An MTU is in use that does not correspond to any well-known standard.
Test the path in the opposite direction to ensure that there are no MTU negotiation conflicts.
If other MTU problems are found, follow the course of action suggested.
On paths that exhibit very high packet loss, re-run the test to ensure MTU consistency.
MTU, or maximum transmission unit, is also known as link MTU. Measured in bytes, it's the maximum packet size that can be sent in one piece over a hop without being fragmented. It differs from path MTU (PMTU) which is the minimum MTU of the hops in a path. MTU issues are direction sensitive, so there could be MTU problems in either direction. Single-ended paths can only test MTU negotiation in the monitoring point-to-target direction, so you'll need to deploy a monitoring point at the other end of the path to test in the opposite direction. In the presence of packet loss the path MTU discovery process can generate an underestimate for the MTU. One way to verify the discovered MTU is to re-run the test a couple times to see if the result is consistent. That said, the fact that the discovered MTU is associated with a known standard suggests that it is not an accidental underestimate.
Possible secondary messages
'Packet loss conditions may have resulted in an underestimate'
'ICMP may be MTU-limited by PIX firewall or similar device'
'ICMP may be MTU-limited by CISCO VPN Network Enhancer or similar device'
'ICMP may be MTU-limited by Checkpoint firewall or similar device'