[LARTC] Question about how TC enforces bandwidth limiting

[Vadtec <vadtec@vadtec.net>]

Hello,

I run one of my PCs as my personal router, with iptables+tc to control 
traffic and be my firewall.

In TC, I use a combination of htb, qdisc and sfq (as well as prio) to 
classify bandwidth. In my current setup, I have 10 classifications of my 
bandwidth. (Even I admit this is probably more than I need, but at this 
point I'm still learning, so I'll just leave them be.) This leads me to 
my question about how TC enforces maximum bandwidth limits.

I am on DSL internet with rates 1.5Mbps/384kbps. I do not make complete 
use of my pipe just in case of a massive burst. I know I will probably 
not burst such a massive burst, but its better to be safe than sorry.

To help understand this, here are the classes and their associated 
bandwidths as I have them set in TC:

   (MAX_RATE is 360kbps)

   $TC qdisc add dev $IFext root handle 1: htb default 90
   $TC class add dev $IFext parent 1: classid 1:1 htb rate $MAX_RATE 
quantum $QUANTUM
   $TC class add dev $IFext parent 1:1 classid 1:10 htb rate 240kbps 
ceil $MAX_RATE prio 0 quantum $QUANTUM
   $TC class add dev $IFext parent 1:1 classid 1:20 htb rate 192kbps 
ceil $MAX_RATE prio 1 quantum $QUANTUM
   $TC class add dev $IFext parent 1:1 classid 1:30 htb rate 80kbps ceil 
$MAX_RATE prio 2 quantum $QUANTUM
   $TC class add dev $IFext parent 1:1 classid 1:40 htb rate 64kbps ceil 
$MAX_RATE prio 3 quantum $QUANTUM
   $TC class add dev $IFext parent 1:1 classid 1:50 htb rate 32kbps ceil 
$MAX_RATE prio 4 quantum $QUANTUM
   $TC class add dev $IFext parent 1:1 classid 1:60 htb rate 20kbps ceil 
$MAX_RATE prio 5 quantum $QUANTUM
   $TC class add dev $IFext parent 1:1 classid 1:70 htb rate 16kbps ceil 
$MAX_RATE prio 6 quantum $QUANTUM
   $TC class add dev $IFext parent 1:1 classid 1:80 htb rate 8kbps ceil 
$MAX_RATE prio 7 quantum $QUANTUM
   $TC class add dev $IFext parent 1:1 classid 1:90 htb rate 2kbps ceil 
$MAX_RATE prio 8 quantum $QUANTUM
   $TC class add dev $IFext parent 1:1 classid 1:100 htb rate 2kbps ceil 
20kbps prio 9 quantum $QUANTUM
   $TC qdisc add dev $IFext parent 1:10 handle 10: sfq perturb 10
   $TC qdisc add dev $IFext parent 1:20 handle 20: sfq perturb 10
   $TC qdisc add dev $IFext parent 1:30 handle 30: sfq perturb 10
   $TC qdisc add dev $IFext parent 1:40 handle 40: sfq perturb 10
   $TC qdisc add dev $IFext parent 1:50 handle 50: sfq perturb 10
   $TC qdisc add dev $IFext parent 1:60 handle 60: sfq perturb 10
   $TC qdisc add dev $IFext parent 1:70 handle 70: sfq perturb 10
   $TC qdisc add dev $IFext parent 1:80 handle 80: sfq perturb 10
   $TC qdisc add dev $IFext parent 1:90 handle 90: sfq perturb 10
   $TC qdisc add dev $IFext parent 1:100 handle 100: sfq perturb 10

Class 90 is the default. Class 100 is a special class, and what my 
question specifically relates to. Class 100 is for bit torrent. I do not 
like the other people in my house using very much bandwidth for 
torrenting as it has a tendency to slow things down to greatly. The 
problem I have is this: when I disable a given torrent clients upload 
limits, the bandwidth climbs to above the 20kbps limit I have set for 
it. When I classify the traffic in iptables, i put it into class 100, so 
it shouldn't getting put into the default class.

While I understand how/why TC enforces minimum bandwidth for a given 
class, why is it that for class 100 TC is not enforcing the cap of 
20kbps to traffic that it is classified at? Is there something else I 
need to do to make TC also enforce arbitrary maximum limits for a given 
classification?

Vadtec

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