Pivoting

[Hal Murray]

gem at rellim.com said:
> I'd have ntpd reject any time prior to EPOCH. 

How do you decide whether to reject it or pivot it into the future?


>> Allow the user to specify the pivot time and/or life time, either
>> at build time or at run time or both.
> EPOCH is used for NMEA, so that is covered at build time.
> I could see adding an option to specify the EPOCH at run time too.

My build time comment was mostly for life time.  I was assuming that EPOCH 
would be used for pivoting.

I know about three pivots to consider.  One is GPS 10 bits for weeks with a 
20 year step size.  Another is 2 digit year numbers with a 100 year step 
size.  The third is 32 bits of seconds in NTP packets with a 136 year step 
size.  Are there any others I've overlooked?

If we want our software to last more than 20 years while talking to crappy 
GPS receivers, we need a way to update the pivot date at run time.  (I'm 
using "last" to mean without rebuilding.)

If we want our software to reject bogus time, we have to balance the tradeoff 
between long life and good filtering.  Run time parameters will allow the 
user to choose.


> As mentioend earlier, PDP-8s still run.  That is late 1960's.  Call it 50
> years. 

It would be interesting to see what those setups are actually doing and if 
they have documentation for something as obscure as NTP.

There is a lot of lab gear running embedded software.  I wonder how much of 
it will be running at its 25th or 50th birthday.  I have a pre-software scope 
that's over 35 years old.


The combination of long life and crappy GPS seems obscure enough that I'm 
willing to document it as a limitation.  It's the kind of code that Eric 
would love to rip out if he found it a year ago.

The documentation issue gets interesting.  A feature isn't any good if you 
can't figure out how to use it.  I wonder if the web will solve that problem. 
 Will NTPsec still be online 20 years from now?  Will we maintain online 
versions of 20 year old releases?

Would anybody notice a warning message from a program that's been running for 
19 years?


-- 
These are my opinions.  I hate spam.