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:
http://caia.swin.edu.au/urp/newtcp/
The
h_ertt Khelp module and this manual page were
written by
David Hayes
<
[email protected]>.
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.