Re: [Cake] openwrt build with latest cake and other qdiscs

[Sebastian Moeller <moeller0@gmx.de>]

Hi Alan,

On May 10, 2015, at 19:29 , Alan Jenkins <alan.christopher.jenkins@gmail.com> wrote:

> Hi again
> 
> I tested Dave's build of cake on wndr3800.  (Didn't try specifying cake2/cake3; I'm guessing tc cake means cake3 on there).  "atm" adjustment worked fine on my adsl.  I can't distinguish it from sqm-scripts with the same configuration.
> 
> SQM seems to win from having "overhead 40", which I can't configure in cake.

	As expected, specifying the overhead is quite important for ATM links, since it shifts the cell padding around (worst case without overhead accounting is to drag in an 47of48 byte payload cell that the accounting did not expect, causing almost 50% unaccounted overhead for small packets, no shaper is going to be happy with such precision ;)

> 
> [table snipped as it did not survive my mailer...]
> IMO it wouldn't hurt for 'tc atm', 'tc adsl', and everything else to default to 'overhead 44'.  (The highest overhead Seb reported; the original worst case overhead[1] plus a vlan header for IPTV).

	Actually there old worst case was 44, with added VLAN tagging this would end up at 48 bytes, but that would also include the ethernet frame check sequence, which I have never seen used over ATM in the real world (which sounds quite weird, without the FCS the payload is not checked for bit errors anymore, so we relay on a) the atm checksums and b) that neither the ethernet to ATM nor the ATM to ethernet repackaging introduces bit errors)… (but I do not claim that I have seen that much). Also I wonder what about double VLAN tagging as is sometimes used bit bitstream access, no idea where the 2nd VLAN gets terminated, one would hope way before the DSL link, but certainty would be nice. I would humbly suggest that we should just get a numerical option for cake to specify the overhead...

Best Regards
	Sebastian

> 
> Alan
> 
> [1] http://ace-host.stuart.id.au/russell/files/tc/tc-atm/#usage
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