In many cases, different URI strings may actually identify the identical resource. For example, the host names used in URL are actually case insensitive, and the URL http://www.XEROX.com is equivalent to http://www.xerox.com. In general, the rules for equivalence and definition of a normal form, if any, are scheme dependent. When a scheme uses elements of the common syntax, it will also use the common syntax equivalence rules, namely that the scheme and hostname are case insensitive and a URL with an explicit ":port", where the port is the default for the scheme, is equivalent to one where the port is elided.

These URI attributes specify EQUALITY matching with

'caseIgnoreMatch' (rather than with 'caseExactMatch') in order to

conform to the spirit of [STD66], which requires case-insensitive

matching on the host part of a URI versus case-sensitive matching

on the remainder of a URI.

These URI attributes follow existing practice of supporting

case-insensitive equality matching for host names in the

associatedDomain attribute defined in [RFC4524].

Metadata Discovery and Compatibility involve comparing URLs. When comparing URLs, processors must use Syntax-Based Normalization as defined in [RFC3968]. Processors must perform Scheme-Based Normalization for HTTP (80) and HTTPS (443) and should perform Scheme-Based Normalization for other well-known schemes.