nanog mailing list archives
Re: 400G forwarding - how does it work?
From: Saku Ytti <saku () ytti fi>
Date: Sun, 7 Aug 2022 12:46:00 +0300
On Sun, 7 Aug 2022 at 12:16, Masataka Ohta <mohta () necom830 hpcl titech ac jp> 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? If we estimate BW and pace TCP window growth at estimated BW, we don't need much buffering at all. 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. 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. Now add to this multiple logical interfaces, each having 4-8 queues, it adds up. Big buffers are bad mmmm'kay is frankly simplistic and inaccurate. 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? -- ++ytti
Current thread:
- RE: 400G forwarding - how does it work? Jeff Tantsura (Aug 03)
- Re: 400G forwarding - how does it work? Jeff Tantsura (Aug 04)
- Re: 400G forwarding - how does it work? Saku Ytti (Aug 05)
- RE: 400G forwarding - how does it work? ljwobker (Aug 05)
- Re: 400G forwarding - how does it work? Saku Ytti (Aug 05)
- RE: 400G forwarding - how does it work? ljwobker (Aug 06)
- Re: 400G forwarding - how does it work? Saku Ytti (Aug 07)
- RE: 400G forwarding - how does it work? ljwobker (Aug 07)
- Re: 400G forwarding - how does it work? Saku Ytti (Aug 05)
- Re: 400G forwarding - how does it work? Jeff Tantsura (Aug 04)
- Re: 400G forwarding - how does it work? Masataka Ohta (Aug 07)
- Re: 400G forwarding - how does it work? Saku Ytti (Aug 07)
- Re: 400G forwarding - how does it work? Masataka Ohta (Aug 07)
- Re: 400G forwarding - how does it work? sronan (Aug 07)
- Re: 400G forwarding - how does it work? Saku Ytti (Aug 07)
- Re: 400G forwarding - how does it work? sronan (Aug 07)
- Re: 400G forwarding - how does it work? Masataka Ohta (Aug 07)
- Re: 400G forwarding - how does it work? Dave Taht (Aug 08)
- Re: 400G forwarding - how does it work? Masataka Ohta (Aug 09)
- Re: 400G forwarding - how does it work? Saku Ytti (Aug 07)
- Re: 400G forwarding - how does it work? Masataka Ohta (Aug 08)
- Re: 400G forwarding - how does it work? Saku Ytti (Aug 08)