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ATM (was Re: too many routes)
Richard Irving <email@example.com> writes: > Ok. I will bite, although I hate to open my mouth, as my shoe always > seems to bee-line for it.. ;} Hehe. > I don't think so.... how about the ability to mix > voice, MPEG, and IP on the same pipe ? Um, I do this now with IP. Admittedly with parallel circuits (virtual or hard) I could send such traffic down different pipes to partition congestion effects, however to do this right I'd really want to use MPLS/tag switching anyway. When I make the decision to use MPLS/tag switching I also have to consider that there is decent queuing available in modern IP routers (that will effectively become hybrid IP routers and MPLS/tag switches) and that I can neatly partition the traffic without using actual or virtual circuits. > Or, how about that with ABR my delay across the ATM > fabric is reduced when I have more bandwidth open. (POTS > is low on utilization, during this "theoretical moment > in time") A couple milliseconds and a few extra Mbs can > count ;) You want bounded delay on some traffic profiles that approach having hard real time requirements. (Anything that has actual hard real time requirements has no business being on a statistically multiplexed network, no matter what the multiplexing fabric is). This can be implemented in routers now, with or without MPLS/tag switching, although having the latter likely makes configuration and maintenance easier. ABR is another attempt to do statistical multiplexing over a substrate that is not well geared to anything other than TDM. It interacts poorly with any protocol that is developed to run over a statistically-multiplexed network (e.g. TCP) and there are little demons with respect to the way RMs are handled that can lead to nasty cases where you really don't get the bandwidth you ought to. The problem again is that mixing TCP and other statmux-smart protocols with ABR introduces two parallel control loops that have no means of communication other than the interaction of varying traffic load, varying delay, and lost data. This often leads to the correct design of additive increase/multiplicative decrease traffic rate response to available bandwidth leading to a stair-step or vacillation as more bandwidth becomes available to each control loop, and a rather serious backing off at the higher level when available bandwidth is decreased even fairly gently. Delay across any fabric of any decent size is largely determined by the speed of light. Therefore, unless ABR is deliberately inducing queueing delays, there is no way your delay can be decreased when you send lots of traffic unless the ATM people have found a way to accelerate photons given enough pressure in the queues. > Oh, 2 things come to mind, my variability throughout an ATM cloud is > greatly reduced versus a routing cloud, a cell requires WAY less time to > cross a switches backplane, versus a packet through a router. And > seriuosly less time to determine where to send it... Um you need to be going very slowly and have huge packets for the passage through a backplane to have any meaning compared to the centisecon propagation delays observed on long distance paths. I know of no modern router that delays packets for anything approaching a handful of microseconds on fast interfaces except in the presence of outbound queues being congested, where if you're running TCP you really want to induce delay rather anyway, so that the transmitter will slow down. > Ok. So, maybe Cisco's Flow Switching approaches VBR having a bad hair > day. (and tuned for SERIOUS tolerance, CDVT=10,000), but certainly not > traditional routing. The analogy between VBR and flow switching confuses me. Could you explain this a bit? Actually, maybe you could explain the rest of the message too, because I think we have a disconnect in terms of vocabulary. :( Sean.