[e2e] Stupid Question: Why are missing ACKs not considered as indicator for congestion?

Francesco Vacirca francesco at net.infocom.uniroma1.it
Thu Feb 1 02:39:11 PST 2007

I'm sorry, I'm quite confused... ;-)
I'll forward to tcpm and iccrg


Michael Welzl wrote:
> Hi,
> I think you're referring to a different email that I sent
> out to ietf tsvwg, dccp, tcpm and irtf iccrg yesterday.
> I suggest to move this discussion to iccrg, where we have
> discussed corruption based rate reaction in the past.
> See below for an answer to your question:
> On Thu, 2007-02-01 at 11:24, Francesco Vacirca wrote:
>> Some link layers use a strongest FEC to protect header. E.g. in some 
>> UMTS coding scheme the link layer employs a 1/3 codification for RLC 
>> header, whereas the payload can use a different scheme (e.g. from 4/5 to 
>> 1/3)... Maybe it could be applied also to TCP. Note that this can 
>> decrease the goodput in case of non lossy links... obviously it depends 
>> on the ratio between useful bits and transmitted bits.
>> In the 802.11 standard some part of the packet (MAC header) is sent with 
>> a different rate to be more protected against channel impairments and 
>> also for compatibility purposes. A cross layer approach could adopt low 
>> rate also for TCP header (also IP obviously)... but I do not think that 
>> the benefits are more than disadvantages.
>> One more thing... in case of Michael experiments, are the packet losses 
>> on the channel due to SNR fluctuations or due to MAC collisions?
>> In the second case, it is quite normal that the whole packet 
>> (header+payload) is corrupted.
> They are generally due to SNR fluctuations, by transmitting
> between two notebooks in ad hoc mode (the only way that
> disabling the CRC worked for us). I remember that Mattia
> also did a quick check with one more notebook, to see if
> MAC influences the result, but it didn't seem to play a role
> (I don't remember if this test made it into the document in
> the end... I think not).
> It would be interesting to know why errors occur in the
> fashion that we saw; we measured, but don't really have
> an explanation. Perhaps it's the PHY coding.
> Anyway, for those of you in who didn't see my previous
> email and are confused, this link should explain it:
> http://www.welzl.at/research/projects/corruption/index.html
> Cheers,
> Michael

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