In Exchange Server 2003, every request for a full offline address book download is served immediately. For example, if a public folder serving 10,000 users gets 1,000 requests in one hour, and the offline address book size is 5 MB, the server will be transmitting 5 GB of data. Depending on the speed of network links and the amount of available bandwidth, such a volume of traffic could potentially overload the network for an extended period.

To prevent the overload of an Exchange server's network adapter or the network to which it is attached, Exchange Server 2003 with SP1 provides a throttling mechanism that allows administrators to limit the network bandwidth used by offline address book downloads by setting a bandwidth threshold.

By default, this throttling feature is turned off. You can activate the feature by adding the following entry to the registry on all public folder servers that host offline address book system folders:

HKLM\System\CurrentControlSet\Services\MSExchangeIS\ParametersSystem

Type: DWORD

Value: OAB Bandwidth Threshold (KBps)

Value Data: bandwidth threshold setting (Range: 0 to 4194304 (decimal))

As you can see from its name, the bandwidth threshold setting is in kilobytes per second (KBps) and should be configured with a decimal value. For example, setting the registry key to a decimal value of 5,000 configures the public folder server to use 5,000 KBps as the bandwidth threshold for offline address book downloads, which is approximately 40,960 kilobits per second (Kbps), or 40.96 megabits per second (Mbps). After the setting has been added and configured, Exchange will dynamically detect the registry entry and begin enforcing the bandwidth limit without requiring an Microsoft Exchange Information Store service restart.

Each time an offline address book download request occurs, administrative rights on the Exchange server are verified for the requestor. If the security context used for the request is the equivalent of local administrator on the Exchange Server computer, it is assumed that an internal function is requesting the download. In this event, the requestor is allowed to proceed with a full download. However, the bytes transmitted to the administrative client are still calculated as part of the average full offline address book bytes downloaded. If the requestor does not have administrative rights, the average full offline address book bytes downloaded over the last ten seconds is determined. If this value is less than the configured threshold, a full download is allowed.

When setting the offline address book download bandwidth threshold, it is recommended that you configure thresholds on the individual servers to values that will not cause an overload of the Exchange server's network adapter or the network. If you have not already gathered and analyzed network and Exchange server performance data, you should do so before you configure the registry entry.

It is also recommended that you monitor the following offline address book performance objects:

• MSExchangeIS | OAB: Full downloads bytes/sec   This performance object can be monitored at times when offline address book downloads are overloading the network. This information allows you to determine how much bandwidth is used in that situation for each public folder server. After you have determined how much bandwidth is being used, you should configure the bandwidth threshold to 60 percent of that value.
  
• MSExchangeIS | OAB: Full download attempts blocked   This performance object allows you to determine how many full download attempts have been blocked because the bandwidth threshold setting has been exceeded. This counter will increment by one for each blocked attempt.
  

Before you implement offline address book download throttling, be aware of the following:

• This feature does not decrease the overall number of downloaded bytes.
  
• This feature throttles full offline address books for the entire server. Administrators of Exchange servers with multihomed networks will need to take this into consideration when planning to use this feature because the throttling will occur against clients on both network segments.
  
• By limiting the bandwidth used by full offline address book downloads, this feature may extend the time it takes for all of the clients to get their updated full offline address books. For this reason, this feature should be used only by customers who need to protect their networks from overloading. Moreover, the threshold value should be set as high as possible.
  
• To revert to the default behavior, delete the registry value.
  

After you apply this registry setting, when an Outlook client tries to download a full offline address book, the public folder store determines the average full offline address book bytes that were downloaded over the previous ten seconds. One of the following two behaviors will occur:

• If the value is less than the configured threshold, the client can continue with the full download.
  
• If the value is more than the configured threshold, the client cannot continue with the full offline address book download, the performance object is incremented by one, and the Outlook client reports the following error message in the Outlook synchronization folder: 'Microsoft Exchange Server' reported error (0x8004010B): 'The function cannot be performed because the Microsoft Exchange Server computer is busy. Try again.' Thereafter, the Outlook client will try to download the full offline address book again, one time every hour, until it succeeds.
  

For detailed steps about how to configure this setting, see Microsoft Knowledge Base article 867623 "Throttling full offline Address Book downloads to limit the effect on a LAN in Exchange Server 2003."

Prior to Service Pack 1 for Exchange Server 2003, company-wide full downloads of the offline address book could occur because of Simple Mail Transfer Protocol (SMTP) address mismatches. In an SMTP address problem, a differential update is created, but Outlook cannot correctly parse the update.

All mail-enabled objects (mailbox, contact, and distribution group) in your organization must have a mail attribute and a proxyAddresses attribute. The proxyAddresses attribute must contain a primary SMTP proxy with the same alias as the mail attribute. For example, if the mail attribute is someone@contoso.com, SMTP:someone@contoso.com must reside in the proxyAddresses attribute. A valid primary SMTP proxy must be in the form "SMTP:<alias>@<domain.com>" (not "smtp:"), where alias is the user's e-mail alias and domain.com is the company's domain name.

Other factors to consider with respect to SMTP addresses include:

• The maximum length of an SMTP relative distinguished name (the portion followed by the at sign (@)) is 63 characters.
  
• The alias and the domain name must not begin with any of these characters: space, !"#$%&`()*+,-./ or any unprintable character. Domain names typically begin with a letter or number, but mistyping a space at the beginning is a common error.
  
• The SMTP problem as it relates to offline address book downloads affects differential updates only. The user's Active Directory® directory service entry must be changed to appear in the differential update file. It is possible for the entry to appear in the differential update (as if a change had been made) if there exists another user in your organization who has the same displayName attribute in Active Directory.
  

In Exchange Server 2003 with SP1, SMTP address mismatches no longer create the need for full offline address book downloads because the users with the mismatched addresses are not added to the offline address book. Instead, when such a user is encountered, the following event is generated in the Application event log of the offline address book server.

Event Source:	MSExchangeSA
Event ID:	9325
Event Type:	Error
Description:	OALGen will skip user entry %1 in address list %2 because the SMTP address %3 is invalid.

To determine whether to add a user to an offline address book, OABGen performs several validation tasks to determine if the SMTP proxy addresses for each user are correct and not malformed or mismatched. If any of the validation tasks fail, the user is not added to the offline address book, and the earlier event log entry is logged.

As a best practice, it is recommended that you regularly monitor offline address book downloads. This includes a periodic review of offline address book event log entries. Prior to Exchange Server 2003 with SP1, offline address books typically recorded successful informational events. Failed events and internal exceptions were not logged, which in some cases made troubleshooting offline address book problems difficult. To increase the usefulness of offline address book event logging, Exchange Server 2003 SP1 includes several changes. For example:

• Exchange logs all offline address book generation errors, even when diagnostic logging is set to None.
  
• Exchange logs events for conditions that force a full offline address book download.
  
• Exchange logs exceptions that are returned to the offline address book subsystem by API programs such as file system, directory, public folder, memory, template issues or template generation exceptions, and added or removed parent distinguished names that force full offline address book downloads.
  
• Exchange logs error codes that are returned from API programs in the description section of the event.
  
• Exchange logs common failures such as missing public folder replicas, out-of-disk-space conditions, and empty address lists.
  
• Exchange adds additional information to each event that shows which offline address book collection failed and which address list was being processed.
  
• Exchange no longer records some of the less useful events that were logged prior to Exchange Server 2003 Service Pack 1.
  

If characters that are in the offline address book configuration object name or in the offline address list name cannot be mapped to the Microsoft Windows Server™ 2003-based server code page, these characters are replaced with a question mark (?) when the event is recorded. This behavior may occur with a character from another language that does not map to the installed code page on the Exchange Server computer. For information about how to resolve this behavior, see Microsoft Knowledge Base article 842126, "Offline Address Book monitoring enhancements in Exchange Server 2003 Service Pack 1."

Outlook 2003 includes several enhancements to offline address books, including support for offline address book v3(a), support for Unicode, and updated group policy settings that enable an administrator to control the offline address book experience.

 New Unicode Format

Using Outlook in Unicode mode is especially advantageous for organizations with multilingual needs and for users who need large Outlook files. Previous versions of Outlook provided support for multilingual Unicode data in the body of e-mail messages. However, Outlook data—such as the To and Subject lines of messages and the ContactName and BusinessTelephoneNumber properties of contact items—was limited to characters defined by your system code page. Outlook 2003 no longer has this limitation, provided Outlook is running in Unicode mode with Exchange Server as the messaging server. As a best practice, it is recommended that Unicode mode be used. Unicode mode is the default mode if the configurations of the user's profile, Exchange server, and administrator settings allow it.

If you want users in your organization to use Unicode for Outlook, you can use several methods to deploy Outlook so users can run in Unicode mode. One deployment option is to encourage Unicode usage by establishing an environment in which, for most users, the criteria that Outlook checks to allow Unicode mode are met. Unicode is used by default if the criteria are met, unless you set options to use ANSI. Another option is to require Unicode for Outlook usage by setting options that create new Unicode user files automatically. Or, you can use a combination of these approaches.

Exchange Server 2003 keeps users' offline address book files up-to-date through full downloads and differential updates. For Outlook clients using OAB v4, fewer situations may cause an e-mail client to download the full offline address book. When your clients download offline address books changes through differential updates rather than full offline address book downloads, network performance and client performance is improved. The following scenarios discuss some cases where full offline address book downloads no longer occur for clients using OAB v4.

• In previous versions of OAB (v2 and v3a), the removal or addition of parent distinguished names in the directory caused a full offline address list download. When clients use OAB v4, these changes are now handled through differential downloads.
  
• In previous version of OAB (v2 and v3a), the modification of the legacyExchangeDN value always caused a full offline address book download. When clients use OAB v4, these changes are now handled through differential downloads.
  
• In previous version of OAB (v2 and v3a), making a single change to most or all client offline address book properties (for example, changing the phone number of all employees) always caused an offline address book download. When clients use OAB v4, these changes are now handled through differential downloads.
  

For more information about the effect that full offline address book downloads can have on a network, see Using Offline Address Books.

 Considerations When a Full Offline Address Book Cannot Be Generated

In versions of Exchange Server earlier than Exchange Server 2003 SP2, a full offline address book download is always forced if a differences file cannot be generated. On servers running Exchange Server 2003 SP2 and later, a full offline address book download is not always forced if a differences file cannot be generated. Specifically, in some cases, although the offline address book server running Exchange Server 2003 SP2 determines that a differences download is more efficient, Exchange Server may be unable to generate a differences file for some clients.

For Outlook clients that use OAB v2 or v3a, if Exchange Server determines that a differences download is more efficient, and if a differences file cannot be generated on a server running Exchange Server 2003 SP2, Exchange Server will not automatically force a full offline address book download. Instead, by default, Exchange logs MSExchangeSA event 9360 indicating that it was unable to produce a differences file and will not produce a full offline address book file. To change this default setting, you must edit the Windows registry as specified in the event log message.

For clients that use OAB v4, the change in the offline address book format resolves the issues that would stop the server from generating a differences file. The server does not attempt to force a full offline address book download by not generating a differences file for clients that are using the new offline address book format. However, Outlook clients that use OAB v4 or later may still decide to do a full download if the size of the differences files is larger than a preset fraction of the full offline address book size.

For more information about this feature, see "Generating a Full Offline Address Book Download File when a Differences File Cannot Be Generated" in Using Offline Address Books.

For information about how to configure this setting, see How to Generate a Full Offline Address Book Download File When a Differences File Cannot Be Generated.

In OAB v4, full offline address book downloads and differential downloads are reduced in size. In most Exchange Server environments, it has been estimated that you will see a 30-40 percent reduction in full offline address book file sizes. This reduction in file sizes significantly improves the performance of offline address book downloads. These improvements are made possible by the adoption of an improved compression mechanism for the offline address book file. Specifically, the newer LZX algorithm (used by Binpatch) is used to compress OAB v4. These improvements are also made possible because the offline address book download files no longer include the indices that allow clients with different local settings to correctly view the offline address book..

In OAB v4, the client uses the locale settings (language and country) that are configured on the client to perform offline address book indexing. Specifically, the client generates the locale-specific indices regardless of what locales are on the server. Using client-based locale settings enables users on the same server (with different locale settings) to correctly view the offline address book, which is sorted based on their locale setting and not the server.

Diagnostic logging improvements make it easier to notice problems that may occur with offline address book downloads. Specifically, events have been added to help you monitor the following problems.

• A Warning event is logged when at least one attribute is removed from the offline address book because it exceeds its size limit. For more information about this event, see "Managing Offline Address Book File Sizes" in Using Offline Address Books.
  
• An Error event is logged when there is a failure in the differential download generation. For more information about this event, see "Generating a Full Offline Address Book Download File When a Difference File Cannot be Generated" in Using Offline Address Books.
  

If you configure Exchange Server 2003 to limit the size of specific property types Exchange Server 2003 SP2 can help you to further improve the performance of OAB v4 downloads. To make this change, in the registry, you can specify the maximum size in bytes for individual property types. Events are logged in the Application Log to help you track changes to these settings. For detailed steps, see How to Manage Offline Address Book File Sizes.

For clients that use OAB v4, differential downloads of the offline address book are permitted if the size of the differences file is no larger than one-half (50 percent) the size of the full offline address list file. For Outlook clients that use OAB v2 or v3a, the default permitted limit is still one-eighth (12.5 percent).