On Wed, Feb 25, 2015 at 12:36 PM, Curtis Villamizar <[email protected]> wrote: > > In message <[email protected]> > Juliusz Chroboczek writes: >> >> > Thought: In general, my feeling is that L2 link status is widely relied >> > upon in commercial product/dpeloyments. If homenet feels it can not rely >> > on it due to the non-commercial nature of its development platforms, >> > thats an interesting aspect, especially because it could impact an IETF >> > standard that we'd like to see commercially implemented and then the >> > constraints could be different... >> >> Are you referring to the routing protocol comparison, or to something else? >> >> I have the impression that Babel and IS-IS behave essentially the same >> wrt. L2 status -- they both converge fast enough after link status has >> been established, and they essentially provide the same facilities for >> application-layer link sensing (IMHO Babel's Hello/IHU subprotocol is >> slightly more flexible, but that's probably not a big deal). >> >> As to wireless links -- as far as I'm aware, making efficient use of >> wireless L2 information in a routing protocol is an open research problem. > > Other than signal strength and collision rate, what L2 information is > available? Per MAC information would be nice for the AP side or any > node in mesh or adhoc mode but that isn't collected anywhere AFAIK. > >> -- Juliusz >> >> _______________________________________________ >> homenet mailing list >> [email protected] >> https://www.ietf.org/mailman/listinfo/homenet > > > ISPs mostly use Ethernet as point-to-point (PTP) links. Including > using 100GbE as PTP. No switches. L2 Link down is one fast indicator > of link down. But for Ethernet over transport gear (ie: OTN) BFD is > almost always used. For short distances L2 over extended reach optics > can be used, including colored optics and WDM. This is also PTP and > in this case BFD should not be needed. To the extent that routers use > SONET or OTN interfaces, these have fast L2 link down indication and > are integrated with L3 link down detection. In all of these detection > is on the order of 10 msec (geographic distance dependent), failover > using FRR is under 50 msec, and IGP convergence is well under a second > (typical 100-200 msec today AFAIK). L3 hellos are way too slow. > > BFD is not heavy weight. L3 hellos (OSPF, ISIS) can be set down to 1s > with detection in 3s (too slow). > > BFD, Hellos, or any form of probe traffic over wireless has the speed > of detection vs overhead issue. At nominal 10 Mb/s (low end today for > wireless) a small probe would take about 0.1 msec (for example, 125 > bytes including all overhead is about 1000 bits). Not bad if running > 100 probes/sec (30 msec detection) unless there are a very large > number of stations using the AP and doing the same thing. In that
You are thinking about it wrong. Wireless-g is only capable of roughly 1300 TXOPs total per second. Bandwidth has NOTHING to do with it. I don't have the figure in my head for n or ac, but it is not a lot, and in the presence of g, falls back to the above figure. losing 10% of the bandwidth - per station - to run BFD is not an option. > case 10 probes per second might be better. A very high collision rate Still too much. Next question? > (typically not due to probes, but to real traffic) might result in a > link down indication. If that is the case, then moving to another AP > would be a good thing. > > Flapping needs to be avoided if an alternate is available (ie: with > 20% loss, in .2*.2*.2 = .008 = 0.8% of intervale a down indication > would occur). If any packet received would bring it back up, then at > 100 probes / sec, a change in IGP link state could occur about once a > second on average. Remembering a link down and holding a down state > for a (longish) while would be a good thing. If there is no alternate > route, not probing at all and/or holding an up state would be good. > OTOH- 20% loss borders on completely unusable. > > Curtis > > _______________________________________________ > homenet mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/homenet -- Dave Täht Let's make wifi fast, less jittery and reliable again! https://plus.google.com/u/0/107942175615993706558/posts/TVX3o84jjmb _______________________________________________ homenet mailing list [email protected] https://www.ietf.org/mailman/listinfo/homenet
