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 the sending host has inhibited such loopback. (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.