RPC over HTTP Interactions on the Client Computer

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Microsoft® Office Outlook® 2003 tries to use either RPC over HTTP or RPC over TCP, depending on your Outlook 2003 profile settings.

For more information about Outlook 2003 profile settings for RPC over HTTP, see Configuring Outlook 2003 for RPC over HTTP (https://go.microsoft.com/fwlink/?LinkId=45954).

If Outlook tries to use either RPC over HTTP or RPC over TCP, Outlook calls the RPC layer on the client computer. Outlook uses a specific protocol sequence to indicate if it must connect to an Exchange server that uses RPC over HTTP or to an Exchange server that use RPC over TCP. If the protocol sequence indicates that Outlook must connect to an Exchange server that uses RPC over HTTP, the RPC layer then creates the HTTP session, controls authentication, and puts the RPC requests inside the HTTP session. Rpcrt4.dll contains support for the RPC layer of RPC over HTTP. Rpcrt4.dll uses any Microsoft Internet Explorer proxy settings that apply to outbound HTTP connections.

Both the HTTP session and the RPC requests inside the HTTP session require authentication. Outlook passes authentication information for the HTTP session and the RPC request to the RPC layer for each request from the client computer to the Exchange server.

You cannot select the type of authentication for the RPC request inside the HTTP tunnel. Outlook always uses NTLM authentication for the RPC request. You can select the type of authentication for the HTTP session.

To select the type of authentication, you must set your Outlook profile to use either Basic authentication or NTLM authentication.

If you use Basic authentication, Outlook prompts you for user name and password information. In the user name field, you must enter your user name information in one of the following formats:

  • <domain>\<user name>

  • user principle name (UPN)

If you use NTLM authentication, Outlook tries to use your current Microsoft Windows® operating system logon information. If your current Windows operating system logon information fails, Outlook prompts you for domain, user name, and password information.

For more information about requirements for RPC over HTTP to use the current Windows operating system logon information, see RPC over HTTP Authentication and Security.

For more information about how to use your current Windows operating system logon information instead of entering logon information, see Microsoft Knowledge Base article 820281, "You must provide Windows account credentials when you connect to Exchange Server 2003 by using the Outlook 2003 RPC over HTTP feature" (https://go.microsoft.com/fwlink/?Linkid=3052&kbid=820281).

After Outlook connects to the RPC proxy server by using the logon information that is provided, Outlook caches the logon information for the current Outlook session. You do not have to re-enter logon information until you exit and restart Outlook.

For detailed steps for how to set the RPC over HTTP settings in your Outlook 2003 profile, see How to Create an Outlook Profile for Users to Use with RPC over HTTP.