I have asked one question already related what is the purpose of payload in Packet Too Big ICMPv6 message. why_PTB_payload
As per the latest IPv6 logo certification I have came across this test case where a sender send ICMPV6 echo request message to destination having some routers in between and the receiver (destination) received the echo request. then,
Receiver send ICMPv6 echo reply but got ICMPV6 packet too big message from router in between with wrong/forged echo reply header appending to the (PTB) packet too big message as a payload. (Deliberately sending wrong payload).
Again the ICMPv6 echo request sent by the sender, now the receiver start fragmenting the echo reply because of the above step1 (i.e without validating the PTB payload the Receiver changed it's MTU value).
According to the test case we should not change the MTU on receiving the wrong or forged payload on PTB message (payload will be the original packet that cannot be forwarded due to less Path MTU)
But this look unnecessary work to validate the PTB for echo reply and seems no proper use-case, this test case look invalid to me as we never store the state of echo reply sent by the kernel to validate the PTB in case PTB carry the wrong ICMPv6 reply header on it's payload.
If this is the valid case then I want to know the logic how we can implement this or if not then why this test case is even there at the IPV6 logo certification.
Link to the document containing the test case IPv6LogoCertificationTestCases (Test case number v6LC.4.1.12)