[ntp:questions] NTP orphan mode with manycast configuration

Steve Kostecke kostecke at ntp.org
Wed Aug 15 02:55:33 UTC 2007

On 2007-08-14, sylcournoyer at gmail.com <sylcournoyer at gmail.com> wrote:

> On Aug 14, 11:25 am, Steve Kostecke <koste... at ntp.org> wrote:

<snip: minimal multicast / orphan howto>

> Thanks for the quick reply.
> With regard to your reply, just to be sure I am understanding the
> right thing and based on my readings, the ntp configuration that
> I want to test/use eventually would use the config parameters
> "manycastclient" and "manycastserver" instead of "multicastclient" and
> "broadcast" :

multicast/broadcast and manycast are entirely different modes.

In multicast/broadcast mode, the client listens on the
multicast/broadcast address. Multicast/broadcast servers send out an
NTP packet every 64 seconds. When the client first hears a server a
temporary unicast association is established between that client and
server. This temporary association is used to calculate the broadcast
delay between the server and the client. Once the broadcast delay is
known the unicast association is discarded and the client resumes
listening on the multicast/broadcast address and now utilizes the time
stamps in the NTP packets.

In manycast mode, the client sends out server discovery probes. Once the
client locates a server a unicast association is established between the
client and server. 

> # I am already on a secure network and I want the smallest IP packets
> # possible
> disable auth

This makes a negligible difference in packet size and leaves these
systems open to a rogue time server.

> # Each computer connected to a local NEMA GPS
> server iburst prefer minpoll 4 maxpoll 10

1. iburst has no effect on ref-clocks

2. prefer is meaningless when you only have one ref-clock

3. 'minpoll 4 maxpoll 10' is almost the default for ntpd. In my
experience 'minpoll 4 maxpoll 4' makes ntpd follow the GPS more closely.

> tos orphan 7
> manycastclient 
> manycastserver

Fine. Any multicast or broadcast address will do.

> driftfile /etc/ntp.drift

It is not considered a good practice to allow daemons to write to /etc

/var/something/ntp.drift is a safer choice

> If all the GPS devices loose their lines of sight with satellites,
> then NTP will automatically enter the orphan mode on each computer,
> which means that one of the computer will be automatically "elected"
> as the "time master"

Based on random values generated by ntpd at start-up.

> and that all the other computers will be synchronized on it (of course
> then the time in the system will drift with respect to the UCT time)

If you have one system which you know has a more stable clock set that
system's orphan stratum to 6 (i.e. one less than everyone else's) so
that it will be prefered in the event that your systems enter orphan

Steve Kostecke <kostecke at ntp.org>
NTP Public Services Project - http://support.ntp.org/

More information about the questions mailing list