--- title: "CDN reducing delay in object retrieval" subject: "Internet and Intranet" chapter: 4 key: "semester_8/internet_and_intranet:cdn-delay-reduction" source: https://www.bctengineer.com/q/semester_8/internet_and_intranet/cdn-delay-reduction site: BCT Engineering --- # CDN reducing delay in object retrieval Internet and Intranet · Chapter 4: HTTP and the Web Services - Asked in: 2073 Bhadra, 8+2 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/internet_and_intranet/cdn-delay-reduction - Whole chapter: https://www.bctengineer.com/md/semester_8/internet_and_intranet/chapter-4 ## Question Describe how content distribution network reduces the delay in receiving a requested object. Will content distribution reduce the delay for all objects requested by a user? Explain your answer with appropriate figure. ## Short answer A CDN caches content on edge servers near users, cutting propagation delay and origin load via DNS redirection. Not all objects benefit: dynamic, uncacheable, or rarely requested content still requires origin trips, and cold misses add an extra hop. ## Answer #### Role of a CDN A **Content Distribution Network (CDN)**, such as Akamai or Cloudflare, is a set of geographically distributed servers (edge or surrogate servers) that store copies of web content. Instead of every user fetching an object from the distant origin server, the request is directed to a nearby CDN server. #### How a CDN reduces delay 1. **Short distance.** The content is physically close to the user, so propagation delay and the number of hops are reduced. 2. **Avoids bottlenecks.** Traffic does not cross congested links to the origin server, and the user's path to the nearest edge server is short and lightly loaded. 3. **Load sharing.** Many servers share the requests, so no single server is overloaded and queuing delay is low. 4. **Caching.** Popular objects stay in the edge cache; the origin is contacted only on a miss or expiry. 5. **Smart redirection.** DNS (or an HTTP redirect) returns the IP address of the best server, chosen by network proximity or load. ```text Origin server (far away) | (only on cache miss) v +-------------+ +-------------+ | CDN edge A | | CDN edge B | +------^------+ +------^------+ | | User 1 User 2 ``` Operation: the user's browser asks for `www.site.com/video`. The site's DNS (a CNAME) points to the CDN's DNS. The CDN DNS picks the nearest server and returns its IP address. The browser fetches the object from that edge server. On a miss, the edge server fetches it from the origin, stores it and serves later users from its cache. #### Does a CDN reduce delay for all objects? **No.** - **Dynamic or personalised content** (bank balance, search results, real-time data) cannot be cached, so each request still goes to the origin server. - **Rarely requested (unpopular) objects** are usually not in the edge cache; the first request is a miss and takes even longer because of the extra hop through the edge server. - If the user is already near the origin, the gain is small. - Objects that have expired or are marked non-cacheable must be revalidated with the origin. So a CDN helps mainly for static, popular and large objects such as images, video, scripts and style sheets. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does a CDN mainly reduce delay? A. By compressing all data B. By serving copies of content from edge servers close to the user, reducing distance and avoiding bottlenecks to the origin C. By encrypting traffic D. By blocking slow users Answer: B. Nearby edge servers cut propagation delay and hops and avoid congested paths to the distant origin server. (Key point: CDN principle) 2. How is the user's request sent to a nearby CDN server? A. The site's DNS (CNAME) points to the CDN's DNS, which returns the IP of the best server B. The user chooses manually C. The origin server forwards it via SMTP D. Using the browser cache only Answer: A. CDN DNS selects a server by proximity or load and returns its IP; the browser fetches from it. (Key point: Redirection mechanism) 3. Does a CDN reduce delay for all objects? A. Yes, always B. No; dynamic or personalised content and rarely requested objects still need the origin C. Yes, but only images D. No; it only helps e-mail Answer: B. Dynamic content cannot be cached and unpopular objects are usually not in the edge cache, so those requests reach the origin. (Key point: Does CDN help all objects) 4. What happens on a cache miss at the edge server? A. The request fails B. The edge fetches the object from the origin, stores it and serves later users from cache C. The user is sent to another country D. The origin deletes the object Answer: B. The first request for an uncached object is slower because of the extra hop, but later users get it from the edge cache. (Key point: Cache miss effect) ## Related in Internet and Intranet - [Redundant network design for insurance company](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Aredundant-network-xyz-insurance) — Internet and Intranet, chapter 5 - [Redundant network for IOE Pulchowk campus](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Aredundant-network-ioe-pulchowk) — Internet and Intranet, chapter 5 ## Related in other subjects - [Reducing power loss in distribution systems](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Apower-loss-reduction-distribution) — Transmission and Distribution System Design, chapter 7, asked 2× - [Information retrieval in cloud](https://www.bctengineer.com/question.md?key=semester_8%2Finformation_system%3Air-importance-in-cloud) — Information System, chapter 7 - [Radial, loop and network distribution merits](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aradial-loop-network-merits) — Transmission and Distribution System Design, chapter 5 - [Flow distribution in five-pipe network](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Anetwork-five-pipes-flow-distribution) — Hydraulics, chapter 3