[ntp:questions] NTP Broadcast/Multicast Benefits

Brad Knowles 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
>  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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