We didn’t have a need to clamp the TCP Maximum Segment Size (MSS) up until now,
because fiber7 uses an MTU of 1500.
Because Path MTU discovery is often broken on the internet, it’s best practice
to limit the Maximum Segment Size (MSS) of each TCP connection, achieving the
same effect (but only for TCP connections).
This change is beneficial when running router7 behind a non-ethernet uplink,
such as a Fritz!Box cable modem.
This has no adverse effect on fiber7: after clamping, the MSS is still 1440, as
without clamping.
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