udpping

How it works

- Every --interval ms, a packet containing a fresh GUID (16 bytes) is
sent to <host>:<port>.
- When a reply arrives, its 16-byte payload is matched back to the GUID
that was sent, and the elapsed time is the round-trip time.
- A packet with no reply within --timeout ms counts as dropped.
- Time is divided into --display-interval ms ticks. Every packet belongs to the tick
it was sent in (not the tick its reply lands in), so a tick's
column keeps updating with new data even after later ticks have
already appeared on screen - a late reply, or a drop, can still land
a mark on a column that's scrolled a few positions to the left.

> Note: the target must actually run a UDP echo service that sends
> back exactly what it received. Most servers on the internet do not run
> this; you'll typically point this at your own echo listener (many
> simple ones exist in a few lines of Python/Go/etc.), or a router/appliance
> that offers RFC 862 echo.