Layered Service Provider

Layered Service Provider (LSP) is a deprecated feature of the Microsoft Windows Winsock 2 Service Provider Interface (SPI). A Layered Service Provider is a DLL that uses Winsock APIs to attempt to insert itself into the TCP/IP . Once in the stack, a Layered Service Provider can intercept and modify inbound and outbound Internet traffic. It allows processing of all the TCP/IP traffic taking place between the Internet and the applications that are accessing the Internet (such as a web browser, the email client, etc.). For example, it could be used by malware to redirect web browers to rogue websites, or to block access to sites like Windows Update. Alternatively, a computer security program could scan network traffic for viruses or other threats. The Winsock Service Provider Interface (SPI) API provides a mechanism for layering providers on top of each other. Winsock LSPs are available for a range of useful purposes, including parental controls and Web content filtering. The parental controls web filter in Windows Vista is an LSP. The layering order of all providers is kept in the Winsock Catalog.

Unlike the well-known Winsock 2 API, which is covered by numerous books, documentation, and samples, the Winsock 2 SPI is relatively unexplored. The Winsock 2 SPI is implemented by network transport service providers and namespace resolution service providers. The Winsock 2 SPI can be used to extend an existing transport service provider by implementing a Layered Service Provider. For example, Quality of Service (QoS) on Windows 98 and Windows 2000 is implemented as an LSP over the TCP/IP protocol stack. Another use for LSPs would be to develop specialized URL filtering software to prevent Web browsers from accessing certain sites, regardless of the browser installed on a desktop. The Winsock 2 SPI allows software developers to create two different types of service providers—transport and namespace. Transport providers (commonly referred to as protocol stacks) are services, which supply functions that set up connections, transfer data, exercise flow control, error control, and so on. Namespace providers are services that associate the addressing attributes of a network protocol with one or more human-friendly names and enable protocol-independent name resolution. The SPI also allows you to develop two types of transport service providers—base and layered service providers.

...
Wikipedia