From: Jiayuan Chen Date: Wed, 22 Apr 2026 12:35:39 +0000 (+0800) Subject: selftests/net: packetdrill: cover RFC 5961 5.2 challenge ACK on both edges X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=cf94b3c0f052c2674328b330309604af2dedd3a0;p=thirdparty%2Fkernel%2Fstable.git selftests/net: packetdrill: cover RFC 5961 5.2 challenge ACK on both edges 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 . 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 Reviewed-by: Eric Dumazet Link: https://patch.msgid.link/20260422123605.320000-3-jiayuan.chen@linux.dev Signed-off-by: Jakub Kicinski --- 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..2776b8728085 --- /dev/null +++ b/tools/testing/selftests/net/packetdrill/tcp_rfc5961_ack-out-of-window.pkt @@ -0,0 +1,48 @@ +// 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 + +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: . + +0 > . 1:1(0) ack 1 + +// Advance SND.UNA past MAX.SND.WND so that the lower edge becomes +// reachable. Issue two 1-MSS writes so each skb is exactly one MSS +// and PSH is set by tcp_push() at the end of each sendmsg, keeping +// the setup independent of the TSO / tcp_fragment split path. + +0 write(4, ..., 1000) = 1000 + +0 > P. 1:1001(1000) ack 1 ++.01 < . 1:1(0) ack 1001 win 1000 + +0 write(4, ..., 1000) = 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: . + +0 > . 2001:2001(0) ack 1