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Official PYQ · 2006Q46Transport Layer: Flow Control, Congestion Control, UDP, TCP, SocketsMCQ1 mark

GATE CS 2006 Q46 — Transport Layer: Flow Control, Congestion Control, UDP, TCP, Sockets

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Station AA needs to send a message consisting of 99 packets to Station BB using a sliding window (window size 33) and go-back-nn error control strategy. All packets are ready and immediately available for transmission. If every 55th packet that AA transmits gets lost (but no acks from BB ever get lost), then what is the number of packets that AA will transmit for sending the message to BB? 

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Answer: C

In Go-Back-NN (GBN) with a window size of 33, if a packet is lost, the receiver discards all subsequent packets. Upon timeout, the sender must retransmit the lost packet and all subsequent outstanding packets in the window.

Since every 5th5^{\text{th}} transmission is lost, let us trace the transmissions step-by-step:

  1. Tx 1: Packet 1 (Received successfully)
  2. Tx 2: Packet 2 (Received successfully)
  3. Tx 3: Packet 3 (Received successfully)
  4. Tx 4: Packet 4 (Received successfully)
  5. Tx 5: Packet 5 (LOST)
  6. Tx 6: Packet 6 (Discarded by receiver, expecting 5)
  7. Tx 7: Packet 7 (Discarded by receiver, expecting 5)

    8. [Timeout for Packet 5 occurs. Sender retransmits from Packet 5]

  8. Tx 8: Packet 5 (Received successfully)
  9. Tx 9: Packet 6 (Received successfully)
  10. Tx 10: Packet 7 (LOST)
  11. Tx 11: Packet 8 (Discarded by receiver, expecting 7)
  12. Tx 12: Packet 9 (Discarded by receiver, expecting 7)

    13. [Timeout for Packet 7 occurs. Sender retransmits from Packet 7]

  13. Tx 13: Packet 7 (Received successfully)
  14. Tx 14: Packet 8 (Received successfully)
  15. Tx 15: Packet 9 (LOST)

    16. [Timeout for Packet 9 occurs. Sender retransmits Packet 9]

  16. Tx 16: Packet 9 (Received successfully)

Thus, a total of 16 transmissions are required to successfully send all 9 packets.

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