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.