The primary recommended method for delivering the Microsoft Skype for Business® client from Citrix Virtual Apps and Desktops (formerly known as XenApp and XenDesktop) is HDX RealTime Optimization Pack 2.x, the product of a close collaboration between Citrix and Microsoft to co-develop a new and improved "v2" delivery architecture for Skype for Business and Citrix customers. This is the only Microsoft-endorsed field-proven solution for delivering Skype for Business in a virtualized environment, supporting not just two-way point-to-point calls but also conference calls and Skype Meetings.

Version 2.x of the HDX RealTime Optimization Pack provides a native Skype for Business client UI experience with very few differences in feature set compared to running the Skype for Business client locally on a Windows device, while maximizing server scalability through redirection of media processing to the user device. This article summarizes the salient variances of this solution from non-virtualized Skype for Business as of 2019 2.4 Long Term Service Release of the Optimization Pack. (This article also applies to the previous 2.3 release and 2.5 or higher.) .

User-Visible Feature Variances

In the v2 architecture, the Skype for Business client user interface (UI) is owned solely by Microsoft and not altered by Citrix software. Nevertheless, there are a few UI differences when the Skype for Business client is virtualized.

• Gallery View multi-view video
  Gallery View, where the video for up to five speakers can be seen (plus pictures of other participants), will not be supported. The Optimization Pack supports “subscription-based video mode” (default since the 2.1 release), where the HDX RealTime Media Engine informs the Audio-Video Conferencing Server which video stream to send to it based on both automatic speaker tracking and user choice, and “active speaker only mode”.
• Convenience Recording
  The HDX RealTime Optimization Pack is designed for compatibility with server/network-based audio recording solutions commonly used by enterprise customers, such as Verba, Numonix or NICE. Audio recording on the user device using the “convenience recording” feature of the Skype for Business client is not supported in a virtualized environment, although visuals presented during a call can be captured.
• Pairing of the Skype for Business client with an IP phone
  The Skype for Business “Better Together over Ethernet” features enable users to control activity on compatible IP phones from the Skype for Business client. For example, users can answer, place, hold and resume calls on their phone using the Skype for Business client. While interoperability with standalone Skype for Business qualified IP phones is supported by the HDX RealTime Optimization Pack, the “Better Together” features and USB connectivity to the user device are not currently supported.
• Video Preview from the conversation window 
  With the Optimization Pack, Video Preview is provided under Options - Video Device, but not directly from the conversation window.
• Screen/app sharing from Skype for Business as a published app
  The Skype for Business client’s data sharing features enable it to share either full screens of the virtual or published desktop, or any application that co-resides with it on the same server. So, when Skype for Business is delivered as a published app into a local desktop, only other published apps running in the same XenApp or XenDesktop session can be shared. Applications running locally on the user device or in other sessions cannot be shared, because the hosted Skype for Business client can only “see” applications running in the same session.
• Producer and presenter roles for Skype Meeting Broadcast
  Skype Meeting Broadcast is a feature of Skype for Business Online (Office 365) for holding meetings with large audiences. Skype Meeting Broadcast producers and presenters must use a non-virtualized Skype for Business client to share and record meeting content, as the HDX RealTime Optimization Pack does not support these roles.

Other Feature Variances

• Media Bypass (i.e. Mediation Server Bypass)
  Media Bypass is an optional feature of Skype for Business that enables call routing directly between the user endpoint and the PSTN gateway, offloading the Mediation Server and thereby reducing latency and improving call quality. The Optimization Pack does not support direct media connections to/from PSTN gateways. If Media Bypass is enabled by Skype for Business Server administrators, PSTN calls involving Optimization Pack users will automatically and transparently route media connections through the Mediation Server. There is no end user impact due to this behavior. This approach has the benefit of ensuring that Optimization Pack deployments are interoperable with a wide variety of third-party PBX and PSTN gateway solutions, but does not offload the Mediation Server. Therefore, network administrators need to account for it when planning network capacity.
• Call Admission Control fallback to PSTN
  Call Admission Control (CAC) is a feature of Skype for Business that determines whether there is sufficient bandwidth between interconnected sites to establish a real-time session of acceptable quality. If the quality would be poor, calls are denied or rerouted. The Optimization Pack supports CAC in all network configurations supported by Microsoft, for both on-premises and remote devices. (On remote endpoints, just as with native Skype for Business clients running remotely, only the internal portions of the media path are subject to CAC bandwidth policies.) However, Optimization Pack 2.3 does not support rerouting calls to the public switched telephone network (PSTN), but reroutes those calls to voicemail.

• Panoramic webcam support
  The Optimization Pack does not support panoramic webcams such as the Polycom CX5000 and CX5100 that deliver a 360-degree view of the meeting room.

Fallback to Generic HDX RealTime

On an exception basis, as a fallback in cases where no HDX RealTime Media Engine is available for the device, (for example, HDX RealTime Media Engine 2.3 is not supported on Chromebooks), the HDX RealTime Optimization Pack can be configured to deliver audio only or both audio and video via Generic HDX RealTime technologies. Generic HDX RealTime refers to a set of technologies that support the delivery of virtually any softphone or Unified Communications application from XenApp or XenDesktop. These technologies deliver a rich user experience, but CPU-intensive video processing on the XenApp or XenDesktop server limits scalability. In the context of Skype for Business, the generic technologies support optional fallback to server-side media processing to avoid denial of service. To reduce server load, fallback can be limited to audio only, or disabled entirely.
If the HDX RealTime Connector has been installed on the XenApp or XenDesktop server, Generic HDX RealTime essentially mirrors the functionality of the HDX RealTime Optimization Pack, and leverages an HDX RealTime Media Engine embedded inside the HDX RealTime Connector. If the HDX RealTime Connector has not been installed, Generic HDX RealTime likewise essentially mirrors the functionality of the native Skype for Business client as when running locally on a Windows device, and uses the Microsoft media engine inside the Skype for Business client running on XenApp/XenDesktop.

Additional Resources

Citrix Documentation - System requirements