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_newreno4Ns-Nslike 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.

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 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 Technology's Centre for Advanced Internet Architectures, Melbourne, Australia. More details are available at:

http://caia.swin.edu.au/urp/newtcp/

AUTHORS

An -nosplit The congestion control module was written by An David Hayes Aq [email protected] . This manual page was written by An Lawrence Stewart Aq [email protected] and An Grenville Armitage Aq [email protected] .