OpenConnect Cisco VPN and MTU issues
At my university, I had the funny issue that screensharing with Cisco Webex did result in a black screen. The other participant did see that someone wanted to share something, but you did not see the content.
It took me quite some time, but as I recently stumbled across MTU issues as well. I checked if this might be the issue here too.
I found out, that this problem occurs when creating a VPN session on a link with a small MTU size. As the VPN reduces the MTU as well, the resulting MTU can be something like 1240 bytes. In this case applications like Cisco Webex do not work correctly (e.g. screensharing stays blank).
While this makes sense for IPv6 (with a minimum MTU of 1280 Bytes), the minimum MTU of IPv4 is 576 Bytes. As my affiliation does not yet support IPv6 at all, I would have assumed that it just works. Especially as most other tools work well with a smaller MTU for IPv4.
I then found out about the --mtu param and the --base-mtu param to openconnect and tried out increasing or decreasing the MTU using this to investigate a workaround.
However, I always ended up with tun0 having an MTU of 1300 on a wifi network with MTU 1440 (so 140 Bytes for Cisco VPN..?).
With the initial problem being found, I am now unsure if the mtu params work as intended. And I am quite sure that Cisco software again has its flaws.
I reported the problems to the projects here: