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the internet has a fundamental assumption that anyone can send anything to anyone. this assumption has of course been broken at the IP layer by firewalls and NAT, causing no end of headaches for application developers and users. that assumption permeates the whole stack - not only can i send a packet to any computer on the public internet, i can email anybody, i can message anybody on any of the messaging platforms. i can tag anybody on social media. on the internet everybody is next door and can come round whenever they want.
but what if they couldn’t?