Microsoft filed suit against TomTom today, alleging that the in-car navigation company's devices violate eight of its patents -- including three that relate to TomTom's implementation of the Linux kernel.

It's believed to be the first time Microsoft has filed a patent suit over Linux, after claiming for years that elements of the open-source operating system violate its patents. However, Microsoft says open-source software is not the intended focal point of the action. Five of the alleged patent violations relate to proprietary software.

Microsoft says it filed the case as a last resort, after trying for more than a year to reach an agreement with TomTom. We've requested comment from TomTom, and we'll update this post with the company's response.

The case, in U.S. District Court in Seattle and the International Trade Commission, is the third time Microsoft has brought such a suit, said Horacio Gutierrez, Microsoft corporate vice president and deputy general counsel for intellectual property. Both of the previous suits settled after they were filed. By comparison, Microsoft has struck more than 500 patent licensing deals in the past five years.

[Update: Here are the complaints filed in federal court: PDF, 10 pages and at the ITC: PDF, 25 pages. A TomTom spokesman declined to comment, citing a policy against discussing pending litigation.]

"In exceptional cases such as this one, when a pragmatic business agreement cannot be reached, we feel that we really have no choice but to pursue legal action to protect our innovations and our partners who license them," Gutierrez said.

Five of the patents in dispute relate to in-car navigation technologies, while the other three involve file-management techniques. Gutierrez said Microsoft has reached licensing agreements with with other in-car navigation vendors over the same patents, and it remains open to "quickly resolving" the TomTom dispute through licensing.

Microsoft has said previously that Linux and other open-source programs violate more than 200 of its patents -- elevating the tension that has long existed between Microsoft and the open-source community. Asked if this TomTom case is the start of a broader legal campaign over those alleged violations, Gutierrez said no.

[Follow-up: Q&A: Microsoft's IP chief on TomTom, Linux and patents.]

"Microsoft respects and appreciates the important role that open-source software plays in our industry and we respect and appreciate the passion and the great contribution that open-source developers make in our industry," Gutierrez said. He said that respect and appreciation is "not inconsistent with our respect for intellectual-property rights."

Microsoft's most recent patent suit was against Primax Technologies, over computer mice. That case settled in December, when the companies reached a licensing agreement.