--- title: 36/60 Days System Design Questions | daily.dev description: A system design question exploring CDN protocol configuration for reducing tail latency in high packet-loss environments. The scenario presents a mobile app... canonical: https://daily.dev/posts/36-60-days-system-design-questions-gbo4pu24m twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: 36/60 Days System Design Questions | daily.dev og:description: A system design question exploring CDN protocol configuration for reducing tail latency in high packet-loss environments. The scenario presents a mobile app... og:url: https://daily.dev/posts/36-60-days-system-design-questions-gbo4pu24m og:image: https://api.daily.dev/og/posts/gBo4Pu24m.png og:image:alt: 36/60 Days System Design Questions og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # 36/60 Days System Design Questions **[Joud Awad](https://daily.dev/sources/iac4jsbu0lv8wbsc85fsh)** · [@joudawad](https://daily.dev/joudawad) · 2 min read · 156 upvotes · 27 comments ## Summary A system design question exploring CDN protocol configuration for reducing tail latency in high packet-loss environments. The scenario presents a mobile app architecture where p99 latency spikes to 800ms+ on lossy networks (4G/5G transitions, flaky WiFi). Readers must choose between HTTP/2-only, HTTP/3-only, or a split HTTP/3-to-clients with HTTP/2-to-origin approach — the strategy used by Netflix and Cloudflare in production. ## Content Your API response went from 320ms to 95ms after a CDN switch. Nothing changed on the server. Same origin. Same payload. The only difference: the CDN started speaking HTTP/3 to your clients. Here's the setup: Mobile app → Load Balancer → Origin (NestJS) → DB Every request goes through the CDN first. Latency is acceptable — until high packet-loss environments (mobile 4G → 5G transitions, flaky WiFi, Asia-Pacific routes). You're seeing 800ms+ tail latency for the p99. The CDN supports HTTP/2 and HTTP/3. What do you enable? A) HTTP/2 only — multiplexing over a single TCP connection eliminates the old HTTP/1.1 head-of-line problem. Proven, widely supported. B) HTTP/3 only — built on QUIC (UDP), eliminates TCP head-of-line blocking entirely, 0-RTT connection resumption. Modern clients handle it fine. C) HTTP/2 to origin, HTTP/2 to clients — maximize multiplexing end-to-end, avoid QUIC instability in enterprise firewalls. D) HTTP/3 to clients, HTTP/2 to origin — QUIC handles the lossy last mile, HTTP/2 handles the stable datacenter leg. One of these is how Netflix, Cloudflare, and most major CDNs actually handle this in production. Pick one — A, B, C, or D — and tell me why. Full breakdown in the comments. If your team is debating CDN config or protocol upgrades, share this — the tradeoff is sharper than most people think. Drop your answer 👇 #30DaysOfSystemDesign #SystemDesign #HTTP #WebPerformance ## Community discussion Top comments from developers on daily.dev. **@joudawad** · 18 upvotes > Answer: D — HTTP/3 to clients, HTTP/2 to origin > > The problem is the last mile. Mobile clients on lossy connections are where TCP falls apart. > > > HTTP/2 fixed HTTP/1.1's application-layer head-of-line blocking by multiplexing streams over one TCP connection. But TCP itself still has head-of-line blocking at the transport layer. A single lost packet stalls ALL streams until it's retransmitted. On a 4G connection dropping 2% of packets, your 20-stream HTTP/2 connection grinds to a halt. > > > HTTP/3 runs on QUIC (UDP). Each stream is independent. A dropped packet stalls only that stream, the other 19... **@joudawad** · 6 upvotes > Why A falls short (HTTP/2 only): > > HTTP/2 multiplexing is a genuine improvement over HTTP/1.1, but it doesn't solve TCP head-of-line blocking. On lossy mobile connections, you're still at the mercy of the TCP retransmission window. For p99 tail latency on bad networks, HTTP/2 won't move the needle. **@joudawad** · 4 upvotes > Why C is wrong (HTTP/2 end-to-end): > > Same problem as A. You've done nothing about the last mile. Consistent HTTP/2 end-to-end sounds clean, but "clean" doesn't show up in your p99. **@joudawad** · 4 upvotes > Why B is wrong (HTTP/3 only): > > HTTP/3 to origin introduces real operational risk. Enterprise firewalls and some cloud load balancers block UDP port 443 or rate-limit it. Your origin infra isn't the bottleneck, the last mile is. Fixing the origin leg with QUIC doesn't help the problem; it just adds complexity and potential breakage. **@joudawad** · 2 upvotes > Also, it would mean a lot to me if you could support my content and stay in touch 🙏 > > - YouTube: [https://www.youtube.com/@system-design-lab](https://www.youtube.com/@system-design-lab) > - LinkedIn: [https://www.linkedin.com/in/joud-awad/](https://www.linkedin.com/in/joud-awad/) > - Medium Blog: [https://joudwawad.medium.com/](https://joudwawad.medium.com/) > - Substack: [https://joudawad.substack.com/](https://joudawad.substack.com/) --- Tags: [#career](https://daily.dev/tags/career) [View this post on daily.dev](https://daily.dev/posts/36-60-days-system-design-questions-gbo4pu24m) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"DiscussionForumPosting","mainEntityOfPage":"https://daily.dev/posts/36-60-days-system-design-questions-gbo4pu24m","headline":"36/60 Days System Design Questions","text":"A system design question exploring CDN protocol configuration for reducing tail latency in high packet-loss environments. The scenario presents a mobile app architecture where p99 latency spikes to 800ms+ on lossy networks (4G/5G transitions, flaky WiFi). Readers must choose between HTTP/2-only, HTTP/3-only, or a split HTTP/3-to-clients with HTTP/2-to-origin approach — the strategy used by Netflix and Cloudflare in production.","url":"https://daily.dev/posts/36-60-days-system-design-questions-gbo4pu24m","datePublished":"2026-06-11T17:40:53.137Z","dateModified":"2026-06-11T17:41:19.511Z","author":{"@type":"Person","name":"Joud Awad","url":"https://daily.dev/joudawad","image":"https://media.daily.dev/image/upload/s--dOB9RaXY--/f_auto/v1773320801/avatars/avatar_iaC4JsBU0lV8wBsc85fSh?_a=BAMAMiiu0","description":"Principal Solution Architecture ","worksFor":{"@type":"Organization","name":"Metalab","logo":"https://www.google.com/s2/favicons?domain=metalab.com&sz=128"},"interactionStatistic":{"@type":"InteractionCounter","interactionType":{"@type":"EndorseAction"},"userInteractionCount":81870}},"image":"https://media.daily.dev/image/upload/s--Ut1eO4iV--/f_auto/v1781199659/posts/gBo4Pu24m?_a=BAMAMiWQ0","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":156},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":27}],"comment":[{"@type":"Comment","text":"Answer: D — HTTP/3 to clients, HTTP/2 to origin\nThe problem is the last mile. Mobile clients on lossy connections are where TCP falls apart.\nHTTP/2 fixed HTTP/1.1’s application-layer head-of-line blocking by multiplexing streams over one TCP connection. But TCP itself still has head-of-line blocking at the transport layer. A single lost packet stalls ALL streams until it’s retransmitted. On a 4G connection dropping 2% of packets, your 20-stream HTTP/2 connection grinds to a halt.\nHTTP/3 runs on QUIC (UDP). Each stream is independent. A dropped packet stalls only that stream, the other 19 keep flowing. Add 0-RTT resumption (subsequent connections skip the handshake entirely) and you shave 1-2 round trips on every reconnect. That’s where the latency gains come from.\nBut your datacenter leg (CDN to origin) is a different story. It’s a stable, low-loss private network. QUIC’s benefits disappear on reliable connections. HTTP/2 multiplexing is mature, well-optimized, and doesn’t carry QUIC’s CPU overhead for cryptography (QUIC encrypts everything at the transport layer, no cleartext, ever).\nThis is exactly how Cloudflare, Fastly, and Akamai operate: HTTP/3 on the edge-facing side, HTTP/2 (or even HTTP/1.1 with keepalive) on the origin-facing side.","datePublished":"2026-06-11T17:41:54.450Z","dateModified":"2026-06-12T06:54:46.782Z","url":"https://daily.dev/posts/gBo4Pu24m#c-Rq3hA6WXt","author":{"@type":"Person","name":"Joud Awad","url":"https://daily.dev/joudawad","image":"https://media.daily.dev/image/upload/s--dOB9RaXY--/f_auto/v1773320801/avatars/avatar_iaC4JsBU0lV8wBsc85fSh?_a=BAMAMiiu0"},"interactionStatistic":{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":18}},{"@type":"Comment","text":"Why A falls short (HTTP/2 only):\nHTTP/2 multiplexing is a genuine improvement over HTTP/1.1, but it doesn’t solve TCP head-of-line blocking. On lossy mobile connections, you’re still at the mercy of the TCP retransmission window. For p99 tail latency on bad networks, HTTP/2 won’t move the needle.","datePublished":"2026-06-11T17:42:00.395Z","url":"https://daily.dev/posts/gBo4Pu24m#c-MEuEvbexM","author":{"@type":"Person","name":"Joud Awad","url":"https://daily.dev/joudawad","image":"https://media.daily.dev/image/upload/s--dOB9RaXY--/f_auto/v1773320801/avatars/avatar_iaC4JsBU0lV8wBsc85fSh?_a=BAMAMiiu0"},"interactionStatistic":{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":6}},{"@type":"Comment","text":"Why C is wrong (HTTP/2 end-to-end):\nSame problem as A. You’ve done nothing about the last mile. Consistent HTTP/2 end-to-end sounds clean, but “clean” doesn’t show up in your p99.","datePublished":"2026-06-11T17:42:26.716Z","url":"https://daily.dev/posts/gBo4Pu24m#c-u0vMx7Vqp","author":{"@type":"Person","name":"Joud Awad","url":"https://daily.dev/joudawad","image":"https://media.daily.dev/image/upload/s--dOB9RaXY--/f_auto/v1773320801/avatars/avatar_iaC4JsBU0lV8wBsc85fSh?_a=BAMAMiiu0"},"interactionStatistic":{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":4}},{"@type":"Comment","text":"Why B is wrong (HTTP/3 only):\nHTTP/3 to origin introduces real operational risk. Enterprise firewalls and some cloud load balancers block UDP port 443 or rate-limit it. Your origin infra isn’t the bottleneck, the last mile is. Fixing the origin leg with QUIC doesn’t help the problem; it just adds complexity and potential breakage.","datePublished":"2026-06-11T17:42:02.751Z","url":"https://daily.dev/posts/gBo4Pu24m#c-ddF7RUh4n","author":{"@type":"Person","name":"Joud Awad","url":"https://daily.dev/joudawad","image":"https://media.daily.dev/image/upload/s--dOB9RaXY--/f_auto/v1773320801/avatars/avatar_iaC4JsBU0lV8wBsc85fSh?_a=BAMAMiiu0"},"interactionStatistic":{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":4}},{"@type":"Comment","text":"Also, it would mean a lot to me if you could support my content and stay in touch 🙏\n\nYouTube: https://www.youtube.com/@system-design-lab\nLinkedIn: https://www.linkedin.com/in/joud-awad/\nMedium Blog: https://joudwawad.medium.com/\nSubstack: https://joudawad.substack.com/","datePublished":"2026-06-11T17:42:39.612Z","url":"https://daily.dev/posts/gBo4Pu24m#c-OLDXX5jQN","author":{"@type":"Person","name":"Joud Awad","url":"https://daily.dev/joudawad","image":"https://media.daily.dev/image/upload/s--dOB9RaXY--/f_auto/v1773320801/avatars/avatar_iaC4JsBU0lV8wBsc85fSh?_a=BAMAMiiu0"},"interactionStatistic":{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":2}}],"isPartOf":{"@type":"WebPage","url":"https://daily.dev/sources/iac4jsbu0lv8wbsc85fsh","name":"Joud Awad"}} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Joud Awad","item":"https://daily.dev/sources/iac4jsbu0lv8wbsc85fsh"},{"@type":"ListItem","position":3,"name":"36/60 Days System Design Questions"}]} ```