What's happening:
In the network environment which the device could both acess the WiFi and cellular network in IPV4-only.
We tried to use cellular interface to send udp packets when both the WiFi and cellular network switch were on.
The host app could send udp packet as usual, however, the app extension, which is a packet tunnel provider(such as VPN process), could not make any udp packet requests.
For more detail, we bind the pdp_ip0 interface. And send some packets to a remote server. I tried to issue a simple udp request in the packet tunnel provier extension when it launches.
After that, we used Wireshark(a network protocol analyzer) to trace the packets. But there was not any packet which we had sent was traced.
It shows the Local Address cellular ip and port, the Foreign Address ip is *:* (interface pdp_ip0), but the packets sent and received are both 0.
More information:
This only happens when using both wifi and cellular(4g) network. The device includes iphone6/7/7plus. The operating system is iOS9/iOS10.