Issue: Unable to share desktop using Skype for Business.
- An error occurred during the screens presentation.
- We couldn’t connect to the presentation because…
- We had some trouble starting your presentation…
User unable to share desktop with external user to internal user. While sharing desktop, user sees above mentioned error messages.
Recently I came across strange issue, were user was unable to share their desktop with other Skype for Business users who was on corporate network or external using Skype for Business client.
UCCAPILOG shows INVITE going out with full candidate list, like Host, Srflx and Relay, and received 200 OK with candidate list like Host, Srflx and Relay from other end, however connectivity check failed and Application sharing never connected and I got above mentioned errors.
Below is the log, which shows, between ACK and BYE took 10 second time however there was no connectivity establish between endpoints.
Client diagnostics says ms-client-diagnostics: 23; reason="Call failed to establish due to a media connectivity failure when one endpoint is internal and the other is remote";
All required port was open, same user was able to share desktop using different network or connecting to his mobile hotspot. That gives sense something was wrong on his network which fails connectivity checks.
When I asked user to connect his home router (Xfinity) and check settings, we found Firewall under gateway setting (Gateway > Firewall > IPv4) it was set as “Maximum Security (High)”. I advise to change it to “Typical Security (Medium)” and tried Desktop sharing again.
This time Desktop sharing works fine, J in fact we were using VBSS (Video Based Screen Sharing) which is using UDP candidate, but both endpoints must VBSS to use. You can refer this TechNet article for VBSS and how it works, as this article not focusing VBSS.
The point here is, users home router (service provider “Comcast”) has firewall setting which was blocking connectivity check and that’s why desktop sharing was affected. Reducing firewall security level to medium resolves issue.
I have attached below screenshot for reference.