144 lines
5.0 KiB
Groff
144 lines
5.0 KiB
Groff
.\"
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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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.\" This documentation was written at the Centre for Advanced Internet
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.\" Architectures, Swinburne University of Technology, Melbourne, Australia by
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.\" 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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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR 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 AUTHOR OR CONTRIBUTORS BE LIABLE FOR
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.\" 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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.\" $FreeBSD$
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.\"
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.Dd January 18, 2012
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.Dt H_ERTT 4
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.Os
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.Sh NAME
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.Nm h_ertt
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.Nd Enhanced Round Trip Time Khelp module
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.Sh SYNOPSIS
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.In netinet/khelp/h_ertt.h
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.Sh DESCRIPTION
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The
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.Nm
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Khelp module works within the
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.Xr khelp 9
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framework to provide TCP with a per-connection, low noise estimate of the
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instantaneous RTT.
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The implementation attempts to be robust in the face of delayed
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acknowledgements, TCP Segmentation Offload (TSO), receivers who manipulate TCP
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timestamps and lack of the TCP timestamp option altogether.
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.Pp
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TCP receivers using delayed acknowledgements either acknowledge every second packet
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(reflecting the time stamp of the first) or use a timeout to trigger the
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acknowledgement if no second packet arrives.
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If the heuristic used by
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.Nm
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determines that the receiver is using delayed acknowledgements, it measures the
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RTT using the second packet (the one that triggers the acknowledgement).
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It does not measure the RTT if the acknowledgement is for the
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first packet, since it cannot be accurately determined.
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.Pp
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When TSO is in use,
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.Nm
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will momentarily disable TSO whilst marking a packet to use for a new
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measurement.
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The process has negligible impact on the connection.
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.Pp
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.Nm
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associates the following struct with each connection's TCP control block:
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.Bd -literal
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struct ertt {
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TAILQ_HEAD(txseginfo_head, txseginfo) txsegi_q; /* Private. */
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long bytes_tx_in_rtt; /* Private. */
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long bytes_tx_in_marked_rtt;
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unsigned long marked_snd_cwnd;
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int rtt;
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int maxrtt;
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int minrtt;
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int dlyack_rx; /* Private. */
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int timestamp_errors; /* Private. */
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int markedpkt_rtt; /* Private. */
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uint32_t flags;
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};
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.Ed
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.Pp
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The fields marked as private should not be manipulated by any code outside of
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the
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.Nm
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implementation.
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The non-private fields provide the following data:
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.Bl -tag -width ".Va bytes_tx_in_marked_rtt" -offset indent
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.It Va bytes_tx_in_marked_rtt
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The number of bytes transmitted in the
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.Va markedpkt_rtt .
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.It Va marked_snd_cwnd
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The value of cwnd for the marked rtt measurement.
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.It Va rtt
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The most recent RTT measurement.
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.It Va maxrtt
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The longest RTT measurement that has been taken.
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.It Va minrtt
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The shortest RTT measurement that has been taken.
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.It Va flags
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The ERTT_NEW_MEASUREMENT flag will be set by the implementation when a new
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measurement is available.
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It is the responsibility of
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.Nm
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consumers to unset the flag if they wish to use it as a notification method for
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new measurements.
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.El
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.Sh SEE ALSO
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.Xr cc_chd 4 ,
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.Xr cc_hd 4 ,
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.Xr cc_vegas 4 ,
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.Xr mod_cc 4 ,
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.Xr hhook 9 ,
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.Xr khelp 9
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.Sh ACKNOWLEDGEMENTS
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Development and testing of this software were made possible in part by grants
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from the FreeBSD Foundation and Cisco University Research Program Fund at
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Community Foundation Silicon Valley.
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.Sh HISTORY
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The
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.Nm
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module first appeared in
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.Fx 9.0 .
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.Pp
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The module was first released in 2010 by David Hayes whilst working on the
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NewTCP research project at Swinburne University of Technology's Centre for
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Advanced Internet Architectures, Melbourne, Australia.
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More details are available at:
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.Pp
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http://caia.swin.edu.au/urp/newtcp/
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.Sh AUTHORS
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.An -nosplit
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The
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.Nm
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Khelp module and this manual page were written by
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.An David Hayes Aq david.hayes@ieee.org .
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.Sh BUGS
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The module maintains enhanced RTT estimates for all new TCP connections created
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after the time at which the module was loaded.
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It might be beneficial to see if it is possible to have the module only affect
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connections which actually care about ERTT estimates.
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