Re: [PATCH net-next] net: advertise TCP MSS from the configured MTU, not the learned PMTU

[Eric Dumazet]

On Fri, Aug 14, 2026 at 8:36 AM Jiayuan Chen <jiayuan.chen@xxxxxxxxx> wrote:
>
> The MSS a host puts in its SYN tells the peer how big a segment it may
> send us. Right now we can shrink it with a PMTU we learned on our own
> send path, which is the wrong direction entirely.
>
> On asymmetric paths this bites - think DSR load balancers, where the
> request side goes through a smaller-MTU overlay. We learn a small PMTU
> going out, then advertise a small MSS, and the peer stays capped for the
> whole connection even though its path back to us is wide. MSS only shows
> up in the SYN and never grows back.
>
> On symmetric paths we lose nothing by dropping it either: the peer runs
> its own PMTU discovery and usually already knows the real path MTU.
>
> So work out the advertised MSS from the configured route or device MTU
> and ignore the learned PMTU. Our send side is unchanged, still clamped by
> tcp_current_mss(). Add ip_dst_mtu_configured()/ip6_dst_mtu_configured()
> and use them from the two default_advmss() paths.
>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxx>


LGTM, but this probably needs Fixes: tags and should be sent to net tree.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Fixes: 164a5e7ad531 ("ipv4: ipv4_default_advmss() should use route mtu")
Cc: stable@xxxxxxxxxxxxxxx

I added the following packetdrill test, please add it in a series.

commit f0c619ca446b669c48b5c2e2c78e23af1a0850b6
Author: Eric Dumazet <edumazet@xxxxxxxxxx>
Date:   Fri Aug 14 08:12:32 2026 +0000

    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@xxxxxxxxxx>

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 0000000000000000000000000000000000000000..f2ef931b77a1c50e77b7568c7f03d10002be4152
--- /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 0000000000000000000000000000000000000000..c7638b11a815b20f70c08a894fed9a65f12327f4
--- /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