IIS 6.0 is the version of IIS shipped with Windows Server 2003. It uses an architecture substantially different from IIS 5 and earlier versions, intended to increase security and performance, called Worker Process Isolation Mode.
Key changes from IIS 5.0:
- Uses HTTP.SYS as a fast, secure kernel-mode listener, cache, and correctness parser for HTTP requests - instead of InetInfo, which ran ISAPIs and user code as LocalSystem
- Isolates all user web code into low-privileged processes, which run as Network Service by default
- Introduces the concept of an Application Pool; Application Pools group IIS applications into worker processes, which helps Admins isolate failures to specific processes and not the whole server (as was the case with InetInfo / Low-isolation applications in IIS 5 and earlier)
- Introduces the Web Service Extension list, which restricts arbitrary executable code
- Uses a central App Pool manager service, W3ADM, which monitors the health of App Pools and recycles them when needed.