So the DNS folks are like "Wouldn't it be great if we could just send large responses over UDP". And it's true. It would. And so it was invented, as part of EDNS0, a general extension mechanism for DNS.
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Some more fun: network paths don't have to be symmetrical, so MTUs don't either. If the client needs to send a large amount of data, this whole process happens for them too. A sender and a receiver can legitimately end up with different limits towards one another.
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Because MTU discovery depends on state, and on ICMP messages being allowed, some folks do something like "MSS clamping" where for TCP they have the network actually meddle with the TCP connection (a little) to offer a different MSS to the other end.
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