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 15.1-STABLE-HBSD May 13, 2021 CC_CDG(4)