How will Citrix support 2008R2 server for XenDesktop DDCs?

The current state of affairs

At present, if you want to build a XenDesktop Desktop Delivery Controller (DDC), it has to be on a Windows 2003 server. This requirement comes from the fact that the XenDesktop code is based on a customised XenApp 5 installation which is Windows Server 2003 only. XenDesktop 4 will have been around for a year soon and we still need to deploy “legacy” 2003 servers when building our shiny new XenDesktop farms. Sounds kind of crazy doesn’t it? But let’s examine why this is and what Citrix might do about it in a future release. Note this article isn’t based on any specific knowledge of future product releases, just analysis of the existing products.

2008R2 challenges

As we know, XenApp 5 can’t be installed onto Windows 2008R2 server – this requires XenApp 6. So, for Citrix to support 2008R2 XenDesktop DDCs, it would seem logical that the XenDesktop code needs to be based upon XenApp 6.

Lets look at the reasons you need a different version of XenApp for Windows Server 2008R2:

1. New RDS API hooks

Microsoft changed the hooks for Terminal Services (now known as Remote Desktop Services or RDS) to remove the “secret” API that Citrix developed and then licensed back to Microsoft. Citrix had to change their ICA stack into RDS to comply with the new APIs which meant a re-write of a number of system-level drivers.

2. 64-bit only OS

Windows Server 2008R2 is a 64-bit only operating system, which means all driver code has to be fully 64-bit and correctly signed.

So a Windows 2008R2 DDC release will be based on the XenApp 6 code?

Seems logical doesn’t it? But having XenApp 6-based DDC’s on Windows 2008 R2 servers will create the same problem XenApp users have in that you can’t mix XenApp 5 and 6 farms. Having to create a separate XenDesktop farm to introduce your first 2008R2 DDC would be a real pain. You would need new AD OU’s, SCP objects and a whole bunch of new desktop groups.

But let’s dig deeper and see if this will really be the case.

XenDesktop DDCs are equivalent to XenApp servers without the terminal services component (although bizarrely you need Terminal Services installed in application mode to install a XenDesktop DDC – a legacy left over from XenApp perhaps?)

So if RDS isn’t required, it should be relatively easy to port the non-RDS dependent components onto Server 2008R2 as it’s the RDS hooks that require the expensive re-write. Having the same “branch” of the code-tree should allow mixed XenDesktop farms with both 2003 and 2008R2 DDCs – something XenApp customers just can’t do due to the RDS changes and the new policy design Citrix also introduced in XenApp 6.

Citrix already offer a 64-bit 2003-based DDC installation so all the 64-bit porting issues will already have been addressed.

Finally, thinking about the Virtual Desktop Agent (VDA). This is like a mini-DDC – it has an IMA stack to communicate with DDCs and also doesn’t utilise RDS having it’s own ICA stack not requiring RDS. Crucially, this is supported on Windows 7, which is the same kernel as Server 2008R2 – so Citrix have most of their XenDesktop 4 stack already working on this kernel version.

So, the final question of this article remains unanswered: Why is it taking Citrix taking so long to release a DDC installation for Windows Server 2008R2?