on a per-connection basis.
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.It Dv TCP_NOPUSH
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By convention, the sender-TCP
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will set the
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.Dq push
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bit and begin transmission immediately (if permitted) at the end of
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every user call to
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.Xr write 2
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or
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.Xr writev 2 .
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The
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.Dv TCP_NOPUSH
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option is provided to allow servers to easily make use of Transaction
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TCP (see
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.Xr ttcp 4 ) .
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When the option is set to a non-zero value,
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.Tn TCP
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will delay sending any data at all until either the socket is closed,
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or the internal send buffer is filled.
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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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or
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.Dv IPPROTO_TCP .
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All options are declared in
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.Aq Pa netinet/tcp.h .
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.Pp
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Options at the
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.Tn IP
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transport 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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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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.Sh MIB VARIABLES
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The
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.Nm
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protocol implements a number of variables in the
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.Li net.inet
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branch of the
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.Xr sysctl 3
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MIB.
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.Bl -tag -width TCPCTL_DO_RFC1644
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.It Dv TCPCTL_DO_RFC1323
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.Pq tcp.rfc1323
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Implement the window scaling and timestamp options of RFC 1323
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(default true).
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.It Dv TCPCTL_DO_RFC1644
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.Pq tcp.rfc1644
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Implement Transaction
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.Tn TCP ,
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as described in RFC 1644.
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.It Dv TCPCTL_MSSDFLT
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.Pq tcp.mssdflt
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The default value used for the maximum segment size
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.Pq Dq MSS
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when no advice to the contrary is received from MSS negotiation.
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.It Dv TCPCTL_SENDSPACE
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.Pq tcp.sendspace
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Maximum TCP send window.
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.It Dv TCPCTL_RECVSPACE
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.Pq tcp.recvspace
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Maximum TCP receive window.
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.It tcp.log_in_vain
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Log any connection attempts to ports where there is not a socket
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accepting connections.
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The value of 1 limits the logging to SYN (connection establishment)
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packets only.
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That of 2 results in any TCP packets to closed ports being logged.
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Any value unlisted above disables the logging
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(default is 0, i.e., the logging is disabled).
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.It tcp.slowstart_flightsize
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The number of packets allowed to be in-flight during the
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.Tn TCP
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slow-start phase on a non-local network.
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.It tcp.local_slowstart_flightsize
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The number of packets allowed to be in-flight during the
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.Tn TCP
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slow-start phase to local machines in the same subnet.
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.It tcp.msl
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The Maximum Segment Lifetime, in milliseconds, for a packet.
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.It tcp.keepinit
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Timeout, in milliseconds, for new, non-established TCP connections.
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.It tcp.keepidle
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Amount of time, in milliseconds, that the connection must be idle
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before keepalive probes (if enabled) are sent.
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.It tcp.keepintvl
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The interval, in milliseconds, between keepalive probes sent to remote
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machines.
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After
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.Dv TCPTV_KEEPCNT
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(default 8) probes are sent, with no response, the connection is dropped.
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.It tcp.always_keepalive
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Assume that
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.Dv SO_KEEPALIVE
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is set on all
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.Tn TCP
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connections, the kernel will
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periodically send a packet to the remote host to verify the connection
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is still up.
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.It tcp.icmp_may_rst
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Certain
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.Tn ICMP
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unreachable messages may abort connections in
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.Tn SYN-SENT
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state.
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.It tcp.do_tcpdrain
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Flush packets in the
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.Tn TCP
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reassembly queue if the system is low on mbufs.
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.It tcp.blackhole
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If enabled, disable sending of RST when a connection is attempted
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to a port where there is not a socket accepting connections.
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See
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.Xr blackhole 4 .
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.It tcp.delayed_ack
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Delay ACK to try and piggyback it onto a data packet.
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.It tcp.delacktime
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Maximum amount of time, in milliseconds, before a delayed ACK is sent.
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.It tcp.newreno
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Enable TCP NewReno Fast Recovery algorithm,
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as described in RFC 2582.
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.It tcp.path_mtu_discovery
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Enable Path MTU Discovery
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.It tcp.tcbhashsize
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Size of the
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.Tn TCP
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control-block hashtable
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(read-only).
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This may be tuned using the kernel option
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.Dv TCBHASHSIZE
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or by setting
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.Va net.inet.tcp.tcbhashsize
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in the
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.Xr loader 8 .
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.It tcp.pcbcount
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Number of active process control blocks
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(read-only).
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.It tcp.syncookies
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Determines whether or not syn cookies should be generated for
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outbound syn-ack packets. Syn cookies are a great help during
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syn flood attacks, and are enabled by default.
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.It tcp.isn_reseed_interval
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The interval (in seconds) specifying how often the secret data used in
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RFC 1948 initial sequence number calculations should be reseeded.
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By default, this variable is set to zero, indicating that
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no reseeding will occur.
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Reseeding should not be necessary, and will break
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.Dv TIME_WAIT
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recycling for a few minutes.
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.It tcp.inet.tcp.rexmit_{min,slop}
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Adjust the retransmit timer calculation for TCP. The slop is
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typically added to the raw calculation to take into account
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occassional variances that the SRTT (smoothed round trip time)
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is unable to accomodate, while the minimum specifies an
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absolute minimum. While a number of TCP RFCs suggest a 1
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second minimum these RFCs tend to focus on streaming behavior
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and fail to deal with the fact that a 1 second minimum has severe
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detrimental effects over lossy interactive connections, such
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as a 802.11b wireless link, and over very fast but lossy
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connections for those cases not covered by the fast retransmit
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code. For this reason we use 200ms of slop and a near-0
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minimum, which gives us an effective minimum of 200ms (similar to Linux).
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.It tcp.inflight_enable
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Enable
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.Tn TCP
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bandwidth delay product limiting. An attempt will be made to calculate
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the bandwidth delay product for each individual TCP connection and limit
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the amount of inflight data being transmitted to avoid building up
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unnecessary packets in the network. This option is recommended if you
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are serving a lot of data over connections with high bandwidth-delay
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products, such as modems, GigE links, and fast long-haul WANs, and/or
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you have configured your machine to accomodate large TCP windows. In such
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situations, without this option, you may experience high interactive
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latencies or packet loss due to the overloading of intermediate routers
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and switches. Note that bandwidth delay product limiting only effects
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the transmit side of a TCP connection.
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.It tcp.inflight_debug
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Enable debugging for the bandwidth delay product algorithm. This may
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default to on (1) so if you enable the algorithm you should probably also
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disable debugging by setting this variable to 0.
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.It tcp.inflight_min
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This puts an lower bound on the bandwidth delay product window, in bytes.
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A value of 1024 is typically used for debugging. 6000-16000 is more typical
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in a production installation. Setting this value too low may result in
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slow ramp-up times for bursty connections. Setting this value too high
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effectively disables the algorithm.
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.It tcp.inflight_max
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This puts an upper bound on the bandwidth delay product window, in bytes.
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This value should not generally be modified but may be used to set a
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global per-connection limit on queued data, potentially allowing you to
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intentionally set a less than optimum limit to smooth data flow over a
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network while still being able to specify huge internal TCP buffers.
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.It tcp.inflight_stab
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The bandwidth delay product algorithm requires a slightly larger window
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than it otherwise calculates for stability. This parameter determines the
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extra window in maximal packets / 10. The default value of 20 represents
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2 maximal packets. Reducing this value is not recommended but you may
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come across a situation with very slow links where the ping time
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reduction of the default inflight code is not sufficient. If this case
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occurs, you should first try reducing
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.Va tcp.inflight_min
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and, if that does not
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work, reduce both
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.Va tcp.inflight_min
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and
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.Va tcp.inflight_stab ,
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trying values of
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15, 10, or 5 for the latter. Never use a value less than 5. Reducing
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.Va tcp.inflight_stab
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can lead to upwards of a 20% underutilization of the link
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as well as reducing the algorithm's ability to adapt to changing
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situations and should only be done as a last resort.
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.El
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.Sh ERRORS
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A socket operation may fail with one of the following errors returned:
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.Bl -tag -width Er
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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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.It Bq Er EAFNOSUPPORT
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when an attempt is made to bind or connect a socket to a multicast
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address.
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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 blackhole 4 ,
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.Xr inet 4 ,
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.Xr intro 4 ,
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.Xr ip 4 ,
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.Xr syncache 4 ,
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.Xr ttcp 4
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.Rs
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.%A V. Jacobson
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.%A R. Braden
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.%A D. Borman
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.%T "TCP Extensions for High Performance"
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.%O RFC 1323
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.Re
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.Rs
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.%A R. Braden
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.%T "T/TCP \- TCP Extensions for Transactions"
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.%O RFC 1644
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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 appeared in
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.Bx 4.2 .
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The RFC 1323 extensions for window scaling and timestamps were added
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in
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.Bx 4.4 .
|