[e2e] Delivery times in mobile networs? Re: Are there any persons interested in TCP over mobile wireless networks in Germany?

Detlef Bosau detlef.bosau at web.de
Thu Dec 13 02:40:37 PST 2012

And particularly to networks:

In HSDPA, we often see channel coding changes between one transmission 
block and the following one.

Compared to, say, Ethernet, this would be analogous to swapping 
interfaces from Ethernet (10 Mbit/s), FE, GE or even 10GE. Not only 
between one packet and the following one but even faster, because in 
mobile networks IP packets may well be split up into several 
transmission blocks.

When we think of wired networks, many of us will hardly see an "internal 
relationship" between observed RTT, when the interfaces are changed. And 
estimating an expectation for the observed RTT from observations with 
different hardware would surely be rejected because we compare peaches 
and oranges in a case like this.

Actually, in TCP there are two ways to handle this situation: Either, 
the RTT drops. Then, after some time, the RTT estimation in a TCP socket 
drops as well. And will (hopefully) settles after some time. (That's, 
what is covered by "weakly stationary" or "quasi stationary" by Edge.)
Or the RTT exhibits a drastic increase. In that case, we are likely to 
see a timeout  - and the situation is salvaged by the timer backoff 
proposed, IIRC, by Karn & Partridge. So, we simply drop the existing RTO 
because it is no longer useful as a confidence interval for RTT. We drop 
the existing RTO, we typically drop the estimations for RTT and RTTVAR 
as well and do timer backoff, until we observe successfull packet 
transmissions, re-initialize RTT and RTTVAR and return to normal operation.

Am 13.12.2012 10:04, schrieb Detlef Bosau:
> Just as an illustration: EWMA filters are used for the analysis of 
> time series.
> And I think all of us are very well aware of typical artefacts in the 
> analysis of time series, the most prominent of which are certainly 
> aliasing phenomena.
> ("Backward spinning wheels in movies.")

Detlef Bosau
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