Network Time Protocol - get atomic clock's precision from the Internet by Girish Venkatachalam
Girish Venkatachalam is a UNIX hacker with more than a decade of
networking and crypto programming experience.
His hobbies include yoga,cycling, cooking and he runs his own
business. Details here:
It is ridiculous that a computer that we buy for several hundred dollars cannot be as accurate as a watch that be bought for less than 10$. That does not have to be the case once you read and understand this article.
Network Time Protocol described in RFC1305 in enough detail and chatty language is the trick behind keeping computers in sync with the most accurate time sources on the plant - atomic clocks on board satellites that are used by GPS gear.
GPS will not even work without sub millisecond accuracy but our computer can be oblivious of time and still do all its work. This is true of even servers including mail servers or web servers.
But it is backward to not have accurate time on your machines. Especially when you can easily get this for free on the Internet.
All you have to do is run the ntp daemon. There are several watered down protocols called daytime and rdate that can help too. For instance, all you have to do is type this as root:
# rdate ptbtime1.ptb.de
That is all there is to it to fix a clock that might have been off by even days. Who knows?
There are also Windows software that can do the same thing. Please google for "free SNTP client". You know what SNTP will stand for if you know NTP. It is Simple Network Time Protocol.
I started my career in Novell in 1998 with a protocol converter project that would make the Netware world talk to the UNIX world. I wrote a program that would act as a bridge for time and translate the proprietary TimeSync format with the NTP time format.
My project suddenly assumed a lot of importance when their Netware server experienced severe time lag due to some driver eating up the timer interrupt.
I think mostly due to corporate politics than anything else, the driver team refused to fix the issue. And the issue was in our hands. I did the right thing by ignoring the timer interrupts and taking time from the hardware clock on the PC. The RTC that is found on most computers.
My work was appreciated in the form of an award though what I did was clearly wrong. The real fix should have been on the driver. But this should tell you something about the commercial corporate world. They put the cart before the horse and see things upside down.
Let us save our crib about the silly commercial world and get back to the wonderful world of UNIX and open source geeks. Before that I wish to point out that my work also was critical in signing a 5 million dollar deal with British Telecom long ago. They had two networks firewalled from one another and time had to be synchronized between them. I suggested using NTP since the Internet has several freely available public NTP stratum 1 servers.
By the way, Novell had a dying need for accurate time due to directory synchronization. NDS which was a precursor to LDAP needed time to be in sync.
But as I said in the beginning, it is ridiculous and somewhat inane not to have your computers in sync with the accurate time. You might not need microsecond accuracy but asking for millisecond accuracy is not too much. Come on, computers are deterministic and this is what they are good at.
There is a command called ntpdate which works similar to rdate on the surface but underneath it is very different.
# ntpdate ptbtime1.ptb.de
I should also talk about the innards just to pique the curiosity of detail oriented peoples. NTP uses source port and destination port UDP 123 for its time query packets. And NTP works in unicast or multicast mode. I have never bothered with multicast configurations. NTP does add traffic to the network but in today's terms it is negligible and inconsequential.
NTP comes with a clock filtering algorithm, a clock selection algorithm when faced with multiple sources that offer different times and it also has protections against malicious tampering by using cryptographic keys.
stratum 1 servers are supposed to be the most accurate followed by stratum 2 or stratum 3 depending on how far away they are from accurate time sources like the atomic clocks. I believe that OpenBSD comes with time sensors that can obtain time through long wave radio and adjust their clocks. This is not available in the entire developed world.
The way NTP does its work is by exchanging UTC time format in the number
of seconds that elapsed from
1900 1970 in a 64 bit field. 32 bits of seconds
and 32 bits of subsecond data. You have leap years, leap seconds and
what not. And the counter will overflow in 2038. People are predicting
the world to end by then...that we will have a revisit of the y2k
problem. Do you care?
Here is the equation for clock correction.
time difference = time at destination - time at source
Time queries are sent as simple UDP packets with time fields on them. The time fields are used to arrive at the correction factor. The computer never sets its clock back. So don't be surprised if your computer does not go back in time. And for NTP to work, the packet latency in the forward and reverse paths have to be the same. Otherwise you won't get accurate time. Think about it, you will know why this is necessary.
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