Lets say we have Alice and Bob communicating via UDP, and Eve indirectly knows the current state of this communication. She also knows the destination MAC,IP and port of those communicating.
Now she wants to send one message to the receiver Bob. This message appears to the receiver totally like if it was sent from Alice.
Now I know we don’t have to worry about the source MAC. But can you spoof the source IP? The ISP can easily check this, but it does not need this information for further routing. My intuition is, that the ISP should not care in general. It would be redundant if the communicating system already protects against such behaviour. It would be pointless if there is atleast one ISP that does not verifiy this, one could VPN tunnel to that ISP Provider and construct an ethernet frame from there.