point-to-point: (fixes #2556) Avoid modification of transmitted packets

Packets sent to channel may later be modified by calling device.  Possible
solutions are to copy before or after sending the packet to the channel.
Adopted solution similar to YansWifiChannel; a Ptr<const Packet> is instead
passed to the channel object, and channel object is responsible for copy.

src/point-to-point/model/point-to-point-channel.cc

@@ -85,7 +85,7 @@ PointToPointChannel::Attach (Ptr<PointToPointNetDevice> device)
 
 bool
 PointToPointChannel::TransmitStart (
-  Ptr<Packet> p,
+  Ptr<const Packet> p,
   Ptr<PointToPointNetDevice> src,
   Time txTime)
 {
@@ -99,7 +99,7 @@ PointToPointChannel::TransmitStart (
 
   Simulator::ScheduleWithContext (m_link[wire].m_dst->GetNode ()->GetId (),
                                   txTime + m_delay, &PointToPointNetDevice::Receive,
-                                  m_link[wire].m_dst, p);
+                                  m_link[wire].m_dst, p->Copy ());
 
   // Call the tx anim callback on the net device
   m_txrxPointToPoint (p, src, m_link[wire].m_dst, txTime, txTime + m_delay);

src/point-to-point/model/point-to-point-channel.h

@@ -78,7 +78,7 @@ public:
    * \param txTime Transmit time to apply
    * \returns true if successful (currently always true)
    */
-  virtual bool TransmitStart (Ptr<Packet> p, Ptr<PointToPointNetDevice> src, Time txTime);
+  virtual bool TransmitStart (Ptr<const Packet> p, Ptr<PointToPointNetDevice> src, Time txTime);
 
   /**
    * \brief Get number of devices on this channel

src/point-to-point/model/point-to-point-remote-channel.cc

@@ -55,7 +55,7 @@ PointToPointRemoteChannel::~PointToPointRemoteChannel ()
 
 bool
 PointToPointRemoteChannel::TransmitStart (
-  Ptr<Packet> p,
+  Ptr<const Packet> p,
   Ptr<PointToPointNetDevice> src,
   Time txTime)
 {
@@ -70,7 +70,7 @@ PointToPointRemoteChannel::TransmitStart (
 #ifdef NS3_MPI
   // Calculate the rxTime (absolute)
   Time rxTime = Simulator::Now () + txTime + GetDelay ();
-  MpiInterface::SendPacket (p, rxTime, dst->GetNode ()->GetId (), dst->GetIfIndex ());
+  MpiInterface::SendPacket (p->Copy (), rxTime, dst->GetNode ()->GetId (), dst->GetIfIndex ());
 #else
   NS_FATAL_ERROR ("Can't use distributed simulator without MPI compiled in");
 #endif

src/point-to-point/model/point-to-point-remote-channel.h

@@ -66,7 +66,7 @@ public:
    * \param txTime Transmit time to apply
    * \returns true if successful (currently always true)
    */
-  virtual bool TransmitStart (Ptr<Packet> p, Ptr<PointToPointNetDevice> src,
+  virtual bool TransmitStart (Ptr<const Packet> p, Ptr<PointToPointNetDevice> src,
                               Time txTime);
 };