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This question already has an answer here:

So, about Heartbleed again, why does SSL/TLS allow the client to specify the message and its length in the heartbeat requests, rather than sending a simple ping?

marked as duplicate by TildalWave, Rory Alsop Sep 28 '14 at 9:30

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migrated from crypto.stackexchange.com Sep 27 '14 at 22:15

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Heartbeat requests are intended for use with connectionless SSL over UDP. A simple ping tells you only that the computer at the other end is up (or not); by including a payload, a program can associate heartbeat requests with specific transactions, detect dropped or out-of-order replies, and do other things that a simple "are you there?" can't do.

That said, I suspect the real reason that heartbeat requests have a payload is that they were modeled on ICMP "echo" requests, without understanding why ICMP echo permits a payload. The ICMP echo payload is intended for low-level network troubleshooting.