CC_CDG(4)              FreeBSD 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
     effectively 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 behaviour.

     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_dctcp(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.

FreeBSD 13.1-RELEASE-p6          May 13, 2021          FreeBSD 13.1-RELEASE-p6