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19. KEY WORDS (Continue on reverse aide If necessary end Identify by block number) Computer networks, layered protocols, soft layering. 20. ABSTRACT (Continue on reverse side If necessary end identify by block number) This report is about the efficiency of protocol layering. It examines the technique of protocol layering in an abstract way " and finds two major sources of inefficiency in protocol imple-mentations which are caused by the imposition on them of a lay-ered structure. The conventional approach to making layered protocol implementations run efficiently-- for avoiding the sources of inefficiency discussed herein-- are all independen