This module implements rate limiting for SIP requests. In contrast to the PIKE module this limits the flow based on a per SIP request type basis and not per source IP. The XML-RPC interface can be used to change tunables while running SER. Currently supported requests are INVITE, REGISTER and SUBSCRIBE.

Limiting the rate messages are processed on a system directly influences the load. The ratelimit module can be used to protect a single host or to protect a SER cluster when run on the dispatching box in front.

A sample configuration snippet might look like this:

if (method=="INVITE" || method="REGISTER" || method=="SUBSCRIBE") {
	if (!rl_check()) {
		sl_send_reply("503", "Service Unavailable");
		break;
	}
};

Upon every incoming request listed above rl_check is invoked. It returns an OK code if the current per request load is below the configured threshold. If the load is exceeded the function returns an error and an administrator can discard requests with a stateless response.

As of writing the ratelimit module supports two different algorithms to be used by rl_check to determine whether a message should be blocked or not.

This is a trivial algorithm that imposes some risks when used in conjunction with long timer intervals. At the start of each interval an internal counter is reset and incremented for each incoming message. Once the counter hits the configured limit rl_check returns an error.

The downside of this algorithm is that it can lead to SIP client synchronization. During a relatively long interval only the first requests (i.e. REGISTERs) would make it through. Following messages (i.e. RE-REGISTERs) will all hit the SIP proxy at the same time when a common Expire timer expired. Other requests will be retransmissed after given time, the same on all devices with the same firmware/by the same vendor.

Random Early Detection tries to circumvent the synchronization problem imposed by the tail drop algorithm by measuring the average load and adapting the drop rate dynamically. When running with the RED algorithm (enabled by default) SER will return errors to the SER routing engine every n'th packet trying to evenly spread the measured load of the last timer interval onto the current interval. As a negative side effect SER might drop messages although the limit might not be reached within the interval. Decrease the timer interval if you encounter this.

The number of allowed requests per interval as well as the interval length itself can be modified over the FIFO interface of SER. In addition rate limiting statistics are provided upon request.