1
Diagnostic step 1
Reproduce with packet captures: capture on client Wi‑Fi interface and on mesh node WAN interface simultaneously to confirm where ICMP Type 3 Code 4 messages are lost or dropped.
FULL AI ANALYSIS
Large HTTPS POSTs stall and timeout only on 5 GHz band after enabling mesh router packet-aggregation
Generated Oct 12, 2026, 10:57:58 PM
Analysis summary
Symptoms point to an MTU/PMTUD failure combined with wireless frame aggregation across mesh hops. The presence of A-MPDU aggregation and lost ICMP 'fragmentation needed' messages indicates that Path MTU Discovery is failing: large TCP segments are being sent but the 'needs-fragmentation' signals are blocked or dropped between the ISP gateway and the wireless client when traversing the mesh node's aggregation logic. Disabling aggregation fixes the problem, suggesting the mesh node or its firmware mishandles fragmented/ICMP messages or reorders frames in a way that prevents the client's TCP stack from reducing MSS. MSS clamping partially helped because it reduced the need for fragmentation.
Possible causes
Mesh node firmware bug in handling A-MPDU/A-MSDU aggregation that drops or fails to forward ICMP 'fragmentation needed' packets.
90%Incorrect MSS/MTU handling on mesh node when offloading or accelerating wireless frames across backhaul links.
55%Driver/firmware mismatch on client Wi‑Fi NIC that interacts poorly with aggregated frames leading to lost MTU signals.
25%Intermediate device (satellite or ISP gateway) blocking ICMP PMTUD messages when aggregation changes payload framing.
10%Suggested fixes
2
Diagnostic step 2
Temporarily disable 802.11 A-MPDU/A-MSDU (frame aggregation) on the affected mesh node to confirm mitigation; keep aggregation disabled until fix is applied if stable.
3
Diagnostic step 3
Keep MSS clamping at 1350–1400 on the router's NAT to work around PMTUD failures (reversible; monitor for broken DNS over HTTPS or VPN MTU implications).
4
Diagnostic step 4
Test with a laptop tethered via Ethernet to the same mesh node backhaul (if possible) to determine whether the problem is strictly wireless aggregation vs backhaul WAN handling.
5
Diagnostic step 5
Collect firmware logs and full packet traces and open a vendor support ticket including timestamps, captures, and a reproduction plan. Escalate to vendor if firmware update or configuration does not resolve.