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  1. GATE CS
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  3. Computer Networks

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 DNS cache and the browser cache are both empty. A. HTTP GET, DNS query, TCP SYN, HTTP GET B. DNS query, HTTP GET, TCP SYN, HTTP GET C. DNS query, TCP SYN, HTTP GET D. TCP SYN, DNS query, HTTP GET

GATE 2016 · Computer Networks · TCP · medium

Answer: C. DNS query, TCP SYN, HTTP GET

  1. DNS resolution must come first: With empty DNS cache, the browser cannot know the server's IP address. It must first send a DNS query (typically via UDP port 53) to resolve the hostname. Without the IP address, no TCP connection can be initiated. This eliminates option D which starts with TCP SYN.
  2. TCP connection establishment comes second: Once the IP address is obtained from DNS, the browser initiates a TCP three-way handshake by sending TCP SYN to port 80 (HTTP) of the web server. HTTP runs over TCP, so the connection must be established before any HTTP request is sent. This eliminates option B (HTTP GET before TCP SYN).
  3. HTTP GET is sent last: After the TCP handshake completes (SYN -> SYN-ACK -> ACK), the browser sends the HTTP GET request. With empty browser cache, the full GET is sent to retrieve the page. The sequence is: DNS query -> TCP SYN -> HTTP GET, matching option C exactly.