|                                                          |
         |              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)                                    |
         |                                                          |

Sent using the same 'Send IP' operation for unicast IP datagrams - an upper level protocol modules merely specifies an IP host group address. Certain extensions may be necessary and or desireable;

1) The service interface should procide a way for aupper-layer protocol to specify the IP time-to-live. It should default to 1 such tha an explicit choice is required.

2) For hosts that may be attatched to one or more networks, the service interface should provide a way for the upper-layer protocol to identify which network interface is to be used for multicast transmission. Only multicast routers should be responsible for forwarding to other networks.

3) For level 2 only, when a host is itself a member of a groupto which a datagram is being sent, the service interface should provide a way for upper-layer protocol to inhibit local delivery of the datagram, ie prevent loopback.

Incoming multicast IP datagrams are received by upper-layer protocol modules using the same 'Receive IP' operation as with unicast datagrams. Selection of destination upper-layer protocals is based on the protocol field in the IP header. However, before any datagrams destined for a particular group is received, an upper-lay protocol must ask the IP module to join that group. Hence two new operations must be included in the IP serivice interface - JoinHostGroup and LeaveHostGroup. Both operations should return immediately (non-blocking) indicating success or failure.

It is permissible to joing the same group on more than one interface (should the host have more than one interface). It is also permissible for more than one upper-layer protocol to request membership in the same group.

Must be extended to recognise IP host group addresses when routing outgoing datagrams. This may need a change in the logic of the IP implementation to include host group recognition and forwarding.

The IP source address must be one of the individual addresses corresponding to the outgoing interface.

The host group address field should NOT be published in the source address field, or anywhere in a source route or recod route option.

A list of host group memberships associated with each network interface should be maintained. Processing of incoming datagrams is the same as that of unicast tranmissions.

Should datagrams be received by the hosts which do not belong to that group, the datagram should be discarded without log nor error. This is also true if a datagram is received on the wrong interface on the correct host. Similarly, an incoming datagram with an IP host group address in its source address field is also discarded. ICMP error messages are never generated in response to a datagram destined to an IP host.

List of group host memberships is updated in response to JoinHostGroup and LeaveHostGroup. Each membership should have an associated reference count or similar to handle multile requests to join and leave the same group. Upon the first and last request to Join and Leave a group respectively, the local network module for the interface should be notified (to update its multicast reception filter)

The IP Module must also implement the IGMP protocol. IGMP is used to is used to keep neighbouring multicast routers informed of the host group memberships present on a particular network. In order to support IGMP, every level 2 host must join the 'all-hosts' group (x.0.0.1) on each network interface at initialisation and remain a member whilst the host is active. Datagrams addressed to the all-hosts network are recognised as a special case and never forwarded beyond a single network, regardless of their ttl.

Incoming multicast packets are delievered to the IP module using the same 'Receive Local' operation as local network unicast packets. To allow the IP module to tell the local network module which multicast packets to accept, the local network service interface is extended to provide two new operations: JoinLocalGroup and LeaveLocalGroup, both with parameter group-address (IP host group address).

The local network module is expected to map the IP host group addresses to local network addresses as required to update its multicast reception filter.

The local network module must not deliver up any mlticast packets that were transmitted from that module - loopback of multicasts is handled at the IP layer or higher.

Networks that directly support multicasting need only to be mapped to the host group. For networks that support broadcast, but not multicast (eg Experimental Ethernet), all IP host group addresses may be mapped to a single local broadcast address.

For point to point links (such as those joining to hosts or a host and a multicast router), multicasts should be transmitted exactly like unicasts.

For store and forward networks (ARPANET or public X.25) all IP host group addresses may be mapped to the well-known address of an IP multicast router - this router would take responsibility for completing multicast delivery.