[PATCH net-next v3 4/4] selftests/net: packetdrill: add TCP MTU probing test

[Yongjian Wang via B4 Relay <devnull+echoowall.gmail.com@kernel.org>]

From: Yongjian Wang <echoowall@gmail.com>

Packetization layer path MTU discovery (net.ipv4.tcp_mtu_probing=2) has
no selftest coverage.

Add a test for an active connection. Check that it starts at
tcp_base_mss, that it sends the first probe half way between
tcp_base_mss and the peer's MSS once cwnd allows it, and that an
acknowledged probe raises the MSS and leads to a larger probe.

The SYN offers timestamps and the scripted SYN-ACK declines them.
This also checks that MTU probing uses the negotiated header length
for the initial send MSS.

Signed-off-by: Yongjian Wang <echoowall@gmail.com>
---
 .../net/packetdrill/tcp_mtu_probing_search.pkt     | 42 ++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/tools/testing/selftests/net/packetdrill/tcp_mtu_probing_search.pkt b/tools/testing/selftests/net/packetdrill/tcp_mtu_probing_search.pkt
new file mode 100644
index 000000000000..a48acff8685e
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_mtu_probing_search.pkt
@@ -0,0 +1,42 @@
+// SPDX-License-Identifier: GPL-2.0
+// Test packetization layer path MTU discovery with tcp_mtu_probing=2: an
+// active connection starts at tcp_base_mss and raises its MSS by binary search
+// as probes are acknowledged.
+
+`./defaults.sh
+sysctl -q net.ipv4.tcp_mtu_probing=2
+sysctl -q net.ipv4.tcp_base_mss=1024`
+
+    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+   +0 fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0
+   +0 connect(3, ..., ...) = -1 EINPROGRESS (Operation is now in progress)
+   +0 > S 0:0(0) <...>
+  +.1 < S. 0:0(0) ack 1 win 65535 <mss 1460,nop,nop,sackOK>
+   +0 > . 1:1(0) ack 1
+   +0 fcntl(3, F_SETFL, O_RDWR) = 0
+
+// The initial window is sent with tcp_base_mss. No probe yet, cwnd is below 11.
+   +0 write(3, ..., 40000) = 40000
+   +0 > P. 1:2049(2048) ack 1
+   +0 > P. 2049:4097(2048) ack 1
+   +0 > P. 4097:6145(2048) ack 1
+   +0 > P. 6145:8193(2048) ack 1
+   +0 > P. 8193:10241(2048) ack 1
+
+// The first probe is half way between tcp_base_mss and the peer's MSS.
+  +.1 < . 1:1(0) ack 10241 win 65535
+   +0 > . 10241:11483(1242) ack 1
+   +0 > P. 11483:13531(2048) ack 1
+   +0 > P. 13531:15579(2048) ack 1
+   +0 > P. 15579:17627(2048) ack 1
+   +0 > P. 17627:19675(2048) ack 1
+   +0 > P. 19675:21723(2048) ack 1
+   +0 > P. 21723:23771(2048) ack 1
+   +0 > P. 23771:25819(2048) ack 1
+   +0 > P. 25819:27867(2048) ack 1
+   +0 > P. 27867:29915(2048) ack 1
+
+// Acknowledging the probe raises the MSS, and the next probe is larger.
+  +.1 < . 1:1(0) ack 29915 win 65535
+   +0 > . 29915:31266(1351) ack 1
+   +0 > P. 31266:33750(2484) ack 1

-- 
Git-157)