CAKE with Adaptive Bandwidth - "autorate"

autorate.sh is a script that automatically adapts CAKE Smart Queue Management (SQM) bandwidth settings by measuring traffic load and RTT times. This is designed for variable bandwidth connections such as LTE, and is not intended for use on connections that have a stable, fixed bandwidth.

CAKE is an algorithm that manages the buffering of data being sent/received by an OpenWrt router so that no more data is queued than is necessary, minimizing the latency ("bufferbloat") and improving the responsiveness of a network.

The CAKE algorithm always uses fixed upload and download bandwidth settings to manage its queues. Variable bandwidth connections present a challenge because the actual bandwidth at any given moment is not known.

People generally pick a compromise bandwidth setting, but typically this means lost bandwidth in exchange for latency control. This compromise is hardly ideal: if the compromise setting is too low, the connection is unnecessarily throttled back to the compromise setting (yellow); if the setting is too high, CAKE will still buffer too much data (green) and induce unwanted latency.

The autorate.sh script periodically measures the load and Round-Trip-Time (RTT) to adjust the upload and download values for the CAKE algorithm.

Theory of Operation

The autorate.sh script runs regularly and adjusts the bandwidth settings of the CAKE SQM algorithm to reflect the current conditions on the bottleneck link. (The script adjusts the upload and download settings independently each time it runs.) The script is typically configured to run once per second and make the following adjustments:

• When traffic is low, the bandwidth setting decays toward a minimum configured value
• When traffic is high, the bandwidth setting is incrementally increased until an RTT spike is detected or until the setting reaches the maximum configured value
• Upon detecting an RTT spike, the bandwidth setting is decreased

Setting the minimum value: Set the minimum value at, or slightly below, the lowest speed observed from the ISP during your testing. This setting will, in general, never result in bufferbloat even under the worst conditions. Under no load, the routine will adjust the bandwidth downwards towards that minimum.

Setting the maximum value: The maximum bandwidth should be set to the lower of the maximum bandwidth that the ISP can provide or the maximum bandwidth required by the user. The script will adjust the bandwidth up when there is traffic, as long no RTT spike is detected. Setting this value to a maximum required level will have the advantage that the script will stay at that level during optimum conditions rather than always having to test whether the bandwidth can be increased (which necessarily results in allowing some excess latency).

To elaborate on the above, a variable bandwidth connection may be most ideally divided up into a known fixed, stable component, on top of which is provided an unknown variable component:

The minimum bandwidth is then set to (or slightly below) the fixed component, and the maximum bandwidth may be set to (or slightly above) the maximum observed bandwidth. Or, if a lower maximum bandwidth is required by the user, the maximum bandwidth is set to that lower bandwidth as explained above.

There is a detailed and fun discussion with plenty of sketches relating to the development of the script and alternatives on the OpenWrt Forum - CAKE /w Adaptive Bandwidth.

Required packages

• iputils-ping for more advanced ping with sub 1s ping frequency
• coreutils-date for accurate time keeping
• coreutils-sleep for accurate sleeping

Installation on OpenWrt

• Install SQM (luci-app-sqm) and enable CAKE on the interface(s) as described in the OpenWrt SQM documentation

• SSH into the router

• Run the following commands to place the script at /root/autorate.sh and make it executable:

  opkg update; opkg install iputils-ping coreutils-date coreutils-sleep
  cd /root
  wget https://raw.githubusercontent.com/lynxthecat/sqm-autorate/main/sqm-autorate.sh
  chmod +x ./sqm-autorate.sh

• Edit the sqm-autorate.sh script using vi or nano to supply information about your router and your ISP's speeds. The minimum bandwidth should be set to at, or below, the lowest observed bandwidth, and the maximum bandwidth set to an estimate of the best possible bandwidth the connection can obtain. Rates are in kilobits/sec - enter "36000" for a 36 mbps link.

  • Change ul_if and dl_if to match the names of the upload and download interfaces to which CAKE is applied These can be obtained, for example, by consulting the configured SQM settings in LuCi or by examining the output of tc qdisc ls.
  • Set minimum bandwidth variables (min_ul_rate and min_dl_rate in the script) to the minimum bandwidth you expect.
  • Set maximum bandwidth (max_ul_rate and max_dl_rate) to the maximum bandwidth you expect your connection could obtain from your ISP.
  • Save the changes and exit the editor

Manual testing

• Run the modified autorate.sh script:

  ./sqm-autorate.sh

• Monitor the script output to see how it adjusts the download and upload rates as you use the connection. (You will see this output if enable_verbose_output is set to '1'. Set it to '0' if you no longer want the verbose logging.)

• Press ^C to halt the process.

Install as a service

You can install this as a service that starts up the autorate process whenever the router reboots. To do this:

• SSH into the router

• Run these commands to install the service file and start/enable it:

  cd /etc/init.d
  wget https://raw.githubusercontent.com/lynxthecat/sqm-autorate/main/sqm-autorate 
  service sqm-autorate start
  service sqm-autorate enable

When running as a service, the autorate.sh script outputs to /tmp/sqm-autorate.log when enable_verbose_output is set to '1'.

Disabling logging when not required is a good idea given the rate of logging.

A Request to Testers

If you use this script I have just one ask. Please post your experience on this OpenWrt Forum thread. Your feedback will help improve the script for the benefit of others.