[time-nuts] Serial or other simple protocols for exchanging time

[Bob kb8tq]

Hi

NTP brings along a bunch of “baggage” that gets you out of the category
of “couple of lines of code” protocol. What I’m suggesting is very much the
same thing as a GPS sentence. Sent once a second with a very minimal 
payload. You just are doing it via UDP instead of RS-232 serial. 

Bob

> On Aug 15, 2019, at 11:35 AM, Tim Shoppa <tshoppa at gmail.com> wrote:
> 
> Bob, ntpd for ages has supported broadcast/multicast UDP.
> https://www.eecis.udel.edu/~mills/ntp/html/assoc.html#broad
> 
> If you care about security it's like a bag of angry cats. And the
> one-way-ness removes the ability to measure round-trip delay. So it's
> pretty rare to see it being used well.
> 
> Tim N3QE
> 
> On Thu, Aug 15, 2019 at 11:08 AM Bob kb8tq <kb8tq at n1k.org> wrote:
> 
>> Hi
>> 
>> Which all sort of begs the question:
>> 
>> Why not a simple “broadcast UDP” once a second time packet approach for a
>> home LAN?
>> 
>> Unless you get really crazy, it’s not going to be a very big packet.
>> Seconds since some
>> arbitrary point in time. Time zone offset. Maybe a leap second count.
>> Server ID maybe.
>> Less than 100 bytes not including the overhead.
>> 
>> Bob
>> 
>>> On Aug 15, 2019, at 5:36 AM, Hal Murray <hmurray at megapathdsl.net> wrote:
>>> 
>>> 
>>> ausserirdischesindgesund at gmail.com said:
>>>> I am a newbie and am wondering what options there are for exchanging
>> time
>>>> on a more basic level than NTP or PTP (that is for situations when a
>>>> full network stack is too complex).
>>> 
>>> You haven't described your problem fully yet.
>>> 
>>> Are you interested in client side or server side?  (or both)
>>> 
>>> What sort of environment are you working in?  What sort of hardware do
>> you
>>> have available?
>>> 
>>> NMEA over a serial port is probably what you want, but...
>>> 
>>> 
>>> Raspberry Pi and similar are not very expensive.  They come with
>> networking
>>> software.  The Pi isn't very nice for time-nut work over the net because
>> the
>>> Ethernet is on USB which adds jitter and/or hanging bridges.  It does
>> have
>>> GPIO.
>>> 
>>> 
>>> There is a lot of things you can do without a "full network stack".
>>> 
>>> What level of hacking is reasonable depends on your environment.  For a
>> setup
>>> at home, you are unlikely to annoy anybody else.
>>> 
>>> The Alto firmware could boot over the (3 MB) Ethernet.  The boot servers
>> would
>>> periodically send a boot-loader packet to a reserved hardware address.
>> The
>>> firmware only had to setup the hardware to receive a packet, wait for
>> one,
>>> sanity check things, and jump to it.
>>> 
>>> If you use UDP rather than TCP, the "stack" level packet format is much
>>> simpler.  Retransmission becomes trivial if you only have one un-ACKed
>> packet
>>> to consider.  Performance on a LAN is OK most of the time.
>>> 
>>> For something like a NTP server, you can avoid routing and ARP by
>> sending the
>>> reply back where it came from.
>>> 
>>> For the client side, the normal problem is finding the server.  If you
>> only
>>> have one server, you can wire in the address.
>>> 
>>> 
>>> 
>>> 
>>> --
>>> These are my opinions.  I hate spam.
>>> 
>>> 
>>> 
>>> 
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