ratelimit Module

Ovidiu Sas

Bogdan Vasile Harjoc

Hendrik Scholz

Edited by

Ovidiu Sas

Bogdan Vasile Harjoc

Hendrik Scholz

   Copyright © 2006 Freenet Cityline GmbH

   Copyright © 2008-2010 VoIP Embedded Inc.
     __________________________________________________________________

   Table of Contents

   1. Admin Guide

        1. Overview
        2. Use Cases
        3. Static Rate Limiting Algorithms

              3.1. Tail Drop Algorithm (TAILDROP)
              3.2. Random Early Detection Algorithm (RED)
              3.3. Network Algorithm (NETWORK)
              3.4. Dynamic Rate Limiting Algorithms
              3.5. Feedback Algorithm (FEEDBACK)

        4. Dependencies

              4.1. Kamailio Modules
              4.2. External Libraries or Applications

        5. Parameters

              5.1. timer_interval (integer)
              5.2. queue (integer:string)
              5.3. pipe (integer:string:integer)

        6. Functions

              6.1. rl_check([pvar])
              6.2. rl_check_pipe(pipe_no)

        7. RPC Commands

              7.1. rl.stats
              7.2. rl.set_pipe
              7.3. rl.get_pipes
              7.4. rl.set_queue
              7.5. rl.get_queues
              7.6. rl.set_pid
              7.7. rl.get_pid
              7.8. rl.push_load
              7.9. rl.set_dbg

        8. Known limitations

   List of Examples

   1.1. Set timer_interval parameter
   1.2. Set queue parameter
   1.3. Set pipe parameter
   1.4. rl_check usage
   1.5. rl_check_pipe usage

Chapter 1. Admin Guide

   Table of Contents

   1. Overview
   2. Use Cases
   3. Static Rate Limiting Algorithms

        3.1. Tail Drop Algorithm (TAILDROP)
        3.2. Random Early Detection Algorithm (RED)
        3.3. Network Algorithm (NETWORK)
        3.4. Dynamic Rate Limiting Algorithms
        3.5. Feedback Algorithm (FEEDBACK)

   4. Dependencies

        4.1. Kamailio Modules
        4.2. External Libraries or Applications

   5. Parameters

        5.1. timer_interval (integer)
        5.2. queue (integer:string)
        5.3. pipe (integer:string:integer)

   6. Functions

        6.1. rl_check([pvar])
        6.2. rl_check_pipe(pipe_no)

   7. RPC Commands

        7.1. rl.stats
        7.2. rl.set_pipe
        7.3. rl.get_pipes
        7.4. rl.set_queue
        7.5. rl.get_queues
        7.6. rl.set_pid
        7.7. rl.get_pid
        7.8. rl.push_load
        7.9. rl.set_dbg

   8. Known limitations

1. 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 RPC interface can be used to change
   tunables while running Kamailio.

   The module implements the pipe/queue policy from BSD's ipfw manual,
   with some simplifications. In principle, each specified method is
   associated with its own queue and a number of queues are connected to a
   certain pipe (see the queue and pipe params).

   Please also take a look at the “pipelimit” module, that implements the
   pipe policy with database support. Note that it doesn't implement the
   queues that exist in this module.

2. 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 an Kamailio cluster when run on the
   dispatching box in front.

   A sample configuration snippet might look like this:
...
        if (is_method("INVITE|REGISTER|SUBSCRIBE") {
                if (!rl_check()) {
                        append_to_reply("Retry-After: 5\r\n");
                        sl_send_reply("503","Limiting");
                        exit;
                };
        };
...

   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.

3. Static Rate Limiting Algorithms

   3.1. Tail Drop Algorithm (TAILDROP)
   3.2. Random Early Detection Algorithm (RED)
   3.3. Network Algorithm (NETWORK)
   3.4. Dynamic Rate Limiting Algorithms
   3.5. Feedback Algorithm (FEEDBACK)

   The ratelimit module supports two different statc algorithms to be used
   by rl_check to determine whether a message should be blocked or not.

3.1. Tail Drop Algorithm (TAILDROP)

   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 retransmitted after
   a given time, the same on all devices with the same firmware/by the
   same vendor.

3.2. Random Early Detection Algorithm (RED)

   The Random Early Detection Algorithm 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) Kamailio will return errors
   to the Kamailio 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 Kamailio might drop messages
   although the limit might not be reached within the interval. Decrease
   the timer interval if you encounter this.

3.3. Network Algorithm (NETWORK)

   This algorithm relies on information provided by network interfaces.
   The total amount of bytes waiting to be consumed on all the network
   interfaces is retrieved once every timer_interval seconds. If the
   returned amount exceeds the limit specified in the modparam, rl_check
   returns an error.

3.4. Dynamic Rate Limiting Algorithms

   When running Kamailio on different machines, one has to adjust the drop
   rates for the static algorithms to maintain a sub 100% load average or
   packets will start getting dropped in the network stack. While this is
   not in itself difficult, it isn't neither accurate nor trivial: another
   server taking a notable fraction of the CPU time will require re-tuning
   the parameters.

   While tuning the drop rates from the outside based on a certain factor
   is possible, having the algorithm run inside ratelimit permits tuning
   the rates based on internal server parameters and is somewhat more
   flexible (or it will be when support for external load factors - as
   opposed to cpu load - is added).

3.5. Feedback Algorithm (FEEDBACK)

   Using the PID Controller model (see Wikipedia page), the drop rate is
   adjusted dynamically based on the load factor so that the load factor
   always drifts towards the specified limit (or setpoint, in PID terms).

   As reading the CPU load average is relatively expensive (opening
   /proc/stat, parsing it, etc), this only happens once every
   timer_interval seconds and consequently the FEEDBACK value is only at
   these intervals recomputed. This in turn makes it difficult for the
   drop rate to adjust quickly. Worst case scenarios are request rates
   going up/down instantly by thousands - it takes up to 20 seconds for
   the controller to adapt to the new request rate.

   Generally though, as real life request rates drift by less, adapting
   should happen much faster.

4. Dependencies

   4.1. Kamailio Modules
   4.2. External Libraries or Applications

4.1. Kamailio Modules

   The following modules must be loaded before this module:
     * SL: Stateless request handling.

4.2. External Libraries or Applications

   The following libraries or applications must be installed before
   running Kamailio with this module loaded:
     * None.

5. Parameters

   5.1. timer_interval (integer)
   5.2. queue (integer:string)
   5.3. pipe (integer:string:integer)

5.1. 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.

   IMPORTANT: A too small value may lead to performance penalties due to
   timer process overloading.

   Default value is 10.

   Example 1.1. Set timer_interval parameter
...
modparam("ratelimit", "timer_interval", 5)
...

5.2. queue (integer:string)

   The format of the queue parameter is "pipe_no:method". For each defined
   method, the algorithm defined by pipe number "pipe_no" will be used.

   To specify a queue that accepts all methods, use “*” instead of METHOD.
   As queues are matched against request methods, you will usually want to
   have this as the last queue added or other queues with specific methods
   will never match. At this time, glob or regexp patterns are not
   supported.

   Example 1.2. Set queue parameter
...
# assign pipe no 0 to method REGISTER
# assign pipe no 3 to method INVITE
# assign pipe no 2 to all other methods
modparam("ratelimit", "queue", "0:REGISTER")
modparam("ratelimit", "queue", "3:INVITE")
modparam("ratelimit", "queue", "2:*")
...

5.3. pipe (integer:string:integer)

   The format of the pipe param is "pipe_no:algorithm:limit". For each
   defined pipe, the given algorithm with the given limit will be used.

   A pipe is characterised by its algorithm and limit (bandwidth, in ipfw
   terms). When specifying a limit, the unit depends on the algorithm used
   and doesn't need to be specified also (eg, for TAILDROP or RED, limit
   means packets/sec, whereas with the FEEDBACK algorithm, it means [CPU]
   load factor).

   Example 1.3. Set pipe parameter
...
# define pipe 0 with a limit of 200 pkts/sec using TAILDROP algorithm
# define pipe 1 with a limit of 100 pkts/sec using RED algorithm
# define pipe 2 with a limit of 50 pkts/sec using TAILDROP algorithm
# define pipe 3 with a limit of load factor 80 using FEEDBACK algorithm
# define pipe 4 with a limit of 10000 pending bytes in the rx_queue
#                                     using NETWORK algorithm
modparam("ratelimit", "pipe", "0:TAILDROP:200")
modparam("ratelimit", "pipe", "1:RED:100")
modparam("ratelimit", "pipe", "2:TAILDROP:50")
modparam("ratelimit", "pipe", "3:FEEDBACK:80")
modparam("ratelimit", "pipe", "4:NETWORK:10000")
...

6. Functions

   6.1. rl_check([pvar])
   6.2. rl_check_pipe(pipe_no)

6.1.  rl_check([pvar])

   Check the current request against the matched ratelimit algorithm. If
   no parameter is provided, the queue will be matched based on method
   type, and then the pipe will be identified based on the matched queue.
   If a pipe number is provided as a parameter, then the given pipe number
   will be used for identifying the ratelimit algorithm. The pipe number
   must be provided as number or via a pseudovariable.

   The method will return an error code if the limit for the matched
   algorithm is reached.

   Meaning of the parameters is as follows:
     * pvar - the pseudovariable holding the pipe id to be used by
       ratelimit.

   This function can be used from ANY_ROUTE.

   Example 1.4. rl_check usage
...
        # perform queue/pipe match for current method
        if (!rl_check()) {
                append_to_reply("Retry-After: 5\r\n");
                sl_send_reply("503","Limiting");
                exit;
        }
...
        # use pipe no 1 for the current method
        # set int pvar to 1
        $var(p) = 1;
        if (!rl_check("$var(p)")) {
                append_to_reply("Retry-After: 5\r\n");
                sl_send_reply("503","Limiting");
                exit;
        }
...

6.2.  rl_check_pipe(pipe_no)

   Check the current request against the matched ratelimit algorithm of
   the pipe provided as parameter. The parameter can be provided as number
   or variable.

   The method will return an error code if the limit for the matched
   algorithm is reached.

   Meaning of the parameters is as follows:
     * pipe_no - the pipe id to be used by ratelimit.

   This function can be used from REQUEST_ROUTE.

   Example 1.5. rl_check_pipe usage
...
        # use pipe no 1 for the current method
        if (!rl_check_pipe("1") {
                append_to_reply("Retry-After: 5\r\n");
                sl_send_reply("503","Limiting");
                exit;
        }
...

7. RPC Commands

   7.1. rl.stats
   7.2. rl.set_pipe
   7.3. rl.get_pipes
   7.4. rl.set_queue
   7.5. rl.get_queues
   7.6. rl.set_pid
   7.7. rl.get_pid
   7.8. rl.push_load
   7.9. rl.set_dbg

7.1.  rl.stats

   Lists the parameters and variables in the ratelimit module.

   Name: rl.stats

   Parameters: none

   RPC Command Format:
                kamcmd rl.stats

7.2.  rl.set_pipe

   Sets the pipe parameters for the given pipe id.

   Name: rl.set_pipe

   Parameters:
     * pipe_id - pipe id.
     * pipe_algorithm - the algorithm assigned to the given pipe id.
     * pipe_limit - the limit assigned to the given pipe id.

   RPC Command Format:
                kamcmd rl.set_pipe 2 RED 10

7.3.  rl.get_pipes

   Gets the list of in use pipes.

   Name: rl.get_pipes

   Parameters:none

   RPC Command Format:
                kamcmd rl.get_pipes

7.4.  rl.set_queue

   Sets the queue parameters for the given queue id.

   Name: rl.set_queue

   Parameters:
     * queue_id - queue id.
     * queue_method - the method assigned to the given queue id.
     * pipe_id - the pipe id assigned to the given queue id.

   RPC Command Format:
                kamcmd rl.set_queue 3 INVITE 2

7.5.  rl.get_queues

   Gets the list of in use queues.

   Name: rl.get_queues

   Parameters: none

   RPC Command Format:
                kamcmd rl.get_queues

7.6.  rl.set_pid

   Sets the PID Controller parameters for the Feedback Algorithm.

   Name: rl.set_pid

   Parameters:
     * ki - the integral parameter.
     * kp - the proportional parameter.
     * kd - the derivative parameter.

   RPC Command Format:
                kamcmd rl.set_pid 0.5 0.5 0.5

7.7.  rl.get_pid

   Gets the list of in use PID Controller parameters.

   Name: rl.get_pid

   Parameters: none

   RPC Command Format:
                kamcmd rl.get_pid

7.8.  rl.push_load

   Force the value of the load parameter. This method is useful for
   testing the Feedback algorithm.

   Name: rl.push_load

   Parameters:
     * load - the forced value of load (it must be greater then 0.0 and
       smaller then 1.0).

   RPC Command Format:
                kamcmd rl.push_load 0.85

7.9.  rl.set_dbg

   This function will enable/disable a WARNING debug log exposing the
   internal counters for each pipe (useful in monitoring the ratelimit
   internals).

   Name: rl.set_dbg

   Parameters:
     * dbg - the debug value (0 means disable and 1 means enable).

   RPC Command Format:
                kamcmd rl.set_dbg 1

8. Known limitations

   The pipes and queues are stored as static vectors, so no more than
   MAX_PIPES/MAX_QUEUES can be added without recompilation.
     * MAX_PIPES - 16
     * MAX_QUEUES - 10
