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How the Sliding Window protocol will operate when using cumulative acknowledgement

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• Review: the necessity for the receiver to transmit (old) ACK frames
  • In the dicsussion of Stop-and-Wait, we learned that:

    When the receiver receives a re-transmission (of an old data frame: The receiver has transmit a "old" ACK frame This must be done to: Recover from the lost of an ACK frame

    Example:

    The ACK frame is lost: The sender will time out and re-transmit the data frame: The receiver must transmit an old ACK to receover from the ACK frame lost !!!!

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• Cumulative ACK scheme: be careful what you acknowledge !!!
  • Recall that in the cumulative ACK scheme:

    ACK n will acknowledge old data frames also !!!

    Important example:

    ACK 3 will also acknowledge the lost data frame #2:

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  • Conclussion:

    When the receiver receives an out-of-order data frame: the receiver must transmit a ACK frame that does not acknowledges a missing data frame

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• How to use a cumulative ACK frame in case of error
  • Problem statement:

    What ACK sequence number does the receiver transmit in the following situation:

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  • Solution: repeat the last ACK

    Example:

    Repeating the last ACK 2 serves 2 purposes:

    ACK 2 can be used to recover a possible lost ACK frame !!! ACK 2 does not acknowledge the lost (missing) frame 1 !!! (because ACK 2 acknowledges data frames with send seq no ≤ 1)