[Cerowrt-devel] [Cake] [Bloat] heisenbug: dslreports 16 flow test vs cablemodems

Sebastian Moeller moeller0 at gmx.de
Tue May 19 12:25:25 EDT 2015

Hi David,

On May 18, 2015, at 17:09 , dpreed at reed.com wrote:

> I'm curious as to why one would need low priority class if you were using fq_codel?  Are the LEDBAT flows indistinguishable?  

	Well, as far as I can tell fq_codel treats all flows the same, but we want LEDBAT flows to basically scavenge the left-overs, not get their fair share on the table ;). Updates by the way are not the best example for this kind of problem as some updates are urgent enough to post-pone everything else for.

> Is there no congestion signalling (no drops, no ECN)? The main reason I ask is that end-to-end flows should share capacity well enough without magical and rarely implemented things like diffserv and intserv.

	As far as I can tell bit torrent tries to do two things: 1) open up quite a number of parallel ingress and egress flows and 2) keep that traffic out of the way of other traffic. fq_codel interferes with how 2) is implemented. Currently, the best of the flawed work-arounds is to have bit torrent tell the network that it should be treated as LEDBAT using TOS bits. This is flawed as we have no gurateee whatsoever on the sanity of TOS bits on our networks ingress (and often networks will re-map the TOS bits anyway, so on ingress the LEDBAT TOS signal might not be in the packets any more, and since one man’s ingress is another man’s egress, basically using TOS bits for keeping bit torrent in the background is a loosing proposition). That said I watched a ripe talk by Peter Lothberg where he proposed for the carriers (DTAG in his case) to encode their TOS bits into the IPv6 addresses and simply ignore the IP TOS bits, so they will not need to re-map those as they are totally neutral for DTAG planned internal network. (And interestingly in DTAG’s IPv6 network RRUL test packets from sweden keep their TOS bits fully intact).

Best Regards

> On Monday, May 18, 2015 8:30am, "Simon Barber" <simon at superduper.net> said:
> I am likely out of date about Windows Update, but there's many other programs that do background downloads or uploads that don't implement LEDBAT or similar protection. The current AQM recommendation draft in the IETF will make things worse, by not drawing attention to the fact that implementing AQM without implementing a low priority traffic class (such as DSCP 8 - CS1) will prevent solutions like LEDBAT from working, or there being any alternative. Would appreciate support on the AQM list in the importance of this.
> Simon
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> On May 18, 2015 4:42:43 AM "Eggert, Lars" <lars at netapp.com> wrote:
> On 2015-5-18, at 07:06, Simon Barber <simon at superduper.net> wrote:
> Windows update will kill your Skype call.
> Really? AFAIK Windows Update has been using a LEDBAT-like scavenger-type congestion control algorithm for years now.
> Lars
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