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

From: Jiayuan Chen <[email-masked]>
Date: Tue, 21 Apr 2026 09:41:01 +0800

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
-- 
2.43.0