Consider a connection to my ISP as being an empty water-pipe, and I only want to measure the flow from the waterworks to me. In this case the waterworks is Rogers in Toronto, and the numbers come from me measuring the link with the Waveform bufferbloat tool.

The ISP promises me 1 Gbit/s of water.  OK, there is no such thing, but you get the idea (;-))

Let's consider the no-latency case. 

• The ISP turns on the tap, and it takes half an RTT to get to me, one way.  Let that be 1 millisecond, 0.001s of delay.
• Once the delay is over, I get (1.0s - 0.001s) * 1 Gbit/s = 0.999 Gb/s. The 0.999 seconds is transfer time, and that transfer is at full speed of the pipe, so it adds up to 0.999 Gb/s
• That's pretty good.

Now let's consider the best possible case where there is latency, but only one delay of 0.456s.  That basically means that only one transfer happens in the second, so there is only once change for latency to hurt me.

• the one-way delay is still 0.001s, but there is also 0.231s of latency, for a delay of 0.232s
• (1.0s - (0.001s + 0.231s) ) * 1 Gbit/s =
• 1.0s - 0.232s = 0.549s  * 1 Gbit/s = 0.768 GB/s
• Cut by a quarter, by one packet's delay

What about the worst case?

• It's not worst, but a pretty common case is a busy link with 1500-byte packets
• One packet is 12,000 bits
• In one second we can transfer 1,000,000,000 bits / 12,000bits/packet = 83,333.3 packets
• Maybe that many delays, too?
• Fortunately, no
  

I personally observed 456.2 Mbit/s, about 54% of a gigabit at home, so it's more like the latency cut my bandwidth in half

--dave

Dave Collier-Brown <dave.collier-Brown@indexexchange.com> wrote:
    >> Dave Collier-Brown via Bloat<bloat@lists.bufferbloat.net> wrote: >
    >> They he said "bandlength"
    >>
    >> > That sounded like an odd name, but the idea was cool:
    >>
    >> > If I have a bandwidth of 1 Mbit/S, but it takes 2 seconds to deliver
    >> 1 > Mbit, do I have a bandlength of only 1/2 Mbit/S?
    >>
    >> Is that because there is 2seconds of delay?

    > Well, 2 seconds elapsed time, 1 of which is delay.

Ah, would that include the delay to ask for the data?
(A DNS request, or an HTTP GET)

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