1. SL Module

Bogdan Iancu

FhG FOKUS

Daniel-Constantin Mierla

asipto.com

1.1. Overview
1.2. Parameters
1.2.1. default_code (int)
1.2.2. default_reason (str)
1.2.3. bind_tm (int)
1.3. Functions
1.3.1. sl_send_reply(code, reason)
1.3.2. send_reply(code, reason)
1.3.3. sl_reply_error()

1.1. Overview

The SL module allows ser 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, ser 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 us set. As time as the timer is valid, The incoming ACK requests will be checked using TO tag value Once the timer expires, all the ACK 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 SER want to reply to it. Than, it will keep the current to-tag, which will be mirrored in ACK. SER will not see its signature and forward the ACK downstream. Caused harm is not bad--just a useless ACK is forwarded.

1.2. Parameters

1.2.1. default_code (int)

Default reply status code.

Default value is 500.

Example 1. default_code example

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

1.2.2. default_reason (str)

Default reply reason phrase.

Default value is 'Internal Server Error'.

Example 2. default_reason example

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

1.2.3. 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 3. bind_tm example

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

1.3. Functions

1.3.1.  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.

Meaning of the parameters is as follows:

  • code - Return code.

  • reason - Reason phrase.

Example 4. sl_send_reply usage

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

1.3.2.  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 the transaction for current request is created, then the reply is sent stateful, otherwise stateless.

Meaning of the parameters is as follows:

  • code - Return code.

  • reason - Reason phrase.

This function can be used from REQUEST_ROUTE, FAILURE_ROUTE, BRANCH_ROUTE.

Example 5. send_reply usage

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

1.3.3.  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 6. sl_reply_error usage

...
sl_reply_error();
...