MTR probes routers on the route path by limiting the number of hops individual packets may traverse, and listening to responses of their expiry.
It will regularly repeat this process, usually once per second, and keep track of the response times of the hops along the path.
MTR relies on Internet Control Message Protocol (ICMP) Time Exceeded (type 11, code 0) packets coming back from routers, or ICMP Echo Reply packets when the packets have hit their destination host.
MTR also has a User Datagram Protocol (UDP) mode that sends UDP packets, with the time to live (TTL) field in the IP header increasing by one for each probe sent, toward the destination host.
When the UDP mode is used, MTR relies on ICMP port unreachable packets (type 3, code 3) when the destination is reached.
Example of output: