How can I send downlink Iperf data over USB tethered latop with Android phone?

Question

I have tethered my laptop with an Android phone. I am using RNDIS in the USB. I can send UL iperf data over the net. But I cannot send DL data to the laptop. DL data can be pushed to the phone but not the laptop. Any idea?

Answer 1

Most native tethering/hotspot functions on smart phones utilize NAT, so the problem you're most likely running into is the laptop is "behind" the NAT service. Unlike other NAT routers/devices there is no native port-forwarding service on a smart phone. I'm sure there is probably a third-party app for Android that can handle port-forwarding, but it might require rooting the device's OS. I know there is no option for Apple products.

Keep in mind that there is always the possibility that your wireless service provider could hand out a private IP address to your smart phone's LTE modem, meaning the phone itself would be hidden behind a NAT service. That doesn't sound like your issue though as you say you can receive download data to the phone from iPerf. But keep in mind there is always that possibility and there would be nothing you can do, except call your wireless service provider and request a static public IP address (at a cost).

You might check and see if your phone is receiving an IPv6 address from your wireless service provider. If yes then it is possible that your smart phone is also providing an IPv6 address to your laptop via the RNDIS connection. Because there is no real NAT in IPv6 it should be completely routable, but there is also a possibility that the smart phone will be filtering ingress traffic to your tethered/hotspot devices.