Yeah? It's an eight byte header. The OS needs something to tag IP packets to get them delivered to the correct application. So you're thinking maybe a four byte header for 50% savings here?
Good point on there needing to be some application-level addressing anyway.
On top of that, I believe the UDP checksum can be omitted as well at least on some OSes (and is arguably not necessary for fully encrypted/authenticated payloads) – leaving really just the two bytes for the "length field".
So we have a checksum of the IP header, a checksum of the UDP header and a port number, an application level stream ID or message ID or whatever the application transport protocol is using, and finally almost certainly an even higher level message ID such as a URI. And that’s before you introduce encryption into it with all that overhead. A level 4 protocol providing full integrity verification, encryption, multi homing, multiplexing, out of band control, and control over transmission reliability would be amazing. But the only way you can experiment with these things is if you use UDP and ports. We take the concept of ports for granted but if you think of ICMP or some other L4 protocols that isn’t the only way to identify the sending and receiving application.
If we just allowed all L4 protocol numbers through and ditched NAT we could have nice things. Or we could kick it up two layers to use QUIC for what SCTP could have been.
There's going to be encryption either way in any modern protocol, and then the header manipulation stuff is already all done in hardware. It's probably more efficient in UDP than as a direct IP protocol, because UDP is fast-pathed in ways protocols other than 6 and 17 aren't.
Having a diversity of IP protocols isn't a nice thing. The designers of TCP/IP made a protocol specifically for doing the thing you wanted to see SCTP do: it was called UDP.
Why isn’t it a nice thing? And SCTP and UDP clearly provide different semantics. I am fine with experimenting with new protocols on top of UDP because it is simple to do but ultimately I think things like SCTP and QUIC should run directly on top of IP.
