How ASP.NET Works with IIS 6.0 (IIS 6.0)
If you run your ASP.NET applications in IIS 6.0, you obtain a significant advantage over running your ASP.NET applications in IIS 5.0. For example, in IIS 5.0, you cannot isolate individual applications into their own worker processes; hence, if one ASP.NET application fails, it can affect all the other applications. IIS 6.0, however, provides a new architecture that allows you to isolate your ASP.NET applications without incurring a performance penalty.
Before you run ASP.NET applications on IIS 6.0, be sure that you understand how ASP.NET works with the two IIS 6.0 isolation modes. When running in IIS 5.0 isolation mode, ASP.NET uses its own request processing model and configuration settings in the Machine.config file — just as it does when ASP.NET runs on IIS 5.x. When running in worker process isolation mode, ASP.NET disables its own request processing model and uses the new worker process architecture of IIS 6.0.
Important: The version of ASP.NET that is included with WindowsServer2003 is not available with the Microsoft® Windows® XP 64-Bit Edition and the 64-bit versions of Windows Server 2003. For more information, see Features unavailable on 64-bit versions of the Windows Server 2003 family in Help and Support Center for Windows Server 2003.
ASP.NET Request Processing Model
When ASP.NET is enabled on a server running Windows Server 2003, the ASP.NET request processing model depends on the application isolation mode in which IIS 6.0 is running. The preferred application isolation mode for IIS 6.0 is worker process isolation mode, which enables you to use application pools, worker process recycling, and application pool health monitoring. When IIS 6.0 is running in worker process isolation mode, ASP.NET uses the IIS 6.0 request processing model settings.
In almost all situations, if your ASP.NET applications are running on Windows Server 2003, configure them to run in worker process isolation mode. However, if your application has compatibility issues in this mode — for example, if your application depends upon read raw ISAPI data filters — you can change your configuration to run IIS 6.0 in IIS 5.0 isolation mode.
If you configure IIS 6.0 to run in IIS 5.0 isolation mode, ASP.NET runs in its own request processing model, Aspnet_wp.exe, and uses its own configuration settings for process configuration, which are stored in the Machine.config configuration file. When your server is running in IIS 5.0 isolation mode, the ASP.NET ISAPI extension implements a request processing model that is similar to worker process isolation mode and contains worker process management capabilities similar to those provided by the WWW service. ASP.NET also provides recycling, health monitoring, and processor affinity.
When ASP.NET is installed on servers running Windows XP or Windows 2000 Server, ASP.NET runs in IIS 5.1 or IIS 5.0, respectively, and runs in the Aspnet_wp.exe process by default. The ASP.NET request processing model provides process recycling, health detection, and support for affinities between worker processes and particular CPUs on a server with multiple CPUs.
When Different Versions of ASP.NET Share the Same Application Pool
When multiple versions of the .NET Framework are installed on a computer that uses IIS 6.0, you might encounter the following error message in the Application Event log:
Note: It is not possible to run different versions of ASP.NET in the same IIS process. Please use the IIS Administration Tool to reconfigure your server to run the application in a separate process.
This error occurs when more than one version of ASP.NET is configured to run in the same process. Different versions of the .NET Framework and run time cannot coexist side by side within the same process. Therefore, an ASP.NET application that uses one version of the run time must not share a process with an application that uses a different version. This error commonly occurs when two or more applications are mapped to different versions of ASP.NET but share the same application pool.
(sursa http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/f1a39358-5ab8-4dc1-a60d-eedea7360780.mspx?mfr=true )