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

Francesco Vacirca francesco at net.infocom.uniroma1.it
Mon Sep 5 04:26:13 PDT 2005

Filipe Abrantes wrote:
> Detlef Bosau wrote:
>> 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.
> Hmmmm, some misunderstanding here i beleive: packet K and K+1 arrive at 
> destination, (K is the seqno of TCP), however packet K has to be 
> retransmitted in some wireless hop in the path, due to transmission 
> errors. What Francesco was saying (if i understood it correctly) is that 
> this restransmission will lead to an increase of the rtt which will be 
> noted by the sender both in packet K and packet K+1 because packet K+1 
> goes imediatly after packet K. So if packet K time's out, then packet 
> K+1 will also timeout. Francesco claimed that this would have some 
> effect on the spurious timeout detection which i didn't quite 
> understood, but im also not that familiar with rto estimation procedures.

My point is a little bit different... I know I'm not good in explain 
that... I'll try again:

One important point:
-usually there is just one timer for on-flight TCP packets

Two different situations:
1) finite number of retransmissions
2) infinite number of retransmissions (or very high)

in case 1) if packet K is dropped because it is reached the maximum 
number of retransmission attempts, K+1 can arrive:

A) before RTO expiration... and K is probably retransmitted with fast 
B) after RTO expiration... and K is retransmitted because of RTO expiration

in case 2) with the same channel condition of 1A, K will arrive before 
RTO expiration... no retransmissions
in case 2) with the same channel condition of 1B, K will arrive after 
RTO expiration... and K is retransmitted because of RTO expiration 
(Spurious retransmission)

 From a TCP point of view, I think that 2) is always better than 1)



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