Re: [LARTC] HTB Weird Shaping Question(Bug?). Please Help!

[Andy Furniss <andy.furniss@dsl.pipex.com>]

Leo wrote:

> In the first test, I limit the SUBCLASS_OUTRATE to 200Kbit. Both pings
> are around 20ms before I start the VoIP services. However, once I start
> the services, the pings jump up to 1800ms.
> 
> In the second test, I limit the SUBCLASS_OUTRATE to 180Kbit. The pings
> jump up to 80ms, which is perfectly acceptable.
> 
> After a few tests, I noticed that 180Kbit is a magic number, anything
> exceed that will generate 1800ms pings, and below it is 80ms.
> 
> In my senario, the weird point is that the determining factor is the
> ceiling, but not the rate. That's the "rate" for other class doesn't
> seem to give bandwidth to packets in the corresponding class unless the
> ceil for the 1:110 is low enough!
> 
> I attached my script and "tc -s class show" below. I truncated part of
> the script and the results to make it short.
> 
> Please shine me a light!

It's because the link is dsl and there are lots of overheads on each 
packet (and they vary with packet size). HTB rates are based on ip 
packet length and with lots of small packets like voip the difference 
can be alot.

The 1800ms latency is not caused by a queue within htb it's in your 
modem/router because it can't send >180kbit ip level for voip.

You can patch HTB and TC to make things perfect - you could set a ceil 
very close to your sync rate then. You need to know exactly what type of 
dsl you are on to find your overhead though. If your modem/router gives 
ATM cell counts you can deduce it from those.

There is a very good thesis and patch info here -

http://www.adsl-optimizer.dk/

Andy.



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