[ntp:questions] NTP Broadcast/Multicast Benefits
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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