Thank you. I am trying to summarize it

core option pmtu_discovery for IPv4

exists a long time
sets socket option IP_PMTUDISC_DO
sets do not fragment bit in outgoing packets
kernel will implement PMTU discovery, initial UDP packets might be dropped while the kernel tries to figure out the MTU
if MTU is found, UDP will be delivered and kernel will store the discovered MTU internally
further packets will use this value
this is probably not really efficient, but probably works

core option pmtu_discovery for IPv6

new introduced in this PR
set socket option IP_PMTUDISC_DO
will not set do not fragment bit in outgoing packets
the kernel will refuse larger packets with EMSGSIZE
no discovery will take place, Kamailio will probably just stop sending the packet

suggestion in this PR for IPv6

use IPV6_PMTUDISC_WANT as socket option
will fragment a datagram if needed according to the path MTU for IPv6
probably could be set by adding a new core option

suggestion in this PR for IPv4

use IP_PMTUDISC_WANT as socket option
will fragment a datagram if needed according to the path MTU, or will set the don't-fragment flag otherwise
probably could be set by adding a new core option

Additionally Kamailio provide options to manually manage the MTU with usual sockets

core setting udp_mtu and udp_mtu_try_proto
will try another protocol if MTU exceeded internally

Additionally Kamailio provide options to manually manage the MTU with RAW sockets

core option udp4_raw_mtu
will fragment the packets internally

My suggestion would be add a new value 2 to the pmtu_discovery core parameter, which sets then the _WANT socket option for IPv4 and IPv6.

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