multiple choice (data communication) 1. Using 6 bit sequence numbers, what is th
ID: 3764723 • Letter: M
Question
multiple choice (data communication)
1. Using 6 bit sequence numbers, what is the maximum size of the send window for a selective repeat protocol ?
a. 32 b. 64 c. 63 d. 31
2. A go-back-n sender sends packets 4,5,6,7,8,9, then waits. The sender then receives an ACK packet with ackNo=7. What does it mean ?
a. packets up to and including 7 have been received correctly and the receiver now expects packet 8
b. packet 7 has been received OK
c. packets up to and including 6 have been received correctly and the receiver now expects packet 7
d. packets up to and including 7 have been received correctly and the sender must wait for the receive buffer to drain before it can send another packet
3. A stop-and-wait protocol runs on a 100 Mbit/s link with a roundtrip time of 10 ms. All packets are 4000 bits long. What is the utilization percentage on the link ?
a. 0.4% b. 8% c. 4% d. 40% e. 5%
4. What can a TCP server (receiver) do if the receiving process is very slow to extract received data from the buffer (possibly because the process is suspended) in order to prevent dropping new incoming segments due to no buffer space ?
a. the receiver TCP cannot prevent buffer overflow on the receiver side
b. the receiver TCP can delay sending back ACK to slow down the client
c. the receiver TCP can temporarily shutdown the receiver window by setting rwnd to 0 in the next packet sent to the TCP client
d. the receiver TCP can send a RST packet back to the client to suspend its transmissions for a while
Explanation / Answer
1) maximum soze with will bw 2^6 -1 == 64-1 = 63. so correct option is c
2) that means packets {4,5,6,7} has been successfully receuved. so correct options is a
3) Tx + propagation + ACK propagation = Tx + RTT
Tx = 4000/100 MB ==> 4000/100 x 10^-6
===>40 x 10^-6 ==> 0.00004 seconds
so given time is 10ms
utilization = 0.00004/(0.01+0.00004) ==> 0.00004/0.01004 ==> 0.00398 ~~ 0.004
so 0.4% is correct answer....option a)
4) d. the receiver TCP can send a RST packet back to the client to suspend its transmissions for a while
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