radiotap header support

[Mathieu Lacage, 2009-04-17 11:38:37]

The idea is great, but the implementation has lots of issues, mainly from a
performance POV. I think that the best way to deal with the performance issue
would be to not add a radiotap header to the packet itself but to write it
directly with the PcapWriter for each packet.

i.e.,:

class PcapWriter 
{
public:
  struct RadiotapHeader {
  ...
  };
  void WriteRadiotapPacket (Ptr<const Packet> p, RadiotapHeader header) {
    .. write each field of the header directly in the outgoing stream
    .. write the packet content now
  }
};

http://codereview.appspot.com/40109/diff/1/10
File src/devices/wifi/yans-wifi-phy.cc (right):

http://codereview.appspot.com/40109/diff/1/10#newcode411
Line 411: h.SetTsft(Now().GetMicroSeconds());
in the real world, the radiotap header is used _only_ for received packets, not
transmitted packets.

http://codereview.appspot.com/40109/diff/1/10#newcode417
Line 417: packet->AddHeader(h);
Adding the header here has lots of really bad consequences but the main one is
that the rx delays are now doing to be calculated _with_ the header at the
receiver.

http://codereview.appspot.com/40109/diff/1/10#newcode582
Line 582: packet->AddHeader(h);
hahah ! Remove + Add ?? This is horribly bad from a performance point of view:
it will trigger a packet deep copy in each receiver.