Microsoft is plannig an extension to the RSS 2.0 spec to make it "multi-directional" - namely, adding synchronisation across different applications.

RSS 2.0 is best known as a way to let Internet users subscribe to content from websites. When content on a site is updated, the RSS feed informs the subscriber, usually with a summary of the updated content and a link to it.

Microsoft's extensions will let it exchange and synchronise content that is updated by two or more parties. The goal is to expand the one-way publishing mechanism to two-way, or three-way etc.

The software giant published version 0.9 of the specification, called Simple Sharing Extensions (SSE), on its site earlier this month and is seeking feedback for a final version.

The extensions "enable feed readers and publishers to generate and process incoming item changes in a manner that enables consistency to be achieved," Microsoft said. "In order to accomplish this, SSE introduces concepts such as per-item change history (to manage item versions and update conflicts) and tombstones (to propagate deletions, and un-deletions)."

The specification could be used to keep contact lists synchronised across a user's various devices, such as a PC, PDA and mobile phone. Or it could be used by family members (or co-workers) to synchronise entries they wish to share from their personal calendars.

Microsoft has a checkered past when it comes to "extending" technologies it does not own, raising inevitable questions about its intentions with RSS 2.0. Sun sued Microsoft for extending Sun's Java technology in a way that prevented some Java applications from running properly on Microsoft's software.

"Microsoft is notorious for developing what it calls 'standards' that are actually Microsoft standards," said Chris Harris-Jones, a principal analyst with Ovum.

Still, Microsoft said its aim is to define "the minimum extensions (to RSS) necessary" to achieve its goal. It released the specification under the Creative Commons license, which is also the license used for RSS 2.0, and it said it is not aware that it owns any patents related to SSE. If it finds any, it said, it will offer a royalty-free patent license on "reasonable and non-discriminatory terms".

Ovum's Harris also wondered why Microsoft picked RSS 2.0 rather than a similar syndication format, Atom. RSS is far more widely used, but Harvard University, which currently owns RSS 2.0, has said it does not plan to update that specification any further, according to Harris. "RSS 2.0 is frozen; it's not going anywhere," he said.