[e2e] Are we doing sliding window in the Internet?

Detlef Bosau detlef.bosau at web.de
Tue Jan 2 11:52:03 PST 2007

Venkata Pingali wrote:
> Server end (i.e, end that has large
> amount of data to transfer):
>     - Most connections are short (90% < 1sec)

Do you have any knowledge of the number of "rounds" the TCP connection 
has seen?  A couple of years ago I saw some similar result (don´t no the 
source at the moment) where 90 % of connections consist of not more than 
20 packets.

Now, consider the initial slowstart, IIRC we start with 2 MSS (?) then 
we have:

Round    CWND
    1              2
    2              4
    3              8
                            total of 14 packets up to now
    4             16
                            total of 24 packets up to now,

thus many flows will finisch before the end of the fourth round which 
would correspond to a CWND of about 6 kByte, 1500 byte MSS assumed.

In short words: Quite a few connections are finished before the end of 
the fist slow start period.

Does this match your observations?

>     - MaxCwnd is < 5KB in > 80% of cases
>     - MaxRTT is distributed almost uniformly
>       in the 0-400ms range.
> Client end (i.e., the end receiving data):
>     - ~ 90% of connections see MaxCwnd < 5KB
>     - < 1% connections see MaxCwnd > 10KB
>     - 90% of connections have MaxRTT < 100ms

Oh, I love it :-)

Last year I had a long argument with someone who told me about the 
benefits of window scaling :-) He talked about extremely large CWNDs by 
several dozens or hundreds of MByte :-)

O.k., that´s a different story because we are talking about greedy 
sources than. However, if that colleague was the only one to activate 
window scaling while surfing from the US and A to good ol´ Europe and 
Cisco et al. had buried hundreds of megabytes of useless queue memory in 
their hardware *blush* this guy perhaps filled the queues the first time 
ever, following the good old paradigm: "Keep the queue full" and that 
way of course outperformed his competitors hopelessly ;-)

> There are some problems with the data:
>     - limited scenarios (web based)
>     - small sample sizes (21K for server, 150K
>       for client)
>     - the website has non-standard distribution
>       of file types and sizes

At least it exists. And reality is often more convincing than standards. 
Particularly in cases were both disagree.

> You can find the various graphs here:
> http://www.isi.edu/aln/e2e.ppt

Just a question: Is it possible to export those slides to a common 
readable format like PDF? I don´t have any M$ products in use here and 
when I opten PowerPoint slides with OpenOffice the results are sometimes 
interesting, sometimes surprising, sometimes hopeless, but nearly always 
quite different from what you wrote :-)



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