How a Session is Maintained

To detect and handle transmission failures, the client must send a HEARTBEAT_REQ message, see HEARTBEAT_REQ, to the server whenever no messages have been sent for the heartbeat interval or sooner. On receipt of a HEARTBEAT_REQ message, the server immediately responds with a HEARTBEAT_CONF message, defined in HEARTBEAT_CONF. If three heartbeats go unconfirmed, the client must declare a session failure and reset the TCP connection.

The heartbeat interval is solely determined by the client and represents a reasonable balance between the speed of failure detection and the network bandwidth consumed by heartbeat messages and their corresponding confirmations. A maximum value of 30 seconds is accepted by the server

The server does not initiate HEARTBEAT_REQ messages. Failure detection on the server is accomplished using the IdleTimeout value from the OPEN_REQ message. When heartbeat messages are implemented, the client must set this value to four times the heartbeat interval. If the server does not receive any messages (including HEARTBEAT_REQ messages) for this period of time, the server declares a session failure and resets the TCP connection.

The server may respond to a HEARTBEAT_REQ message with a FAILURE_CONF message. This indicates to the client that the server is off-line, and the client must reset the TCP connection.

Note	Heartbeat messages must be sent regardless of line activity or in-activity.

Figure 1 depicts the heartbeat message flow.

Note	You need to synchronize the time of the client and the server and you need to make sure the intervals between the heartbeats are identical.