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AWS Certified Advanced Networking - Specialty · AWS
Select 2 answers.
This question asks for 2 answers, but our data marks only 1. Compare with the community discussion below for the full answer.
Written by other learners. Comments can disagree with the marked answer — check the official documentation when in doubt.
Path MTU discovery (PMTUD) relies on ICMP “fragmentation needed” messages to inform the sender to reduce packet size. If the EC2 security group (or any firewall on the path) blocks ICMP, those ICMP messages never reach the sender and PMTUD cannot work. With DF set, the sender will keep sending large packets that get dropped, causing intermittent file-transfer failures.
That could be a valid point if the issue was persistent.
The question isn't about packets dropping but about file transfer failure. Client requests file, server sends jumbo back towards client and it gets dropped but with ICMP fragmentation required but DF bit set. If ICMP isn allowed to the servers they'll adjust MTU and send non-jumbo allowing the file transfer to succeed so D is valid. The question says "source IP addresses are within a VPC, on the internet, or on premises." There is no mention of VPN in the question so I think this must be the trick.