The traffic shown in the Alibaba Cloud CDN console (via Monitoring & Usage Analytics or Usage Statistics) reflects actual network-layer traffic, which is also the basis for billing. The response size field in your access logs captures only application-layer traffic — which is always lower. Alibaba Cloud CDN bills you for 1.1 times your logged traffic to account for the overhead between these two layers.
Why the numbers differ
Application-layer logs record the payload bytes that your application receives. Network-layer traffic includes additional bytes that the application never sees:
TCP and IP packet header overhead
Before transmission, application-layer data is encapsulated into TCP packets and then into IP packets. Each IP packet can carry up to 1,500 bytes, of which 40 bytes are consumed by headers — 20 bytes for the TCP header and 20 bytes for the IP header. These 40 bytes are not recorded in the response size log field.
The minimum header overhead is 40 ÷ (1,500 − 40) ≈ 2.74% of logged traffic. Because not all packets are full-sized, the typical overhead is around 3%.
TCP retransmission
Under real-world Internet conditions, network congestion and device failures cause packet loss. The OS kernel protocol stack retransmits lost packets — a process invisible to the application layer and not captured in access logs. Approximately 3% to 10% of packets are lost due to these conditions.
How Alibaba Cloud CDN calculates billed traffic
TCP/IP header overhead and TCP retransmission together add 7%–15% to raw logged traffic. Alibaba Cloud CDN applies a fixed overhead factor of 10%, so your billed traffic is:
Billed traffic = logged traffic × 1.1