Multicast is not yet available in FreeRTOS+TCP.
What it would need is a few changes:
Add some code in NetworkInterface.c to tell the NIC which multicast addresses you want to use. You will probably see code that allows the LLMNR multicast address 126.96.36.199 (01-00-5E-00-00-FC).
Note that there are often two ways of filtering MAC-addresses: usually there is an array of e.g. 4 MAC-addresses. And sometimes there you can enter hash values (Multicast hash match).
Secondly: make sure that the library allows packets that have a multicast target address.
This happens in
. Again you will see code that allow LLMNR traffic:
#if( ipconfigUSELLMNR == 1 )
/* Is it the LLMNR multicast address? */
( ulDestinationIPAddress != ipLLMNR
IP_ADDR ) &&
And finally: the address resolution: when replying, make sure that the proper MAC address is found.
Normally, MAC addresses are obtained in either of two ways: address couples are stored upon reception. So when you reply to a UDP message, the ARP table probably contains the IP-address of the peer.
But when you’re the first one to send, an ARP request will be created to lookup the IP-address (and the first UDP packet will get lost). This will not work for Multicast. So that will need a change in the ARP lookup routine. You can read here
about IPv4 multicast address resolution.
After making the above changes, you can use an ordinary UDP socket to communicate.