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[iccrg] ECN statistics for Mac OS X

Stuart Cheshire <cheshire@apple.com> Mon, 23 March 2015 20:53 UTC

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From: Stuart Cheshire <cheshire@apple.com>
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Date: Mon, 23 Mar 2015 15:53:11 -0500
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Subject: [iccrg] ECN statistics for Mac OS X
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As requested today, here is some information about ECN on Mac OS X:

1. To enable ECN, type:

sysctl -w net.inet.tcp.ecn_negotiate_in=1
sysctl -w net.inet.tcp.ecn_initiate_out=1

2. To make this change persistent, put following lines in /etc/sysctl.conf:

net.inet.tcp.ecn_initiate_out=1
net.inet.tcp.ecn_negotiate_in=1

3. To see the results, use netstat:

% netstat -sp tcp
tcp:
...
  12558 connections negotiated ECN
    0 times congestion notification was sent using ECE
    1983 times CWR was sent in response to ECE

This implies that 1983 times my laptop has received acks with ECE set, which suggests that some of its outbound packets experienced congestion and were marked “CE”. I have not yet identified which device or devices are marking these packets (if any — it’s possible it could just be a bug in the statistics reporting).

Stuart Cheshire

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