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GATE CS 2014 Set 2 · Computer Networks

Question 37

MCQPrinted as CS Q.272 marksWrong answer: −2/3Medium (estimate)

Question

An IP machine Q has a path to another IP machine H via three IP routers R1, R2, and R3.

Q−R1−R2−R3−H\text{Q}-\text{R1}-\text{R2}-\text{R3}-\text{H}

H acts as an HTTP server, and Q connects to H via HTTP and downloads a file. Session layer encryption is used, with DES as the shared key encryption protocol. Consider the following four pieces of information:

[I1] The URL of the file downloaded by Q
[I2] The TCP port numbers at Q and H
[I3] The IP addresses of Q and H
[I4] The link layer addresses of Q and H

Which of I1, I2, I3, and I4 can an intruder learn through sniffing at R2 alone?

Choose one option
Official answer and solution
Official answer
C
From
Official GATE 2014 answer key

Solution

Identify the protocol header visibility at each layer of the OSI/TCP-IP stack when session-layer encryption is employed.

  1. Scope of Encryption:

    • Session layer encryption encrypts everything at the presentation and application layers (such as the HTTP payload and HTTP request headers, including the URL).
    • Thus, an intruder sniffing packets at router R2 cannot decrypt the application data, so [I1] (The URL) CANNOT be learned.

  2. Transport Layer Visibility:

    • TCP headers reside below the session layer and remain completely unencrypted.
    • Router R2 can inspect the TCP segment headers, revealing [I2] (The TCP port numbers at Q and H).

  3. Network Layer Visibility:

    • IP headers reside at the network layer and are necessary for intermediate routing.
    • Router R2 examines IP packet headers, revealing the original end-to-end source and destination IP addresses, [I3] (The IP addresses of Q and H).

  4. Data Link Layer Visibility:

    • Link layer (MAC) addresses change on each hop between network devices. When a frame arrives at R2 from R1, the link-layer header contains the MAC address of R1 as source and R2 as destination; when forwarded to R3, it has R2 as source and R3 as destination.
    • Neither end-host MAC address (Q or H) is present on the link between R1, R2, and R3. Thus, [I4] CANNOT be learned at R2.

  5. Conclusion:
    The intruder learns only I2 and I3, which corresponds to option (C).

Final answer

C

Common trap

Assuming MAC addresses travel end-to-end like IP addresses, whereas link-layer addresses are local to each physical link.

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

Source

Paper
GATE 2014 Computer Science and Information Technology (CS) Session 2
Question
37 of 65, printed as CS Q.27
Section
Computer Science and Information Technology

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