Notes on enabling ECN (slightly OT)

Installing and Using OpenWrt Network and Wireless Configuration
41

Well heck. No, I hadn't thought about [the lack of] sudo being the issue. sigh My apologies, and thanks for setting me straight there. :slight_smile:

32706 client connections attempted to negotiate ECN
		1725 client connections successfully negotiated ECN
		10959 times graceful fallback to Non-ECN connection
		49275 times lost ECN negotiating SYN, followed by retransmission
		188 server connections attempted to negotiate ECN
		171 server connections successfully negotiated ECN
		17 times lost ECN negotiating SYN-ACK, followed by retransmission
		170 times received congestion experienced (CE) notification
		3 times CWR was sent in response to ECE
		4349 times sent ECE notification
		92 connections received CE atleast once
		3 connections received ECE atleast once
		1164 connections using ECN have seen packet loss but no CE
		61 connections using ECN have seen packet loss and CE
		34 connections using ECN received CE but no packet loss
		2 connections fell back to non-ECN due to SYN-loss
		44 connections fell back to non-ECN due to reordering
		1 connection fell back to non-ECN due to excessive CE-markings
		1 connection fell back caused by connection drop due to RST
		0 connection fell back due to drop after multiple retransmits
		6 connections fell back due to RST after SYN
42

Guess how I thought about asking? I learned the same way :wink:

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43

I have to admit that report does not make a whole lot of sense. The ecn attempt failures was really high, seeing loss and ecn marks also quite weird. I am assuming this was over wifi?

44

Affirmative--the node I output the report from is on wireless 99.9% of the time.

45

On macOS Tahoe 26.1 ECN is enabled by default:

% sysctl -a | grep ecn
net.inet.tcp.ecn_timeout: 5
net.inet.tcp.ecn_setup_percentage: 100
net.inet.tcp.accurate_ecn: 0
net.inet.tcp.ecn_initiate_out: 1
net.inet.tcp.ecn: 1
net.inet.ipsec.ecn: 0
net.inet.mptcp.probecnt: 5
net.inet6.ipsec6.ecn: 0

% sysctl -a | grep heur
net.inet.tcp.disable_tcp_heuristics: 0
net.inet.tcp.link_heuristics_flags: 63
net.inet.tcp.link_heuristics_rto_min: 3000
net.link.generic.system.link_heuristics_flags: 9
net.link.generic.system.link_heuristics_lqm_max: 0
net.link.generic.system.link_heuristics_delay: 30000

Notes:

net.inet.tcp.ecn_negotiate_in doesn’t exist anymore. It should be removed from a plist, if you’re using that.

net.inet.tcp.ecn is new. Probably should be added if you want to be sure ECN remains on.

net.inet.tcp.disable_tcp_heuristics remains disabled by default. I wouldn’t be so eager with enabling it, because there could be servers out there without ECN. It also affects other TCP stuff like TCP Fast Open (TFO) and MPTCP. When these fail to negotiate OS will put hosts on a (temporary?) blacklist and perform regular TCP.

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