1// SPDX-License-Identifier: GPL-2.0 2// 3// RFC 5961 Section 5.2 / RFC 793 Section 3.9: an incoming segment's 4// ACK value must lie in [SND.UNA - MAX.SND.WND, SND.NXT]; otherwise 5// the receiver MUST discard the segment and send a challenge ACK 6// back. Exercise both edges of that window in a single connection. 7 8`./defaults.sh 9sysctl -q net.ipv4.tcp_invalid_ratelimit=0 10` 11 12 0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3 13 +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0 14 +0 bind(3, ..., ...) = 0 15 +0 listen(3, 1) = 0 16 17// Three-way handshake. Peer advertises rwnd = 1000 (no wscale), so 18// MAX.SND.WND is tracked as 1000. 19 +0 < S 0:0(0) win 1000 <mss 1000,sackOK,nop,nop,nop,wscale 0> 20 +0 > S. 0:0(0) ack 1 <...> 21+.1 < . 1:1(0) ack 1 win 1000 22 +0 accept(3, ..., ...) = 4 23 24// ---- Upper edge: SEG.ACK > SND.NXT -------------------------------- 25// Server has sent nothing yet, so SND.UNA = SND.NXT = 1. 26// Peer sends a pure ACK with SEG.ACK = 2, beyond SND.NXT. 27 +0 < . 1:1(0) ack 2 win 1000 28// Expect a challenge ACK: <SEQ = SND.NXT = 1, ACK = RCV.NXT = 1>. 29 +0 > . 1:1(0) ack 1 30 31// Advance SND.UNA past MAX.SND.WND so that the lower edge becomes 32// reachable. Issue two 1-MSS writes so each skb is exactly one MSS 33// and PSH is set by tcp_push() at the end of each sendmsg, keeping 34// the setup independent of the TSO / tcp_fragment split path. 35 +0 write(4, ..., 1000) = 1000 36 +0 > P. 1:1001(1000) ack 1 37+.01 < . 1:1(0) ack 1001 win 1000 38 +0 write(4, ..., 1000) = 1000 39 +0 > P. 1001:2001(1000) ack 1 40+.01 < . 1:1(0) ack 2001 win 1000 41// Now SND.UNA = SND.NXT = 2001, MAX.SND.WND = 1000, bytes_acked = 2000. 42 43// ---- Lower edge: SEG.ACK < SND.UNA - MAX.SND.WND ------------------ 44// SND.UNA - MAX.SND.WND = 2001 - 1000 = 1001, so SEG.ACK = 1000 falls 45// below the acceptable range. 46 +0 < . 1:1(0) ack 1000 win 1000 47// Expect a challenge ACK: <SEQ = SND.NXT = 2001, ACK = RCV.NXT = 1>. 48 +0 > . 2001:2001(0) ack 1 49