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../../principles/reliability.rst:790
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The `selective repeat` sender maintains a sending buffer that can store up to `W` unacknowledged frames. These frames are sent as long as the sending buffer is not full. Several implementations of a `selective repeat` sender are possible. A simple implementation associates one retransmission timer to each frame. The timer is started when the frame is sent and canceled upon reception of an acknowledgment that covers this frame. When a retransmission timer expires, the corresponding frame is retransmitted and this retransmission timer is restarted. When an acknowledgment is received, all the frames that are covered by this acknowledgment are removed from the sending buffer and the sliding window is updated.
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../../principles/reliability.rst:792
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The figure below illustrates the operation of `selective repeat` when frames are lost. In this figure, `C(OK,x)` is used to indicate that all frames, up to and including sequence number `x` have been received correctly.
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../../principles/reliability.rst:798
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Selective repeat : example
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../../principles/reliability.rst:802
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Pure cumulative acknowledgments work well with the `go-back-n` strategy. However, with only cumulative acknowledgments a `selective repeat` sender cannot easily determine which frames have been correctly received after a data frame has been lost. For example, in the figure above, the second `C(OK,0)` does not inform explicitly the sender of the reception of `D(2,c)` and the sender could retransmit this frame although it has already been received. A possible solution to improve the performance of `selective repeat` is to provide additional information about the received frames in the acknowledgments that are returned by the receiver. For example, the receiver could add in the returned acknowledgment the list of the sequence numbers of all frames that have already been received. Such acknowledgments are sometimes called `selective acknowledgments`. This is illustrated in the figure above.
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../../principles/reliability.rst:811
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In the figure above, when the sender receives `C(OK,0,[2])`, it knows that all frames up to and including `D(0,...)` have been correctly received. It also knows that frame `D(2,...)` has been received and can cancel the retransmission timer associated to this frame. However, this frame should not be removed from the sending buffer before the reception of a cumulative acknowledgment (`C(OK,2)` in the figure above) that covers this frame.
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../../principles/reliability.rst:817
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A reliable protocol that uses `n` bits to encode its sequence number can send up to :math:`2^n` successive frames. However, to ensure a reliable delivery of the frames, `go-back-n` and `selective repeat` cannot use a sending window of :math:`2^n` frames. Consider first `go-back-n` and assume that a sender sends :math:`2^n` frames. These frames are received in-sequence by the destination, but all the returned acknowledgments are lost. The sender will retransmit all frames. These frames will all be accepted by the receiver and delivered a second time to the user. It is easy to see that this problem can be avoided if the maximum size of the sending window is :math:`{2^n}-1` frames. A similar problem occurs with `selective repeat`. However, as the receiver accepts out-of-sequence frames, a sending window of :math:`{2^n}-1` frames is not sufficient to ensure a reliable delivery. It can be easily shown that to avoid this problem, a `selective repeat` sender cannot use a window that is larger than :math:`\frac{2^n}{2}` frames.
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../../principles/reliability.rst:863
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Reliable protocols often need to send data in both directions. To reduce the overhead caused by the acknowledgments, most reliable protocols use `piggybacking`. Thanks to this technique, a datalink entity can place the acknowledgments and the receive window that it advertises for the opposite direction of the data flow inside the header of the data frames that it sends. The main advantage of piggybacking is that it reduces the overhead as it is not necessary to send a complete frame to carry an acknowledgment. This is illustrated in the figure below where the acknowledgment number is underlined in the data frames. Piggybacking is only used when data flows in both directions. A receiver will generate a pure acknowledgment when it does not send data in the opposite direction as shown in the bottom of the figure.
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../../principles/reliability.rst:869
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Piggybacking example
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../../principles/reliability.rst:879
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The size of the sliding window can be either fixed for a given protocol or negotiated during the connection establishment phase. Some protocols allow to change the maximum window size during the data transfer. We will explain these techniques with real protocols later.
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