[e2e] Congestion control as a hot topic in IETF
detlef.bosau at web.de
Fri Mar 8 15:41:38 PST 2013
Am 08.03.2013 15:28, schrieb Jon Crowcroft:
> actually packets stored in the channel may not have contended for the
> channel, whereas packets arriving at a queue are contending for the
> output to that queue....
Even that is simply wrong.
In Wifi, packets are sent in the whole: One IP packet is encapsulated in
one 802.11 packet. This simply holds not true in mobile networks.
E.g. in HSDPA you have 2 ms timeslots and you may change the line coding
scheme, the puncturing and the wrapping about channels each and every 2 ms.
You have transport blocks the payload of which may vary from about 150
bit to 12000 bit, very roughly I need to look it up.
Than you have opportunistic scheduling where channels may be suspended
for some time.
What I read so far in CS books is simply orthogonal to reality in many
cases. And the model to assign a packet a "serialization delay" is
hopelessly naive and, physically, simply as wrong as could be.
That's completely different from wired networks.
And I really cannot imagine that I'm the only one here to see this.
One immediate consequence is that the annoying discussion about
"spurious time outs" is nonsense.
The RTO in TCP is a confidence interval derived, by Cantelli's
inequality, from expectation and variance of the/, so assuemed,
//stationary /RTT. (See Edge's paper, the congavoid paper etc.)
(Cantelli's inequality is a special case of Chebysheff's inequality.)
Of course, you can put RTT "measurements" into an EWMA filter (garbage
in garbage out principle), put this into Cantelli's inequality and get
(again garbage in garbage out) an RTO statistics.
You can easier take 15. (No matter if seconds or hours.)
This is as wrong as the other RTO formulae but easier to implement.
However, if we use an RTO determined by pure hand waving, how can we
really claim this to be a confidence interval and say that ACKs are "late"?
And what's particular annoying: I'm neither the first one to claim so
neither the only one. E.g. Raj Jain and Lixia Zhang published on the
deficiencies of timers nearly 25 years ago! This is really old and
accepted basic knowledge! Why is this ignored?
You referred to Dave Reed some posts ago, from quite some PM exchange I
very well know Dave's anger about formulae, which are mistaken for
reality, and the ignorance of the community against physical reality
which is around everywhere!
And for decades we modelled "bandwidths" in our simulators as
"serialization delay" in wired networks and mobile networks as well.
except some very few extensions, e.g. the EURANE extensions to the NS2,
and wonder why simulators yield exactly the wrong results we previously
encoded in them!
As you may notice, I'm extremely upset about this!
It may sound cynical, however I'm not employed with any university at
the moment, so there is no professor to fire me tomorrow morning because
I wrote this post. And believe me, this attitude is anything than
unfamiliar to me. If someone points to physical reality and violates
some dogmatic "laws", he is most likely to be at least severely berated
he must not damage the reputation of his employer.
However, wrong statements are not corrected by berating students, PhD
students or research assistants!
Wrong statements are only corrected by correct ones!
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