On 04/05/16 12:41, Jonathan Morton wrote: >> On 4 May, 2016, at 12:57, Kevin Darbyshire-Bryant >> <ke...@darbyshire-bryant.me.uk> wrote: >> >> In essence my (mis)understanding of this code is something like: We've >> got here because we've been dropping and codel is telling is to continue >> to drop. With that decided we enter a do..while, the first thing to >> happen is to ECN mark and let that marked packet escape to send the >> signal. Otherwise we appear to iterate around the loop. So here's the >> nub of my question: the INET_ECN_set_ce is done on every iteration of >> that loop...with its potential early escape..do we escape on every >> iteration? Do we need to twiddle the ECN bits on every packet that >> we're about to drop? And we seem to mark the packet on exit of the loop >> anyway. > It’s rather oddly structured code, to be sure. > > The vital clue for you may be that you can only set CE on an IPv{4,6} packet > which already has something *other* than Not-ECT set; it’s impossible for > non-IP packets, and not ECN compliant for Not-ECT IP packets. So > INET_ECN_set_ce() returns true only when it succeeds. > > On a UDP flood stream, typically Not-ECT is set, so the early-out never > triggers. Instead Codel drops a packet, schedules the next drop, and checks > whether the next drop schedule has already been reached (which can happen for > high drop rates and/or slow transmission rates). > > It then attempts to set CE on the first packet transmitted after a drop > sequence, just in case it was a mixed ECT/Not-ECT stream; strenuous efforts > to get the congestion signal heard as early as possible. This is more likely > when flow isolation isn’t in use, or when it is per-host instead of per-flow. > That’s also why the first attempt to set CE is within the drop loop. > > - Jonathan Morton > Thanks Jonathan. Appreciate the time taken for the explanation. Much happier :-)
Kevin
smime.p7s
Description: S/MIME Cryptographic Signature
_______________________________________________ Cake mailing list Cake@lists.bufferbloat.net https://lists.bufferbloat.net/listinfo/cake