Echo request and reply messages are used by both the controller and the switch to maintain and verify the liveliness of the controller-switch connection. Echo messages are also used to calculate the latency and bandwidth of the controller-switch connection.
To send echo request and reply messages, the TCP/TLS connection establishment procedure defined in the Connection setup on TCP and TLS recipe of Chapter 1, OpenFlow Channel Connection Establishment (Part 1) should be used.
As echo messages are transmitted by both the switch and the controller, the controller should be able to send, receive, and process these messages. The following sections explain these procedures in detail.