> wondering what the exact meaning of the outlet of [netreceive] in TCP
> mode is:
>
> Does it indicate whether Pd through the OS was able to open a port

The right outlet  indicates that the port is open and the other end is listening.

> or
> does it indicate if a TCP packet arrived at the destination?
>
> And: Am I correct assuming that netsend only outputs a message when it
> got a (send ...) message into its inlet? So there is no way to indicate
> whether the receiving machine is online unless I keep sending
> heartbeat/ping messages?
>

If the other end drops the connection, then the right outlet will output zero, but it might not happen right away.
Especially with wireless it's better to have some kind of heartbeat ping-pong going at a higher level than the OS.

Martin