| Hold on a minute > possibly (my chronology is fuzzy) was approximately as good as the mathematics would have reasonably allowed Just because Van Jacobsen's papers spew forth great volumes of mathematics, doesn't mean there is any robust mathematics behind TCP. Read to the end of his paper "Congestion Avoidance and Control". Read past all of the impressive plots graphs and equations. Read to the conclusions. "The 1-packet increase has less justification than the 0.5 decrease. In fact, it’s almost certainly too large." This statement shows how little formal consideration went into the entire algorithm. The 1-packet increase is not simply too big or too small, it just doesn't make any sense. For starters, how big is the packet? Oh, it isn't defined anywhere. Even if we just go with the de facto internet packet size of 1542 bytes (you know, the old limit for 10Mbit Ethernet)... Could that one packet increase per roundtrip make equal sense for a 10Gbit path to India with a 400ms round trip time, as it does for a 56Kb link between Berkeley and MIT (his test case)? Of course it doesn't make sense. And it gives lie to any notion that there is a formal underpinning for TCP. They tweaked it until it worked, and then put on a nice mathematical show to feel better about it. Quoth Van Jacobsen: "We have simulated the above algorithm and it appears to perform well". Oh now I feel better. Second point, which Braham is covering: TCP makes the assumption that router queue lengths are reasonable. TCP says, fill up the router queues until they drop packets. But router queues have been getting longer and longer as memory gets cheaper. These queues can create additional seconds of delay to layer on top of the 10ms-400ms speed of light delays we see on the internet itself. EDIT: In that 10Gb to India example, it takes TCP literally DAYS to fill up the pipe because of that "1 packet per roundtrip" window increase. Days, by the way, of no incidental packet loss, because it all gets reset on a loss. EDIT: I spent 5 years of my life working on the fact that latency was never really factored into the design of most network protocols. |