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Unformatted text preview: CSCI 415 Computer Networks Homework 4 Solution Saad Mneimneh Hunter College of CUNY Problem 1: Sliding window algorithm The sliding window algorithm with unbounded sequence numbers is given in the notes. Upon receiving an RN , the sender checks if RN > SN and slides the window. Similarly, upon receiving an SN , the receiver checks if SN = RN and slides the window. Describe the modifications needed (if any) for these two rules when the sequence numbers are sent modulo p (bounded). ANSWER : The rule for the receiver does not need to change. However, the rule for the sender will not work as it is possible to receive a smaller subsequent num- ber after performing the modulo p operation. More precisely, the sender needs to check if the received RN is equal to any of SN + 1 mod p...SN + n mod p , and slide the window accordingly. Problem 2: Modulus without FIFO Assume that a sliding window algorithm is used but the FIFO property does not hold. Instead, consider only frames that are sent and received (possibly with error). If a frame is the i th frame sent and the j th frame received, then | i- j | K . Note that when K = 0, we have the FIFO property. Heres an example scenario with K = 3. 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 (a) Show that if the receiver receives SN at time t , then RN ( t )- n- K SN RN ( t ) + n- 1 ANSWER : If frame SN was received at time t , consider the time t < t at which the received frame SN was transmitted. We can establish three inequal-was transmitted....
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- Spring '08
- Computer Networks