[ntp:questions] NTP on LAN

David L. Mills mills at udel.edu
Mon Sep 29 02:44:18 UTC 2008


David,

I'm not sure which configuration you are considering; the documentation 
clearly shows, even in the diagrams, that there can be more than one 
orphan server and more than one orphan client. The expressed intent is 
that, should all sources of time be lost, that onlyu all orphan clients 
select the same orphan server. It's as simple as that.

Orphan mode was motivated by certain misguided configurations that tried 
to engineer failover configurations using engineered local clock stratum 
assignments. These are dangerous, fragile and will form loops unless 
specifically and carefully done. Usually under some failure scenario or 
other a cycle forms between two local clocks and in the best 
Bellman-Ford tradition, they count to infinity.

Orphan mode was specifically designed for the most common case where a 
LAN has multiple Internet servers and where one or more of them can 
become disconnected. The (single) orphan server becoms the source of 
last resort.

Dave

David Woolley wrote:

> Steve Kostecke wrote:
> 
>>
>> Nonsense.
>>
>> A single ntpd can be configured in Orphan Mode to serve as the "server"
>> for a time island.
> 
> 
> I hadn't considered the degenerate case.  However, if one looks at the 
> key selling points for orphan mode, in the documentaion, one finds that 
> they all depend on having multiple orphans at the same stratum:
> 
>>    There  are  many  disadvantages using the local clock driver: multiple
>>    source  redundancy  is  not  possible  and the subnet is vulnerable to
>>    single-point failures. Orphan mode is intended to replace the need for
> 
> 
> If it really has benefits in contexts where there is only one orphan 
> mode server, the documentation needs amending to mention them.
> 
> Note two orphans at the same stratum aren't possible here, as the 
> requirement included a strict precedence and two at different strata is 
> another degenerate case.
> 




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