h_ertt(4) manual page

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)

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/

Manual Pages — H_ERTT

NAME

CONTENTS

SYNOPSIS

DESCRIPTION

SEE ALSO

ACKNOWLEDGEMENTS

HISTORY

AUTHORS

BUGS

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.