Sun Java System Web Server 6.1 extends NSAPI by introducing a new filter interface that complements the existing Server Application Function (SAF) interface. Filters make it possible to intercept and possibly modify data sent to and from the server. The server communicates with a filter by calling the filter's filter methods. Each filter implements one or more filter methods. A filter method is a C function that performs a specific operation, such as processing data sent by the server.