Manual Reference Pages - CC_CDG (4)
- CDG Congestion Control Algorithm
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
CDG is therefore incrementally deployable and suitable for use on shared
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
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.
The algorithm exposes the following variables in the
branch of the
Current algorithm/implementation version number.
Delay-based window decrease factor as a percentage (on delay-based backoff, w =
w * beta_delay / 100).
Default is 70.
Loss-based window decrease factor as a percentage (on loss-based backoff, w =
w * beta_loss / 100).
Default is 50.
Scaling parameter for the probabilistic exponential backoff.
Default is 2.
Number of samples used for moving average smoothing (0 means no smoothing).
Default is 8.
Number of consecutive delay-gradient based congestion episodes which will
trigger loss-based CC compatibility.
Default is 5.
Number of consecutive delay-gradient based congestion episodes to hold the
window backoff for loss-based CC compatibility.
Default is 5.
If non-zero, this enables an experimental mode where CDGs window increase
factor (alpha) is increased by 1 MSS every
RTTs during congestion avoidance mode.
to 1 results in the most aggressive growth of the window increase factor over
values for slower growth.)
Default is 0.
Revisiting TCP Congestion Control using Delay Gradients
Minimising RTT across homogeneous 802.11 WLANs with CAIA Delay-Gradient TCP (v0.1)
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.
congestion control module first appeared in
.Fx 9.2 .
The module was first released in 2011 by David Hayes whilst working on the
NewTCP research project at Swinburne University of Technologys Centre for
Advanced Internet Architectures, Melbourne, Australia.
More details are available at:
congestion control module was written by
.An David Hayes Aq firstname.lastname@example.org .
This manual page was written by
.An Lawrence Stewart Aq lstewart@FreeBSD.org
.An Grenville Armitage Aq email@example.com .
The underlying algorithm and parameter values are still a work in progress and
may not be optimal for some network scenarios.
Visit the GSP FreeBSD Man Page Interface.
Output converted with manServer 1.07.