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Computer Networking : A Top-Down Approach

Computer Networking : A Top-down Approach

Authors: James F. Kurose, Keith W. Ross
ISBN: 9780132856201
Edition: 6
Chapter: Transport Layer
Exercise: Problems
Question: 6

 

Question

Consider our motivation for correcting protocol rdt2.1. Show that the receiver, shown in Figure 3.57, when operating with the sender shown in Figure 3.11, can lead the sender and receiver to enter into a deadlock state, where each is waiting for an event that will never occur.


Answer

Consider the states of sender and receiver as follows:

Sender state as “wait for call 1 from above”.

Receiver state as “wait for 1 from below”.

 

  • Initially sender sends a packet with sequence number 1.
  • After sending the packet, the state of the sender will be “wait for ACK or NAK 1” (indicates that sender waits for acknowledgement ACK or NAK from receiver).
  • The receiver receives the packet 1 and sends the ACK.
  • After acknowledging the packet, the state of the receiver is “wait for 0 from below” (indicates that the receiver is waiting for packet 0).
  • Assume that sender receives the corrupted ACK sent by the receiver for packet 1.
  • Since the sender received the corrupted ACK, it transmits the packet 1 again.
  • The receiver sends a NAK as it received packet with sequence number 1 again instead of packet with sequence number 0.
  • The sender sends the packet with sequence number 1 again.
  • The receiver sends NAK again to sender.

 

Therefore, the sender repeats the sending packet with sequence number 1 and the receiver sends NAK again to sender. This raises a deadlock stage for both sender and the receiver.

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