Riverbed's Steelhead appliances complement Outlook 2003 deployments, with or without Microsoft’s new Outlook cache mode implemented. Many users have the misunderstanding that Outlook 2003's cache mode will fix WAN performance issues, but cache mode simply hides these issues.

"Cache Mode" attempts to hide performance issues over the WAN associated by preemptively downloading all new e-mails and their attachments from the Exchange server to a cache of storage on the local hard disk of the Outlook client workstation. The Outlook 2003 client does not display the new e-mail to the end-user until it has been completely downloaded, along with all its associated attachments. When the end-user finally sees the new e-mail and desires to open it, the data is fetched from the local hard drive cache of the Outlook client workstation. This gives the illusion of fast performance.

While Microsoft introduced cache mode to mitigate some of the impacts of latency on Outlook, nevertheless a decision to enable cache mode can increase the importance of Riverbed technology. The following observations illustrate the value of using Riverbed technology with Microsoft Outlook/Exchange 2003, with or without cache mode:

• Outlook 2003's cache mode will increase network utilization – not reduce data transfers.
  Cache mode proactively pulls down new e-mails over the network. Some of those e-mails and their accompanying attachments may not be of interest to the recipient (e.g., spam, useless attachments, etc...), and would not have been retrieved over the network without cache mode. Riverbed takes bytes off the wire, effectively reversing increased load that Outlook 2003 cache mode will create for the network. In the typical case, 65-95% of the raw traffic formerly seen on the network will be eliminated through use of Steelhead appliances.

• Cache Mode will lead to more spikes in network traffic.
  E-mails sent to aliases that include multiple recipients at the same site will result in the same e-mail being sent over the WAN repeatedly, at least once per recipient. Worse yet, with cache mode enabled, those e-mails will be downloaded at the same time, causing the network to be hammered with multiple simultaneous requests to the Exchange server. Using Steelhead appliances, the raw data forming those e-mails will only be sent once over the WAN, regardless of the number of recipients at a remote site. Thus, the network will not suffer over-utilization regardless of the number of recipients of a single e-mail message.

• Outlook 2K3 Cache mode doesn't address performance issues.
  Outlook merely hides the slow performance of the chatty MAPI protocol. Hiding the problem can still lead to a performance issue that affects productivity. For example, most end-users don't carry around USB flash memory around with them, so a typical way of distributing files and documents to coworkers in the office is through e-mail. But at a remote site, just e-mailing a large document to a colleague in a neighboring cubicle will result in that colleague waiting for several minutes as the document travels over the network to the exchange server, and then back over the same network to the colleague's workstation (the “boomerang effect”). Using Riverbed, the colleague will receive the e-mail attachment almost instantaneously, with or without cache mode, eliminating the boomerang effect.

• Using Riverbed with Outlook 2003 provides benefits for non-email applications.
  Using Riverbed with Exchange 2003 will of course have the beneficial side-effect of warming the data store for transfers of the same data using other protocols, including file access through Microsoft Windows (CIFS), web traffic, FTP, etc.
  
  A common scenario involves a remote employee who receives a file attachment from Outlook and stores it on his or her local file server. When that file server is backed up over the WAN to a backup server in the data center, that file attachment has already been seen by the Steelhead appliance in the remote office, and the redundant data does not need to be sent again over the WAN in order to complete the backup. This is also true if that employee then decides to copy the file attachment through a mapped CIFS drive to a different server in the data center.

Summary

Steelhead appliances will accelerate Exchange 2003, just as it does Exchange 5.5 or Exchange 2000. Outlook cache mode does not accelerate Exchange; it merely hides the latency-induced reduction in performance by not displaying email until it has been delivered.