North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical RE: Fast TCP?
> Glad this came up as I have been reading this paper - > > Does Figure 1 in > > http://netlab.caltech.edu/pub/papers/fast-030401.pdf > > seem reasonable ? Will 100 RED TCP flows really only fill 90% of a 155 > Mbps pipe but 87% of a 2.4 Gbps connection > and 75% of a 4.8 Gbps connection ? This seems strangely non-linear to > me. > > A more fundamental question is, is this really useful except in the > case of very high bandwidth single flows (such as > e-VLBI or particle physics or uncompressed HDTV). > After all, isn't the current standard practice not to come close to > fully utilizing backbone bandwidth ? I think the idea is that (similar to the 1Gb/s single-stream test a few months ago) that the concerns of academics are not exactly inline with those of network operators. The idea with a non-stablized TCP Vegas on a very fast pipe [with a small number of streams] is that as delays get large (relative to the size of the network connection) you have a very long/impossible window to grow into to fully utilize the full bandwidth. With TCP Reno (which it seems they have the biggest fault with) a single packet drop causes far more severe problems. Since RED causes packet drops, high speed streams that get RED'd are in an immense world of pain. Further, since a typically delayed ack window is only 100ms, this is a lot of data that isn't transmitted over the network or retransmitted and resequenced. If you have many streams (where each one represents a small portion of your network link, whether backbone or CPE) you can easily fill your pipe, this is common experience. If you aren't using RED [or similar] to manage congestion, you are good with a smaller number of streams. When you have a single (or small number of streams) you need larger windows, more tolerance for latency, and a large willingness to buffer data rather than drop it. I think this is all well understood at a common-sense level. I think the academics (practice, not people) are the ones that will figure out some idealized set of variables for a slightly modified equation from the ones we all use wrt to bits-in-flight calculations. I think they mention in the paper that they will start by stablizing TCP Vegas for a high latency, high speed link. I could be wrong (about my understanding or what is considered common-sense). I am not sure why sending a single large/high speed stream today (>1Gb/s) is such an improvement over sending multiple today-streams of data, but I guess that is the difference between a get-it-done-right and a get-it-done-now mentality. Deepak Jain AiNET
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