Analysis Of Differences In Bandwidth Assurance Between Japanese Bidirectional CN2 And Unidirectional CN2

This article outlines the differences in bandwidth behavior between two common international routing modes in actual transmission, analyzes the technical and business factors affecting bandwidth assurance, how to verify line quality through testing and SLAs, and provides selection recommendations for different business scenarios to facilitate decision-making by technical leaders and promoters when purchasing and deploying international links in Japan.

In most scenarios, Japanese bidirectional CN2 is easier to achieve the nominal bandwidth guarantee than unidirectional CN2. The reason is that bidirectional CN2 usually ensures symmetrical packet paths (China uses CN2/MPLS channels for both out/out/out/in/out), thereby guaranteeing a unified strategy between the control plane and bearer plane, low packet loss rates, and stable latency. This is especially important for TCP-based transmissions, as TCP performance is affected by both forward data and reverse ACK channel quality. In contrast, one-way CN2 uses the high-quality CN2 path in only one direction, and the backhaul may go through the public internet or other links, and backhaul congestion or packet loss directly reduces bidirectional throughput.

The TCP protocol relies on ACK acknowledgments to advance the window. If there is packet loss, jitter, or suppression in the return path, delayed or lost ACK will trigger retransmission and rate drops, resulting in a decrease in effective throughput. Additionally, unidirectional premium paths cannot guarantee symmetrical QoS strategies (such as MPLS-TE or QoS markings are handled uniformly at both ends), operators lack control over backhaul, and SLA items (latency/packet loss/availability) are hard to guarantee end-to-end, so actual bandwidth protection is often lower than factory metrics.

Bottlenecks usually occur on the return trip into other backbone or public network hubs, international entry/exit exchange points, and access links in the destination country. If the return route detours through multiple carriers or crosses congested switching points, even going forward via CN2 cannot guarantee overall performance. The Japan direction is also affected by local IX and carrier interconnection strategies. If the destination CDN or cloud area lacks good direct connections, the last mile or intra-city link can become a key point affecting bandwidth protection.

Common methods include: using iperf/iperf3 for long-term throughput testing (one-way and bidirectional), analyzing packet loss and paths with ping/traceroute, recording latency/jitter/packet loss time series using SLA monitoring tools, and retesting at high concurrency and different time periods to cover network conditions. When comparing tests, pay attention to whether the bidirectional ACK paths are consistent. It is recommended to conduct both forward and reverse directions tests simultaneously in both data centers to record TCP throughput, retransmission rate, and RTT changes to truly reflect bandwidth protection differences.

The loss magnitude is related to the quality of the return trip: if there is no obvious congestion on the return trip, the difference between the two can be about 5%-15%; If backhaul congestion is severe, the effective throughput of one-way CN2 may decrease by 30%-70%. Real-time services (VoIP/video conferencing) are sensitive to jitter and packet loss; even if average bandwidth seems sufficient, jitter can degrade the experience. Therefore, when discussing SLA and billing, it is important to distinguish between peak bandwidth, committed bandwidth (CIR/Committed), and actual stable rate.

Request the full SLA text from the operator (including latency, packet loss, availability, and compensation terms), provide end-to-end routing screenshots (BGBP AS PATH, MPLS tags), and request performance verification during the trial period. It is recommended to include test scenarios in the contract: including testing tools, time periods, statistical criteria, and acceptance thresholds. If only one-way CN2 is purchased, the return route should be specified, including return route and SLA, or alternative options requiring two-way protection.

If the business is sensitive to latency and packet loss (financial transactions, real-time voice/video, database synchronization, large file bidirectional transmission), bidirectional CN2 is preferred for end-to-end stable bandwidth protection; For unidirectional distribution scenarios (such as large volumes of content from China to Japan, CDN returns that rely less on backhaul), unidirectional CN2 can be a more cost-effective solution, but it requires integration with backhaul strategies and monitoring methods, and if necessary, can be combined with CDN or two-hop direct connections to reduce uncertainty.

A hybrid strategy can be adopted: the main link uses bidirectional CN2 to ensure critical business, while the backup link uses lower-cost one-way or internet links for disaster recovery or peak shaving for sudden traffic spikes; At the same time, dynamic routing policies (BGP priority, routing weights) and traffic engineering (QoS, offloading) are enabled, automatically switching according to policies through monitoring. This not only controls procurement costs but also maintains bandwidth protection and business continuity at critical moments.