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

[Michael Welzl]

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