I'm currently trying to find a way to prove, that ECN (https://docs.cumulusnetworks.com/display/DOCS/Buffer+and+Queue+Management#BufferandQueueManagement-e
...) mechanism is actually working.
I have 4 servers, all connected to one switch with 10GE ports. I have enabled ECN on all servers and on switch itself for relevant ports. Now I want to make sure, that ECN is actually doing something. So, I emulated interface overload - from one server I started 3 iperf TCP sessions to 3 another servers (so, 3 servers are sending towards one server).
I expect to see, that eventually the receiving server's interface on the switch will be overloaded and it will start marking IP packets with '11 - Congestion Encountered' bits. But, I don't see any on the server (running tcpdump like this: tcpdump -i ens4f0 '(ip & 3 == 3)' ).
I even reduced ECN threshold to 1000 bytes on switch - ecn.ecn_port_group.max_threshold_bytes = 1000 with no visible result.
Maybe, anybody could advise, how to actually emulate and see the result of working ECN?
Thank you in advance