To fully understand the new architecture of OAB v4, it is important to understand the underlying reasons for the changes to the offline address book. This section covers the main support issues currently encountered by Exchange customers working with OAB v2 or v3a.

The biggest issue confronting administrators is the preponderance of unexpected full offline address book downloads in pre-OAB v4 environments. There are a variety of root causes for an unexpected full offline address book download, but the most prevalent causes in OAB v2 and v3a include:

• Changes in Active Directory® directory service.
  This, in itself, has several underlying sub-causes, including, but not limited to, the following two items:
  
  • Addition or removal of parent distinguished names.
    The differential download mechanism cannot accommodate a new parent distinguished name in Active Directory, and the Exchange OAB Generation (OABGen) process does not generate a differential offline address book on the day after the parent distinguished names change.
    
  • Differential offline address book file larger than one-eighth the size of the full offline address book.
    The offline address book differential file contains an entire user record even if the changes to the record are small. For example, if the only change is the office location, the entire record including the certificates is added to the differential offline address book. Therefore, clients can hit the one-eighth rule with relatively small changes in Active Directory.
    
• Failure to generate the differential offline address book update.
  Outlook always downloads a full offline address book whose sequence number is higher than the sequence number stored on the client whenever a differential update cannot be located with a higher sequence number than the client.
  
• Template changes.
  Outlook clients update their offline address book templates only when they download a full offline address book. Unless an administrator is willing to wait until all clients eventually download a full offline address book, they have to trigger a company-wide full OAB download to make the template changes effective in a timely fashion.
  

Any one of these problems can cause a full offline address book download on every Outlook client that runs in cached mode in the Exchange organization, flooding the network in the process.

OAB v4 is a new addition to the offline address book infrastructure. You must have Exchange Server 2003 Service Pack 2 installed on your computers that run Exchange Server if you are to use the new features. For Outlook 2003 clients to use these new OAB v4 features, they must be running Outlook 2003 SP2.

For the most part, only the format of the offline address book data has changed in OAB v4. Exchange System Attendant is still responsible for generating the offline address book by calling Oabgen.dll and the Exchange Store is still responsible for storing the offline address book system folders.

By default, every client request for a full offline address book download is served immediately, and the public store does not limit the number of concurrent full offline address book downloads that can occur.

Because of this architecture for full offline address book downloads, Exchange Server 2003 SP1 introduced the ability to throttle full offline address book downloads. When you limit the bandwidth that is used by full offline address book downloads, this feature may extend the time elapsed until all clients receive their updated full offline address book data. Therefore, you should only use this feature if you have a very large offline address book and you must protect your LANs from overloading.

For more information about the full offline address book throttling mechanism, see Microsoft Knowledge Base article 867623, Throttling full offline Address Book downloads to limit the effect on a LAN in Exchange Server 2003.

The throttling mechanism included with Exchange Server 2003 SP1 is only mentioned in this bulletin because of the client-side response changes in Outlook 2003 SP2. This feature in Exchange Server 2003 SP1 does not rely on either the server or client using OAB v4.

OAB v4 files generated by Exchange Server 2003 SP2 will be up to 33 percent smaller than comparable files generated by earlier versions of Exchange Server. This file size reduction is primarily due to three architectural changes in the offline address book files.

• LZX compression method used
  Because of the use of BinPatch technology in OAB v4, the use of the LZX compression method is now used for offline address book files. The BinPatch technology is also known as Binary Delta Compression and is the same one used by Windows Update.
  For additional information about Binary Delta Compression technology, see Using Binary Delta Compression (BDC) Technology to Update Windows Operating Systems.
  
• Indexes generated by the Outlook client
  As a result of using BinPatch technology, the responsibility of generating ambiguous name resolution (ANR) and Browse indexes for OAB v4 belongs to the Outlook client. This reduces the number of attachments stored in the offline address book public folder posts.
  
• Limiting property sizes
  The OAB v4 generation code in Exchange Server 2003 SP2 truncates overly long strings, removes overly long binary values, and limits the number of values in a multiple-valued attribute.
  If a single-valued string attribute exceeds the size limit, the string is truncated at the limit. If a binary attribute exceeds the size limit, it is not included in the offline address book. For multiple-valued attributes, if one value exceeds the size limit, all the values are removed. In Outlook 2003 SP2, the client will display nothing for removed attributes.
  The default OAB v4 attribute size limit can be configured through registry modifications on the computer that runs Exchange Server 2003 SP2.
  

As in earlier versions of the offline address book, a full offline address book and a differential offline address book are generated every morning, by default, at 05:00 local server time. The difference in the OAB v4 files is that they are generated using BinPatch technology.

In contrast to pre-OAB v4 versions, the server checks to see if there have been any changes since the last generated full offline address book. If no changes occurred, the following two actions take place during the offline address book generation process:

• The full offline address book is not generated. Instead, the existing full offline address book has a couple of properties updated on it to keep the server from deleting it prematurely.
  
• The differential offline address book from the day before is left untouched.
  

Because the sequence numbers in this scenario do not change, this feature ensures clients do not download offline address book data that they do not need.

As in pre-OAB v4 versions, the offline address book public folder contains only one post for the full offline address book. Each offline address book post has the following file attachments:

• Data.oab
  The Data.oab file contains all of the offline address book recipient data. In contrast to pre-OAB v4 versions, the Data.oab file is the only attachment containing compressed recipient data. There are no attachments for indexes into that data.
  
• LngXXXX.oab (multiple files)
  
• MacXXXX.oab (multiple files)
  In the “Lng” and “Mac” file attachments, XXXX is the hexadecimal language locale ID. These files contain the addressing and display template information.
  

As in pre-OAB v4 versions, the offline address book public folder contains one post for each of the last 30 days’ differential offline address book files (unless there are no offline address book changes and an offline address book is not built on a particular day). However, the post for the OAB v4 differential file has the following new characteristics:

• The attachment file containing the differential update is called Binpatch.oab.
  The Binpatch.oab file is a delta of the current day’s Data.oab from the previous day’s Data.oab file. It is unaware of the properties or records that were changed.
  
• If the Address Book templates have changed, the server will add LngXXXX.oab and MacXXXX.oab file attachments.
  The presence of these templates signals the client that it needs to update its local templates. The client then downloads the updated templates from the post as an extra step in the differential offline address book download.
  Placing the updated templates in the differential offline address book download is another method of minimizing full offline address book downloads. In pre-OAB v.4 versions of the offline address book, anytime the offline address book templates change on the server, you had to initiate a full offline address book download so that the clients would receive the updated templates.
  

  Note:
  In the case where there are absolutely no changes in Active Directory, but there is a change in the template, the server generates a Binpatch.oab file containing no changes in addition to the new template. Even though there are no changes to the actual offline address book data, Outlook performs a differential download and then regenerates the indexes using the new template.

After the SP2 update is installed on the computers that run both Office 2003 and Exchange Server 2003, Outlook will automatically download the full OAB v4.

If you work with OAB v4, Outlook will automatically fall back to OAB v3a if OAB v4 is missing on the server and to OAB v2 if both OAB v4 and OAB v3a are not available. You can control this behavior by editing the registry data on the Outlook client.

Because of the new BinPatch technology used with OAB v4, it is important to understand how Outlook decompresses and extracts the offline address book files after they are downloaded from Exchange Server.

• Full offline address book downloads
  During the offline address book download, Outlook decompresses the Data.oab and Lngxxxx.oab files. Then it creates the following offline address book files in the same folder as the .oab file location (the default folder is %userprofile%\Local Settings\Application Data\Outlook folder) configured on the client.
  
  • Details.tmp (created from Data.oab)
    
  • Browse.tmp (generated from details.tmp)
    
  • Anrdex.tmp (generated from details.tmp)
    
  • Rdndex.tmp (generated from details.tmp)
    
  • Pdndex.tmp (generated from details.tmp)
    
  • Tmplts.tmp (created from Lngxxxx.oab file)
    
  After the download completes, Outlook renames these files to the corresponding .oab files, overwriting the existing .oab files.
  
• Differential offline address book downloads
  Outlook downloads and decompresses the following files from the server during a differential offline address book download.
  
  • Binpatch.oab (one or more)
    This file is downloaded as the Diff<seq>.tmp file on the client where <seq> represents the sequence number of the differential file in decimal.
    Outlook downloads multiple Binpatch.oab files from the server together and applies the changes to the Udetails.oab file on the client chunk by chunk. The chunking is an optimization to make the client experience better.
    
  • Lngxxxx.oab
    This file is converted into Utmplts.oab on the client side. Outlook downloads this template if it is present in the differential post on the server.
    
  Outlook creates the following .tmp files on the client before writing them into the corresponding .oab files.
  
  • Details.tmp (merged file from all Diff<seq>.tmp files and the existing Udetails.oab file)
    
  • Browse.tmp (re-generated from Details.tmp)
    
  • Anrdex.tmp (re-generated from Details.tmp)
    
  • Rdndex.tmp (re-generated from Details.tmp)
    
  • Pdndex.tmp (re-generated on the client side – this can be a dummy file for a single pass or it can contain temporary data for the two pass memory optimization)
    
  • Tmplts.tmp (created from the Lngxxx.oab file)
    
  After the update is completed, Outlook copies the .tmp files to the corresponding .oab files including a “u” prefix.
  
• Building ANR and Browse indexes
  The following files make up the indexes built and used by Outlook for name resolution and name browsing in the offline address book.
  

  Note:
  Each file has some internal changes dealing with the new SP2 architecture but the only file for which there is significant change in SP2 is Updndex.oab.

  • UBrowse.oab
    The UBrowse.oab file holds all the records that can be browsed in the address book, in browse order.
    
  • URDNdex.oab
    The URdndex.oab file is used to open address book entries and to lookup recipients from their Exchange or SMTP addresses.
    
  • UANRdex.oab
    The UANRdex.oab file is used to perform name resolution.
    
  • UPDNdex.oab
    OAB v2 and OAB v3a UPDNdex.oab files served the purpose of holding a portion of the distinguished name strings stored in the Urdndex.oab file. Because the OAB v4 version of the URDNdex.oab file no longer stores strings, the UPDNdex.oab file is no longer necessary in OAB v4 OAB. So, the file acts as a placeholder to ensure the rest of the Outlook code that relies on the presence of this file continues to behave correctly.
    
• Full details vs. no details offline address book downloads
  In OAB v4, there is no longer any support for a no details offline address book. OAB v4 only supports a full details version on the server. Outlook will download the full details offline address book even if a no details offline address book is selected.
  For the end user, this means the selection for the type of download in the Offline Address Book dialog box is available. But, regardless of the selection, a full details offline address book download is always performed.
  
  
  However, Outlook will transition to the no details format in the event of a downgrading (such as missing OAB v4 files on the server) to OAB v3a or OAB v2 if the no details option is selected.
  

Invariably, customers need to modify the registry default settings for the different mechanisms involved in the offline address book download process. This section discusses only the new functionality available with OAB v4. Unless noted, all custom modifications for the OAB v2 or OAB v3a still apply to OAB v4.

This section covers the following configuration settings:

• Full vs. differential offline address book download threshold
  
• Server-side throttling full offline address book downloads (not entirely specific to SP2)
  
• Client-side throttling full offline address book downloads (not specific to SP2)
  
• Random full offline address book request timer (OAB v2, OAB v3a, and OAB v4)
  
• Policy to download only Unicode OAB v4 from the server
  
• Enable Cyclic Redundancy Checks (CRC )
  

In pre-OAB v4 versions of the offline address book, Outlook uses a default threshold of one-eighth to determine when to not perform a differential offline address book download and instead perform a full offline address book download. If you are using Outlook 2003 SP2 and OAB v4, this threshold has been increased to 50 percent.

Outlook calculates a percentage ratio of the total size of all the compressed differential offline address book files on the server to the current total size of the compressed full offline address book on the server. If the percentage ratio exceeds the threshold value, Outlook performs a full download instead of downloading the differentials.

The threshold can be customized using the following client registry settings.

Key:	HKeyCurrentUser\Software\Microsoft\Exchange\Exchange Provider
DWORD:	OAB v4 Full Download Threshold
Values:	0: Default (Same as registry data not present. Outlook uses the default value of 50 percent.) 1-64 (hexadecimal): Any number between 1 and 64 (hexadecimal), which equates to numbers between 1 and 100 (decimal).

In Exchange Server 2003 SP1 and later, you can override the default behavior and limit the bandwidth used by full offline address book downloads from a particular public server. After Exchange Server 2003 SP1, the server maintains a performance counter for full offline address book downloads. When Outlook is about to perform a full offline address book download, Outlook does a ropGetAttachmentTable Remote Procedure Call (RPC) and the server checks the performance counter to see whether the traffic is below the limit specified in the registry key. If the limit is exceeded, the server returns MAPI_E_BUSY.

To customize this threshold, the following registry data is added to the Exchange server storing the offline address book data.

Key:	HKeyLocalMachine\System\CurrentControlSet\Services\MSExchangeIS\ParametersSystem\
DWORD:	OAB Bandwidth Threshold (KBps)
Values:	For example, use 5000 as the value to configure the server to use 5000 kilobytes per second (KBps) as the bandwidth threshold. Note: 5000 KBps is approximately 40,960 kilobits per second (Kbps), or 40.96 megabits per second (Mbps).

After you apply this registry setting, when an Outlook client tries to download a full offline address book, the public store determines the average full offline address book bytes that were downloaded over the previous 10 seconds. Then, one of the following events occurs.

• If the value is less than the registry setting, the client can continue with the full offline address book download at full speed.
  
• If the value is more than the registry setting, the client cannot continue with the full offline address book download. Depending on the client you use, you will get the following client-side behavior.
  

  Resultant action	Outlook 2003 SP1 or earlier	Outlook 2003 SP2 or later
  Error	Microsoft Exchange Server reported error (0x8004010B) : Unknown Error 0x8004010B	The function cannot be performed because the Microsoft Exchange Server computer is busy. Try again later.
  offline address book download retry	One time every hour, until it succeeds	New retry algorithm to distribute the secondary attempts in a more random fashion. Outlook 2003 SP2 retries after a random interval (non-configurable) that varies between 5 minutes and 1 hour.

Outlook 2003 SP2 introduces a new registry policy setting that allows you to disperse full offline address book download requests to randomly start between 1 hour and a specified number of hours (which can span multiple days). If the registry policy is set and a full offline address book download is required, Outlook performs the full offline address book download in a random time between 1 hour and the number of hours specified in the registry.

To implement this feature, use the following registry data.

Key:	HKCU\Software\Microsoft\Exchange\Exchange Provider
DWORD:	Max Full OAB Download Wait
Values:	Integer >=1

The following registry settings ensure Outlook will download only OAB v4 data from the Exchange server.

Key:	HKCU\Software\Microsoft\Exchange\Exchange Provider
DWORD:	OAB v4 Only
Values:	0: Default (Same as registry data not present, or set to any value other than 1) Outlook performs the same fallback logic as it does today for Unicode mode (try to get the highest version of the offline address book that is available currently on the server). 1: Outlook uses only OAB v4, regardless of the other offline address book versions on the server. If no OAB v.4 files are present on the server and this value is set, Outlook does not use an offline address book but instead uses the online global address list (GAL).

OAB v4 Browse and ANR indexing is based upon the locale setting (language and country) of the client and not the server. With OAB v4, the client generates the locale specific indexes for local .oab files regardless of what locales are on the server. These indexes are regenerated in their entirety after each full download or differential download.

The OAB v4 data also provides for improved name resolution and browsing by using the PR_GIVEN_NAME attribute. This attribute is a new property for Outlook 2003 SP2 to help resolve first names in directories where the display name is set to Lastname, Firstname.

Here is one example where this behavior is a problem in non-OAB v4 clients.

User Info:

     Pica

     Pica, Guido

     GPica

     gpica@exchange.contoso.com

     /o=Contoso/ou=APPS/cn=recipients/cn=gpica

In pre-OAB v4 clients, you cannot find this user if you type "Guido” in the To field of a message. If you are using OAB v4, this same text will resolve the name for Pica, Guido.