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H_ERTT(4)	       FreeBSD Kernel Interfaces Manual		     H_ERTT(4)

     h_ertt -- Enhanced	Round Trip Time	Khelp module

     #include <netinet/khelp/h_ertt.h>

     The h_ertt	Khelp module works within the 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 acknowl-
     edgements,	TCP Segmentation Offload (TSO),	receivers who manipulate TCP
     timestamps	and lack of the	TCP timestamp option altogether.

     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 h_ertt 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

     When TSO is in use, h_ertt	will momentarily disable TSO whilst marking a
     packet to use for a new measurement.  The process has negligible impact
     on	the connection.

     h_ertt associates the following struct with each connection's TCP control

     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;

     The fields	marked as private should not be	manipulated by any code	out-
     side of the h_ertt	implementation.	 The non-private fields	provide	the
     following data:

	   bytes_tx_in_marked_rtt  The number of bytes transmitted in the

	   marked_snd_cwnd	   The value of	cwnd for the marked rtt	mea-

	   rtt			   The most recent RTT measurement.

	   maxrtt		   The longest RTT measurement that has	been

	   minrtt		   The shortest	RTT measurement	that has been

	   flags		   The ERTT_NEW_MEASUREMENT flag will be set
				   by the implementation when a	new measure-
				   ment	is available.  It is the responsibil-
				   ity of h_ertt consumers to unset the	flag
				   if they wish	to use it as a notification
				   method for new measurements.

     cc_chd(4),	cc_hd(4), cc_vegas(4), mod_cc(4), hhook(9), khelp(9)

     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.

     The h_ertt	module first appeared in FreeBSD 9.0.

     The module	was first released in 2010 by David Hayes whilst working on
     the NewTCP	research project at Swinburne University of Technology's Cen-
     tre for Advanced Internet Architectures, Melbourne, Australia.  More
     details are available at:

     The h_ertt	Khelp module and this manual page were written by David	Hayes

     The module	maintains enhanced RTT estimates for all new TCP connections
     created after the time at which the module	was loaded.  It	might be bene-
     ficial to see if it is possible to	have the module	only affect connec-
     tions which actually care about ERTT estimates.

FreeBSD	10.2		       January 18, 2012			  FreeBSD 10.2


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