[En-Nut-Discussion] About Multicast packet receive

[Dusan Ferbas]

You need IGMP only if you are looking for traffic reduction (IGMP 
Snooping). Then you need router(s) and switch(es) that have IGMP 
implemented. Otherwise switch will handle multicasts as broadcasts. This 
floods network.

One of your routers (switches) has to be IGMP quuerier which periodically 
multicasts requests to all hosts. Hosts then respond with reports to which 
multicast group they belongs. Switch then know which groups are on which of 
its ports. When multicast arrives it is forwarded only to those ports hosts 
with same group are on. There are predefined multi-groups - all hosts, all 
routers (used e.g. by RIP2 when running in RIP2 native mode /not 
compatibility with v1 mode/).

IGMP has version 1 and 2 and if they should coexist on a same network more 
complex implementation is necessary on queriers. Also a good solution 
handles cases where new queriers appears on a network or when querier 
shutdowns. If you require simple implementation just equip Nut/OS with a 
new protocol and ability to respond to an IGMP query request with IGMP 
report (several bytes).

>control register (bank 0 offset 4). For a full implementation you will
>also need to implement IGMP and join the multicast groups of interest,
>otherwise my understanding is you will not cross routers with your
>packets.

Dusan