h_ertt(9)	       FreeBSD Kernel Developer's Manual	     h_ertt(9)


     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(4), cc_chd(4), cc_hd(4), cc_vegas(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 9.0		       February 15, 2011		   FreeBSD 9.0

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