1.\" 2.\" Copyright (c) 2010-2011 The FreeBSD Foundation 3.\" All rights reserved. 4.\" 5.\" This documentation was written at the Centre for Advanced Internet 6.\" Architectures, Swinburne University of Technology, Melbourne, Australia by 7.\" David Hayes under sponsorship from the FreeBSD Foundation. 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 19.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR 22.\" ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28.\" SUCH DAMAGE. 29.\" 30.\" $FreeBSD$ 31.\" 32.Dd January 18, 2012 33.Dt H_ERTT 4 34.Os 35.Sh NAME 36.Nm h_ertt 37.Nd Enhanced Round Trip Time Khelp module 38.Sh SYNOPSIS 39.In netinet/khelp/h_ertt.h 40.Sh DESCRIPTION 41The 42.Nm 43Khelp module works within the 44.Xr khelp 9 45framework to provide TCP with a per-connection, low noise estimate of the 46instantaneous RTT. 47The implementation attempts to be robust in the face of delayed 48acknowledgements, TCP Segmentation Offload (TSO), receivers who manipulate TCP 49timestamps and lack of the TCP timestamp option altogether. 50.Pp 51TCP receivers using delayed acknowledgements either acknowledge every second packet 52(reflecting the time stamp of the first) or use a timeout to trigger the 53acknowledgement if no second packet arrives. 54If the heuristic used by 55.Nm 56determines that the receiver is using delayed acknowledgements, it measures the 57RTT using the second packet (the one that triggers the acknowledgement). 58It does not measure the RTT if the acknowledgement is for the 59first packet, since it cannot be accurately determined. 60.Pp 61When TSO is in use, 62.Nm 63will momentarily disable TSO whilst marking a packet to use for a new 64measurement. 65The process has negligible impact on the connection. 66.Pp 67.Nm 68associates the following struct with each connection's TCP control block: 69.Bd -literal 70struct ertt { 71 TAILQ_HEAD(txseginfo_head, txseginfo) txsegi_q; /* Private. */ 72 long bytes_tx_in_rtt; /* Private. */ 73 long bytes_tx_in_marked_rtt; 74 unsigned long marked_snd_cwnd; 75 int rtt; 76 int maxrtt; 77 int minrtt; 78 int dlyack_rx; /* Private. */ 79 int timestamp_errors; /* Private. */ 80 int markedpkt_rtt; /* Private. */ 81 uint32_t flags; 82}; 83.Ed 84.Pp 85The fields marked as private should not be manipulated by any code outside of 86the 87.Nm 88implementation. 89The non-private fields provide the following data: 90.Bl -tag -width ".Va bytes_tx_in_marked_rtt" -offset indent 91.It Va bytes_tx_in_marked_rtt 92The number of bytes transmitted in the 93.Va markedpkt_rtt . 94.It Va marked_snd_cwnd 95The value of cwnd for the marked rtt measurement. 96.It Va rtt 97The most recent RTT measurement. 98.It Va maxrtt 99The longest RTT measurement that has been taken. 100.It Va minrtt 101The shortest RTT measurement that has been taken. 102.It Va flags 103The ERTT_NEW_MEASUREMENT flag will be set by the implementation when a new 104measurement is available. 105It is the responsibility of 106.Nm 107consumers to unset the flag if they wish to use it as a notification method for 108new measurements. 109.El 110.Sh SEE ALSO 111.Xr cc_chd 4 , 112.Xr cc_hd 4 , 113.Xr cc_vegas 4 , 114.Xr mod_cc 4 , 115.Xr hhook 9 , 116.Xr khelp 9 117.Sh ACKNOWLEDGEMENTS 118Development and testing of this software were made possible in part by grants 119from the FreeBSD Foundation and Cisco University Research Program Fund at 120Community Foundation Silicon Valley. 121.Sh HISTORY 122The 123.Nm 124module first appeared in 125.Fx 9.0 . 126.Pp 127The module was first released in 2010 by David Hayes whilst working on the 128NewTCP research project at Swinburne University of Technology's Centre for 129Advanced Internet Architectures, Melbourne, Australia. 130More details are available at: 131.Pp 132http://caia.swin.edu.au/urp/newtcp/ 133.Sh AUTHORS 134.An -nosplit 135The 136.Nm 137Khelp module and this manual page were written by 138.An David Hayes Aq david.hayes@ieee.org . 139.Sh BUGS 140The module maintains enhanced RTT estimates for all new TCP connections created 141after the time at which the module was loaded. 142It might be beneficial to see if it is possible to have the module only affect 143connections which actually care about ERTT estimates. 144