How often does MD5 sum check occur

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Once the transport protocol is established between nodes , connection header is exchanged by both the nodes to verify md5 check sum and message type . However, I would like to know how often this is done ? Like is md5 sum check done for every message that publisher transmits or is it done once for a particular link and particular publish-subscribe node pair till the end of transmission ? Please explain this in both topic and service contexts