Offline address books in Exchange 2016

 

Applies to: Exchange Online, Exchange Server 2016

Topic Last Modified: 2017-02-15

Learn how offline address books are created and distributed in Exchange 2016.

An offline address book (OAB) is a local copy of an address list collection. OABs are used for address book queries by Outlook clients that are configured in cached Exchange mode. OABs are the only option for Outlook clients that are disconnected from the Exchange server, but they're also queried first by connected Outlook clients as a way to help reduce the workload on Exchange servers. You can configure which address lists are included in an OAB, access to specific OABs, how frequently the OABs are generated, and where the OABs are distributed from.

By default, a new installation of Exchange creates an OAB named Default Offline Address Book on the server. This OAB is also the default OAB, which means it's the OAB that's used by mailboxes and mailbox databases that don't have an OAB assigned to them.

OABs in Exchange 2016 are improved over OABs in Exchange 2010. These changes were introduced in Exchange 2013:

  • Only web-based distribution is supported (public folder distribution is no longer available). Web-based distribution allows:

    • Support for more concurrent downloads by client computers.

    • Reduced bandwidth usage.

    • More control over the OAB distribution points.

  • Only OAB version 4 is supported. This version of the OAB is Unicode, and allows clients to receive differential updates, instead of always using full downloads. All versions of Outlook that are supported by Exchange fully support OAB version 4.

  • A mailbox assistant (not the Microsoft Exchange System Attendant service) is the process that's responsible for generating OABs. This allows OAB generation to run or pause based on the workload of the server (workload management).

  • OAB generation occurs in a designated arbitration mailbox (not on a designated OAB generation server). These mailboxes can use database availability groups (DAGs) to help prevent a single point of failure for OAB generation and downloads.

For OAB procedures, see Procedures for offline address books in Exchange 2016.

To learn more about address lists, see Address lists in Exchange 2016.

OAB generation is controlled by the mailbox assistant named OABGeneratorAssistant that runs under the Microsoft Exchange Mailbox Assistants service. OAB generation occurs in a designated arbitration mailbox that has the OrganizationCapabilityOABGen value for the PersistedCapability property. An arbitration mailbox with this capability is also known as an organization mailbox.

By default, OABs are generated every 8 hours. To change the OAB generation schedule, see Change the offline address book generation schedule in Exchange 2016. To manually update an OAB, see Use the Exchange Management Shell to update offline address books.

The arbitration mailbox named SystemMailbox{bb558c35-97f1-4cb9-8ff7-d53741dc928c} is the first organization mailbox in your organization. By default, this organization mailbox is responsible for generating all OABs (the first OAB named Default Offline Address Book, and any new OABs that you create).

You can create additional organization mailboxes to generate OABs. Exchange 2016 contains the improvements to OAB generation that were introduced in Exchange 2013 Cumulative Update 7 (CU7):

  • You can configure multiple OABs to be generated by the same organization mailbox, but you can't configure an OAB to be generated by more than one organization mailbox. If you configured an OAB with multiple organization mailboxes, each copy of the OAB had a different unique identifier. So, a full OAB download was required whenever a client was proxied to a different organization mailbox location.

  • You can configure an OAB to allow a read-only copy (also known as a shadow copy) to be distributed to all organization mailboxes in the organization (also known as shadow distribution). All copies of the OAB have the same unique identifier, so full a OAB download isn't required when a client is proxied to a different organization mailbox location.

    Typically, shadow copies are only required in multi-site Exchange organizations. You configure an organization mailbox in each site, and you configure shadow distribution for an OAB to help prevent cross-site OAB download requests by clients (likely over slow WAN links). To create additional organization mailboxes, see Use the Exchange Management Shell to create organization mailboxes.

    Shadow distribution is described in detail in the next section.

To find all organization mailboxes, and the organization mailbox that's defined for an OAB, see Use the Exchange Management Shell to find organization mailboxes.

The OAB files are generated and stored in the designated organization mailbox, so the destination for OAB download requests is the Mailbox server that holds the active copy of the organization mailbox. The OAB files are copied from the organization mailbox to %ExchangeInstallPath%ClientAccess\OAB\<OAB GUID> for retrieval by clients. Clients never connect directly to this backend location. Client requests for the OAB are proxied by the Client Access (frontend) services on a Mailbox server to this backend location.

By default, Outlook clients are configured to download the OAB every 24 hours, or users can initiate a manual download from Outlook at any time.

OAB distribution to clients depends on Internet Information Services (IIS) virtual directories and the Autodiscover service. The IIS virtual directory that's used for client access to OABs is located in the default web site in the Client Access (frontend) services on the Mailbox server, and is named OAB (Default Web Site). This virtual directory is automatically created when you install Exchange, and is configured to service internal clients at the URL https://<ServerName>/oab (for example, https://mailbox01.contoso.com/oab). You'll need to manually configure the external URL that's used to distribute OABs to external clients. For more information, see Configure external URLs.

In the properties of the OAB, you can configure the OAB virtual directories that are available to distribute the OAB to clients. The default setting restricts OAB distribution to the OAB virtual directories on the server that holds the OAB's organization mailbox. However, the Client Access services on any Mailbox server can proxy incoming OAB download requests to the correct location. Therefore, we recommend that you configure all OAB virtual directories to accept requests to download the OAB. For instructions, see Use the Exchange Management Shell to configure any virtual directory in the organization to accept download requests for the OAB.

The Autodiscover service advertises the OAB URLs that you've configured. Autodiscover is supported by all versions of Outlook and virtually all mobile devices that are currently by Exchange. Here's a summary of the OAB distribution process:

  1. Outlook receives the OAB URL from Autodiscover, and connects to the Client Access (frontend) services on a Mailbox server.

  2. The Client Access services on the Mailbox server that accepted the connection performs these steps:

    1. Queries Active Directory to find the organization mailbox that's responsible for generating the user's OAB (the default OAB, the OAB that's specified for the mailbox database, or the OAB that's specified for the mailbox).

    2. Queries Active Directory again to find the mailbox database that hosts the organization mailbox for the OAB, and the Mailbox server that currently holds the active copy of the database.

    3. Proxies the OAB download request to the identified Mailbox server.

    4. Retrieves the OAB files from the backend location %ExchangeInstallPath%ClientAccess\OAB\<GUID> and proxies them back to the client.

If a shadow copy of the OAB exists in an organization mailbox in the local Active Directory site (the site where the user is connecting from), then a local Mailbox server is used to download the OAB. However, synchronization of the shadow copy between organization mailboxes is performed on-demand. Here's how it works:

  1. Let's say the organization mailbox doesn't have a suitable shadow copy of the OAB. This can be caused by the following conditions:

    • A client has never requested a download of the shadow copy.

    • The shadow copy is out of date. Shadow copies are aware when an updated copy of the parent OAB has been generated and published (manually, or by the default 8 hour OAB generation schedule). The affected Mailbox servers will stop distributing the outdated shadow copy to clients.

  2. The first client tries to download the shadow copy will receive error 0x80190194 (BG_E_HTTP_ERROR_404) in Outlook. This will trigger a full copy of the OAB from the parent to the shadow copy. The following events are reported:

    • Event ID: 102

      Source: MSExchange OABRequestHandler

      Description: The OABRequestHandler has begun downloading the OAB <GUID> from the server <Server>.

    • Event ID: 103

      Source: MSExchange OABRequestHandler

      Description: The OABRequestHandler has finished downloading the OAB <GUID>.

  3. The OABRequestHandler will make up to three immediate attempts to copy the OAB files from the Mailbox server that holds the parent OAB generation mailbox. If all three attempts fail, the OABRequestHandler will retry the copy after one hour. The following events are reported:

    • Event ID: 104

      Source: MSExchange OABRequestHandler

      Description: Download of the OAB <GUID> failed. The job will be re-submitted. The error was: BG_ERROR_CONTEXT=BE_ERROR_CONTEXT_REMOTE_FILE; error code=0x80190194

    • Event ID: 105

      Source: MSExchange OABRequestHandler

      Description: Download of the OAB <GUID> has failed too many times. The job will not be resubmitted for the next hour.

  4. If the OAB is configured for shadow distribution, but there's no organization mailbox in the local Active Directory site (the site where the user is connecting from), the Client Access services will proxy the OAB download request back to the Mailbox server that holds the organization mailbox for the parent OAB.

The improvements to OABs typically require clients to download OAB updates, not the full and complete OAB. However, full OAB downloads are sometimes required. For example:

  • The Changes.oab files are greater than or equal to half the size of the full OAB files. Outlook compares the total size of the compressed Changes.oab files that are required to update the OAB to the total size of the compressed full OAB files on the server.

  • There's no OAB on your computer (for example, during the initial setup of Outlook).

  • A differential file is missing on the server. Missing differential files can be caused by the following conditions:

    • You haven't used Outlook to connect to your Exchange mailbox in more than 30 days (by default, the differential files are stored on the server for 30 days).

    • The server couldn't generate the differential file for a day that's required to update your local copy of the OAB.

  • A more recent version of the OAB is available on the server (for example, your mailbox was upgraded from Exchange 2010, and your local copy of the OAB is version 3).

  • Applying changes to the OAB failed. For example, differential files are corrupted on the server (the server crashed during differential file generation).

  • The OAB is not present on your computer (for example, you manually deleted one or more local OAB files).

  • A previous full download failed, so Outlook has to start over.

  • You initiated a manual download of the full OAB.

Whether you use a single OAB or multiple OABs, consider the following factors as you plan and implement your OAB strategy:

  • Th size of each OAB in your organization. OAB sizes can vary from a few megabytes to hundreds of megabytes. The following factors can affect the size of the OAB:

    • The usage of certificates in your organization. The more public key infrastructure (PKI) certificates, the larger the OAB. PKI certificates range from 1 kilobyte (KB) to 3 KB. They're the single largest contributor to the OAB size.

    • The number of mail recipients in your organization.

    • The number of groups in your organization.

    • User information that your organization adds to each recipient object. For example, some organizations configure full address and contact details for each user.

  • The number of OAB downloads.

  • The number and frequency of parent distinguished name changes for recipient objects in Active Directory.

  • SMTP address mismatches.

  • The overall number of changes that you make to Active Directory.

  • Recipients that you've hidden in Active Directory by using methods outside of Exchange will be visible in OABs (for example, by using the Windows security descriptor). To effectively hide recipients in OABs, configure the Hide from address lists property for the recipient in the Exchange admin center (EAC) or the HiddenFromAddressListsEnabled parameter in the corresponding recipient management cmdlet in the Exchange Management Shell. For more information, see Hide recipients from address lists. Or, you can create an address list that doesn't include the hidden recipients, assign the address list to the OAB, and assign the OAB to users (directly or by making the OAB the default). For more information about creating address lists, see Create address lists.

In Exchange 2010, moving OAB generation to another server required you to specify a different generation server in the properties of the OAB. But in Exchange 2013 and Exchange 2016, OAB generation occurs in a designed organization mailbox, not on a designated server. To move OAB generation to another server, you need to move the organization mailbox. For example:

Remember, you can configure multiple OABs to use the same organization mailbox, but you can't configure an OAB to use more than one organization mailbox. If you need multiple copies of the OAB in different locations (typically, in different Active Directory sites), verify that an organization mailbox is exists in the site, and enable shadow distribution for the OAB. For more information, see Use the Exchange Management Shell to enable shadow distribution for offline address books.

 
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