[e2e] Question about propagation and queuing delays

Detlef Bosau detlef.bosau at web.de
Tue Aug 23 06:25:47 PDT 2005

Ian McDonald wrote:

> As a lucky guy doing a PhD on congestion control I couldn't resist the bait :-)
> I may be missing something but we need congestion control as long as we have networks. In the USA

I´m totally with you.

> and in Europe you may all have unlimited bandwidth available at virtually no cost to you but in the
> rest of the "real world" it doesn't quite work like that. So as long as you are bandwidth
> constrained you will need congestion control. I think others are out of touch of reality....
> </flame bait off>

Excuse me, where is the flame bait? What you say is abolutley correct 
and I totally agree with you!

I will only give one example (I always tend to write too much...). When 
I worked as a network adminstrator in northern Germany we had to attach 
some points of sales to a company network which were situated in the 
Czech Republik.

It is interesting to observe people who always talk about ISDN, DSL, 
backbondes with large bandwdith, and now you are informed: "We do not 
yet know whether 9k6 can be achieved, we have to check the old POTS 
line, it may be too noisy."

Depending on where you are, you may perfectly encounter different 
realities! Even here in Germany you may encounter stone aged POTS lines 
in some rural areas.

When I read what you say, I would like to invite you into my ISP´s 
support newsgroup, I think much of the readers can learn a lot from you!

Just to give one example from there: We recently had a discussion about 
"Fastpath". In DSL lines, you need error recovery on the last mile. Now, 
  to save overhead you do codespreading/interleaing. Some "well informed 
guys" want the ISP to turn interleaving off in order to spare some "ping 
time". First of all, it´s simply ridiculous, theat individual customers 
without any technical knowledge will prescribe the provider the 
appropriate line coding for one individual wire pair. Second: Not only 
these customers may be affected by increasing error rates: These guys 
flood large portions of the network with defictive frames, more 
precisely with defective ATM cells with corrupted payload, which is 
eventually being detected at the customers AAL 5 peer. (At least AFAIK.)
This is thoughtless waste of bandwidth, but it is nearly impossible to 
convince those guys that this is malicious in quite a number of cases!

What is even more disastrous: In fact, in DSL TCP appears to be based 
upon AAL5/UBR. Unspecified bitrate. Hence, all congestion control is 
done at the TCP endpoints. I´m totally with you that this requires well 
behaved participants in a network. IIRC, LANE works with ABR and that 
will alleviate the problem.

> Seriously traffic can be constrained for many different reasons apart from backbones:
> - link at other end (e.g. web server) is on a "slow" link
> - mobile networks
> - link between ISP and upstream ISP (a particular problem in NZ at the moment)
> - slow speed link at consumer premises

Could you _please_ join this newsgroup :-)

> Most backbones are over provisioned in the developed world but less so in more remote corners and
> even less so in developing countries. I have seen presentations showing >50% packet loss in parts of
> Asia and Africa on this list in the last few months - surely you need congestion control for that!

Excuse me, but I don´t mind congestion control! Of course we need it! 
Perhaps my command of the englisch language is rather poor. But I 
sincerely hope that no one had misunderstood me that way that I denied 
the necessity of congestion control!

The problem _I_ expect is, that congestion control and even proper 
retransmission control can run into severe problems, when TCP timers 
don´t work.

And when I talked about the Internet as it is perceived in Europe and 
the US, I concluded that in _this_ area TCP works fine.

Whether this holds true all about the world and in all kinds of networks 
is highly questionable.

So, I really don´t see a flame bait here. Perhaps you understood me in a 
different way,
but from what you wrote I couldn´t agree with you even better!


Detlef Bosau
Galileistrasse 30
70565 Stuttgart
Mail: detlef.bosau at web.de
Web: http://www.detlef-bosau.de
Mobile: +49 172 681 9937

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