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 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 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 Program 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.

Debian                                          January 18, 2012                                       H_ERTT(4)