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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<book id="sl" xmlns:xi="http://www.w3.org/2001/XInclude">
<bookinfo>
<title>The SL Module - Statless request handling</title>
<authorgroup>
<author>
<firstname>Bogdan</firstname>
<surname>Iancu</surname>
<affiliation><orgname>FhG FOKUS</orgname></affiliation>
<address>
<email>iancu@fokus.fraunhofer.de</email>
</address>
</author>
<author>
<firstname>Daniel-Constantin</firstname>
<surname>Mierla</surname>
<affiliation><orgname>asipto.com</orgname></affiliation>
<address>
<email>miconda@gmail.com</email>
</address>
</author>
</authorgroup>
<copyright>
<year>2003</year>
<holder>FhG FOKUS</holder>
</copyright>
</bookinfo>
<toc></toc>
<chapter>
<title>Admin Guide</title>
<section id="sl.overview">
<title>Overview</title>
<para>
The <acronym>SL</acronym> 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.
</para>
<para>
The <acronym>SL</acronym> 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.
</para>
<para>
To speed up the filtering process, the module uses a timeout
mechanism. When a reply is sent, a timer us 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.
</para>
<para>
The ACK filtering may fail in some rare cases. If you think these
matter to you, better use stateful processing (<acronym>TM</acronym>
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. SER will not see
its signature and forward the ACK downstream. Caused harm is not
bad--just a useless ACK is forwarded.
</para>
</section>
<xi:include href="sl_params.xml"/>
<xi:include href="sl_functions.xml"/>
<xi:include href="sl_stats.xml"/>
</chapter>
</book>
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