Why was that dumb of SCTP? What should it have done instead?

Used a UDP port. Making new IP protocols is literally what UDP is for.

Other than getting through broken routers, what's the advantage of building a protocol on top of UDP rather than IP?

("Getting through broken routers" is certainly a significant advantage; I'm just asking if that's the only good reason to not build directly on IP.)

The only "advantage" to declaring an IP protocol is that you might save 8 bytes and a trivial checksum. Mostly, though, declaring an IP protocol is a vanity decision.

I'm wondering the reverse: what's the advantage to building on UDP, other than passing routers that for some reason are inclined to pass UDP but reject IP protocols they don't know? You said that this was "what UDP was for", and I was hoping for some more detail there on why UDP helps. As you said, it's just 8 bytes and a trivial checksum.

The advantage to building on UDP is that it gets through (some) middleboxes; that's what the 4 bytes of port numbers buys you. That, and the fact that UDP is designed to be the TCP/IP escape hatch for things like SCTP that don't want TCP's stream and congestion control semantics.

So there's no advantage other than getting through broken middleboxes?

No, another advantage is that you can do UDP entirely in userland without privilege.

What allocating a new IP protocol says is pretty close to "all bets are off, and we're carefully taking responsibility for how every system that interacts with TCP/IP headers will handle these packets". Since SCTP doesn't need that, there's no upside. It's vanity and bloody-mindedness.

Very true. I'd of course run tests, but I would guess port 80 would work these days because of QUIC. Even port 53 is probably locked down to whitelisted hosts.

Private ones usually do. (outgoing)

Corporate ones sure don't. Even outgoing HTTP is passed through proxies on most bigco networks I've worked at.

Yes. But I don't think that corporate firewalls matter when it comes to user adaption. If you need to work with SCTP in your company, you'll get your firewall rule, if not you don't.

If they want to only allow proxied HTTP, that's their decision and developers should respect that, and not mask everything under HTTP. Their administrators made a substantial effort to forbid everything, why would honest person try to overcome their effort?

Home routers is another matter, home users don't make conscious decision about it, but in my experience UDP works just fine (because a lot of games use it) and it should be good enough for any protocol.

Because in many cases, those administrators don't even realize they're breaking the network. They just run a botched config that they inherited from their predecessors or something like that.

You might say that they're clueless and shouldn't be allowed near networking equipment (and you might even be right) but it's not going to change a thing. For the foreseeable future, working around broken environments is all we can do.

It's not. Talk to them. If you really need port XYZ open because your task requires that, you'll get the port open.