--- title: "Proxy array load balancing types for big ISP" subject: "Internet and Intranet" chapter: 5 key: "semester_8/internet_and_intranet:proxy-array-lb-mechanisms-isp" source: https://www.bctengineer.com/q/semester_8/internet_and_intranet/proxy-array-lb-mechanisms-isp site: BCT Engineering --- # Proxy array load balancing types for big ISP Internet and Intranet · Chapter 5: Designing Internet Systems and Servers - Asked in: 2074 Magh, 4+6 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/proxy-array-lb-mechanisms-isp - Whole chapter: https://www.bctengineer.com/md/semester_8/internet_and_intranet/chapter-5 ## Question Explain different types of proxy array load balancing mechanism and also explain in detail which mechanism is appropriate for a big ISP. ## Short answer Proxy array load balancing types: DNS round robin, hardware switch, hash-based CARP, redundant mirrored array, and ICP hierarchy. For a large ISP, a CARP-style hash-based array with redundant layer-4 load balancers is best: it avoids duplicate caches, scales linearly, and health checks rehash traffic on failure. ## Answer #### Proxy array load balancing A **proxy array** is a group of proxy (cache) servers that work together as one logical cache. Load balancing decides which member serves which request, so that no proxy is overloaded and the total cache is used efficiently. **1. DNS round robin.** The proxy name has several A records; DNS returns them in rotation. Simple and cheap, but not aware of load or failures (cached DNS answers continue to point to a dead proxy), and the same URL may be cached in many proxies (duplicate copies, low hit rate). **2. Hardware / layer-4 or layer-7 switch (load balancer).** A switch in front of the proxies spreads connections by round robin, least connections or URL-based rules. It detects failed proxies, but it may be a single point of failure and costs money. **3. Hash-based (non-redundant) array, e.g. CARP.** The URL (or host name) is hashed and the result selects exactly one proxy. Each object lives in only one proxy, so the total cache size is the sum of all members and the hit rate is high. Adding or removing a proxy remaps only a small fraction of the URLs. **4. Redundant array (mirrored / replicated).** Each object is stored in two or more proxies, or traffic is directed to a pair. A proxy failure is hidden from users, but the effective cache is smaller. **5. Hierarchical / ICP (sibling) caching.** Proxies ask their neighbours with ICP before going to the origin. This gives some sharing but adds inter-proxy traffic and delay. ```text Clients --> [Front end: switch / PAC file] | | | hash(URL) selects one proxy [P1] [P2] [P3] ... --> Internet ``` #### Mechanism appropriate for a big ISP A **hash-based array (CARP)** combined with a **layer-4 load balancer for redundancy** is best. - An ISP has millions of users and a huge set of objects. With hashing, every URL is cached once, so the combined disk of all proxies acts as one very large cache and the hit ratio is high; round robin would duplicate popular objects and waste disk space. - It scales linearly: proxies are added as traffic grows, and because of the hashing scheme only about 1/n of the cached URLs are lost when one is added. - There is no inter-proxy query traffic (unlike ICP), so no extra delay or load. - Requests are distributed according to configurable weights (capacity), which suits proxies of different sizes. - Failure handling: health checks remove a dead proxy and its share is re-hashed to the others; a pair of load balancers (VRRP/HSRP) or WCCP routers avoids a single point of failure. - Transparent redirection (WCCP or policy routing) removes the need to configure client browsers, which is impractical for an ISP. Hence an ISP uses a CARP-style non-redundant array for efficiency, with redundant front-end devices and health monitoring for availability. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a key weakness of DNS round robin for proxy load balancing? A. It is not aware of load or failures, and cached DNS answers can point to a dead proxy B. It is very expensive C. It cannot use more than two proxies D. It requires every client to run a PAC script Answer: A. Round robin only rotates addresses; it does not detect failures and cached answers keep pointing to dead proxies. (Key point: DNS round robin weakness) 2. Why does a hash-based (CARP) array give a high hit rate? A. Every object is copied to all proxies B. Each URL is cached in exactly one proxy, so the total cache is the sum of all members C. Proxies query each other with ICP D. The origin server decides where objects are stored Answer: B. Hashing assigns each URL to one proxy, so there are no duplicates and the combined disk acts as one large cache. (Key point: Hash-based array) 3. What is the disadvantage of ICP sibling caching? A. It cannot share any objects B. It needs client browsers to be configured C. It adds inter-proxy traffic and delay D. It stores each object only once Answer: C. ICP proxies ask neighbours before the origin, which gives sharing but adds extra queries and delay. (Key point: ICP drawback) 4. Which mechanism does the answer recommend for a big ISP? A. Plain DNS round robin B. Port-based redundancy only C. ICP hierarchy of siblings D. Hash-based array (CARP) with a layer-4 load balancer for redundancy Answer: D. CARP gives a large single-copy cache, scales linearly and has no ICP traffic; redundant front-ends give availability. (Key point: Choice for big ISP) ## Related in Internet and Intranet - [Proxy load balancing benefits; non-redundant array](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Aproxy-lb-benefits-non-redundant) — Internet and Intranet, chapter 5 - [Short note: Proxy load balancing](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Anote-proxy-load-balancing) — Internet and Intranet, chapter 5, asked 2× - [RADIUS; proxy load balancing with CARP](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Aradius-proxy-lb-carp) — Internet and Intranet, chapter 5