Silicon Valley start-up Alacritech has won a preliminary injunction against Microsoft over the use of advanced networking technologies due to feature in Longhorn, the next version of Windows. The ruling (PDF), which is subject to appeal, temporarily blocks Microsoft from developing or selling systems based on the disputed technology.

Alacritech sued Microsoft in August 2004, alleging that Microsoft's existing and future operating systems containing the "Chimney" TCP offload architecture use Alacritech’s proprietary SLIC technology. The disputed code, designed to eliminate network processing bottlenecks, crops up in the Scalable Networking Pack for Windows Server 2003 as well as Longhorn. Alacritech argues that Microsoft's software violates two of its patents relating to scalable networking, US Patent No. 6,427,171 and US Patent No. 6,987,868, both entitled Protocol Processing Stack for use with Intelligent Network Interface Device.

A US District Court in San Francisco granted Alacritech's request for a preliminary injunction on 5 April. Microsoft has 21 days to appeal. ®

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