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1.                 From The Free On-line Dictionary of Computing (19 January 2023):
                   
   
   
                   
                     
   Generic Routing Encapsulation
   GRE
      <networking, protocol> (GRE) A protocol which allows an
      arbitrary network protocol A to be transmitted over any other
      arbitrary network protocol B, by encapsulating the packets
      of A within GRE packets, which in turn are contained within
      packets of B.
      Defined in RFC 1701 and RFC 1702 (GRE over IP).
      (1998-07-19)