To support the sending of multicast IP datagrams, the IP module must be extended to recognize IP host group addresses when routing outgoing datagrams. Most IP implementations include the following logic:
if IP-destination is on the same local network, send datagram locally to IP-destination else send datagram locally to GatewayTo( IP-destination )
To allow multicast transmissions, the routing logic must be changed to:
if IP-destination is on the same local network or IP-destination is a host group, send datagram locally to IP-destination else send datagram locally to GatewayTo( IP-destination )
If the sending host is itself a member of the destination group on the outgoing interface, a copy of the outgoing datagram must be looped-back for local delivery, unless inhibited by the sender. (Level 2 implementations only.)
The IP source address of the outgoing datagram must be one of the individual addresses corresponding to the outgoing interface.
A host group address must never be placed in the source address field or anywhere in a source route or record route option of an outgoing IP datagram.