CC_CDG(4)						   BSD Kernel Interfaces Manual 						 CC_CDG(4)

NAME

     cc_cdg -- CDG Congestion Control Algorithm

DESCRIPTION

     CAIA-Delay Gradient (CDG) is a hybrid congestion control algorithm which reacts to both packet loss and inferred queuing delay.  It attempts
     to operate as a delay-based algorithm where possible, but utilises heuristics to detect loss-based TCP cross traffic and will compete effec-
     tively as required.  CDG is therefore incrementally deployable and suitable for use on shared networks.

     During delay-based operation, CDG uses a delay-gradient based probabilistic backoff mechanism, and will also try to infer non congestion
     related packet losses and avoid backing off when they occur.  During loss-based operation, CDG essentially reverts to cc_newreno(4)-like be-
     haviour.

     CDG switches to loss-based operation when it detects that a configurable number of consecutive delay-based backoffs have had no measurable
     effect.  It periodically attempts to return to delay-based operation, but will keep switching back to loss-based operation as required.

MIB Variables
     The algorithm exposes the following variables in the net.inet.tcp.cc.cdg branch of the sysctl(3) MIB:

     version		Current algorithm/implementation version number.

     beta_delay 	Delay-based window decrease factor as a percentage (on delay-based backoff, w = w * beta_delay / 100).	Default is 70.

     beta_loss		Loss-based window decrease factor as a percentage (on loss-based backoff, w = w * beta_loss / 100).  Default is 50.

     exp_backoff_scale	Scaling parameter for the probabilistic exponential backoff.  Default is 2.

     smoothing_factor	Number of samples used for moving average smoothing (0 means no smoothing).  Default is 8.

     loss_compete_consec_cong
			Number of consecutive delay-gradient based congestion episodes which will trigger loss-based CC compatibility.	Default is
			5.

     loss_compete_hold_backoff
			Number of consecutive delay-gradient based congestion episodes to hold the window backoff for loss-based CC compatibility.
			Default is 5.

     alpha_inc		If non-zero, this enables an experimental mode where CDG's window increase factor (alpha) is increased by 1 MSS every
			alpha_inc RTTs during congestion avoidance mode.  (Setting alpha_inc to 1 results in the most aggressive growth of the
			window increase factor over time.  Use higher alpha_inc values for slower growth.)  Default is 0.

SEE ALSO

     cc_chd(4), cc_cubic(4), cc_hd(4), cc_htcp(4), cc_newreno(4), cc_vegas(4), h_ertt(4), mod_cc(4), tcp(4), khelp(9), mod_cc(9)

     D. A. Hayes and G. Armitage, "Revisiting TCP Congestion Control using Delay Gradients", Networking 2011 Proceedings, Part II, 328-341, May
     2011.

     N. Khademi and G. Armitage, Minimising RTT across homogeneous 802.11 WLANs with CAIA Delay-Gradient TCP (v0.1), CAIA Technical Report
     121113A, http://caia.swin.edu.au/reports/121113A/CAIA-TR-121113A.pdf, November 2012.

ACKNOWLEDGEMENTS

     Development and testing of this software were made possible in part by grants from the FreeBSD Foundation and The Cisco University Research
     Program Fund, a corporate advised fund of Silicon Valley Community Foundation.

HISTORY

     The cc_cdg congestion control module first appeared in FreeBSD 9.2.

     The module was first released in 2011 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_cdg congestion control module was written by David Hayes <david.hayes@ieee.org>.  This manual page was written by Lawrence Stewart
     <lstewart@FreeBSD.org> and Grenville Armitage <garmitage@swin.edu.au>.

BUGS

     The underlying algorithm and parameter values are still a work in progress and may not be optimal for some network scenarios.

BSD
								   July 2, 2013 							       BSD