On 2017-01-10 18:17, Dave Taht wrote: > In the case of wifi I have 3 issues with this line of thought. > > multicast in wifi has generally supposed to be unreliable. This makes > it reliable. reliability comes at a cost - > > multicast is typically set at a fixed low rate today. unicast is > retried at different rates until it succeeds - for every station > listening. If one station is already at the lowest rate, the total > cost of the transmit increases, rather than decreases. > > unicast gets block acks until it succeeds. Again, more delay. > > I think there is something like 31 soft-retries in the ath9k driver.... If I remember correctly, hardware retries are counted here as well. > what happens to diffserv markings here? for unicast CS1 goes into the > BE queue, CS6, the VO queue. Do we go from one flat queue for all of > multicast to punching it through one of the hardware queues based on > the diffserv mark now with this patch? > > I would like it if there was a way to preserve the unreliability > (which multiple mesh protocols depend on), send stuff with QoSNoack, > etc - or dynamically choose (based on the rates of the stations) > between conventional multicast and unicast. > > Or - better, IMHO, keep sending multicast as is but pick the best of > the rates available to all the listening stations for it. The advantage of the multicast-to-unicast conversion goes beyond simply selecting a better rate - aggregation matters a lot as well, and that is simply incompatible with normal multicast. Some multicast streams use lots of small-ish packets, the airtime impact of those is vastly reduced, even if the transmission has to be duplicated for a few stations. - Felix
