Microsoft is extending the popular RSS 2.0 web syndication format to make it 'multidirectional', allowing it to be used for synchronising information such as contacts and calendar entries across different applications, the company has said.

RSS 2.0 is best known as a way to let internet users subscribe to content from websites that support RSS (Really Simple Syndication) feeds. When content on a site is updated, the RSS feed informs the subscriber, often with a summary of the updated content and a link to it.

Microsoft is developing a set of extensions to RSS so that it can be used for exchanging and synchronising content that is updated by two or more parties. Its goal is to take what is essentially a one-way publishing mechanism and make it multidirectional.

The company published version 0.9 of the specification, called SSE (Simple Sharing Extensions) for RSS 2.0, on its website earlier this month and is seeking feedback for a final version.

To understand what the extensions hope to achieve, imagine two PC users who wish to share and co-edit a list of items using an RSS feed. Both people publish their lists using RSS with the sharing extensions, and both also subscribe to the other's feed.

Whenever either of the two updates their list, the changes are added to their feed and incorporated into the list of the other subscriber.

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 firm added.

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, explained Ray Ozzie, Microsoft's recently hired chief technical officer, in a posting on his blog.

Ozzie's involvement in SSE is no surprise – he created Lotus Notes, which lets workers update and synchronise calendars, documents and other files with each other. Notes was part of the inspiration for SSE, Ozzie said.

After joining Microsoft he met with some of its product teams, including Exchange and Outlook, and thought about ways of synchronising information among Microsoft products, as well as with those of other companies, he wrote. Soon after, SSE was born.

"In just a few weeks time, several Microsoft product groups... built prototypes and demos, and found that it works and interoperates quite nicely," Ozzie wrote. It's too early to say which Microsoft products will use SSE, and code won't be released until version 1.0 is ready at a future, unspecified date, he said.

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, for example, 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 at UK research company Ovum.

Nevertheless, 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 licence, which is also the licence 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 licence on "reasonable and non-discriminatory terms".

Harris-Jones 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.

Atom, on the other hand, was submitted this year to the Internet Engineering Task Force for standardisation. "It would be nice if Microsoft would support Atom, and then submit SSE for standardisation alongside it, so it all becomes part of an internationally recognised standard," Harris-Jones said.

"Otherwise – and I'm being very cynical here – you might end up with Microsoft's RSS, which only Microsoft uses, and Atom, which everyone else uses. So you end up with two standards, one for Microsoft and one for everyone else. But then maybe that's too cynical, even for an analyst."

Microsoft said it picked RSS in part because it is a very simple technology. The SSE extensions could be used with Atom "in principle", the company said, although the version developed does not support it.

The SSE specification can be used with the OPML (Outline Processor Markup Language) format, used for creating hierarchical lists such as categorised music playlists, Microsoft said.

An FAQ document about SSE, including an explanation of how it works, is available here.

The SSE efforts are distinct from Microsoft's other RSS work, the firm said, such as the planned support for RSS within Vista, and Simple List Extensions to RSS, which can be used to let websites publish lists, such as photo albums or music playlists.