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

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

On 8/14/26 5:21 PM, Eric Dumazet wrote:
> On Fri, Aug 14, 2026 at 8:36 AM Jiayuan Chen <jiayuan.chen@linux.dev> 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@linux.dev>
>
> 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@vger.kernel.org
>
> I added the following packetdrill test, please add it in a series.
>
> commit f0c619ca446b669c48b5c2e2c78e23af1a0850b6
> Author: Eric Dumazet <edumazet@google.com>


Thanks Eric, I will do it after cold time.