for ipv6 linux stack helper

diff -r 2bbac970450e src/internet/model/ipv6-interface.cc
--- a/src/internet/model/ipv6-interface.cc	Sat Jan 04 22:41:34 2014 +0900
+++ b/src/internet/model/ipv6-interface.cc	Tue Jan 07 00:23:58 2014 +0900
@@ -116,8 +116,16 @@
       return; /* no NDISC cache for ip6-localhost */
     }
 
-  Ptr<Icmpv6L4Protocol> icmpv6 = m_node->GetObject<Ipv6L3Protocol> ()->GetIcmpv6 ();
-  m_ndCache = icmpv6->CreateCache (m_device, this);
+  Ptr<IpL4Protocol> proto = m_node->GetObject<Ipv6> ()->GetProtocol (Icmpv6L4Protocol::GetStaticProtocolNumber ());
+  Ptr<Icmpv6L4Protocol> icmpv6;
+  if (proto)
+    {
+      icmpv6 = proto->GetObject <Icmpv6L4Protocol> ();
+    }
+  if (icmpv6)
+    {
+      m_ndCache = icmpv6->CreateCache (m_device, this);
+    }
 }
 
 void Ipv6Interface::SetNode (Ptr<Node> node)
@@ -216,7 +224,12 @@
       if (!addr.IsAny () || !addr.IsLocalhost ())
         {
           /* DAD handling */
-          Ptr<Icmpv6L4Protocol> icmpv6 = m_node->GetObject<Ipv6L3Protocol> ()->GetIcmpv6 ();
+          Ptr<IpL4Protocol> proto = m_node->GetObject<Ipv6> ()->GetProtocol (Icmpv6L4Protocol::GetStaticProtocolNumber ());
+          Ptr<Icmpv6L4Protocol> icmpv6;
+          if (proto)
+            {
+              icmpv6 = proto->GetObject <Icmpv6L4Protocol> ();
+            }
 
           if (icmpv6 && icmpv6->IsAlwaysDad ())
             {
diff -r 2bbac970450e src/internet/model/ipv6.h
--- a/src/internet/model/ipv6.h	Sat Jan 04 22:41:34 2014 +0900
+++ b/src/internet/model/ipv6.h	Tue Jan 07 00:23:58 2014 +0900
@@ -31,6 +31,7 @@
 
 #include "ns3/ipv6-address.h"
 #include "ipv6-interface-address.h"
+#include "ns3/ip-l4-protocol.h"
 
 namespace ns3 {
 
@@ -304,6 +305,13 @@
   virtual void SetForwarding (uint32_t interface, bool val) = 0;
 
   /**
+   * \brief Get L4 protocol by protocol number.
+   * \param protocolNumber protocol number
+   * \return corresponding Ipv6L4Protocol or 0 if not found
+   */
+  virtual Ptr<IpL4Protocol> GetProtocol (int protocolNumber) const = 0;
+
+  /**
    * \brief Register the IPv6 Extensions.
    */
   virtual void RegisterExtensions () = 0;