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6.2 Extensions to the IP Module

6.2. Extensions to the IP Module​

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.