GATE CS 2014 Set 2 — Question 37
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Computer Networks → Network Fundamentals → OSI Model
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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
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?
Q—R1—R2—R3—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?
Correct answer
(C) Only I2 and I3
Solution
An intruder sniffing at router R2 can capture all packets that pass through it. The path is Q—R1—R2—R3—H. Let's analyze what information is visible in the packets at R2.
1.[I3] The IP addresses of Q and H: The IP header of every packet contains the source IP address (Q's IP) and the destination IP address (H's IP). This information is essential for routing and is not encrypted. R2 itself must read the destination IP address to forward the packet. Therefore, an intruder at R2 can learn I3.
2.[I2] The TCP port numbers at Q and H: The IP packet's payload is a TCP segment (since HTTP runs over TCP). The TCP header contains the source and destination port numbers. Like the IP header, the TCP header is not encrypted by session-layer encryption (like TLS/SSL). The encryption applies to the TCP payload (the application data). Therefore, an intruder at R2 can learn I2.
3.[I1] The URL of the file downloaded by Q: The URL is part of the HTTP GET request, which is application-layer data. The problem states that "Session layer encryption is used". This implies that the application data, including the HTTP request containing the URL, is encrypted. This is standard practice in protocols like HTTPS (HTTP over TLS/SSL). Since the URL is in the encrypted payload of the TCP segment, an intruder at R2 cannot read it. Therefore, the intruder cannot learn I1.
4.[I4] The link layer addresses of Q and H: Link layer addresses (like MAC addresses) are used for addressing on a single physical link or network segment. The link layer frame is re-created at each hop (router).
- The packet from Q to R1 has Q's MAC as source and R1's MAC as destination.
- The packet from R1 to R2 has R1's MAC as source and R2's MAC as destination.
- The packet from R2 to R3 has R2's MAC as source and R3's MAC as destination.
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