diff options
| author | Eric Dumazet <edumazet@google.com> | 2026-08-15 15:15:08 +0800 |
|---|---|---|
| committer | Jakub Kicinski <kuba@kernel.org> | 2026-08-20 12:14:23 -0700 |
| commit | e5c8e301b497930e25cc4085fbc7ea2223b37030 (patch) | |
| tree | 034830550e0b38e77fad3faae3ddf0545666ad22 /tools/testing | |
| parent | 2640e64195948a601430d230c9864f5426574cde (diff) | |
selftests: net: packetdrill: add tests for advertised MSS with PMTU exceptions
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>
Link: https://patch.msgid.link/20260815071532.301908-1-jiayuan.chen@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Diffstat (limited to 'tools/testing')
| -rw-r--r-- | tools/testing/selftests/net/packetdrill/tcp_advmss_pmtu_ipv4.pkt | 67 | ||||
| -rw-r--r-- | tools/testing/selftests/net/packetdrill/tcp_advmss_pmtu_ipv6.pkt | 67 |
2 files changed, 134 insertions, 0 deletions
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 |