[Bloat] ADSL, ATM drivers, bloat, education & confusion

[Jesper Dangaard Brouer]

On Sat, 6 Jun 2015 16:29:15 +0200 Sebastian Moeller <moeller0 at gmx.de> wrote:

> On Jun 6, 2015, at 15:38 , Kevin Darbyshire-Bryant <kevin at darbyshire-bryant.me.uk> wrote:
> 
[...]
> > The problem I have is setting outbound rate limiting.  I was hoping
> > that 'cake' without the 'bandwidth' parameter would work on the
> > 'backpressure' from the ATM(?) driver, sadly this wasn't the case
> > and so setting a bandwidth limit (I'm not in a position to test the
> > new keywords for ATM encapsulation etc yet) was the only way
> > forward.  
> 
>  This is rather important to get right, ATM’s arcane 48/53
> encapsulation only leaves 100*48/53 = 90.5% of the sync rate for
> useable bits, and even those need to contain all the headers specific
> to your line (plus AAL5’s unfortunate choice of fitting each packet
> into an integer number of ATM cells), mean that without AQM taking
> the link layer encapsulation into account you need to aim for roughly
> 80-85% of the sync rates on and ATM link. With a link that disappears
> often I currently would recommend sqm-scripts as weapon of choice
> (you should be able to get cake into sqm-scripts) as the IFB needs to
> be set up again after the “connected” interface reappears, which
> current sqm-scripts should do for you...

That is true, the ATM overhead on ADSL is very important to get right
for your ratelimiting work as intended (that is you gain control over
the queue).

The iproute2 "tc" have supported option "linklayer atm" and "overhead"
for quite some time now (since 2008).  All the rate_tables based
schedulers (e.g. HTB, TBF) have these options.

There is also the more generic "stab" that allow linklayer adaptation to
work for any qdisc. See man tc-stab [1]

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Sr. Network Kernel Developer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer

[1] http://manpages.ubuntu.com/manpages/precise/man8/tc-stab.8.html