[ntp:questions] NTP Broadcast/Multicast Benefits
brad at stop.mail-abuse.org
Tue Aug 2 17:21:09 UTC 2005
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
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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