Cookies

A cookie is a 64-bit value used by the controller, uninterpreted by the switch, which is used to modify and delete commands in the OFP_FLOW_MOD message. One of the applications of the cookie is to integrate the OpenFlow flow information with other flow export formats on the switch, such as NetFlow. Correlating these messages with the underlying flow proves to be difficult because of the conflict in the information generated from the message format or the use of wildcards. The problem is complex, considering it is almost impossible to synchronize with the flow table of the switch due to lack of reliability in flow expiration messages. Flow cookies make this correlation possible.

Another application is identifying a rule's source using flow cookies to gather statistics for access-lists hit count. An access-list defined in the controller may represent many OpenFlow rules. These access-lists contain OpenFlow rules, which can be tagged with a single flow cookie, making it easier to identify the source of an OpenFlow rule.