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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical RE: packet reordering at exchange points
> > To transfer 1Gb/s across 100ms I need to be prepared to buffer at least > > 25MB of data. According to pricewatch, I can pick up a high density 512MB > > Why ? > > I am still waiting (after many years) for anyone to explain to me the issue > of buffering. It appears to be completely unneccesary in a router. > > Everyone seems to answer me with 'bandwidth x delay product' and similar, > but think about IP routeing. The intermediate points are not doing any form > of per-packet ack etc. and so do not need to have large windows of data etc. > > I can understand the need in end-points and networks (like X.25) that do > per-hop clever things... > > Will someone please point me to references that actually demonstrate why an > IP router needs big buffers (as opposed to lots of 'downstream' ports) ? Sure, see the original Van Jacobson-Mike Karels paper "Congestion Avoidance and Control", at http://www-nrg.ee.lbl.gov/papers/congavoid.pdf. Briefly, TCP end systems start pumping packets into the path until they've gotten about RTT*BW worth of packets "in the pipe". Ideally these packets are somewhat evenly spaced out, but in practice in various circumtances they can get clumped together at a bottleneck link. If the bottleneck link router can't handle the burst then some get dumped. -- Jim
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