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<section><title>Introduction</title>
<para>This document describes usage of SIP Express Router as a presence server.
<!-- TODO: What is presence, ... (use doc from PIC-SE wiki?) -->
</para>

<section><title>Main features</title>
<para><itemizedlist>
	<listitem><para>presence events with XCAP authorization and watcher info
	support</para></listitem>
	<listitem><para>resource list server (only for presence now)</para></listitem>
	<listitem><para>B2BUA for presence events (no resource list support now)</para></listitem>
	<listitem><para>MESSAGE authorization (via XCAP)</para></listitem>
</itemizedlist>
</para>
</section>
		
<section><title>SER presence basics</title>

<para>Presence is one of quite important components of SER. It allows users to
watch presence state of other users, process lists of them and manipulate one's 
own presence state.</para>

<para>Configuration data can be stored on a XCAP server <xref
linkend="pres_draft_xcap"/> by user's client software and processed by SER. This
may be useful for lists of watched users (resource lists) and for authorization
rules.</para>

<para>There is a few of modules which serve as parts of "presence":
<itemizedlist>
	<listitem><para><link linkend="pa">PA</link> acts as a presence
	server. Its main function is processing of subscriptions to presence
	state of standalone users and processing presence state publications for
	them.</para></listitem>
	<listitem><para><link linkend="pres_rls">RLS</link> - Resource list server - this
	module processes subscriptions to lists of resources. It gets presence
	information for standalone users from internal queries to PA module or
	remote presence server queries and build them
	together into list notifications.</para></listitem>
	<listitem><para><link linkend="presence_b2b">PRESENCE_B2B</link> can be used
	to subscribe to presence state on remote server. It can be used by RLS for
	remote presence server queries.</para></listitem>
	<listitem><para><link linkend="xcap_module">XCAP</link> offers internal functions
	for querying XCAP server. <!-- TODO caching responses and monitoring changes
	in documents stored on XCAP server. --></para></listitem>
	<listitem><para><emphasis>DIALOG</emphasis> module is a helper module used
	by other presence modules for some
	dialog operations. It was intended to contain dialog management functions
	but it was not finished.</para></listitem>
<!--	<listitem><para><emphasis>RPA </emphasis>Reg events server. Will be used by
	PA instead of direct callbacks into usrloc. Will be added
	soon.</para></listitem>-->
</itemizedlist>
</para>

<para>All <quote>presence</quote> modules share common code stored in SER
libraries. Their interface is described in standalone documents.
</para>

<section><title>Persistence</title>
<para>Modules can store their status (working data) into database. This data is
automatically reloaded on startup, so it is possible to restart SER and
clients don't note it. Established SIP dialogs are stored in database too.</para>
<para>Details about database storage are described for each module separately in
module documentation.

<!-- TODO: db modes (cache mode) -->
</para>
</section>

<section><title>Authorization</title>
<para>Authorization is very important in presence services. The server
must take care about authorization rules defined by user about whom and
whom not allow access to user's presence status. More about authorization
rules may be found in <xref linkend="pres_draft_common_auth"/> and 
<xref linkend="pres_draft_auth"/>.</para>

<!-- TODO: types of authorization common for all presence modules -->

<para>Only XCAP storage of authorization rules is supported at this moment. It is 
<emphasis>not fully implemented</emphasis> now - only basic rule conditions, no sphere 
and time conditions. Transformations defined in <xref linkend="pres_draft_auth"/> are 
ignored.
May be, that in the
future will be possible to use other variants like webdav or storing
authorization rules in SER's own database.</para>

<!-- TODO: XCAP module and authorization ? at least link into XCAP doc? -->

</section>

</section> <!-- presence basics -->

</section> <!-- introduction -->
