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by umvi 2270 days ago
> Finding a use case for this would be tricky...

I can tell you a real life use case that already exists because I built this exact thing already (though a closed-source version): satellite internet terminals.

With satellite internet, uplink (upload) is generally more complicated than downlink (download). This is because a terminal can just passively listen for packets intended for him, but with uplink you have to make sure you only transmit in your designated time slot.

Because uplink is more complicated than downlink, more often than not uplink will get in a degraded or broken state, while downlink is still operational.

Now say I get a call from a customer service rep saying a customer's terminal is stuck in a degraded state. How can I recover it? TCP (and therefore HTTP) is a no-go because it requires uplink to be fully functional. So I can't visit the terminal's embedded webserver or issue commands to the HTTP API.

However, using a udp to tcp proxy like this, I can issue commands to the terminal's HTTP API "blindly" which it will then execute to (hopefully) recover itself. This can also be done with other "one-way" protocols like SNMP.

3 comments

Wow. I was literally about to reply with the same exact thing (built my own as well)! No value in this comment other than to validate your comment.
I feel like the real solution is your terminal should ping every hour, and if a ping test fails it should auto-reset itself and file a bug for the developers with whatever state information is needed for them to fix the bug.
Unfortunately satellite internet systems are way more complicated than you probably think. It can be very hard to tell why a terminal is experiencing issues. Sometimes it's because of a nearby cell phone tower and you have to adjust some terminal configurations to shift frequencies slightly. Maybe there's just a lots of bad weather in that particular part of the country and rain fade is degrading the signal and the terminal isn't transmitting at max power like it should. Maybe your neighbor has a police radar detector in his car interfering with the signal. Maybe a horse has bumped the dish and it's slightly mispointed at the satellite now. Maybe it's an issue on the gateway side. (all true stories).

Unless it's a mass issue across a whole beam, it usually requires some investigation to figure out why an individual terminal state is degraded. And sometimes (more than I'm comfortable with) during the course of your investigation you make the terminal state even worse (because you tweaked some config parameter the wrong way). You can always tell when you done messed up because suddenly you get disconnected from your ssh session or you can no longer reach the embedded web server from your browser. In those cases sometimes the only way to recover is to precisely undo what you did, and an auto-ping-reset will not fix it (unless the system keeps track of most recent config changes or something). Your options are: use udp/tcp proxy or snmp to attempt to undo the config parameter you foobared or send out a technician and fix it on site...

I'm not saying our system is perfect. Surely we could improve fault detection and recovery. I'm just saying one-way communication protocols are real nice to have sometimes because of these scenarios.

We run a bunch of remote paging basestations that utilize satellite, however, they are also backed up with 4G routers which are used instead if the satellite link fails for any reason (rain fade, nuclear missile attack on the satellite hub/network etc). I guess TV over 4G would use a lot more bandwidth and potentially be very expensive.
Or it could be a few nuts. https://www.youtube.com/watch?v=cZkAP-CQlhA

( yes, I know that's a terrestrial, microwave relay dish and not a satellite antenna, but it could happen.)

Wouldn't such a system be a big security hole?
No because you can still use cryptographic signatures to ensure the packets are valid before acting on them.