Analysis of International Roaming Network Issues and the QunarNDT Diagnostic Tool

Instructor Duan Kai, a system operations engineer at Qunar, introduced his background in load balancing, automation, log analysis, and CDN management.

The scenario described an overseas user who, despite having international roaming enabled, encountered frequent video upload failures on the Qunar app, a problem not seen domestically.

Qunar has deployed service nodes in Hong Kong and utilizes CDN acceleration for overseas users, but to investigate the failure they developed a network detection tool called QunarNDT, available at http://qops.qunarcdn.com/ , which can be accessed via a mobile browser.

The diagnostic process involves opening the tool, clicking "Detect Now," and reviewing the generated report, which reveals the user's IP, DNS, and connectivity to Qunar's main sites.

Analysis of the report showed that the user's outbound IP and DNS remained those of a domestic carrier, meaning the request was routed back to domestic service interfaces; the failure was ultimately caused by interface timeout due to high latency and large upload size.

Conclusions: (1) International roaming traffic often reaches the same domestic service nodes as in‑country traffic; (2) Cross‑region and cross‑carrier latency, combined with large data transfers, leads to timeouts.

The article further explains the roaming authentication process, inter‑carrier routing, and settlement mechanisms that cause overseas traffic to be tunneled back to domestic networks.

It also compares routing behaviors of major Chinese carriers (e.g., China Mobile routes to local networks, while China Unicom and China Telecom route back to the home region).

Finally, the author recommends using overseas SIM cards or portable Wi‑Fi devices for better performance, noting that these options allow direct access to local networks and Qunar's Hong Kong nodes, reducing latency.