Unless the number of hops to trace is explicitly set with the hop option, mtrace first attempts to trace the full reverse path by default. If there is no response within a 3 second timeout interval, a "*" is displayed and the probing switches to hop-by-hop mode. Trace queries are issued starting with a maximum hop count of one and increasing by one until the full path is traced or no response is received. At each hop, multiple probes are sent. The first attempt is made with the unicast address of the host running mtrace as the destination for the response. Since the unicast route may be blocked, the remainder of attempts request that the response be multicast to mtrace.mcast.net (224.0.1.32) with the TTL set to 32 more than what is needed to pass the thresholds seen so far along the path to the receiver. For the final attempts, the TTL is increased by another 32.
Alternatively, the TTL may be set explicitly with the TTL option.
The output of mtrace is a short listing of the hops in the order they are queried, that is, in the reverse of the order from the source to the receiver. For each hop, a line is displayed showing:
the hop number (counted negatively to indicate that this is the reverse path)
the multicast routing protocol
the threshold required to forward data (to the previous hop in the listing as indicated by the up-arrow character)
the cumulative delay for the query to reach that hop (valid only if the clocks are synchronized)
The response ends with a line showing the round-trip time, which measures the interval from the time the query is issued until the response is received, both derived from the local system clock.
Mtrace packets use special IGMP packets with IGMP type codes of 0x1E and 0x1F.