OnDataGenerationEvent(Data d) - 
    queue.add(d, {state => PENDING_TRANSFER} )
    process(queue)

OnDataReceivedEvent(Data reply) - 
   lock(queue) {
      d = dequeue()
      if reply.status = OK then
          # no Op
      else # status is NAK
          # add back to front of queue
          queue.add_front(d,{state => PENDING_TRANSFER})
      endif
          
   }
   process(queue)

sub process(queue)
   lock(queue ){
       d = dequeue(queue)
       if d.state = PENDING_TRANSFER then 
            transfer(d)
            
            queue.add(d, {state => PENDING_REPLY } )
       endif
   }