There is a device in the path which was detected via the traceroute phase of Delivery monitoring but which does not respond suitably for testing purposes.
- Verify that the device under test exists and is powered on.
- Ensure that ping and traceroute are operational along the test path.
- Ensure that appropriate protocols are enabled, i.e. ICMP Echo, ICMP Echo Reply and ICMP TTL Expired messages are enabled along the entire test path.
- If it is not possible to enable these protocols, turn on the advanced test setting "Continue test if unable to connect to target". Delivery monitoring will only be able to test the path up to the target, but will not be able to test the target.
- Deploy a target Monitoring Point on the target device to test targets without ICMP Echo connectivity.
The echo reply option may be turned off for this router.
There is another rare routing condition that can cause this symptom. The hop that responded obviously is on the path to the target, and is able to establish a path back to the Monitoring Point. However, packets that are generated by the Monitoring Point and that are addressed directly to the discovered hop may not find a path to that device. If this is the case, usually "destination unreachable" messages are returned. We sometimes see this situation when Internet providers use unpublished addresses in the middle of their network.
The "traceroute" phase of Delivery monitoring works by sending packets with low time-to-live (TTL) values in the IP headers. The TTL value specifies how many hops the packet is allowed before it is returned. When a packet can't reach its destination because the TTL value is too low, the last host returns the packet and identifies itself with an ICMP "time exceeded" message. By sending a series of packets and incrementing the TTL value with each successive packet, Delivery monitoring finds out who all the intermediary hosts are. In this case, although the host is there, Delivery monitoring cannot elicit a response from it with subsequent test packets.
Possible secondary messages
- "Device cannot be measured directly" This secondary message is used in conjunction with other secondary messages when the target cannot be tested.
- "Ensure that ICMP is enabled such that ping and traceroute operate correctly" The target does not respond at all to test packets. Ensure that ICMP is enabled along the test path.
- "Measured MTU is too small for testing" Since Delivery monitoring requires a variety of test packet sizes for testing, the MTU must be sufficiently large to accommodate the variations. The measured MTU in this case is too small for testing.
- "Device severely limits test packet size" The measured MTU is within an acceptable range for testing, but ICMP packets are severely limited in size by a device, to the point that it prevents testing.
- "Response packet size does not match sent packet size"