[sr-dev] [kamailio/kamailio] Path MTU discovery for IPv6 (PR #3141)

[vanrein]

@sergey-safarov You did not say how it "did not work as expected".

With what I've learnt, I was a bit surprised about the IPv4 original code, which says

```
optval= (pmtu_discovery) ? IP_PMTUDISC_DO : IP_PMTUDISC_DONT;
```

WIth `ptmu_discover != 0` this code sets the DF flag on all IPv4 traffic, and makes it behave like in IPv6 where this is implied.  But I am not so sure if it picks up any ICMP messages *Fragmentation needed and DF set* are processed.  An unconnected socket has no way of using that.

Had it been `IP_PMTUDISC_WANT`, like I used for IPv6, then at least any such messages would have been learnt by the OS for the route and influence the resend.

I did not want to change the existing code, but given that you do not experience a benefit, this might make sense.  It is much less clear than in the case for IPv6, though, so I am uncertain.

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