--- title: What Is HTTP/3? QUIC-Based HTTP Explained description: Learn what is HTTP/3, how HTTP3 QUIC works, and the key differences between HTTP/2 vs HTTP/3 for faster web performance. image: https://blog.blazingcdn.com/hubfs/Gemini-Blog/image-Nov-02-2026-10-30-49-5393-PM.jpeg --- [![BlazingCDN](https://blog.blazingcdn.com/hubfs/Logo/blog-logo-w.png)](https://blog.blazingcdn.com?hsLang=en-us) [📘 Learn ▾](https://blog.blazingcdn.com/cdn-learn?hsLang=en-us) [CDN Fundamentals](https://blog.blazingcdn.com/cdn-fundamentals?hsLang=en-us) [By Content Type](https://blog.blazingcdn.com/by-content-type?hsLang=en-us) [Advanced Concepts](https://blog.blazingcdn.com/advanced-concepts?hsLang=en-us) [📖 Glossary](https://blog.blazingcdn.com/glossary?hsLang=en-us) [🎬 Video & Streaming ▾](https://blog.blazingcdn.com/video-streaming-cdn?hsLang=en-us) [🔴 Live Streaming](https://blog.blazingcdn.com/live-streaming?hsLang=en-us) [📺 VOD & OTT](https://blog.blazingcdn.com/vod-ott?hsLang=en-us) [🎞️ Encoding & Players](https://blog.blazingcdn.com/encoding-players?hsLang=en-us) [🎧 Audio & Podcasts](https://blog.blazingcdn.com/audio-podcasts?hsLang=en-us) [💰 Bandwidth & Costs](https://blog.blazingcdn.com/bandwidth-costs?hsLang=en-us) [🏗️ Infrastructure ▾](https://blog.blazingcdn.com/cdn-infrastructure?hsLang=en-us) [🗄️ Storage & Origin](https://blog.blazingcdn.com/storage-origin?hsLang=en-us) [📦 Large File Delivery](https://blog.blazingcdn.com/large-file-delivery?hsLang=en-us) [💸 Cloud Costs & Egress](https://blog.blazingcdn.com/cloud-costs-egress?hsLang=en-us) [🌐 Networking & ISP](https://blog.blazingcdn.com/networking-isp?hsLang=en-us) [⚙️ DevOps & Cloud Infra](https://blog.blazingcdn.com/devops-cloud-infra?hsLang=en-us) [⚡ Web Performance](https://blog.blazingcdn.com/web-performance?hsLang=en-us) [🏭 Industries ▾](https://blog.blazingcdn.com/cdn-industry-insights?hsLang=en-us) [📺 Media & Broadcasting](https://blog.blazingcdn.com/media-broadcasting?hsLang=en-us) [💾 Software & SaaS](https://blog.blazingcdn.com/software-saas?hsLang=en-us) [📣 AdTech & Advertising](https://blog.blazingcdn.com/adtech-advertising?hsLang=en-us) [🎮 Gaming & Esports](https://blog.blazingcdn.com/gaming-esports?hsLang=en-us) [📱 Mobile Apps & Developers](https://blog.blazingcdn.com/mobile-apps-developers?hsLang=en-us) [🤖 AI & Machine Learning](https://blog.blazingcdn.com/ai-machine-learning?hsLang=en-us) [🏛️ Enterprise & Corporate](https://blog.blazingcdn.com/enterprise-corporate?hsLang=en-us) [📚 E-Learning & EdTech](https://blog.blazingcdn.com/e-learning-edtech?hsLang=en-us) [🏟️ Sports & Live Events](https://blog.blazingcdn.com/sports-live-events?hsLang=en-us) [💰 Pricing & Compare ▾](https://blog.blazingcdn.com/cdn-pricing-and-cdn-costs?hsLang=en-us) [Provider Pricing](https://blog.blazingcdn.com/provider-pricing?hsLang=en-us) [Cost Optimization](https://blog.blazingcdn.com/cost-optimization?hsLang=en-us) [Decision Support](https://blog.blazingcdn.com/decision-support?hsLang=en-us) [⚡ Compare](https://blog.blazingcdn.com/cdn-comparison?hsLang=en-us) [Provider Comparisons](https://blog.blazingcdn.com/provider-comparisons?hsLang=en-us) [Strategy Comparisons](https://blog.blazingcdn.com/strategy-comparisons?hsLang=en-us) [📊 Benchmarks](https://blog.blazingcdn.com/cdn-ratings-and-benchmarks?hsLang=en-us) [🔒 Security ▾](https://blog.blazingcdn.com/cdn-security?hsLang=en-us) [Attack Protection](https://blog.blazingcdn.com/attack-protection?hsLang=en-us) [Encryption & Access](https://blog.blazingcdn.com/encryption-access?hsLang=en-us) [Video & DRM Security](https://blog.blazingcdn.com/video-drm-security?hsLang=en-us) [🛠️ Tools](https://blog.blazingcdn.com/cdn-tools?hsLang=en-us) [Get Started](https://blazingcdn.com/sign-up-contact-form/) [Infrastructure](https://blog.blazingcdn.com/en-us/tag/infrastructure) [Infrastructure - Networking & ISP](https://blog.blazingcdn.com/en-us/tag/infrastructure-networking-isp) # What Is HTTP/3? QUIC-Based HTTP Explained Vadym Borovskyi Nov 2, 2026, 11:32:31 PM ![](https://blog.blazingcdn.com/hubfs/Gemini-Blog/image-Nov-02-2026-10-30-49-5393-PM.jpeg) HTTP 3 (HTTP/3, RFC 9114) is the third major version of HTTP. It keeps HTTP semantics (methods, status codes, headers) but runs over QUIC, a UDP-based transport, instead of TCP. QUIC merges transport and TLS 1.3 handshakes into one round trip and recovers lost packets per stream. On a 100 ms mobile link, that removes about 100 ms of setup and stops one lost packet from stalling every request. As of 2026, HTTP/3 is supported by default in Chrome, Edge, Firefox and Safari, and by the large edge networks, including Cloudflare, Fastly, Akamai, Amazon CloudFront and Google Cloud CDN. Browsers discover it through an Alt-Svc response header or an HTTPS DNS record and fall back to HTTP/2 over TCP when UDP port 443 is blocked. ## **How HTTP 3 works over QUIC** HTTP/3 maps each request and response onto its own bidirectional QUIC stream. QUIC (RFC 9000, May 2021) provides reliable, ordered delivery inside each stream but not across streams. Each stream is independent, so a lost UDP datagram only delays the stream whose bytes it carried. Under HTTP/2, the same loss stalls every multiplexed stream until TCP retransmits. Four mechanisms do most of the work: - **Combined handshake.** TLS 1.3 runs inside QUIC (RFC 9001). Transport and crypto parameters are negotiated together in 1 RTT, versus 2 RTT for a TCP handshake followed by TLS 1.3. Resumed sessions can send 0-RTT data with the first flight. - **Per-stream loss recovery.** Packet numbers are never reused and acknowledgments carry explicit ranges, which removes TCP's retransmission ambiguity and makes RTT estimates cleaner under loss. - **Connection IDs.** Connections are identified by IDs rather than the IP and port four-tuple. A phone moving from Wi-Fi to LTE can migrate the connection without a new handshake. - **QPACK header compression** (RFC 9204). This replaces HTTP/2's HPACK. HPACK assumes in-order delivery across the whole connection, which QUIC deliberately does not provide. ### Why HTTP/3 helps on lossy mobile networks HTTP/3 helps on lossy mobile networks because a lost packet only blocks the stream it belonged to, and setup costs one round trip instead of two. On high-RTT, 1 to 2% loss links typical of congested cellular cells, both effects compound. Wired, low-loss connections see much smaller gains. A worked example shows the scale. Assume 1% random loss, a 100-packet congestion window and six concurrent requests on one connection. - The probability that a window contains at least one loss is 1 minus 0.99 to the power of 100, which is about 63%. - Under HTTP/2, each such event can stall all six streams for at least one RTT. - Under HTTP/3, only the one stream carrying the lost packet stalls. - Expected streams blocked per loss event therefore drop from 6 to 1. - At 100 ms RTT, setup also falls from 200 ms (TCP plus TLS 1.3) to 100 ms, or to zero added RTT on 0-RTT resumption. ## **HTTP3 and QUIC: where each sits in the stack** The HTTP3 QUIC pairing replaces two layers of the classic stack. HTTP/2 runs over TLS, which runs over TCP in the kernel. HTTP/3 runs over QUIC, which carries TLS 1.3 inside it, over UDP. In most deployments the QUIC layer lives in user space. Moving the transport into user space is what lets QUIC evolve without operating system upgrades. It also lets QUIC encrypt nearly all transport headers, which blocks middlebox ossification. The cost is CPU: kernel TCP benefits from decades of offloads. Efficient UDP serving depends on generic segmentation offload (GSO), receive offload and careful batching. Operators should expect higher per-gigabit CPU on HTTP/3 than on HTTP/2. ## **HTTP/2 vs HTTP/3: what actually changes** | Property | HTTP/2 | HTTP/3 | | --- | --- | --- | | Transport | TCP plus TLS | QUIC over UDP, TLS 1.3 built in | | New connection setup | 2 RTT with TLS 1.3 | 1 RTT, 0-RTT on resumption | | Head-of-line blocking | At the TCP layer, across all streams | Per stream only | | Header compression | HPACK | QPACK | | Network change | Connection breaks | Migrates via connection ID | | Server CPU per Gb/s | Lower, kernel offloads | Higher, user-space crypto and pacing | HTTP/3 trades extra server CPU for lower setup latency and loss isolation, so its gains concentrate on high-RTT, lossy client networks. **HTTP/3 vs. QUIC:** QUIC is the general-purpose transport. HTTP/3 is one application mapping onto it. DNS over QUIC and Media over QUIC use the same transport without HTTP/3. **HTTP/3 vs. SPDY and gQUIC:** Google's gQUIC carried an HTTP/2-like mapping from about 2013. IETF QUIC redesigned the crypto handshake and wire format, so gQUIC and HTTP/3 do not interoperate. ## **How to check if a site uses HTTP/3** To check if a site uses HTTP/3, open the browser developer tools Network panel, enable the Protocol column and reload the page. Requests served over HTTP/3 show "h3". The first load often shows "h2" because the browser only learns about HTTP/3 from that first response, so reload before concluding anything. Three further signals confirm server-side support: - The response carries an alt-svc header advertising h3 on port 443, with a max-age (ma) value in seconds, for example 86400. - The domain publishes an HTTPS DNS record (RFC 9460) whose alpn parameter lists h3. This lets clients use QUIC on the very first connection. - A command-line HTTP client built with HTTP/3 support, run in its HTTP/3-only mode, completes the request rather than failing over to TCP. On nginx 1.25 or later with the QUIC module, enabling it takes three changes. Add the quic parameter to a listen directive on port 443 alongside the existing TLS listener, set an add\_header directive that emits alt-svc with h3, and open UDP 443 in every firewall and security group. Forgetting that last step is the most common reason a correctly configured server never negotiates h3. ## **Common HTTP/3 misconceptions** **"HTTP/3 is always faster."** On clean, low-RTT wired links the median difference is small, often within a few milliseconds. On saturated large downloads, a well-tuned TCP stack can still match or beat QUIC on throughput per server core. **"UDP means unreliable."** QUIC implements acknowledgments, retransmission, flow control and congestion control (CUBIC or BBR are both common). UDP is only the envelope that gets packets through existing middleboxes. **"0-RTT is free latency."** 0-RTT data can be replayed by an attacker. Servers should accept it only for idempotent requests, and many CDNs restrict it to GET. **"Apps must be rewritten."** Request and response semantics are identical to HTTP/2. Application code is untouched; only the server, load balancer and firewall layers change. ## **FAQ: HTTP/3 and QUIC** ### What is HTTP/3 in simple terms? HTTP/3 is the same HTTP that browsers already use, carried over QUIC on UDP instead of TCP. QUIC builds reliability, encryption with TLS 1.3, and independent streams into the transport. Connections start in one round trip, and one lost packet no longer delays every request sharing the connection. It was standardized as RFC 9114 in 2022. ### Is HTTP/3 faster than HTTP/2? HTTP/3 is faster than HTTP/2 mainly on high-latency or lossy networks, such as mobile connections with 1% or more packet loss. Savings come from one fewer setup round trip and from isolating loss to a single stream. On fast wired links the difference is usually small, and servers spend more CPU per gigabit on QUIC. ### Do all browsers support HTTP/3? All major browsers support HTTP/3 by default as of 2026: Chrome, Edge, Firefox and Safari. Clients fall back to HTTP/2 over TCP automatically when UDP port 443 is blocked, which still happens on some corporate and public networks. That fallback means enabling HTTP/3 server-side carries little compatibility risk for end users. ### Why does my site show h2 instead of h3 in developer tools? A site usually shows h2 on the first request because the browser learns about HTTP/3 from the alt-svc header in that response. Reload and check again. If h3 never appears, confirm UDP 443 is open end to end, that alt-svc advertises h3, and that no proxy or VPN in the path forces TCP. ## **Measure HTTP/3 against your real client mix** This week, split your real user monitoring data by negotiated protocol (h2 versus h3) and by connection type. Compare time to first byte and largest contentful paint at p75 and p95 for mobile users only. If h3 sessions on cellular show a p95 gap of 100 ms or more, prioritize HTTP/3 for your HTML and API edge. If the gap is negligible, your traffic is dominated by large objects or wired clients, and throughput per server matters more than the transport. For that large-object case, BlazingCDN delivers over HTTP/2 and TLS 1.3 from NVMe SSD servers. It does not offer HTTP/3, by design. Its [**CDN performance metrics for 100 MB HTTPS downloads**](https://blazingcdn.com/performance-metrics/) list the June 2026 measurements of 1.12 Gb/s in Europe and 1.11 Gb/s in North America. More protocol definitions live in the [**CDN and web protocol glossary**](https://blog.blazingcdn.com/glossary?hsLang=en-us). 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All rights reserved. ```json { "@context" : "https://schema.org", "@id" : "https://blazingcdn.com/#organization", "@type" : "Organization", "description" : "High-volume CDN for video, live streaming, OTT/IPTV, software, games, SaaS and large-file delivery.", "logo" : { "@id" : "https://blazingcdn.com/#logo", "@type" : "ImageObject", "height" : 560, "url" : "https://blazingcdn.com/wp-content/uploads/2024/08/logo-560-560.png", "width" : 560 }, "name" : "BlazingCDN", "sameAs" : [ "https://www.linkedin.com/company/blazingcdn", "https://x.com/BlazingCdn", "https://www.reddit.com/r/BlazingCDN/", "https://www.wikidata.org/wiki/Q141711074" ], "url" : "https://blazingcdn.com/" } ``` ```json { "@context" : "https://schema.org", "@id" : "https://blog.blazingcdn.com/#website", "@type" : "WebSite", "inLanguage" : "en-US", "name" : "BlazingCDN Blog", "publisher" : { "@id" : "https://blazingcdn.com/#organization" }, "url" : "https://blog.blazingcdn.com/" } ``` ```json { "@context" : "https://schema.org", "@id" : "https://blog.blazingcdn.com/en-us/what-is-http-3-quid-based-http-explained#article", "@type" : "BlogPosting", "author" : { "@id" : "https://blog.blazingcdn.com/en-us/author/blazingcdn#person", "@type" : "Person", "description" : "Vadym Borovskyi is Chief Product and Marketing Officer at BlazingCDN, where he has led product strategy and growth since 2021. 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QUIC-Based HTTP Explained", "image" : "https://143144902.fs1.hubspotusercontent-eu1.net/hubfs/143144902/Gemini-Blog/image-Nov-02-2026-10-30-49-5393-PM.jpeg", "inLanguage" : "en-us", "isPartOf" : { "@id" : "https://blog.blazingcdn.com/#website" }, "mainEntityOfPage" : { "@id" : "https://blog.blazingcdn.com/en-us/what-is-http-3-quid-based-http-explained", "@type" : "WebPage" }, "publisher" : { "@id" : "https://blazingcdn.com/#organization" }, "wordCount" : 1553 } ``` ```json { "@context" : "https://schema.org", "@id" : "https://blog.blazingcdn.com/en-us/what-is-http-3-quid-based-http-explained#breadcrumb", "@type" : "BreadcrumbList", "itemListElement" : [ { "@type" : "ListItem", "item" : "https://blog.blazingcdn.com/en-us", "name" : "Blog", "position" : 1 }, { "@type" : "ListItem", "item" : "https://blog.blazingcdn.com/en-us/what-is-http-3-quid-based-http-explained", "name" : "What Is HTTP/3? QUIC-Based HTTP Explained", "position" : 2 } ] } ``` ```json { "@context" : "https://schema.org", "@id" : "https://blog.blazingcdn.com/en-us/what-is-http-3-quid-based-http-explained#faq", "@type" : "FAQPage", "mainEntity" : [ { "@type" : "Question", "acceptedAnswer" : { "@type" : "Answer", "text" : "HTTP/3 is the same HTTP that browsers already use, carried over QUIC on UDP instead of TCP. QUIC builds reliability, encryption with TLS 1.3, and independent streams into the transport. Connections start in one round trip, and one lost packet no longer delays every request sharing the connection. It was standardized as RFC 9114 in 2022." }, "name" : "What is HTTP/3 in simple terms?" }, { "@type" : "Question", "acceptedAnswer" : { "@type" : "Answer", "text" : "HTTP/3 is faster than HTTP/2 mainly on high-latency or lossy networks, such as mobile connections with 1% or more packet loss. Savings come from one fewer setup round trip and from isolating loss to a single stream. 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