[e2e] Why do we need TCP flow control (rwnd)?

David P. Reed dpreed at reed.com
Wed Jul 2 14:21:39 PDT 2008

I can't resist critiquing the following, because it captures a serious 
thinking difficulty that really, seriously harms research in most fields 
where random processes are involved:

Mark Crovella wrote:
> However -- the only studies I've seen of
> *session* arrivals show that session arrivals *are* typically well
> modeled as Poisson.  
We've already mentioned that there are times and places when session 
arrivals are definitely NOT Poisson, even if sometimes they seem so.  So 
if arrivals seem Poisson, except when they are not, what is the person 
claiming?  Well, it seems that they are discarding the data that 
disagrees with their hypothesis!   In the US, scientists who do that get 
*prosecuted for fraud* in many cases.   And when they do not, are they 
doing good science?

If you clean that statement up - "except under condition X, arrivals are 
well modeled as Poisson", you must first have a sufficient and 
defensible precondition X that you can write down without using the idea 
"under the conditions that arrivals are Poisson, they can be modeled as 
Poisson", which is circular reasoning: assuming that which is to be proved.

And there is another problem in this statement: the phrase "*are* ... 
well modeled as .." cannot be true in any serious statistical 
publication.  The reason is simple:  You need to specify the purpose to 
which the model will be put.  That's another precondition that must be 
explicitly made, if one is to avoid egregious error.

Consider an easier to understand example - that of a pseudo-random 
number generation (PRNG) algorithm.   Does the output model a random 
process?  Of course not.  It's completely deterministic and 
predictable.   Yet for *some* limited purposes, we treat the output as 
if it were actually random.  But in each case, one must determine 
whether the use of a PRNG rather than randomness is valid and safe.

We can use PRNG's in some analyses without bad results instead of random 
numbers.  But we all know that PRNG's make bad one-time-pads for 
encryption.  Thus, the *use* matters.

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