[sgjohnson]

can't spoof the source IP in TCP communication, as the handshake cannot happen.

With UDP you can send whatever, but obviously you won't be able to receive the response.

[Sesse__]

It used to be possible back in the days when sequence numbers were easily guessable. (You'd obviously not be able to receive, only send, so you couldn't do TLS, but TLS had hardly been invented at the time.) Now operating systems are way too good for that. :-)