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

David L. Mills mills at udel.edu
Thu Jul 8 16:55:02 UTC 2010


The basic definition of SNTP has not changed over the yeas, although 
rfc5905 does clarify the intended scope and role of primary servers, 
secondary servers and clients. It was the expected, but not required, 
model that the Unix adjtime() system call be used if the offset was less 
than an unspecified value and the settimeofday() if greater. There was 
no intent, either in the earlier SNTP specifications or rfc5905 to 
specify the SNTP clock discipline algorithm itself.


David Woolley wrote:

> 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 
> specification.
> 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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