Setting queue-capacity in Kea

Kea 1.5 adds a new feature, dhcp-queue-control.

This feature was added to mitigate a problem that may occur when a large number of devices reboot at once, flooding the DHCP server with an unusual spike of requests. Recovery after a network failure can also cause a surge of requests.

Congestion occurs when servers are subjected to client queries faster than they can be fulfilled. Subsequently, the servers begin accumulating a backlog of pending queries. The longer the high rate of traffic continues, the farther behind the servers fall. Depending on the client implementation, those that fail to get leases either give up or simply continue to retry forever. In the former case, the server may eventually recover. The latter case is vicious cycle from which the server is unable to escape.

The goal of congestion handling is to help the servers mitigate the peak in traffic by fulfilling as many of the most relevant requests as possible until it subsides.

The Kea Administrator Reference Manual explains how to set the value:

"Dhcp4":
{
    ...
   "dhcp-queue-control": {
       "enable-queue": true,
       "queue-type": "kea-ring4",
       "capacity" : 250
    },
    ...
}

This feature is disabled by default because enabling it and setting any type of queuing may adversely impact performance. Be sure to monitor your servers and clients closely when enabling queue controls.