1*e5c8e301SEric Dumazet// SPDX-License-Identifier: GPL-2.0 2*e5c8e301SEric Dumazet// 3*e5c8e301SEric Dumazet// Test that IPv4 advertised MSS in SYN-ACK is derived from the configured 4*e5c8e301SEric Dumazet// interface MTU (1500 -> MSS 1460), not the ICMP-learned Path MTU. 5*e5c8e301SEric Dumazet 6*e5c8e301SEric Dumazet--ip_version=ipv4 7*e5c8e301SEric Dumazet 8*e5c8e301SEric Dumazet`./defaults.sh 9*e5c8e301SEric Dumazetethtool -K tun0 tso off 10*e5c8e301SEric Dumazet` 11*e5c8e301SEric Dumazet 12*e5c8e301SEric Dumazet// 13*e5c8e301SEric Dumazet// Connection 1: Learn PMTU exception (MTU 1200 -> MSS 1160) 14*e5c8e301SEric Dumazet// 15*e5c8e301SEric Dumazet 0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3 16*e5c8e301SEric Dumazet +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0 17*e5c8e301SEric Dumazet +0 bind(3, ..., ...) = 0 18*e5c8e301SEric Dumazet +0 listen(3, 1) = 0 19*e5c8e301SEric Dumazet 20*e5c8e301SEric Dumazet +0 < S 0:0(0) win 65535 <mss 1460,sackOK,nop,nop,nop,wscale 8> 21*e5c8e301SEric Dumazet +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8> 22*e5c8e301SEric Dumazet +.1 < . 1:1(0) ack 1 win 257 23*e5c8e301SEric Dumazet +0 accept(3, ..., ...) = 4 24*e5c8e301SEric Dumazet 25*e5c8e301SEric Dumazet// Send a full 1460-byte segment 26*e5c8e301SEric Dumazet +0 write(4, ..., 1460) = 1460 27*e5c8e301SEric Dumazet +0 > P. 1:1461(1460) ack 1 28*e5c8e301SEric Dumazet 29*e5c8e301SEric Dumazet// ICMP Fragmentation Needed arrives indicating next-hop MTU 1200 30*e5c8e301SEric Dumazet +0 < icmp unreachable frag_needed mtu 1200 [1:1461(1460)] 31*e5c8e301SEric Dumazet 32*e5c8e301SEric Dumazet// Local host retransmits using the learned MTU 1200 (MSS = 1200 - 40 = 1160) 33*e5c8e301SEric Dumazet +0 > . 1:1161(1160) ack 1 34*e5c8e301SEric Dumazet +0 > P. 1161:1461(300) ack 1 35*e5c8e301SEric Dumazet +0 < R 1:1(0) ack 1461 win 0 36*e5c8e301SEric Dumazet 37*e5c8e301SEric Dumazet// Close connection 1 and listener 38*e5c8e301SEric Dumazet +0 close(4) = 0 39*e5c8e301SEric Dumazet +0 close(3) = 0 40*e5c8e301SEric Dumazet 41*e5c8e301SEric Dumazet// 42*e5c8e301SEric Dumazet// Connection 2: New connection from the same peer 43*e5c8e301SEric Dumazet// 44*e5c8e301SEric Dumazet +0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3 45*e5c8e301SEric Dumazet +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0 46*e5c8e301SEric Dumazet +0 bind(3, ..., ...) = 0 47*e5c8e301SEric Dumazet +0 listen(3, 1) = 0 48*e5c8e301SEric Dumazet 49*e5c8e301SEric Dumazet +0 < S 0:0(0) win 65535 <mss 1460,sackOK,nop,nop,nop,wscale 8> 50*e5c8e301SEric Dumazet 51*e5c8e301SEric Dumazet// Verify: SYN-ACK MUST advertise configured MSS 1460, NOT the learned PMTU MSS 1160 52*e5c8e301SEric Dumazet +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8> 53*e5c8e301SEric Dumazet +0 < . 1:1(0) ack 1 win 257 54*e5c8e301SEric Dumazet +0 accept(3, ..., ...) = 4 55*e5c8e301SEric Dumazet 56*e5c8e301SEric Dumazet// Verify: Outgoing transmit MSS is still constrained by the learned PMTU 1200 57*e5c8e301SEric Dumazet +0 write(4, ..., 1460) = 1460 58*e5c8e301SEric Dumazet +0 > . 1:1161(1160) ack 1 59*e5c8e301SEric Dumazet +0 > P. 1161:1461(300) ack 1 60*e5c8e301SEric Dumazet +0 < . 1:1(0) ack 1461 win 257 61*e5c8e301SEric Dumazet 62*e5c8e301SEric Dumazet// Clean up 63*e5c8e301SEric Dumazet +0 close(4) = 0 64*e5c8e301SEric Dumazet +0 > F. 1461:1461(0) ack 1 65*e5c8e301SEric Dumazet +0 < F. 1:1(0) ack 1462 win 257 66*e5c8e301SEric Dumazet +0 > . 1462:1462(0) ack 2 67*e5c8e301SEric Dumazet +0 close(3) = 0 68