RE: [PATCH v9 net-next 15/15] selftests/net: packetdrill: add TCP Accurate ECN cases

-----Original Message-----
From: Neal Cardwell <ncardwell@google.com> 
Sent: Tuesday, January 20, 2026 8:35 PM
To: Jakub Kicinski <kuba@kernel.org>
Cc: Chia-Yu Chang (Nokia) <redacted>; pabeni@redhat.com; edumazet@google.com; parav@nvidia.com; linux-doc@vger.kernel.org; corbet@lwn.net; horms@kernel.org; dsahern@kernel.org; kuniyu@google.com; bpf@vger.kernel.org; netdev@vger.kernel.org; dave.taht@gmail.com; jhs@mojatatu.com; stephen@networkplumber.org; xiyou.wangcong@gmail.com; jiri@resnulli.us; davem@davemloft.net; andrew+netdev@lunn.ch; donald.hunter@gmail.com; ast@fiberby.net; liuhangbin@gmail.com; shuah@kernel.org; linux-kselftest@vger.kernel.org; ij@kernel.org; Koen De Schepper (Nokia) <redacted>; g.white@cablelabs.com; ingemar.s.johansson@ericsson.com; mirja.kuehlewind@ericsson.com; cheshire <redacted>; rs.ietf@gmx.at; Jason_Livingood@comcast.com; Vidhi Goel <redacted>
Subject: Re: [PATCH v9 net-next 15/15] selftests/net: packetdrill: add TCP Accurate ECN cases


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On Tue, Jan 20, 2026 at 1:53 PM Jakub Kicinski [off-list ref] wrote:

quoted

On Mon, 19 Jan 2026 19:58:52 +0100 chia-yu.chang@nokia-bell-labs.com
wrote:

quoted

Linux Accurate ECN test sets using ACE counters and AccECN options 
to cover several scenarios: Connection teardown, different ACK 
conditions, counter wrapping, SACK space grabbing, fallback schemes, 
negotiation retransmission/reorder/loss, AccECN option drop/loss, 
different handshake reflectors, data with marking, and different sysctl values.

Thank you for closing the packetdrill side, and big thanks to Neal for 
prioritizing getting it reviewed and merged!

I updated the packetdrill build in netdev CI and looks like one of the 
cases is flaking a little. Since it looks like you'll have to respin, 
please try to fix:

# 1..2
# tcp_accecn_client_accecn_options_lost.pkt:32: error handling packet: 
timing error: expected outbound packet in relative time range 
+0.020000~+0.500000 sec but happened at +0.015816 sec # script packet:  
0.181936 .5 1:1013(1012) ack 1 <ECN e1b 1 ceb 0 e0b 1,nop> # actual 
packet:  0.177752 .EA 1:1013(1012) ack 1 win 1050 <ECN e1b 1 ceb 0 e0b 
1,nop> # not ok 1 ipv4 # tcp_accecn_client_accecn_options_lost.pkt:32: 
error handling packet: timing error: expected outbound packet in 
relative time range +0.020000~+0.500000 sec but happened at +0.015800 
sec # script packet:  0.181952 .5 1:1013(1012) ack 1 <ECN e1b 1 ceb 0 
e0b 1,nop> # actual packet:  0.177752 .EA 1:1013(1012) ack 1 win 1050 
<ECN e1b 1 ceb 0 e0b 1,nop> # not ok 2 ipv6 # # Totals: pass:0 fail:2 
xfail:0 xpass:0 skip:0 error:0

https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fnetd
ev-ctrl.bots.linux.dev%2Flogs%2Fvmksft%2Fpacketdrill%2Fresults%2F48220
1%2F115-tcp-accecn-client-accecn-options-lost-pkt%2Fstdout&data=05%7C0
2%7Cchia-yu.chang%40nokia-bell-labs.com%7Cf125ccafc7134b620bad08de585b
1a35%7C5d4717519675428d917b70f44f9630b0%7C0%7C0%7C639045345459758258%7
CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlA
iOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=kn3Pb
VXw%2Bkznnf7VaYzwP%2FL3IO2LYGYQzZOWS3HRZ6w%3D&reserved=0

Probably this is happening because the SRTT is around 56ms:

.050 * 7/8 + 1/8 * .1 = .05625 sec

So the RACK fast recovery starts afte rabout 15ms due to .25 * srtt being about 14ms:
(.050 * 7/8 + 1/8 * .1) * .25 = .0140625 sec

If we make the SRTT 100ms then the fast retransmit should be around:

(.1 * 7/8 + 1/8 * .1) * .25 = .025 sec

So I'd suggest changing the timing of the SYNACK from 50ms to 100ms:

old:
+0.05 < [ect0] SW. 0:0(0) ack 1 win 32767 <mss 1024,ECN e0b 1 ceb 0
e1b 1,nop,nop,nop,sackOK,nop,wscale 8>

new:
+.1 < [ect0] SW. 0:0(0) ack 1 win 32767 <mss 1024,ECN e0b 1 ceb 0 e1b
1,nop,nop,nop,sackOK,nop,wscale 8>

neal

Thanks Neal and Eric, I've fixed this issue as well as the concerned raised in patch 10 of listen socket.
All AccECN packetdrill still pass from my end, so I will submit v10.

Thanks.
Chai-Yu