Sent from my iPhone
> On 20. mars 2015, at 16:31, Jim Gettys <[email protected]> wrote: > > > >> On Fri, Mar 20, 2015 at 10:54 AM, Michael Welzl <[email protected]> wrote: >> Folks, >> >> I think I have just seen this statement a little too often: >> >> > That’s right, Jim. The “some packet loss is good” part is from what I have >> > seen the hardest thing for people to understand. People have been trained >> > to believe that any packet loss is terrible (..) >> >> I understand the "wrong mindset" thing and the idea of AQM doing something >> better. Still, I'd like people to understand that packet loss often also >> comes with delay - for having to retransmit. This delay is not visible in >> the queue, but it's visible in the end system. It also comes with >> head-of-line blocking delay on the receiver side: at least with TCP, >> whatever has been received after a dropped packet needs to wait in the OS >> for the hole to be filled before it can be handed over to the application. >> >> Here we're not talking a few ms more or less in the queue, we're talking an >> RTT, when enough DupACKs are produced to make the sender clock out the >> missing packet again. Else, we're talking an RTO, which can be much, much >> more than an RTT, and which is what TLP tries to fix (but TLP's timer is >> also 2 RTTs - so this is all about delay at RTT-and-higher magnitudes). >> >> Again, significant delay can come from dropped packets - you just don't see >> it when all you measure is the queue. ECN can help. > > And without AQM, the RTT's are often many times the actual speed of light > RTT's, sometimes measured in seconds. And you eventually get the losses > anyway, as the bloated queues overflow. > not necessarily with ecn. and where in a burst loss occurs also matters > So without AQM, you are often/usually in much, much, much worse shape; > better to suffer the loss, and do the retransmit than wait forever. sure!! > - Jim > >> >> Cheers, >> Michael >
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