# The SL Module - Stateless request handling

## Admin Guide

### 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 wants 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.

### Event routes

#### `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");
    }
    ...

#### `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");
    }
    ...