The traditional mechanism for Messaging API (MAPI) clients such as Outlook to access Exchange Server mailboxes has been to use remote procedure call (RPC) over TCP/IP connections. However, Exchange Server 2003 introduces a new mechanism: RPC over HTTP connections, which provides great flexibility for Microsoft Office Outlook 2003 clients. Using RPC over HTTP, Outlook 2003 clients can connect from virtually any network, across any firewall, provided that HTTP outbound access is available on that network.
The basic concept behind RPC over HTTP is straightforward. An Outlook 2003 client connects over HTTP to an RPC proxy server. Acting on behalf of the client, the RPC proxy server establishes RPC connections to the back-end Exchange server, which is the Exchange server that hosts the requesting client's mailbox. (If you'd like more background information about this type of connection, see the Windows & .NET Magazine article "Exchange 2003 RPC over HTTP Access," September 2003, InstantDoc ID 39770.)
Although the basic concept behind RPC over HTTP connectivity isn't difficult to grasp, this new form of access introduces new chances for connections to fail. To help you troubleshoot RPC over HTTP connections, I've created a checklist that targets the common causes of failures in RPC over HTTP connections. The items in the checklist fall into five broad categories:
- Check the prerequisites.
- Check the connection type.
- Check the client's configuration.
- Check the servers' configuration.
- Check for connectivity.
Check the Prerequisites
RPC over HTTP connectivity has a number of prerequisites, and making sure that these prerequisites are met is the first task when you're troubleshooting an RPC over HTTP connection. The first prerequisite is that the client PC must be running Outlook 2003 and Windows XP Service Pack 2 (SP2) or XP SP1 with the appropriate patch—Quick Fix Engineering (QFE) patch 331320—which resolves a problem with client delays. (To learn more about the problem and its patch, see the Microsoft article "Outlook 2003 Performs Slowly or Stops Responding When Connected to Exchange Server 2003 Through HTTP" at http://support.microsoft.com/?kbid=331320.)
To make sure you have the correct OS patch version, you can check the version of the rpcrt4.dll file in the \%windir%\system32 directory on the client PC. In Windows Explorer, find the rpcrt4.dll file, right-click the file, and choose Properties. In the Properties dialog box, click the Version tab, then select File Version in the Item name list. You need to have version 5.1.2600.1142 or later.
The second prerequisite is that the RPC proxy server must be running Windows Server 2003, as must any domain controllers (DCs) or Global Catalog (GC) servers that the RPC proxy server will use to authenticate the client connection. In addition, any GC servers to which the user might be referred need to run Windows 2003. The back-end Exchange server or any other Exchange servers (e.g., public folder servers) must also be running Windows 2003 and Exchange 2003. So, the simplest rule of thumb is to make sure your entire infrastructure is using Windows 2003 and Exchange 2003.
Check the Connection Type
The goal of RPC over HTTP connectivity is to make connections between Outlook 2003 and Exchange 2003 as seamless as possible to the end user. That is, the user should detect no difference between an RPC over MAPI connection and an RPC over HTTP connection. For this reason, if Outlook 2003 can't make an RPC over HTTP connection, it automatically fails over to the TCP/IP protocol and tries to make an RPC over MAPI connection.
Although this behavior is desirable during the routine operation of the Exchange system, it's undesirable when you're first attempting to configure RPC over HTTP access. Because the failover is completely transparent, you won't know whether your RPC over HTTP connection worked or whether it failed and you're connected through an RPC over MAPI connection.
When you're on a client that has Outlook 2003 already running, there's a simple trick you can use to determine how the client is connected to the Exchange server. While holding down the Ctrl key, right-click the Outlook icon in the system tray and select Connection Status. This action yields a Connection Status dialog box that identifies the current connection type in use.
When you need to configure an RPC over HTTP connection, you can suppress this automatic failover feature by editing the registry on the Outlook 2003 client PC. In the HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\RPC subkey, create the DisableRpcTcpFallback entry. Set DisableRpcTcpFallback to the DWORD value of 1 to disable RPC/TCP fallback. After you know the system is running properly, set DisableRpcTcpFallback to 0 to allow failover.
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