In the Mikrotik Manual - Manual:Multicast detailed example - There is a section about “Multicast and Wireless” pasted below.
Pertinent statements are highlighted
Multicast (and broadcast) over wireless works over wireless depending on who is transmitting multicast packet:
If AP transmits multicast packet, packet is transmitted over air with multicast receiver address (and yes - only single copy of packet is transmitted no matter how many clients are registered). As there is no single recipient of packet, it does not get acked - therefore delivery is not reliable (no retransmissions in case somebody does not receive packet). Due to this unreliable delivery, lowest basic rate is used to ensure as reliable delivery as possible. So even if you can send unicast stream between AP and Station using 54Mpbs air rate, multicasts are sent only using 6Mbps air rate (assuming that 6Mbps is lowest basic rate).
If Station transmits multicast packet, it is transmitted over air with unicast AP receiver address (as Stations always transmit all packets to AP, and always using unicast receiver address). Due to this, delivery of multicast packets from Station to AP is reliable and subject to normal rate selection process - max throughput can be used. What complicates the matter - when AP receives multicast packet from Station, it processes it locally, but it also must send it back over the air so that the rest of stations in wireless network see it (think of AP as switch in ethernet, except that is does not have to send multiple copies of packet for all clients to receive). This causes AP to execute multicast transmission procedure as described above, therefore every multicast packet gets transmitted over air twice - first time by Station (using its transmit rate) and second time - by AP using lowest basic rate. This behaviour can be altered by disabling “forwarding” option for particular Station on AP (or for all clients using “default-forwarding” option) - disabling forwarding causes AP not to forward traffic between clients and also disables “sending back” multicasts and broadcasts received from client, because that can be considered special case of forwarding traffic between clients.
If multicast traffic is forwarded across WDS link, it is transmitted over air with unicast receiver address (remote end of WDS link) and therefore is reliable and subject to normal rate selection.
From above we can draw some conclusions, how to increase wireless network throughput:
Ensure that unicast receiver address is used when transmitting multicast
Increase lowest basic rate if feasible.
Some ideas on how to apply above conclusions:
In case multicast traffic is to be delivered over point-to-point link (e.g. some backbone link), you should ensure by some means that multicasts are delivered over wireless using unicast receiver address. This can be achieved by using tunnels (as previously discussed) or by using WDS links (either AP-to-AP WDS or AP-to-StationWDS WDS link). If WDS links are used, care must be taken not to inject multicasts in regular wireless interface (to avoid regular AP multicast transmission procedure).
In case WDS can not be used, network should be planned so that Station is transmitting multicasts and AP should have forwarding disabled.
In case multicasts are to be delivered to multiple destinations in wireless network, it is best to organize network such that AP is transmitting multicasts (because AP will transmit just one copy) and increase lowest basic rate, if throughput is not enough.
Questions:
- What are the settings in Winbox or at the command line to disable forwarding to a specific client - OR how do you disable it for all clients using the default option?
- How does a user “take care” not to inject multicasts in regular wireless interface? Is disabling forwarding all that is needed to avoid regular AP multicast procedure?
I have been reading over the manual trying to figure out where I would disable forwarding to a specific client or where to set the default forwarding option!
Any help would be greatly appreciated!