[RAI] SIP Option tags and the old P-Headers

[Alan Johnston <alan at sipstation.com>]

I've had some hallway discussions with folks this week, and I wanted to 
bring this to the list for discussion.

With the old P-headers, the definition of a SIP option tag was not 
allowed.  This made some sense since an option tag could mean anything, 
and as a result could affect interoperability.

However, option tags are also used in SIP for feature discovery and 
negotiation, which actually helps interoperabilty and reduces the need 
for profiles and the like.  That is, the use of OPTIONS, Supported 
header fields, and caller prefs are useful mechanism which rely on 
option tags.

I would like to see this relaxed for some new SIP header fields that 
would have been done as P-headers in the past.  For these header fields, 
an option tag could be defined as long as it only meant that the UA 
understands/supports the header field.  As such, this wouldn't change 
basic SIP behavior - it would just be used to indicate and discover 
support for this header field, which by definition only provides 
additional information about the SIP request or response.

Comments?

Thanks,
Alan