5. MODEL OF A HOST IP IMPLEMENTATION

   The multicast extensions to a host IP implementation are specified in
   terms of the layered model illustrated below.  In this model, ICMP
   and (for level 2 hosts) IGMP are considered to be implemented within
   the IP module, and the mapping of IP addresses to local network
   addresses is considered to be the responsibility of local network
   modules.  This model is for expository purposes only, and should not
   be construed as constraining an actual implementation.

         |                                                          |
         |              Upper-Layer Protocol Modules                |
         |__________________________________________________________|

      --------------------- IP Service Interface -----------------------
          __________________________________________________________
         |                            |              |              |
         |                            |     ICMP     |     IGMP     |
         |             IP             |______________|______________|
         |           Module                                         |
         |                                                          |
         |__________________________________________________________|

      ---------------- Local Network Service Interface -----------------
          __________________________________________________________
         |                            |                             |
         |           Local            | IP-to-local address mapping |
         |          Network           |         (e.g., ARP)         |
         |          Modules           |_____________________________|
         |      (e.g., Ethernet)                                    |
         |                                                          |

   To support level 1 multicasting, a host IP implementation must
   support the transmission of multicast IP datagrams.  To support level
   2 IP multicasting, a host must also support the reception of
   multicast IP datagrams.  Each of these two new services is described
   in a separate section, below.  For each service, extensions are
   specified for the IP service interface, the IP module, the local
   network service interface, and an Ethernet local network module.
   Extensions to local network modules other than Ethernet are mentioned
   briefly, but are not specified in detail.