--- a/src/internet/model/ipv4-l3-protocol.cc	Thu Oct 11 12:21:17 2012 -0700
+++ a/src/internet/model/ipv4-l3-protocol.cc	Mon Oct 15 14:19:03 2012 -0400
@@ -532,6 +532,10 @@ 
                                 Ptr<Ipv4Route> route)
 {
   NS_LOG_FUNCTION (this << packet << ipHeader << route);
+  if (Node::ChecksumEnabled ())
+    {
+      ipHeader.EnableChecksum ();
+    }
   SendRealOut (route, packet, ipHeader);
 }