[ntp:questions] SNTP with 1ms of precision?

David Woolley david at ex.djwhome.demon.invalid
Thu Jul 8 06:50:26 UTC 2010

Danny Mayer wrote:
> On 6/16/2010 5:22 PM, Maarten Wiltink wrote:
>> "Marcelo Pimenta" <marcelopimentacs at gmail.com> wrote in message
>> news:AANLkTilQ6M8ApEoaSIbr-o8mhwiFqKFV9xyf6MUDraDt at mail.gmail.com...
>> [...]
>>> The NTP algorithm is much more complicated than the SNTP algorithm.
>> The short, short version: there is no SNTP algorithm. SNTP is NTP
>> _without_ the algorithms. Using NTP means continuously adjusting the
>> speed of your clock so it tracks real time as best you can make it,
>> while SNTP is simply asking what time [they think] it is.
> This is a totally inaccurate statement. See RFC 5905 Section 14. SNTP is

That RFC was published after this thread was started! You can't go 
changing the definitions just for you convenience.  Even if it had been 
published, say six months earlier, the reality is that de facto and 
historic definitions would still dominate the market.

> merely a subset of the full NTP protocol. An SNTP server is one with
> it's own refclock and not dependent on any other upstream servers while
> and SNTP client is one with a single upstream server and no dependent
> clients. An SNTP client or an SNTP server should be disciplining the
> clock in the same way as an NTP server. An SNTP server should
> continuously adjust the speed of your clock otherwise it's not SNTP
> compliant.

In reality, most SNTP clients step the clock.  A few may use a simple 
frequency control scheme.  Once you go much beyond that, it becomes 
simpler to use a full NTP client, but maybe configure only one server.

In fact, RFC 1305 doesn't require any specific clock discipline for 
NTPv3 clients; that is in an appendix, rather than in the main 

The important clarification about SNTP, ignoring any recent attempt to 
redefine it, is that it doesn't specify an algorithm, rather than that 
it requires the use of only a trivial algorithm.

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