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TBut how about the PC side? What's the data sink? I suggest to perform some basic tests with PCs if the intended throughput is feasible.
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+1 on testing with PC's. Just write a simple transmit program and receive program that connect to each other over a raw ethernet or UDP socket. Have the receiver verify the data so it knows if packets start being dropped. Have the transmitter be able to throttle to a given data rate. You can also configure the NIC's to a specific MTU size.
With this configuration, you can easily test the effects of different protocol and frame size combinations.