GATE | GATE-CS-2014-(Set-2) | Question 65

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?

(A) Only I1 and I2
(B) Only I1
(C) Only I2 and I3
(D) Only I3 and I4


Answer: (C)

Explanation:

An Intruder can’t learn [I1] through sniffing at R2 because 
URLs and Download are functioned at Application layer of OSI Model.

An Intruder can learn [I2] through sniffing at R2 because
Port Numbers are encapsulated in the payload field of IP Datagram.

An Intruder can learn [I3] through sniffing at R2 because IP 
Addresses and Routers are functioned at network layer of OSI Model.

An Intruder can’t learn [I4] through sniffing at R2 because 
it is related to Data Link Layer of OSI Model.


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