WAN packet overhead is the time a packet carrying your data takes to pass through a wide area network to reach its destination.
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MTU is the packet size transmitted from the device. Ethernet common size is 1500 bytes. Smaller packet sizes can be used and those 1500 bytes can be broken apart downstream to accommodate the network path. The device does not always recieve packets of 1500 bytes from the internet. https://en.wikipedia.org/wiki/Maximum_transmission_unit