The best place to begin when explaining LDAP is to examine how it got its name. Let's start at the beginning. The latest incarnation of LDAP (Version 3) is defined in a set of nine documents outlined in RFC 3377. This list includes:
The original core set of LDAPv3 RFCs
"Authentication Methods for LDAP"
"Lightweight Directory Access Protocol (v3): Extension for Transport Layer Security"
"Lightweight Directory Access Protocol (v3): Technical Specification"
Why is LDAP considered lightweight? Lightweight compared to what? (As we look at LDAP in more detail, you'll certainly be asking how something this complex could ever be considered lightweight.) To answer these questions, it is necessary to look at LDAP's origins. The roots of LDAP are closely tied to the X.500 directory service; LDAP was originally designed as a lighter desktop protocol used to gateway requests to X.500 servers. X.500 is actually a set of standards; anything approaching thorough coverage of X.500 is beyond the scope of this book.
 Understanding X.500?The Directory, by David W. Chadwick, provides a good explanation of X.500 directories. While the book itself is out of print, an HTML version of it can be accessed from http://www.salford.ac.uk/its024/X500.htm.
X.500 earned the title "heavyweight." It required the client and server to communicate using the Open Systems Interface (OSI) protocol stack. This seven-layered stack was a good academic exercise in designing a network protocol suite, but when compared to the TCP/IP protocol suite, it is akin to traveling the European train system with four fully loaded footlockers.
 For a quick, general comparison of the OSI model and the TCP/IP protocol stack, see Computer Networks, by Andrew S. Tannenbaum (Prentice Hall).
LDAP is lightweight in comparison because it uses low overhead messages that are mapped directly onto the TCP layer (port 389 is the default) of the TCP/IP protocol stack. Because X.500 was an application layer protocol (in terms of the OSI model), it carried far more baggage, as network headers were wrapped around the packet at each layer before it was finally transmitted on the network (see Figure 1-1).
 A connectionless version of LDAP that provided access via UDP was defined by an Internet-Draft produced by the LDAP Extension Working Group of the IETF. However, the current draft expired in November, 2001. You can access the group's web site at http://www.ietf.org/html.charters/ldapext-charter.html.
LDAP is also considered lightweight because it omits many X.500 operations that are rarely used. LDAPv3 has only nine core operations and provides a simpler model for programmers and administrators. Providing a smaller and simpler set of operations allows developers to focus on the semantics of their programs without having to understand rarely used features of the protocol. In this way, LDAP designers hoped to increase adoption by providing easier application development.
Network directory services are nothing new; we're all familiar with the rise of DNS. However, a directory service is often confused with a database. It is easy to understand why. Directory services and databases share a number of important characteristics, such as fast searches and an extendable schema. They differ in that a directory is designed to be read much more than it is written; in contrast, a database assumes that read and write operations occur with roughly the same frequency. The assumption that a directory is read often but written rarely means that certain features that are essential to a database, such as support for transactions and write locks, are not essential for a directory service such as LDAP.
At this point, it's important to make the distinction between LDAP and the backend used to store the persistent data. Remember that LDAP is just a protocol; we'll discuss what that means shortly, but essentially, it's a set of messages for accessing certain kinds of data. The protocol doesn't say anything about where the data is stored. A software vendor implementing an LDAP server is free to use whatever backend it desires, ranging from flat text files on one extreme to highly scalable, indexed relational databases on the other. So when I say that LDAP doesn't have support for transactions and other features of databases, I mean that the protocol doesn't have the messages that you would need to take advantage of these features (remember, it's lightweight) and doesn't require that the backend data store provide these features.
The point is that the client will never (and should never) see or even know about the backend storage mechanism (see Figure 1-2). For this reason, LDAP-compliant clients written by vendor A should interoperate with an LDAP-compliant server written by vendor Z. Standards can be a wonderful thing when followed.
It has been suggested that an LDAP server could be used as backend storage for a web server. All HTML and graphic files would be stored within the directory and could be queried by mutiple web servers. After all, a web server typically only reads files and sends them to clients; the files themselves change infrequently. While it's certainly possible to implement a web server that uses LDAP to access its backend storage, a special type of directory already exists that is better suited to meet the needs of serving files, namely a filesystem. So, for example, while an LDAP directory might not be a good location for storing spooled files in transit to a printer, using it to store printer configuration settings (e.g., /etc/printcap) shared among clients would be a big win.
This brings up two good points about the intended function of LDAP:
LDAP is not a generalized replacement for specialized directories such as filesystems or DNS.
While storing certain types of binary information (e.g., JPEG photos) in directories can be useful, LDAP is not intended for storing arbitrary "blobs" (Binary Lumps of Bits).
What about storing individual application settings for roaming users on an LDAP server? It is a judgment call whether this is better served by a filesystem or a directory. For example, it is possible to store basic application settings for Netscape Communicator in LDAP. Such things as an address book, a bookmarks file, and personal preference settings are certainly appropriate for storage in a directory. However, using your directory as a location for browser cache files would violate rule #2.
All of this talk of directory services makes it is easy to forget that LDAP is a protocol. It is not uncommon to hear someone refer to an LDAP server or LDAP tree. I have done so and will continue to do so. LDAP does provide a treelike view of data, and it is this treelike view to which people refer when speaking of an LDAP server.
This introduction won't go into the specifics of the actual protocol. It is enough to think of LDAP as the message-based, client/server protocol defined in RFC 2251. LDAP is asynchronous (although many development kits provide both blocking and nonblocking APIs), meaning that a client may issue multiple requests and that responses to those requests may arrive in an order different from that in which they were issued. Notice in Figure 1-3 that the client sends Requests 1 and 2 prior to receiving a response, and the response to Request 3 is returned before the response to Request 2.
More aspects of programming with LDAP operations will be covered in Chapter 10.