[Cake] active sensing queue management

Alan Jenkins alan.christopher.jenkins at gmail.com
Wed Jun 10 21:05:44 EDT 2015

On 10/06/15 21:54, Sebastian Moeller wrote:
> Hi Dave,
> On Jun 10, 2015, at 21:53 , Dave Taht <dave.taht at gmail.com> wrote:
>> http://dl.ifip.org/db/conf/networking/networking2015/1570064417.pdf
>> gargoyle's qos system follows a similar approach, using htb + sfq, and
>> a short ttl udp flow.
>> Doing this sort of measured, then floating the rate control with
>> "cake" would be fairly easy (although it tends to be a bit more
>> compute intensive not being on a fast path)
>> What is sort of missing here is trying to figure out which side of the
>> bottleneck is the bottleneck (up or down).
> 	Yeah, they relay on having a reliable packet reflector upstream of the “bottleneck” so they get their timestamped probe packets returned.

They copy & frob real IP headers.  They don't _say_ how the reflection 
works, but I guess low TTL -> ICMP TTL exceeded, like traceroute.  Then 
I read Gargoyle also use ICMP TTL exceeded and I thought my guess is 
quite educated 8).

Note the size of the timestamp, a generous 8 bytes.  It "just happens" 
that ICMP responses are required to include the first 8 bytes of the IP 
payload 8).

>   In the paper they used either uplink or downlink traffic so figuring where the bottleneck was easy at least this is how I interpret “Experiments were performed in the upload (data flowing from the users to the CDNs) as well as in the download direction.". At least this is what I get from their short description in glossing over the paper.

Ow!  I hadn't noticed that.  You could reduce both rates proportionally 
but the effect is inelegant.  I wonder what Gargoyle does...

2012 gargoyle developer comment says "There are not settings for active 
congestion control on the uplink side. ACC concentrats on the download 
side only."

Random blog post points out this is sufficient to fix prioritization 
v.s. bufferbloat.  "In upstream direction this is not a big problem 
because your router can still prioritize which packet should be sent 
first".  (Yay, I get to classify every application I care about /s and 
still get killed by uploads in http).

One solution would be if ISPs made sure upload is 100% provisioned. 
Could be cheaper than for (the higher rate) download.

> 	Nice paper, but really not a full solution either. Unless the ISPs cooperate in supplying stable reflectors powerful enough to support all downstream customers.

I think that's a valid concern.  Is "TTL Exceeded" rate-limited like 
Echo (because it may be generated outside the highest-speed forwarding 
path?), and would this work as tested if everyone did it?

>   But if the ISPs cooperate, I would guess, they could eradicate downstream buffer bloat to begin with. Or the ISPs could have the reflector also add its own UTC time stamp which would allow to dissect the RTT into its constituting one-way delays to detect the currently bloated direction. (Think ICMP type 13/14 message pairs "on steroids", with higher resolution than milliseconds, but for buffer bloat detection ms resolution would probably be sufficient anyways). Currently, I hear that ISP equipment will not treat ICMP requests with priority though.
> 	Also I am confused what they actually simulated: “The modems and CMTS were equipped with ASQM, CoDel and PIE,” and “However, the problem pop- ularly called bufferbloat can move about among many queues some of which are resistant to traditional AQM such as Layer 2 MAC protocols used in cable/DSL links. We call this problem bufferbloat displacement.” seem to be slightly at odds. If modems and CTMS have decent AQMs all they need to do is not stuff their sub-IP layer queuesand be done with it. The way I understood the cable labs PIE story, they intended to do exactly that, so at least the “buffer displacement” remedy by ASQM reads a bit like a straw man argument. But as I am a) not of the cs field, and b) only glossed over the paper, most likely I am missing something important that is clearly in the paper...
> Best Regards
> 	Sebastian

I had your reaction about pie on the modem.

We could say there is likely room for improvement in any paper, that 
claims bufferbloat eliminated with a "target" parameter of 100ms :p.  
Results don't look that bad (why?) but I do see 25ms bloat v.s. 
codel/pie.  It may be inevitable but deserves not to be glossed over 
with comparisons to the unrelated 100ms default parameter of codel, 
which in reality is the one called "interval" not "target" :).  Good QM 
on the modem+cmts has got to be the best solution.


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