[ntp:hackers] smearing the leap second

Brian Utterback brian.utterback at oracle.com
Sun Jun 21 15:23:29 UTC 2015


On 6/21/2015 9:56 AM, Terje Mathisen wrote:
> If you ever allow a mix of smearing and non-smearing servers, then 
> you've already lost!
>
> The smearing server(s) will smear, hopefully all using the same 
> (Google) slew formula and hide the leap second announcement, while the 
> standard servers will set the leap bit and not smear.
>
> There is _no_ way you can expect this to work in combination, unless 
> you also replace the clients with a sw version which knows about 
> "SMRx" REFIDs and how to extract the current smear offset. In the 9 
> days remaining until Jun 30 this will _not_ happen. :-(
>
> Smearing only makes sense when you administer a strict inhouse 
> environment, where you control all ntp paths for all internal systems.
>
> Terje
>

All that you say is true, but the reality is that if we add this feature 
to the reference implementation, there will be networks with a mix. I 
might even venture that they will be in the majority. For this to be 
robust, it is very important that the servers know when the time that 
they are being served has been smeared. We don't want the smear to be 
applied twice in the chain, we don't want both smearing and non-smearing 
servers in the chain.

What we do want is that if all software in the network were updated and 
leap smear aware, to do the right thing. Then, replacing all of the 
clients with old software, that again it would do the right thing. Then 
with a mix of servers. What happens if the clients have a leap second 
file? Updated clients should be able to have a choice of actions. We can 
do all of this with an extension field. Using only a refid field doesn't 
allow for this and breaks loop detection for the duration of the smear.

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