[core] duration type comments

[Peter Bigot <[e-mail]>]

Rather than spend a lot of time on this one:

1. It is suggested that many options require a duration.  I could find only
two: subscription-lifetime and max-age.  Before determining the encoding,
the characteristics of these durations should be reviewed.  I suspect
virtually every use of subscription-lifetime will be either "forever" or
"stop".  What range of durations are reasonably expected for Max-Age for
resources that reflect measurements of real-world data?

2. While we could select an encoding that supports only those durations
we're thinking about now, I would prefer one that could handle other natural
uses of duration, like "how long until a scheduled event".  For that use, a
set of representable values that is not closed under subtraction is
unsuitable.

3. A detailed description of the subscription-lifetime option encoding in
variable-length-integer format takes one third of a page in
draft-hartke-coap-observe-02.  The pseudo-FP approach takes three pages to
motivate and describe, plus five pages to list its values.  It's a nice
solution, but any claim that this decreases complexity relative to
variable-length-integer is indefensible.

4. I'm not going to bother to implement this right now to see how much space
it takes, but if we continue to create novel encodings that require, for
example, three times as much ROM as a comparable straightforward encoding,
CoAP will be unimplementable on the very devices where it's most needed.

Peter