Apple Mac OS X Server (Administrator’s Guide) - Manual (Page 73)

Directory Services 73
In fact, Open Directory can provide information about network services both from service
discovery protocols and from directory domains. To accomplish this, Open Directory simply
asks all its sources of information for the type of information requested by a Mac OS X
process. The sources that have the requested type of information provide it to Open
Directory, which collects all the provided information and hands it over to the Mac OS X
process that requested it.
For example, if Open Directory requests information about file servers, the file servers on the
network respond via service discovery protocols with their information. A directory domain
that contains relatively static information about some file servers also responds to the
request. Open Directory collects the information from the service discovery protocols and
the directory domains.
When Open Directory requests information about a user, service discovery protocols don’t
respond because they don’t have user information. (Theoretically, AppleTalk, Rendezvous,
SMB, and SLP could provide user information, but in practice they don’t have any user
information to provide.) The user information that Open Directory collects comes from
whatever sources have it—from directory domains.
Directory Domain Protocols
Administrative data needed by directory services is stored on Mac OS X Servers in Open
Directory databases. An Open Directory database is one type of directory domain. Open
Directory can use either of two protocols to store and retrieve directory data:
Directory
services File server
File server
Directory
domain