Sorry I wasn't specific enough, I didn't mean ICMP ping but some application protocol specific periodic request-acknowledgment exchange within a TCP connection in order to check that the connection is still alive regardless of what the OS socket layer says. Many application protocols have this, at least older ones, and I always thought the reason for it is that OS TCP stacks are sometimes unreliable or have unreasonable timeouts.

It's been a long time I've done network programming, though...