diff --git a/doc/publican/sources/Protocol.xml b/doc/publican/sources/Protocol.xml
index 97e9ba29..0b27082d 100644
--- a/doc/publican/sources/Protocol.xml
+++ b/doc/publican/sources/Protocol.xml
@@ -194,6 +194,21 @@
 	</varlistentry>
       </variablelist>
     </para>
+    <para>
+      The protocol does not specify the exact position of the ancillary data
+      in the stream, except that the order of file descriptors is the same as
+      the order of messages and <code>fd</code> arguments within messages on
+      the wire.
+    </para>
+    <para>
+      In particular, it means that any byte of the stream, even the message
+      header, may carry the ancillary data with file descriptors.
+    </para>
+    <para>
+      Clients and compositors should queue incoming data until they have
+      whole messages to process, as file descriptors may arrive earlier
+      or later than the corresponding data bytes.
+    </para>
   </section>
   <xi:include href="ProtocolInterfaces.xml" xmlns:xi="http://www.w3.org/2001/XInclude"/>
   <section id="sect-Protocol-Versioning">