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I had assumed that since IPPROTO_MPTCP(262)!=IPPROTO_TCP(6), they were using another protocol number for everything, but you're right that there's a TCP option involved too.

But also, plenty of stateful firewalls will strip unknown options. Here's how to enable Ciscos to forward it: https://www.cisco.com/c/en/us/support/docs/ip/transmission-c....

Edit: here's checkpoint: https://support.checkpoint.com/results/sk/sk114666

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IPPROTO_MPTCP (262) is too big to fit into the byte that's reserved for the protocol number. I'm not sure what the reason is Linux has this protocol number, I'm guessing it's so that user mode sockets can easily request MPTCP sockets?

I'm not too surprised about Cisco and Checkpoint. I suppose corporate networks are intentionally always a pain when it comes to new protocols and features.

I think you're right that it serves only as a way to communicate to the kernel that userspace wants MTCP socket.

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