RE: [LARTC] newbie - shaping a PSTN connection

[Dancer Vesperman <dancer@anthill.echidna.id.au>]

No worries. Why does it work?
Every packet leaving an interface is up to MTU bytes in length. Let's
say that a byte is ten bits (8+ 1start+1stop). So a 1500byte packet is
15000 bits. On a 28.8K connection, that means a 1500 byte packet takes
0.52 seconds to transmit. Assuming that the queueing discipline is FIFO
(first-in-first out) and the queue length on the interface is, say, 10
packets, a packet can take 10*0.52 seconds to get to the head of the
queue. Plus transmission time, that's 5.2 (for queuing) +0.52 for
transmission. Quite a delay.

You may be able to do better by shortening the packet queue on the
interface (ip link set ppp0 qlen 4) for example. Or attaching an SFQ
queueing discipline (tc qdisc add dev ppp0 root handle 1: sfq perturb
10) - although for SFQ, I think you'd want a more substantive queue
length.


On Tue, 2003-09-09 at 18:52, Daniel Horth wrote:
> wow - from 8 second ping responses on my heavily loaded link to 2 second
> just by changing the MTU - sure I'll be able to improve further on that with
> experimentation.
> 
> thanks for that!
> 
> - dan.
> 
> 
> -----Original Message-----
> From: Dancer Vesperman [mailto:dancer@anthill.echidna.id.au]
> Sent: Tuesday, 9 September 2003 6:37 PM
> To: Daniel Horth
> Subject: Re: [LARTC] newbie - shaping a PSTN connection
> 
> 
> I'd seriously advise cutting your MTU/MRU down. A lot. Say as low as 562
> or 168. If your MTU/MRU is defaulting to 1500, it's going to be hard to
> get any effective shaping on your link.
> 
> On Tue, 2003-09-09 at 18:21, Daniel Horth wrote:
> > Hi - I've been going through the LARTC how-to and have successfully used
> the
> > sample scripts (and wondershaper) with my xDSL links.
> 
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