Re: [Cake] COBALT implementation in ns-3 with results under different traffic scenarios

[Shefali Gupta <shefaligups11@gmail.com>]

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Hi Jonathan,

Thanks for your inputs.

We re-looked into the COBALT implementation to understand why it drops the
first packet later than CoDel.

There was a bug in the data that was collected in 'drop timestamp files'.
We tried using a different approach to store packet drop times, and now we
see that COBALT indeed drops the first packet prior to CoDel's first packet
drop (image below). So the issue was that our previous approach of storing
the packet drop times in a file was not correct.

Let us know your opinion.
[image: image.png]
Regards,
Shefali Gupta
Jendaipou Palmei

On Fri, Dec 21, 2018 at 6:18 PM Jonathan Morton <chromatix99@gmail.com>
wrote:

> On 21 Dec, 2018, at 12:37 pm, Shefali Gupta <shefaligups11@gmail.com>
> wrote:
>
> In the meantime, we have added the following plots to our wiki:
>
> 1. Number of packet drops per time interval
>
> Link:
> https://github.com/Daipu/COBALT/wiki/Proactive-Drop-Count-per-time-interval-graphs
>
> 2. A file showing the timestamp of each drop
>
> Link: https://github.com/Daipu/COBALT/wiki/Drop-Timestamp-Files
>
>
> Interesting - but very very odd.  COBALT is apparently not behaving
> anything like as designed.
>
> As an immediate point, the first drop occurs 2 whole seconds later than
> either Codel or PIE, which is completely at odds with the excellent control
> of the initial slow-start phase seen in other graphs.
>
> I took the timestamp files, dumped them into a spreadsheet column each,
> and plotted 1/(T(n)-T(n-1)) for each drop event against the raw
> timestamps.  This yields a view of the instantaneous drop frequency.
> Because PIE sometimes drops multiple packets at once, yielding very high
> values here, I truncated the Y-axis at 1000 Hz.
>
> The Codel implementation shows the expected behaviour of a linear ramp of
> drop frequency over time during its dropping phases.  The COBALT
> implementation does not.  Indeed, during a single phase, COBALT's dropping
> frequency appears to vary chaotically, as if it is implementing random-drop
> instead of timed drops.
>
> jendaipoupalmei@gmail.com
>
>  - Jonathan Morton
>
>

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