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.

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