1. Huge topic, nobody likes it, but difficult to avoid this (I am facing it for last 26 years of my life).

    Maybe you will find it interesting:

    - Multicasts and PMTUD - not sure if anything has changed since I last touched the topic (the big multicast packet is silently dropped w/o sending back any ICMP message and it was as RFC stated) and we need to fragment the multicast packets

    -if fragmentation is required on the tunnel we can either fragment inner (before encapsulation) or outer packets (encapsulated)- consequences are different.
    When inner packet is fragmented the endpoint can reassemble the packets; when outer packet is fragmented the other side of the tunnel is reponsible for it as we need to reassemble packet before de-encapsulating it

    - when fragmenting multicast packets - can the endpoint host reassemble them? Always?

    - implementation matters and all the kludges vendor have for this

    - etc
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