A worker process is user-mode code whose role is to process requests, such as processing requests to return a static page, invoking an ISAPI extension or filter, or running a Common Gateway Interface (CGI) handler.

In both application isolation modes, the worker process is controlled by the WWW service. However, in worker process isolation mode, a worker process runs as an executable file named W3wp.exe, and in IIS 5.0 isolation mode, a worker process is hosted by Inetinfo.exe. Figure 2.2, which depicts the architecture for IIS 5.0 isolation mode, uses a dashed line to suggest the relationship between the worker process and the WWW service.

Worker processes use HTTP.sys to receive requests and to send responses by using HTTP. Worker processes also run application code, such as ASP.NET applications and XML Web services. You can configure IIS to run multiple worker processes that serve different application pools concurrently. This design separates applications by process boundaries and helps achieve maximum Web server reliability.

By default, worker processes in worker process isolation mode run under the Network Service account, which has the strongest security (least access) compatible with the functionality that is required. For more information about Network Service accounts, including Windows Server 2003 built-in security accounts, see Running IIS 6.0 as an Application Server, and IIS and Built-in Accounts.