Sufficient Buffer Sizes

Tue Jan 2 23:21:02 UTC 2024

On Tue, Jan 2, 2024 at 3:02 PM Mike Hammett <nanog at ics-il.net> wrote:
> While attempting to ascertain how big of switch buffers I needed in a 100G switch, I rediscovered this article where I first learned about switch buffers.
>
> https://fasterdata.es.net/network-tuning/router-switch-buffer-size-issues/#:~:text=Optimum%20Buffer%20Size&text=The%20general%20rule%20of%20thumb,1G%20host%20across%20the%20WAN.
>
> It suggests that 60 meg is what you need at 10G. Is that per interface? Would it be linear in that I would need 600 meg at 100G?
>
> Some 100G switches I was looking at only had 36 megs, so that's insufficient either way you look at it.

Hi Mike,

My thoughts:

1. 50 ms is -way- too much buffer. A couple links like that in the
path and the user will suffer jitter in excess of 100ms which is
incredibly disruptive for interactive applications.

2. The article discussed how much buffer to apply to the -slower-
interfaces, not the faster ones, the idea being that data entering
from the faster interfaces could otherwise overwhelm the slower ones
resulting in needless retransmission and head-end blocking. Are the
100G interfaces on your switch the -slower- ones?

I don't know the best number, but I suspect the speed at which packets
clear an interface is probably a factor in the equation, so that the
reasonable buffer depth in ms when a packet clears in 1ms is probably
different than the reasonable buffer depth when a packet clears in 1
us.

Regards,
Bill Herrin

-- 
William Herrin
bill at herrin.us
https://bill.herrin.us/