[Cake] flow isolation for ISPs

Toke Høiland-Jørgensen toke at toke.dk
Thu Apr 6 08:14:10 EDT 2017

Pete Heist <peteheist at gmail.com> writes:

>  On Apr 6, 2017, at 12:50 PM, Toke Høiland-Jørgensen <toke at toke.dk> wrote:
>  Pete Heist <peteheist at gmail.com> writes:
>  The fq_codel qdisc does have support for arbitrary tc filters to replace
>  the default hashing, BTW. If you don't need the cake shaper, that might
>  be a solution?
>  I see, I found mention of it in Chapter 6 of a draft RFC that it looks
>  like you wrote, actually
>  (https://tools.ietf.org/html/draft-ietf-aqm-fq-codel-06#section-6). :)
>  To try it out, am I heading the right direction by looking at tc
>  filter’s skbedit action, or is that just for MQ devices?
>  (http://man7.org/linux/man-pages/man8/tc-skbedit.8.html)
>  I also saw this mention of “We are not aware of any deployments
>  utilising the custom classification feature"
>  https://tools.ietf.org/html/draft-ietf-aqm-fq-codel-02#section-5.1.1,
>  so not sure how often this has been tried. :)
>  Yeah, haven't actually heard of anyone using the feature in production.
>  It's basically this section from the 'classful qdiscs' section of 'man
>  tc':
>        When a packet enters a classful qdisc it can be classified to one of the classes within. Three criteria are available, although not all qdiscs will use all three:
>        tc filters
>               If tc filters are attached to a class, they are consulted first for relevant instructions. Filters can match on all fields of a packet header, as well as on the firewall mark applied by ipchains or iptables.
>  So you can basically use the full capabilities of tc-filter in place of
>  the built-in hashing of fq_codel. The tc-u32 man page has some examples,
>  which is probably a good starting point.
>  If you do try this out and feel like writing up a small
>  example/tutorial, I'm happy to add a link (or the whole thing) somewhere
>  on bufferbloat.net :)
> Sure, if I get it working I’ll include an example in my paper,


> but I’m still a little confused. Is fq_codel actually a classful
> qdisc?

Yup. The hash buckets are the classes, basically. You can get per-flow
stats by doing `tc -s class show dev eth0` (only works when a flow has
built a queue).

> I get the part about matching with tc-filter and the u32 selector (as
> intuitive as that is :), but am not sure of the action the filter
> needs to take. However, I do see the example towards the bottom of the
> tc-u32 man page where a hash table is created and filters move packets
> into the right buckets. Perhaps it will be eventually decipherable
> from this… :)

The filter classifies a packet into a class. The 'minor number' of this
is the queue number (so needs to be less than the number of flows you
configured for fq_codel - 1024 by default).


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