Everyone is familiar with CDNs—they cache images, videos, and scripts on nodes closest to users to accelerate webpage loading. However, standard CDNs have a fatal flaw: they focus solely on acceleration while ignoring security. When faced with high-volume attacks, the CDN nodes themselves become targets; instead of accelerating content, they can actually facilitate service outages.

　　High-security CDNs (or DDoS-protected CDNs) upgrade this model by integrating "acceleration" and "security protection" into a single process. All user requests first reach the CDN edge nodes; legitimate traffic is served by the node or forwarded to the origin server, while malicious traffic is filtered out at the nodes and scrubbing centers, ensuring only clean traffic reaches the origin.

　　This mechanism relies on the collaborative operation of a four-layer architecture: the global scheduling layer acts as the "navigator," the edge access layer handles "screening," the distributed scrubbing centers perform "filtering," and the origin-pull control layer acts as the "gatekeeper." Let’s break down each layer.

　　Layer 1: Global Scheduling Layer—The "Brain" of the Network

　　The global scheduling layer serves as the command center for the high-security CDN, directing traffic to the appropriate destinations. It operates using three key technologies: DNS scheduling, HTTP scheduling, and Anycast routing.

　　DNS scheduling is the traditional method—it returns the IP address of the node closest to the user based on their geographic location and ISP. HTTP scheduling offers greater flexibility, enabling granular traffic distribution at the application layer. Anycast routing is a critical weapon in high-security scenarios: the same IP address is advertised across multiple nodes, and the BGP protocol automatically routes attack traffic to the node nearest the attack source, enabling "near-source scrubbing."

　　The scheduling system continuously collects network-wide data every second, monitoring metrics such as bandwidth utilization, CPU load, network latency, and whether a node is currently under attack. If a node becomes overwhelmed by a massive traffic surge, the scheduling system redirects incoming requests to healthy nodes within seconds. This dynamic scheduling capability is what enables high-security CDNs to withstand distributed attacks reaching the terabit (Tbps) scale.

　　Layer 2: Edge Access Layer—The First "Sentry Post"

　　The edge node layer is the tier closest to the user, comprising hundreds or even thousands of Points of Presence (POPs) distributed globally. It performs two main functions: acceleration via caching and preliminary filtering.

　　Acceleration is a familiar concept: static resources are cached on the nodes, so when users access them repeatedly, the data is retrieved directly from the node rather than the origin server. This improves response speeds and conserves origin server bandwidth.

　　Regarding security, each edge node is equipped with a lightweight detection module. Traffic with obvious malicious characteristics—such as malformed TCP packets, clearly spoofed IPs, or port scans—is dropped immediately at the edge layer, eliminating the need to waste computing resources by sending it to downstream scrubbing centers.

　　The edge layer also offers the benefit of elastic scalability. Under normal operating conditions, it functions routinely; however, in the event of a traffic surge or an attack, the system can automatically allocate additional resources to nodes in the affected region, preventing the infrastructure from being overwhelmed by a sudden influx of traffic.

　　Layer 3: Distributed Scrubbing Center—The True "Heavy Artillery"

　　If the edge node acts as the initial sieve, the scrubbing center is the "water purification plant" that thoroughly filters out the contaminants. Deployed at the core nodes of the backbone network and boasting bandwidth capacities in the Tbps range, it serves as the backbone of the entire defense system.

　　Detection at the scrubbing center proceeds in progressive, layered stages:

　　Network Layer (L3) Protection: This layer targets volumetric attacks—such as SYN floods, UDP floods, ICMP floods, and DNS/NTP reflection amplification attacks—by utilizing packet fingerprint matching and traffic baseline comparisons to rapidly identify and discard anomalous packets. For SYN floods, SYN Cookie technology is employed; this prevents the allocation of half-open connection resources until the three-way handshake is complete, thereby exhausting the attacker's resources at the protocol level.

　　Transport Layer (L4) Granular Control: Through TCP protocol stack validation, the system identifies spoofed source IPs and anomalous handshake requests, effectively mitigating connection-exhaustion attacks.

　　Application Layer (L7) Deep Behavioral Analysis: This is the primary battleground for defending against CC (Challenge Collapsar) attacks. Simple rate limiting is no longer sufficient, as CC attack requests often appear nearly identical to those from legitimate users. Scrubbing centers perform three key tasks: frequency analysis (tracking request counts per IP per minute), path analysis (evaluating whether access patterns are normal), and behavioral modeling (comparing activity against the "profile" of a typical user to detect anomalies). For suspicious traffic, they may inject unobtrusive JavaScript challenges or CAPTCHAs; only real humans can execute the necessary scripts, instantly exposing automated tools.

　　It is worth highlighting JA3/JA4 fingerprinting technology. This method extracts parameters from the TLS handshake—such as version numbers, cipher suites, and extension lists—to generate a unique fingerprint. Most automated attack tools (like Python’s Requests library or Go’s HttpClient) produce highly consistent fingerprints; once a match is detected, the connection is severed during the handshake, effectively stifling the CC attack before it even begins.

　　Layer 4: Origin-Pull Control Layer—The Origin Server’s "Final Gatekeeper"

　　While the first three layers handle the bulk of the heavy lifting, vigilance cannot be relaxed at the final stage. The origin-pull control layer secures the communication link between CDN nodes and the origin server.

　　First, the origin server's IP address is completely masked. User requests terminate at the CDN nodes; attackers see only the CDN’s IP addresses and cannot locate the actual origin server.

　　Second, origin-pull requests undergo an additional round of filtering. Even if attack traffic manages to "penetrate" the first three layers (a highly unlikely scenario), the origin server performs a final check—applying measures such as per-IP rate limiting and blocking suspicious User-Agent strings.

　　Third, the origin-pull link itself is encrypted. A TLS-encrypted channel is established between the CDN nodes and the origin server to prevent man-in-the-middle attacks from tampering with data or stealing sensitive information.

　　Fourth, the origin server accepts traffic exclusively from CDN nodes. An IP whitelist mechanism ensures that only CDN node IP addresses can access the origin server, while all other requests are blocked. Even if an attacker discovers the origin server's IP address, they cannot launch an attack without the necessary whitelist authorization.

 　　How the Four Layers Collaborate: A Walkthrough of the Full Process

　　Imagine your website is protected by a high-defense CDN. Suddenly, a million requests arrive, a mix of legitimate traffic and malicious attacks—specifically SYN floods and CC attacks.

　　Step 1 (Scheduling Layer): The DNS system directs user requests to the nearest edge node, while Anycast automatically diverts a portion of the attack traffic to various scrubbing centers.

　　Step 2 (Edge Layer): Malformed TCP packets and obviously forged requests are dropped immediately at the edge.

　　Step 3 (Scrubbing Center): SYN flood attacks are filtered via protocol validation, while CC attacks are intercepted using JA3 fingerprinting and behavioral analysis.

　　Step 4 (Origin-Pull Layer): After passing through the first three layers, only clean traffic remains; this is forwarded to the actual origin server via an encrypted channel.

　　Final Result: Despite the massive volume of attack traffic, the origin server receives only legitimate user requests. The entire process is transparent to the user—the website loads smoothly, allowing users to place orders or watch videos without interruption.

　　Summary: All four layers are indispensable. The high-defense CDN’s four-layer architecture is essentially a defense strategy based on "luring the enemy in, filtering layer by layer, and cloaking the origin server." Each layer performs the task it is best suited for; only by working together can the system withstand a wide range of threats, from Terabit-scale DDoS attacks to sophisticated CC attacks. Understanding this architecture gives you confidence when selecting products and provides a clear direction for troubleshooting should issues arise.