Max UDP packet size + Java: weirdness

Chuck Swiger cswiger at
Tue Feb 9 20:48:47 UTC 2010

On Feb 9, 2010, at 12:41 PM, Nerius Landys wrote:
> First, I noticed that I was able to send data that is 9216 in length
> between 2 FreeBSD 7.1 hosts ("far apart" in network distance) running
> Sun JDK 1.5.0_16-p9 (compiled myself from /usr/ports/java/jdk15).
[ ... ]
> For my particular application, I would really benefit from being able
> to send large UDP packets.  Does anyone know how to increase the max
> allowed size of UDP packets in a Java program?  I assume it's
> something outside of the Java program, in the system or kernel or
> whatnot.

You've encountered:

% sysctl net.inet.udp.maxdgram
net.inet.udp.maxdgram: 9216

However, increasing it will guarantee that you will exceed even normal jumbo frame size and thus depend upon IP fragmentation / reassembly for the traffic.  I don't consider that to be a good idea, but it might be OK for local traffic....


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