But the structure of those spaces can be different! The only structure IP imposes is that every packet has a source and destination address. It's up to each pro

But the structure of those spaces can be different! The only structure IP imposes is that every packet has a source and destination address. It's up to each protocol whether it has port numbers (like TCP, UDP, and SCTP), or not (like ICMP and IGMP), or some other mechanism for identifying flows.