Leap second coming at the end of June, watch your databases 😉

Yes, negative delay in ntpd. Packets travelling back in time when going from the server to your client. Fasten your seat belts people, Captain Kirk is back 😉

OK, seriously now. As you can expect, I was quite surprised to see this, but it appears to be expected in certain conditions.

Here's the full story. … Continue reading →

Running ntpq -p to check how ntpd is working is one of the first things a sysadmin learns when trying to debug problems with ntpd. One of the first questions which arises is "how do delay, offset and jitter relate?", or even "what do delay, offset and jitter mean?". Well, today I stumbled across a graph that gives me the opportunity to explain this in practice.

Let's start with explaining these three terms as bare-bones as possible. Note that definitions given below are here just to give you an idea and are no way accurate; please go to the NTP documentation, NTP FAQ or community support sites to get the real ones.

• delay is an estimate of how many milliseconds an NTP packet takes to travel from a server to your client.
• offset is an estimate of how many milliseconds your computer's clock differs from UTC.
• jitter is an estimate of how these measurements are accurate, and it's non-negative: the smaller the jitter, the better accuracy.

Now look at this graph: … Continue reading →

It was some time I didn't go through the NTP docs and I felt I was missing some of the new interesting features of ntpd. So I decided to freshen my knowledge and go through them once again, and I allocated some time every week to read the docs.

Recently I started reading "Event messages and status words". I almost immediately found one thing I could not understand: apparently, the codes in the system status word and peer status word exceeded the assigned capacity of four bits. But I was tired and I decided to wait until the week after and re-read it.

Yesterday I started reading it again, and I also went through my notes from last time I read the doc. No luck, it was not that I was tired: there really were codes that could not be represented using just four bits. But, again, I thought I was mistaken and sent a message to the questions@ntp.org list.

And, to my surprise, it was actually a bug!

This drives me to two considerations.

The first: read the docs, ask yourself questions, then ask for clarifications if you need them, and report bugs if you happen to find one.

The second is actually a joke: only a pain-in-the-ass like me could bother enough to read that document, and count the bits and the codes, and find a mismatch. What a poor reputation I am building for myself 🙂

Quoting a friend of mine regarding this:

The international scientific community worked their ass off to provide the world with an accurate time, accepting some discontinuities so that it won't drift too much from the "solar time". And what does Google do? They invent the gummy second!!!

😆