Spyglass SDI (Software Development Interface) is the main competitor to NCSA Mosaic CCI (Common Client Interface). The expectation is that SDI and CCI will eventually converge into a common specification. In addition to what CCI offers, SDI also provides a protocol for initiating messages from the browser to the client application. This protocol causes the application to be informed when the certain events occur on the browser or actually diverts handling of those events to the application. This addition makes SDI an appropriate protocol for invoking a new client application from a browser. The Netscape Navigator web browser supports SDI under Windows and MacOS.