[e2e] Spurious Timeouts in mobile wireless networks, was: Re: Retransmission Timouts revisited

[Detlef Bosau]

Francesco Vacirca wrote:
> I'm agree with that... what I'm trying to say is that L2 retransmissions 
> are due to transmission errors on the wireless channel... and with all 
> kind of ARQ protocols (from Stop'n Wait to Selective Repeat) if you drop 
> packet K (because one or more PDUs belonging to that TCP-SDU reached the 
> max number of retransmissions N), that implies that if packet K+1 
> crosses the wireless channel it will arrive after the moment that K 
> would arrived with some more retransmissions... and this implies that if 



I admit: I cannot follow.

Packet K is lost. Packet K+1 not. What´s the problem?

And what do you mean with "K+1 .... will arrive after the moment K would 
arrived with...."?

K did not arrive.

So K´s arrivale time in case of ... does not matter.

> the RTO would expire for K with infinite retransmissions, it would 
> expire also for K+1 with N retransmissions... with no advantages for 

Why?

I do not the the connection from your maximum retransmission count N and 
the TCP RTO. In addition, N is purely a L2 topic. For TCP/IP, a packet 
is either delivered and experiences a certian latency or it is lost.


> TCP,  but with some small disadvantages that I do not think could 
> influence the overall goodput.
> Moreover with FRTO (or other mechanisms) TCP can recover a spurious 
> timeout, but a normal timeout (due to real loss) can just be recovered 
> by slow-start because it is a real loss with RTO expiration and there is 
> no possibility to distinguish it from a congestion event (without 
> explicit notification).


Nevertheless, there are situations where a slow start is not necessary.

Basically, my question is: Do spurious timeouts happen really that often?

Or in different terms: From what I´ve learned during the last few weeks,
3G networks are basically understood, spurious timeouts do happen, 
occasionally, now and then. If so, we are able to detect them and 
recover from, e.g. unnecessar congestion handling.

So, for the case of TCP over 3G networks: Is this case closed?

Just a question.

Detlef

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Detlef Bosau
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