helper+docs+examples: New helper to simplify link failing

Thanks to Saran Tarnoi for providing base implementation of the code
diff --git a/docs/source/examples.rst b/docs/source/examples.rst
index be27f0b..41142c9 100644
--- a/docs/source/examples.rst
+++ b/docs/source/examples.rst
@@ -6,6 +6,9 @@
 .. note::
    :red:`!!! This page only shows up examples of how to config topology and perform basic operations in ndnSIM (an example equivalent to "Hello, world1") !!!  These are **NOT** examples of real experimentations (just like "Hello, world!" is not a real program).`
 
+.. note::
+   If you compiled ndnSIM with examples (``./waf configure --enable-examples``) you can directly run the example without putting scenario into ``scratch/`` folder.
+
 .. _simple-scenario:
 
 Simple scenario
@@ -46,6 +49,9 @@
 
      NS_LOG=ndn.Consumer:ndn.Producer ./waf --run=ndn-simple
 
+.. note::
+   If you compiled ndnSIM with examples (``./waf configure --enable-examples``) you can directly run the example without putting scenario into ``scratch/`` folder.
+
 .. _9-node-grid-example:
 
 9-node grid example
@@ -95,6 +101,9 @@
 
     NS_LOG=ndn.Consumer:ndn.Producer ./waf --run=ndn-grid
 
+.. note::
+   If you compiled ndnSIM with examples (``./waf configure --enable-examples``) you can directly run the example without putting scenario into ``scratch/`` folder.
+
 .. _9-node-grid-example-using-topology-plugin:
 
 9-node grid example using topology plugin
@@ -155,6 +164,8 @@
 
     NS_LOG=ndn.Consumer:ndn.Producer ./waf --run=ndn-grid-topo-plugin
 
+.. note::
+   If you compiled ndnSIM with examples (``./waf configure --enable-examples``) you can directly run the example without putting scenario into ``scratch/`` folder.
 
 6-node bottleneck topology
 --------------------------
@@ -195,6 +206,9 @@
 
         NS_LOG=ndn.Consumer:ndn.Producer ./waf --run=ndn-congestion-topo-plugin
 
+.. note::
+   If you compiled ndnSIM with examples (``./waf configure --enable-examples``) you can directly run the example without putting scenario into ``scratch/`` folder.
+
 .. _11-node 2-bottleneck topology with custom forwarding strategy:
 
 11-node 2-bottleneck topology with custom forwarding strategy
@@ -260,6 +274,9 @@
 
         ./waf --run=ndn-congestion-alt-topo-plugin --visualize
 
+.. note::
+   If you compiled ndnSIM with examples (``./waf configure --enable-examples``) you can directly run the example without putting scenario into ``scratch/`` folder.
+
 3-level binary tree with packet-level trace helpers
 ---------------------------------------------------
 
@@ -309,6 +326,33 @@
 
      tcpdump -r ndn-simple-trace.pcap
 
+.. note::
+   If you compiled ndnSIM with examples (``./waf configure --enable-examples``) you can directly run the example without putting scenario into ``scratch/`` folder.
+
+.. _Simple scenario with link failures:
+
+Simple scenario with link failures
+----------------------------------
+
+The following example (``ndn-simple-with-link-failure.cc``) shows how to "fail" links in ndnSIM simulation.
+The basic idea is to set ndn::Faces that correspond to the failed link to DOWN state.
+ndnSIM now includes a simple helper that simplifies this process.
+
+.. literalinclude:: ../../examples/ndn-simple-with-link-failure.cc
+   :language: c++
+   :linenos:
+   :lines: 21-31,52-
+   :emphasize-lines: 54-56
+
+If this code is placed into ``scratch/ndn-simple-with-link-failure.cc`` and NS-3 is compiled in debug mode, you can run and see progress of the
+simulation using the following command (in optimized mode nothing will be printed out)::
+
+     NS_LOG=ndn.Consumer:ndn.Producer:ndn.LinkControlHelper ./waf --run=ndn-simple-with-link-failure
+
+.. note::
+   If you compiled ndnSIM with examples (``./waf configure --enable-examples``) you can directly run the example without putting scenario into ``scratch/`` folder.
+
+
 25-node tree topology with L2Tracer
 -----------------------------------
 
diff --git a/docs/source/faq.rst b/docs/source/faq.rst
index c660492..a7ce4bd 100644
--- a/docs/source/faq.rst
+++ b/docs/source/faq.rst
@@ -133,47 +133,7 @@
 Right now, NS-3 does not provide ability to actually "break" the link between nodes in NS-3.
 However, exactly the same effect can be achieved by making an interface (:ndnsim:`ndn::Face`) up or down (:ndnsim:`ndn::Face::SetUp(true)` or :ndnsim:`ndn::Face::SetUp(false)`).
 
-Here is an example of function to "fail" a point-to-point link between two NDN nodes:
-
-.. code-block:: c++
-
-    // hijacker is more than an application. just disable all faces
-    void
-    FailLinks (Ptr<Node> node1, Ptr<Node> node2)
-    {
-      Ptr<ndn::L3Protocol> ndn1 = node1->GetObject<ndn::L3Protocol> ();
-      Ptr<ndn::L3Protocol> ndn2 = node2->GetObject<ndn::L3Protocol> ();
-
-      // iterate over all faces to find the right one
-      for (uint32_t faceId = 0; faceId < ndn1->GetNFaces (); faceId++)
-        {
-          Ptr<ndn::NetDeviceFace> ndFace = ndn1->GetFace (faceId)->GetObject<ndn::NetDeviceFace> ();
-          if (ndFace == 0) continue;
-
-          Ptr<PointToPointNetDevice> nd1 = ndFace->GetNetDevice ()->GetObject<PointToPointNetDevice> ();
-          if (nd1 == 0) continue;
-
-          Ptr<Channel> channel = nd1->GetChannel ();
-          if (channel == 0) continue;
-
-          Ptr<PointToPointChannel> ppChannel = DynamicCast<PointToPointChannel> (channel);
-
-          Ptr<NetDevice> nd2 = ppChannel->GetDevice (0);
-          if (nd2->GetNode () == node1)
-            nd2 = ppChannel->GetDevice (1);
-
-          if (nd2->GetNode () == node2)
-            {
-              Ptr<ndn::Face> face1 = ndn1->GetFaceByNetDevice (nd1);
-              Ptr<ndn::Face> face2 = ndn2->GetFaceByNetDevice (nd2);
-
-              face1->SetUp (false);
-              face2->SetUp (false);
-              break;
-            }
-        }
-    }
-
+You can use :ndnsim:`ndn::LinkControlHelper` to schedule failing links.  For example, refer to :ref:`Simple scenario with link failures` example.
 
 General questions
 -----------------
diff --git a/examples/ndn-simple-with-link-failure.cc b/examples/ndn-simple-with-link-failure.cc
new file mode 100644
index 0000000..161e735
--- /dev/null
+++ b/examples/ndn-simple-with-link-failure.cc
@@ -0,0 +1,104 @@
+/* -*-  Mode: C++; c-file-style: "gnu"; indent-tabs-mode:nil; -*- */
+/*
+ * Copyright (c) 2012 University of California, Los Angeles
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation;
+ *
+ * This program 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 this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ *
+ * Author: Alexander Afanasyev <alexander.afanasyev@ucla.edu>
+ *       : Saran Tarnoi <saran.tarnoi@gmail.com>
+ */
+// ndn-simple-with-link-failure.cc
+#include "ns3/core-module.h"
+#include "ns3/network-module.h"
+#include "ns3/point-to-point-module.h"
+#include "ns3/ndnSIM-module.h"
+
+// for LinkStatusControl::FailLinks and LinkStatusControl::UpLinks
+#include "ns3/ndn-link-control-helper.h"
+
+using namespace ns3;
+
+/**
+ * This scenario simulates a very simple network topology:
+ *
+ *
+ *      +----------+     1Mbps      +--------+     1Mbps      +----------+
+ *      | consumer | <------------> | router | <------------> | producer |
+ *      +----------+         10ms   +--------+          10ms  +----------+
+ *
+ *
+ * Consumer requests data from producer with frequency 10 interests per second
+ * (interests contain constantly increasing sequence number).
+ *
+ * For every received interest, producer replies with a data packet, containing
+ * 1024 bytes of virtual payload.
+ *
+ * To run scenario and see what is happening, use the following command:
+ *
+ *     NS_LOG=ndn.Consumer:ndn.Producer ./waf --run=ndn-simple
+ */
+
+int 
+main (int argc, char *argv[])
+{
+  // setting default parameters for PointToPoint links and channels
+  Config::SetDefault ("ns3::PointToPointNetDevice::DataRate", StringValue ("1Mbps"));
+  Config::SetDefault ("ns3::PointToPointChannel::Delay", StringValue ("10ms"));
+  Config::SetDefault ("ns3::DropTailQueue::MaxPackets", StringValue ("20"));
+
+  // Read optional command-line parameters (e.g., enable visualizer with ./waf --run=<> --visualize
+  CommandLine cmd;
+  cmd.Parse (argc, argv);
+
+  // Creating nodes
+  NodeContainer nodes;
+  nodes.Create (3);
+
+  // Connecting nodes using two links
+  PointToPointHelper p2p;
+  p2p.Install (nodes.Get (0), nodes.Get (1));
+  p2p.Install (nodes.Get (1), nodes.Get (2));
+
+  // Install NDN stack on all nodes
+  ndn::StackHelper ndnHelper;
+  ndnHelper.SetDefaultRoutes (true);
+  ndnHelper.InstallAll ();
+
+  // Installing applications
+
+  // Consumer
+  ndn::AppHelper consumerHelper ("ns3::ndn::ConsumerCbr");
+  // Consumer will request /prefix/0, /prefix/1, ...
+  consumerHelper.SetPrefix ("/prefix");
+  consumerHelper.SetAttribute ("Frequency", StringValue ("10")); // 10 interests a second
+  consumerHelper.Install (nodes.Get (0)); // first node
+
+  // Producer
+  ndn::AppHelper producerHelper ("ns3::ndn::Producer");
+  // Producer will reply to all requests starting with /prefix
+  producerHelper.SetPrefix ("/prefix");
+  producerHelper.SetAttribute ("PayloadSize", StringValue("1024"));
+  producerHelper.Install (nodes.Get (2)); // last node
+
+  // The failure of the link connecting consumer and router will start from seconds 10.0 to 15.0
+  Simulator::Schedule (Seconds (10.0), ndn::LinkControlHelper::FailLink, nodes.Get (0), nodes.Get (1));
+  Simulator::Schedule (Seconds (15.0), ndn::LinkControlHelper::UpLink,   nodes.Get (0), nodes.Get (1));   
+  
+  Simulator::Stop (Seconds (20.0));
+
+  Simulator::Run ();
+  Simulator::Destroy ();
+
+  return 0;
+}
diff --git a/examples/wscript b/examples/wscript
index 437ed26..49a0c5c 100644
--- a/examples/wscript
+++ b/examples/wscript
@@ -91,3 +91,6 @@
         'ndn-simple-api.cc',
         'custom-apps/ndn-api-app.cc'
         ]
+
+    obj = bld.create_ns3_program('ndn-simple-with-link-failure', all_modules)
+    obj.source = 'ndn-simple-with-link-failure.cc'
diff --git a/helper/ndn-link-control-helper.cc b/helper/ndn-link-control-helper.cc
new file mode 100644
index 0000000..cf2b99d
--- /dev/null
+++ b/helper/ndn-link-control-helper.cc
@@ -0,0 +1,139 @@
+/* -*-  Mode: C++; c-file-style: "gnu"; indent-tabs-mode:nil; -*- */
+/*
+ * Copyright (c) 2013 University of California, Los Angeles
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation;
+ *
+ * This program 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 this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ *
+ * Author: Alexander Afanasyev <alexander.afanasyev@ucla.edu>
+ *       : Saran Tarnoi <saran.tarnoi@gmail.com>
+ */
+
+#include "ndn-link-control-helper.h"
+
+#include "ns3/assert.h"
+#include "ns3/names.h"
+#include "ns3/point-to-point-net-device.h"
+#include "ns3/point-to-point-channel.h"
+#include "ns3/channel.h"
+#include "ns3/log.h"
+
+#include "ns3/ndn-l3-protocol.h"
+#include "ns3/ndn-net-device-face.h"
+
+NS_LOG_COMPONENT_DEFINE ("ndn.LinkControlHelper");
+
+namespace ns3 {
+namespace ndn {
+
+void
+LinkControlHelper::FailLink (Ptr<Node> node1, Ptr<Node> node2)
+{
+  NS_LOG_FUNCTION (node1 << node2);
+  
+  NS_ASSERT (node1 != 0);
+  NS_ASSERT (node2 != 0);
+  
+  Ptr<ndn::L3Protocol> ndn1 = node1->GetObject<ndn::L3Protocol> ();
+  Ptr<ndn::L3Protocol> ndn2 = node2->GetObject<ndn::L3Protocol> ();
+
+  NS_ASSERT (ndn1 != 0);
+  NS_ASSERT (ndn2 != 0);
+
+  // iterate over all faces to find the right one
+  for (uint32_t faceId = 0; faceId < ndn1->GetNFaces (); faceId++)
+    {
+      Ptr<ndn::NetDeviceFace> ndFace = ndn1->GetFace (faceId)->GetObject<ndn::NetDeviceFace> ();
+      if (ndFace == 0) continue;
+
+      Ptr<PointToPointNetDevice> nd1 = ndFace->GetNetDevice ()->GetObject<PointToPointNetDevice> ();
+      if (nd1 == 0) continue;
+
+      Ptr<Channel> channel = nd1->GetChannel ();
+      if (channel == 0) continue;
+
+      Ptr<PointToPointChannel> ppChannel = DynamicCast<PointToPointChannel> (channel);
+
+      Ptr<NetDevice> nd2 = ppChannel->GetDevice (0);
+      if (nd2->GetNode () == node1)
+        nd2 = ppChannel->GetDevice (1);
+
+      if (nd2->GetNode () == node2)
+        {
+          Ptr<ndn::Face> face1 = ndn1->GetFaceByNetDevice (nd1);
+          Ptr<ndn::Face> face2 = ndn2->GetFaceByNetDevice (nd2);
+
+          face1->SetUp (false);
+          face2->SetUp (false);
+          break;
+        }
+    }
+}
+void
+LinkControlHelper::FailLinkByName (const std::string &node1, const std::string &node2)
+{
+  FailLink (Names::Find<Node> (node1), Names::Find<Node> (node2));
+}
+
+void
+LinkControlHelper::UpLink (Ptr<Node> node1, Ptr<Node> node2)
+{
+  NS_LOG_FUNCTION (node1 << node2);
+
+  NS_ASSERT (node1 != 0);
+  NS_ASSERT (node2 != 0);
+  
+  Ptr<ndn::L3Protocol> ndn1 = node1->GetObject<ndn::L3Protocol> ();
+  Ptr<ndn::L3Protocol> ndn2 = node2->GetObject<ndn::L3Protocol> ();
+
+  NS_ASSERT (ndn1 != 0);
+  NS_ASSERT (ndn2 != 0);
+
+  // iterate over all faces to find the right one
+  for (uint32_t faceId = 0; faceId < ndn1->GetNFaces (); faceId++)
+    {
+      Ptr<ndn::NetDeviceFace> ndFace = ndn1->GetFace (faceId)->GetObject<ndn::NetDeviceFace> ();
+      if (ndFace == 0) continue;
+
+      Ptr<PointToPointNetDevice> nd1 = ndFace->GetNetDevice ()->GetObject<PointToPointNetDevice> ();
+      if (nd1 == 0) continue;
+
+      Ptr<Channel> channel = nd1->GetChannel ();
+      if (channel == 0) continue;
+
+      Ptr<PointToPointChannel> ppChannel = DynamicCast<PointToPointChannel> (channel);
+
+      Ptr<NetDevice> nd2 = ppChannel->GetDevice (0);
+      if (nd2->GetNode () == node1)
+        nd2 = ppChannel->GetDevice (1);
+
+      if (nd2->GetNode () == node2)
+        {
+          Ptr<ndn::Face> face1 = ndn1->GetFaceByNetDevice (nd1);
+          Ptr<ndn::Face> face2 = ndn2->GetFaceByNetDevice (nd2);
+
+          face1->SetUp (true);
+          face2->SetUp (true);
+          break;
+        }
+    }
+}
+
+void
+LinkControlHelper::UpLinkByName (const std::string &node1, const std::string &node2)
+{
+  UpLink (Names::Find<Node> (node1), Names::Find<Node> (node2));
+}
+
+}
+}
diff --git a/helper/ndn-link-control-helper.h b/helper/ndn-link-control-helper.h
new file mode 100644
index 0000000..619fb1c
--- /dev/null
+++ b/helper/ndn-link-control-helper.h
@@ -0,0 +1,103 @@
+/* -*-  Mode: C++; c-file-style: "gnu"; indent-tabs-mode:nil; -*- */
+/*
+ * Copyright (c) 2013 University of California, Los Angeles
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation;
+ *
+ * This program 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 this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ *
+ * Author: Alexander Afanasyev <alexander.afanasyev@ucla.edu>
+ *       : Saran Tarnoi <saran.tarnoi@gmail.com>
+ */
+
+#ifndef NDN_LINK_CONTROL_HELPER_H
+#define NDN_LINK_CONTROL_HELPER_H
+
+#include "ns3/ptr.h"
+#include "ns3/node.h"
+
+namespace ns3 {
+namespace ndn {
+
+/**
+ * @brief Helper class to control the up or down statuss of an NDN link connecting two specific nodes
+ * @ingroup Ndn
+ */
+class LinkControlHelper
+{ 
+public:
+  /**
+   * @brief Fail NDN link between two nodes
+   *
+   * The helper will attempt to find NDN link between node1 and
+   * node2 and set NDN face to DOWN state
+   *
+   * Note that only PointToPointChannels are supported by this helper method
+   *
+   * @param node1 one node
+   * @param node2 another node
+   */
+  static void
+  FailLink (Ptr<Node> node1, Ptr<Node> node2);
+
+  /**
+   * @brief Fail NDN link between two nodes
+   *
+   * The helper will attempt to find NDN link between node1 and
+   * node2 and set NDN face to DOWN state
+   *
+   * Note that only PointToPointChannels are supported by this helper method
+   *
+   * This variant uses node names registered by Names class
+   *
+   * @param node1 one node's name
+   * @param node2 another node's name
+   */
+  static void
+  FailLinkByName (const std::string &node1, const std::string &node2);
+
+  /**
+   * @brief Re-enable NDN link between two nodes
+   *
+   * The helper will attempt to find NDN link between node1 and
+   * node2 and set NDN face to UP state
+   *
+   * Note that only PointToPointChannels are supported by this helper method
+   *
+   * @param node1 one node
+   * @param node2 another node
+   */
+  static void
+  UpLink (Ptr<Node> node1, Ptr<Node> node2);
+  
+  /**
+   * @brief Re-enable NDN link between two nodes
+   *
+   * The helper will attempt to find NDN link between node1 and
+   * node2 and set NDN face to UP state
+   *
+   * Note that only PointToPointChannels are supported by this helper method
+   *
+   * This variant uses node names registered by Names class
+   *
+   * @param node1 one node's name
+   * @param node2 another node's name
+   */
+  static void
+  UpLinkByName (const std::string &node1, const std::string &node2);
+}; // end: LinkControlHelper
+
+
+} // ndn
+} // ns3
+
+#endif // NDN_LINK_CONTROL_HELPER_H
diff --git a/wscript b/wscript
index c0dbf11..6f200dc 100644
--- a/wscript
+++ b/wscript
@@ -124,6 +124,7 @@
         "helper/ndn-header-helper.h",
         "helper/ndn-face-container.h",
         "helper/ndn-global-routing-helper.h",
+        "helper/ndn-link-control-helper.h",
 
         "apps/ndn-app.h",