<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" 
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<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>
