Re: [PATCH net v2 1/2] net: advertise TCP MSS from the configured MTU, not the learned PMTU

On Sat, Aug 15, 2026 at 9:04 AM Jiayuan Chen <[email protected]> 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.
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Fixes: 164a5e7ad531 ("ipv4: ipv4_default_advmss() should use route mtu")
> Cc: [email protected]
> Signed-off-by: Jiayuan Chen <[email protected]>

Reviewed-by: Eric Dumazet <[email protected]>