RE: RFC3819 > The assumption is that L2 will do a reasonably good and efficient job of > multicast/broadcast - certainly better than L3 or other layers would.
What I think Juliuz is trying to point out is that the RFC doesn't talk about how good the performance of L2 multicast needs to be - for instance in terms of BER/packet loss. It does talk about the impact of BER on TCP throughput, but not on multicast. How good an error rate do the IPv6 related multicasts need? Adrian summarized the causes of packet loss: "1. The basic "packet error rate" (before Ack) designed to is 10% in the case of unicast. This is a very broad statement and almost so simplistic as to be misleading. Errors are composed of 1) collisions, 2) interference, 3) rate/MCS adaptation errors, 4) sudden changes in the radio environment such as shadowing. The residual error for unicast is less because of the retry mechanism. 2. Multicast is generally sent at a low "basic" (one intended to be receivable by all STAs in the AP's network) rate. This improves reliability in some of the error causes cited above, but not all. ...." If it takes two missed RAs to decide that you aren't connected to the router any more, that has about a 1% chance of happening with a 10% packet error rate (though the multicast error rate is probably somewhat better since it is sent at the lower rate). This assumes that the error probability for the two RAs is independent; they occur far enough apart in time that this doesn't seem unreasonable. Why does it take two lost RAs rather than one? Presumably because the protocol assumes that sometimes an RA is lost, just not with as high a probability as 10%. 1% seems high - how good does it need to be for acceptable behavior? One could improve the behavior by adjusting parameters so that more than 2 had to be lost. Of course there is a tension between bandwidth for multicast heartbeats, wanting to detect reasonably quickly that the device is no longer on the subnet (e.g. because it is mobile), and not losing the context unnecessarily. Without guidance on how good the multicast packet loss rate should be, it is difficult to define the best solution . _______________________________________________ homenet mailing list homenet@ietf.org https://www.ietf.org/mailman/listinfo/homenet
