Enhance text which explains how to debug networking

problems when setting up PXE.

Reviewed by: gjb

en_US.ISO8859-1/books/handbook/Makefile

@@ -49,6 +49,7 @@ IMAGES_EN = advanced-networking/isdn-bus.eps
 IMAGES_EN+= advanced-networking/isdn-twisted-pair.eps
 IMAGES_EN+= advanced-networking/natd.eps
 IMAGES_EN+= advanced-networking/net-routing.pic
+IMAGES_EN+= advanced-networking/pxe-nfs.png
 IMAGES_EN+= advanced-networking/static-routes.pic
 IMAGES_EN+= bsdinstall/bsdinstall-adduser1.png
 IMAGES_EN+= bsdinstall/bsdinstall-adduser2.png

en_US.ISO8859-1/books/handbook/advanced-networking/chapter.sgml

@@ -4590,7 +4590,7 @@ myhost.example.com:/b/tftpboot/FreeBSD/install       /         nfs      ro
 	them.</para>
     </sect2>
 
-    <sect2>
+    <sect2 id="network-pxe-setting-up-dhcp">
       <title>Setting up the DHCP Server</title>
 
       <para>PXE requires a <acronym>TFTP</acronym> server and a
@@ -4659,11 +4659,64 @@ subnet 192.168.0.0 netmask 255.255.255.0 {
 	</step>
 
 	<step>
-	  <para>Use the
-	    <filename role="package">net/wireshark</filename> port to
-	    debug the <acronym>DHCP</acronym> and
-	    <acronym>TFTP</acronym> network traffic to look for any
-	    problems.</para>
+	  <para>Use the <filename role="package">net/wireshark</filename>
+	    port to debug the network traffic involved during the
+	    <acronym>PXE</acronym> booting process, which is
+	    illustrated in the diagram below.
+	    In <xref linkend="network-pxe-setting-up-dhcp">, an
+	    example configuration is shown where the
+	    <acronym>DHCP</acronym>, <acronym>TFTP</acronym>, and
+	    <acronym>NFS</acronym> servers are actually on the same
+	    machine.  However, these severs can be on separate
+	    machines.</para>
+
+	  <figure>
+	    <title>PXE Booting process with NFS root mount</title>
+	    <mediaobjectco>
+		<imageobjectco>
+		  <areaspec units="calspair">
+		    <area id="co-pxenfs1" coords="2873,8133 3313,7266">
+		    <area id="co-pxenfs2" coords="3519,6333 3885,5500">
+		    <area id="co-pxenfs3" coords="4780,5866 5102,5200">
+		    <area id="co-pxenfs4" coords="4794,4333 5102,3600">
+		    <area id="co-pxenfs5" coords="3108,2666 3519,1800">
+		  </areaspec>
+		  <imageobject>
+		    <imagedata fileref="advanced-networking/pxe-nfs">
+		  </imageobject>
+		  <calloutlist>
+		    <callout arearefs="co-pxenfs1">
+		      <para>Client broadcasts DHCPDISCOVER.</para>
+		    </callout>
+		    <callout arearefs="co-pxenfs2">
+		      <para>DHCP server responds with IP address,
+			<literal>next-server</literal>,
+			<literal>filename</literal>, and
+			<literal>root-path</literal>.</para>
+		    </callout>
+		    <callout arearefs="co-pxenfs3">
+		      <para>Client sends <acronym>TFTP</acronym>
+			request to <literal>next-server</literal>
+			asking to retrieve
+			<literal>filename</literal>.</para>
+		    </callout>
+		    <callout arearefs="co-pxenfs4">
+		      <para>TFTP server responds and sends
+			<literal>filename</literal> to client.</para>
+		    </callout>
+		    <callout arearefs="co-pxenfs5">
+		      <para>Client executes
+			<literal>filename</literal> which is
+			&man.pxeboot.8;.  &man.pxeboot.8; loads the
+			kernel.  When the kernel executes, the root
+			filesystem specified by
+			<literal>root-path</literal> is mounted over
+			<acronym>NFS</acronym>.</para>
+		    </callout>
+		  </calloutlist>
+		</imageobjectco>
+	    </mediaobjectco>
+	  </figure>
 	</step>
 
 	<step>