Hi Maxim,

>
> This may indicate different network paths, with
> some of them
> filtering ICMP frag-needed packets, and others
> don't (or at least
> doing MSS clamp). Alternatively, blackhole
> detection might come
> and magically fix things.
>

Indeed, blackhole detection solved my issue. Thanks for your expertise !

# echo 2 > /proc/sys/net/ipv4/tcp_mtu_probing

and also in sysctl.conf.

Now, TCP packets have a size of 564 octets (probably due to tcp_base_mss default value), which may not be optimal but is ok for now.

Bruno