CC_CDG(4)

📦 version 12.2, in debian bookworm, debian trixie, ubuntu jammy, ubuntu noble, ubuntu questing, ubuntu resolute, ubuntu stonking

💖 CC_CDG 💖

okay bestie, welcome to CC_CDG(4), straight out of the Kernel Interfaces Manual ✨ this is the CDG Congestion Control Algorithm and she is so much smarter than she looks 🧠💅

🌸 NAME 🌸

cc_cdg — the CDG Congestion Control Algorithm. that's it, that's the name, iconic 👑

🦋 DESCRIPTION 🦋

so CAIA-Delay Gradient (CDG) is a hybrid congestion control algorithm — she reacts to both packet loss and inferred queuing delay. main character energy fr fr 🌈

her whole thing is: she tries to operate as a delay-based algorithm wherever possible, but she's got heuristics on deck to detect loss-based TCP cross traffic, and she will compete effectively as required 😤 CDG is therefore incrementally deployable and suitable for use on shared networks. no cap.

during delay-based operation, CDG uses a delay-gradient based probabilistic backoff mechanism, and she'll also try to infer non congestion related packet losses and avoid backing off when they occur 🫶 like, she's not gonna panic over a loss that wasn't even about congestion, be serious.

during loss-based operation, CDG essentially reverts to cc_newreno(4)-like behaviour. yeah she switches lanes, it's giving versatility 💅

CDG switches to loss-based operation when she detects that a configurable number of consecutive delay-based backoffs have had no measurable effect. then she periodically attempts to return to delay-based operation, but she'll keep switching back to loss-based operation as required. she's flexible like that 🔥

✨ 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. literally just the version, bestie.
  • 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 🌈

and the ✨academic reads✨, we love a scholarly queen:

  • 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. thank u funders, we appreciate u 🫶💖

⭐ 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>. queens, all of them 💅✨

💀 BUGS 💀

The underlying algorithm and parameter values are still a work in progress and may not be optimal for some network scenarios. 🙈 she's still growing, okay?? be nice!!

and that's the whole page bestie, no skips, no summaries, we ate and left no crumbs 🍓

signing off from the Debian edition of this manual, dated July 2, 2013 — yeah she's vintage but she still slays 💖 mwah 😘

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