cnp3-ebook/principles/reliability

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157 ../../principles/reliability.rst:696

	English Dealing with corrupted frames 29/10000 · 29
158 ../../principles/reliability.rst:698

	English The receiver FSM of the Alternating bit protocol discards all frames that contain an invalid CRC. This is the safest approach since the received frame can be completely different from the frame sent by the remote host. A receiver should not attempt at extracting information from a corrupted frame because it cannot know which portion of the frame has been affected by the error. 379/10000 · 379
159 ../../principles/reliability.rst:701

	English The figure below illustrates the operation of the alternating bit protocol. 75/10000 · 75
160 ../../principles/reliability.rst:731

	English The Alternating Bit Protocol can recover from the losses of data or control frames. This is illustrated in the two figures below. The first figure shows the loss of one data frame. 180/10000 · 180
161 ../../principles/reliability.rst:758

	English The second figure illustrates how the hosts handle the loss of one control frame. 81/10000 · 81
162 ../../principles/reliability.rst:794

	English The Alternating Bit Protocol can recover from transmission errors and frame losses. However, it has one important drawback. Consider two hosts that are directly connected by a 50 Kbits/sec satellite link that has a 250 milliseconds propagation delay. If these hosts send 1000 bits frames, then the maximum throughput that can be achieved by the alternating bit protocol is one frame every :math:`20+250+250=520` milliseconds if we ignore the transmission time of the acknowledgment. This is less than 2 Kbits/sec ! 514/10000 · 514
163 ../../principles/reliability.rst:804

	English Go-back-n and selective repeat 30/10000 · 30
164 ../../principles/reliability.rst:806

	English To overcome the performance limitations of the alternating bit protocol, reliable protocols rely on `pipelining`. This technique allows a sender to transmit several consecutive frames without being forced to wait for an acknowledgment after each frame. Each data frame contains a sequence number encoded as an `n` bits field. 325/10000 · 325
165 ../../principles/reliability.rst:812

	English Pipelining improves the performance of reliable protocols 57/10000 · 57
166 ../../principles/reliability.rst:814

	English `Pipelining` allows the sender to transmit frames at a higher rate. However this higher transmission rate may overload the receiver. In this case, the frames sent by the sender will not be correctly received by their final destination. The reliable protocols that rely on pipelining allow the sender to transmit `W` unacknowledged frames before being forced to wait for an acknowledgment from the receiving entity. 414/10000 · 414
167 ../../principles/reliability.rst:816

	English This is implemented by using a `sliding window`. The sliding window is the set of consecutive sequence numbers that the sender can use when transmitting frames without being forced to wait for an acknowledgment. The figure below shows a sliding window containing five frames (`6,7,8,9` and `10`). Two of these sequence numbers (`6` and `7`) have been used to send frames and only three sequence numbers (`8`, `9` and `10`) remain in the sliding window. The sliding window is said to be closed once all sequence numbers contained in the sliding window have been used. 566/10000 · 566
168 ../../principles/reliability.rst:822

	English The sliding window 18/10000 · 18
169 ../../principles/reliability.rst:824

	English The figure below illustrates the operation of the sliding window. It uses a sliding window of three frames. The sender can thus transmit three frames before being forced to wait for an acknowledgment. The sliding window moves to the higher sequence numbers upon the reception of each acknowledgment. When the first acknowledgment (`OK0`) is received, it enables the sender to move its sliding window to the right and sequence number `3` becomes available. This sequence number is used later to transmit the frame containing `d`. 528/10000 · 528
170 ../../principles/reliability.rst:831

	English Sliding window example 22/10000 · 22
171 ../../principles/reliability.rst:834

	English In practice, as the frame header includes an `n` bits field to encode the sequence number, only the sequence numbers between :math:`0` and :math:`2^{n}-1` can be used. This implies that, during a long transfer, the same sequence number will be used for different frames and the sliding window will wrap. This is illustrated in the figure below assuming that `2` bits are used to encode the sequence number in the frame header. Note that upon reception of `OK1`, the sender slides its window and can use sequence number `0` again. 529/10000 · 529
172 ../../principles/reliability.rst:841

	English Utilisation of the sliding window with modulo arithmetic 56/10000 · 56
173 ../../principles/reliability.rst:846

	English Unfortunately, frame losses do not disappear because a reliable protocol uses a sliding window. To recover from losses, a sliding window protocol must define : 159/10000 · 159
174 ../../principles/reliability.rst:848

	English a heuristic to detect frame losses 34/10000 · 34
175 ../../principles/reliability.rst:849

	English a `retransmission strategy` to retransmit the lost frames 57/10000 · 57
176 ../../principles/reliability.rst:854

	English The simplest sliding window protocol uses the `go-back-n` recovery. Intuitively, `go-back-n` operates as follows. A `go-back-n` receiver is as simple as possible. It only accepts the frames that arrive in-sequence. A `go-back-n` receiver discards any out-of-sequence frame that it receives. When `go-back-n` receives a data frame, it always returns an acknowledgment containing the sequence number of the last in-sequence frame that it has received. This acknowledgment is said to be `cumulative`. When a `go-back-n` receiver sends an acknowledgment for sequence number `x`, it implicitly acknowledges the reception of all frames whose sequence number is earlier than `x`. A key advantage of these cumulative acknowledgments is that it is easy to recover from the loss of an acknowledgment. Consider for example a `go-back-n` receiver that received frames `1`, `2` and `3`. It sent `OK1`, `OK2` and `OK3`. Unfortunately, `OK1` and `OK2` were lost. Thanks to the cumulative acknowledgments, when the sender receives `OK3`, it knows that all three frames have been correctly received. 1082/10000 · 1082

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