[Tmrg] Round-table PFLDnet submission

[Lachlan Andrew]

Greetings all,

Does silence mean people are happy with my new proposal to measure
load in terms of simultaneous sessions in a processor sharing M/G/1
queue?

We're aiming to have this settled within a week, so now would be a
good time to comment on this or any other issues with the document
(see attached .dvi).

Also, I'd ask all authors to commit regularly to CVS so that we can
all see the latest.

Currently it looks like the RTT section is entirely empty.  Sally, do
you mind if I cut-and-paste the discussion of RTTs from your section
into that section?  Again, I'll take silence as permission :)  (We can
always back it out of CVS.)

Cheers,
Lachlan

On 28/11/2007, Lachlan Andrew <lachlan.andrew at gmail.com> wrote:
> Greetings Sally and everyone,
>
> In the description of delay/throughput tradeoff, it talks about
> "moderate congestion" as 1-2% packet loss with NewReno.  Unless I'm
> mistaken, that says "windows should be about 1/sqrt(0.01)=10 packets"
> (to within a small factor).  I'd prefer not to quantify the load that
> way.  Consider some scenarios:
>
> 56kbit/s:  10 packets of 12000 bits  > 200ms.  That means that for 56k
> tests with inter-city RTTs (50ms), a moderate level of load would be
> *half* of one flow.
>
> 100Mbit/s bottleneck, 100ms path.  "Moderate" congestion would be when
> 2000 flows each gets about 50kbit/s.  To me, that is very heavy load.
> Indeed, however large the bottleneck bandwidth is, "moderate"
> congestion would be when 100ms paths give 50kbit/s per user.
>
>
> I'd much prefer to specify the load in terms of the offered load as a
> fraction of bandwidth.
>
> I propose an alternative:  The "load" is the average number of flows
> if the traffic was served by an  M/G/1 queue with an ideal
> processor-sharing service discipline.
>
> My reasons are:
> 1. This scales properly as capacity increases, and is correctly
> independent of RTT
>
> 2. A processor-sharing M/G/1 queue is a model of roughly what we're
> aiming for with a single bottleneck (equal instantaneous rates).
>
> 3. For loads like 10%, this simply corresponds to 10% of the bandwidth.
>
> 4.  It reflects that, even at extreme overload, we want to consider a
> system whose average number of flows doesn't increase with time.
> Otherwise, the results would be   very   sensitive to duration, and we
> agreed that we should try to design tests which are   not   sensitive
> to the parameters.
>
> Thoughts?


-- 
Lachlan Andrew  Dept of Computer Science, Caltech
1200 E California Blvd, Mail Code 256-80, Pasadena CA 91125, USA
Ph: +1 (626) 395-8820    Fax: +1 (626) 568-3603
http://netlab.caltech.edu/~lachlan
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