[Cake] Why are target & interval increased on the reduced bandwidth tins?

[Sebastian Moeller]

Hi Kevin,

so the way codel is designed target is best understood as a function of interval (allowing 5-10% of interval as standing queue allows a fine trade-off between bandwidth utilization and latency under load increase).
Now, interval is basically akin to the time you are willing to give a flow to react to signals, it should be in the same order of magnitude as the path RTT. Now reducing the bandwidth allocation for a traffic class will increase its saturation load RTT and hence increasing the target seems justified; target just follows along due to still wanting a reasonable bandwidth/latency trade-off.
So in short these scale the shaper to work well under loaded conditions. But Jonathan & Toke will be able to give the real explanation ;)

Best Regards
	Sebastian



> On Jun 24, 2020, at 16:33, Kevin Darbyshire-Bryant <kevin at darbyshire-bryant.me.uk> wrote:
> 
> Genuine question.  For the reduced bandwidth tins in diffserv3/4/8 a different rate and hence different target & interval values are also calculated.  I get why a target/interval calculation is desirable for the ‘main’ tin - this forms a ‘best case’ of how long each byte takes to transmit and is fundamental to the shaper.  What I’m less clear on is why increased targets & intervals are used for the reduced threshold tins.
> 
> To my mind it means those tins can be more ‘bursty’ before codel jumps on them.  That’s possibly ok on an egress path but I’m not so convinced on an ingress path.
> 
> Please point out the error in my thinking!
> 
> 
> Cheers,
> 
> Kevin D-B
> 
> gpg: 012C ACB2 28C6 C53E 9775  9123 B3A2 389B 9DE2 334A
> 
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