XCLN: How MAPI Clients Access Active Directory

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Article ID: 256976 - View products that this article applies to.
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SUMMARY

In Microsoft Exchange Server 4.0, 5.0, and 5.5, the directory service is on the same server as the server where MAPI clients log on and look up addresses in the global address list. In Exchange 2000, the directory service integrates into the Microsoft Windows 2000 operating system (Active Directory). As a result, the directory service may or may not be located on the Exchange 2000 server. In this environment, a MAPI client accesses the directory and logs onto mailboxes differently from previous versions of Exchange Server. This article explains how each version of Outlook and Exchange Client accesses Active Directory.

MORE INFORMATION

There are two mechanisms in Exchange 2000 to provide MAPI clients access to Active Directory. The first one is the referral service, which is used to refer Outlook clients to the global catalog server. The second one is the proxy service, which bridges all remote procedure calls (RPCs) between earlier versions of MAPI clients and the directory service server. Both of these methods are implemented in the DSProxy component.

Upon start-up of an Exchange 2000 server, the DSProxy component checks whether the Name Service Provider Interface (NSPI) is registered locally. If it is, that means that this server is a global catalog server and the referral and proxy services are not needed. Otherwise, DSProxy registers its endpoint with the RPC End-Point-Mapper (EPM) Manager and initializes its referral and proxy services. From this point on, DSProxy listens to incoming NSPI requests from MAPI clients.

Outlook 2000

Initially, when a user uses Outlook 2000 to log on to an Exchange 2000 server, Outlook expects to find the directory service on its home server. After the client contacts the server, the DSProxy service passes back a referral to the client. From that point on, all future directory requests are sent directly to the referral server instead. The referral server in this case is the global catalog server.

Upon receiving the referral from DSProxy, Outlook saves the referral in its MAPI profile in the following registry location:
HKEY_CURRENT_USER\software\Microsoft\Windows NT\CurrentVersion\Windows Messaging Subsystem\Profiles\<profile_name>\dca740c8c0421...
Value Name : 001e6602
Value Type : STRING (REG_SZ)
Value Data : Name of Global Catalog Server sent back by the DSPROXY component.
NOTE: For computers that use Windows 95, Windows 98, and Windows 98 Second Edition, upon receiving the referral from DSProxy, Outlook saves the referral in its MAPI profile in the following registry location:
HKEY_CURRENT_USER\software\Microsoft\Windows Messaging Subsystem\Profiles\<profile_name>\dca740c8c0421...
Value Name : 001e6602
Value Type : STRING (REG_SZ)
Value Data : Name of Global Catalog Server sent back by the DSPROXY component.
In case Active Directory is not available for some reason, you must restart Outlook. When you restart Outlook, it contacts the Exchange 2000 server again, and the server passes back a new referral to enable Outlook to contact Active Directory in the future.

Outlook 97, Outlook 98, Exchange Client 4.0, and Exchange Client 5.0

For Outlook 97, Outlook 98, and Exchange Client 4.0 and 5.0 to access the directory service, the DSProxy component on the Exchange 2000 server relays all the MAPI-DS requests to the global catalog. Because Active Directory supports many different methods and protocols, including MAPI, for accessing its database, DSProxy simply forwards the requests to the global catalog without doing any alteration to the packets.

The proxy process is as follows:
  1. The MAPI client sends a request for a name lookup.
  2. The Exchange 2000 DSProxy component transmits this request to the local global catalog.
  3. The global catalog returns the result to the Exchange 2000 server.
  4. Exchange 2000 sends the results back to the MAPI client.
  5. The MAPI client sends an acknowledgement to Exchange 2000 server.
  6. Exchange 2000 then transmits the acknowledgement back to the global catalog.
This process also applies when the MAPI client browses the global address list. If you use Network Monitor to manage or view the network traffic, you might see a few extra packets being sent over the wire as a user scrolls through the global address list. Overall, the overhead for the entire proxy process is minimal and should be seamless to users.

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Article ID: 256976 - Last Review: January 27, 2007 - Revision: 3.4
APPLIES TO
  • Microsoft Exchange 2000 Server Standard Edition
  • Microsoft Outlook 97 Standard Edition
  • Microsoft Outlook 98 Standard Edition
  • Microsoft Outlook 2000 Standard Edition
  • Microsoft Exchange Client 5.5
  • Microsoft Exchange Client 5.0
Keywords: 
kbhowto KB256976

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