Internet Control Message Protocol, designed for control and diagnostic messages
In computer networking, the MTU (Maximum Transmission Unit) of a communications protocol of a layer is the size (in bytes) of the largest protocol data unit that the layer can pass onwards.
"Any device in the path of communication between the sender and receiver whose mtu is smaller than the packet will drop such packets and reply the sender with icmp destination unreachable datagram too big message containing the device s mtu"
from question "When sending data larger than the SendBufferSize, how will the data be received?"
"What happen when icmp is disabled in an router and when packet size greater than mtu how the router fragments that packet"
from question "Will TCP header be present in IP fragmented packets"