[ntp:questions] Broadcast mode

Martin Townsend mtownsend1973 at gmail.com
Mon Sep 12 10:34:28 UTC 2016


Hi Martin,

Thank you for your swift response.  I see that I may have been a bit
naive in my initial choice :) I will try client/server with your
suggestions.

Cheers,
Martin.

On Mon, Sep 12, 2016 at 11:07 AM, Martin Burnicki
<martin.burnicki at burnicki.net> wrote:
> Hi,
>
> Martin Townsend wrote:
>> Hi,
>>
>> We have a closed network of embedded devices that we want to keep in
>> sync, the actual time is not important but the fact that they are all
>> on the same time is.  One device is always the master so this is the
>> device that we want the others to synchronise to.
>>
>> Reading the documentation broadcast mode looks perfect.  I'm currently
>> trying it out with the following configuration:
>>
>> # Broadcast Server
>> broadcast 192.168.2.255
>> ttl 1
>>
>> server 127.127.1.0
>> fudge 127.127.1.0 stratum 14
>>
>> ...
>>
>> # Broadcast Client
>> broadcastclient
>>
>> server 127.127.1.0
>> fudge 127.127.1.0 stratum 14
>>
>> ...
>>
>>
>> I've seen what look like valid messages being exchanged using tcpdump
>> but haven't seen any evidence that the times are converging but I
>> haven't really left it long enough. In the meantime I have read this
>> link
>> http://www.ntp.org/ntpfaq/NTP-s-config-adv.htm#AEN3658
>> which seems to indicate that this setup is not possible.  So my question is
>> Is broadcast mode with LCL a non started and if so what is the best
>> method for a private network that you need to keep in sync?
>>
>> I'm using the latest version 4.2.8p8, built using Yocto for an ARM cortex A8.
>
> Broadcast mode requires some kind of authentication by default, so the
> client know the broadcast packet has really been sent by the trusted
> server. Without authentication any node on the network could start
> sending broadcast packets with a wrong time, and the clients might
> follow that wrong time.
>
> So you either have to configure authentication properly on the server
> *and* on each client, or you you have to disable authentication
> explicitly on each client.
>
> The question still remains why you think broadcast mode is better than
> normal client / server mode. in broadcast mode the client is unable to
> determine and compensate the packet delay on the network, so the
> resulting accuracy is worse than with client / server mode.
>
> Foe client / server mode you'd just need:
>
> on the server:
>
> server 127.127.1.0
> fudge 127.127.1.0 stratum 0
>
> on each non-Windows client:
>
> server aa.bb.cc.dd iburst
>
> on each Windows client:
>
> server aa.bb.cc.dd iburst minpoll 6 maxpoll 6
>
> Of course you had to replace "aa.bb.cc.dd" by the real IP address of
> your server. "minpoll 6 maxpoll 6" avoid some limitations on Windows
> clients.
>
> The stratum of the local clock (127.127.1.0) on the server should be
> fudged to 0 so the server is visible as stratum 1. If you fudge the
> local clock to 14 then the server will be stratum 15, which is only one
> level above "not available".
>
> This is not accepted by some NTP implementations, and I'm not sure if
> ntpd as client would even accept a stratum 5 server, broadcast or not.
>
>
> Martin


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