Traditionally, Microsoft Exchange Server has supported only Messaging API (MAPI) remote procedure calls (RPCs) over protocols such as NetBIOS over TCP (NetBT). This model assumes that clients connect to servers across perfect networks or at least assumes a consistent standard of service in a corporate LAN. Many of us understand the frustration of watching Microsoft Outlook report frequent problems connecting to Exchange when we use slow or high-latency links such as a RAS dial-in. The RPC interchange is also fragile and prone to timeouts and failures.
A combination of Outlook 11 and Exchange Server 2003 (formerly code-named Titanium)—and to a lesser extent, a combination of Outlook 11 and Exchange 2000 Server—will deliver significant improvements in these networking areas. Both Outlook 11 and Exchange 2003 are currently scheduled for release in mid-2003, so the information that I report here is subject to change as Microsoft refines the products before their final release. The good news is that the new products offer such far-reaching enhancements that the idea of all these improvements being absent from the final code is inconceivable. We have much to look forward to.
Outlook 11 Networking Improvements
RPCs are sensitive to network latency—a problem that can affect both Exchange clients and servers. From a client perspective, RPC problems (most of which are due to excessive latency) become apparent when you see a pop-up window reporting that Outlook is waiting for Exchange to respond. Outlook might communicate with several Exchange servers during one session, as users move between their mailbox and public folders. Latency can affect any of these connections, so the existence of pop-up windows isn't necessarily an indication that a problem exists between the client and the mailbox server.
Because of the nature of RPCs, you can use low-bandwidth networks to connect Outlook to Exchange and transfer large items across the link, albeit slowly. (For example, many people use 9.6Kbps dial-up connections through cell phones.) To handle transfers over low-bandwidth links, RPCs split large items into multiple packets and transmit the packets in parallel. Unfortunately, applications might need to wait for the RPC transaction to complete and a response to return—a phenomenon you often see in Outlook-to-Exchange communications across high-latency links. In Outlook XP, Microsoft improved Outlook's ability to continue working while waiting for a transaction to complete, but room to improve further exists.
Among other improvements that Microsoft wanted to introduce in Outlook 11, making the client smarter about how it consumes network resources was important to the company, primarily because of the increasing demand for connectivity for mobile devices that use wireless networks—or even the latest cell phone networks. Devices such as the Tablet PC also pose interesting challenges for applications that need persistent connectivity with servers and become temperamental if they experience a network interruption. Outlook falls squarely into this category.
The four major areas of networking improvement in Outlook 11 are the use of HTTP to transport RPCs, the introduction of cached replication, improved compression and best-body support, and the addition of bandwidth profiles.
The use of HTTP to transport RPCs. Outlook 11 puts an HTTP or HTTP Secure (HTTPS) wrapper around the MAPI RPCs it sends to Exchange 2003. You can connect Outlook 11 to an Exchange 2003 server any time you're able to connect Outlook Web Access (OWA) to Exchange—with no requirement to remap RPC ports or establish a VPN. However, because Microsoft has changed the way RPCs work to permit this functionality, you must deploy Outlook 11 on Windows XP Service Pack 1 (SP1) or later on the desktop and connect to Exchange 2003 servers to enable secure authentication. In a front-end/back-end configuration, the front-end Exchange 2003 servers in the demilitarized zone (DMZ) can proxy the HTTP-wrapped RPCs to back-end Exchange 2003 servers. Also, the Exchange 2003 servers must run on Windows 2003 Server, as must the Global Catalog (GC) servers that Outlook uses. This functionality isn't available if you run Exchange 2000—you must upgrade to Exchange 2003 before you can connect Outlook to Exchange over HTTP.
Cached replication. Outlook 11 can fetch data from Exchange 2003, Exchange 2000, and Exchange Server 5.5 servers to maintain a local cache on the PC. The client then uses this cache to access messages and attachments. I discuss how the local cache works in the next section.
Improved compression and best-body support. New code in Outlook 11 optimizes the flow of RPCs exchanged with the server, so the amount of data sent over the wire during synchronization operations is reduced. Microsoft's efforts to reduce the level of Outlook's communication with Exchange should come as no surprise if you've ever monitored the flow of bytes across a dial-up connection as client and server patiently move messages from one to the other. Better compression and smarter use of network resources mean that you see fewer dialog boxes reporting that Outlook is waiting for Exchange to respond.