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Microsoft has made available what could be the final beta of Windows Server 2003 R2, which is slated to ship by the end of the year.

The server operating system release candidate, which is typically a final beta before general availability of the software, was made available for download from Microsoft's Web site.

Windows Server 2003 R2 release candidate 1 (RC1) is built on Windows Server 2003 SP1 and is actually the second release candidate. The first was RC0, which was released August 30. The first public beta was made available in May.

Key to R2 is an identity technology called Active Directory Federation Services. ADFS is one of the final pieces of Microsoft's identity management platform and it also represents the company's first implementation of the WS-Federation protocol it developed with IBM. Microsoft has positioned WS-Federation as the linchpin for integrating security infrastructures between companies.

Also part of R2 is branch office support so servers can be remotely managed from a central location, integration of Services for Unix into the core operating system, new storage management features, support for .Net Framework 2.0, and enhancements to the Windows SharePoint Services collaboration technology.

Virtualization

The R2 release is the first so-called "interim" release of the server OS since Microsoft initiated a release cycle in May 2004 that called for a major OS upgrade every four years with a lesser "R2" release in between.

Last week, Microsoft said it would use R2 as the launching pad for its new licensing for virtualization, which takes effect December 1. Under the plan, users can run four virtual machines on top of the Enterprise Edition of Windows Server 2003 R2 without any additional licensing costs beyond the base server. Users can run any Windows server version within those virtual machines, including Windows NT and Windows 2000.

Microsoft officials said there have already been 25,000 public downloads of Beta 2 and RC0 combined.

The R2 release candidate software is available for both x64 and x86 versions, but cannot be installed on anything other than software versions of Windows Server 2003 with Service Pack 1. The R2 release candidate 1 is available for evaluation for 180 days.