535 lines
16 KiB
Groff
535 lines
16 KiB
Groff
.\" Copyright (c) 1983, 1991, 1993
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.\" The Regents of the University of California.
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.\" Copyright (c) 2010-2011 The FreeBSD Foundation
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.\" All rights reserved.
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.\"
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.\" Portions of this documentation were written at the Centre for Advanced
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.\" Internet Architectures, Swinburne University of Technology, Melbourne,
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.\" Australia by David Hayes under sponsorship from the FreeBSD Foundation.
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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 November 14, 2011
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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.
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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.
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.Tn TCP
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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,
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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 utilizing the
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.Tn TCP
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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.
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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.
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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
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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
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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.
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The address assigned to 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.
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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 ".Dv TCP_CONGESTION"
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.It Dv TCP_INFO
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Information about a socket's underlying TCP session may be retrieved
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by passing the read-only option
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.Dv TCP_INFO
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to
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.Xr getsockopt 2 .
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It accepts a single argument: a pointer to an instance of
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.Vt "struct tcp_info" .
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.Pp
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This API is subject to change; consult the source to determine
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which fields are currently filled out by this option.
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.Fx
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specific additions include
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send window size,
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receive window size,
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and
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bandwidth-controlled window space.
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.It Dv TCP_CONGESTION
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Select or query the congestion control algorithm that TCP will use for the
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connection.
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See
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.Xr mod_cc 4
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for details.
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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
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.No receiver- Ns Tn 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_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.
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The boolean 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
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.No sender- Ns Tn 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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When this 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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.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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.Fx
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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
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.Pq Dv AF_INET
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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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or
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.Dv IPPROTO_TCP .
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All options are declared in
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.In 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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.Pp
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The default congestion control algorithm for
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.Tn TCP
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is
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.Xr cc_newreno 4 .
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Other congestion control algorithms can be made available using the
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.Xr mod_cc 4
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framework.
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.Ss MIB Variables
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The
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.Tn TCP
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protocol implements a number of variables in the
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.Va net.inet.tcp
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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 ".Va TCPCTL_DO_RFC1323"
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.It Dv TCPCTL_DO_RFC1323
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.Pq Va rfc1323
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Implement the window scaling and timestamp options of RFC 1323
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(default is true).
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.It Dv TCPCTL_MSSDFLT
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.Pq Va 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 Va sendspace
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Maximum
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.Tn TCP
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send window.
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.It Dv TCPCTL_RECVSPACE
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.Pq Va recvspace
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Maximum
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.Tn TCP
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receive window.
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.It Va 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
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.Tn SYN
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(connection establishment) packets only.
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That of 2 results in any
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.Tn TCP
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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 Va msl
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The Maximum Segment Lifetime, in milliseconds, for a packet.
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.It Va keepinit
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Timeout, in milliseconds, for new, non-established
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.Tn TCP
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connections.
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.It Va 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 Va keepintvl
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The interval, in milliseconds, between keepalive probes sent to remote
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machines, when no response is received on a
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.Va keepidle
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probe.
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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 Va 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 Va 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 Va 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 Va 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 Va delayed_ack
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Delay ACK to try and piggyback it onto a data packet.
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.It Va delacktime
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Maximum amount of time, in milliseconds, before a delayed ACK is sent.
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.It Va path_mtu_discovery
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Enable Path MTU Discovery.
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.It Va tcbhashsize
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Size of the
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.Tn TCP
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control-block hash table
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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 Va pcbcount
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Number of active process control blocks
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(read-only).
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.It Va syncookies
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Determines whether or not
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.Tn SYN
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cookies should be generated for outbound
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.Tn SYN-ACK
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packets.
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.Tn SYN
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cookies are a great help during
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.Tn SYN
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flood attacks, and are enabled by default.
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(See
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.Xr syncookies 4 . )
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.It Va 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 Va rexmit_min , rexmit_slop
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Adjust the retransmit timer calculation for
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.Tn TCP .
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The slop is
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typically added to the raw calculation to take into account
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occasional variances that the
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.Tn SRTT
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(smoothed round-trip time)
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is unable to accommodate, while the minimum specifies an
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absolute minimum.
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While a number of
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.Tn TCP
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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.
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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
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.Tn Linux ) .
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.It Va rfc3042
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Enable the Limited Transmit algorithm as described in RFC 3042.
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It helps avoid timeouts on lossy links and also when the congestion window
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is small, as happens on short transfers.
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.It Va rfc3390
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Enable support for RFC 3390, which allows for a variable-sized
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starting congestion window on new connections, depending on the
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maximum segment size.
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This helps throughput in general, but
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particularly affects short transfers and high-bandwidth large
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propagation-delay connections.
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.It Va sack.enable
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Enable support for RFC 2018, TCP Selective Acknowledgment option,
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which allows the receiver to inform the sender about all successfully
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arrived segments, allowing the sender to retransmit the missing segments
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only.
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.It Va sack.maxholes
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Maximum number of SACK holes per connection.
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Defaults to 128.
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.It Va sack.globalmaxholes
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Maximum number of SACK holes per system, across all connections.
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Defaults to 65536.
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.It Va maxtcptw
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When a TCP connection enters the
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.Dv TIME_WAIT
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state, its associated socket structure is freed, since it is of
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negligible size and use, and a new structure is allocated to contain a
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minimal amount of information necessary for sustaining a connection in
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this state, called the compressed TCP TIME_WAIT state.
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Since this structure is smaller than a socket structure, it can save
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a significant amount of system memory.
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The
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.Va net.inet.tcp.maxtcptw
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MIB variable controls the maximum number of these structures allocated.
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By default, it is initialized to
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.Va kern.ipc.maxsockets
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/ 5.
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.It Va nolocaltimewait
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Suppress creating of compressed TCP TIME_WAIT states for connections in
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which both endpoints are local.
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.It Va fast_finwait2_recycle
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Recycle
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.Tn TCP
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.Dv FIN_WAIT_2
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connections faster when the socket is marked as
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.Dv SBS_CANTRCVMORE
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(no user process has the socket open, data received on
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the socket cannot be read).
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The timeout used here is
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.Va finwait2_timeout .
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.It Va finwait2_timeout
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Timeout to use for fast recycling of
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.Tn TCP
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.Dv FIN_WAIT_2
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connections.
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Defaults to 60 seconds.
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.It Va ecn.enable
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Enable support for TCP Explicit Congestion Notification (ECN).
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ECN allows a TCP sender to reduce the transmission rate in order to
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avoid packet drops.
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.It Va ecn.maxretries
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Number of retries (SYN or SYN/ACK retransmits) before disabling ECN on a
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specific connection. This is needed to help with connection establishment
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when a broken firewall is in the network path.
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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
|
|
.Xr getsockopt 2 ,
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.Xr socket 2 ,
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.Xr sysctl 3 ,
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|
.Xr blackhole 4 ,
|
|
.Xr inet 4 ,
|
|
.Xr intro 4 ,
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|
.Xr ip 4 ,
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.Xr mod_cc 4 ,
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.Xr syncache 4 ,
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.Xr setkey 8
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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 "A. Heffernan"
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|
.%T "Protection of BGP Sessions via the TCP MD5 Signature Option"
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.%O "RFC 2385"
|
|
.Re
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|
.Rs
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.%A "K. Ramakrishnan"
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|
.%A "S. Floyd"
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|
.%A "D. Black"
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|
.%T "The Addition of Explicit Congestion Notification (ECN) to IP"
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.%O "RFC 3168"
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.Re
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.Sh HISTORY
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|
The
|
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.Tn TCP
|
|
protocol appeared in
|
|
.Bx 4.2 .
|
|
The RFC 1323 extensions for window scaling and timestamps were added
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in
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.Bx 4.4 .
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The
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.Dv TCP_INFO
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option was introduced in
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.Tn Linux 2.6
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and is
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|
.Em subject to change .
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