QUdpSocket error

[DeepDiver]

What is the return value of writeDatagram and readDatagram?
In general: you shall take care about return values - they have a meaning.

Here is the Qt documentation on writeDatagram:
Datagrams are always written as one block. The maximum size of a datagram is highly platform-dependent, but can be as low as 8192 bytes. If the datagram is too large, this function will return -1 and error() will return DatagramTooLargeError.

Sending datagrams larger than 512 bytes is in general disadvised, as even if they are sent successfully, they are likely to be fragmented by the IP layer before arriving at their final destination.

Believe it or not: ip packet loss is part of udp - if you need reliability: use tcp.

[mdecandia]

I take care of return values and they are about 20000 (bytes) on writeDatagram on linux and windows client.
Server receives that data size only when linux client is running. processPendingDatagram slot is not called when windows client is running becauese readyRead() signal is not emitted (Udp datagram isn't received entirely).

[DeepDiver]

MTU on Ethernet is ~1500 bytes.
Meaning the IP packets have a max size of 1500 bytes.
In case you send data bigger than the MTU - you will get fragmentation.

This is what we see from your ethereal trace.

UDP doesn't take care about ordering of packet delivery and/or guarantee of delivery.

This is what we see in your ethereal trace as well.

As soon as one ip fragment is lost - the whole datagram is gone.

My advice: try TCP or send the 20000 bytes in chunks of less than 1480 bytes and re-assemble the whole data pack on server side on your own.

And believe me: not loosing data in the linux-linux setup is pure luck.