Alibaba Cloud CDN uses IETF QUIC to accelerate short video and image delivery in the Taobao app, achieving 20% faster video downloads and 10% less stuttering. This article explains how QUIC works, how it compares to TCP-based protocols, and how the XQUIC library is deployed on mobile devices with Alibaba Cloud CDN.
Background
HTTP has evolved from HTTP/1.0 to HTTP/3, with each version addressing the limitations of its predecessor. HTTP/3 replaces TCP at the transport layer with QUIC, which brings lower latency, no head-of-line (HOL) blocking, and resilience to network changes. Alibaba Cloud CDN has tracked this evolution from Google QUIC (gQUIC) to IETF QUIC, which is standardized in RFC 9000 (May 2021) and deployed across CDN POPs.
The move to IETF QUIC was a deliberate choice: proprietary protocol solutions are harder for users to evaluate and adopt, while standardized protocols benefit from broader design review and security analysis. The XQUIC library was built from this collaboration. The following timeline summarizes the key milestones:
How QUIC works
HTTP evolution
HTTP has gone through four major upgrades:
HTTP/1.0 to HTTP/1.1 — Persistent connections and pipelined requests reduce the overhead of repeated TCP handshakes.
HTTP to HTTPS — Transport Layer Security (TLS) adds end-to-end encryption, but also adds handshake round trips.
HTTP/1.1 to HTTP/2 — Multiplexing and header compression improve concurrency over a single connection. TCP remains the transport layer.
HTTP/2 to HTTP/3 — HTTP/3 adopts QUIC as the transport. QUIC solves HOL blocking and provides 0-RTT (zero round-trip time) connection resumption, multiplexing, and connection migration.
QUIC features
Low connection latency
QUIC is based on UDP and supports 0-RTT data transfer on session resumption. By comparison, HTTPS with TLS 1.3 early data requires 1-RTT, and a full TLS 1.2 handshake requires 3-RTT.
No HOL blocking
HTTP/2 multiplexes streams over a single TCP connection. If a packet is lost, all streams stall until retransmission completes. QUIC eliminates this by design — each stream is independent at the transport layer. HTTP/3 also replaces HPACK with QPACK header encoding, further reducing HOL blocking for HTTP headers.
User-space congestion control
QUIC implements congestion control at the application layer instead of the kernel. Different algorithms and parameters can be applied per connection or scenario, without waiting for OS updates.
Connection migration
When a device switches networks — for example, from Wi-Fi to 4G — the IP address changes and TCP connections drop. QUIC uses connection IDs (CIDs) to maintain connections across network changes, making the transition invisible to users.
QUIC vs TCP
| Dimension | TCP | QUIC |
|---|---|---|
| Packet number space | Shared | Separate per encryption level |
| RTT measurement | Send/receive timestamps, ignores host delay | Delayed ACK mechanism, more precise |
| Selective acknowledgment | Up to 3 SACK ranges | More ACK ranges — faster recovery under high packet loss |
| Loss recovery | Retransmission timeout (RTO) + tail loss probe (TLP) | Probe timeout (PTO) |
| Minimum congestion window | 1 packet | 2 packets (reduces stalls; rate drops exponentially in sustained congestion) |
| ACK idempotence | Not guaranteed | Enforced — each packet number is unique per encryption level |
QUIC packet numbers increase strictly and encode transmission order directly. When a packet carrying an ACK frame is detected as lost, QUIC retransmits the necessary frames in a new packet with a new packet number. This enables accurate RTT measurement and eliminates false retransmissions that TCP can encounter with selective acknowledgment (SACK).
Use cases
| Scenario | Benefit |
|---|---|
| Short videos | Faster initial load, less stuttering |
| Images | Shorter download time |
| Dynamic requests (API calls, signaling) | Lower interaction latency |
| Live streaming | Reduced playback stuttering, more stable stream ingest |
XQUIC library
XQUIC is a lightweight, high-performance, cross-platform QUIC protocol library built to the IETF QUIC standard. It has been widely deployed for shopping guides and short videos on the Taobao app — for example, product searches in Taobao Mobile and video browsing in Taobao Time both use XQUIC for faster network data transmission.
| Characteristic | Details |
|---|---|
| Binary size | Under 400 KB on Android and iOS |
| Protocol stack | Full IETF QUIC stack: transport layer, encryption layer, and application layer |
| Platform support | Linux, Android, iOS, macOS (Windows support planned) |
| Integration | Client-side SDK; server-side module for Tengine or NGINX |
Results
After deploying IETF QUIC with XQUIC across short video scenarios in the Taobao app:
20% reduction in short video download time
10% reduction in video stuttering
Next steps
To enable QUIC on your Alibaba Cloud CDN domain, see Configure the QUIC protocol.