model: PacketHeader<T> and interface with NS-3

This commit creates a compatible interface to convert ndn-cxx Interest
and Data structures into NS-3's Ptr<Packet> and back.

Refs: #2048, #2230
diff --git a/utils/ndn-ns3-packet-tag.hpp b/utils/ndn-ns3-packet-tag.hpp
new file mode 100644
index 0000000..af9213f
--- /dev/null
+++ b/utils/ndn-ns3-packet-tag.hpp
@@ -0,0 +1,56 @@
+/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
+/**
+ * Copyright (c) 2011-2014  Regents of the University of California.
+ *
+ * This file is part of ndnSIM. See AUTHORS for complete list of ndnSIM authors and
+ * contributors.
+ *
+ * ndnSIM is free software: you can redistribute it and/or modify it under the terms
+ * of the GNU General Public License as published by the Free Software Foundation,
+ * either version 3 of the License, or (at your option) any later version.
+ *
+ * ndnSIM is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
+ * without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
+ * PURPOSE.  See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * ndnSIM, e.g., in COPYING.md file.  If not, see <http://www.gnu.org/licenses/>.
+ **/
+
+#ifndef NDN_NS3_PACKET_TAG_HPP
+#define NDN_NS3_PACKET_TAG_HPP
+
+#include "ns3/packet.h"
+#include "ns3/ptr.h"
+#include <ndn-cxx/tag.hpp>
+
+namespace ns3 {
+namespace ndn {
+
+class Ns3PacketTag : public ::ndn::Tag {
+public:
+  static size_t
+  getTypeId()
+  {
+    return 0xaee87802; // md5("Ns3PacketTag")[0:8]
+  }
+
+  Ns3PacketTag(Ptr<const Packet> packet)
+    : m_packet(packet)
+  {
+  }
+
+  Ptr<const Packet>
+  getPacket() const
+  {
+    return m_packet;
+  }
+
+private:
+  Ptr<const Packet> m_packet;
+};
+
+} // namespace ndn
+} // namespace ns3
+
+#endif // NDN_NS3_PACKET_TAG_HPP