The day my ping took countermeasures

Summary

Once my holidays had passed, I found myself reluctantly reemerging into the world of the living. I powered on a corporate laptop, scared to check on my email inbox. However, before turning on the browser, obviously, I had to run a ping. Debugging the network is a mandatory first step after a boot, right? As expected, the network was perfectly healthy but what caught me off guard was this message: I was not expecting ping to take countermeasures that early on in a day. Gosh, I wasn't expecting any countermeasures that Monday!Once I got over the initial confusion, I took a deep breath and collected my thoughts. You don't have to be Sherlock Holmes to figure out what has happened. I'm really fast - I started ping before the system NTP daemon synchronized the time. In my case, the computer clock was rolled backward, confusing ping.While this doesn't happen too often, a computer clock can be freely adjusted either forward or backward. However, it's pretty rare for a regular network utility, like ping, to try to manage a situation like this. It's even less common to call it "taking countermeasures". I would totally expect ping to just print a nonsensical time value and move on without hesitation.Ping developers clearly put some thought into that. I wondered how far they went. Did they handle clock changes in both directions? Are the bad measurements excluded from the final statistics? How do they test the software?I can't just walk past ping "taking countermeasures" on me. Now I have to understand what ping did and why. An investigation like this starts with a quick glance at the source code: * P I N G . C * * Using the InterNet Control Message Protocol (ICMP) "ECHO" facility, * measure round-trip-delays and packet loss across network paths. * * Author - * Mike Muuss * U. S. Army Ballistic Research Laboratory * December, 1983 Ping goes back a long way. It was originally written by Mike Muuss while at the U. S. Army Ballistic Research Laboratory, in 1983, before I was born...