## IIS Core Functionality

IIS defines a basic functionality that you can use to build Web applications. Active Server Pages (ASP) and other Microsoft technologies have extended this basic functionality to create a rich environment for application development. The basic server functionality is exposed through the Internet Server Application Programmer Interface (ISAPI). ISAPI is described in detail in the Microsoft Platform SDK.

The core functions which IIS provides include:

- Establishing and maintaining HTTP connections.
- Reading HTTP requests and writing HTTP responses.
- Modifying HTTP headers.
- Obtaining client certificate information.
- Managing asynchronous connections.
- Mapping Uniform Resource Locators (URLs) to physical paths.
- Managing and running applications.
- Transmitting files.

ASP extends this functionality by providing a link to the COM architecture and thus the other participants in Windows DNA. Similarly, you can extend the IIS architecture by defining a custom set of functions using ISAPI. The relationship between the IIS core functionality, ASP, and extended architectures is depicted in the following graphic:

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