Category: tc

As I’ve discussed before, the broadband service we receive from Virgin is subject to caps on the traffic on the line during certain times of day (see this post I wrote). Virgin have a relatively complex system for throttling your speed based on the amount you download and upload within certain times. I have a script which performs a backup of my data from my server at home to my work computer using rsync over SSH. This means that I sometimes have a large amount of data to upload during the day. Since Virgin’s upload restrictions apply between 1500 and 2000 during which time you are permitted to upload a maximum of 1.5GB. What this means is that for five hours, the average transfer rate must not exceed 300MB per hour, which equates to a transfer speed of 83.3 kB/s. I searched for a way to limit my server’s upload speed and came across tc (which is a kernel extension which must be enabled in your .config). Slackware’s kernel config includes the necessary parts as modules, so when you use the tc command, they are automatically loaded.

I found a good example which described what I wanted to achieve (limit my uploads to 83.3 kB/s) here. Using the examples on that page, I wrote a little script to allow me to start and stop the limited uploads easily:

#!/bin/bash

# Script to throttle uploads during the times when Virgin
# won’t allow unlimited uploads.
#
# Those times are between 1500 and 2000 no more than
# 1.5GB must be uploaded, so the upload speed needs to be
# capped at 83.3kB/s.

maxRate=83.3kbps
burstRate=100000kbps
interface=eth0

clearRule(){
        # First things first, clear the old rule
        tc qdisc del dev eth0 root
}

makeRule(){
        # Now add the throttling rule
        tc qdisc add dev $interface root handle 1:0 htb default 10
        tc class add dev $interface parent 1:0 classid 1:10 htb rate $maxRate ceil $burstRate prio 0
}

listRules(){
        tc -s qdisc ls dev $interface
}

case “$1” in
        ‘start’)
                clearRule
                makeRule
        ;;
        ‘stop’)
                clearRule
        ;;
        ‘status’)
                listRules
        ;;
        *)
                echo “Usage: $0 {start|stop|status}”
        ;;
esac

You’ll note I’ve set a $burstRate value of 100MB/s. This is probably not necessary, but with the same $burstRate value as used in $maxRate, I was seeing significant slowdown in the responsiveness of the remote session; I hope this high burst rate will alleviate that slowdown.

I saved this script somewhere where root only could get it and then added the following cronjobs to root’s crontab:

# Add networking throttling between 1500 and 2000
00 15 * * * /root/throttle_uploads.sh start
00 20 * * * /root/throttle_uploads.sh stop

So far, the process appears to be working, insofar as my daily uploads from home to work have slowed to approximately 80kB/s during the times when Virgin monitor uploads.