According to it's definition:
Announce to TCP sessions running over the tunnel that they should limit
their send packet sizes such that after OpenVPN has encapsulated them,
the resulting UDP packet size that OpenVPN sends to its peer will not
exceed maxbytes. The default value is 1450.The max parameter is
interpreted in the same way as the –link-mtu parameter, i.e. the UDP
packet size after encapsulation overhead has been added in, but not
including the UDP header itself. Resulting packet would be at most 28
bytes larger for IPv4 and 48 bytes for IPv6 (20/40 bytes for IP header
and 8 bytes for UDP header). Default value of 1450 allows IPv4 packets
to be transmitted over a link with MTU 1473 or higher without IP level
fragmentation.
So if the default value is 1450 and the ipv4 header is + 28 = 1478
and the guide continues with "allows IPv4 packets to be transmitted over
a link with MTU 1473 or higher"
Is this a typo?
I have a setup where the OpenVPN server is behind a link with MTU 1350.
The VPN uses UDP.
What can be the maximal mssfix size then, 1322 bytes?
Also would you care explaining what is the advantage of setting the
MSSFIX over setting the tunnel MTU to the highest possible value?
The problems are that OpenVPN does not force you to do anything with the
MTU, it even defaults to 1500bytes like it would be ethernet but when
sending oversized packets inside the tunnel they will just get lost and
you have to play around with this until apps start behaving correctly.
Thanks
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