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