What would happen if we didn’t use TCP or UDP?

At some point, I wondered—what if I sent a packet using a transport protocol that didn’t exist? Not TCP, not UDP, not even ICMP—something completely made up. Would the OS let it through? Would it get stopped before it even left my machine? Would routers ignore it, or would some middlebox kill it on sight? Could it actually move faster by slipping past common firewall rules?

No idea.

So I had to try.
↫ Hawzen

Okay so the end result is that it’s technically possible to send a packet across the internet that isn’t TCP/UDP/ICMP, but you have to take that literally: one packet.