selftests/net: packetdrill: add tcp_rcv_wnd_shrink_allowed.pkt

This test verifies the sequence number checks using the maximum
advertised window sequence number when net.ipv4.tcp_shrink_window
is enabled.

Signed-off-by: Simon Baatz <gmbnomis@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260309-tcp_rfc7323_retract_wnd_rfc-v3-5-4c7f96b1ec69@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/packetdrill/tcp_rcv_wnd_shrink_allowed.pkt

@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: GPL-2.0
+
+--mss=1000
+
+`./defaults.sh
+sysctl -q net.ipv4.tcp_shrink_window=1
+sysctl -q net.ipv4.tcp_rmem="4096 32768 $((32*1024*1024))"`
+
+   0 `nstat -n`
+
+// Establish a connection.
+  +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
+
+  +0 < S 0:0(0) win 32792 <mss 1000,nop,wscale 7>
+  +0 > S. 0:0(0) ack 1 <mss 1460,nop,wscale 10>
+  +0 < . 1:1(0) ack 1 win 257
+
+  +0 accept(3, ..., ...) = 4
+
+  +0 < P. 1:10001(10000) ack 1 win 257
+   * > .  1:1(0) ack 10001 win 15
+
+  +0 < P. 10001:11024(1023) ack 1 win 257
+   * > .  1:1(0) ack 11024 win 13
+
+// Max window seq advertised 10001 + 15*1024 = 25361, last advertised: 11024 + 13*1024 = 24336
+
+// Segment beyond the max window is dropped
+  +0 < P. 11024:25362(14338) ack 1 win 257
+   * > .  1:1(0) ack 11024 win 13
+
+// Segment using the max window is accepted
+  +0 < P. 11024:25361(14337) ack 1 win 257
+   * > .  1:1(0) ack 25361 win 0
+
+// Check LINUX_MIB_BEYOND_WINDOW has been incremented once
+  +0 `nstat | grep TcpExtBeyondWindow | grep -q " 1 "`