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.\" 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.
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.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.