The default keep alive time for RPC connections uses the IIS idle connection timeout, which is 15 minutes. This usually doesn't cause a problem on local LAN or WAN connections, but routers and switches that are used to connect Internet clients to internal Exchange servers often have more aggressive timeouts. Typically these network devices have a 5 minute timeout which causes problems for external clients, particularly Outlook Anywhere, iPhone, and iPad clients. Symptoms include messages stuck in the Outbox and poor email performance on the remote clients, and high CPU utilization on the Exchange Client Access Servers (CAS).
The new best practice is to adjust the RPC keep alive timeout value on the Client Access Server from 15 minutes to 2 minutes. Since RPC is a function of Windows, not Exchange, this value is adjusted under the Windows NT registry key. The value is located here:
Normally the MinimumConnectionTimeout DWORD value does not exist, which means RPC uses the default value of 900 seconds (15 minutes). To adjust it, create or modify the MinimumConnectionTimeout value and set the value to decimal 120 (seconds, or 2 minutes). IIS must be restarted on the CAS to affect the change.
The following command will create the appropriate values:
reg add "HKLM\Software\Policies\Microsoft\Windows NT\RPC" -v "MinimumConnectionTimeout" -t REG_DWORD -d 120
The Outlook and ActiveSync clients honor this new timeout during the connection to the CAS, so both client and server now send a Keep-Alive packet after two minutes of inactivity, effectively maintaining both TCP connections needed.
A colleague of mine works for a large global company that was affected by this. They have several thousand iPads connecting to nine load balanced CAS servers and all the CAS were peaking at 100% CPU utilization. Once they implemented this change the average load on the CAS is now 20-30% and the iPad performance is much improved.
This is my new best practice and I make this change on every Exchange CAS deployment. For more information about RPC over HTTP see Configuring Computers for RPC over HTTP on TechNet.