cnp3-ebook/principles/reliability

Exit Zen

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
177 ../../principles/reliability.rst:856

	English The figure below shows the FSM of a simple `go-back-n` receiver. This receiver uses two variables : `lastack` and `next`. `next` is the next expected sequence number and `lastack` the sequence number of the last data frame that has been acknowledged. The receiver only accepts the frame that are received in sequence. `maxseq` is the number of different sequence numbers (:math:`2^n`). 385/10000 · 385
178 ../../principles/reliability.rst:890

	English A `go-back-n` sender is also very simple. It uses a sending buffer that can store an entire sliding window of frames [#fsizesliding]_. The frames are sent with increasing sequence numbers (modulo `maxseq`). The sender must wait for an acknowledgment once its sending buffer is full. When a `go-back-n` sender receives an acknowledgment, it removes from the sending buffer all the acknowledged frames and uses a retransmission timer to detect frame losses. A simple `go-back-n` sender maintains one retransmission timer per connection. This timer is started when the first frame is sent. When the `go-back-n sender` receives an acknowledgment, it restarts the retransmission timer only if there are still unacknowledged frames in its sending buffer. When the retransmission timer expires, the `go-back-n` sender assumes that all the unacknowledged frames currently stored in its sending buffer have been lost. It thus retransmits all the unacknowledged frames in the buffer and restarts its retransmission timer. 1011/10000 · 1011
179 ../../principles/reliability.rst:932

	English The operation of `go-back-n` is illustrated in the figure below. In this figure, note that upon reception of the out-of-sequence frame `D(2,c)`, the receiver returns a cumulative acknowledgment `C(OK,0)` that acknowledges all the frames that have been received in sequence. The lost frame is retransmitted upon the expiration of the retransmission timer. 354/10000 · 354
180 ../../principles/reliability.rst:938

	English Go-back-n : example 19/10000 · 19
181 ../../principles/reliability.rst:941

	English The main advantage of `go-back-n` is that it can be easily implemented, and it can also provide good performance when only a few frames are lost. However, when there are many losses, the performance of `go-back-n` quickly drops for two reasons : 245/10000 · 245
182 ../../principles/reliability.rst:943

	English the `go-back-n` receiver does not accept out-of-sequence frames 63/10000 · 63
183 ../../principles/reliability.rst:944

	English the `go-back-n` sender retransmits all unacknowledged frames once it has detected a loss 88/10000 · 88

Shortcut	Action
`?`	Open available keyboard shortcuts.
`Alt` + `Home`	Navigate to the first translation in the current search.
`Alt` + `End`	Navigate to the last translation in the current search.
`Alt` + `PageUp` or `Ctrl` + `↑` or `Alt` + `↑` or `Cmd` + `↑` or	Navigate to the previous translation in the current search.
`Alt` + `PageDown` or `Ctrl` + `↓` or `Alt` + `↓` or `Cmd` + `↓` or	Navigate to the next translation in the current search.
`Ctrl` + `Enter` or `Cmd` + `Enter`	Submit current form; this works the same as pressing Save and continue while editing translation.
`Ctrl` + `Shift` + `Enter` or `Cmd` + `Shift` +`Enter`	Unmark translation as Needing edit and submit it.
`Alt` + `Enter` or `Option` + `Enter`	Submit the string as a suggestion; this works the same as pressing Suggest while editing translation.
`Ctrl` + `E` or `Cmd` + `E`	Focus on translation editor.
`Ctrl` + `U` or `Cmd` + `U`	Focus on comment editor.
`Ctrl` + `M` or `Cmd` + `M`	Shows Automatic suggestions tab.
`Ctrl` + `1` to `Ctrl` + `9` or `Cmd` + `1` to `Cmd` + `9`	Copies placeable of a given number from source string.
`Ctrl` + `M` followed by `1` to `9` or `Cmd` + `M` followed by `1` to `9`	Copy the machine translation of a given number to current translation.
`Ctrl` + `I` followed by `1` to `9` or `Cmd` + `I` followed by `1` to `9`	Ignore one item in the list of failing checks.
`Ctrl` + `J` or `Cmd` + `J`	Shows the Nearby strings tab.
`Ctrl` + `S` or `Cmd` + `S`	Focus on search field.
`Ctrl` + `O` or `Cmd` + `O`	Copy the source string.
`Ctrl` + `Y` or `Cmd` + `Y`	Toggle the Needs editing checkbox.
`→`	Browse the next translation string.
`←`	Browse the previous translation string.