The Heartbeat message is used as a keepalive for the connection, to make sure that the other party is still responsive after a period of inactivity. The heartbeat interval is specified in the original Logon <35=A> message.

When either end of a FIX connection has not sent any data for the heartbeat interval, it will transmit a Heartbeat message. When either end of the connection has not received any data for the heartbeat interval plus some reasonable transmission time, it will transmit a Test Request <35=1> message. If there is still no Heartbeat message received after the heartbeat interval plus some reasonable transmission time then the connection should be considered lost and applications should attempt to recover.

If the heartbeat interval is set to zero then no regular Heartbeat messages will be generated. In such case, a Test Request <35=1> message can still be sent regardless, which will force a Heartbeat message.

Heartbeats issued in response to a Test Request <35=1> must contain the TestReqID (112) present in the request. This is useful to verify that the Heartbeat is the result of the Test Request <35=1> and not as the result of a regular timeout.