[ntp:questions] Re: NTP Multicast server/client?

David L. Mills mills at udel.edu
Mon Oct 3 15:06:06 UTC 2005


Danny,

I really like it when folks send in plain, unformatted ASCII. It's much 
easier to read and better resists spam filtering.

By default, to mobilize a broadcast client requires authentication using 
either symmetric or public key cryptography. Ordinarily, eight packets 
are exchanged with the server at two-second intervals. This is 
sufficient to both accurately calibrate the propagation delay and run 
the authentication protocol. This can be disabled with the novolley 
option in the broadcastclient command. If so or if the server does not 
respond, a default delay is assumed and life goes on.

Dave

Dave

Danny Mayer wrote:

> David Woolley wrote:
> 
>> In article <433E0D98.6070004 at gis.net>, mayer at gis.net (Danny Mayer) wrote:
>>
>>
>>> Wilhelm Greiner wrote:
>>>
>>>> Also i see when ntpd with debugging runs that it transmits any packets
>>>> to find out is the way back reachable.
>>>>
>>>
>>> I don't understand what you mean here. Can you explain?
>>>
>>>
>>>> Can i turn off this initial trials??
>>>>
>>>
>>> I also don't understand this question.
>>
>>
>>
>> When a broadcast client is starting it makes a normal access to the
>> server to calibrate the round trip delay and therefore estimate the
>> downstream delay.  It will fall back to the configured delay if this
>> fails.
>>
> 
> I'm not sure that it does send a packet if authentication is not 
> enabled, but I'd have to check.
> 
>> Also, I believe that if you enable authentication, and that is strongly
>> encouraged for broadcast, this exchange is mandatory to negotiate keys.
>> (I vaguely remember that authentication is defaulted on for broadcast.)
>>
> 
> Authentication is enabled by default for both broadcast and multicast. 
> Turning it off means that you have accepted the risks of doing so.
> 
> When you authenticate that means that you are exchanging a bunch of NTP 
> packets between broadcast/multicast client and server. It's described as 
> the "key dance" in the documentation. I don't remember how many packets 
> are exchanged, but it's more than two.
> 
> Danny
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