2806 parameter optionality

Hi Henning, James, folks,
   whilst working on a parameter for use with the tel: URI scheme, I 
was slightly confused by the classification of mandatory and optional 
parameters.

It seems to me that there are actually three classes of 
externally-defined parameters:

*    Parameters that are mandatory in action
- the recipient must both understand AND to act on the parameter, or 
else reject use of the whole URI; action on the parameter is a MUST

*    Parameters that are mandatory to understand
- the recipient must understand the parameter, but is free to ignore 
it if it has good reason so to do; action on the parameter is a SHOULD

*    Parameters that are purely optional. (This class implies the 
same behaviour as specified in the original RFC2806)
- the recipient of such a parameter can simply ignore the presence of 
the parameter if it is not understood; action on the parameter is a 
MAY

In RFC2806bis, mandatory parameters are proposed to have 'm-' as the 
first two characters of their label.

Examples of this kind of parameter are the ones in James' parameter draft).
James, although this is off-topic for the main 'tel:' URI update, how 
do you see your parameters?

Do folks think that:
(a) the presence of a 'm-' string in a parameter label puts it into 
the first or second category?
(b) there should be a way of indicating the other mandatory category?

all the best,
   Lawrence
-- 
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