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The Sender Window Algorithm using cumulative ACKs

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• Recall: Sender window

  • Sender window:

    Sender window = the serie of send sequence numbers that the sender can use to identify a outstanding frame

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  • Fact:

    The sender must retain all outstanding data frames in its send buffer (In case the receiver requests the data frame be re-transmitted)

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• How the sender window changes (= "moves")

  • Update algorithm for the sender window:

    Let Sender Window = [ A .. B ] Sender receives an ACK frame Let X = recv seq no in received ACK frame if ( X > A ) { /* =================================================== Frames A, A+1, ... , X−1 have been acknowledged !! =================================================== */ for ( each Y = A, A+1, ..., X−1 ) do { /* ------------------------------------ Frame Y is acknowledged !!! ------------------------------------ */ Release buffer for frame Y } Sender Window = [X .. X + (B-A) +1)] // I.e.: set sender window to [X .....] }

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    (Use black board to show examples in class !!!)

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  • Example 1:

    Suppose the current sender window is: Sender window = [1, 2, 3, 4] The sender transmits the next data frame: The next data frame will use the first available number in the sender window is its send seq. no.!!! When the sender receives the ACK frame, the send window will be updated to [2,3,4,5]:

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  • Example 2:

    Suppose: Sender window = [1,2,3,4] Events: The sender transmits frames 1,2,3,4 But: Frame 1 is lost The receiver transmits back ACK frames 1,1,1 !!! because: ACK 1 does not acknowledge the lost frame 1 How the sender process the received ACK frames: Sender window = [1 .. 4] ACK 1: 1 > 1 ? No (do not update sender window) ACK 1: 1 > 1 ? No (do not update sender window) ACK 1: 1 > 1 ? No (do not update sender window) Graphically: Note: The sender window does not slide forward !!!         (because no send buffer has been released !!!).

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  • Example 3: previous example continues

    The sender times out and retransmits frame 1 and frame 1 was received correctly Current sender/receiver windows: Sender window = [1,2,3,4] Events: The sender retransmits frame 1 Frame 1 is received The receiver transmits ACK frame 5 !!! How the sender process the received ACK frame (#5): Send window = [1 .. 3] ACK 5: 5 > 1 ? Yes for each frame Y = 1, 2, 3, 4 do: Y = 1 ===> Release buffer for frame 1 Y = 2 ===> Release buffer for frame 1 Y = 3 ===> Release buffer for frame 1 Y = 4 ===> Release buffer for frame 1 Sender Window = [5 .. 8] (Because ACK 5 acknowledges frames 1,2,3,4 !!!) Graphically: Result: The sender will release all 4 buffered frames (1,2,3,4) Now the sender window will slide forward by 4 positions The sender will now buffer the data frames 5,6,7,8