4.1. General Considerations

A matching rule is applied to attribute values through an AttributeValueAssertion or MatchingRuleAssertion [RFC4511]. The conditions under which an AttributeValueAssertion or MatchingRuleAssertion evaluates to Undefined are specified elsewhere [RFC4511]. If an assertion is not Undefined, then the result of the assertion is the result of applying the selected matching rule. A matching rule evaluates to TRUE, and in some cases Undefined, as specified in the description of the matching rule; otherwise, it evaluates to FALSE.

Each assertion contains an assertion value. The definition of each matching rule specifies the syntax for the assertion value. The syntax of the assertion value is typically, but not necessarily, the same as the syntax of the attribute values to which the matching rule may be applied. Note that an AssertionValue in a SubstringFilter [RFC4511] conforms to the assertion syntax of the equality matching rule for the attribute type rather than to the assertion syntax of the substrings matching rule for the attribute type. Conceptually, the entire SubstringFilter is converted into an assertion value of the substrings matching rule prior to applying the rule.

The definition of each matching rule indicates the attribute syntaxes to which the rule may be applied, by specifying conditions the corresponding ASN.1 type of a candidate attribute syntax must satisfy. These conditions are also satisfied if the corresponding ASN.1 type is a tagged or constrained derivative of the ASN.1 type explicitly mentioned in the rule description (i.e., ASN.1 tags and constraints are ignored in checking applicability), or is an alternative reference notation for the explicitly mentioned type. Each rule description lists, as examples of applicable attribute syntaxes, the complete list of the syntaxes defined in this document to which the matching rule applies. A matching rule may be applicable to additional syntaxes defined in other documents if those syntaxes satisfy the conditions on the corresponding ASN.1 type.

The description of each matching rule indicates whether the rule is suitable for use as the equality matching rule (EQUALITY), ordering matching rule (ORDERING), or substrings matching rule (SUBSTR) in an attribute type definition [RFC4512].

Each matching rule is uniquely identified with an object identifier. The definition of a matching rule should not subsequently be changed. If a change is desirable, then a new matching rule with a different object identifier should be defined instead.

Servers MAY implement the wordMatch and keywordMatch matching rules, but they SHOULD implement the other matching rules in Section 4.2. Servers MAY implement additional matching rules.

Servers that implement the extensibleMatch filter SHOULD allow the matching rules listed in Section 4.2 to be used in the extensibleMatch filter and SHOULD allow matching rules to be used with all attribute types known to the server, where the assertion syntax of the matching rule is the same as the value syntax of the attribute.

Servers MUST publish, in the matchingRules attribute, the definitions of matching rules referenced by values of the attributeTypes and matchingRuleUse attributes in the same subschema entry. Other unreferenced matching rules MAY be published in the matchingRules attribute.

If the server supports the extensibleMatch filter, then the server MAY use the matchingRuleUse attribute to indicate the applicability (in an extensibleMatch filter) of selected matching rules to nominated attribute types.