What are upstream and downstream connections? A comparison of all types and performance of optimized routes in mainland China.

A comprehensive comparison of the advantages and disadvantages of three carrier-optimized lines and third-party international bandwidth, from CN2 GIA to CUG 9929 and CMIN2. In the server network field, upstream and downstream are two fundamental but easily confused concepts.

Upstream typically refers to carriers or service providers located near the core layer of the internet backbone, providing higher-level network interconnection. For cross-border data flows, data packets travel from the user's end to an overseas server, and the network nodes along the way can be divided into upstream and downstream: nodes near the core backbone network with higher routing decision-making authority are upstream, while nodes near the end user are downstream. Specifically, in cross-border access scenarios, upstream carriers refer to large ISPs (Internet Service Providers) with international export bandwidth, such as China Telecom International Limited, China Unicom International, China Mobile International, and international Tier 1 backbone network operators such as NTT, PCCW, and Telia. These carriers connect to the global internet through submarine cable systems and transnational fiber optic cables, determining the main transmission paths for cross-border traffic.

Downstream refers to service providers closer to the end user, including branch carriers in various provinces and cities, small and medium-sized ISPs, and IDC data centers. Downstream operators typically lease bandwidth or exchange traffic with upstream operators, and their routing strategies are largely constrained by upstream providers.

Understanding the upstream and downstream logic helps to analyze the value of optimized routes: When mainland China accesses overseas servers, data packets travel from the user end through various downstream nodes to the upstream international exit, crossing submarine fiber optic cables to reach overseas data centers; the return journey flows in the opposite direction. So-called mainland China optimized routes essentially involve operators specifying specific upstream international exits and routing strategies to ensure data packets avoid congested nodes during peak hours, reducing packet fragmentation hops and thus lowering latency and packet loss rates.

Inbound to Server and Outbound to China are two metrics that must be considered separately when evaluating route quality. Inbound reflects the path from a Chinese user's request to an overseas server; outbound reflects the path from the server's return data to the Chinese user. High-quality optimized routes must ensure that both ends use priority paths, rather than just one-way optimization.

Mainland China optimized routes are mainly divided into two systems: premium routes owned by the three major operators and third-party international optimized routes.

China Telecom Series

The 163 backbone network (AS4134) is China Telecom's basic public backbone network, carrying the vast majority of traffic for ordinary household broadband users. It boasts abundant bandwidth resources and low cost. However, congestion and packet loss are prone to occur at international exit points during peak evening hours, resulting in significant latency fluctuations. It is not recommended for business scenarios with specific requirements for domestic access quality.

CN2 GT (Global Transit, AS4809) is China Telecom's basic optimized line. Its technical route uses the 163 network for domestic segments, utilizing CN2 nodes only in the exit or return segments. Since the domestic segment still uses AS4134, congestion still occurs in intra-provincial and inter-provincial segments during peak evening hours, offering limited improvement in access experience, but at a lower cost than GIA. Real-world testing shows that CN2 GT uses CN2 nodes in the return or international segments, providing some optimization at the exit segment, but latency and congestion aggregation still easily occur in domestic provincial backbone networks.

CN2 GIA (Global Internet Access, AS4809) is China Telecom's premium line. Data packets travel entirely through AS4809 nodes from the access point, operating completely independently from the domestic provincial backbone to the international gateway, without passing through the 163 network. Real-world testing shows that GIA latency from South China to Hong Kong is consistently between 12-20ms, with a packet loss rate below 0.5%. The bandwidth cost of CN2 GIA is approximately 2 to 3 times that of CN2 GT, bandwidth resources are relatively scarce, and the risk of outbound congestion is extremely low.

Key identification points: A genuine CN2 GIA should show CN2 nodes starting with 59.43 throughout the entire traceroute link, and should not show 163 nodes starting with 202.97.

China Unicom Series

China Unicom's backbone network system is the most complex among the three major operators, mainly involving three lines.

The CU standard line (AS4837, China169) is China Unicom's public backbone network, carrying the vast majority of ordinary broadband traffic. International outbound traffic experiences significant congestion during evening peak hours, with large latency fluctuations and packet loss rates potentially reaching 5%-10%. As an upgrade solution, the CUG international outbound + 4837 domestic inbound (AS4837 + AS10099) route connects to the CUG international gateway after exiting the country, resulting in a significant improvement in backhaul performance. This is considered a cost-effective basic optimization solution.

The CUG premium line (AS10099 + AS9929, abbreviated as 9929/CUII/A network) is China Unicom's highest-level premium backhaul network. The overseas segment directly connects to the CUG international outbound, while the domestic segment is routed to the low-load AS9929 premium backbone network, ultimately delivering to users via AS4837. Latency to North America remains stable at 150-170ms, with a peak packet loss rate below 0.5%. AS9929 domestic segment node IPs begin with 218.105 or 210.51; these can be identified by checking the traceroute.

China Mobile Series

CMI Optimized Line (AS58453, China Mobile International) is an optimized routing solution provided by China Mobile International for cross-border business. It offers excellent latency performance in the Asia-Pacific region, especially in Southeast Asia, supports high-bandwidth scenarios, and offers better value than CN2 GIA. Note that CMI is part of China Mobile's international backbone network, and may be subject to QoS rate limiting during peak hours.

CMIN2 (AS58807) is a premium line launched by China Mobile to compete with CN2 GIA. It enjoys higher priority and maintains high speeds even during evening peak hours. Its bandwidth is typically more abundant than the CN2 GIA solution, making it a high-performance option added in recent years.

Third-Party International Optimized Lines

In addition to the three major operators' self-operated backbone networks, optimized lines from third-party international operators are also worth considering.

PCCW (PCCW Global / HKT) is a Hong Kong telecommunications service provider. PCCW offers significant optimization for access to mainland China, especially in southern regions like Guangdong and Fujian. Latency for China Telecom, China Unicom, and China Mobile is between 30-60ms. Speeds to North America are stable at around 140ms. Some PCCW lines offer intelligent routing and dynamic routing.

NTT, SoftBank, and IIJ are all Tier 1 backbone network operators in Japan, possessing strong interconnectivity capabilities in East Asia. Overseas VPS providers like RFCHost integrate with international operators such as IIJ, PCCW, SoftBank, Telstra, and NTT to achieve bypass optimization and intelligent routing, improving cross-border transmission quality between China and Japan, and between China and Singapore.

CN2 Series Sub-Level Differences: Within the CN2 family, Pure CN2 GIA is the highest level, using the AS4809 premium standard across all three networks. Some service providers offer premium CN2 networks that further integrate China Mobile's CMIN2 and China Unicom's 9929 tri-network optimized backhaul, resulting in better latency control and packet loss rate compared to traditional single GIA-covered lines. However, early mainland China optimized CN2 routes for China Mobile and China Unicom were still based on older generation networks, which may cause congestion during peak hours when accessing between networks.

Some products on the market package CN2 GT as a CN2 optimized solution. The true way to identify this is to use the `traceroute` or `mtr` command to trace the backhaul route. If nodes starting with 202.97 appear in the domestic segment, it is generally not a full CN2 GIA solution.

Furthermore, some vendors advertise direct connection to all three networks, but in reality, they do not use the new CMIN2 or 9929 premium networks in the China Mobile and China Unicom segments. Before purchasing, you must clearly confirm with customer service the ASN identifier of each operator's domestic backbone network access and whether the backhaul is optimized for both directions, rather than just one-way outbound optimization used only to reduce explicit latency.