Hi Dave,
> On Apr 10, 2023, at 01:04, David Lang via Bloat <bloat@lists.bufferbloat.net>
> wrote:
>
> TCP ramps up it's speed fairly slowly, and backs off fairly drastically when
> it is told (via ECN or a dropped packet) that it has hit the limit.
[SM] That is the prevailing narrative, yes. However during slow start
most/many TCPs double the congestion window every RTT and since it takes up to
a full RTT for a drop or CE mark to be fed back to the sender, TCPs can end up
with a congestion window that is up to twice as large as sustainable, so the
"classic" halving of the congestion window when dropping out of slow-start
seems not like a bad idea... after that the congestion window increase rate
will be somewhat slower...
> As a result, a single TCP session is not going to fully utilize a connection.
[SM] In my experience even single TCP flows can get close to saturation
(but I typically do such tests over short paths...)
> There are people who do large, high speed transfers over long distances on a
> regular basis (think movie studios sending uncompressed movie footage around
> the world for processing).
[SM] Yes, that is not me ;)
> To fully utilize their bandwidth, they use protocols that involve lots of
> connections operating in parallel
[SM] Or stick to TCP but run a few connections in parallel, if these
are not synchronized the aggregate ends up saturating a path pretty well
(within reason, over long RTT paths the control loop simply is not as "tight"
as over short RTT paths...)
Regards
Sebastian
>
> David Lang
>
> On Sun, 9 Apr 2023, Dave Collier-Brown via Bloat wrote:
>
>> Date: Sun, 9 Apr 2023 16:06:54 -0400
>> From: Dave Collier-Brown via Bloat <bloat@lists.bufferbloat.net>
>> Reply-To: Dave Collier-Brown <dave.collier-br...@indexexchange.com>
>> To: bloat@lists.bufferbloat.net
>> Subject: Re: [Bloat] Hey, all, what about bandlength?
>> 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
>>
>>
>> On 4/8/23 22:32, Michael Richardson via Bloat wrote:
>>
>>
>> Dave Collier-Brown
>> <dave.collier-br...@indexexchange.com><mailto:dave.collier-br...@indexexchange.com>
>> wrote:
>> >> Dave Collier-Brown via
>> Bloat<bloat@lists.bufferbloat.net><mailto: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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