5.2.1.3 Multicast

If the destination is an IP multicast, the following steps are taken.

Note that the main differences between the forwarding of IP unicasts and the forwarding of IP multicasts are

• IP multicasts are usually forwarded based on both the datagram's source and destination IP addresses,

• IP multicast uses an expanding ring search,

• IP multicasts are forwarded as Link Level multicasts, and

• ICMP errors are never sent in response to IP multicast datagrams.

Note that the forwarding of IP multicasts is still somewhat experimental. As a result, the algorithm presented below is not mandatory, and is provided as an example only.

   (5a) Based on the IP source and destination addresses found in the
        datagram header, the router determines whether the datagram has
        been received on the proper interface for forwarding.  If not,
        the datagram is dropped silently.  The method for determining
        the proper receiving interface depends on the multicast routing
        algorithm(s) in use.  In one of the simplest algorithms, reverse
        path forwarding (RPF), the proper interface is the one that
        would be used to forward unicasts back to the datagram source.

   (6a) Based on the IP source and destination addresses found in the
        datagram header, the router determines the datagram's outgoing
        interfaces.  To implement IP multicast's expanding ring search
        (see [INTERNET:4]) a minimum TTL value is specified for each
        outgoing interface.  A copy of the multicast datagram is
        forwarded out each outgoing interface whose minimum TTL value is
        less than or equal to the TTL value in the datagram header, by
        separately applying the remaining steps on each such interface.

   (7a) The router decrements the packet's TTL by one.

   (8a) The forwarder performs any IP option processing that could not
        be completed in step (3).

   (9a) The forwarder performs any necessary IP fragmentation, as
        described in Section [4.2.2.7].

   (10a) The forwarder determines the Link Layer address to use in the
        Link Level encapsulation.  The mechanisms for doing this are
        Link Layer-dependent.  On LANs a Link Level multicast or
        broadcast is selected, as an algorithmic translation of the
        datagrams' IP multicast address.  See the various IP-over-xxx
        specifications for more details.

   (11a) The forwarder encapsulates the packet (or each of the fragments
        thereof) in an appropriate Link Layer frame and queues it for
        output on the appropriate interface.