cc_hd(4) [freebsd man page]
CC_HD(4) BSD Kernel Interfaces Manual CC_HD(4) NAME
cc_hd -- HD Congestion Control Algorithm DESCRIPTION
The HD congestion control algorithm is an implementation of the Hamilton Institute's delay-based congestion control which aims to keep net- work queuing delays below a particular threshold (queue_threshold). HD probabilistically reduces the congestion window (cwnd) based on its estimate of the network queuing delay. The probability of reducing cwnd is zero at hd_qmin or less, rising to a maximum at queue_threshold, and then back to zero at the maximum queuing delay. Loss-based congestion control algorithms such as NewReno probe for network capacity by filling queues until there is a packet loss. HD com- petes with loss-based congestion control algorithms by allowing its probability of reducing cwnd to drop from a maximum at queue_threshold to be zero at the maximum queuing delay. This has been shown to work well when the bottleneck link is highly multiplexed. MIB Variables The algorithm exposes the following tunable variables in the net.inet.tcp.cc.hd branch of the sysctl(3) MIB: queue_threshold Queueing congestion threshold (qth) in ticks. Default is 20. pmax Per packet maximum backoff probability as a percentage. Default is 5. qmin Minimum queuing delay threshold (qmin) in ticks. Default is 5. SEE ALSO
cc_chd(4), cc_cubic(4), cc_htcp(4), cc_newreno(4), cc_vegas(4), h_ertt(4), mod_cc(4), tcp(4), khelp(9), mod_cc(9) L. Budzisz, R. Stanojevic, R. Shorten, and F. Baker, "A strategy for fair coexistence of loss and delay-based congestion control algorithms", IEEE Commun. Lett., 7, 13, 555-557, Jul 2009. ACKNOWLEDGEMENTS
Development and testing of this software were made possible in part by grants from the FreeBSD Foundation and Cisco University Research Pro- gram Fund at Community Foundation Silicon Valley. FUTURE WORK
The Hamilton Institute have recently made some improvements to the algorithm implemented by this module and have called it Coexistent-TCP (C- TCP). The improvements should be evaluated and potentially incorporated into this module. HISTORY
The cc_hd congestion control 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 cc_hd congestion control module and this manual page were written by David Hayes <david.hayes@ieee.org>. BSD
September 15, 2011 BSD
Check Out this Related Man Page
H_ERTT(4) BSD 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 manipu- late 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 Pro- gram 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. BSD
January 18, 2012 BSD