"Is there CC?" is the wrong question...
2014-03-07 08:20:50 GMT
Let me define two terms: traffic or applications are "elastic" if increasing loss causes decreasing presented load and "regenerative" if increasing loss causes increased load (from the sender to the loss point). Clearly regenerative traffic is a bad thing because it can cause "latchup" and congestion collapse like behaviors. You would think that regenerative protocols would be rare, but they are not.
The problem is that it is trivial to construct applications that are regenerative. For example, if you use TCP to deliver low rate CBR (constant bit rate) traffic, then the TCP retransmissions are regenerative until the loss rate is sufficient to cause the system to fail. This sounds like a corner case, but it is not. Suppose you are serving 250 kb/s flows to forty thousand individual near clients (20 mS RTT). These flows will do a reasonable job of meeting their target rates at up to several percent loss (I'm guessing 3%). If I vary the load by increasing the number of flows into a fixed 10 Gb/s shared bottleneck, the total loss rate will suddenly spiral out of control once the aggregate target goodput exceeds the link capacity. The loss rate will rise until some or all of flows fail to meet the required performance.
As long as the tunnel does not itself do something regenerative (for example by repairing losses to protect its payload) the tunnel does not change the extent to which its traffic is regenerative.
The best way to predict the future is to create it. - Alan Kay
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