All About OAB v4

Two recent updates--Microsoft Office 2003 Service Pack 2 (SP2) and Exchange Server 2003 SP2--together deliver a new version of the Offline Address Book (OAB). The OAB is a crucial component for Microsoft Office Outlook 2003's Cached Exchange Mode, in which the default behavior is for Outlook to perform most of its address operations against the local OAB rather than making a round trip to the server.

OAB version 4 (v4) provides three major benefits. Administrators and users alike will be happy to see fewer full downloads of the OAB. Both full and partial OAB download files will be smaller. And users will be able to use first names for address resolution when the Global Address List (GAL) is arranged in Last Name, First Name order (a much requested enhancement).

To reduce the size of OAB downloads and the frequency of full downloads, Microsoft has changed the way Exchange 2003 generates the OAB. The OAB generation process now produces two OAB v4 files: a data.oab file that contains the full OAB, and a binpatch.oab file that contains the changes from the previous data.oab file (generally the data.oab file from the previous day). The files use LZX compression and the same Binary Delta Compression (BDC) technology that Windows Update downloads use. Microsoft says you can expect full OAB download sizes to be 30 to 40 percent smaller with OAB v4.

On the client, the first time Outlook 2003 SP2 starts and detects that the server hosting the OAB associated with the user's mailbox store is running Exchange 2003 SP2, Outlook will initiate a full OAB v4 download. (If Office 2003 SP2 has been applied but the Exchange 2003 server isn't running SP2, Outlook will stick with OAB v3.) Outlook then will extract the individual .oab files from the data.oab file download.

Subsequently, Outlook will download only the binpatch.oab files and use them to update its local .oab files. If the user has been offline for several days, Outlook will download the binpatch.oab files for all those days and use them to bring the local OAB up to date. Outlook will download the most recent full data.oab file only when one or more binpatch.oab files since the last data.oab download are missing or when the user hasn't updated the OAB for more than a month. Before Outlook 2003 SP2, Outlook's default behavior was to download the full OAB whenever the size difference of the files was greater than one-eighth of the full OAB. Outlook 2003 SP2 increases that limit by a factor of four; SP2 will continue to use the binpatch updates unless they total more than one-half the size of the full OAB.

Another reason the OAB download files are smaller is that they no longer include indexes. OAB v4 moves the index-generation process from the Exchange server to the Outlook client. After each full or binpatch download, Outlook builds the indexes for browsing the OAB and performing address resolution. One side benefit of building the index on the client is that users in multilanguage organizations can search the address book by using an index sorted for their language locale rather than the server's locale.

Yet another factor that controls the size of OAB downloads is that Exchange can be set with registry values to truncate string and binary properties that exceed a certain limit. The default limits are as follows:
- Single string property: 3400 bytes
- Single binary property: 32kb
- Multivalued string property: 64kb
- Multivalued binary property: 64kb

The key properties that Outlook needs to resolve addresses aren't subject to size limits or truncation.

Note that OAB v4, like OAB v3, generates only Unicode files. To use Version 4, the Outlook client must be using Cached Exchange Mode with a Unicode .ost file (which Outlook will create automatically for an Exchange 2003 mailbox).

By coordinating SP2 releases for both Outlook 2003 and Exchange 2003, Microsoft has made it possible to relieve the burden on servers that were seeing heavy OAB downloads and to improve OAB functionality for Outlook users. Performing the differential update and generating the indexes does require some processing resources on the client, of course. While Microsoft says that beta users didn't see any performance hit, you'll want to watch out for that possibility--especially if your organization has a very large OAB. Also be aware of one other side effect of this new OAB generation method: Outlook dialog boxes still show a "no details" download option for the OAB, but that option no longer exists in OAB v4.

OAB Version 4 in Exchange Server 2003 Service Pack 2 http://www.microsoft.com/technet/prodtechnol/exchange/2003/insider/oab4.mspx

How Outlook 2003 SP2 and Exchange Server 2003 SP2 OAB Version 4 Work Together http://www.microsoft.com/technet/prodtechnol/exchange/2003/insider/outlook-oab.mspx

Offline Address Book Best Practices Guide http://www.microsoft.com/technet/prodtechnol/exchange/guides/offlineaddressguide/fcfba91c-d56d-43bd-a7b7-69949950f430.mspx