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 provide level 1 multicasting, a host IP implementation must support the transmission of multicast IP datagrams. To provide level 2 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.