[R-C] [Iccrg] draft-alvestrand-rtcweb-congestion-01 - A Google Congestion Control Algorithm for Real-Time Communication on the World Wide Web

[Saverio Mascolo]

hi all,

the model could be weak....however before going into "theoretical
questions" can we look at  measurements?


thanks
saverio

On Mon, Nov 7, 2011 at 2:58 AM, Wesley Eddy <wes at mti-systems.com> wrote:

> On 11/1/2011 10:34 AM, Lars Eggert wrote:
>
>> This may be of interest to the RG:
>> http://wiki.tools.ietf.org/**html/draft-alvestrand-rtcweb-**congestion-01<http://wiki.tools.ietf.org/html/draft-alvestrand-rtcweb-congestion-01>
>>
>>
>
> Here are my comments on this (also copied to the authors and the
> mailing list for RTCWEB congestion control):
>
> (1)
> I think the model tries to correct for some effects later, but this
> part only makes sense if all frames are the same size, and none of
> the timestamps are subject to jitter being introduced by OS scheduling,
> MAC layer (e.g. wireless) issues, etc:
> """
>   We define the (relative) inter-arrival time, d(i) as
>
>     d(i) = t(i)-t(i-1)-(T(i)-T(i-1))
> """
>
> (2)
>
> This part doesn't quite make sense to me, but I think it's just in
> the wording and not in the math:
> """
>   Since the time ts to send a frame of size L over a path with a
>   capacity of C is
>
>     ts = L/C
>
>
>   we can model the inter-arrival time as
>
>              L(i)-L(i-1)
>     d(i) = -------------- + w(i) = dL(i)/C+w(i)
>                  C
> """
> That's not a model of the inter-arrival time, it's a model of the
> difference between the delta timestamps and delta reception times
> based on difference in packet length.
>
>
> (3)
> In the definition of w(i), do you also intend to account for other
> sources of delays in the end systems' software?  Why do we think that
> white noise is the right model?  This seems like a big assumption,
> unless there's actually data that shows it holds under a very wide
> range of settings.
>
> (4)
> It should probably be understood clearly that the i values are sample
> numbers, but that the generation of samples comes from the behavior of
> the plant, and they aren't taken at the filter/controller regularly.
> This seems to be quite different than when the document says:
> """
>   In this document we make the assumption that the rate control
>   subsystem is executed periodically and that this period is constant.
> """
>
> (5)
> The algorithm at the receiver side is clueless about packet losses,
> and seems like it would see them only when the loss period ends (when
> a packet survives) and then would see them as really long inter-arrival
> times rather than losses?  (or did I miss something really important?)
>
>
> (6)
> The rules about 2% and 10% in the sender-side algorithm seem ... odd.
> Unless the report interval is sufficiently long in relation to the frame
> rate, the percentages may not work well.  These parameters have to be
> balanced, or the algorithm will be fragile.
>
> (7)
> Shouldn't the sender estimate of available bandwidth be able to go lower
> than the TFRC equation output if the delay has been increasing (though
> packets are not yet being lost)?
>
> (8)
> Startup is a big topic that isn't yet addressed.
>
>
>
>
> --
> Wes Eddy
> MTI Systems
>
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> Iccrg at cs.ucl.ac.uk
> http://oakham.cs.ucl.ac.uk/**mailman/listinfo/iccrg<http://oakham.cs.ucl.ac.uk/mailman/listinfo/iccrg>
>



-- 
Prof. Saverio Mascolo
Dipartimento di Elettrotecnica ed Elettronica
Politecnico di Bari
Via Orabona 4, 70125 Bari Italy
Tel. +39 080 5963621
Fax. +39 080 5963410
email:mascolo at poliba.it

http://c3lab.poliba.it


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