pub const OPTION_TAG: &str = "gruu";Expand description
The option tag a UA offers to ask for a GRUU.
§4.1: a compliant UA “MUST include the Supported header field” in every REGISTER and “the value of that header field MUST include ‘gruu’ as one of the option tags”. A registrar that does not see it has been told nothing was asked for, and §5.2 has it attach nothing.