[LARTC] Using TBF to throttle a PC to 5kbps

[LARTC] Using TBF to throttle a PC to 5kbps

[Lee Sanders]

Hi Everyone,

This is a simple question but I don't understand why the below tbf is not 
working as expected by throttling traffic to 5kbps

If I throttle a PC's traffic using the below when traffic exceeds 5kbps 
packets start getting dropped (as they should) but all traffic gets dropped. 
not just the bit over 5kbps.

TC="tc add dev ppp0"
$TC parent 8001:2 handle 2: tbf rate 5kbps latency 50ms burst 1540

I can see the "dropped" counter increasing, the overlimits counter stays the 
same and there is a "backlog" counter with 21p or similar each time which 
also stays the same. To get traffic working again I have to delete the qdisc 
and add it again.

What am I doing wrong ?

This is a Kubuntu machine running 2.6.14 kernel, the tbf is a leaf qdisc for a 
WRR class. 

Thanks in Advance
Lee

p.s. if anyone can explain what the backlog is, I can't find it in the docco.
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Re: [LARTC] Using TBF to throttle a PC to 5kbps

[Andy Furniss]

Lee Sanders wrote:
> Hi Everyone,
> 
> This is a simple question but I don't understand why the below tbf is not 
> working as expected by throttling traffic to 5kbps
> 
> If I throttle a PC's traffic using the below when traffic exceeds 5kbps 
> packets start getting dropped (as they should) but all traffic gets dropped. 
> not just the bit over 5kbps.
> 
> TC="tc add dev ppp0"
> $TC parent 8001:2 handle 2: tbf rate 5kbps latency 50ms burst 1540
> 
> I can see the "dropped" counter increasing, the overlimits counter stays the 
> same and there is a "backlog" counter with 21p

That's strange - this is a count of the packets queued, but why 21 - the 
settings you use make for a very small buffer 5kbps and 50ms is < 1 bulk 
packet.

do a tc -s -d qdisc ls dev ppp0 and you may see tbf has optimised it to 
even less.

Try with 500ms latency.

Andy.

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