[ntp:hackers] Fw: Internet delays affecting NTP timing - feature or bug?

Peter Martinez peter.martinez at btinternet.com
Sun May 26 09:46:40 UTC 2019

Repeating my last message:

----- Original Message ----- 
From: "Peter Martinez" <peter.martinez at btinternet.com>
To: <hackers at lists.ntp.org>
Sent: Thursday, May 23, 2019 7:14 AM
Subject: Internet delays affecting NTP timing - feature or bug?

> Greeting to the Hackers list from a new member.
> I have recently been writing code for NTP, both client and server, as a 
> follow-up to building a GPS-based time reference.  I wanted to use the GPS 
> to explore the performance of the NTP system globally.  I worked from the 
> RFCs and have not studied the "reference" software.  I have noticed a 
> strange and worrying phenomenon which I would like to describe and see if 
> others can see it too.  Maybe something needs to be done about it.
> I first spotted that if I poll a distant NTP server from an NTP client on 
> one PC on my LAN, I see a different offset and delay compared with another 
> instance of the same client, either on the same PC or another PC on the 
> same LAN.  But ONLY on a small number of distant servers.  This led to the 
> discovery that it was difference in the local port addresses of the two 
> PCs that was triggering the effect - by modifying the client software so 
> that I could change the local port address, I discovered that polling one 
> of the affected servers then changing the port address, would change the 
> indicated offset and delay.  On other servers, changing the port address 
> has no effect.
> I have a lot more detail I can describe, but for this post I just want to 
> ask if anyone else has seen this.  If not, maybe someone would care to 
> look for it.  Modify your NTP client software to let you choose the local 
> port address in real time, poll a few servers around the world and see 
> what shows up.
> I am fairly sure that this isn't a bug in my software, and I am fairly 
> sure it's not in the affected servers.  It seems to be "out there" in the 
> big wide internet.  If I am right, then this will affect NTP accuracy.  I 
> could ask the question "how is it that some part of an internet route can 
> introduce a spurious delay to a packet of data which is triggered by the 
> content of the packet?".  Is it a feature or a bug? If it's a bug, can it 
> be fixed?  NTP will be better if it can be fixed.
> regards
> Peter Martinez
> NW England

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