400G forwarding - how does it work?
dip
diptanshu.singh at gmail.com
Sun Aug 7 17:12:07 UTC 2022
Disclaimer: I often use the M/M/1 queuing assumption for much of my work to
keep the maths simple and believe that I am reasonably aware in which
context it's a right or a wrong application :). Also, I don't intend to
change the core topic of the thread, but since this has come up, I couldn't
resist.
>> With 99% load M/M/1, 500 packets (750kB for 1500B MTU) of
>> buffer is enough to make packet drop probability less than
>> 1%. With 98% load, the probability is 0.0041%.
To expand the above a bit so that there is no ambiguity. The above assumes
that the router behaves like an M/M/1 queue. The expected number of packets
in the systems can be given by
[image: image.png]
where [image: image.png] is the utilization. The probability that at least
B packets are in the system is given by [image: image.png] where B is the
number of packets in the system. for a link utilization of .98, the packet
drop probability is .98**(500) = 0.000041%. for a link utilization of 99%,
.99**500 = 0.00657%.
>> When many TCPs are running, burst is averaged and traffic
>> is poisson.
M/M/1 queuing assumes that traffic is Poisson, and the Poisson assumption is
1) The number of sources is infinite
2) The traffic arrival pattern is random.
I think the second assumption is where I often question whether the traffic
arrival pattern is truly random. I have seen cases where traffic behaves
more like self-similar. Most Poisson models rely on the Central limit
theorem, which loosely states that the sample distribution will approach a
normal distribution as we aggregate more from various distributions. The
mean will smooth towards a value.
Do you have any good pointers where the research has been done that today's
internet traffic can be modeled accurately by Poisson? For as many papers
supporting Poisson, I have seen as many papers saying it's not Poisson.
https://www.icir.org/vern/papers/poisson.TON.pdf
https://www.cs.wustl.edu/~jain/cse567-06/ftp/traffic_models2/#sec1.2
On Sun, 7 Aug 2022 at 04:18, Masataka Ohta <mohta at necom830.hpcl.titech.ac.jp>
wrote:
> Saku Ytti wrote:
>
> >> I'm afraid you imply too much buffer bloat only to cause
> >> unnecessary and unpleasant delay.
> >>
> >> With 99% load M/M/1, 500 packets (750kB for 1500B MTU) of
> >> buffer is enough to make packet drop probability less than
> >> 1%. With 98% load, the probability is 0.0041%.
>
> > I feel like I'll live to regret asking. Which congestion control
> > algorithm are you thinking of?
>
> I'm not assuming LAN environment, for which paced TCP may
> be desirable (if bandwidth requirement is tight, which is
> unlikely in LAN).
>
> > But Cubic and Reno will burst tcp window growth at sender rate, which
> > may be much more than receiver rate, someone has to store that growth
> > and pace it out at receiver rate, otherwise window won't grow, and
> > receiver rate won't be achieved.
>
> When many TCPs are running, burst is averaged and traffic
> is poisson.
>
> > So in an ideal scenario, no we don't need a lot of buffer, in
> > practical situations today, yes we need quite a bit of buffer.
>
> That is an old theory known to be invalid (Ethernet switches with
> small buffer is enough for IXes) and theoretically denied by:
>
> Sizing router buffers
> https://dl.acm.org/doi/10.1145/1030194.1015499
>
> after which paced TCP was developed for unimportant exceptional
> cases of LAN.
>
> > Now add to this multiple logical interfaces, each having 4-8 queues,
> > it adds up.
>
> Having so may queues requires sorting of queues to properly
> prioritize them, which costs a lot of computation (and
> performance loss) for no benefit and is a bad idea.
>
> > Also the shallow ingress buffers discussed in the thread are not delay
> > buffers and the problem is complex because no device is marketable
> > that can accept wire rate of minimum packet size, so what trade-offs
> > do we carry, when we get bad traffic at wire rate at small packet
> > size? We can't empty the ingress buffers fast enough, do we have
> > physical memory for each port, do we share, how do we share?
>
> People who use irrationally small packets will suffer, which is
> not a problem for the rest of us.
>
> Masataka Ohta
>
>
>
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