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

Francesco Vacirca francesco at net.infocom.uniroma1.it
Fri Sep 2 08:54:07 PDT 2005

Filipe Abrantes wrote:
>> Well, packet losses are the main problem... and they should be hidden 
>> to TCP with wireless link layer protocol and persistant 
>> retransmissions...
> Well, if packet loss is hidden by L2 retransmissions, then, packet loss 
> would become delay, which can become spurious timeouts (depending on the 
> delay increase) right? Then spurious timeouts won't be that marginal...

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


> Best regards
> Filipe
>> Spurious timeouts are a second (marginal) problem that in my opinion 
>> (and in my research) are not frequent events in a well engineered 
>> GPRS/UMTS network.
>> Francesco
>>> This can be due to
>>> - scheduling (voice is priorized over data)
>>> - route change (I don´t know whether this is that important, because 
>>> much of this is micromobility)
>>> - changes in physical path properties.

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