[e2e] Are Packet Trains / Packet Bursts a Problem in TCP?

Detlef Bosau detlef.bosau at web.de
Thu Sep 28 16:35:26 PDT 2006

Sireen Malik wrote:
> A few pointers....
> Web-pages have large variance - enough to start another discussion of 
> heavy-tailed distributions ;-) This is one source of burstiness. Hard 
> to cure! Or, is it?

I wonder, if this needs a cure at all!

What do we learn from the heavy tailed distributions of web pages? I 
don´t know the numbers, but does this mean that e.g. 95 % of webpages 
contain less than 20 kBytes of data? Of course, wenn these pages are 
visited and thousands of senders send some kbytes of data, the traffic 
might be "not very smooth". But I think, if this caused a problem, we 
did something fundamentally wrong.
It´s perhaps the basic question: How telco-like shall the Internet become?

> TCP is bursty, too. It's like an on-off source: A bunch of packets are 
> sent in a window in the on-phase, etc. 
Not quite. This discussion ignores ACK clocking. Ideally, TCP should not 
be bursty. Ideally (Freds remaks on TDM like behaviour) TCP packets and 
ACK packets shall form an evenly spaced packet train which wraps around 
downstream and upstream.
At least, that´s the idea behind the congavoid paper and following ones.

And that´s the idea behind a huge number of NS2 simulations with some 
few flows which are well behaved and work as desired.

On the other hand, people look at the Internet, observe things like 
"self similarity" and I don´t know whether we unterstand the reasons for 
this claimed ( I´m  not fully convinced of that.  Many of theses papers 
present a more or less "few observations proof", so I´m somewhat 
reluctant on this one...) behaviour.

> People ahve argued about "shaping" TCP packet generation process to 
> address this burstiness at small time scales. Look for Biplab Sikdar 
> paper from this century. My ex-colleague at TUHH Dirk Abendroth also 
> did a take on this.
> Mark Crovella, et. al showed that under heavy loss conditions, TCP 
> causes pseduo self-similarity.
Does he give a formal proof for this? And what are the lessons learned 
from that?

Perhaps it´s a very personal problem: I once suggested a mechanism 
called PTE. And now, I´m looking for a problem suitable for this 
solution. (I´m afraid there could be none.) And during this search, I 
thought about burstiness of TCP again and again. Now, one question is: 
Does PTE, pacing, whatever significantly impact burstiness? Or is 
burstiness on a single flow irrelevant (it most likely is)? A second 
question is: Does burstiness stem from one major reason or are there a 
couple of reasons? (I think, it´s a couple of reasons.) Third: How can 
we keep burstiness in acceptable limits?


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