ConnectionFailed
class ConnectionFailed(reasonCode: ReasonCode, message: String, cause: Throwable? = null) : MqttException
The connection attempt failed before the broker accepted or refused it.
Covers transport-level failures during MqttClient.connect — DNS resolution, TCP connect, TLS handshake, a socket that closed mid-handshake, and a CONNACK that never arrived. The broker never expressed an opinion, so the attempt is worth retrying; contrast ConnectionRejected, which carries the broker's own refusal and will fail the same way against the same configuration.