| // 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 |
