Re: [PATCH net 1/2] Revert "selftests: net: packetdrill: add tests for advertised MSS with PMTU exceptions"

[Jiayuan Chen <jiayuan.chen@linux.dev>]

On 10/26/26 3:23 PM, Esko Mobius via B4 Relay wrote:
> From: Esko Mobius <echoowall@gmail.com>
>
> This reverts commit e5c8e301b497 ("selftests: net: packetdrill: add
> tests for advertised MSS with PMTU exceptions").
>
> These tests assert the behaviour introduced by commit 2640e6419594
> ("net: advertise TCP MSS from the configured MTU, not the learned
> PMTU"), which is reverted in the next patch because it causes a
> regression. Remove them first so the series stays bisectable.
>
> Signed-off-by: Esko Mobius <echoowall@gmail.com>


Reviewed-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 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
> deleted file mode 100644
> index f2ef931b7..000000000
> --- a/tools/testing/selftests/net/packetdrill/tcp_advmss_pmtu_ipv4.pkt
> +++ /dev/null
> @@ -1,67 +0,0 @@
> -// 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
> deleted file mode 100644
> index c7638b11a..000000000
> --- a/tools/testing/selftests/net/packetdrill/tcp_advmss_pmtu_ipv6.pkt
> +++ /dev/null
> @@ -1,67 +0,0 @@
> -// 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
>