Subject: RE: Every time ..., was Re: General formula

From: Alan Lloyd <[email-masked]>
Date: Thu, 3 Jun 1999 08:54:42 +1000

Ed - this is all too hard for me ... I cannot even predict how long my
wife will be on the phone tonight or the number of times my mobile will
drop out when using it (hands free of course) in the car.

As to certificate usage and lifetime - theory is useful - but practice
makes perfect. 

What is the lifetime of a Bus or Train ticket... It depends on the
attitude and hunger of the damn machines that read them...QED

regards as always.

PS - This person that predicted phone calls and phone connections - gee
he must be "one from on high" - can you ask him how big my next month's
bill will be :-)

	PPS good luck with the paper - is it signed - and how many
attributes does it have.?

> -----Original Message-----
> From:	Ed Gerck 
> Sent:	Thursday, June 03, 1999 4:12 AM
> To:	Alan Lloyd
> Cc:	Bob Blakley; Graham Klyne; PKIX
> Subject:	Every time ..., was Re: General formula
> 
> 
> 
> Alan Lloyd wrote:
> 
> > Ed I  still do not know how anyone on this planet ...
> > can predict the life time of a cert.
> 
> Every time you make a phone call, it gets through because
> somebody on this planet was able to predict its duration.
> And yet, how can anybody on this planet be able to
> predict the lifetime (Lebendauer) of that connection?
> 
> The answer: By following very similar (though not
> equal) arguments as the ones I used. So, there is
> nothing magical about it.
> 
> I suggest you read my original posting here and its
> inlined reference [1]. The full paper will hopefully
> make it clearer also by examples.
> 
> Cheers,
> 
> Ed Gerck
> ______________________________________________________________________
> Dr.rer.nat. E. Gerck                                 [email-masked]
>   ---  Meta-Certificate Group member -- http://www.mcg.org.br  ---
>

---

From: Ed Gerck <[email-masked]>
Date: Wed, 02 Jun 1999 20:47:56 -0700

Alan Lloyd wrote:

> wife will be on the phone tonight

Alan:

Theory says that "how long your wife will be on the phone" does NOT
depend when you first noted that she was on the phone -- if this is
of any use to you, though it may already relate to your experience ;-)

This also means that if you call someone and it is busy, you could call
right afterwards (just add the round-trip total delay time -- say, 5 sec)
and your sucess rate should be the same as if you would wait some
minutes (as people normally do).

The reason in both cases is simple -- since phone statistics is given by
a Poison distribution and you don't know when the conversation actually
*started*, its end does not depend on the duration of your observation.

So, theory is nice -- when it corresponds to reality (as the above two
examples do). It allows us to use elevators, credit and rely on chemistry.

> or the number of times my mobile will drop out when using it (hands free
> of course) in the car.

I might be tempted to model it ;-)

> As to certificate usage and lifetime - theory is useful - but practice

Oh, not the cliche again! Don't have anything better down there?

> attitude and hunger of the damn machines that read them...QED

No -- their lifetime is actually the expected duration of their existence as
tickets, ie, the expected duration of time that you can still use them. Well,
I do have a metro ticket for Washington DC that I use for three months
now... and I sure expect them to be valid this whole year, their lifetime.

> he must be "one from on high" - can you ask him how big my next month's

His name was Agner Krarup Erlang -- Eralng for short. And his name is also
the unit for phone traffic.

 A.K. Erlang was the first person to study the problem of telephone networks.
 By studying a village telephone exchange he worked out a formula, now known
 as Erlang's formula, to calculate the fraction of callers attempting to call someone
 outside the village that must wait because all of the lines are in use. Although
 Erlang's model is a simple one, the mathematics underlying today's complex
 telephone networks is still based on his work.

Check in
 http://pass.maths.org.uk/issue2/erlang/index.html

> attributes does it have.?

That is why long postings must be very carefully written; the more
you write the less it tends to live without at least one correction. I hope
this may be a good enough excuse for my long postings.... I risk more
when they are long ;-)

Cheers,

Ed Gerck
  ---  Meta-Certificate Group member -- http://www.mcg.org.br  ---

---

From: David Chia <[email-masked]>
Date: Thu, 3 Jun 1999 16:01:14 +1000 (EST)

> attitude and hunger of the damn machines that read them...QED

You seem to have missed the point. The aim is not to predict 'exactly'
the life time of any 'particular' certificate.  The exercise is to
determine if collectively the phenomon exibits predictable behaviour
and to estimate the 'expected' or the statistical maximum likelihood
value (together with the standard error of estimation if possible) so
that cost can be evaluated and alternatives can be compared.

When I buy hardware I usually also check the mean time between failure
value beside cost and warrantee.

David Chia, RMIT University

---

From: Ben Laurie <[email-masked]>
Date: Thu, 03 Jun 1999 10:02:05 +0100

> *started*, its end does not depend on the duration of your observation.

I don't believe this for a moment. To take an extreme example, if people
make, on average, 1 phone call a year, with a mean duration of 1 minute,
then it is intuitively obvious that if I wait a day after they are busy,
I have an almost 100% chance that they won't be busy, whereas 5 seconds
later the chances are noticably less than 100%. Of course, I believe the
second statement, but the first is not a consequence of it, IMNSHO.

You're just making this stuff up as you go along, aren't you?

Cheers,

Ben.

--
http://www.apache-ssl.org/ben.html

"My grandfather once told me that there are two kinds of people: those
who work and those who take the credit. He told me to try to be in the
first group; there was less competition there."
     - Indira Gandhi

---

From: Ed Gerck <[email-masked]>
Date: Thu, 03 Jun 1999 13:09:09 -0700

Ben Laurie wrote:

> I have an almost 100% chance that they won't be busy,

Ben Laurie:

Is this how people really use the phone where you live? Is this "phone
statistics"? C'mon, give me a break-- and realize that the statistics of
your "extreme example" (what an understatement) is not even given by
a Poisson pdf.

> whereas 5 seconds

You remind me of a guy that drowned in a lake that had just one inch
of water depth, on average. Just too bad he did not realize it was an
average, though. Or, even, what an average was.

Since phone traffic statistic is hardly on-topic here, I might help you off
list if you consider this important for your understanding of certificate
lifetimes.

And, Ben, next time you don't understand something, just ask and say what
part of my text you did not understand. You don't need to become aggressive
in order to receive my reply.

Cheers,

Ed Gerck

---

From: Ben Laurie <[email-masked]>
Date: Sun, 06 Jun 1999 12:46:02 +0100

> your "extreme example" (what an understatement) is not even given by

That's the whole point: phone statistics don't follow a Poisson pdf for
an individual user. The Poisson pdf applies to the phone system as a
whole (i.e. the combined statistics for all users). Naturally for an
individual user the call frequency and duration are unrelated (i.e. not
Poisson).

> in order to receive my reply.

But I did understand it, I simply don't believe it is correct. In
particular, the statement "since phone statistics is given by a Poison
distribution" is incorrectly used to support an argument about a single
phone user.

However, I do apologise to you and the list for the tone I used.

Cheers,

Ben.