Multicast allows to transmit data to a group of receivers, unlike unicast which transmits to just one receiver and broadcast which transmits to everyone. Multicast is used for applications like IPTV, internet radio or group communication and more. Also IPv6 relies on multicast for its basic services.
Without optimization, switches (including batman-adv) typically implement multicast as broadcast. If everyone gets the packet, all group members get it too. Consequently, all layer 2 switches are multicast capable, but that comes with a cost: Even if there are only a few multicast listeners interested, the whole mesh network is unnecessarily bothered with these multicast packets. Considering that broadcast transmissions are particularly expensive operations on WiFi in general and wireless mesh networks (in terms of bandwidth) reducing the amount of multicast traffic by only forwarding traffic to its multicast group members will greatly improve performance for these services.
The first step in optimizing multicast is to find out who is in which multicast group. Fortunately, IP multicast provides mechanisms to learn about the presence and number of these so called multicast listeners. This is interesting for layer 3 IP multicast routers, but also professional layer 2 switches make use of this to reduce multicast overhead, advertising these products as "multicast aware" or "IGMP/MLD snooping switches".
In the field observations revealed that a typical multicast session is actually only having one, two or even zero hosts interested in it. Therefore, optimizing these cases is the first step, with more sophisticated optimizations to be implemented later.
The current multicast optimizations in batman-adv provide a basic infrastructure for multicast awareness. So far, this basic optimization reduces overhead in the following two ways:
Discards IPv4/IPv6 packets with no multicast listener¶
batman-adv detects whether there are any potential IPv4/IPv6 multicast listeners interested in traffic to the given multicast destination address. If not then the packet is never forwarded into the mesh network.
Forwards IPv4/IPv6 packets with a single multicast listener via unicast¶
If a single multicast listener was detected then the packet is forwarded directly to the corresponding batman-adv node.
In the following cases multicast packets are still distributed via classic flooding:
- If there is more than one listener for a specific multicast group.
- If the multicast packets have a scope greater than link-local.
(These things are work in progress and will be supported at a later time.)
Additionally, in bridged scenarios multicast packets are still flooded in the following cases:
- If there is no IGMP/MLD querier.
- If the IPv4 multicast destination is between 188.8.131.52 and 184.108.40.206.
- If the IPv6 multicast destination is ff02::1.
How to activate the multicast optimizations?¶
By default multicast optimizations are activated. You can force the old behaviour, classic flooding of all multicast frames, on a certain node via the following command, executed as root (here: bat0):
echo 0 > /sys/class/net/bat0/mesh/multicast_mode
Note: This only affects multicast traffic originating from this specific node.