Microsoft's web application technology, called ASP.Net, has won substantial market share. In March 2004, the internet survey site Netcraft reported that the number of sites using ASP.Net has overtaken the number using Java Server Pages or Java Servlets.

Until now, ASP.Net has required a Windows web server, but this is set to change next month with the first full release of Novell's Mono, an open-source implementation of the Microsoft dot-Net platform.

Mono runs on Linux and Mac OS X as well as on Windows, and integrates with Apache - by far the most popular web server on the internet. Mono ushers in a new world of choice for ASP.Net developers, and early feedback indicates that the platform is technically strong. But is Mono legal?

All eyes are on Microsoft. When dot-Net was launched in 2002, the firm made great play of how it was making the new dot-Net runtime platform and the associated C# language an open standard, and it has indeed worked with standards bodies ECMA and ISO to follow through on this promise. It has also indicated that royalty-free licences are available to third-parties that wish to implement the platform.

So far, so good, although licensing is a complex topic and some open-source advocates remain wary of the legal implications of using dot-Net. A larger complication is that not everything in Microsoft's dot-Net platform is covered by the standardised specs. This includes ASP.Net, a fact acknowledged by the Mono team, which refers to it in the FAQ on Mono's web site as a "controversial element".

In a recent blog posting, lead Mono developer Miguel de Icaza wrote, "We do not plan on infringing patents, and if we were to infringe [a] patent, we will either find prior art that renders that particular claim invalid, or rewrite the code to work in a different form. We do not like software patents, but we will abide by the legal rules."

The bottom line is that nobody knows whether Mono's ASP.Net is safe to use. It is a defining moment for Microsoft. The company can either stand by the spirit of its dot-Net standardisation and welcome this rival implementation, or it can wheel out the legal guns and attempt to shoot it down.

Some within the company will be spooked at the idea of sharing some of its technology jewels with competitors, but from the outside, the right answer is obvious.

The industry's number one reason for not using Microsoft systems is that they are too tightly bound to Windows, raising both security concerns and fear of costly lock-in. ASP.Net for Linux and Apache is a huge boost for the platform, in both the enterprise and the low-end shared hosting market where Microsoft has little penetration.

There is no need for Microsoft to provide the only implementation of ASP.Net. To succeed it need only offer the best implementation, a challenge the firm should relish.

As for Mono, Novell owes its customers a clear explanation of whether this interesting new product is based on a sound legal framework.