kamailio/doc/tutorials/serdev/msg_parser.xml

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<!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" 
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<section id="msg_parser" xmlns:xi="http://www.w3.org/2001/XInclude">
    <sectioninfo>
	<revhistory>
	    <revision>
		<revnumber>$Revision$</revnumber>
		<date>$Date$</date>
	    </revision>
	</revhistory>
    </sectioninfo>
    
    <title>The SIP Message Parser</title>
    <para>
	In this section we will discuss internals of the <acronym>SIP</acronym>
	message header parser implemented in the server. Message parsing is
	very important and one of the most time consuming operations of a
	<acronym>SIP</acronym> server. We have been trying to make the parser
	as fast as possible.
    </para>
    <para>
	A header field parser can be either in the server core or in a
	module. By convention, parser that is needed by the core itself or is
	needed by at least two modules will be in the core. Parsers contained
	in modules will be not described in this section.
    </para>
    <para>
	There is a <filename>parser</filename> subdirectory that contains all
	the parsers and related stuff.
    </para>
    <para>
	The following parsers can be found under <filename>parser</filename>
	subdirectory:
    </para>

    <section id="sip_msg_structure">
	<title>Structure of a <acronym>SIP</acronym> Message</title>
	<para>
	    A <acronym>SIP</acronym> message consists of message header and
	    optional message body.  The header is separated from the body with
	    a empty line (containing CRLF only).
	</para>
	<para>
	    Message header consists of the first line and one or more header
	    fields. The first line determines type of the message. Header
	    fields provide additional information that is needed by clients and
	    servers to be able to process the message.
	</para>
	<para>
	    Each header field consists of header field name and header field
	    body. Header field name is delimited from header field body by a
	    colon (":"). For example, "Server: SIP Express Router" - in this
	    case "Server" is header field name and "SIP Express Router" is
	    header field body.
	</para>
    </section> <!-- sip-msg-structure -->
    <section id="parser_structure">
	<title>Parser Structure</title>
	<para>
	    The server implements what we call <emphasis>incremental
		parsing</emphasis>. It means that a header field will be not
	    parsed unless it is really needed. There is a minimal set of
	    header that will be parsed every time. The set includes:
	    <itemizedlist>
		<listitem>
		    <para>
			The first line - the server must know if the message is
			request or response
		    </para>
		</listitem>
		<listitem>
		    <para>
			Via header field - Via will be needed for sure. We must
			add ourself to Via list when forwarding the message.
		    </para>
		</listitem>
	    </itemizedlist>
	</para>
	
	<xi:include href="fline_parser.xml"/>
	<xi:include href="hfname_parser.xml"/>

	<xi:include href="to_parser.xml"/>
	<xi:include href="from_parser.xml"/>
	<xi:include href="cseq_parser.xml"/>
	<xi:include href="event_parser.xml"/>
	<xi:include href="expires_parser.xml"/>
	<xi:include href="via_parser.xml"/>
	<xi:include href="contact_parser.xml"/>
	<xi:include href="digest_parser.xml"/>
    </section> <!-- parser-organization -->
</section>