A user-friendly and administrator-friendly tool
Exchange 2000 Server's Offline Address Book (OAB) is an invaluable tool for road warriors and other users who spend a considerable amount of time disconnected from the network. Surprisingly, administrators don't have to do a lot of work to provide this tool to users. You just need to teach your users how to download the OAB from their clients, and you need to make sure that Exchange 2000 is generating and replicating the OAB to a sufficient number of servers. To accomplish these tasks, you need to know what the OAB contains and the various OAB download options. You also need to know what happens behind the scenes—how Exchange 2000 generates and stores the OAB and what happens when a client requests an OAB download. Finally, you need to know how to set up OAB replication.
What the OAB Contains
The OAB is a snapshot of an Active Directory (AD) store that Messaging API (MAPI) clients (primarily Outlook) download to enable offline validation of email addresses. Users typically don't perceive any practical differences between the OAB and the online Global Address List (GAL), except perhaps that the OAB is a slimmed-down version of the GAL. Both the OAB and the GAL originate from AD. The AD store holds hundreds of attributes for accounts. The OAB doesn't hold all this data—if it did, Outlook clients would have to download a huge file. Instead, the OAB contains only the data users need to address messages offline. If a user opts to have the client download all possible data, the OAB contains the information that appears on the General, Phone/Notes, and E-mail Addresses tabs of the user's Properties dialog box. However, the OAB doesn't contain any data that depends on pointers to other AD objects. For example, the OAB doesn't include data about hidden recipients or data about distribution group membership. Distribution groups are collections of pointers to other user, contact, and group objects. Distribution group membership is probably the feature users miss most when they work with the OAB.
Table 1 lists the six files that form the OAB and describes each file's purpose. These files are from the OAB that I use at Hewlett-Packard (HP). The 190,638KB of data in these six files represents about 275,000 entries (i.e., mail-enabled accounts, contacts, and groups) in the HP GAL. Outlook clients download compressed versions of the OAB files to minimize network demand. Typically, the compressed files are roughly half the size of the files on disk, but even so, fetching this data can take a long time if the OAB is large or if clients download the OAB across a slow dial-up connection.
Within the OAB, Exchange 2000 (as well as all other versions of Exchange) use distinguished names (DNs) in legacy format (e.g., /O=HP/OU=Central/CN=Recipients/CN=Tredmond) rather than AD format to identify objects. Exchange writes the DNs into message headers after it validates the addresses. Using DNs in this way might seem strange, but this approach allows the display names of users, contacts, and groups to change without rendering OAB entries invalid. Using the legacy format might also seem strange, but this format lets Microsoft maintain backward compatibility not only in the OAB but also in MAPI profiles. This format also lets users reply to messages sent from older servers.
The legacyExchangeDN attribute makes this system possible. This attribute stores the legacy-format DNs for AD objects and participates in address validation through the name-resolution process. Therefore, when an Outlook client reconnects after creating offline messages, Exchange 2000 uses the legacyExchangeDN attribute to revalidate the addresses. However, if you've never used Exchange Server 5.5 (or earlier) in your Exchange organization, Exchange 2000 doesn't use the legacyExchangeDN attribute. Instead, Exchange 2000 uses the SMTP address format, which is the default format for Exchange 2000 and later.
The OAB Download Options
To access the OAB download options in Outlook XP, users simply need to select Send/Receive on the Tools menu, then click Download Address Book. (In Outlook 2000, users will find the Download Address Book option under Synchronize on the Tools menu.) Figure 1 shows an example of the dialog box that appears. In the Information to download section, users can select the amount of data they want to download. If users want all possible information, they need to select Full Details. If users want to minimize the amount of data downloaded, they need to select No Details. The No Details option, however, makes the OAB much less useful. Although users can still validate email addresses, other data—such as the X.509 certificates that contain public keys for users registered for advanced security—is unavailable.
Unless an OAB is corrupted or hasn't been updated in a long time, users typically don't need to download the complete OAB. Instead, they can select the Download changes since the last Send/Receive check box. When a user selects this option, the Outlook client downloads a smaller file that contains the updates, then applies those updates to the local OAB files. However, the Outlook client always downloads a full copy of the OAB if changes have been made to more than 6 percent (approximately) of the total directory entries since the most recent full download.
Outlook clients must use an OAB version that's compatible with the server that holds the users' mailboxes. So, for example, if Mary's mailbox is on an Exchange 2000 server and Larry's mailbox is on an Exchange 5.5 server, the Outlook client automatically downloads the Exchange 2000 OAB for Mary and the Exchange 5.5 OAB for Larry.
Outlook XP introduces Send/Receive groups, which let users define how they want to synchronize their offline folders with Exchange. When users create these definitions, they can specify whether they want the OAB downloaded. The default behavior is for Outlook XP to download a new copy of the OAB each time synchronization occurs. Although this default behavior ensures that the OAB is up-to-date, synchronization takes a lot longer. Thus, if your users have Outlook XP, have them review the default Send/Receive group settings to make sure those settings are what they want. I like to change the default functionality by making the OAB download an explicit action, so I define a special Send/Receive group that includes the OAB. An alternative approach is to periodically update the OAB manually by selecting Tools, Send/Receive, Download Address Book.
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