[ntp:questions] NTP Broadcast/Multicast Benefits

Matt Kinard kinard at raytheon.com
Tue Aug 2 17:41:51 UTC 2005

Is there a threshold that one doesn't want to cross?  How many clients is too
many for one server?  How does one judge?  I guess that these are the questions
that I've got.

Brad Knowles wrote:

> At 10:24 AM -0500 2005-08-02, Matt Kinard wrote:
> >     I was wondering if anyone could enumerate on the reasons why one
> >  would want to configure an ntp in a broadcast/multi-cast mode.
>         The overhead of handling one-to-one communications between each
> client and server pair can get excessive, and interfere with the
> ability to keep good quality time -- due to excessive UDP packet
> loss, latency and jitter due to heavily loaded network buffers,
> etc....  Even if you were using TCP, the jitter and latency would be
> excessive, due to retransmits and overflowing buffers, and TCP would
> add a whole host of it's own problems.  Moreover, there comes a time
> when you'd run out of memory and network I/O capacity on the
> server(s), regardless of all the other issues.
>         Using broadcast and multicast modes allows the server(s) to
> handle a much larger number of clients and still keep good time.
>         Of course, broadcast mode won't cross subnets, unless you're
> doing something strange with your routers.  Multicast would cross
> subnets, but would require additional support that is not typically
> configured into many routers -- at least not by default.
>         Using "manycast" mode would allow the clients auto-discover the
> "closest" time servers to their topological location, helping to
> balance the load across the servers on your network.
>         However, it also depends on multicast support at the routers.
> Moreover, "manycast" mode ends up setting up unicast one-to-one
> relationships between the clients and the servers, so it may be less
> desirable in that respect.
> >  Do we see siginifcant network performance issues in client/server mode?
>         Yes.  See above.
> >  I know that the management of the NTP configuration could be simplified
> >  if the bc/mc mode is used, but are there any other benefits?  What about
> >  drawbacks?
>         I hope that some others will also speak up and help fill in some
> of the gaps I may have left.  But I believe that, on a large network,
> the advantages greatly outweigh the disadvantages.
> --
> Brad Knowles, <brad at stop.mail-abuse.org>
> "Those who would give up essential Liberty, to purchase a little
> temporary Safety, deserve neither Liberty nor Safety."
>      -- Benjamin Franklin (1706-1790), reply of the Pennsylvania
>      Assembly to the Governor, November 11, 1755
>    SAGE member since 1995.  See <http://www.sage.org/> for more info.

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