loss of original packet type info when tracing multiple protocols.

[Sjoerd Janssen <s.a.janssen@student.utwente.nl>]

Hi,

- How does one retain the original information in the common header -
the packet type being the most important- when using nodes consisting of
multiple protocol layers and tracing for some of these layers is
required?

Consider for example a node containing a MAC layer and a TCP agent.. The
ns/cmu trace objects generate their traces by looking at the packet type
field in the common header. For a MAC packet, the type therefore needs
to be changed from PT_TCP  to PT_MAC in the sending MAC object (I know
this is being done in mac-802_11.cc when a Packet is passed to the
channel). On the receiving MAC side, the packet type should be changed
back to PT_TCP but I don't see how this information can be retained. I
also don't see how / if  this is being done by the MAC object in
mac-802_11.cc

I guess there are ways to avoid it, e.g. packing the original (TCP/IP)
packet into the data field of the Packet transmitted by MAC (and
unpacking at the MAC receiver) or using an additional header to store
the information contained in the original common header,  but I'd really
like to know how the people from CMU did this in mac-802_11

Regards, Sjoerd Janssen.