Video-based Screen Sharing (VbSS) uses server video port range and NOT the server application sharing port range…
In the June 2016 cumulative update for Skype for Business Server 2015, Microsoft introduced support for Video-based Screen Sharing (VbSS). Loads of people have covered VbSS in detail, so I don’t want to get into that but I want to cover something I experienced and that is the effect it might have on port ranges and QoS marking.
One thing that slipped past me when we upgraded to take advantage of VbSS was that any VbSS media sent from the Skype server uses the server video port range and NOT the server application sharing port range.
So, depending on how you have QoS configured in your environment, you could suddenly see a lot more traffic in your video class than you did previously. In my case, instead of application sharing traffic being marked with AF31 (DSCP 26) it was instead marking as AF41 (DSCP 34) when using VbSS.
For us this meant several reports of poor quality content and poor quality video where the video class was being exceeded.
A few Wireshark traces later and I was able to confirm that whilst VbSS was in action, the server video port range was being used when it left the server. If I forced the meeting back to RDP the application sharing range was used again. TechNet confirmed this:
To get around this I had to add an additional Application Sharing QoS policy specifically for VbSS which specified both the video port range and the ASMCUSVC.exe application with the appropriate DSCP value (in my case, DSCP 26).
After this, application traffic, whether is was using VbSS or RDP, whilst using different port ranges was marking correctly as AF31 (DSCP 26).