How Outlook Connects (Overview)

The Client Summary

Outlook talks to Exchange Servers via Remote Procedure Calls (RPCs).  RPC is functionality implemented by the Operating System.  Traditional RPC calls travel over the TCP/IP network stack.  Enhancements made in Windows XP SP1 and higher (not available for Windows 2000) enable RPC to travel over the HTTP network stack.  This makes enhanced RPCs internet routable.  Outlook can take advantage of this enhancement to connect to an Exchange Server over the internet.  For more information on the OS implementation, see this MSDN Article.

The Server Summary

These RPC enhancements are also prerequisite for all machines sending and receiving the enhanced RPC over HTTP enabled packets.  This mandates that all machines Outlook talks to must also be running Windows.NET Server 2003 for the RPC to be recognized by the Server.  This includes GCs, DCs, Exchange Front End and Back End Servers and all Exchange Public Folder stores as well.  Because the RPC enhancement proxies using IIS version 6, and Exchange 2000 only runs in IIS 5 Compatibility Mode, any Exchange Server receiving RPC over HTTP packets must be running IIS version 6, which equates to Exchange Titanium or higher.

Miscellaneous Nuggets of Information

Abstraction and Implementation

The beauty of RPC over HTTP is its abstraction in technology from the actual client and server that use its technology.  In an Outlook/Exchange communication, all RPCs are initiated by the client and responded to by the server.  This means that only Outlook needs to make changes to how it calls RPC since Exchange will merely respond in the same format it received in.  In addition, Outlook only needs to pass a few more parameters to the RPC API in order to take advantage of the enhanced functionality.  This can all be done in a modular way such that changes in the client to support RPC over HTTP are minimized.  This also means that once you're connected using RPC over HTTP, there's really no additional client code running.  All behavior should be very similar to RPC over TCP/IP.

Configuration and Deployment

All the information required for Outlook to connect to Exchange over HTTP is currently configured by the Office administrator and deployed to the clients.  So, in a typical enterprise scenario, the end-user need not be familiar with RPC over HTTP in any way.  Simply an internet connection will allow him or her to access their Exchange Server.  This was not possible before RPC over HTTP.