Is it the application,or the layer underneath?
Three monitor types kept deliberately separate from application logic: ping for reachability and path quality, TCP for an open port, DNS for the right answer. When a site will not load, these tell you where to start.
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Three types, three questions
Ping
Does the host answer at all, how long does the round trip take, and how steady is the path?
TCP port
Is the service listening on that port, and does the connection complete in reasonable time?
DNS record
Does the name return the value you expect, and how quickly does the answer arrive?
Ping-specific conditions
Reachability alone is not the whole story; path quality is measurable too.
Packet loss
Set a threshold above which the check fails. Empty means off; zero means any loss is a failure.
Jitter
The spread of reply times, for latency-sensitive services such as voice and video.
Worst round trip
Separately from the average, the slowest packet in that check can carry its own threshold.
Average round trip
An overall ceiling on response time, as with the other monitor types.
Vantage points and resolvers
Inside and outside
The same host is checked from different vantage points; a difference means a path problem, not a service problem.
Custom resolver
You can point a check at a specific DNS server to measure how that resolver behaves.
Precise failure reasons
Name not found, DNS timeout, connection refused and no replies are each recorded as their own reason.
Where this stops
ICMP is not always allowed
Some networks block ping; there the result proves nothing about service health and TCP is the better check.
No arbitrary commands
A monitor runs only these defined checks; for a deeper look at the path there is a separate diagnostics tool.
We watch from outside
These report the view from the outside network, not the CPU or memory of the server itself.
Related features
Separate the network layer
Add a ping or TCP monitor next to the HTTP one for the same service, so an outage tells you which layer broke.
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