Ability to describe some of the background leading to the development of LDAP and X.500.
Try This: Skim through a couple of the RFCs listed below. Try to get a feel for the evolution of LDAP.
RFC 1202 RFC 1248 RFC 1274 RFC 1275 RFC 1276 RFC 1277 RFC 1278 RFC 1279 RFC 1484 RFC 1485 RFC 1486 RFC 1487 RFC 1488 RFC 1777 RFC 1778 RFC 1779 RFC 1823 RFC 1959 RFC 1960 RFC 2251 RFC 2252 RFC 2254 RFC 2255 RFC 2256
Factoid: "DAS" (Directory Assistance Service) and "Dixie" were precursors to LDAP. DAS is described in RFC 1202; Dixie is described in RFC 1249.
| # Historical Background
### X.500 The first widely recognized directory service was the X.500 Directory Access Protocol. This was a very powerful, all-purpose directory service that incorporated many good ideas about what a directory service should be able to do. X.500 organizes directory entries in a hierarchical name space capable of supporting large amounts of information. It also defines powerful search capabilities to make retrieving information easier. Because of its functionality and scalability, X.500 is often used together with add-on modules for interoperability between incompatible directory services. Unfortunately, X.500 proved to be ill-suited for deployment on the Internet. It was difficult to implement and was too complicated for many of the simpler tasks that seemed like logical candidates for directory services. X.500 specifies that communication between the directory client and the directory server uses the directory access protocol (DAP). However, as an application layer protocol, the DAP requires the entire OSI protocol stack to operate. Supporting the OSI protocol stack requires more resources than are available in many small environments. Therefore, an interface to an X.500 directory server using a less resource intensive or lightweight protocol was desired. X.500 is described in RFCs 1274 through 1279.
### Early LDAP The complexity of X.500 lead to the development of a simplified front-end designed to hide some of its uglier features. This work resulted in the Lightweight Access Protocol described in RFCs 1484 through 1488. This protocol depended on an X.500 server for all real directory processing and merely provided a more tractable interface for programmers and directory-enabled applications. The Directory Access Protocol was eventually replaced by the Lightweight Directory Access Protocol (LDAP v1) which was first formally described in RFC 1777. Developed at the University of Michigan at Ann Arbor in conjunction with the Internet Engineering Task Force, LDAP is an Internet standard directory service that runs over TCP/IP. LDAP databases can be distributed across multiple platforms to increase availability to directory data. The LDAP directory service is based on a client-server model. One or more LDAP servers contain the data that make up the LDAP directory tree. An LDAP client connects to an LDAP server and submits a query regarding the directory data. The server responds with the answer, or with a referral to another LDAP server where the client can possibly have the query serviced. LDAP was designed to provide 90% of the functionality of X.500 at 10% of the cost in terms of complexity and overhead. It can be thought of as "fat-free X.500".
### Modern LDAP LDAP was accepted much more readily by the computer industry than X.500. Its dependence on an underlying X.500 server was eventually replaced by full stand-alone capabilities. LDAP version 2 offered a more complete programming environment and was much easier to integrate with existing applications. It was at this point that LDAP started to be recognized as a viable network-based service that justified the cost of its implementation. LDAP v2 is described in RFC 1823. Multiple LDAP servers can contain identical directory entries using a mechanism called replication. If you configure your LDAP servers to use replication, one and only one LDAP server is responsible for modifications to the directory. This server is known as the supplier server. Other LDAP servers that receive replicated directory data are known as consumer servers. All consumer servers refer modification requests back to the supplier server. LDAP v3 is described in RFCs 2251 through 2256.

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