400G forwarding - how does it work?

Dave Taht dave.taht at gmail.com
Mon Aug 8 12:51:12 UTC 2022

On Sun, Aug 7, 2022 at 11:24 PM Masataka Ohta
<mohta at necom830.hpcl.titech.ac.jp> wrote:
> sronan at ronan-online.com wrote:
> > There are MANY real world use cases which require high throughput at
> > 64 byte packet size.
> Certainly, there were imaginary world use cases which require
> to guarantee so high throughput of 64kbps with 48B payload
> size for which 20(40)B IP header was obviously painful and 5B
> header was used. At that time, poor fair queuing was assumed,
> which requires small packet size for short delay.
> But as fair queuing does not scale at all, they disappeared
> long ago.

What do you mean by FQ, exactly?

"5 tuple FQ" is scaling today on shaping middleboxes like preseem and
LibreQos to over 10gbits. ISP reported results of customer calls about
speed simply vanish. Admittedly the AQM is dropping or marking some
.x% of packets, but tests with fq with short buffers vs aqm alone
showed the former the clear winner, and fq+aqm took it in for the

On linux fq_codel is the near universal default, also. The linux tcp
stack does fq+pacing at nearly 100gbits today on "BIG" tcp.

"disappeared", no. invisible, possible. transitioning from +10 gbit
down to 1gbit or less, really, really, useful. IMHO, and desparately
needed, in way more places.

Lastly VOCs and LAG and switch fabrics essentially FQ ports. In the
context of aggregating up to 400Gbit from that, you are FQing also.

Now fq-ing inline against the ip headers at 400gbit appeared
impossible until this convo when the depth of the pipeline and
hardware hashing was discussed, but I'll settle for more rfc7567
behavior just in stepping down from that, to 100, and from 100
stepping down, adding in fq+aqm.

>  > Denying those use cases because they don’t fit
>  > your world view is short sighted.
> That could have been a valid argument 20 years ago.
>                                                 Masataka Ohta

FQ World Domination pending: https://blog.cerowrt.org/post/state_of_fq_codel/
Dave Täht CEO, TekLibre, LLC

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