MTU: Match a Jumbo-Frame Segment

Backups to the storage array start fine and then hang. The storage segment runs jumbo frames, and this host's end of it was never configured for them. Small packets fit and get through, but the array's full-size frames are dropped. The array lives in a network namespace on this box, so you can test from its side.

/root/case/assignment.env names the storage interface, the segment's MTU and the addresses (all differ per session; /root/case/brief.txt has the details). On the live system:

1. Set the storage interface's MTU to exactly the segment MTU. Leave every other interface alone, especially the uplink.

2. Prove a full-size packet crosses without fragmentation, from the array's side: ip netns exec <namespace> ping -c1 -M do -s <payload> <host address>.

3. Write the largest ICMP payload (in bytes) that fits in one unfragmented IPv4 packet at that MTU to the file /root/case/max_payload.txt, as one number. The packet also carries an IPv4 header and an ICMP header.

Grading reads the live MTUs and sends that don't-fragment ping itself. Nothing needs to survive a reboot.