H_ERTT(4) FreeBSD Kernel Interfaces Manual H_ERTT(4)
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.
FreeBSD 15.1-STABLE-HBSD January 18, 2012 H_ERTT(4)