Use of Android VpnService MTU parameter

Question

I'm building a VPN application which acts as a packet sniffer and then forwards the packets to the active device.network. Gmail is having intermittent attachment download issues when my VPN is on. I do not set the MTU on the VPN.

Android VpnService.Builder exposes a method to set the MTU https://developer.android.com/reference/android.net/VpnService.Builder#setMtu(int)

While creating a VPN service, what can be the issues if this parameter is not set. The documentation says,

If it is not set, the default value in the operating system will be used.

I'm trying to figure out if there can be issues related to speed or Android DownloadManager, if the MTU is not set properly.

Answer 1

I found out the problem to be a known bug in Android related to split tunnelling, existent on Android 8 - 9.

https://issuetracker.google.com/issues/111574523