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RCM6760 Detect Ethernet cable disconnected

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I'm attempting to create more robust TCP/IP connections to our Rabbit RCM6760. It maintains a passive connection where application software connects and stays connected for long periods of time transferring data back and forth via a raw TCP connection. It works fine under most operating conditions.

We are having some issues with the connection locking up, where there is a connection error of some type and we cannot re-connect even after closing the application software. These are still being diagnosed on the software side, but it would be good for our Rabbit to take a more active role in analyzing the status of the connection and closing it if it's in a strange state.

Most specifically, I noticed tcp_tick() and sock_fastread() don't seem to detect when the ethernet cable is disconnected. I found this post:
http://cms.digi.com/support/forum/60345/can-you-check-to-see-if-an-ethernet-cable-connected-the-rabbit
that recommends using pd_havelink() to detect that condition at least, but I'm surprised tcp_tick() and sock_established() don't call this at a lower level before checking for new data. Or if they do, I'm not aware of the right status to check that's updated/returned by them.

Are there general tips for maintaining a robust connection using the Rabbit's tcpip API?

Conditions we can ideally detect:
- Ethernet cable disconnected from Rabbit (can do using above)
- No connection to a network (Ethernet cable disconnected past a switch/hub, or no other devices on network)
- Application software has attempted to disconnect, but has done so incompletely (*RST packets have at least been sent both directions)

Any help is appreciated, thanks!

- James
asked Jan 9 in Rabbit Software by jrmclaugh New to the Community (1 point)

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1 Answer

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After further research, it looks like the only two real options in our use case are:
- pd_havelink() to detect immediate physical connection being lost
- tcp_keepalive() to detect anything between the Rabbit module and the application being lost

This works for us since we're always on closed networks and don't have a heavy load, so keepalive packets every 60sec or so won't hurt. Our connections tend to be persistent, anyway.

Are there any other things I should be aware of using tcp_keepalive(), other than increased network usage?

Thanks!
answered Jan 10 by jrmclaugh New to the Community (1 point)
Started my answer and then saw your response waiting in the moderation queue.

Yes, tcp_keepalive() is a good solution for detecting a TCP socket going down.  It should add minimal network usage, but usage might be a consideration if you're on a cellular network.
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