How is LDAP related to X.500 and DAP, and why is it called "lightweight"?
X.500 is the ITU-T family of directory standards, including the Directory Access Protocol (DAP); LDAP took over X.500's data model but runs directly over TCP/IP with a much simpler encoding, which is what makes it lightweight.
X.500 (from the telecommunication standardisation sector of the International Telecommunication Union, ITU-T) defined a complete global directory: the information model (entries, attributes, a tree of names) and the protocols to access it, among them DAP. DAP ran on the full OSI protocol stack, which was heavy to implement on the desktop computers of the early 1990s.
LDAP was designed as a gateway-friendly subset:
- It keeps the X.500 information model: the tree, distinguished names, object classes and attribute names such as
cn,ou,oandc. - It runs directly over TCP/IP instead of the OSI stack.
- It simplifies the operations and the encoding.
That is why LDAP names look like X.500 names (cn=John Doe, ou=users, o=acme): the naming scheme is inherited, only the access protocol was slimmed down. X.500 survives most visibly in X.509 certificates, whose subject names use the same syntax.
Go deeper:
X.500 (Wikipedia) — the protocols (DAP, DSP, ...) and data model LDAP inherited.- LDAP history (Wikipedia) — how LDAP started as a lightweight front end to X.500 directories.