UDP reconnect logic with PX4

[al2950]

I think this issue has probably caught a few people out in the past. But when using UDP with PX4, PX4 will record the port & ip address of the first heartbeat it receives. It will then send any status and responses to this address. This address is set once, and only once, and so if you disconnect and reconnect with a different local port (on client), you will not receive any messages until the PX4 is rebooted.

Suggest adding a default port to MavSDK which it attempts to bind to, currently it is set to 0 so OS picks a 'sensible' random port.

[JonasVautherin]

Isn't that a PX4 issue? If PX4 only supports one UDP client and needs to be rebooted to move to a new one, then it has to be fixed on the PX4 side, doesn't it?

[julianoes]

One thing to note is that PX4 re-uses the same ports, so they are both set statically, while MAVSDK chooses the local port randomly.

[al2950]

Isn't that a PX4 issue? If PX4 only supports one UDP client and needs to be rebooted to move to a new one, then it has to be fixed on the PX4 side, doesn't it?

Maybe, but I dont think its a bug in PX4.

Firstly (in PX4) most of the logic assumes a single client, allowing multiple clients would add complexity (which in turn decreases reliability) Secondly adding logic to monitor a client, and potentially drop it if it does not hear from it may result in unintended dropped clients due to a network hitch... which would be a safety concern.

I have not discussed this on the PX4 github, but my pov was that it would be way easier to just allow a user to specify a local port with MAVSDK. If not then i suggest adding this note to the troubleshooting docs

[JonasVautherin]

Disclaimer: I am not the one who needs to be convinced, I'll leave the final word to Julian 😇. But I'd like to share my opinion, which is that PX4 having to reboot in order to swap to a different client is a bug. And allowing to set the local port with MAVSDK would be a hack. The way I see client-server connections is that the server exposes a port and the client contacts it over this port. When I connect to GitHub, my browser does not set a local port to make it easier for GitHub to answer, and I don't need to reboot GitHub between two calls 🙈.

This said, without changing anything, I'm pretty sure that you can solve your problem with mavlink-router. Did you consider that?

[julianoes]

I agree with @JonasVautherin but I'm also aware that we can't just change the way PX4 is set up, and we want to be compatible with it, so we should find a way.

@al2950 can you describe what steps I would take to reproduce this? I need to play with this to figure out exactly what is happening and then think through it.

[julianoes]

I think I tried to fix this in PX4 a while ago: PX4/PX4-Autopilot#21608

[al2950]

I agree with @JonasVautherin but I'm also aware that we can't just change the way PX4 is set up, and we want to be compatible with it, so we should find a way.

@al2950 can you describe what steps I would take to reproduce this? I need to play with this to figure out exactly what is happening and then think through it.

Hi @julianoes, thanks for the reply. I am currently away and I dont have any access to a computer (and trying to limit any screen time). I will put together a test for you when I get back in a couple of weeks.

However it would be useful what, if any, PX4 hardware you have access to? Or if you are comfortable setting up PX4 SITL with gazebo or similar