Fragmentation and UDP/RAW

Intermediate routers cannot fragment packets and UDP/RAW transports do not perform retransmission. To attempt to ensure that a UDP/RAW packet is not dropped because of fragmentation, one of the following conditions can occur:

z/OS® Communications Server always sends the packet using the minimum MTU (1280) unless the MTU for the destination is learned from an ICMPv6 Packet Too Big message.
An application sends a packet using the IPV6_DONTFRAG socket option.

For example, a situation can occur where the MTU was learned by way of Path MTU discovery. In that case, the network topology changes, reducing the MTU to this particular destination. UDP/RAW sends with the original learned MTU and receives a Packet Too Big message. In this case, the packet is dropped, but subsequent sends learn the changed MTU and send with the appropriate size.