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GATE CSE 2024 Session 1 Q16: Order of DNS, TCP SYN and HTTP GET packets when loading a page

Question

A user starts browsing a webpage hosted at a remote server. The browser opens a single TCP connection to fetch the entire webpage from the server. The webpage consists of a top-level index page with multiple embedded image objects. Assume that all caches (e.g., DNS cache, browser cache) are all initially empty. The following packets leave the user's computer in some order.

(i) HTTP GET request for the index page (ii) DNS request to resolve the web server's name to its IP address (iii) HTTP GET request for an image object (iv) TCP SYN to open a connection to the web server

Which one of the following is the CORRECT chronological order (earliest in time to latest) of the packets leaving the computer?

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Answer: C (official GATE 2024 final answer key)

Concept used: Name resolution precedes connection set-up, which precedes HTTP requests.

First step: The browser needs the server's IP address before it can send a TCP SYN, and learns of the images only after reading the index page.

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Source: official GATE 2024 Session 1 CS question paper and final answer key, organised by IISc Bangalore. Question paper · Answer key

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