[e2e] Rationale for EWMA filters in RTTM

Detlef Bosau detlef.bosau at web.de
Fri Aug 5 01:06:45 PDT 2011

Hi to all,

perhaps this sounds stupid, however, I would like to understand this.

I'm still to understand the rationale behind the EWMA filters for SRTT 
and RTTVAR as suggested in RFC 2988.

 From what I've read, these filters basically assume a Poisson model for 
a TCP flow. So, I assumed a TCP flow with window 1 segment, i.e. stop'n 
wait, so that the arrival process of ACKs at the sender should be 
simple: All the interarrival times are i.i.d. and exponentially distributed.

So, I used octave to generate some random numbers from an exponentially 
distribution and submitted these to the RTT estimator from RFC 2988.

Any misconception of mine up to now?


O.k. The results are...........

When I applied some fat there, perhaps I could pass the result as an 
opus posthumous by Joseph Beuys.

"Strange drawing with fat."

(For those, who have no idea about Beuys:
look at the right picture.)

Of course, I plaid around with the gain alpha. (I only considered RTT 
yet. I don't want to waste all the butter in my fridge for an artistic 
redesign of my results...)

Actually, this had some influence on the results. E.g. chosing a 
alpha=1/200 appeared to be somewhat more convincing. However, this is 
playing around and no science.

Do I have a basic misconception here? I would appreciate any comment.

Detlef Bosau
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