TomTom, a Dutch manufacturer of in-car navigation systems, has countersued Microsoft in a legal spat that could ultimately have implications for tech vendors' ability to freely use the open source Linux OS, or parts of it, without first paying a toll to Redmond.TomTom sued Microsoft this week in U.S. District Court in Virginia, alleging that the software maker's Streets & Trips navigation software violates four of its patents. The countersuit comes weeks after Microsoft sued TomTom for allegedly violating its own navigation-related patents and also, significantly, patents that relate to file transfer systems.

Microsoft claimed that TomTom's Linux-based file transfer system is a knock off of patented Microsoft file transfer technology known as FAT. If Microsoft prevails in the case, it could limit tech manufacturers' ability to use Linux without paying royalties or licensing fees to Microsoft.

TomTom's countersuit, filed March 16, does not address Microsoft's Linux-related claims directly. Rather, it contends that Microsoft's selling of Streets & Trips "directly or indirectly infringes" on four TomTom patents. TomTom is asking a judge to bar Microsoft from continuing to use the allegedly offending technology and is also seeking unspecified damages.

While some open source advocates have expressed disappointment that TomTom did not address Microsoft's allegations regarding the use of Linux directly, such sentiments are likely premature. Since TomTom doesn't own Linux, its defense of its right to use the software would more properly be part of its forthcoming, formal response to Microsoft's original complaint.

"By fighting, TomTom will focus attention on Microsoft's (at best) schizophrenic strategy relating to open source software," said Andrew Updegrove, a pro-open source attorney who is a partner at Boston-based Besmer Updegrove, in a blog post.

Microsoft has made several overtures to the open source community in recent months, including its launch last year of the Open Source Interoperability Initiative--a forum that includes labs, plug fests, technical content, and "opportunities for ongoing cooperative development." Microsoft also promised free access to certain Windows interoperability protocols for non-commercial open source developers.

Still, Microsoft has not backed off its claim that it owns 42 patents used in the creation of the Linux kernel, and its suit against TomTom indicates that the company intends to defend that claim in court, if necessary.