Question

2. (i) [3] What are the three properties of Reliable Data Transfer?
(ii) [2+2] Suppose host A sends a packet to host B. How can A be ensured that the packet did not corrupt in the channel? If the packet was corrupted, what can be done?
(iii) [2+2] In our rdt protocols, why do we need to introduce sequence numbers and timers?
(iv) [5] Draw the Finite State Machine (FSM) for the receiver side of protocol rdt3.0.

Answer 1

2.(i) The properties of Reliable Data Transfer are:

• No packet loss
• No bit corruption
• The packets are received in the same order they were send

(ii) The positive acknowledgements and helps A to understand that the packet is not corrupted in the channel. If the packet was corrupted then the retransmission of the same packet is done.

(iii) Sequence numbers are required for a receiver to find out whether the arriving packet contains new data or it is a retransmission. Timers are used to handle losses in the channel. If the ACK for a transmitted packet is not received within the duration of the timer for the packet, the packet (or its ACK or NACK) is assumed to have been lost. Hence, the packet is retransmitted.

(iv) Finite State Machine (FSM) for the receiver side of protocol rdt3.0. (iv) Finite State Machine (FSM) for the receiver side of protocol rdt3.0.