Minimum MTU verification for IPv6

[LionelJouin]

The minimum MTU allowed with IPv6 is 1280. If the MTU is set to a lower value, issues will happen with the connection, no error will be returned to the NSC and this error will be in the forwarder logs:

failed to set /proc/sys/net/ipv6/conf/proxy.load-fe75/disable_ipv6 = 0: open /proc/sys/net/ipv6/conf/proxy.load-fe75/disable_ipv6: no such file or directory

Raised here:

sdk-kernel/pkg/kernel/networkservice/connectioncontextkernel/ipcontext/ipaddress/common.go

Line 91 in 0125eaf

return errors.Wrapf(err, "failed to set %s = 0", disableIPv6Filename)

I think there should be some checks somewhere in the chain to verify the connection this MTU property is valid.

[zolug]

It seems when requesting a lower than 1280 bytes mtu, the network stack "realizes" that the interface can't support ipv6.
So the interface won't be included under /proc/sys/net/ipv6/conf/, therefore disable_ipv6 fails (https://github.com/networkservicemesh/sdk-kernel/blob/release/v1.3.1/pkg/kernel/networkservice/connectioncontextkernel/ipcontext/ipaddress/common.go#L88)
If using a lower mtu should be supported, then probably the code would need to check if the interface was already running "without ipv6".