Why does my MulticastSocket receive only every 256th packet?

Question

I subscribe to UDP multicast stream in MATLAB. This cannot be done initially, so instead I use an object java.net.MulticastSocket. Each UDP packet is tagged with some metadata, in particular the number of sequences. Every time my data source sends a UDP packet, this counter incrementally increments.

Here is my skeletal code:

s = java.net.MulticastSocket(50001);
s.setSoTimeout(15000);
s.setReuseAddress(1);
s.setReceiveBufferSize(32768);
s.joinGroup(java.net.InetAddress.getByName('239.255.0.4'));
p = java.net.DatagramPacket(zeros(1, 1600, 'int8'), 1600);

ii = 1;
d = cell(10000,1);

while ii < 10000
    s.receive(p);
    d{ii} = p.getData;
    d{ii} = d{ii}(1:p.getLength);

    ii = ii + 1;
end

As soon as I catch all the data, I can process it; this bit is not important.

After you catch 10,000 packets like this, I look at the sequence count and it turns out that I am missing packets. This is true; it is UDP after all, so there is no guarantee of receiving traffic. However, what is really interesting, I get only every 256th packet:

sequenceCnt =
    ...
    56637
    56893
    57149
    57405
    57661
    57917
    58173
    58429
    58685
    58941
    59197
    59453
    ...

Wireshark , . , (.. , UDP ), MATLAB/Java .

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Answer 1

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