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Sep 25 / The Architect

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?


Leave a comment
  1. Stephane Thirion / Sep 26 2010

    DDCs in Windows 2008 R2 ?
    Without IMA and Terminal Server components it will be easy, isn’t it ?

  2. AGVirt / Feb 10 2011

    As mentioned in the Neil’s article, one issue with Citrix XenApp 6 is that, unlike previous versions, it doesn’t support mixed server farms (server clusters that contain more than one XenApp and/or Windows server version in the same farm.). That leaves 3 possible options for upgrading existing XenApp sites:

    Option 1: Upgrade all servers to XenApp 6 all at once (since XenApp 6 only runs on Windows Server 2008 R2). This is not easy to accomplish in a production environment, which can afford little or no downtime. And if you are hosting an application that isn’t very compatible with 64-bit platforms (Windows Server 2008 R2 is 64-bit only) then you’re truly stuck.

    Option 2: Create a secondary farm for the XenApp 6 servers; then distribute the clients between the two using a Web interface. This introduces 3 challenges: (1) Since XenApp 6 has new management consoles and a whole new way of doing configuration and scripting, each farm will need to be managed separately using a different tool-set. (2) Determining how to properly distribute the clients between each of the two farms requires close monitoring; otherwise users may not be able to connect to the proper farm. (3) In all likelihood the combined size of both farms will potentially be much larger than a single farm would have been.

    Option 3: Switch to Ericom’s PowerTerm WebConnect. With a three-tier architecture, PowerTerm WebConnect (certified by Microsoft for Windows Server 2008 R2) can support mixed farms containing Windows Server 2003, 2008, 2008 R2 and future versions of Windows as well. So it can actually be easier, safer and more cost-effective to not only choose this solution over Citrix’s in the first place, but even to upgrade an existing XenApp farm to PowerTerm WebConnect than to XenApp 6!

    You can read more and download a free evaluation at:


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