Ways of communication between two Arduinos?
Let the ultrasound-module broadcast measurements and not care about whether anyone are listening or not. If you are using wifi, you can send to the broadcast address of the network you are using. Then the ultrasound-module does not need to know anything about the receiver, like ip address or number of receivers. The only thing you need to make sure is that they are on the same subnet. If you want to, you can later put another module on the same subnet and it will also receive the data.
If you establish a handshake protocol it complicates things, because there are so many cases you need to consider. What should happen if communication link dies after handshake, but before data are transmittet? Should the receiver acknowledge the data? What should the sender do, if it does not receive the acknowledge? etc...
If the only thing the sender is using the handshake for is to know whether to send data or not, then just send it and forget about it. If it was to do something special, in case there are no receiver, like logging data and send the log when the receiver comes back online, then there is a use case for the handshaking.