Re: [LARTC] Patch accurate packet scheduling for ATM/ADSL

[Philippe <lartc@rootsucks.com>]

Hi ! (Sorry for my bad english...)

> I use the patch
> (http://ace-host.stuart.id.au/russell/files/tc/tc-atm/) for accurate
> the packet scheduling on ATM/ADSL link and i think I've found a bug.

I use it too, this patch is great! :o)

But, with TBF, both burst and limit parameters are "affected" by the
patch (or I clearly missed something).

tc qdisc add dev eth1 root handle 1:0 tbf rate 100kbit \
    burst 1514b limit 1514b mpu 64 atm overhead -4

tc qdisc add dev eth1 parent 1:1 handle 10:0 pfifo

Before the patch (without "atm overhead -4"), using PPPoA + VC/Mux and
MSS set to 1438, TBF was working normally.

Just after the patch (with "atm overhead -4"), it stopped working :
packets were blocked until I raise up burst and limit to 1696 (32 * 53).

In this case Linux sees 1492 bytes packets on eth1 (1438 + 20 IP + 20
TCP + 14 Ethernet)

Same effect with a lower MSS : burst and limit must be set to 742b (14 *
53) when MSS = 574.

Pretty sure I do not unsterdand something... and wondering why I cannot
set set burst/limit to 628b (only for TCP streams).

To get mimimal burst/limit values, an extra atm cell is needed :

mss = 1150
eth1 packet = 1204b (1150 + 40 + 14)
modem packet = 1200b ( 1150 + 40 + 2 PPP + 8 AAL5)
Atm cells = 25 (1200 / 48)

Add one cell... and...

min burst/limit = 1378 ( 26 * 53 )

TBF rate also seems to be affected: I currently have it set to 159kbit
on a 128kbits upstream link and it works like a charm! :o)

This is not really a problem (if it works, don't touch it) but I would 
like to understand...

Thanks!

Philippe





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