3cb73e3d8b
avoidance:
- Enable setting the RXCSUM and TXCSUM flags for loopback interfaces;
set both by default.
- When RXCSUM is set, flag packets sent over the loopback interface as
having checked and valid IP, UDP, TCP checksums so that higher
protocol layers won't check them.
- Always clear CSUM_{IP,UDP_TCP} checksum required flags on transmit,
as they will have gotten there as a result of TXCSUM being set.
This is done only for packets explicitly sent over the loopback, not
simulated loopback via if_simloop() due to !SIMPLEX interfaces, etc.
Note that enabling TXCSUM but not RXCSUM will lead to unhappiness, as
checksums won't be generated but will be validated.
Kris reports that this leads to significant performance improvements
in loopback benchmarking with TCP and UDP for throughput:
RXCSUM RXCSUM+TXCSUM
TCP 15% 37%
UDP 10% 74%
Update man page.
Reviewed by: sam
Tested by: kris
MFC after: 1 week
91 lines
3.4 KiB
Groff
91 lines
3.4 KiB
Groff
.\" Copyright (c) 1983, 1991, 1993
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.\" The Regents of the University of California.
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.\" Copyright (c) 2009 Robert N. M. Watson
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.\" 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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.\" 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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.\" @(#)lo.4 8.1 (Berkeley) 6/5/93
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.\" $FreeBSD$
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.\"
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.Dd March 15, 2009
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.Dt LO 4
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.Os
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.Sh NAME
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.Nm lo
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.Nd software loopback network interface
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.Sh SYNOPSIS
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.Cd "device loop"
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.Sh DESCRIPTION
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The
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.Nm loop
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interface is a software loopback mechanism which may be
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used for performance analysis, software testing, and/or local
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communication.
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As with other network interfaces, the loopback interface must have
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network addresses assigned for each address family with which it is to be used.
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These addresses
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may be set or changed with the
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.Dv SIOCSIFADDR
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.Xr ioctl 2 .
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The loopback interface should be the last interface configured,
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as protocols may use the order of configuration as an indication of priority.
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The loopback should
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.Em never
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be configured first unless no hardware
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interfaces exist.
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.Pp
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If the transmit checksum offload capability flag is enabled on a loopback
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interface, checksums will not be generated by IP, UDP, or TCP for packets
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sent on the interface.
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.Pp
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If the receive checksum offload capability flag is enabled on a loopback
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interface, checksums will not be validated by IP, UDP, or TCP for packets
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received on the interface.
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.Pp
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By default, both receive and transmit checksum flags will be enabled, in
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order to avoid the overhead of checksumming for local communication where
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data corruption is unlikely.
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If transmit checksum generation is disabled, then validation should also be
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disabled in order to avoid packets being dropped due to invalid checksums.
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.Sh DIAGNOSTICS
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.Bl -diag
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.It lo%d: can't handle af%d.
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The interface was handed
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a message with addresses formatted in an unsuitable address
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family; the packet was dropped.
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.El
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.Sh SEE ALSO
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.Xr inet 4 ,
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.Xr intro 4
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.\" .Xr ns 4
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.Sh HISTORY
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The
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.Nm
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device appeared in
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.Bx 4.2 .
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The current checksum generation and validation avoidance policy appeared in
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.Fx 8.0 .
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