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 persons are numbered sequentially in some order:
i. HTTP GET request for the index page
ii. DNS request to resolve the web server name to its IP address
iii. HTTP GET request for an image object
iv. TCP SYN to open a connection to the server
Which one of the following is the CORRECT chronological order (earliest in time to latest) of the packets leaving the computer?
A. (iv), (i), (ii), (iii)
B. (ii), (iv), (i), (iii)
C. (ii), (i), (iv), (iii)
D. (iv), (ii), (i), (iii)
GATE 2024 · Computer Networks · Network Protocols · medium
DNS must come first (all caches empty): Since all caches are empty, the browser has no IP address for the web server. It must send a DNS request (ii) first to resolve the hostname to an IP address. Without this, no TCP connection can be established.
TCP SYN to open connection: After receiving the IP address from DNS, the browser initiates a TCP connection by sending the SYN packet (iv). The 3-way handshake must complete before any HTTP data can be sent.
HTTP GET for index page: Once the TCP connection is established, the browser sends an HTTP GET request (i) for the top-level index page. This is the first HTTP message and retrieves the HTML that lists embedded images.
HTTP GET for image objects: After receiving and parsing the index HTML, the browser discovers the embedded image URLs and sends HTTP GET requests (iii) for each image — all over the same TCP connection that was already opened.