| // 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. 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: <SEQ = SND.NXT = 2001, ACK = RCV.NXT = 1>. |
| +0 > . 2001:2001(0) ack 1 |
