H_ERTT(4)						   BSD Kernel Interfaces Manual 						 H_ERTT(4)

NAME

     h_ertt -- Enhanced Round Trip Time Khelp module

SYNOPSIS

     #include <netinet/khelp/h_ertt.h>

DESCRIPTION

     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 acknowledgements, TCP Segmentation Offload (TSO), receivers who manipu-
     late 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 determined.

     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 block:

     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 outside 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 markedpkt_rtt.

	   marked_snd_cwnd	   The value of cwnd for the marked rtt measurement.

	   rtt			   The most recent RTT measurement.

	   maxrtt		   The longest RTT measurement that has been taken.

	   minrtt		   The shortest RTT measurement that has been taken.

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

SEE ALSO

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

ACKNOWLEDGEMENTS

     Development and testing of this software were made possible in part by grants from the FreeBSD Foundation and Cisco University Research Pro-
     gram Fund at Community Foundation Silicon Valley.

HISTORY

     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
     Centre for Advanced Internet Architectures, Melbourne, Australia.	More details are available at:

     http://caia.swin.edu.au/urp/newtcp/

AUTHORS

     The h_ertt Khelp module and this manual page were written by David Hayes <david.hayes@ieee.org>.

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.

BSD
								 January 18, 2012							       BSD