[e2e] Congestion control as a hot topic in IETF
Jon Crowcroft
Jon.Crowcroft at cl.cam.ac.uk
Sun Mar 10 03:52:21 PDT 2013
you misunderstand -
there are very good _MODELS_
of wireless behaviour -
there isn't just one single magic FORMULA
because the systems are so complex in reality
e.g. simply considering a simple path loss exponent
doesn't work in a building because you need to ray trace
from transmit to all potential receiver to compute
the multipath interference effects - so we don't do this
unless we're planning a very robust deployment
however, you can use the MODEL to choose robust designs
of modulation, coding etc
(given spectrum, power budgets and antennae constraints)
and bound the capacity (and delay) seen in a wide range of
circumstances
the designs are usually adaptive....
(as per the WiFi discussion we just had)....
so building a +predictive+ online, realtime wireless channel
equation based system is almost impossible for any real world
deployment, but we can explain the behaviour of the system
just fine...
(oh, and look at any movemone on the order of physical scale of the
wavelength of the signal, or any atmospheric effects
(ionosphere/scintillation) to make life more fun...
but that does not mean we don't understand it =
it just maskes the design space more fun....
coz you have to evaluate a range of techniques over a
wide range of circumstances...
however, it also does mean you can base a
congestion feedback at layer 4 on info
passed by layer 1/2/3 up, at a receiver
to infer channel contention - and, using
aforesaid models, use this to trigger
feedback to layer four senders (e.g. via ECN)
to factor in layer 2 troubl, just the way
you can factor in queue/buffer build problem
(e.g. via a virtual queue occupancy estimation)...
its not black magic or a dark art - its just not
as simple as shannon, coz there's a lot of devils
in the details....but that is what engineering
so often is...
In missive <513B98F0.5020801 at web.de>, Detlef Bosau typed:
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>>Am 09.03.2013 11:28, schrieb Jon Crowcroft:
>>>
>>> no, its a basis for sound system design.
>>
>>Up to now, and I'm looking for this for a decade now and asked many,
>>many researchers, I don't know a formula which derives the possible
>>throughput over a wireless channel depending on the SNR.
>>
>>And no, Shannon-Hartley doesn't.
>>> >>> eventually the signal (to background noise) is too low level to carry any info in any way distinguishable
>>>
>>> >>>
>>> >>> >>Data corruption is a phenomenon which occurs at the receiver. The
>>> >>> >>problem is that the receiver cannot successfully rebuild a packet from
>>> >>> >>what he received. The air interface has no idea of which waves are
>>> >>> >>travelling along and whether they make any sense at all.
>>> >>>
>>> >>> you're confusing interference with other sources and misreading the honorably Dave Reed
>>> >>
>>> >>I'm quite sure that I'm not misreading Dave Reed.
>>> >>
>>>
>>> you are conflating two (or three) completely different facets of
>>> wireless nets...
>>
>>Just the opposite is true.
>>
>>The problem is that we often observe _one_ phenomenon, e.g. packets are
>>not ACKed in time, which can be the consequence of
>>- collision, i.e. a MAC problem,
>>- corruption, caused by noise, shading, interference etc.,
>>- congestion,
>>and believe that there is _the one single reason_ for the observed
>>phenomenon and afterwards identify this by an educated guess or divine
>>inspiration.
>>
>>This is sometimes called "ratio ex post" and is one of the two most
>>often made mistakes in science.
>>(The other one is to mistake coincidence for correlation and even more
>>causal relation. Take this and "ratio ex post" - and I'm convinced you
>>can falsify the vast majority of medical studies currently being
>>published.)
>>
>>When I understand Dave correctly, this is what Dave sometimes calls
>>"confirmation bias".
>>
>>>
>>> >>> secondly, you are ignoring absorption (e.g. by water vapour which gets a little bit hotter)
>>> >>> and also _self_ interferance (aka Ricean fading) and scattering (rayleigh fading)
>>> >>>
>>> >>
>>> >>So, a model which correctly describes wireless channels is that flexible
>>> >>that it fits anything - and has no use at all.
>>>
>>> incorrect - it is a sound basis for design.
>>
>>No. A model with dozens of variables, hardly any of which can be
>>estimated in a sound way doesn't prove anything.
>>
>>A typical example is the loss differentiation debate.
>>
>>There are literally hundreds of papers around which try to determine
>>whether a packet loss is due to corruption or congestion.
>>
>>Take any of them - and look for "ratio ex post" - I don't know at least
>>one single paper which holds.
>>
>>
>>--
>>------------------------------------------------------------------
>>Detlef Bosau
>>Galileistraße 30
>>70565 Stuttgart Tel.: +49 711 5208031
>> mobile: +49 172 6819937
>> skype: detlef.bosau
>> ICQ: 566129673
>>detlef.bosau at web.de http://www.detlef-bosau.de
>>
>>
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>> <meta content="text/html; charset=ISO-8859-1"
>> http-equiv="Content-Type">
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>> <div class="moz-cite-prefix">Am 09.03.2013 11:28, schrieb Jon
>> Crowcroft:<br>
>> </div>
>> <blockquote cite="mid:E1UEH0X-0003SN-V1 at mta0.cl.cam.ac.uk"
>> type="cite">
>> <pre wrap="">
>>
>>no, its a basis for sound system design.
>></pre>
>> </blockquote>
>> <br>
>> Up to now, and I'm looking for this for a decade now and asked many,
>> many researchers, I don't know a formula which derives the possible
>> throughput over a wireless channel depending on the SNR. <br>
>> <br>
>> And no, Shannon-Hartley doesn't.<br>
>> <blockquote cite="mid:E1UEH0X-0003SN-V1 at mta0.cl.cam.ac.uk"
>> type="cite">
>> <pre wrap="">
>> >>> eventually the signal (to background noise) is too low level to carry any info in any way distinguishable
>>
>> >>>
>> >>> >>Data corruption is a phenomenon which occurs at the receiver. The
>> >>> >>problem is that the receiver cannot successfully rebuild a packet from
>> >>> >>what he received. The air interface has no idea of which waves are
>> >>> >>travelling along and whether they make any sense at all.
>> >>>
>> >>> you're confusing interference with other sources and misreading the honorably Dave Reed
>> >>
>> >>I'm quite sure that I'm not misreading Dave Reed.
>> >>
>>
>>you are conflating two (or three) completely different facets of
>>wireless nets...</pre>
>> </blockquote>
>> <br>
>> Just the opposite is true.<br>
>> <br>
>> The problem is that we often observe <u>one</u> phenomenon, e.g.
>> packets are not ACKed in time, which can be the consequence of<br>
>> - collision, i.e. a MAC problem,<br>
>> - corruption, caused by noise, shading, interference etc.,<br>
>> - congestion,<br>
>> and believe that there is <u>the one single reason</u> for the
>> observed phenomenon and afterwards identify this by an educated
>> guess or divine inspiration.<br>
>> <br>
>> This is sometimes called "ratio ex post" and is one of the two most
>> often made mistakes in science.<br>
>> (The other one is to mistake coincidence for correlation and even
>> more causal relation. Take this and "ratio ex post" - and I'm
>> convinced you can falsify the vast majority of medical studies
>> currently being published.) <br>
>> <br>
>> When I understand Dave correctly, this is what Dave sometimes calls
>> "confirmation bias". <br>
>> <br>
>> <blockquote cite="mid:E1UEH0X-0003SN-V1 at mta0.cl.cam.ac.uk"
>> type="cite">
>> <pre wrap="">
>>
>> >>> secondly, you are ignoring absorption (e.g. by water vapour which gets a little bit hotter)
>> >>> and also _self_ interferance (aka Ricean fading) and scattering (rayleigh fading)
>> >>>
>> >>
>> >>So, a model which correctly describes wireless channels is that flexible
>> >>that it fits anything - and has no use at all.
>>
>>incorrect - it is a sound basis for design. </pre>
>> </blockquote>
>> <br>
>> No. A model with dozens of variables, hardly any of which can be
>> estimated in a sound way doesn't prove anything.<br>
>> <br>
>> A typical example is the loss differentiation debate.<br>
>> <br>
>> There are literally hundreds of papers around which try to determine
>> whether a packet loss is due to corruption or congestion.<br>
>> <br>
>> Take any of them - and look for "ratio ex post" - I don't know at
>> least one single paper which holds.<br>
>> <br>
>> <br>
>> <pre class="moz-signature" cols="72">--
>>------------------------------------------------------------------
>>Detlef Bosau
>>Galileistraße 30
>>70565 Stuttgart Tel.: +49 711 5208031
>> mobile: +49 172 6819937
>> skype: detlef.bosau
>> ICQ: 566129673
>><a class="moz-txt-link-abbreviated" href="mailto:detlef.bosau at web.de">detlef.bosau at web.de</a> <a class="moz-txt-link-freetext" href="http://www.detlef-bosau.de">http://www.detlef-bosau.de</a>
>>
>></pre>
>> </body>
>></html>
>>
>>--------------090201050506010700000902--
cheers
jon
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