Exit Zen
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../../principles/reliability.rst:784
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The receiving window with selective repeat
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../../principles/sharing.rst:1220
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In practice, a router is connected to multiple input links. The figure below shows an example with two hosts.
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../../glossary.rst:90
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UDP
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../../principles/reliability.rst:786
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A `selective repeat` receiver discards all frames having an invalid CRC, and maintains the variable `lastack` as the sequence number of the last in-sequence frame that it has received. The receiver always includes the value of `lastack` in the acknowledgments that it sends. Some protocols also allow the `selective repeat` receiver to acknowledge the out-of-sequence frames that it has received. This can be done for example by placing the list of the correctly received, but out-of-sequence frames in the acknowledgments together with the `lastack` value.
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../../principles/sharing.rst:1279
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In general, the links have a non-zero delay. This is illustrated in the figure below where a delay has been added on the link between `R` and `C`.
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../../glossary.rst:92
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User Datagram Protocol is a protocol of the transport layer in the TCP/IP protocol suite that provides an unreliable connectionless service that includes a mechanism to detect corruption
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../../principles/linkstate.rst:98
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Static or dynamic link metrics ?
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../../principles/reliability.rst:788
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When a `selective repeat` receiver receives a data frame, it first verifies whether the frame is inside its receiving window. If yes, the frame is placed in the receive buffer. If not, the received frame is discarded and an acknowledgment containing `lastack` is sent to the sender. The receiver then removes all consecutive frames starting at `lastack` (if any) from the receive buffer. The payloads of these frames are delivered to the user, `lastack` and the receiving window are updated, and an acknowledgment acknowledging the last frame received in sequence is sent.
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../../principles/sharing.rst:1326
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Footnotes
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../../glossary.rst:18
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unicast
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../../principles/linkstate.rst:100
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As link state packets are flooded regularly, routers are able to measure the quality (e.g. delay or load) of their links and adjust the metric of each link according to its current quality. Such dynamic adjustments were included in the ARPANET routing protocol [MRR1979]_ . However, experience showed that it was difficult to tune the dynamic adjustments and ensure that no forwarding loops occur in the network [KZ1989]_. Today's link state routing protocols use metrics that are manually configured on the routers and are only changed by the network operators or network management tools [FRT2002]_.
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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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../../glossary.rst:20
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a transmission mode where an information is sent from one source to one recipient
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../../principles/linkstate.rst:104
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When a link fails, the two routers attached to the link detect the failure by the absence of HELLO messages received during the last :math:`k \times N` seconds. Once a router has detected the failure of one of its local links, it generates and floods a new LSP that no longer contains the failed link. This new LSP replaces the previous LSP in the network. In practice, the two routers attached to a link do not detect this failure exactly at the same time. During this period, some links may be announced in only one direction. This is illustrated in the figure below. Router `E` has detected the failure of link `E-B` and flooded a new LSP, but router `B` has not yet detected this failure.
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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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../../glossary.rst:292
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vnc
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../../principles/reliability.rst:798
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Selective repeat : example
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../../principles/sharing.rst:1331
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Some networking technologies allow to adjust dynamically the bandwidth of links. For example, some devices can reduce their bandwidth to preserve energy. We ignore these technologies in this basic course and assume that all links used inside the network have a fixed bandwidth.
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../../glossary.rst:294
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A networked application that allows to remotely access a computer's Graphical User Interface. See https://en.wikipedia.org/wiki/Virtual_Network_Computing
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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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