The TCP communications in the IRI sends and receives packets with 16 additional CRC32 bytes after the transaction data.
Internet TCP/IP data is already check-summed. Furthermore, malformed data will already be rejected by the IRI in processing.
It is a very fast algorithm and anyone can add the same data in their packets very quickly and easily. However, from what I can tell, there is neither a security advantage nor a data processing integrity advantage to including the CRC32 sum data.
What is the purpose of adding additional CRC32 data onto the TCP data exchanged?
edit: Just to add, the IRI node sends 1000 times more UDP packets than TCP packets and none of the UDP packets add the CRC32 checksum to the transmitted data.