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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
     acknowledgements, TCP Segmentation Offload (TSO), receivers who
     manipulate TCP timestamps and lack of the TCP timestamp option

     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
     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

           marked_snd_cwnd             The value of cwnd for the marked rtt

           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

     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
     Centre 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
     beneficial to see if it is possible to have the module only affect
     connections which actually care about ERTT estimates.

FreeBSD 11.0-PRERELEASE        January 18, 2012        FreeBSD 11.0-PRERELEASE


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