This Unsigned32 value, carried in OpenFlow connection notifications, is the auxiliary connection ID that OpenFlow uses to distinguish a switch's supplementary connections to a controller from its single primary (auxiliary ID 0) connection. Admins use it to tell apart the main control channel from any additional auxiliary channels used for parallel processing of OpenFlow messages to the same controller. For example, a switch with an auxiliary connection carrying a value of 1 alongside a primary connection entry with auxiliary ID 0 indicates it has established a secondary channel to the same controller for improved throughput.