The sys admin's daily grind – Trickle

Blown Away

Article from Issue 160/2014
Author(s):

If your data traffic suffers from congestion at times, don't worry. Now you can shoot down programs that are heavy on traffic to free up the inflow and outflow.

I am over 40 years old and am starting to mellow in my old age. No, I'm only joking; certain phenomena still drive me up the wall. For example, when I am using SSH on a server to edit a configuration file and the bandwidth is so pathetic that the landing zone is a matter of luck when you try to position the cursor – that really makes me mad.

I know, I know, today even a line to a Black Forest village has enough bandwidth for an SSH connection, if you have exclusive access. Because hell, as Sartre already knew, is other people: In my case, it's the HTTP connections that are pushing my poor little SSH to the edge. I could turn to Mosh [1], but that helps with shaky connections rather than crowded lines. My remedy for traffic jams goes by the name of trickle [2] [3].

This traffic-shaping tool uses LD_PRELOAD to redirect some standard library calls, such as socket() and therefore only works with dynamically linked binaries. However, that practically includes all programs that the typical user deploys to eat up bandwidth. In the simplest case, I might even be one of these users myself; then, I can practice self-restraint when calling traffic-producing programs. To this end, I can start Firefox, for example, with:

trickle -u 32 -d 256 firefox

This command limits the upload speed (-u) to 100KBps and the download speed (-d) to 300KBps. Beware: These are actually kilobytes, not bits. I can also reduce speed in one direction only, if I am not worried about the other direction. Figure 1 shows the successful application of a download limit to 2Mbps.

Figure 1: Speedometer shows how a download succeeds at a speed of around 2Mbits per second.

Late Throttle

Trickle's boons naturally only occur to me when the download is already running and the SSH session hangs. Luckily, trickle has a daemon mode. Therefore, I launch trickle when I boot the machine with

trickled -u 32 -d 256

Of course, the values must be adapted to match the available bandwidth. When launched, the trickle daemon searches for /etc/trickled.conf, which can look like Listing 1. It assigns certain protocols a priority and does some tweaking.

Listing 1

trickled.conf

[ssh]
Priority = 1
Time-Smoothing = 0.1
Length-Smoothing = 1
[ftp]
Priority = 5
Time-Smoothing=3
Length-Smoothing=5
[www]
Priority = 10
Time-Smoothing = 8
Length-Smoothing = 10

The values that follow Time-Smoothing and Length-Smoothing determine how great the fluctuations can be over a certain interval. The smaller the value, the greater the benefits are for interactive protocols such as SSH. With larger values, sometimes a major outlier is permissible in both upward and downward directions. Trickle has some disadvantages compared with real traffic shaping, but it's fine for home use – maybe I really am getting soft.

Infos

  1. "Charly's Column: Mosh" by Charly Kühnast. Linux Magazine, November 2013, pg. 46: http://www.linuxpromagazine.com/Issues/2013/156/Charly-s-Column-Mosh
  2. Trickle: http://monkey.org/~marius/pages/?page=trickle
  3. "Traffic shaping with Trickle" by Oliver Frommel. Linux Magazine, January 2006, pg. 70: http://www.linux-magazine.com/Issues/2006/62/Traffic-shaping-with-Trickle

The Author

Charly Kühnast is a Unix operating system administrator at the Data Center in Moers, Germany. His tasks include firewall and DMZ security and availability. He divides his leisure time into hot, wet, and eastern sectors, where he enjoys cooking, freshwater aquariums, and learning Japanese, respectively.

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