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GATE CS 2023 · Computer Networks

Question 52

MSQ2 marksNo negative markingHard (estimate)

Question

Suppose in a web browser, you click on the www.gate-2023.in URL. The browser cache is empty. The IP address for this URL is not cached in your local host, so a DNS lookup is triggered (by the local DNS server deployed on your local host) over the 3-tier DNS hierarchy in an iterative mode. No resource records are cached anywhere across all DNS servers.

Let RTT denote the round trip time between your local host and DNS servers in the DNS hierarchy. The round trip time between the local host and the web server hosting www.gate-2023.in is also equal to RTT. The HTML file associated with the URL is small enough to have negligible transmission time and negligible rendering time by your web browser, which references 10 equally small objects on the same web server.

Which of the following statements is/are CORRECT about the minimum elapsed time between clicking on the URL and your browser fully rendering it?

Choose every correct option
Official answer and solution
Official answer
C and D
From
Official GATE 2023 answer key

Solution

Sum DNS resolution RTTs and HTTP fetching RTTs under non-persistent parallel vs persistent pipelined modes.

  1. DNS Resolution Time:
    Iterative resolution over a 3-tier DNS hierarchy (Root DNS, TLD .in DNS, Authoritative DNS) without local or intermediate caching requires 3 sequential queries from the local DNS server on the host, taking:

    DNS delay=3×RTT\text{DNS delay} = 3 \times \text{RTT}

  2. Case 1: Non-persistent HTTP with 5 parallel TCP connections:

    • Fetching base HTML file: 1 RTT (TCP handshake) + 1 RTT (HTTP request/response) = 2 RTTs.
    • Base HTML references 10 objects. With 5 parallel connections available:
      • First batch of 5 objects: 1 RTT (TCP handshakes) + 1 RTT (HTTP requests/responses) = 2 RTTs.
      • Second batch of 5 objects: 1 RTT (TCP handshakes) + 1 RTT (HTTP requests/responses) = 2 RTTs.
    • Total elapsed time = 3 (DNS)+2 (base)+2+2=9 RTTs3\text{ (DNS)} + 2\text{ (base)} + 2 + 2 = 9\text{ RTTs}. (Statement C is CORRECT)

  3. Case 2: Persistent HTTP with pipelining:

    • Fetching base HTML file: 1 RTT (TCP handshake) + 1 RTT (HTTP request/response) = 2 RTTs.
    • All 10 objects are requested over the already-open TCP connection in a single pipelined request burst: 1 RTT.
    • Total elapsed time = 3 (DNS)+2 (base)+1 (objects)=6 RTTs3\text{ (DNS)} + 2\text{ (base)} + 1\text{ (objects)} = 6\text{ RTTs}. (Statement D is CORRECT)
Final answer

Options (C, D)

Common trap

Forgetting the initial TCP 3-way handshake RTT for the web server or assuming all 10 objects download in parallel when connection limit is 5.

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

Source

Paper
GATE 2023 Computer Science and Information Technology (CS)
Question
52 of 65
Section
Computer Science and Information Technology

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