12. Congestion Handling

12.1. What is Congestion?

Congestion occurs when servers are subjected to client queries faster than those queries can be processed. As a result, 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 implementations, those that fail to get leases either give up or simply continue to retry forever. In the former case, the server may eventually recover, but the latter case is a vicious cycle from which the server is unable to escape.

In a well-planned deployment, the number and capacity of servers is matched to the maximum client loads expected. As long as capacity is matched to load, congestion does not occur. If the load is routinely too heavy, then the deployment needs to be re-evaluated. Congestion typically occurs when there is a network event that causes overly large numbers of clients to simultaneously need leases, such as recovery after a network outage.

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

Prior to Kea 1.5, kea-dhcp4 and kea-dhcp6 read inbound packets directly from the interface sockets in the main application thread, which meant that packets waiting to be processed were held in socket buffers themselves. Once these buffers filled, any new packets were discarded. Under swamped conditions, the servers ended up processing client packets that were no longer relevant, or worse, were redundant. In other words, the packets waiting in the FIFO socket buffers became increasingly stale.

12.2. Configuring Congestion Handling

Kea 1.5 introduced the Congestion Handling feature. Congestion handling offers the ability to configure the server to use a separate thread to read packets from the interface socket buffers. As the thread reads packets from the buffers, they are added to an internal packet queue, and the server’s main application thread processes packets from this queue rather than from the socket buffers. By structuring it this way, a configurable layer has been introduced which can make decisions on which packets to process, how to store them, and the order in which they are processed by the server.

The default packet queue implementation for both kea-dhcp4 and kea-dhcp6 is a simple ring buffer. Once it reaches capacity, new packets get added to the back of the queue by discarding packets from the front of the queue. Rather than always discarding the newest packets, Kea now always discards the oldest packets. The capacity of the buffer, i.e. the maximum number of packets the buffer can contain, is configurable. A reasonable starting point would be to match the capacity to the number of leases per second a specific installation of Kea can handle. Please note that this figure varies widely depending on the specifics of an individual deployment.

As there is no one algorithm that will best handle the dynamics of all sites, and because over time new approaches will evolve, the packet queue is implemented as a plug-in, which can replaced by a custom queue implementation via a hook library. This should make it straightforward for interested parties to experiment with their own solutions. (Developers can refer to isc::dhcp::PacketQueue and isc::dhcp::PacketQueueMgr, described in the Kea Developer’s Guide.)

Packet queue behavior is configured in both kea-dhcp4 and kea-dhcp6 servers through an optional, top-level, configuration element, dhcp-queue-control. Omitting this element disables packet queueing:

"dhcp-queue-control": {
    "enable-queue": true|false,
    "queue-type": "queue type",
    "capacity" : n
}

where:

• enable-queue true|false - enables or disables packet queueing. When true, the server processes packets from the packet queue, which is filled by a separate thread. When false, the server processes packets directly from the socket buffers in the main thread. It is disabled by default.
• queue-type - name of the queue implementation to use. This value exists so that custom implementations can be registered (via a hook library) and then selected. There is a default packet queue implementation that is pre-registered during server start up: “kea-ring4” for kea-dhcp4 and “kea-ring6” for kea-dhcp6.
• capacity = n [packets] - this is the maximum number of packets the queue can hold before packets are discarded. The optimal value for this is extremely site-dependent. The default value is 64 for both kea-ring4 and kea-ring6.

The following example enables the default packet queue for kea-dhcp4, with a queue capacity of 250 packets:

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

The following example enables the default packet queue for kea-dhcp6, with a queue capacity of 300 packets:

"Dhcp6":
{
    ...
   "dhcp-queue-control": {
       "enable-queue": true,
       "queue-type": "kea-ring6",
       "capacity" : 300
    },
    ...
}