[e2e] Codel and Wireless

[Andrew Mcgregor]

Empirically, for fq_codel, long RTT flows work fine so long as RTT < 5
intervals, roughly speaking, and it degrades very slowly.  So 100ms is
about right for the internet.


On 4 December 2013 10:10, Daniel Havey <dhavey at yahoo.com> wrote:

> On Tuesday, December 3, 2013 2:41 PM, Andrew Mcgregor <
> andrewmcgr at google.com> wrote:
>  All of which is why fq_codel is so much better... because flows queue
> independently, and drops are calculated per flow (although overall queue
> size is included implicitly via the sojourn time), the RTT delay has far
> less impact.  CoDel is an ingredient of an AQM system, not a desirable AQM
> on its own.
>
> >Makes sense to me.  We need to get the worst case RTT right.  If we set
> the interval to 100ms then the user with the users with larger RTTs may
> have issues.
>
>
> On 4 December 2013 08:11, Daniel Havey <dhavey at yahoo.com> wrote:
>
>
>
>
>
>
> On Tuesday, December 3, 2013 12:22 PM, Detlef Bosau <detlef.bosau at web.de>
> wrote:
>
> To my understanding, the "sojourn time" considered in CoDel is the
> difference between the time when a packet/leaves /a queue and the time,
> when this packet has /arrived /at the queue. In other words: The time a
> packet spends in the queue.
>
> When this time is unusually high, CoDel sees an imminent congestion and
> drops packets.
>
> The problem is that CoDel makes no difference whether the "sojourn time"
> is caused by a huge number of packets in the queue, i.e. congestion, or
> by a huge delivery time resulting from corruption loss and necessary
> retransmissions.
>
> CoDel parameters are interval and target.  If the queue drains before the
> interval then there shouldn't be any drops.   Also there is "leverage" from
> Red in a Different Light.  If CoDel decides to drop a packet from a flow
> that is in congestion avoidance, fast retransmit or slow start then the
> window is halved and the queue drains quickly.  If the flow doesn't have
> enough data to trigger fast retransmit then that is unfortunate for that
> user since they now have to wait an RTT for that packet, and it does not
> drain the queue very much.
>
>
> Hence, we have the good old loss differentiation problem. And because
> CoDel is particularly intended for edge routers, the disaster is placed
> exactly there where it is expected to happen......8-)
>
> Detlef
>
> Am 01.12.2013 22:16, schrieb Andrew Mcgregor:
> > I mean sojourn time, one way in the particular queue, as per CoDel,
> rather
> > than anything TCP-related.  Clearance rate is fairly simply related to
> > sojourn time, of course, given enough integration time for the statistics
> > to converge.
> >
> >
> > On 2 December 2013 02:57, Detlef Bosau <detlef.bosau at web.de> wrote:
> >
> >> Am 01.12.2013 06:05, schrieb Andrew Mcgregor:
> >>> The actual clearance rate from the queue (or the sojourn time), if that
> >>> matters for your AQM scheme.  That way you are not assuming a known
> line
> >>> rate.
> >> Clearance rate or sojourn time?
> >>
> >> Clearance rate may apply for a packet delivery rate. From a TCP point of
> >> view, the sojourn time is the difference between the arrival of the
> >> according ACK and the time a data packet left the sender.
> >>
> >> So you omit any recovery latency.
> >>
> >>
> >>
> >>> On 30 November 2013 00:13, Detlef Bosau <detlef.bosau at web.de> wrote:
> >>>
> >>>>  Am 29.11.2013 00:24, schrieb Andrew Mcgregor:
> >>>>
> >>>> In which case... measure, don't assume.  Served us well for 802.11
> >>>> modulation selection, I don't see why it shouldn't work for AQM.
> >>>>
> >>>>
> >>>> What do you want to measure?
> >>>>
> >>>
> >>
> >> --
> >> ------------------------------------------------------------------
> >> Detlef Bosau
> >> Galileistraße 30
> >> 70565 Stuttgart                            Tel.:   +49 711 5208031
> >>                                            mobile: +49 172 6819937
> >>                                            skype:     detlef.bosau
> >>                                            ICQ:          566129673
> >> detlef.bosau at web.de                    http://www.detlef-bosau.de
>
> >>
> >>
> >
>
>
> --
> ------------------------------------------------------------------
> Detlef Bosau
> Galileistraße 30
> 70565 Stuttgart                            Tel.:   +49 711 5208031
>                                            mobile: +49 172 6819937
>                                            skype:     detlef.bosau
>                                            ICQ:          566129673
> detlef.bosau at web.de                    http://www.detlef-bosau.de
>
>
>
>
> --
> Andrew McGregor | SRE | andrewmcgr at google.com | +61 4 8143 7128
>
>
>


-- 
Andrew McGregor | SRE | andrewmcgr at google.com | +61 4 8143 7128