The SL Module - Stateless request handling

Bogdan Iancu


Daniel-Constantin Mierla

Table of Contents

1. Admin Guide
1. Overview
2. Parameters
2.1. default_code (int)
2.2. default_reason (str)
2.3. bind_tm (int)
3. Functions
3.1. sl_send_reply(code, reason)
3.2. send_reply(code, reason)
3.3. sl_reply_error()
3.4. sl_forward _reply([ code, [ reason ] ])
4. Statistics
4.1. 1xx_replies
4.2. 200_replies
4.3. 202_replies
4.4. 2xx_replies
4.5. 300_replies
4.6. 301_replies
4.7. 302_replies
4.8. 3xx_replies
4.9. 400_replies
4.10. 401_replies
4.11. 403_replies
4.12. 404_replies
4.13. 407_replies
4.14. 408_replies
4.15. 483_replies
4.16. 4xx_replies
4.17. 500_replies
4.18. 5xx_replies
4.19. 6xx_replies
4.20. xxx_replies
4.21. sent_replies
4.22. sent_err_replies
4.23. failures
4.24. received_ACKs
5. Event routes
5.1. sl:filtered-ack
5.2. sl:local-response

List of Examples

1.1. default_code example
1.2. default_reason example
1.3. bind_tm example
1.4. sl_send_reply usage
1.5. send_reply usage
1.6. sl_reply_error usage
1.7. send_reply usage

Chapter 1. Admin Guide

1. Overview

The SL module allows the SIP server to act as a stateless UA server and generate replies to SIP requests without keeping state. That is beneficial in many scenarios, in which you wish not to burden server's memory and scale well.

The SL module needs to filter ACKs sent after a local stateless reply to an INVITE was generated. To recognize such ACKs, Kamailio adds a special "signature" in to-tags. This signature is sought for in incoming ACKs, and if included, the ACKs are absorbed.

To speed up the filtering process, the module uses a timeout mechanism. When a reply is sent, a timer is set. As long as the timer is valid, the incoming ACK requests will be checked using TO tag value. Once the timer expires, all the ACK messages are let through - a long time passed till it sent a reply, so it does not expect any ACK that have to be blocked.

The ACK filtering may fail in some rare cases. If you think these matter to you, better use stateful processing (TM module) for INVITE processing. Particularly, the problem happens when a UA sends an INVITE which already has a to-tag in it (e.g., a re-INVITE) and the server want to reply to it. Then, it will keep the current to-tag, which will be mirrored in ACK. Kamailio will not see its signature and forward the ACK downstream. Caused harm is not bad, just a useless ACK is forwarded.

2. Parameters

2.1. default_code (int)

Default reply status code.

Default value is 500.

Example 1.1. default_code example

modparam("sl", "default_code", 505)

2.2. default_reason (str)

Default reply reason phrase.

Default value is 'Internal Server Error'.

Example 1.2. default_reason example

modparam("sl", "default_reason", "Server Error")

2.3. bind_tm (int)

Controls if SL module should attempt to bind to TM module in order to send stateful reply when the transaction is created.

Default value is 1 (enabled).

Example 1.3. bind_tm example

modparam("sl", "bind_tm", 0)  # feature disabled

3. Functions

3.1.  sl_send_reply(code, reason)

For the current request, a reply is sent back having the given code and text reason. The reply is sent stateless, totally independent of the Transaction module and with no retransmission for the INVITE's replies.

If the code is in the range 300-399 (redirect reply), the current destination set is appended to the reply as Contact headers. The destination set contains the request URI (R-URI), if it is modified compared to the received one, plus the branches added to the request (e.g., after an append_branch() or lookup("location")). If the R-URI was changed but it is not desired to be part of the destination set, it can be reverted using the function revert_uri().

Custom headers to the reply can be added using append_to_reply() function from textops module.

Meaning of the parameters is as follows:

  • code - Return code.

  • reason - Reason phrase.

Example 1.4. sl_send_reply usage

sl_send_reply("404", "Not found");

3.2.  send_reply(code, reason)

For the current request, a reply is sent back having the given code and text reason. The reply is sent stateful or stateless, depending of the TM module: if a transaction exists for the current request, then the reply is sent statefully, otherwise stateless.

Meaning of the parameters is as follows:

  • code - Return code.

  • reason - Reason phrase.

This function can be used from REQUEST_ROUTE and FAILURE_ROUTE. It can be used on ONREPLY_ROUTE executed by tm module (upon a t_on_reply() callback).

Example 1.5. send_reply usage

send_reply("404", "Not found");
send_reply("403", "Invalid user - $fU");

3.3.  sl_reply_error()

Sends back an error reply describing the nature of the last internal error. Usually this function should be used after a script function that returned an error code.

Example 1.6. sl_reply_error usage


3.4.  sl_forward _reply([ code, [ reason ] ])

Forward statelessly the current received SIP reply, with the option to change the status code and reason text. The new code has to be in the same class. The received reply is forwarded as well by core when the config execution ended, unless it is dropped from config.

Meaning of the parameters is as follows:

  • code - Status code.

  • reason - Reason phrase.

This function can be used from ONREPLY_ROUTE.

Example 1.7. send_reply usage

    sl_forward_reply("404", "Not found");

4. Statistics

4.1. 1xx_replies

Number of 1xx replies.

4.2. 200_replies

Number of 201 replies.

4.3. 202_replies

Number of 202 replies.

4.4. 2xx_replies

Number of 2xx replies.

4.5. 300_replies

Number of 300 replies.

4.6. 301_replies

Number of 301 replies.

4.7. 302_replies

Number of 302 replies.

4.8. 3xx_replies

Number of 3xx replies.

4.9. 400_replies

Number of 400 replies.

4.10. 401_replies

Number of 401 replies.

4.11. 403_replies

Number of 403 replies.

4.12. 404_replies

Number of 404 replies.

4.13. 407_replies

Number of 407 replies.

4.14. 408_replies

Number of 408 replies.

4.15. 483_replies

Number of 483 replies.

4.16. 4xx_replies

Number of 4xx replies.

4.17. 500_replies

Number of 500 replies.

4.18. 5xx_replies

Number of 5xx replies.

4.19. 6xx_replies

Number of 6xx replies.

4.20. xxx_replies

Number of replies whose code don't match any above.

4.21. sent_replies

Number of all sent replies.

4.22. sent_err_replies

Number of sent error replies.

4.23. failures

Number of failures.

4.24. received_ACKs

Number of received ACKs filtered by SL module.

5. Event routes

5.1. sl:filtered-ack

Executed when ACK to locally generated reply is recognized and absorbed.

event_route[sl:filtered-ack] {
    xlog("sl:filtered-ack ACK to local reply absorbed\n");

5.2. sl:local-response

Executed after sl module reply function has sent local reply.

event_route[sl:local-response] {
    xlog("sl:local-response replied locally\n");