Update description of forwarding behavior in ARM

(cherry picked from commit 30e126ad02)

doc/arm/Bv9ARM-book.xml

@@ -516,18 +516,35 @@
 	  </para>
 
 	  <para>
-	    There may be one or more forwarders,
-	    and they are queried in turn until the list is exhausted or an
-	    answer
-	    is found. Forwarders are typically used when it is undesirable
-	    for all the servers at a given site to interact directly with the
-	    rest of
-	    the Internet's servers. A typical scenario involves
-	    internal <acronym>DNS</acronym> servers and an
-	    Internet firewall. Servers unable
-	    to pass packets through the firewall forward their requests to the server
-	    that can, and that server queries the Internet <acronym>DNS</acronym> servers
-	    on the internal servers' behalf.
+	    Forwarders are typically used when an administrator does not
+	    wish for all the servers at a given site to interact
+	    directly with the rest of the Internet. For example, a
+	    common scenario is when multiple internal DNS servers are
+	    behind an Internet firewall. Servers behind the firewall
+	    forward their requests to the server with external access,
+	    which queries Internet DNS servers on the internal servers'
+	    behalf.
+	  </para>
+
+	  <para>
+	    Another scenario (largely now superseded by Response Policy
+	    Zones) is to send queries first to a custom server for RBL
+	    processing before forwarding them to the wider Internet.
+	  </para>
+
+	  <para>
+	    There may be one or more forwarders in a given setup. The
+	    order in which the forwarders are listed in
+	    <filename>named.conf</filename> does not determine the
+	    sequence in which they are queried; rather,
+	    <command>named</command> uses the response times from
+	    previous queries to select the server that is likely to
+	    respond the most quickly. A server that has not yet been
+	    queried is given an initial small random response time to
+	    ensure that it is tried at least once. Dynamic adjustment of
+	    the recorded response times ensures that all forwarders are
+	    queried, even those with slower response times.  This
+	    permits changes in behavior based on server responsiveness.
 	  </para>
 	</section>