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diff --git a/share/man/man4/h_ertt.4 b/share/man/man4/h_ertt.4 new file mode 100644 index 0000000..3dbedab --- /dev/null +++ b/share/man/man4/h_ertt.4 @@ -0,0 +1,143 @@ +.\" +.\" Copyright (c) 2010-2011 The FreeBSD Foundation +.\" All rights reserved. +.\" +.\" This documentation was written at the Centre for Advanced Internet +.\" Architectures, Swinburne University of Technology, Melbourne, Australia by +.\" David Hayes under sponsorship from the FreeBSD Foundation. +.\" +.\" Redistribution and use in source and binary forms, with or without +.\" modification, are permitted provided that the following conditions +.\" are met: +.\" 1. Redistributions of source code must retain the above copyright +.\" notice, this list of conditions and the following disclaimer. +.\" 2. Redistributions in binary form must reproduce the above copyright +.\" notice, this list of conditions and the following disclaimer in the +.\" documentation and/or other materials provided with the distribution. +.\" +.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND +.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE +.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE +.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR +.\" ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL +.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS +.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) +.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT +.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY +.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF +.\" SUCH DAMAGE. +.\" +.\" $FreeBSD$ +.\" +.Dd January 18, 2012 +.Dt H_ERTT 4 +.Os +.Sh NAME +.Nm h_ertt +.Nd Enhanced Round Trip Time Khelp module +.Sh SYNOPSIS +.In netinet/khelp/h_ertt.h +.Sh DESCRIPTION +The +.Nm +Khelp module works within the +.Xr khelp 9 +framework to provide TCP with a per-connection, low noise estimate of the +instantaneous RTT. +The implementation attempts to be robust in the face of delayed +acknowledgements, TCP Segmentation Offload (TSO), receivers who manipulate TCP +timestamps and lack of the TCP timestamp option altogether. +.Pp +TCP receivers using delayed acknowledgements either acknowledge every second packet +(reflecting the time stamp of the first) or use a timeout to trigger the +acknowledgement if no second packet arrives. +If the heuristic used by +.Nm +determines that the receiver is using delayed acknowledgements, it measures the +RTT using the second packet (the one that triggers the acknowledgement). +It does not measure the RTT if the acknowledgement is for the +first packet, since it cannot be accurately determined. +.Pp +When TSO is in use, +.Nm +will momentarily disable TSO whilst marking a packet to use for a new +measurement. +The process has negligible impact on the connection. +.Pp +.Nm +associates the following struct with each connection's TCP control block: +.Bd -literal +struct ertt { + TAILQ_HEAD(txseginfo_head, txseginfo) txsegi_q; /* Private. */ + long bytes_tx_in_rtt; /* Private. */ + long bytes_tx_in_marked_rtt; + unsigned long marked_snd_cwnd; + int rtt; + int maxrtt; + int minrtt; + int dlyack_rx; /* Private. */ + int timestamp_errors; /* Private. */ + int markedpkt_rtt; /* Private. */ + uint32_t flags; +}; +.Ed +.Pp +The fields marked as private should not be manipulated by any code outside of +the +.Nm +implementation. +The non-private fields provide the following data: +.Bl -tag -width ".Va bytes_tx_in_marked_rtt" -offset indent +.It Va bytes_tx_in_marked_rtt +The number of bytes transmitted in the +.Va markedpkt_rtt . +.It Va marked_snd_cwnd +The value of cwnd for the marked rtt measurement. +.It Va rtt +The most recent RTT measurement. +.It Va maxrtt +The longest RTT measurement that has been taken. +.It Va minrtt +The shortest RTT measurement that has been taken. +.It Va flags +The ERTT_NEW_MEASUREMENT flag will be set by the implementation when a new +measurement is available. +It is the responsibility of +.Nm +consumers to unset the flag if they wish to use it as a notification method for +new measurements. +.El +.Sh SEE ALSO +.Xr cc_chd 4 , +.Xr cc_hd 4 , +.Xr cc_vegas 4 , +.Xr mod_cc 4 , +.Xr hhook 9 , +.Xr khelp 9 +.Sh ACKNOWLEDGEMENTS +Development and testing of this software were made possible in part by grants +from the FreeBSD Foundation and Cisco University Research Program Fund at +Community Foundation Silicon Valley. +.Sh HISTORY +The +.Nm +module first appeared in +.Fx 9.0 . +.Pp +The module was first released in 2010 by David Hayes whilst working on the +NewTCP research project at Swinburne University of Technology's Centre for +Advanced Internet Architectures, Melbourne, Australia. +More details are available at: +.Pp +http://caia.swin.edu.au/urp/newtcp/ +.Sh AUTHORS +.An -nosplit +The +.Nm +Khelp module and this manual page were written by +.An David Hayes Aq david.hayes@ieee.org . +.Sh BUGS +The module maintains enhanced RTT estimates for all new TCP connections created +after the time at which the module was loaded. +It might be beneficial to see if it is possible to have the module only affect +connections which actually care about ERTT estimates. |
