Small requests succeed but large transfers stall over a tunnel. How would you investigate MTU issues?

Tunneling adds headers and can reduce the effective path MTU. If oversized packets cannot be fragmented as needed and path-MTU feedback is blocked or mishandled, larger transfers may stall while small traffic works. Inspect packet captures, interface MTUs, retransmissions, and relevant ICMP messages from the affected path. Test appropriate packet sizes without assuming every network permits echo requests. Correct tunnel MTU or supported MSS handling based on evidence. Avoid globally lowering arbitrary values because that may mask the fault and reduce performance elsewhere.