HI Michael,
On Aug 1, 2014, at 06:21 , Michael Richardson <m...@sandelman.ca> wrote: > > On symmetric links, particularly PPP ones, one can use the LCP layer to do > echo requests to the first layer-3 device. This can be used to measure RTT > and through some math, the bandwidth. Sure. > > On assymetric links, my instinct is that if you can measure the downlink > speed through another mechanism, that one might be able to subtract, but I > can't think exactly how right now. > I'm thinking that one can observe the downlink speed by observing packet > arrival times/sizes for awhile --- the calculation might be too low if the > sender is congested otherwise, but the average should go up slowly. If you go this rout, I would rather look at the minimum delay between incoming packets as a function of the size of the second packet. > > At first, this means that subtracting the downlink bandwidth from the uplink > bandwidth will, I think, result in too high an uplink speed, which will > result in rate limiting to a too high value, which is bad. But given all the uncertainties right now finding the proper shaping bandwidths is an iterative process anyway, but one that is best started with a decent initial guess. My thinking is that with binary search I would want to definitely see decent latency under load after the first reduction... > > > But, if there something wrong with my notion? > > My other notion is that the LCP packets could be time stamped by the PPP(oE) > gateway, and this would solve the asymmetry. If both devices are time synchronized to a close enough delta that would be great. Initial testing with icmp timestamp request makes me doubt the quality of synchronization (at least right now). > This would take an IETF action > to make standard and a decade to get deployed, but it might be a clearly > measureable marketing win for ISPs. But if the “grown ups” can be made to act wouldn’t we rather see nice end-user query-able SNMP information about the current up and downlink rates (and in what protocol level, e.g. 2400Mbps down, 1103Kbps up ATM carrier) (For all I know the DSLAMs/BRASes might already support this) Best Regards Sebastian > > -- > Michael Richardson > -on the road- > > > > _______________________________________________ > Cerowrt-devel mailing list > Cerowrt-devel@lists.bufferbloat.net > https://lists.bufferbloat.net/listinfo/cerowrt-devel _______________________________________________ Cerowrt-devel mailing list Cerowrt-devel@lists.bufferbloat.net https://lists.bufferbloat.net/listinfo/cerowrt-devel