The mechanism
Either side sends a Ping frame (0x9). Recipient must reply with Pong (0xA), echoing the payload. Missing pong = connection considered dead.
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Interval choice
30-60 seconds is standard. Under 15 = wasted bandwidth on mobile. Over 120 = slow to detect broken connections.
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Which side sends
Historically server. But client-initiated pings work too, and detect network partitions from the client side. Bidirectional pings + timeout each side is safest.