What happens if the data transmitted in an Ethernet Frame has been altered, but the FCS is still valid for this data? (FCS collision)
Is that the reason why some download sites provide MD5 / SHA sums for their downloads?
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What will happen is that the frame will be considered valid by the data link layer and its payload will be passed to the upper layers. The upper layers might have their own error detection mechanisms (e.g. TCP/IP checksum) so the might detect the error themselves and throw out the frame, but then again, they might not. If they don't, then the application will actually receive faulty data.
The chances of that happening are rather low, but I'm sure happens due to the sheer number of packets sent across networks.
