Facebook dropping MSS on congestion

Wed Mar 20 19:21:52 UTC 2019

Good day,

I am writing here, as in technical support ticket I will most likely end 
up to the outsourcing guys, who will try to write some formal reply and 
close the ticket quickly to keep KPI high:)
I have a faint hope that someone will read and listen. It may also be 
useful to colleagues.
I noticed at last few month, if some congestion occurs on the network 
(specific subnet), facebook reduces the maximum segment size (MSS), even 
down to 256 bytes. Purely academically, on paper - this will reduce 
latency.
In reality - it will cause avalanche effect.
If ISP have CGNAT, or some other appliances - with great probability 
they will encounter the fact that pps will increase 4-5 times, and might 
hit pps limit on hardware. Additionally, overhead on IP headers will 
increase significantly, especially on ipv6, and this will further 
aggravate the congestion.
Facebook don't do that, please. And thank you, if you listen to 
suggestions.

Denys