[Codel] [Bloat] The challenge

[Jim Gettys]

On 05/09/2012 04:06 PM, Fred Baker wrote:
> On May 9, 2012, at 12:16 AM, Simon Barber wrote:
>
>> One question now remains - will codel AQM be sufficient on it's own in getting delays down to levels that users are happy with for the common latency sensitive interactive traffic - VoIP, gaming and Skype for example - or are the further reductions that can be had with traffic classification and smart queuing algorithms necessary?
> In my opinion, AQM is enough to get traffic into the "reasonable" range, but if you're looking for a specific SLA that might be applicable to gaming etc, you will need to do some engineering in terms of diffserv etc.
> _______________________________________________
>
I used to think that, before I understood:
    1) what the web was doing (again), by ignoring the 2 old TCP
connection rule, crossed by web site "sharding".  This is why I wrote
the IW10 considered Evil draft last fall.
    2) what the NIC offload engines were doing to generate line rate
packet trains and injecting them into the net, where they can land
"spat" at the customer end.
    3) Ledbat won't help once an effective AQM is in place; the delay
it's keying off of goes away, and then it competes like Reno with
regular flows.  100 BitTorrent flows competing with your traffic can
ruin your entire day.  We get to revisit this topic.
    4) a busy 802.11 net (or using one where the range to the AP drops
the bandwidth to low rate) means even single big packets has 12
milliseconds of latency @ 1Mbps (even ignoring other 802.11 effects,
just the bit transfer time).

Even on high speed (50Mbps broadband) I see transients of > 100ms just
browsing image heavy web sites.

So while CoDel will help (a lot, particularly policing TCP so that it
will respond quickly rather than suffering the quadratic responsiveness
problem), I now firmly believe we have to probably delve into "fair"
queuing and classification if we want a low latency edge to the Internet.

This doesn't mean I believe we'll have to do all this beyond the
broadband edge box (e.g. CMTS/DSLAM/FIOS) ; but I think these and the
home network have to actually be careful.  thankfully, at the edge, you
have way more cycles/packet you can *afford* to use on such schemes.

Having an AQM that actually works well enough to be "on" by default will
help most of the other instances of bloat I know of elsewhere, but the
edge, it's a different story....
                        - Jim