Debugging APP Load Failures: MTU/MSS Analysis with Wireshark

The author introduces a recent user‑reported problem where a company’s app continuously fails to load content, then reviews basic networking concepts: MTU (maximum transmission unit) is the largest frame payload (1500 bytes for Ethernet) and MSS (maximum segment size) is the TCP payload declared in the SYN packet, which can be altered by middleboxes (MSS clamping).

During investigation, a packet capture shows the client repeatedly retransmitting packet 16 while the server never acknowledges it. Earlier packets from the server have a Length of 1506 bytes, suggesting that a middle device altered the client’s MSS to 1452 bytes, causing the full‑size 1514‑byte frames to be dropped.

Further evidence is gathered by sending a ping with the ping -M do -s 1472 -c 3 -i 0.2 ip command, which returns “Message too long, mtu=1492”, confirming an MTU mismatch. The article also shows how to lower the payload size or specify a TTL (e.g., ping -M do -s 1472 -c 3 -i 0.2 ip -t 3 ) to locate the problematic hop.

Analysis of the HTTP API GET request (decrypted TLS) reveals that the new app version added very large parameters, causing requests to reach the full‑size MTU and be discarded, whereas the older version’s requests were well below the 1460 byte limit. Rolling back to the old version eliminates the failure.

Tips are provided for decrypting TLS traffic in Wireshark by setting the SSLKEYLOGFILE environment variable, which is supported by Chrome, Curl, Firefox, etc.

The article concludes that packet capture and systematic analysis are powerful debugging techniques, encouraging readers to apply the demonstrated methods to similar network issues.