GATE CS 2024 Set 1 | Question 16
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 ?
Explanation
Answer: (C)
Follow what the browser needs at each step:
- (ii) It knows only the server’s name and the DNS cache is empty, so it must first send a DNS request for the IP address.
- (iv) With the address known, it opens the TCP connection by sending a SYN.
- (i) Over that connection it requests the index page.
- (iii) Only after the index page arrives does the browser learn which images are embedded, so an image GET comes last.
So the order is (ii), (iv), (i), (iii). Options A and D send the SYN before the address is known, and option B requests an image before the index page.
Answer: C