It sends to this back to the sender ... the DNS server in this case. And so the sender has a clue what even triggered the error, it includes the top part of the packet that triggered the error.
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It also means that fragments of the message look strange to many parts of the network, like firewalls and switches. UDP packets without UDP headers! How are they supposed to know whether the packet is allowed or not? How is flow-switching supposed to work? The internet shrugs.
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We often end up building in the ability to relate these packets to one another in many places, and since that's expensive, they also have to be rate-limited. It's deeply messy.
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That's what's going on down there though, and understanding that full picture is key to diagnosing some common network mysteries. O.k. I have a team meeting now, more later!
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Meeting postponed! o.k. so what happens when the DNS server sends a 4K response and the MTU is 1200 bytes? Well the DNS server gets the error, and it fixes the *next* response, but that first is lost. Super annoying. So the client has to retry, but then it works.
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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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