1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
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 Mt 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.
|
