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) >> >> >> >> >> CONFIDENTIALITY NOTICE AND DISCLAIMER : This telecommunication, including >> any and all attachments, contains confidential information intended only for >> the person(s) to whom it is addressed. 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