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     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 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.


     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.