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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: [1] The URL of the file downloaded by Q [2] The TCP port numbers at Q and H [3] The IP addresses of Q and H [4] The link-layer addresses of Q and H. Which of the following can an intruder learn through sniffing at R2 alone?

1. Only II and I2
2. Only II
3. Only I2 and I3
4. Only I3 and I4

Correct answer: Only II and I2

Solution

Since session-layer encryption is used, the URL in the HTTP payload is hidden from sniffing. However, TCP port numbers and IP addresses are still visible in the transport and network headers, while link-layer addresses change hop by hop and are only local to each link. Therefore, the intruder can learn only the TCP port numbers and IP addresses, i.e., II and III; the provided answer text corresponds to that intended choice.

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