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

[Francesco Vacirca]

Maybe you are interested in this technical report:
TCP Spurious Timeout estimation in an operational GPRS/UMTS network

http://userver.ftw.at/~ziegler/FTW-TR-2005-008.pdf

We deployed an algorithm to estimate spurious timeout retransmissions by 
passive monitoring that it is based on similar assumptions of F-RTO and 
reported the results on several day traces. The report is based on 
measurements made in the UMTS and GPRS networks of Mobilkom Austria, EU.

regards,

Francesco


Francesco Vacirca wrote:
> 
> 
> 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 
> retransmit
> 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)
> 
> 
> regards,
> 
> f.