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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: Mark Allman: Internet measurement: what next?
Eddy, E.B. Dreger wrote: You mean something like RouteScience [supposedly] does or ECN influencing next hop? Guilty as charged, of course :-) Yes, I know, that would get ugly for transit ASes.Well, the measurement itself isn't ugly - it's just a question of what you choose to do with the information. Some more conservative folks just take the data over time, and use it for peer or transit elimination, for example. Degenerate seems a little strong, even in its mathematical sense - rather like an auto maker calling car drivers the "degenerate" case of mechanics. But on this list, I suppose I'll let it pass :-)For the degenerate case of a leaf AS, though, things become simpler... I'll agree that the constraints on active use of measurements for route change are lighter when you have no BGP downstreams. But it's still possible to use good end to end measurements, with very strong damping, to make changes in a transit routing context. Others in this thread have been talking about bandwidth measurement along the variety of paths you use across other peoples' networks. To me, that's a good answer to one of Mark's original questions: What do we need to be able to measure that we cannot measure very well today? That is, I consider end to end bandwidth one of those quantities we'd love to measure widely and often, but practically, we cannot. Available techniques are ugly and unscalable, but more disconcertingly, estimates of bandwidth appear to have a shelf-life comparable to certain sub-atomic particles. That's not to say it's a lost cause, of course. Many end to end paths are constrained primarily by first hop (which you can usually measure directly with SNMP) and last hop (which is roughly constant, and next to impossible to influence). For the remainder of the path in between, it's important to characterize it, but accurately assessing bandwidth is hard. Measuring probability of packet loss over time is a very practical proxy. It won't give you all that precise a projection of your goodput if you throw, say, a multi-gigabyte ftp down the pipe, but for short or light transactions, it's not bad at all. After all, once you've accounted for first and last hop, your traffic is probably a pretty minor fraction of total load on each link along the chain. Mike Lloyd CTO, RouteScience
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