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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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12.4.1 LDAP: Lightweight access to X.500<br />

The OSI directory st<strong>and</strong>ard, X.500, specifies that communication between the<br />

directory client <strong>and</strong> the directory server uses the Directory Access Protocol<br />

(DAP). However, as an application layer protocol, DAP requires the entire OSI<br />

protocol stack to operate, which requires more resources than are available in<br />

many small environments. Therefore, an interface to an X.500 directory server<br />

using a less resource-intensive or lightweight protocol was desired.<br />

LDAP was developed as a lightweight alternative to DAP, because it requires<br />

the more popular <strong>TCP</strong>/<strong>IP</strong> protocol stack rather than the OSI protocol stack. LDAP<br />

also simplifies some X.500 operations <strong>and</strong> omits some esoteric features. Two<br />

precursors to LDAP appeared as RFCs issued by the IETF, RFC 1202 –<br />

Directory Assistance Service <strong>and</strong> RFC 1249 – DIXIE Protocol Specification.<br />

These were both informational RFCs which were not proposed as st<strong>and</strong>ards.<br />

The directory assistance service (DAS) defined a method by which a directory<br />

client communicates to a proxy on an OSI-capable host, which issues X.500<br />

requests on the client's behalf. DIXIE is similar to DAS, but provides a more<br />

direct translation of the DAP.<br />

The first version of LDAP was defined in RFC 1487 – X.500 Lightweight Access,<br />

which was replaced by RFC 1777 – Lightweight Directory Access Protocol. Much<br />

of the work on DIXIE <strong>and</strong> LDAP was carried out at the University of Michigan,<br />

which provides reference implementations of LDAP <strong>and</strong> maintains LDAP-related<br />

Web pages <strong>and</strong> mailing lists. Since then, LDAPv2 has been replaced by LDAP<br />

Version 3. LDAPv3 is summarized in RFC 4510, but the technical specifications<br />

are divided into multiple subsequent RFCs listed in Table 12-4.<br />

Table 12-4 LDAP-related RFCs<br />

RFC number Content<br />

4510 <strong>Technical</strong> Specification Road Map<br />

4511 The Protocol<br />

4512 Directory Information Models<br />

4513 Authentication Methods <strong>and</strong> Security Mechanisms<br />

4514 String Representation of Distinguished Names<br />

4515 String Representation of Search Filters<br />

4516 Uniform Resource Locator<br />

4517 Syntaxes <strong>and</strong> Matching Rules<br />

4518 Internationalized String Preparation<br />

460 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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