Symmetric messages are initiated by either the switch or the controller and sent without solicitation. There are three symmetric message subtypes in OpenFlow protocol as follows:

Hello

Hello messages are exchanged between the switch and controller upon connection setup.

Echo

Echo request/reply messages can be sent from either the switch or the controller, and must return an echo reply. These messages can be used to indicate the latency, bandwidth, and/or liveliness of a controller-switch connection (that is a heartbeat).

Vendor

These messages provide a standard way for OpenFlow switches to offer additional functionality within the OpenFlow message type space for future revisions of OpenFlow.