[ns] How to change the packet sent at a node, add new fields to it

[hiba alame]

Hi,
  I want to add an additional field to the packet sent by a node and I 
want to do these changes at the LL layer, so where do I need to apply 
these changes i.e. in which files? 
   
  Any sort of help is useful, this is a very urgent matter. If you can 
refer me to any documentation on this issue I would be more than 
greatful to you.
   
  Thank you all!!!

       
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