GATE CS 2016 Set 2 Q.35 (CS Q.25): Application Layer: DNS, HTTP, SMTP, FTP

Identify the correct sequence in which the following packets are transmitted on the network by a host when a browser requests a webpage from a remote server, assuming that the host has just been restarted.

HTTP GET request, DNS query, TCP SYN

DNS query, HTTP GET request, TCP SYN

DNS query, TCP SYN, HTTP GET request

TCP SYN, DNS query, HTTP GET request

Official answer and solution

Official answer: C
From: Official GATE 2016 answer key

Solution

Trace the protocol stack dependencies required to fetch a webpage starting from a fresh reboot.

When a host has just restarted, its local caches (DNS cache, ARP cache) are empty.

Trace the chronological transmission of network packets:

Step 1 (DNS Query): The browser only has the server's domain name (e.g., www.gate.ac.in). It cannot send IP packets to the server without its destination IP address. Hence, it first sends a DNS query packet to resolve the domain name.

Step 2 (TCP SYN): Once the DNS response provides the remote server's IP address, HTTP requires an established TCP connection. The host initiates the TCP three-way handshake by transmitting a TCP SYN packet to the server.

Step 3 (HTTP GET request): After the three-way handshake completes (SYN $\to$ SYN-ACK $\to$ ACK), the application layer sends the HTTP GET request packet over the established TCP stream.

Thus, the transmission sequence is:

\text{DNS query} \longrightarrow \text{TCP SYN} \longrightarrow \text{HTTP GET request}

This matches option (C).

Final answer

Common trap

Placing the HTTP GET request first because it is the initiating action in the browser, forgetting lower-layer prerequisites.

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2016 Computer Science and Information Technology (CS) Session 2
Question: 35 of 65, printed as CS Q.25
Section: Computer Science and Information Technology

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