[PATCH net v2 2/2] selftests: net: packetdrill: add tests for advertised MSS with PMTU exceptions
From: Jiayuan Chen <jiayuan.chen@linux.dev>
Date: 2026-08-15 07:16:00
Also in:
linux-kselftest, lkml
Subsystem:
kernel selftest framework, networking [general], the rest · Maintainers:
Shuah Khan, Shuah Khan, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds
From: Eric Dumazet <edumazet@google.com> Add packetdrill tests for IPv4 and IPv6 to verify that the advertised MSS in SYN-ACK is derived from the configured interface/route MTU, and is not shrunk by learned Path MTU exceptions from previous outbound connections. Signed-off-by: Eric Dumazet <edumazet@google.com> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev> --- .../net/packetdrill/tcp_advmss_pmtu_ipv4.pkt | 67 +++++++++++++++++++ .../net/packetdrill/tcp_advmss_pmtu_ipv6.pkt | 67 +++++++++++++++++++ 2 files changed, 134 insertions(+) create mode 100644 tools/testing/selftests/net/packetdrill/tcp_advmss_pmtu_ipv4.pkt create mode 100644 tools/testing/selftests/net/packetdrill/tcp_advmss_pmtu_ipv6.pkt
diff --git a/tools/testing/selftests/net/packetdrill/tcp_advmss_pmtu_ipv4.pkt b/tools/testing/selftests/net/packetdrill/tcp_advmss_pmtu_ipv4.pkt
new file mode 100644
index 000000000000..f2ef931b77a1
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_advmss_pmtu_ipv4.pkt@@ -0,0 +1,67 @@ +// SPDX-License-Identifier: GPL-2.0 +// +// Test that IPv4 advertised MSS in SYN-ACK is derived from the configured +// interface MTU (1500 -> MSS 1460), not the ICMP-learned Path MTU. + +--ip_version=ipv4 + +`./defaults.sh +ethtool -K tun0 tso off +` + +// +// Connection 1: Learn PMTU exception (MTU 1200 -> MSS 1160) +// + 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 65535 <mss 1460,sackOK,nop,nop,nop,wscale 8> + +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8> + +.1 < . 1:1(0) ack 1 win 257 + +0 accept(3, ..., ...) = 4 + +// Send a full 1460-byte segment + +0 write(4, ..., 1460) = 1460 + +0 > P. 1:1461(1460) ack 1 + +// ICMP Fragmentation Needed arrives indicating next-hop MTU 1200 + +0 < icmp unreachable frag_needed mtu 1200 [1:1461(1460)] + +// Local host retransmits using the learned MTU 1200 (MSS = 1200 - 40 = 1160) + +0 > . 1:1161(1160) ack 1 + +0 > P. 1161:1461(300) ack 1 + +0 < R 1:1(0) ack 1461 win 0 + +// Close connection 1 and listener + +0 close(4) = 0 + +0 close(3) = 0 + +// +// Connection 2: New connection from the same peer +// + +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 65535 <mss 1460,sackOK,nop,nop,nop,wscale 8> + +// Verify: SYN-ACK MUST advertise configured MSS 1460, NOT the learned PMTU MSS 1160 + +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8> + +0 < . 1:1(0) ack 1 win 257 + +0 accept(3, ..., ...) = 4 + +// Verify: Outgoing transmit MSS is still constrained by the learned PMTU 1200 + +0 write(4, ..., 1460) = 1460 + +0 > . 1:1161(1160) ack 1 + +0 > P. 1161:1461(300) ack 1 + +0 < . 1:1(0) ack 1461 win 257 + +// Clean up + +0 close(4) = 0 + +0 > F. 1461:1461(0) ack 1 + +0 < F. 1:1(0) ack 1462 win 257 + +0 > . 1462:1462(0) ack 2 + +0 close(3) = 0
diff --git a/tools/testing/selftests/net/packetdrill/tcp_advmss_pmtu_ipv6.pkt b/tools/testing/selftests/net/packetdrill/tcp_advmss_pmtu_ipv6.pkt
new file mode 100644
index 000000000000..c7638b11a815
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_advmss_pmtu_ipv6.pkt@@ -0,0 +1,67 @@ +// SPDX-License-Identifier: GPL-2.0 +// +// Test that IPv6 advertised MSS in SYN-ACK is derived from the configured +// interface MTU (1520 -> MSS 1460), not the ICMPv6-learned Path MTU. + +--ip_version=ipv6 + +`./defaults.sh +ethtool -K tun0 tso off +` + +// +// Connection 1: Learn PMTU exception (MTU 1280 -> MSS 1220) +// + 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 65535 <mss 1460,sackOK,nop,nop,nop,wscale 8> + +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8> + +.1 < . 1:1(0) ack 1 win 257 + +0 accept(3, ..., ...) = 4 + +// Send a full 1460-byte segment + +0 write(4, ..., 1460) = 1460 + +0 > P. 1:1461(1460) ack 1 + +// ICMPv6 Packet Too Big arrives indicating next-hop MTU 1280 + +0 < icmp packet_too_big mtu 1280 [1:1461(1460)] + +// Local host retransmits using the learned MTU 1280 (MSS = 1280 - 40 - 20 = 1220) + +0 > . 1:1221(1220) ack 1 + +0 > P. 1221:1461(240) ack 1 + +0 < R 1:1(0) ack 1461 win 0 + +// Close connection 1 and listener + +0 close(4) = 0 + +0 close(3) = 0 + +// +// Connection 2: New connection from the same peer +// + +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 65535 <mss 1460,sackOK,nop,nop,nop,wscale 8> + +// Verify: SYN-ACK MUST advertise configured MSS 1460, NOT the learned PMTU MSS 1220 + +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8> + +0 < . 1:1(0) ack 1 win 257 + +0 accept(3, ..., ...) = 4 + +// Verify: Outgoing transmit MSS is still constrained by the learned PMTU 1280 + +0 write(4, ..., 1460) = 1460 + +0 > . 1:1221(1220) ack 1 + +0 > P. 1221:1461(240) ack 1 + +0 < . 1:1(0) ack 1461 win 257 + +// Clean up + +0 close(4) = 0 + +0 > F. 1461:1461(0) ack 1 + +0 < F. 1:1(0) ack 1462 win 257 + +0 > . 1462:1462(0) ack 2 + +0 close(3) = 0
--
2.43.0