Re: [Cake] fq_codel on 3g network in Mauritius

[Jonathan Morton <chromatix99@gmail.com>]

> On 24 Jul, 2016, at 17:40, moeller0 <moeller0@gmx.de> wrote:
> 
>>> In theory interval can be different for ingress and egress (think old-school SAT-internet with modem upload) it probably is easiest to only configure one interval setting for the time being.
>> 
>> Since the interval parameter depends on the RTT, not the one-way delay, it should always be the same both ways (except for inter-packet-time effects).
> 
> 	Thanks for setting this straight. I was confused; the thing that lingered at the back of my mind was that if we do the target extension for one direction and correct the interval in that direction, we should also correct the interval in the other direction, especially since as Jonathan points out the interval describes the full back-and-forth path…

To be more precise:

- The overhead compensation may differ in each direction, either due to asymmetric quirks of a particular link technology (DOCSIS), or because different links are used for each direction (unidirectional satellite).

- The “target" parameter must be set above one MTU serialisation delay, otherwise a “perfect” flow where each packet is enqueued just as the previous one is dequeued will trigger Codel.  Hence the ideal target may differ per direction, if bandwidth is relatively low and asymmetric.  Cake automatically performs this adjustment.

- The “interval” parameter should be higher than “target” by some minimum factor, which for significant asymmetry may also cause it to differ per direction.  Cake also automatically performs this adjustment, but does not rigidly hold it to the original recommendation of 20:1.

Hence, when using Cake, it is sufficient to set an “rtt” parameter (or associated keywords) consistent with expected path RTTs, and this should be the same in both directions.

 - Jonathan Morton