Using Japanese cloud servers during peak hours resulted in issues such as webpage loading, SSH connection lag, and slow data transfer. Latency that was initially as low as 50 milliseconds suddenly spiked to over 200 milliseconds, significantly reducing its cost-effectiveness. What technical reasons lie behind this precipitous shift from "easy to use" to "difficult to use"? This article will provide an in-depth analysis.
Communication between China and Japan relies on multiple submarine fiber optic cable systems, the physical foundation of the international internet. However, these cables are extremely fragile. Fishing trawling operations and ship anchoring can directly damage submarine cables. Japanese government reports also show that several failures occur annually due to anchor chain dragging or natural wear and tear. Once a submarine cable breaks, communication data that should be directly connected is forced to take a detour, transiting through the United States or Southeast Asia, significantly increasing latency and packet loss rates. This is one of the fundamental reasons why "optimized direct connections" can suddenly fail.
The so-called "optimized high bandwidth" on the market essentially refers to service providers signing high-quality interconnection agreements (peering) with China Telecom, China Unicom, and China Mobile, such as CN2, IIJ, or SoftBank lines, and optimizing policies at the BGP level. The average latency of these optimized BGP lines can be as low as 50 to 80 milliseconds.
However, excellent routing strategies are a double-edged sword—when the optimized path fails or experiences a traffic storm, BGP dynamic routing will automatically switch to a backup path within seconds. If the backup path is of poor quality, although the server may not crash, the user experience will still plummet. Unfortunately, there have been several recent notifications of network fluctuations targeting Japanese data centers, confirming that abnormal network lines from China Unicom and China Telecom have caused significant increases in latency and packet loss on routes from Japan back to China.
This is the most common cause. The total international outbound bandwidth between China and Japan is fixed, and the number of ports interconnected with China's three major operators is limited. During peak evening hours (typically 8 PM to 11 PM Beijing time), a massive number of users are online simultaneously, inevitably causing congestion on outbound bandwidth. This results in data packets queuing in the backbone network nodes, causing overall latency to spike to over 150 milliseconds. When bandwidth is severely insufficient or peak traffic exceeds port bandwidth, congestion and packet loss occur.
Many users overlook a mechanism at the cloud platform level: proactive bandwidth throttling. When the CPU usage of your purchased cloud server consistently exceeds the baseline threshold, or your monthly bandwidth package is exhausted, the cloud service provider will proactively limit the instance's frequency or restrict outbound bandwidth to ensure fair usage for other users on the physical machine. This is one of the main reasons why "sudden performance degradation" occurs without any identifiable hardware faults. Furthermore, domestic and international firewall review rules, strict packet inspection mechanisms, or outdated firmware can also cause latency in inbound traffic.
In cross-border network operations, DDoS attacks remain a significant threat. Japanese data centers commonly face this risk, with a single attack peaking at 580Gbps. When a data center's core IP is subjected to sustained high-volume DDoS attacks, firewalls or DDoS protection systems will automatically initiate traffic redirection and cleaning, or even black-hole blocking, causing abnormal access to normal services. Furthermore, when large international network service providers' data centers perform planned maintenance or software upgrades, they also reroute traffic; even a brief increase in latency can be perceived as "lag" by users.
In such situations, the best first step is to perform route tracing (tracert or mtr) to pinpoint whether packet loss is occurring within the domestic LAN, the international gateway, or within the Japanese data center. If the issue is line congestion, consider adding a CN2 GIA relay solution or adding a relay node with better network quality between the user and the server, but this will increase costs. Simultaneously, check the current bandwidth monitoring panel to confirm if there are instances of "saturation" or "instantaneous over-limit" bandwidth usage, ruling out server CPU and memory performance bottlenecks.
The network lines of Japanese cloud servers are not always "optimal"; they are actually a dynamic collaborative system highly dependent on physical infrastructure, routing policies, carrier load, and upper-layer risk control. Understanding these underlying technical reasons can help you be less anxious and more composed in troubleshooting and resolving network lag issues.
