[ntp:questions] Broadcast mode

Martin Burnicki martin.burnicki at burnicki.net
Mon Sep 12 10:07:06 UTC 2016


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
> ttl 1
> server
> fudge stratum 14
> ...
> # Broadcast Client
> broadcastclient
> server
> fudge 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:

fudge 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

The stratum of the local clock ( 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.


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