Dax Kelson of Guru Labs reported that 8% of the Internet hosts may not be reachable from an ECN capable client. It seems likely from the nature of the original test that the cause of the observed problem is not the IP ECN bits (in the ToS field), but the TCP bits (in the flags field) that are used to negotiate ECN. In an attempt to pin down precisely what the cause of the observed problem might be, we ran the following tests using tbit (http://www.aciri.org/tbit/): 1. Send www.foobar.com a SYN packet with the TCP ECN_ECHO and CWR flags set. 2. If a SYN/ACK is received, check to see if ECN_ECHO flag is set. Procced to step 3. If no SYN/ACK is received (after three retries), or an early RESET is received declare failure. 3. Ignoring the fact that the remote host probably didn't negotiate ECN usage, send a data packet containing a valid HTTP request, with ECT and CE bits set in the IP ToS field. 4. If an ACK is received, check to see if ECN_ECHO flag is set. Otherwise (after three retries), declare failure. Before running the test against a given host, we checked to see if the host was reachable from our site, and would ACK a SYN packet sent without ECN_ECHO and CWR flags set. The tests were run from moose.aciri.org, which runs FreeBSD 4.1. The test results are available at http://www.aciri.org/tbit/. The hosts are selected from those listed at the site www.100hot.com. We are currently running the test against the original list of about 35,000 hosts posted by Dan. We would appreciate any comments and feedback. - Jitu, Mark and Sally.