[c-nsp] MTU and PMTUD

[Marcin Kurek]

Hi Saku,

> I am embarrassed to say, but I _just_ found out, like literally weeks
> ago, that Junos BGP TCP window is 16kB, I did also find hidden command
> (https://github.com/ytti/seeker-junos) to bump it up to 64kB. I am
> working with JNPR to have public documentation for the hidden command
> to improve supportability and optics. I am separately working on hopes
> of getting TCP window scaling.
> I know that we are limited by TCP window, as the BGP transfer speed is
> within 0.5% of theoretical max, and increasing 16kB to 64kB increases
> BGP transfer speed exactly 4 times, being still capped by TCP window.
> I think Cisco can go to 130k, but won't by default.
> Maybe after that issue is remedied I will review packet size.

Thanks for sharing, that's some great info here. Probably I'll have to 
take a closer look at the TCP window setting as well.

> I've seen Cisco presentations in the 90s and early 00s showing
> significant benefit from it. I have no idea how accurate it is
> today,nor why it would have made a difference in the past, like was
> the CPU interrupt rate constrained?

I'm sorry, I didn't get that part about constrained CPU interrupt rate?
My simple way of looking into that is that if we bump up the MTU, we end 
up with fewer packets on the wire, so less processing on both sides.

> You can do 'tcp mss X' under neighbor stanza.

Thanks, gave it a try today, but I'm getting some strange results.

Testing using XR 7.5.2 and older IOS XE, resulting MSS depends on who is 
passive/active.


Kind regards,
Marcin