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README
Ratelimit Module Hendrik Scholz <hscholz@raisdorf.net> Copyright � 2006 Freenet Cityline GmbH Revision History Revision $Revision$ $Date$ _________________________________________________________________ Overview 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. Use Cases 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. Rate Limiting Algorithms 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. Tail drop 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 (RED) 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. Parameters timer_interval (integer) The initial length of a timer interval in seconds. All amounts of messages have to be divided by this timer to get a messages per second value. Default value is 5 seconds. invite_limit (integer) The initial number of allowed INVITE requests per timer interval. Once this number has been reached within one interval rl_check returns an error. If running in Random Early Detection (RED) mode (default) rl_check preemtively starts dropping packets. Default value is 0 which means unlimited requests are permitted. register_limit (integer) The initial number of allowed REGISTER requests per timer interval. Once this number has been reached within one interval rl_check returns an error. If running in Random Early Detection (RED) mode (default) rl_check preemtively starts dropping packets. Default value is 0 which means unlimited requests are permitted. subscribe_limit (integer) The initial number of allowed SUBSCRIBE requests per timer interval. Once this number has been reached within one interval rl_check returns an error. If running in Random Early Detection (RED) mode (default) rl_check preemtively starts dropping packets. Default value is 0 which means unlimited requests are permitted. Functions rl.check() Add the current request to the internal counters and determine if this request is above the limit. The ratelimit module has different internal counters for the supported request types and if a counter is exceeded the function returns an error code. Example 1. rl.check usage ... if (method==INVITE) { if (!rl_check()) { sl_send_reply("503", "Service Unavailable"); break; } } ... FIFO Interface 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. * rl.stats - Get current per request limits as well as current load levels for all request types. * rl.invite_limit - Set the number of allowed INVITE requests per interval to the given value as the sole parameter. A limit of 0 turns of rate limiting of INVITE messages. * rl.register_limit - Set the number of allowed REGISTER requests per interval to the given value as the sole parameter. A limit of 0 turns of rate limiting of REGISTER messages. * rl.subscribe_limit - Set the number of allowed SUBSCRIBE requests per interval to the given value as the sole parameter. A limit of 0 turns of rate limiting of SUBSCRIBE messages.