[e2e] What are the shared resources? Re: Codel and Wireless

Detlef Bosau detlef.bosau at web.de
Tue Dec 24 06:24:08 PST 2013

Am 23.12.2013 22:02, schrieb Detlef Bosau:
> Am 23.12.2013 01:30, schrieb Andrew Mcgregor:
>> Look up all the research on 'effective bandwidth' and 'grade of service'
> Frankly spoken, I was upset by your post.
> What is "effective bandwidth"? Some other average - from what you derive
> short term forecasts from long term observations.

Perhaps, I'm allowed to look at "effective bandwidth" a bit closer.
Apparently, this term characterizes an "achievable throughtput" in
contrast to a "maximum throughtput".

It makes sense to determine an achievable throughput in contrast to a
maximum throughput because the maximum throughput is usually shared
between several flows, in addition, retransmissions and protocol
overhead must be taken into account. Neither of the aforementioned is
assessed e.g. by packet pair or packet train approaches.

Characterizing a link by an "achievable throughtput" is not new, I did
so myself years ago:

booktitle = "KiVS Kurzbeiträge und Workshop 2005",
year = 2005,
title = "{Path Tail Emulation: An Approach to Enable End--to--End
          Congestion Control for Split Connections and Performance
          Enhancing Proxies}",
author =  "D.~Bosau",
address = "Kaiserslautern, Germany",
pages = "33-40"

After my talk I was asked the simple question (and I'm grateful for this
question): "How do you know the achievable throughput
about a wireless link?"

I uttered quite some nonsense about continuous monitoring and the like -
and eventually had to understand that this question actually debunked my
whole idea as nonsense.

Actually, and I'm grateful for any correction here, it is actually
impossible to asses somethink like an "effective bandwidth" or
"achievable throughput" for a mobile wireless link.

And although this is annoying: A resource
distribution/control/management scheme which does not work for a single
link hardly works for a path consisting of a sequence of links.

The idea of hiding links or more complex parts of an Internet path
behind some mechanism which emulates the part behind a "black box" with
an "effective bandwidth" or an "effective latency" is appealing. We
could distribute resources between flows, we wouldn't have any loss
differentiation problems, no spurious timeouts, we had a "Kodachrome"
network (credits to Simon & Garfunkel).

IIRC, at the same conference Matthias Scheidegger presented an approach
which simulated larger parts of the Internet with some kind of
emulation model for the NS2.

As I said. The idea is appealing.

However, it doesn't work for all links.

Scheidegger's approach however is useful for simulation purposes. One
can simulate an observed throughput. So you can simulate what has
already happened.

My fault was that I tried to emulate an "effective throughput" for an
operational mobile wireless link.

I would greatly appreciate someone who tells me, that I'm wrong. I would
be eager to proceed the work with PTE - however, I didn't to so because
I had to understand that my approach was wrong.

Detlef Bosau
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detlef.bosau at web.de                     http://www.detlef-bosau.de

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