All things related to multicast

[debbie fligor <>]

Of course, as soon as I post, I get more complete answers, so here's  
what Meru has to say:


These questions are from our I2/Joint Techs conferences that we  
participate in and are driven by applications like iTunes, IPTV, etc.   
This information will be shared with our peers and will help us better  
understand IP multicast over WiFi.

- How is multicast sent to AP from the controller?
[Sivaram] Tunneled unicast, if the AP has joined the multicast group  
(IGMP snooping).
- Is multicast flooded to all APs in the SSID or just to AP with  
interest?
[Sivaram] Only APs of interest (IGMP snooping).
- Is IGMP snooping implemented?  What version(s)?
[Sivaram] Yes, IGMP v2 and v3.
- What about IPv6?  IPv6 requires multicast.
[Sivaram] We support IPv6 passthrough, for both unicast and multicast.
- Is MLD snooping implemented?
[Sivaram] No
- Can multicast base rate be set higher than the broadcast rate?  What  
are the limitations?
[Sivaram] Yes. This is a function of the current data rate of the  
associated devices in the multicast group.
- How does multicast work w/WPA2 security?  Broadcast group key?  
Multiple unicast?
[Sivaram]  In the shared BSSID mode, with broadcast group key at min  
data rate among all clients of interest. In per-station BSSID mode,  
individual transmissions for each client at the data rate of the client.
- How does this work with 802.11n multipath?
[Sivaram] Very well, since virtual cell ensures that at any time the  
client is connected to the AP with the highest data rate, and  
multicast is a function of the client data rate, not the minimum base  
rate.
- Is multiple unicast better due to retransmissions at 802.11 layer?
[Sivaram] We support both modes - with per-station BSSID option, you  
get individual transmissions for each device; with shared BSSID  
option, you get group transmission.



It is important to understand that Meru has some unique solutions for  
multicast and broadcast.

1. We virtually eliminate all control/management broadcast over the  
air. For example, we do proxy ARP in both directions, convert  
broadcast to unicast DHCP messages over the air, etc.
2. We transmit multicast and broadcast at a data rate that is a  
function of the minimum of the current data rates for the associated  
devices, not at the min base rate.
3. We minimize probe response and other broadcast traffic over the air  
by only transmitting it from the assigned AP (enabled because of our  
virtual cell architecture).
4. APs that do not have clients assigned to them minimize beacon  
traffic.
5. At the application layer, we only allow configured broadcast ports  
to go through.
6. With IGMP snooping, we minimize the propagation of multicast  
traffic over the backend infrastructure.
7. With per-station BSSID option, we can convert multicast into  
individual unicasts at the current data rate of the client, which  
works very well for sparse mode multicast join, and for high data rate  
clients.


- Sivaram



-----
-debbie
Debbie Fligor, n9dn       Network Engineer, CITES, Univ. of Il
email: 

          <http://www.uiuc.edu/ph/www/fligor>
                   "My turn."  -River Tam