> I'd think that all the intermediate routers would want to have an opinion about my packet, caring, at the very least, that it's either TCP or UDP.

They absolutely don't. Routers are layer 3 devices; TCP & UDP are layer 4. The only impact is that the ECMP flow hashes will have less entropy, but that's purely an optimization thing.

Note TCP, UDP and ICMP are nowhere near all the protocols you'll commonly see on the internet — at minimum, SCTP, GRE, L2TP and ESP are reasonably widespread (even a tiny fraction of traffic is still a giant number considering internet scales).

You can send whatever protocol number with whatever contents your heart desires. Whether the other end will do anything useful with it is another question.

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lxgr 20 hours ago | parent [-]

> They absolutely don't. Routers are layer 3 devices;

Idealized routers are, yes.

Actual IP paths these days usually involve at least one NAT, and these will absolutely throw away anything other than TCP, UDP, and if you're lucky ICMP.

	▲	eqvinox 16 hours ago \| parent [-]
		See nearby comment about terminology. Either we're discussing odd IP protocols, then the devices you're describing aren't just "routers" (and particularly what you're describing is not part of a "router"), or we're not discussing IP protocols, then we're not having this thread. And note the GP talked about "intermediate routers". That's the ones in a telco service site or datacenter by my book.