Re: [PATCH net 0/2] Revert advertising the TCP MSS from the configured MTU

[Eric Dumazet]

Le lun. 26 oct. 2026 à 08:23, Esko Mobius via B4 Relay
<devnull+echoowall.gmail.com@xxxxxxxxxx> a écrit :
>
> Commit 2640e6419594 ("net: advertise TCP MSS from the configured MTU,
> not the learned PMTU") breaks TCP for clients behind networks that
> filter inbound ICMP, when the server reaches them through a hop with a
> smaller MTU: the handshake completes, then the connection stalls as
> soon as the client sends full-size segments. This was reported in [1]
> and confirmed on 7.3-rc7; reverting the commit on top of 7.3-rc7 fixes
> it. Details and numbers are in patch 2.
>
> The asymmetric (DSR-style) case that commit targets can still be served
> by the existing per-route advmss metric, which overrides the default
> after the revert; patch 2 includes a test of that as well. If you would
> rather keep the new behaviour as an explicit opt-in some other way, I
> am happy to test or rework this.
>
> Patch 1 removes the packetdrill tests that assert the reverted
> behaviour, patch 2 is the revert itself.
>
> [1] https://lore.kernel.org/regressions/4F0FAEB6-A13B-47F3-BFF1-C706686C6F64@xxxxxxxxx/

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>

Even if the reverted behavior was closer to RFC 2923 / RFC 1122, it
was a long-standing
user visible behavior and should not have been tagged for stable.

Relays should really clamp MSS (nft "tcp option maxseg size set rt mtu"),
but a revert is the right call for net and stable trees.

If needed, the asymmetric/DSR case can be revisited in net-next as an opt-in.