address = scheme ":" [user] "@" host [ gruu-parameter ]
scheme = "sip" / "sips"
user = 1*( alphanum / escaped / "-" / "+" / "_" / "~" / "." )
escaped = "%" HEXDIG HEXDIG
host = *( domainlabel "." ) toplabel [ "." ]
domainlabel = alphanum / (alphanum *( alphanum / ( *("-") alphanum) ) )
toplabel = ALPHA / (ALPHA *( alphanum / (*("-") alphanum ) ) )
gruu-parameter = ";gr=" gruu-value
gruu-value = 1*( alphanum / "-" / "_" / ":" )
alphanum = ALPHA / DIGIT
