3. Syntaxes

Syntax definitions constrain the structure of attribute values stored in an LDAP directory, and determine the representation of attribute and assertion values transferred in the LDAP protocol.

Syntaxes that are required for directory operation, or that are in common use, are specified in this section. Servers SHOULD recognize all the syntaxes listed in this document, but are not required to otherwise support them, and MAY recognise or support other syntaxes. However, the definition of additional arbitrary syntaxes is discouraged since it will hinder interoperability. Client and server implementations typically do not have the ability to dynamically recognize new syntaxes.

3.1. General Considerations

The description of each syntax specifies how attribute or assertion values conforming to the syntax are to be represented when transferred in the LDAP protocol [RFC4511]. This representation is referred to as the LDAP-specific encoding to distinguish it from other methods of encoding attribute values (e.g., the Basic Encoding Rules (BER) encoding [BER] used by X.500 [X.500] directories).

The LDAP-specific encoding of a given attribute syntax always produces octet-aligned values. To the greatest extent possible, encoding rules for LDAP syntaxes should produce character strings that can be displayed with little or no translation by clients implementing LDAP. However, clients MUST NOT assume that the LDAP-specific encoding of a value of an unrecognized syntax is a human-readable character string. There are a few cases (e.g., the JPEG syntax) when it is not reasonable to produce a human-readable representation.

Each LDAP syntax is uniquely identified with an object identifier [ASN.1] represented in the dotted-decimal format (short descriptive names are not defined for syntaxes). These object identifiers are not intended to be displayed to users. The object identifiers for the syntaxes defined in this document are summarized in Appendix A.

A suggested minimum upper bound on the number of characters in an attribute value with a string-based syntax, or the number of octets in a value for all other syntaxes, MAY be indicated by appending the bound inside of curly braces following the syntax's OBJECT IDENTIFIER in an attribute type definition (see the <noidlen> rule in [RFC4512]). Such a bound is not considered part of the syntax identifier.

For example, "1.3.6.1.4.1.1466.115.121.1.15{64}" in an attribute definition suggests that the directory server will allow a value of the attribute to be up to 64 characters long, although it may allow longer character strings. Note that a single character of the Directory String syntax can be encoded in more than one octet, since UTF-8 [RFC3629] is a variable-length encoding. Therefore, a 64-character string may be more than 64 octets in length.

3.2. Common Definitions

The following ABNF rules are used in a number of the syntax definitions in Section 3.3.

PrintableCharacter = ALPHA / DIGIT / SQUOTE / LPAREN / RPAREN /
                     PLUS / COMMA / HYPHEN / DOT / EQUALS /
                     SLASH / COLON / QUESTION / SPACE
PrintableString    = 1*PrintableCharacter
IA5String          = *(%x00-7F)
SLASH              = %x2F  ; forward slash ("/")
COLON              = %x3A  ; colon (":")
QUESTION           = %x3F  ; question mark ("?")

The <ALPHA>, <DIGIT>, <SQUOTE>, <LPAREN>, <RPAREN>, <PLUS>, <COMMA>, <HYPHEN>, <DOT>, <EQUALS>, and <SPACE> rules are defined in [RFC4512].