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CS310 Computer Networks Short Test 3 (22/4/2013) Total marks: 10 Time allowed: 15 minutes Name: ID: 1) Consider the following overly simplified view of TCP buffer on the sender's side showing the sliding window used to implement flow control. Bytes 202, 203, 204, 205 have been sent but are unacknowledged. The size of congestion window (cwnd) is 15 bytes. 201 202 203 204 205 206 207 208 209 210 211 Sent and acknowledged Next byte to be sent Can't be sent until window opens more [1 mark] a) What is the size of receiver window (rwnd). b) Suppose the sender receives a packet with an acknowledgment value of 206, and rwnd remains the same. If cwnd becomes 8 bytes, sketch the state of the new window. How many bytes can the sender effectively send? [3 marks]

2) Consider sending data packets over a communication channel, which can introduce bit errors with a low probability of p. The channel however does not lose any packets. Apart from checksum, what are the two other mechanisms needed to facilitate reliable data transfer over such a channel. [1 mark] 3) Suppose the slow start threshold (ssthresh) for TCP congestion control is set to 32 segments. The sender sends 32 segments and receives acknowledgment for all the segments. How many segments can the sender send in the next transmission round? (Assume amount of data on the sender side, and the rwnd is infinitely large). [1 mark] 4) TCP provides reliable data transport to the application layer data. For the following scenarios, sketch and complete the interactions between the TCP sender and TCP receiver, showing how they handle segment loss. For scenario 2 fill in the blank as well. [4 marks]

Host A Scenario 1 Seq=3500 Seq=4700 Seq=5009 Seq=5500 Seq=3800 Host B | Host A Scenario 2 Seq=600 data 200 bytes Ack= X Host B