[Sip] INFO issues: Backwards Compatibility and Forwards Detection

[Eric Burger <eburger at standardstrack.com>]

The current text states that if a UA does not support receipt of any  
Info Packages, it MUST drop the Info-Recv header.  Likewise, if the UA  
does not support sending any Info Packages, as would be the case if a  
UAS does not support any of the Info Packages offered by the UAC, it  
MUST drop the Info-Send header.

As an extreme example, if a UAS does not want to send any Info  
Packages to a UAC and simultaneously the UAS does not support any of  
the Info Packages offered by the UAC, the UAS will have neither an  
Info-Send nor an Info-Recv header.  In this case, the UAC cannot  
disambiguate between a legacy UAS and an Info Package-aware UAS that  
simply does not want to receive INFO messages.

Is this a problem?  On the one hand, one could argue a UAS that does  
not support any INFO packages may still support proprietary INFO  
packages or the legacy, standards track INFO usages.  On the other  
hand, one could argue a UAS that supports INFO packages yet 
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