Nichols, Jacobson: Controlling Queue Delay
Posted May 20, 2012 22:31 UTC (Sun) by farnz
In reply to: Nichols, Jacobson: Controlling Queue Delay
Parent article: Nichols, Jacobson: Controlling Queue Delay
One of the challenges is determining the "correct" spacing; for Ethernet, it's trivial, as to send at a given speed, you just maintain a steady inter-packet spacing. For other link layers, however, it gets more complex.
Assume that your sender is on 10G Ethernet (a nice fast server for a web company, for example). Put the bottleneck link on VDSL2 (often used as the copper end of FTTC connectivity). VDSL2 has a fixed 4 kilobaud signalling rate, so 80MBit/s down (the current fastest in the UK) is provided as 20,000 bits per symbol. A standard ISP MTU in the UK is 1,500 bytes, or 9,000 bits. You therefore need to send a burst of 3 packets to guarantee enough bits buffered to sustain the full 80MBit/s down, then spacing, then another burst.
And note that you can't rely on link speeds to identify what the connection technology in the bottleneck is - if I run a VDSL2 link at 25kbit/symbol, I get 100MBit/s, same as 100BASE-TX; but 100BASE-TX sends at under 1 bit/symbol, so can saturate with a single packet buffered, while VDSL2 at 25kbit/symbol and an Ethernet-compatible 1,500 byte MTU needs 3 packets buffered to saturate.
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