Update CHANGES.html and RELEASE_NOTES

CHANGES.html

@@ -54,6 +54,7 @@ us a note on ns-developers mailing list.</p>
 <h1>Changes from ns-3.30 to ns-3.31</h1>
 <h2>New API:</h2>
 <ul>
+<li> New attributes for <b> Ipv4L3Protocol</b> have been added to enable RFC 6621-based duplicate packet detection (DPD) (<b>EnableRFC6621</b>) and to control the cache expiration time (<b>DuplicateExpire</b>).</li>
 </ul>
 <h2>Changes to existing API:</h2>
 <ul>

RELEASE_NOTES

@@ -22,6 +22,8 @@ To be determined.
 
 New user-visible features
 -------------------------
+- (internet) An option to enable IPv4 hash-based multicast duplicate packet 
+  detection (DPD) based on RFC 6621 has been added.
 
 Bugs fixed
 ----------