Subject: Re: [PATCH net v2 2/2] selftests/net: packetdrill: cover RFC 5961 5.2 challenge ACK on both edges

From: Jiayuan Chen <[email-masked]>
Date: Wed, 22 Apr 2026 10:38:22 +0800

On 4/21/26 9:41 AM, Jiayuan Chen wrote:
> RFC 5961 Section 5.2 / RFC 793 Section 3.9 require a challenge ACK
> whenever an incoming SEG.ACK falls outside
> [SND.UNA - MAX.SND.WND, SND.NXT].  There is currently no packetdrill
> coverage for either edge.
>
> Add tcp_rfc5961_ack-out-of-window.pkt, which in a single passive-open
> connection exercises:
>
>    - Upper edge (SEG.ACK > SND.NXT): peer ACKs data that was never
>      sent before the server has transmitted anything.
>    - Lower edge (SEG.ACK < SND.UNA - MAX.SND.WND): after the server
>      has sent 2000 bytes (the peer-advertised rwnd forces two 1000-byte
>      segments, both acknowledged), peer sends an ACK that is older
>      than the acceptable window.
>
> Both cases must elicit a challenge ACK
> <SEQ = SND.NXT, ACK = RCV.NXT, CTL = ACK>.  The per-socket RFC 5961
> Section 7 rate limit is disabled for the duration of the test so that
> both challenge ACKs can fire back-to-back.
>
> Signed-off-by: Jiayuan Chen <[email-masked]>
> Reviewed-by: Eric Dumazet <[email-masked]>
> ---
>   .../tcp_rfc5961_ack-out-of-window.pkt         | 46 +++++++++++++++++++
>   1 file changed, 46 insertions(+)
>   create mode 100644 tools/testing/selftests/net/packetdrill/tcp_rfc5961_ack-out-of-window.pkt
>
> diff --git a/tools/testing/selftests/net/packetdrill/tcp_rfc5961_ack-out-of-window.pkt b/tools/testing/selftests/net/packetdrill/tcp_rfc5961_ack-out-of-window.pkt
> new file mode 100644
> index 000000000000..44d54c812820
> --- /dev/null
> +++ b/tools/testing/selftests/net/packetdrill/tcp_rfc5961_ack-out-of-window.pkt
> @@ -0,0 +1,46 @@
> +// SPDX-License-Identifier: GPL-2.0
> +//
> +// RFC 5961 Section 5.2 / RFC 793 Section 3.9: an incoming segment's
> +// ACK value must lie in [SND.UNA - MAX.SND.WND, SND.NXT]; otherwise
> +// the receiver MUST discard the segment and send a challenge ACK
> +// back.  Exercise both edges of that window in a single connection.
> +
> +`./defaults.sh
> +sysctl -q net.ipv4.tcp_invalid_ratelimit=0
> +`
> +
> +   0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
> +  +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
> +  +0 bind(3, ..., ...) = 0
> +  +0 listen(3, 1) = 0
> +
> +// Three-way handshake.  Peer advertises rwnd = 1000 (no wscale), so
> +// MAX.SND.WND is tracked as 1000.
> +  +0 < S 0:0(0) win 1000 <mss 1000,sackOK,nop,nop,nop,wscale 0>
> +  +0 > S. 0:0(0) ack 1 <...>
> ++.1 < . 1:1(0) ack 1 win 1000
> +  +0 accept(3, ..., ...) = 4
> +
> +// ---- Upper edge: SEG.ACK > SND.NXT --------------------------------
> +// Server has sent nothing yet, so SND.UNA = SND.NXT = 1.
> +// Peer sends a pure ACK with SEG.ACK = 2, beyond SND.NXT.
> +  +0 < . 1:1(0) ack 2 win 1000
> +// Expect a challenge ACK: <SEQ = SND.NXT = 1, ACK = RCV.NXT = 1>.
> +  +0 > . 1:1(0) ack 1
> +
> +// Advance SND.UNA past MAX.SND.WND so that the lower edge becomes
> +// reachable.  Write 2000 bytes; the peer's rwnd of 1000 forces two
> +// 1000-byte segments, each acknowledged in turn.
> +  +0 write(4, ..., 2000) = 2000
> +  +0 > P. 1:1001(1000) ack 1
> ++.01 < . 1:1(0) ack 1001 win 1000
> +  +0 > P. 1001:2001(1000) ack 1
> ++.01 < . 1:1(0) ack 2001 win 1000
> +// Now SND.UNA = SND.NXT = 2001, MAX.SND.WND = 1000, bytes_acked = 2000.
> +
> +// ---- Lower edge: SEG.ACK < SND.UNA - MAX.SND.WND ------------------
> +// SND.UNA - MAX.SND.WND = 2001 - 1000 = 1001, so SEG.ACK = 1000 falls
> +// below the acceptable range.
> +  +0 < . 1:1(0) ack 1000 win 1000
> +// Expect a challenge ACK: <SEQ = SND.NXT = 2001, ACK = RCV.NXT = 1>.
> +  +0 > . 2001:2001(0) ack 1

https://sashiko.dev/#/patchset/20260421014128.289362-1-jiayuan.chen%40linux.dev

I don't think the AI's concern holds, but I think I can split 
write(2000) into write(1000)
so the test doesn't rely on kernel internals.