Computer Networks | Set 3

3

Following Questions have been asked in GATE 2011 CS exam.

1) A layer-4 firewall ( a device that can look at all protocol headers up to the transport layer) CANNOT
(A) block HTTP traffic during 9:00PM and 5:00AM
(B) block all ICMP traffic
(C) stop incoming traffic from a specific IP address but allow outgoing traffic to same IP
(D) block TCP traffic from a specific user on a specific IP address on multi-user system during 9:00PM and 5:00AM

Answer (A)
HTTP is an application layer protocol. Since firewal is at layer 4, it cannot block HTTP data.



2) Consider different activities related to email.

   m1:Send an email from a mail client to mail server
   m2:Download an email from mailbox server to a mail client
   m3:Checking email in a web browser

Which is the applicable level protocol user in each activity?
(A) m1:HTTP, m2:SMTP, m3:POP
(B) m1:SMTP, m2:FTP, m3:HTTP
(C) m1:SMTP, m2:POP, m3:HTTP
(D) m1:POP, m2:SMTP, m3:IMAP

Answer (C)
Simple Mail Transfer Protocol (SMTP) is typically used by user clients for sending mails.
Post Office Protocol (POP) is used by clients for receiving mails.
Checking mails in web browser is a simple HTTP process.



3) Consider a network with five nodes, N1 to N5, as shown below.

The network uses a a Distance Vector Routing Distance Vector Routing protocol. Once the routes have stabilized, the distance vectors at different nodes are as following.
N1:(0, 1, 7, 8, 4)
N2:(1, 0, 6, 7, 3)
N3:(7, 6, 0, 2, 6)
N4:(8, 7, 2, 0, 4)
N5:(4, 3, 6, 4, 0)
Each distance vector is the distance of the best known path at that instance to nodes, N1 to N5, where the distance to itself is 0. Also, all links are symmetric and the cost is identical in both directions. In each round, all nodes exchange their distance vectors with their respective neighbours. Then all nodes update their distance vectors. In between two rounds, any change in cost of a link will cause the two incident nodes to change only that entry in their distance vectors.

The cost of link N2-N3 reduces to 2 (in both directions). After the next round of update what will be the new distance vector at node, N3?
(A) (3, 2, 0, 2, 5)
(B) (3, 2, 0, 2, 6)
(C) (7, 2, 0, 2, 5)
(D) (7, 2, 0, 2, 6)

Answer (A)
In the next round, every node will send and receive distance vectors to and from neighbors, and update its dostance vector.
N3 will recieve (1, 0, 2, 7, 3) from N2 and it will update distances to N1 and N5 as 3 and 5 respectively.



4) After the update in the previous question, the link N1-N2 goes down. N2 will reflect this change immediately in its distance vector as cost, ∞. After the NEXT ROUND of update, what will be cost to N1 in the distance vector of N3?
(A) 3
(B) 9
(C) 10
(D) ∞

Answer (C)
In the next round, N3 will receive distance from N2 to N1 as infinite. It will receive distance from N4 to N1 as 8. So it will update distance to N1 as 8 + 2.

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Please write comments if you find any of the answers/explanations incorrect, or you want to share more information about the topics discussed above.

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