Microsoft has been blocked from adding a new networking feature into its next release of Windows by a Californian court.

The TCP offload technology, codenamed Chimney, was intended to be added to Longhorn and a service pack for Windows Server 2003 but San Francisco District Court has granted a preliminary injunction against the software giant by Alacritech.

Alacritech bills itself as "the world’s leading supplier of TCP Offload Engine (TOE) solutions that enable the highest performance and efficiency in networked systems" and claims it discussed its technology with Microsoft in 1998 and that Microsoft subsequently cut off communication with the company.

In May 2003, Microsoft demonstrated a technology it called "Chimney" that Alacritech said was similar to its own intellectual property. Alacritech offered Microsoft a licence, but Microsoft rejected Alacritech's terms, and in August 2004, Alacritech filed suit claiming patent infringement. In November, it filed a motion for a preliminary injunction to keep Microsoft from infringing the patent or inducing others to do so.

This is hardly the first time Microsoft has been accused of holding talks with companies and then breaking them off only to introduce a very similar technology in its own products further down the line, but this is believed to be the first time a court has pre-empted its inclusion.

Microsoft claims however that Chimney was independently developed by Microsoft engineers. It is reviewing its options, a spokeswoman said. The Scalable Networking Pack that was due to contain Chimeny has not yet shipped, but the spokeswoman did not know whether the lawsuit had stalled its release.

The preliminary injunction prevents Microsoft from "making, using, offering for sale, selling, importing or inducing others to use" Chimney.

TCP is the key protocol used in most IP (Internet Protocol) data networks. TCP offload technology shifts the burden of handling TCP tasks from the main processor of a server or workstation to a network interface card or other component, leaving the main processor more capacity for application processing. Alacritech promotes its patented SLIC Technology architecture as scalable enough to meet future networking and I/O needs at 10G bit/s and beyond.