2 .\" Copyright (c) 2010-2011 The FreeBSD Foundation
3 .\" All rights reserved.
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.
9 .\" Redistribution and use in source and binary forms, with or without
10 .\" modification, are permitted provided that the following conditions
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.
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
37 .Nd Enhanced Round Trip Time Khelp module
39 .In netinet/khelp/h_ertt.h
43 Khelp module works within the
45 framework to provide TCP with a per-connection, low noise estimate of the
47 The implementation attempts to be robust in the face of delayed
48 acknowledgements, TCP Segmentation Offload (TSO), receivers who manipulate TCP
49 timestamps and lack of the TCP timestamp option altogether.
51 TCP receivers using delayed acknowledgements either acknowledge every second packet
52 (reflecting the time stamp of the first) or use a timeout to trigger the
53 acknowledgement if no second packet arrives.
54 If the heuristic used by
56 determines that the receiver is using delayed acknowledgements, it measures the
57 RTT using the second packet (the one that triggers the acknowledgement).
58 It does not measure the RTT if the acknowledgement is for the
59 first packet, since it cannot be accurately determined.
63 will momentarily disable TSO whilst marking a packet to use for a new
65 The process has negligible impact on the connection.
68 associates the following struct with each connection's TCP control block:
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;
78 int dlyack_rx; /* Private. */
79 int timestamp_errors; /* Private. */
80 int markedpkt_rtt; /* Private. */
85 The fields marked as private should not be manipulated by any code outside of
89 The 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
92 The number of bytes transmitted in the
94 .It Va marked_snd_cwnd
95 The value of cwnd for the marked rtt measurement.
97 The most recent RTT measurement.
99 The longest RTT measurement that has been taken.
101 The shortest RTT measurement that has been taken.
103 The ERTT_NEW_MEASUREMENT flag will be set by the implementation when a new
104 measurement is available.
105 It is the responsibility of
107 consumers to unset the flag if they wish to use it as a notification method for
118 Development and testing of this software were made possible in part by grants
119 from the FreeBSD Foundation and Cisco University Research Program Fund at
120 Community Foundation Silicon Valley.
124 module first appeared in
127 The module was first released in 2010 by David Hayes whilst working on the
128 NewTCP research project at Swinburne University of Technology's Centre for
129 Advanced Internet Architectures, Melbourne, Australia.
130 More details are available at:
132 http://caia.swin.edu.au/urp/newtcp/
137 Khelp module and this manual page were written by
138 .An David Hayes Aq Mt david.hayes@ieee.org .
140 The module maintains enhanced RTT estimates for all new TCP connections created
141 after the time at which the module was loaded.
142 It might be beneficial to see if it is possible to have the module only affect
143 connections which actually care about ERTT estimates.